I was inside RBMK 1500, and have a few questions :)

[Salvador]

So as I'm interested in nuclear physics , I and a few other friends that work in the tech field we went to a now shutdown and in the process of decommissioning NPP. For various reasons I won't mention the stations name or location but those of you who know much about reactor types etc will probably have a clue where it was.

So we walked through all the main parts , the reactor hall , controls room , support facilities , feedwater pumps, and the turbine generator or simply machine hall.Long story short , when at site I was amazed at the vast scale and monstrous size of everything starting from corridors and support buildings to the machine hall which was so big I had problems seeing the other end of it even from the middle of that hall.
the reactor is the RBMK channel type.
So here's the questions.

1) In the active zone I spent about 1 hour and that gave me about 2 uSv of dosage , if my calculations are correct that's not much overall although I would like to hear some feedback on this because it doesn't seem a lot in terms of average annual dosage for a person but the time scale in which I got that is also very short so how does that add up?

2) The RBMK and the CANDU (according to wikipedia) are the only two reactors in the world that can refuel while in operation , I saw the huge and tall crane which is at the rector hall which does the refueling, while it's done nobody stays in the reactor hall but instead the operator of the crane watches through a very thick glass window in a room that is sealed off to prevent radiation exposure.the question is what is the most prevelent type of radiation near the fuel cassette when it's taken out of the core and how far it reaches from the source also it's penetrating power , because I got some contradictory opinions about that yesterday.

3) Question about dust. the reactor hall had a floor entirely out of stainless steel all corridors up to the hall had the same , is that done because such a floor is easier to clean (decontaminate) in case some radiation source dust have settled on it , like for example when refueling?

4) Radiation being part of the EM field needs a source , say for example that dust particles from a radioactive metal like uranium have scattered across a given territory like in Chernobyl , is it true that the radiation intensity is dependent on the size of the particle/s and their overall quantity in that given area ?
so a larger single piece of radioactive material would have a larger amount of radiation emitted than a smaller piece of that same material.

5) the RBMK is said to have been a "good design" for the soviets because it gave them the chance of both producing civilian electricity and creating weapons grade plutonium at the same time hence the refueling while in operation. I read in "world-nuclear.org" that the only naturally fissile metal is uranium which is in the form of uranium ore which consists mostly of fertile U238 which cannot undergo a chain reaction itself.So the U235 does the chain reaction and the neutrons from that reaction hit the U238 which turns into the Pu239 which can then be used for weapons.
Does it become weapons grade in the reactor or does it need to undergo more "purifying" elsewhere, like the uranium that is enriched ?

and the last one for now , when they dig out the uranium if most of it's mass consists of the non fissile U238 using a gas centrifuge I read that they can separate the U238 from U235 simply because the U238 is heavier and in high rpm of the gas is located more to the outside , so they have to make the uranium in gaseous form before they can enrich it using this process ?

 

[SteamKing]

5) the RBMK is said to have been a "good design" for the soviets because it gave them the chance of both producing civilian electricity and creating weapons grade plutonium at the same time hence the refueling while in operation. I read in "world-nuclear.org" that the only naturally fissile metal is uranium which is in the form of uranium ore which consists mostly of fertile U238 which cannot undergo a chain reaction itself.So the U235 does the chain reaction and the neutrons from that reaction hit the U238 which turns into the Pu239 which can then be used for weapons.
Does it become weapons grade in the reactor or does it need to undergo more "purifying" elsewhere, like the uranium that is enriched ?

Weapons grade fissile material is usually purified to > 90% of the fissile isotope, either U-235 or Pu-239. In the case of U-235, this process is extremely expensive, involving either electromagnetic separation, separation by gaseous diffusion, or separation by centrifuge. Pu-239 can be separated from U-238 and U-235 using chemical means, since it is a different element altogether. Enrichment in any event does not take place inside the reactor.

and the last one for now , when they dig out the uranium if most of it's mass consists of the non fissile U238 using a gas centrifuge I read that they can separate the U238 from U235 simply because the U238 is heavier and in high rpm of the gas is located more to the outside , so they have to make the uranium in gaseous form before they can enrich it using this process ?

For ease of handling, uranium metal is usually converted to uranium hexafluoride gas (UF₆), which substance can be used in a gaseous diffusion plant to separate U-235 from U-238.

For more information on enrichment technologies, see this article:

https://en.wikipedia.org/wiki/Enriched_uranium

 

[mfb]

1) In the active zone I spent about 1 hour and that gave me about 2 uSv of dosage , if my calculations are correct that's not much overall although I would like to hear some feedback on this because it doesn't seem a lot in terms of average annual dosage for a person but the time scale in which I got that is also very short so how does that add up?

Natural background radiation is about 2000µSv/year, or 0.2µSv/hour, but depending on the place where you live. Within this hour your dose rate was higher, but if you compare it to the dose over a year, it is completely negligible. Moving from one place to another gives you larger changes in natural dose rate within a day, and airplane flights give doses significantly above 2 µSv.

2) The RBMK and the CANDU (according to wikipedia) are the only two reactors in the world that can refuel while in operation , I saw the huge and tall crane which is at the rector hall which does the refueling, while it's done nobody stays in the reactor hall but instead the operator of the crane watches through a very thick glass window in a room that is sealed off to prevent radiation exposure.the question is what is the most prevelent type of radiation near the fuel cassette when it's taken out of the core and how far it reaches from the source also it's penetrating power , because I got some contradictory opinions about that yesterday.

All radiation types contribute, but alpha and beta don't get far enough to be relevant, unless radioactive dust gets distributed. I guess a steel floor makes cleaning easier.

4) Radiation being part of the EM field needs a source , say for example that dust particles from a radioactive metal like uranium have scattered across a given territory like in Chernobyl , is it true that the radiation intensity is dependent on the size of the particle/s and their overall quantity in that given area ?
so a larger single piece of radioactive material would have a larger amount of radiation emitted than a smaller piece of that same material.

It does not depend on the shape, it just depends on the amount of radioactive material. More radioactive material, more radiation.

5) the RBMK is said to have been a "good design" for the soviets because it gave them the chance of both producing civilian electricity and creating weapons grade plutonium at the same time hence the refueling while in operation. I read in "world-nuclear.org" that the only naturally fissile metal is uranium which is in the form of uranium ore which consists mostly of fertile U238 which cannot undergo a chain reaction itself.So the U235 does the chain reaction and the neutrons from that reaction hit the U238 which turns into the Pu239 which can then be used for weapons.
Does it become weapons grade in the reactor or does it need to undergo more "purifying" elsewhere, like the uranium that is enriched ?

