Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[TCups]

The IAEA is reporting:

- core damage at units 1 and 3, situation "very serious"
- fuels rods exposed in units 4,5 and 6
- total of 4 units have core damage
- radiation levels rising

Of course, the only fuel rods to be "exposed" at unit 4 would now be in the SFP, which brings into question the last reported water temp of 183ºF in the SPF. And the rods in the SPF aren't just those unloaded at the recent shutdown, but also, a large portion of the stored, spent fuel rods from the last 40 years or so of operation. So things are heating up.

 

[AntonL]

2. The explosions in Japan have destroyed the reactor buildings, leaving the reactor cores mostly intact due to their containment structures. The explosion in Chernobyl destroyed the reactor core due to the lack of containment, ejecting the core material itself into the surrounding area (and air).

Who cares if the reactor cores are in tact or not.

The greater problem is that in unit 4, 5 and 6 being shut down the reactor fuel is stored in a swimming pool on the 4th floor of the respective buildings, all in all a further 1760 tonnes of spent fuel is stored in open swimming pools http://www.nirs.org/reactorwatch/accidents/6-1_powerpoint.pdf"

These storage tanks are slowly boiling dry (or leaking dry after the explosions) causing the fuel to burn as it happened in Unit 4.

Nobody has talked about the contaminated cooling water that is allowed to return to the pacific ocean. Sushi and seaweed will disappear from the Japanese cuisine

 

[Bob S]

I just read that the Ministry of Health Labour and Welfare has increased the legal limit of exposure for plant workers from 100mSv to 250mSv/hr..

Really? I thought 250 mSv (milli-Sieverts)/hr was equivalent to 25 rads (or rem)/hr. The U.S. radiation worker is limited to about 5 rem (whole body)/year. 200-400 rads is ~50% mortality.

See Table 2-1 in http://www.hss.energy.gov/publications/rcm/ch21.html

Bob S

 

[lapsedgeo]

As a non-nuclear boffin this is starting to feel out of control which is scary, when you guys get jumpy I get really jumpy.

 

[Astronuc]

I couldn't find the information on that page. Could you please give the exact location?

The plant stata are at ANS Nuclear Cafe. Toward top - and attributed to FEDERATION OF ELECTRIC POWER COMPANIES OF JAPAN Washington DC Office, Update to information sheet regarding Tohoku earthquake as of 10:15AM EST (05:15 GMT), March 16, 2011:

Meanwhile - http://www.world-nuclear-news.org/RS_Second_fire_reported_at_unit_4_1603111.htm

It's not clear to me that the spent fuel at Units 5 and 6 are exposed. The SFP temp of 183ºF is uncomfortably close to boiling 212ºF at 1 atm. I don't know if that is pool water or atmosphere above.


See also - http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1300189582P.pdf
from http://www.jaif.or.jp/english/ - updated plant/unit status reports.

http://www.tepco.co.jp/en/press/corp-com/release/11031608-e.html

 

[Astronuc]

The IAEA is reporting:

- core damage at units 1 and 3, situation "very serious"
- fuels rods exposed in units 4,5 and 6
- total of 4 units have core damage
- radiation levels rising

There are 6 units at FK Daiichi.

Based on published information only - Units 1,2 and 3 have some core damage - as yet unknown. Unit 4 is defueled - no fuel in core. That fuel was removed for a normal outage in November. However, the fuel is in the spent fuel pool and could be at risk of failure IF the water level exposes the fuel. Units 5 and 6 have fuel but are in cold shutdown. Their SFP has older, slightly cooler fuel - probably from 3 or 4 cycles ago at the youngest.

 

[Reno Deano]

Raising worker allowable whole body does equivalents is routine procedure for emergency workers so they can respond to within plant events and make necessary repairs/midigating actions. They (TEPCO) use the same regulatory frame work and international radiological health guidance as the US. They have already assigned older workers to the higher does rate activities.

 

[Reno Deano]

Let put a nuclear accident in perspective:

As I have stated before, the nuclear events in Japan - if left to run to their conclusion without interdiction would not even closely rival the loss-of-life in one town hit by the tsunami (fear that 10,000 out of 164,000 people lost their lives). Many more Japanese towns face similar catastrophe losses. Not even for long term radiation exposure health risks.

http://www.foxnews.com/world/2011/03/16/japans-uncertain-following-radiation-level-panic/"

The Japanese nuclear accidents will be a vey good study and design fodder for years to come. More than likey all BWR 1 thru 3's will be shut down due to biased fears.