Most reactor types also produce other isotopes of plutonium, which prevent building a nuclear weapon out of them without expensive enrichment afterwards. If you can take out fuel during operation and extract the plutonium, you can get a better isotopic composition, and save time and money because processing gets easier.

 

[Hiddencamper]

2 uSv is around 0.2 mRem. That's about what I get doing a tour in my BWR (not counting lower elevations of containment). So that's pretty typical.

For spent fuel, using water you need at least 7 feet to reduce it down to "normal"/"occupational" dose levels (on the order of mRem or uSv). It's really nasty stuff, especially when you just pull it out of the core, and when unshielded it has the potential to deliver lethal doses in seconds to minutes.

 

[Salvador]

seems like the very enrichment , purifying process is the most complicated in all of the cycle from digging out of ground to putting inside formed pellets into the core until the critical mass and the start of the chain reaction which is possible the easiest step.
like for example if a criminal entity with limited knowledge and resources wanted to make a bomb , enriching uranium would be the step that would most likely fail them but if they got already enriched uranium I assume it would be comparably easy to then make a bomb if efficiency and maximum yield is not the prime target.

That's about what I get doing a tour in my BWR

What do you mean by that , sounds like you have a personal nuclear reactor in your basement ? :D that's probably not what you meant but I just had a little giggle about the idea of that.

Well yes they had large ponds deep into the sublevels of the RBMK were they keep the rods taken out of the active core and they store them there for 5 years until then they are carried further to the storage facility site.
also they built a pretty unique thing for transportation of still active and usable fuel rods, since the EU wanted them to shut down the RBMK yet the reactors were in good shape and just bit over half their primary lifetime expectancy of 30 years.a small railroad vessel with radiation absorbing walls they simply places the still usable fuel into it from the first reactor which was to be shut down first and transported it to the second were using the crane they simply inserted the rods into the core while the reactor was working.
Ok here's a follow up question.If I were to take out a cassette which has something like 36 ,or maybe less, can't remember , fuel assembly rods , what would be the average radiation intensity in the vicinity of such a cassette and if there was no walls or barriers anywhere around such a thing how far away would the radiation still be dangerous from it's source?
This is probably one of the most confusing things to the general public , the question of how far away from a highly energetic ionizing radiation source the radiation levels falls off to a safe one. I do understand this is dependent as mfb affirmed before on the mass of the source in question and it's atomic mass number properties (either U235 or 238)

Speaking about different radioactive elements, one more question from me , I have seen the question and thought myself about what elements are more dangerous, the ones with short half life or the ones with a long one.
From my own thinking and from what I read elsewhere it seems that the ones with a short half life are the most energetic ones since they release more radiation in a shorter amount of time instead of releasing the same amount or less in a very long amount of time, so if you are in their vicinity you get a much higher dose than from a longer half life one correct?
But they also say that the ones with the long half life are more dangerous in the long run not because you would receive a deadly or dangerous dosage but because they accumulate slowly in the environment if not taken care of and can later have the risk of being digested and accumulate in the human body , is this general viewpoint I just wrote correct?

And also if U235 has a half life of 700 million years then it means that naturally it's a very long half life metal which makes it a very very low power radiation source , my question is, does enriched uranium have the same half life as it's natural ore partner (I assume it has )
so that means that it a natural fuel with a high energy density that can be stored for extremely long periods of time without significant degradation in it's energy density up until the point when critical mass is reached and a chain reaction starts ?

here's the part which I start to misunderstand , what happens with uranium's half life after it is undergoing chain reaction ? I read that only small amount of it's total energy density is used in a typical reactor, which to me would indicate that most of the U235 is still there in the material but somehow unable to undergo further fission only I don't know why exactly , my guess would be because other isotopes of U235 have been formed which catch up the neutrons necessary for further sustained chain reaction ?

thank you so far for answers :)

 

[Hiddencamper]

I'm a senior reactor operator at a BWR. Operators will tour the plant shiftly verifying all the equipment is working properly, making adjustments to valves, adding oil to pumps, etc. I occasionally am out with my operators doing tours, or just do one on my own if I'm not running the control room.

For spent fuel, I do not have exact numbers. I have been quoted that a spent fuel bundle that is freshly irradiated in open air can deliver a whole body dose of over 1 million rad/hr.

It's important to distinguish between radiation, and radiation sources. The products inside of nuclear fuel are radiation sources. As long as they remain in the fuel rod, and there is shielding, they will never impact the health and safety of the public. If the shielding is lost, then the area directly around the fuel is dangerous, but it will not migrate outside. If the fuel rod ruptures, then it will go outside, and that's when there is a public health impact.

As for the fuel itself, my BWR fuel is enriched close to 5% U-235. About 92-94% U-238, and the rest is additives or gadolinium poison. By the time we discharge it from the core, it is around .6-.7% U-235, and around .6-.7% Pu-239 (it breeds some plutonium during operation that is usable as fuel). Fuel depletion is a main part of why we discharge fuel from the core. Other reasons include fission product buildup as you said, tested lifetime limits, and internal changes to the fuel that limit it's linear heat generation rate limits. It has nothing to do with half-life, that's a radioactive decay process, we use fission in power reactors.

 

[mfb]

Speaking about different radioactive elements, one more question from me , I have seen the question and thought myself about what elements are more dangerous, the ones with short half life or the ones with a long one.
From my own thinking and from what I read elsewhere it seems that the ones with a short half life are the most energetic ones since they release more radiation in a shorter amount of time instead of releasing the same amount or less in a very long amount of time, so if you are in their vicinity you get a much higher dose than from a longer half life one correct?
But they also say that the ones with the long half life are more dangerous in the long run not because you would receive a deadly or dangerous dosage but because they accumulate slowly in the environment if not taken care of and can later have the risk of being digested and accumulate in the human body , is this general viewpoint I just wrote correct?

They are dangerous in different ways. Per atom, a short-living isotope will lead to more radiation in the short term. Which is bad if you stand next to it, but it is good if the atom decays within the reactor lifetime, so you don't have to worry about containing it for longer timescales.

The lifetime of nuclei does not depend on their environment (apart from a few exceptions that are not relevant here). A 235U nucleus simply does not care if it is somewhere in a natural uranium deposit or somewhere in a processing plant. For technical applications, the half-life of both 238U and 235U is so long that it does not matter.

here's the part which I start to misunderstand , what happens with uranium's half life after it is undergoing chain reaction ? I read that only small amount of it's total energy density is used in a typical reactor, which to me would indicate that most of the U235 is still there in the material but somehow unable to undergo further fission only I don't know why exactly , my guess would be because other isotopes of U235 have been formed which catch up the neutrons necessary for further sustained chain reaction ?

Try to make a fire with a few pieces of wood in a huge pile of ash. It just won't work for similar reasons.

235U is a single isotope, there are no "other isotopes of 235U".

 

[jim hardy]

Ok here's a follow up question.If I were to take out a cassette which has something like 36 ,or maybe less, can't remember , fuel assembly rods , what would be the average radiation intensity in the vicinity of such a cassette and if there was no walls or barriers anywhere around such a thing how far away would the radiation still be dangerous from it's source?

I've only been around a PWR so don't know what is a "cassette" .

Spent fuel fresh out of the reactor is highly active . Here's an image from Forbes magazine and that's about what they look like... (i was never able to get such a good photo.)
http://www.forbes.com/sites/jamesco...nuclear-power-plant-be-refueled/#7cbfb650482c

The radiation from that fuel element as you see makes the water glow blue,
i daresay nobody would get close enough to measure its level with a meter,

Probably someone here could give you an educated estimate.
Must be thousands of Sv ?

On a practical note - With no water in between it and me, i wouldn't want to be near enough to see it , a few hundred meters.
I hope that gives you at least a gut feel for your question.

 

[Hiddencamper]

I've only been around a PWR so don't know what is a "cassette" .

Spent fuel fresh out of the reactor is highly active . Here's an image from Forbes magazine and that's about what they look like... (i was never able to get such a good photo.)
http://www.forbes.com/sites/jamesco...nuclear-power-plant-be-refueled/#7cbfb650482c

The radiation from that fuel element as you see makes the water glow blue,
i daresay nobody would get close enough to measure its level with a meter,

Probably someone here could give you an educated estimate.
Must be thousands of Sv ?

On a practical note - With no water in between it and me, i wouldn't want to be near enough to see it , a few hundred meters.
I hope that gives you at least a gut feel for your question.

That's a great picture of a BWR core. Probably Columbia generating station? It's definitely a "newer" BWR.

 

[Astronuc]

And also if U235 has a half life of 700 million years then it means that naturally it's a very long half life metal which makes it a very very low power radiation source , my question is, does enriched uranium have the same half life as it's natural ore partner (I assume it has )
so that means that it a natural fuel with a high energy density that can be stored for extremely long periods of time without significant degradation in it's energy density up until the point when critical mass is reached and a chain reaction starts ?

here's the part which I start to misunderstand , what happens with uranium's half life after it is undergoing chain reaction ? I read that only small amount of it's total energy density is used in a typical reactor, which to me would indicate that most of the U235 is still there in the material but somehow unable to undergo further fission only I don't know why exactly , my guess would be because other isotopes of U235 have been formed which catch up the neutrons necessary for further sustained chain reaction ?

U-235, like each radionuclide, has a unique half-life that does not change due to irradiation. A mass of U-235 would decay naturally, whether in a reactor or not. However, in a reactor, the U-235 will absorb a neutron, and either fission (about 84% probability), or emit a gamma and become U-236 in a less excited state. U-236 can absorb a neutron and become U-237, otherwise, U-236 emits an alpha particle producing Th-232, and U-237 emits a beta particle to become Np-237. U-238 may absorb a neutron and become U-239, which emits a beta particle and becomes Np-239, which emits a beta particle becoming Pu-239, which most likely fissions.

During operation in an LWR and graphite-moderated reactors fueled by enriched uranium, the U-235 is depleted, while some U-238 is converted to transuranics including Pu-239/-240/-241 and higher mass isotopes. Isotopes like Pu-239 and Pu-241 are fissile, so they begin to make up for some of the lost U-235.

The level of enrichment depends on the power density in the core, how long the reactor operates before refueling, and the fraction of the fuel replaced. Commercial reactors have a regulated/licensed limit of 5 wt% U-235 in U, or equivalent if MOX (U,Pu)O₂ is used. The 5 wt% limit is applied on a pellet and fuel rod basis, i.e., no commercial pellet (UO2) is fabricated with more than 5 wt% U-235, and in fact, BWR fuels tend to use a max of 4.90 and PWR fuels a max of 4.95%. Some special fuel can be fabricated with higher enrichment for special tests, and some special fuel for small research reactors use higher enrichments, but less than 20% U-235.

Commercial plants will shutdown periodically to remove some fuel, usually the most depleted, and add fresh fuel. The depleted or spent fuel not only has less U-235, but it has a lot of fission products, which parasitically absorb neutrons in competition with the fissile species. Typically, commercial plants consume about 5 to 5.5% of the uranium (most of the U-235, and some of the U-238) on an assembly basis. The peak burnup fuel rods consume about 6 % of the uranium, and some pellets consume about 6.5 to 7.5% of the uranium. The relative amounts of fuel rods and pellets depend on the power (and burnup) distribution in the core, which is radially and axially peaked, with the peak shifting according to operation.

 

[Astronuc]

Probably Columbia generating station?

That appears to be CGS. I can confirm with the author.

 

[Astronuc]

and the last one for now , when they dig out the uranium if most of it's mass consists of the non fissile U238 using a gas centrifuge I read that they can separate the U238 from U235 simply because the U238 is heavier and in high rpm of the gas is located more to the outside , so they have to make the uranium in gaseous form before they can enrich it using this process ?

Traditionally, uranium oxide is converted to UF₆, which has a relatively low boiling/vapor point. It makes it easy to process the stream. Centrifuge systems are fairly common for gaseous systems, and they make use of the different in mass (3 amu) between U-235 and U-238.

 

[jim hardy]

That's a great picture of a BWR core. Probably Columbia generating station? It's definitely a "newer" BWR.

Makes sense. I'm a PWR guy, and we don't leave the head studs in place.
I take it your BWR refueling water is not borated?

Was looking for a good picture of Cherenkov..

 

[Garlic]

I have a question, if the fuel rods are removed, no further reaction in the reactor can take place, right?
If that's true, why can't you shut down a nuclear reactor when it's not needed? You would just take the fuel rod and put it in a nearby unterground nuclear dump storage. Is it so, that the fuel rod is so active that there is no practical methods for moving them? If they are still critical, you would break them so that they are subcritical.