No amount of Ad hominem circumstantial statements and uneducated guesstimates will change the out come.

 

[TCups]

Let put a nuclear accident in perspective:

. . . "More than likey all BWR 1 thru 3's will be shut down due to biased fears." . . .

No amount of Ad hominem circumstantial statements and uneducated guesstimates will change the out come.

So which is that statement? Ad hominem circumstantial or uneducated guesstimate?

Here is my guesstimate:
The designs were flawed. Multiple reactor facilities have been damaged and filled with salt water. Fires and explosions have occurred. Radiation has been released. Many thousands of stored fuel rods are not contained within quake/tsunami/fire/explosion-proof storage facilities. It doesn't take any fear or bias to "guess", with a high degree of probability, that these reactor facilities will remain shut down forever.

Containment and cleanup + new, more robust reactor facilities and waste storage facilities will be the order of the day.

 

[bondboy]

Astronuc, could you comment on the latest news regarding the possibility of the spent fuels "going critical"? I was under the assumption the spent fuel is depleted and that while still dangerous, should not or could not go critical.

NewScientist reports
http://www.newscientist.com/blogs/shortsharpscience/2011/03/nuclear-crisis-radioactive-fue.html

1820 GMT, 16 March 2011
Michael Marshall, environment reporter

The situation at Japan's Fukushima Daiichi nuclear power plant has become extremely unnerving. The Tokyo Electric Power Company has now admitted that the spent fuel rods could go critical - that is, a nuclear chain reaction could restart.

We have known since yesterday that the reactors themselves were coming under control, and that the biggest threat came from the spent fuel ponds, where the water level has fallen and temperatures have risen. That could lead to the stored fuel rods breaking open, releasing their radioactive contents.

Kyodo News reports:
Tokyo Electric Power Co. said Wednesday it is considering spraying boracic acid by helicopter to prevent spent nuclear fuel rods from reaching criticality again, restarting a chain reaction, at the troubled No. 4 reactor of its quake-hit Fukushima No. 1 nuclear power plant.

''The possibility of recriticality is not zero,'' TEPCO said as it announced the envisaged step against a possible fall in water levels in a pool storing the rods that would leave them exposed.

 

[Reno Deano]

Members and lurkers on this site are directed to the NRC's Home page and new reactor certifications, to view the latest safety features incorporated into BWR and PWR coming online in the future (distant probably).

All New Reactor Certifications: http://www.nrc.gov/reactors/new-reactors/design-cert.html"

Advanced BWR Certification: http://adamswebsearch2.nrc.gov/idmws/DocContent.dll?library=PU_ADAMS^pbntad01&LogonID=531be9e690af0e3981f946651c81516a&id=080740229

 

[turbo]

It seems that a new power line could be up and running soon, so that at least any undamaged cooling systems could be supplied by electrical pumps. That might reduce the number of critical problems and give TEPCO a cushion in addressing cooling of the spent fuel pools.

http://news.yahoo.com/s/ap/20110316/ap_on_re_as/as_japan_earthquake;_ylt=Au3_Pnymt2OFbsV3_6SfexCs0NUE;_ylu=X3oDMTFoOGhpYnVlBHBvcwMyNARzZWMDYWNjb3JkaW9uX3RvcF9zdG9yaWVzBHNsawNuZXdwb3dlcmxpbmU-

 

[Azael]

Normally that is true, when one reloads a core one only ever put spent fuel into the fuel pool at BWR's. The case is special with reactor number 4 though, because reactor number 4 was shut down for some kind of maintenance and they moved all the core inventory into the spent fuel pool. That means you got a lot of fuel bundles that only spent one or two cycles in the care. The one cycle fuel bundles could plausibly go critical during some circumstances. Especially if the bundles where not kept inside the general lattice of the pool after the earthquake.

Astronuc, could you comment on the latest news regarding the possibility of the spent fuels "going critical"? I was under the assumption the spent fuel is depleted and that while still dangerous, should not or could not go critical.

NewScientist reports
http://www.newscientist.com/blogs/shortsharpscience/2011/03/nuclear-crisis-radioactive-fue.html

1820 GMT, 16 March 2011
Michael Marshall, environment reporter

The situation at Japan's Fukushima Daiichi nuclear power plant has become extremely unnerving. The Tokyo Electric Power Company has now admitted that the spent fuel rods could go critical - that is, a nuclear chain reaction could restart.

We have known since yesterday that the reactors themselves were coming under control, and that the biggest threat came from the spent fuel ponds, where the water level has fallen and temperatures have risen. That could lead to the stored fuel rods breaking open, releasing their radioactive contents.

Kyodo News reports:
Tokyo Electric Power Co. said Wednesday it is considering spraying boracic acid by helicopter to prevent spent nuclear fuel rods from reaching criticality again, restarting a chain reaction, at the troubled No. 4 reactor of its quake-hit Fukushima No. 1 nuclear power plant.

''The possibility of recriticality is not zero,'' TEPCO said as it announced the envisaged step against a possible fall in water levels in a pool storing the rods that would leave them exposed.

 

[Astronuc]

Astronuc, could you comment on the latest news regarding the possibility of the spent fuels "going critical"? I was under the assumption the spent fuel is depleted and that while still dangerous, should not or could not go critical.

. . . .

Tokyo Electric Power Co. said Wednesday it is considering spraying boracic acid by helicopter to prevent spent nuclear fuel rods from reaching criticality again, restarting a chain reaction, at the troubled No. 4 reactor of its quake-hit Fukushima No. 1 nuclear power plant.

''The possibility of recriticality is not zero,'' TEPCO said as it announced the envisaged step against a possible fall in water levels in a pool storing the rods that would leave them exposed.

Generally the racks in which spent fuel is stored contain neutron absorbing materials, so if for some reason, the pool was flooded by pure water, the SFP would not go critical. The spent fuel is generally depleted - but each assembly could have some residual positive reactivity, especially as Xe-135 decay. Xe-135 is one of the strongest neutron absorbers, which during operation is in equilibrium - at steady-state, it's production rate = depletion rate.

http://en.wikipedia.org/wiki/Xenon-135

I believe when Unit 4 was shutdown, they did a full-core offload, which means that there is some fuel which still has considerable life left. That would compound the heat burden on the pool, and it would add positive reactivity in the pool.

If TEPCO is concerned, then I'd be concerned too. However, I am not familiar with the practices or their current SFP configuration.

Reactivity is a measure of how a material can contribute to criticality. The multiplication factor, k, is a way to measure criticality. k = 1 means critical (constant power), k < 1 means subcritical (neutrons (neutron flux) and power decreases), and k > 1 means supercritical (neutrons (flux) and power increases).

Positive reactivity increases k, while negative reactivity decreases k.

http://en.wikipedia.org/wiki/Neutron_multiplication_factor

In the spent fuel pool, we want k < 1, always!

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html#c4
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fiscon.html

See chapter 18 of this book
Nuclear engineering handbook By Kenneth D. Kok
http://books.google.com/books?id=EMy2OyUrqbUC&pg=PA596&lpg=PA596
https://www.amazon.com/dp/1420053906/?tag=pfamazon01-20

 

[Reno Deano]

So which is that statement? Ad hominem circumstantial or uneducated guesstimate?

Here is my guesstimate:
The designs were flawed. Multiple reactor facilities have been damaged and filled with salt water. Fires and explosions have occurred. Radiation has been released. Many thousands of stored fuel rods are not contained within quake/tsunami/fire/explosion-proof storage facilities. It doesn't take any fear or bias to "guess", with a high degree of probability, that these reactor facilities will remain shut down forever.

Containment and cleanup + new, more robust reactor facilities and waste storage facilities will be the order of the day.

The storage of spent fuel, whether within dry casks or spent fuel pools at commerical reactor near populated areas will most likely "fuel" activities to get Yucca Mtn spent fuel/waste storage project back on track.

I personally cannot fathom the amount of debris (non-nuke related) created by the tsaumi and how it will be disposed of by Japan. Nuke workers will definitely be in demand in Japan for their reactor cleanup and decommissioning. Overall the Japanese nuclear program came out in relatively good shape considering they have over 50 nuke plants on the island.

 

[Reno Deano]

For those interested in the release of radionuclides during a PWR or BWR reactor accident the WASH 1400 report is invaluable. Very big download, but details situational time-lines for fuel rod gap and meltdown releases, including reactor vessel melt through.

http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr75-014/appendix-vii-viii-ix-x/"

 

[russ_watters]

Who cares if the reactor cores are in tact or not.

I care because that's what made Chernobyl Chernobyl and that's a major difference between this and Chernobyl.