And one more question, if the reactors can't be shut down for a little time, can't we basically redirect the energy from the main power line to something else. I mean, it's easy to lose unwanted energy.

 

[Salvador]

thank you guys for contributing here , I appreciate that.

@Hiddencamper , that's interesting to know about your workplace.I'm definitely sure that a spent fuel rod has a very high radiation intensity around it I was more interested in knowing how far this radiation can go and still be dangerous , if there is no obstacle inbetween and the radiation source is not some secondary dust etc but just the spent fuel taken out of the core.
I ask this because when a catastrophic core meltdown and rupturing of pipes and explosions as a result of all this happens like in Chernobyl the radiation was literally out of this world in terms of dose levels and most of the dosimeters at site gave up and their pointing arrows went "through the roof", but as we know Chernobyl unit 4 aftermath was so deadly mostly because due to fire and at least two powerful explosions much of the radiation source was pulverized and as dust went into the atmosphere were it then traveled huge amounts of territory ,
now a purely theoretical question if for example there was the same core meltdown and explosions etc but somehow nothing turned to dust and no dust or no radiation source big or small went anywhere outside the reactor , I wonder what would then the radiation levels be say 1km from the site , 5km and so on ?@Jim and others , is that the spent fuel pond in the photo , or is that part of the active zone of a BWR ? if it's the second case does the BWR really work something similar like a electrical heating spiral dropped in a kettle which makes the water flow from the lower cold region pas the heating element and then it flows upwards and boils at the surface creating steam ?
if the fuel is really underwater all the time I guess it has to be tightly and perfectly sealed from water entering the fuel right?

A cassette is what they call each of the fuel bundles inside the RBMK core , not sure whether it's an official term or something that some workers like to refer to but it's just that and nothing more.@Astronuc and mfb , so the spent fuel can no longer be used in a traditional reactor mostly because the U235 which is the main fission target has underwent fission which has turned most of the U235 into other elements with similar yet slightly different atomic masses and most of those elements absorb neutrons without being able to further undergo fission themselves , except for a few like the Pu but since it takes up only a small portion of all the mass of the depleted fuel it's effect is small correct?
But if the Pu which forms in the fuel while the U235 undergoes fission is there in small quantities and fissions itself then in reactors like the RBMK they would have to take the fuel out before it's completely spent and somehow extract the Pu out of it otherwise if they leave it till the end doesn't the Pu239 degrade much like the U235?

Also I wonder what is the enrichment level for military naval reactors since limited space is a big factor there and I assume the most energy density per given size is needed.?

 

[Salvador]

@Garlic , if all the fuel is removed from the core the core no longer has reactions going on in it.But you can't completely shut down a nuclear reactor like you can simply turn off a light bulb , even when the moderator control rods are fully inserted upon the operator's wish to shut down the reactor there is still some small amount of nuclear reactivity going on.To my knowledge a nuclear reactor is said to be shut down when it's core thermal energy output is at it's minimal possible output with all the moderator rods inserted , no usable electricity can be generated at this point but the reactor still needs active cooling because there is the so called decay heat that needs to be taken away otherwise the core will overheat and melt.
I assume this is what happened in Japan in Fukushima plant , they lost the backup electrical power for the cooling system after the reactor was shut down and due to this loss of coolant flow the fuel overheated by the heat it still produced even in the shut down state and so core meltdown happened which further released some gasses under pressure which then broke the containment and escaped into the atmosphere.

P.S. When I was at the RBMK they said that even the fuel which has been fully spent and is inserted into the spent fuel reservoir , the reservoir itself has some active cooling because the fuel still produces some heat overtime due to the decay reactions that are still happening inside of it.ofcourse this cooling is not as big as the one taking place inside the reactor core but it's still there.

you can move fuel even when it's not fully spent , for example I said earlier they built this railway wagon in which they placed the good still capable fuel rods from the first reactor and transported them to the second reactor and put them there so that they could further fission and produce electricity.

Why would you need to waste produced electricity ? It's not like we have a tree full of electricity and we don't know were to put it , the grid will always need some extra electricity atleast here so none of it goes to waste and if for some time the grid is full they can simply lower the energy output and produce less at that moment.

 

[Garlic]

...they can simply lower the energy output and produce less at that moment.

Oh, I diddn't know that.

Is there a reactor that redirects some of its energy directly to the active cooling system of it, so that the reactor works without needing any electricity source to power its active cooling system?

And a while ago I have red an article from popular science, there it says there is a reactor type (that hasn't been implemented yet), in which when the meltdown occurs, the molten salts flow into a chamber where it's safely contained, not harming anyone. Why doesn't anyone use this?

"If a reactor loses power, a freeze plug melts, draining the radioactive fuel into a tank where it cools to a solid."

Wait.. Can the radioactive fuel cool to a solid despite it's enormous decay heat?

 

[Salvador]

all mainstream nuclear reactors that I know of use the electricity they produce to cool themselves with electrical pumps that feed the water through the reactor because that very water not only cools the reactor but also produces the heat and steam necessary to run the turbine in the first place.It's a steam cycle only the heat source is radioactive instead of being coal or gas or anything else.
the problems begin when the reactor is shut down and doesn't produce it's own electricity and if besides that somehow also the grid to which the station is connected fails somehow then it needs a reserve electricity source for the coolant pumps , which is normally a diesel generator.
the Chernobyl accident happened because they were testing the ability to power the coolant pumps using the leftover inertia of the turbine to cover the time that it takes from emergency shutdown of the reactor to the full power from the backup diesel generators.

uranium inside the cores of nuclear reactors, atleast most of which I know, is in solid state , in formed metal pellets not in liquid form , it's melting point is quite high so it normally stays solid and is kept that way.
as for the molten salt reactors and my questions I will wait and let the more experienced forum members answer and I suggest you do the same.

 

[Hiddencamper]

Jim: BWRs don't use boron during refuel. The control rods remain fully inserted in the core, and the spent fuel pool is either spaced to ensure k effective is < 0.95 or boron plated fuel racks are used. During operation, boron injection is only used during ATWS scenarios (failure to scram) at high power/with core instabilities/if containment is challenged. Boron injection can also be used to provide supplemental inventory control during loss of high pressure injection scenarios, and some plants will inject boron to adjust ph of reactor coolant during a LOCA which improves the ability of water to absorb iodine released from fuel damage and minimize offsite release.

In that picture you can see the feedwater spargers (upper rings), and directly next to the top of fuel you can also see the core spray ECCS spargers. The steam lines are also easily visible. This picture has the steam separator and core shroud head removed (to get access to the fuel).

Garlic: when you scram a commercial light water reactor, the core is shut down in a couple seconds. RBMKs are slower at scram times, but the average BWR/PWR scram is less than three seconds to put all the rods in. The core is subcritical in less than 3 seconds in these cases.

When the core is shut down, you get a prompt power drop from full power to around 7%. Neutron flux then rapidly drops with a -80 to -110 second period, reaching the source range within the next 20-30 minutes. The heat from fission is negligible after the first minute or so and nearly all heat is from decay heat.

Salvador: when temperatures get hot enough to start melting fuel, ceramic fuel pellets begin to almost vaporize. So when the fuel cladding ruptures you end up with a fair amount of radioactive dust. Additionally the noble gas inventory in the fuel rod is not a dust but also gets ejected, which is a large part of the initial release during a LOCA. You also get various noncondensibles or other fission products that can be carried outside by steam in the core.

That is the BWR core by the way. Water is forced through the bottom using jet pumps driven by recirculation pumps. Water flows upwards past the fuel and boils. Then it passes through an in core steam separator and steam dryer until it gets to the top head of the vessel steam some where it can go down the steam lines. The fuel is under water at all times. Normal water level in a BWR is roughly 20 feet above the top of the fuel. The fuel is only uncovered during accidents when steam cooling is being used to cool the core, or during scram failure when you may partially uncover the top of the core to help lower power and try to bring the core subcritical on temperature and steam voids. BWR cores can remain adequately cooled with the top 1/3rd of the core uncovered, or more, if there is adequate steam flow or if the core spray system is in operation.

Also nearly all plants have a non electric core cooling system. For example, nearly all BWRs have a steam driven emergency feed pump which can use decay heat to operate for several days. These feed pumps are what kept the four reactors at Fukushima Daini cooled for the first couple days after the March 2011 tsunami.

Some plants use diesel driven emergency feed pumps. Some plants simply have passive steam generators with gravity makeup tanks. But the bottom line is that you have some form of non electric cooling for a period of time.

After a scram you don't have enough heat to drive the main turbine/generator. But you can operate large and small turbine driven feed pumps for a few hours, and the small feed pumps for days. If we lose normal pressure control of a BWR, one way we will depressurize the core is to just operate a large turbine driven feed pump on the minimum flow line at high speed to draw steam from the core. This allows us to avoid using relief valves.

 

[Salvador]

Seems like the BWR is simpler with respect to CANDU or others that it doesn't have all these small coolant tubes running through the core which complicates the design and also brings in more chance of hardware failure. If I understand correctly the BWR is simply U235 fuel formed in rods which form a core together with the moderator rods which is simply made watertight and placed inside a large cylindrical vessel with thick walls. Correct?
So the primary loop water is simply pumped in at the bottom of the vertical vessel then it goes through the core and as being heated partly due to the flow partly due to heat flows upwards until boils at the surface where then a steam separator takes away the steam and the water simply condenses and falls back into the vessel ?

Also because of this system does the BWR operate it's primary loop under pressure and if yes what kind of pressure ?

The way I see it is that a PWR keeps the coolant under high pressure in the primary to prevent boiling and then simply let's the secondary loop to boil because sooner or later one needs to boil the water to create steam but the BWR (hence the name boiling) boils the water already in he very reactor vessel top aka the primary loop and then directly drives the turbine with that boiled steam after the turbine it's condensed with the help of auxillary water and sent back to the very reactor vessel.
Seems like this reactor then has one loop less in the steam cycle.Sounds like should give better efficiency ?

 

[mfb]

I have a question, if the fuel rods are removed, no further reaction in the reactor can take place, right?
If that's true, why can't you shut down a nuclear reactor when it's not needed? You would just take the fuel rod and put it in a nearby unterground nuclear dump storage. Is it so, that the fuel rod is so active that there is no practical methods for moving them? If they are still critical, you would break them so that they are subcritical.

You still have the decay heat, but that is not even the main problem (you can cool the core). If you shut down a reactor, you get xenon poisoning. At full power, some xenon-135 is produced from the decay of iodine-135. It is a strong neutron absorber, so it quickly absorbs a neutron and becomes xenon-136.
If you shut down a reactor, xenon-135 keeps getting produced for about a day, but the neutrons to get rid of it are missing, so it accumulates in the reactor. This makes restarting the reactor problematic: you would have to remove more control rods than usual to counter the neutron absorption of xenon-135. But then you start "burning away" this xenon isotope, your reactor power increases, you burn away even more, the reactor power increases even more...

This is exactly what happened at Chernobyl.
If you shut down a reactor, you have to wait before restarting it.

And one more question, if the reactors can't be shut down for a little time, can't we basically redirect the energy from the main power line to something else. I mean, it's easy to lose unwanted energy.

Not on a scale of a GW. That is sufficient to boil 500 kg of water per second, for example. A swimming pool of water vapor per second.

@Astronuc and mfb , so the spent fuel can no longer be used in a traditional reactor mostly because the U235 which is the main fission target has underwent fission which has turned most of the U235 into other elements with similar yet slightly different atomic masses and most of those elements absorb neutrons without being able to further undergo fission themselves , except for a few like the Pu but since it takes up only a small portion of all the mass of the depleted fuel it's effect is small correct?

In most reactors, only a small fraction of the available U-235 is used. Reprocessing can separate it from the fission products to recycle most of the fuel rods, but making new fuel rods is often cheaper.

But if the Pu which forms in the fuel while the U235 undergoes fission is there in small quantities and fissions itself then in reactors like the RBMK they would have to take the fuel out before it's completely spent and somehow extract the Pu out of it otherwise if they leave it till the end doesn't the Pu239 degrade much like the U235?

For nuclear weapons, that's exactly what is done: get the fuel rod out early, extract plutonium, put the rest in again. For nuclear reactors, using the plutonium is a great additional source of fission.

Also I wonder what is the enrichment level for military naval reactors since limited space is a big factor there and I assume the most energy density per given size is needed.?

The size of fuel rods is probably negligible.

Why would you need to waste produced electricity ? It's not like we have a tree full of electricity and we don't know were to put it , the grid will always need some extra electricity atleast here so none of it goes to waste and if for some time the grid is full they can simply lower the energy output and produce less at that moment.

Sometimes we do, and electricity can be sold at negative prices (due to subsidies).