 

[Astronuc]

From Nuclear and Industrial Safety Agency
Seismic Damage Information (the 26th Release)
(As of 14:00 March 16th, 2011)
http://www.nisa.meti.go.jp/english/files/en20110317-1.pdf

 

[Astronuc]

Who cares if the reactor cores are in tact or not.

If the fission products were confined to containment, then it would be a TEPCO problem. As soon as there is a leakage path from the core to the environment, then it becomes a health and safety issue, a concern for the authorities, and a major concern for the public who may be exposed.

 

[Reno Deano]

Nuclear Energy Institute fact sheet on "Used Nuclear Fuel Storage at the Fukushima Daiichi Nuclear Power Plant," March 16, 2011.

http://resources.nei.org/documents/japan/Used_Fuel_Pools_Key_Facts_March_16_Update.pdf"

 

[PietKuip]

Nuclear Energy Institute fact sheet on "Used Nuclear Fuel Storage at the Fukushima Daiichi Nuclear Power Plant," March 16, 2011.

http://resources.nei.org/documents/japan/Used_Fuel_Pools_Key_Facts_March_16_Update.pdf"

So what is being reported now (that a basin has boiled dry) cannot have happened?

 

[Reno Deano]

Informative article on the activities to control the reactor accidents at the Japanese nuclear plants:

Below is a summary [for the general public] on the Fukushima situation prepared by Dr Josef Oehmen, a research scientist at MIT, in Boston.