 

[Hiddencamper]

In a BWR, there are only fuel rods. The water in the core acts as the moderator. Feedwater is pumped to the outer portion of the vessel, the steam downcomer. Water from the downcomer region is pumped using recirculation and jet pumps into the bottom of the core where it goes up vertically and boils just like you said.

The BWR is pressured. At full power, reactor pressure is around 1000-1050 PSIG, or around 6.8-7.2 MPa.

You get slightly worse thermodynamic efficiency in a BWR due to lower operating temperature, but you have less equipment overall, you only have 2 coolant pumps for the core, and other differences that make up the savings on electrical usage and cost of plant. BWRs are also simpler to operate during both online conditions and transients. You always have direct observation of core water level, you don't worry about steam voids in the core, you don't have thermodynamic balance issues like you do with PWRs during transients (trying to balance primary/secondary during complicated steam generator tube ruptures in a PWR is challenging). Also spatial xenon oscillations are self-suppressing in a BWR, so you don't really have to worry about xenon instabilities like you would in a PWR.

https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Bwr-rpv.svg/350px-Bwr-rpv.svg.png

This picture is very good, except the "downcomer region" is not in the right place. The actual downcomer is the area where feedwater enters the vessel, outside of the core shroud.

 

[Salvador]

in the picture at the bottom it says control rods but from your explanation I got that the reactor uses no control rods so which one is it ?
as for the water moderator I understand that since steam is many times less dense than water allowing lower portions of the core boil reduces the power because it produces more fast neutrons which escape the chain reaction but pumping in more water so that the boiling point is moved upwards above the core top increases the reactor thermal output.is this correct ? it comes off wikipedia so I ask , because in Chernobyl this positive void feedback was doing the exact opposite , the steam in the core increased the reactivity and hence the thermal output, which ofcourse gave even more steam so up until the point of hardware breakdown point.
Or was it the xenon poison that the steam void faster neutrons burned away and hence the increase in thermal power?So the water boiling temperature changes in a BWR according to the output power ? Because I see this as you slow down the generator that puts more backpressure on the turbine which then puts more pressure on the incoming steam so the pressure in the reactor vessel increases and hence the boiling point of water must do the same?

 

[jim hardy]

Jim: BWRs don't use boron during refuel.

Thanks !
We use heavily borated water for refueling. Our PWR head bolts are not stainless so we take them out of the cavity before floodup. I did notice those still in place . Or are they PCV closure bolts?

@Jim and others , is that the spent fuel pond in the photo , or is that part of the active zone of a BWR ?

You are looking down into the core of a BWR through probably thirty feet of water. That element has been lifted out and is en route probably to storage in the spent fuel pond.
I'll let BWR guy address your further questions. My knowledge of BWR's is scant, from a one day introductory class. I don't pretend any expertise on BWR's.

 

[Astronuc]

So the water boiling temperature changes in a BWR according to the output power ? Because I see this as you slow down the generator that puts more backpressure on the turbine which then puts more pressure on the incoming steam so the pressure in the reactor vessel increases and hence the boiling point of water must do the same?

No, not directly. The boiling point of water changes with pressure. The point of a boiling water reactor is that the core is mostly boiling at saturated conditions, except for the lower portion (bottom ~1 m), where the coolant enters to core with subcooling.

In a BWR, the coolant undergoes nucleate boiling, then bulk boiling as the coolant passes along the fuel rod. In a PWR, some nucleate boiling is allowed, but departure from nucleate boiling is not.

 

[Astronuc]

in the picture at the bottom it says control rods but from your explanation I got that the reactor uses no control rods so which one is it ?

BWRs use control rods in-core for reactivity control, but they can also use flow for reactivity control. Flow can be reduced, which allows more voiding in the core, and this means less moderation. With less moderation, the neutron flux 'hardens', i.e., the proportion of fast neutrons increases, but the reactor is still critical while producing power. A harder neutron spectrum can be used to convert U-238 to Pu-239, and this deliberate operation is know as 'spectral shift'.

PWRs use soluble boron (in the form of boric acid with a LiOH buffer) in the coolant for reactivity control, since control rods are normally withdrawn from the core during operation. Some control rods may be inserted during startup or certain power maneuvers. In some PWRs, there are special axial power shaping rods which are used during operation, but these are not common. Some PWR designs use 'grey' (regulating) rods for rapid power changes associated with load-following or frequency control. The term 'grey' means the rods are not as strongly absorbing a 'black' control rods, with the latter used primarily for shutdown.

Both BWR and PWR fuel also use 'burnable absorbers/poisons' distributed in the fuel rods for reactivity control with respect to power distribution within the fuel assembly and among the assemblies in the core. The burnable absorber/poison depletes in conjunction with the depletion of the fissile species.

In a graphite-moderated reactor, e.g., RBMK, the water coolant actually adds some 'negative' reactivity, since the water does absorb neutrons (in addition to moderation by the hydrogen), but in an RBMK, most of the moderation occurs in the graphite. Graphite-moderated, water-cooled reactors have a 'positive void coefficient', i.e., if the water becomes too hot at pressure (or if pressure becomes too low at temperature), and the water boils (voids).

The Chernobyl accident was an experiment gone awry. To overcome Xe-poisoning, more reactivity would have to be added, which would mean control elements would be removed from the core. When the Xe-135 decays, the negative reactivity it provides the core dissipates, and at some point, without control elements or other neutron absorbers present, at the point where reactivity of the core becomes positive, the power would increase.

 

[Hiddencamper]

in the picture at the bottom it says control rods but from your explanation I got that the reactor uses no control rods so which one is it ?
as for the water moderator I understand that since steam is many times less dense than water allowing lower portions of the core boil reduces the power because it produces more fast neutrons which escape the chain reaction but pumping in more water so that the boiling point is moved upwards above the core top increases the reactor thermal output.is this correct ? it comes off wikipedia so I ask , because in Chernobyl this positive void feedback was doing the exact opposite , the steam in the core increased the reactivity and hence the thermal output, which ofcourse gave even more steam so up until the point of hardware breakdown point.
Or was it the xenon poison that the steam void faster neutrons burned away and hence the increase in thermal power?So the water boiling temperature changes in a BWR according to the output power ? Because I see this as you slow down the generator that puts more backpressure on the turbine which then puts more pressure on the incoming steam so the pressure in the reactor vessel increases and hence the boiling point of water must do the same?

You said "moderator rods" earlier. A BWR uses a liquid water moderator, there are no moderator rods. There are fuel rods and control rods. The control rods are movable and are inserted for either flux shaping, or for overall power control.

Because BWRs are water moderated, that means as water boils to steam, the decrease in moderator density causes a drop in reactivity. More boiling = less reactivity, and vice versa. Because BWR core power follows moderator density and steam voids, it means we can control reactor power by adjusting core recirculation flow. Raising flow will raise power, and lowering flow will reduce power. During certain high pressure transients like a load reject without bypass or a turbine trip ATWS, the recirculation pumps will automatically trip to low speed or off to rapidly void the core and help reduce power to prevent exceeding Minimum Critical Power Ratio or reactor pressure vessel pressure limits.

The boiling temperature does change slightly. The turbine/generator for a BWR is slaved to the reactor. This is backwards for most power plants, where turbine load is controlled separately from boiler output. In a BWR, the turbine throttles are controlled with a reactor pressure control regulator. So if I want to lower generator output, I will first lower reactor power. The reduction in steam flow causes a slight reduction in steam pressure, which the pressure regulator senses. The regulator will then throttle the turbine valves closed to maintain steam header pressure. Steam pressure is sensed right before the turbine, at the steam equalizing header, so that means reactor pressure will always be higher than the turbine building steam header, based on your throttling losses across the steam lines. For my BWR, our no load pressure is set around 917 PSIG. At full power we are around 942 PSIG at the steam header, and 1025 PSIG in the reactor steam dome.

The following is an example of how reactor pressure is controlled against steam flow:
http://imgur.com/VabG33w

As reactor power and pressure change, you will have slight changes in reactor coolant temperature, as RCS temps follow saturation pressure. After a shut down, we cool down the reactor by reducing steam pressure, and allowing the flashing of water to steam to absorb latent/sensible heat from the RCS and lower coolant temperature.

The turbine has to follow reactor pressure for a water moderated BWR. If you were to raise turbine load with no power change, when pressure lowers the flashing to steam would cause core voiding, and reactor power would lower. If you were to reduce turbine load with no power change, pressure would increase, causing increased reactor power. BWR plants are not stable with the turbine load being controlled independent from the reactor, so instead the turbine is set up to automatically follow the reactor.

 

[Hiddencamper]

Thanks !
We use heavily borated water for refueling. Our PWR head bolts are not stainless so we take them out of the cavity before floodup. I did notice those still in place . Or are they PCV closure bolts?

Those are the reactor head bolts. The drywell (PCV) head bolts are attached to the drywell head itself and are not in the picture. If you look around the outside of the bolts, you will see flanged off lines. Those are for the reactor head vent line, the RCIC (aux feed) head spray line, and the reactor steam dome pressure instruments. That all fits between the reactor head and the drywell (PCV) head.

 

[Salvador]

@Astronuc and so it increased , and so rapidly that the team did a manual scram but since the control rod flaw did actually further increase reactivity for the first few secs of insertion the whole thing just went "absolutely bananas"
from reading about the accident over the years I have understood that it was actually sort of like winning a lottery , so many factors had to all come together to produce this result if anyone of the important factors was avoided the result would have been different.

Also I've never actually fully understood the exact way some fission reactor cores are constructed one of which is the RBMK.I looked through the CANDU papers you gave earlier and there were some good pictures of the core assembly.In the BWR I now see that the core assembly if we can call it that way simply sits inside a water filled and sealed vessel which is under pressure as the water starts to heat up and boil.
Now for the RBMK , when I was at the reactor hall I walked around quite a bit while talking to the station scientists and operators about the way everything works there and I saw the fuel rod assemblies (spare ones) hanging down from the reactor hall ceiling right nest to the wall were the automatic crane operates and takes them one by one and inserts into the core while taking the used ones out.They seemed like long (about 14m in length) tubes of some grey colored metal alloy (probably zircalloy)
yet still I didn't get the chance to fully understand them up close.
the way I see the RBMK is this, and please correct my view , it has this big graphite core some 7m in height and it has holes in it running vertically , some 1600 of them each hole through the core carries a fuel assembly some carry control rods , each fuel assembly consists of multiple fuel rods which are zirconium alloy tubes in each of which U235 pellets stacked one upon another roughly 7 m long , if I understand only 7m is the pellet stack height in each fuel tube/rod since 7m is also the height of the core active zone and the above 7m is for apparatus moving the controls rods and other things also the upper steel reinforced concrete biological shield plate.
now the part that I don't get is were does the water flow by , since the water to my understanding should flow by the very fuel assembly tubes in which the U235 fuel is since those probably are the hottest in the core and graphite not so much because as much as I know graphite conducts heat less than metal.
Does the graphite touch the water at all or is the water flowing by in each fuel assembly inside , pas each of the multiple fuel rods in that assembly?

@Hiddencamper well by moderator rods I meant control rods since some call them moderating rods I assume , but basically with moderator I understand everything that can alter or influence the chain reaction aka reactivity.
it's interesting that the BWR has the reverse logic as most other reactors were in pressurized two loop reactors you would want to avoid boiling int he primary loop in a BWR you want to have boiling in case of emergency shut down.one thing I find good is that if you want more thermal output from the reactor simply increase the coolant flow and rise the boiling height to a higher level that seems interesting.

When I look at the RBMK it seems like it too has just one coolant loop - the primary which directly drive the turbine , what about boiling point in RBMK , seems like the boiling should also take place inside the core only in the upper very part of the core or maybe above it which I'm not sure so then it goes through the pipes into the steam separator which is located very closely to the reactor hall.
But then it looks like the RBMK needs to be controlled carefully in order for the boiling not to go lower down the reactor core because that would cause an increase in reactivity which would need to be counteracted if it were to take the thermal power over the design limit max as it did in Chernobyl only in Chernobyl I assume it happened so fast the operators didn't have any chance of decreasing it from the state in which they were in at the moment.

 

[Dr_Zinj]

I think I have some answers to a couple of your questions.3.) While dust clean up is important, stainless steel flooring prevents contaminants from easily sinking in and being absorbed by the flooring material. Most tiles are somewhat porous, and concrete even more so. A spillage of any kind would require ripping out the flooring to remove the contamination; and the more porous or longer the contaminate has to soak in, the more material needs to be removed.4. All other things remaining equal, larger particles of radioactive material will give off a larger total amount of radiation.5. Once upon a time, my father worked as a engineer for Exxon in their uranium enrichment program up near Malta in upstate New York. They had to convert uranium metal into a gaseous form (uranium hexafluoride) and I think they were working on the gaseous diffusion process, not centrifuge. The spinning centrifuges do indeed separate the different isotopes based on their mass. However, the process itself is only about 1% efficient, so it has to be repeated multiple times to concentrate the isotopes. The Manhattan Project estimated over 50,000 centrifuges running to produce a single kilogram of U-235 per day; but the engineering problems were too great so the Manhattan Project got their enriched uranium primarily through the thermal diffusion process. Cost about 20 times more than the centrifuge process would have cost.A kilogram of uranium metal makes a humongous amount of gas; and the Little Boy dropped on Hiroshima took 64 kilograms of highly enriched U-235. If my molar conversion is correct, that should be equal to about 4000 liters of Uranium Hexafluoride. That’s just for the enriched metal. Figure about 400,000 liters (or more) at the beginning of the process.

 

[Astronuc]

Also I've never actually fully understood the exact way some fission reactor cores are constructed one of which is the RBMK.I looked through the CANDU papers you gave earlier and there were some good pictures of the core assembly.In the BWR I now see that the core assembly if we can call it that way simply sits inside a water filled and sealed vessel which is under pressure as the water starts to heat up and boil.
Now for the RBMK , when I was at the reactor hall I walked around quite a bit while talking to the station scientists and operators about the way everything works there and I saw the fuel rod assemblies (spare ones) hanging down from the reactor hall ceiling right nest to the wall were the automatic crane operates and takes them one by one and inserts into the core while taking the used ones out.They seemed like long (about 14m in length) tubes of some grey colored metal alloy (probably zircalloy)
yet still I didn't get the chance to fully understand them up close.
the way I see the RBMK is this, and please correct my view , it has this big graphite core some 7m in height and it has holes in it running vertically , some 1600 of them each hole through the core carries a fuel assembly some carry control rods , each fuel assembly consists of multiple fuel rods which are zirconium alloy tubes in each of which U235 pellets stacked one upon another roughly 7 m long , if I understand only 7m is the pellet stack height in each fuel tube/rod since 7m is also the height of the core active zone and the above 7m is for apparatus moving the controls rods and other things also the upper steel reinforced concrete biological shield plate.
now the part that I don't get is were does the water flow by , since the water to my understanding should flow by the very fuel assembly tubes in which the U235 fuel is since those probably are the hottest in the core and graphite not so much because as much as I know graphite conducts heat less than metal.
Does the graphite touch the water at all or is the water flowing by in each fuel assembly inside, pass each of the multiple fuel rods in that assembly?

http://www.world-nuclear.org/inform...-power-reactors/appendices/rbmk-reactors.aspx
Fission reactors come in a variety of designs according to fuel form, moderator and coolant.

Graphite-moderated reactors can be water-cooled or gas-cooled, or even liquid/molten salt, if the fuel form happens to be a solution of U or Pu salt in some solution. Normally, in water-cooled, graphite-moderated reactors, the water coolant is separated from the graphite moderator. The water is in a pressure tube, as shown in the diagram in the link from world-nuclear.org. The fuel resides in the pressure tube, and the water coolant flows through a lattice or array of fuel rods, which comprises the fuel assembly. The fuel rod is a hermetically sealed metal tube, usually welded to endplugs formed from barstock of the same alloy. The cooling water flows through to array of fuel rods to carry heat out of the core to a heat exchanger and is then recirculated to the fuel rods. The coolant is circulated in the 'primary' cooling system. The core is the plurality of fuel assemblies surrounded by the moderator and coolant, which are the same in an LWR.

In the CANDU system, the fuel is located in horizontal pressure tubes through which the coolant passes. The moderator, heavy water, is located on the outside of the pressure tubes inside a calandria vessel.

 

[Salvador]

Well I guess those of you who follow nuclear stuff more closely will probably be aware of this supposedly German woman by the nickname "bionerd23" I actually want to make a thread were I want to ask some of your opinion on what she does and the levels of radioactivity she puts herself through and what do you think about it but as for now and this thread I just found a picture on her flickr account of a piece of graphite that seems like from a RBMK , although to be honest I'm not sure since I've never had the chance to go deep enough in an actual reactor or to see it's parts outside of it.

https://www.flickr.com/photos/bionerd/8196527482/

I must say over the years watching her youtube channel I have learned quite a bit about radiation's real life nature than any textbook or theory could tell.She's like somekind of a weird modern Marie Curie.

Also , in the RBMK 1500 plant that I was they had pipes running from the reactor to the local city and when the reactor was running they heated the city in winter entirely out from the reactors so called waste heat , the one which after the turbines couldn't be used for any more electricity generation due to it being not hot enough and condensed back to water , much like a modern cogeneration station does.
this way the reactor heat was used to it's fullest potential , I wonder why don't other NPP around the world don't use similar practices ?
Like for example many reactor designs use the large cooling towers and we can see fog which is I assume condensed water rising from the tops but I assume those towers are there to waste some leftover heat so the water in the primary cycle could go back to the reactor at the neede level, why they don't use that heat somewhere else instead of pushing it into the atmosphere?

 

[etudiant]

Hi Salvador,
The direct use of waste heat requires the reactor to be close by the city to be heated. Siting rules in the US and Europe precluded that. But here in Manhattan there are steam pipes under many streets, distributing waste heat from fossil fuel plants to nearby apartments and offices.
One consequence is that the US and European reactors need either a large body of water or an expensive cooling tower installation to dump the waste heat.
In Florida, the manatees flock in winter to the warm pools created that way. On the other hand, reactors here have been curtailed because of the lack of river flow for cooling water in a drought.

 

[Salvador]

yes soviets built these special cities for workers near every RBMK or VVER reactor , they were on average about 3km from the very reactor so the accident in Chernobyl meant the city had to be evacuated.

 

[nikkkom]

Also , in the RBMK 1500 plant that I was they had pipes running from the reactor to the local city and when the reactor was running they heated the city in winter entirely out from the reactors so called waste heat , the one which after the turbines couldn't be used for any more electricity generation due to it being not hot enough and condensed back to water , much like a modern cogeneration station does.
this way the reactor heat was used to it's fullest potential , I wonder why don't other NPP around the world don't use similar practices?

Because this requires certain level of trust in what nuclear people are doing at the plant. I am not at all thrilled to have a possibility to learn one fine day that someone "made a small mistake" and my heating radiator was also emitting some gammas. Thank you very much.