He is a PhD Scientist, whose father has also extensive experience in Germany ’s nuclear industry.

~~~~~~~~~~~~~~~~~~
What happened at Fukushima
I will try to summarize the main facts. The earthquake that hit Japan was 7 times more powerful than the worst earthquake the nuclear power plant was built for (the Richter scale works logarithmically; the difference between the 8.2 that the plants were built for and the 8.9 that happened is 7 times, not 0.7). So the first hooray for Japanese engineering, everything held up.

When the earthquake hit with 8.9, the nuclear reactors all went into automatic shutdown. Within seconds after the earthquake started, the control rods had been inserted into the core and nuclear chain reaction of the uranium stopped. Now, the cooling system has to carry away the residual heat. The residual heat load is about 3% of the heat load under normal operating conditions.

The earthquake destroyed the external power supply of the nuclear reactor. That is one of the most serious accidents for a nuclear power plant, and accordingly, a “plant black out” receives a lot of attention when designing backup systems. The power is needed to keep the coolant pumps working. Since the power plant had been shut down, it cannot produce any electricity by itself any more.

Things were going well for an hour. One set of multiple sets of emergency Diesel power generators kicked in and provided the electricity that was needed. Then the Tsunami came, much bigger than people had expected when building the power plant (see above, factor 7). The tsunami took out all multiple sets of backup Diesel generators.
When designing a nuclear power plant, engineers follow a philosophy called “Defense of Depth”. That means that you first build everything to withstand the worst catastrophe you can imagine, and then design the plant in such a way that it can still handle one system failure (that you thought could never happen) after the other. A tsunami taking out all backup power in one swift strike is such a scenario. The last line of defense is putting everything into the third containment (see above), that will keep everything, whatever the mess, control rods in our out, core molten or not, inside the reactor.
When the diesel generators were gone, the reactor operators switched to emergency battery power. The batteries were designed as one of the backups to the backups, to provide power for cooling the core for 8 hours. And they did.

Within the 8 hours, another power source had to be found and connected to the power plant. The power grid was down due to the earthquake. The diesel generators were destroyed by the tsunami. So mobile diesel generators were trucked in.

This is where things started to go seriously wrong. The external power generators could not be connected to the power plant (the plugs did not fit). So after the batteries ran out, the residual heat could not be carried away any more.

At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event”. It is again a step along the “Depth of Defense” lines. The power to the cooling systems should never have failed completely, but it did, so they “retreat” to the next line of defense. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator, right through to managing a core meltdown.

It was at this stage that people started to talk about core meltdown. Because at the end of the day, if cooling cannot be restored, the core will eventually melt (after hours or days), and the last line of defense, the core catcher and third containment, would come into play.

But the goal at this stage was to manage the core while it was heating up, and ensure that the first containment (the Zircaloy tubes that contains the nuclear fuel), as well as the second containment (our pressure cooker) remain intact and operational for as long as possible, to give the engineers time to fix the cooling systems.

Because cooling the core is such a big deal, the reactor has a number of cooling systems, each in multiple versions (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and the emergency core cooling system). Which one failed when or did not fail is not clear at this point in time.

So imagine our pressure cooker on the stove, heat on low, but on. The operators use whatever cooling system capacity they have to get rid of as much heat as possible, but the pressure starts building up. The priority now is to maintain integrity of the first containment (keep temperature of the fuel rods below 2200°C), as well as the second containment, the pressure cooker. In order to maintain integrity of the pressure cooker (the second containment), the pressure has to be released from time to time. Because the ability to do that in an emergency is so important, the reactor has 11 pressure release valves. The operators now started venting steam from time to time to control the pressure. The temperature at this stage was about 550°C.

This is when the reports about “radiation leakage” starting coming in. I believe I explained above why venting the steam is theoretically the same as releasing radiation into the environment, but why it was and is not dangerous. The radioactive nitrogen as well as the noble gases do not pose a threat to human health.

At some stage during this venting, the explosion occurred. The explosion took place outside of the third containment (our “last line of defense”), and the reactor building. Remember that the reactor building has no function in keeping the radioactivity contained. It is not entirely clear yet what has happened, but this is the likely scenario: The operators decided to vent the steam from the pressure vessel not directly into the environment, but into the space between the third containment and the reactor building (to give the radioactivity in the steam more time to subside). The problem is that at the high temperatures that the core had reached at this stage, water molecules can “disassociate” into oxygen and hydrogen – an explosive mixture. And it did explode, outside the third containment, damaging the reactor building around. It was that sort of explosion, but inside the pressure vessel (because it was badly designed and not managed properly by the operators) that lead to the explosion of Chernobyl . This was never a risk at Fukushima . The problem of hydrogen-oxygen formation is one of the biggies when you design a power plant (if you are not Soviet, that is), so the reactor is build and operated in a way it cannot happen inside the containment. It happened outside, which was not intended but a possible scenario and OK, because it did not pose a risk for the containment.

So the pressure was under control, as steam was vented. Now, if you keep boiling your pot, the problem is that the water level will keep falling and falling. The core is covered by several meters of water in order to allow for some time to pass (hours, days) before it gets exposed. Once the rods start to be exposed at the top, the exposed parts will reach the critical temperature of 2200 °C after about 45 minutes. This is when the first containment, the Zircaloy tube, would fail.

And this started to happen. The cooling could not be restored before there was some (very limited, but still) damage to the casing of some of the fuel. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started melting. What happened now is that some of the byproducts of the uranium decay – radioactive Cesium and Iodine – started to mix with the steam. The big problem, uranium, was still under control, because the uranium oxide rods were good until 3000 °C. It is confirmed that a very small amount of Cesium and Iodine was measured in the steam that was released into the atmosphere.

It seems this was the “go signal” for a major plan B. The small amounts of Cesium that were measured told the operators that the first containment on one of the rods somewhere was about to give. The Plan A had been to restore one of the regular cooling systems to the core. Why that failed is unclear. One plausible explanation is that the tsunami also took away / polluted all the clean water needed for the regular cooling systems.

The water used in the cooling system is very clean, demineralized (like distilled) water. The reason to use pure water is the above mentioned activation by the neutrons from the Uranium: Pure water does not get activated much, so stays practically radioactive-free. Dirt or salt in the water will absorb the neutrons quicker, becoming more radioactive. This has no effect whatsoever on the core – it does not care what it is cooled by. But it makes life more difficult for the operators and mechanics when they have to deal with activated (i.e. slightly radioactive) water.

But Plan A had failed – cooling systems down or additional clean water unavailable – so Plan B came into effect. This is what it looks like happened:

In order to prevent a core meltdown, the operators started to use sea water to cool the core. I am not quite sure if they flooded our pressure cooker with it (the second containment), or if they flooded the third containment, immersing the pressure cooker. But that is not relevant for us.

The point is that the nuclear fuel has now been cooled down. Because the chain reaction has been stopped a long time ago, there is only very little residual heat being produced now. The large amount of cooling water that has been used is sufficient to take up that heat. Because it is a lot of water, the core does not produce sufficient heat any more to produce any significant pressure. Also, boric acid has been added to the seawater. Boric acid is “liquid control rod”. Whatever decay is still going on, the Boron will capture the neutrons and further speed up the cooling down of the core.
The plant came close to a core meltdown. Here is the worst-case scenario that was avoided: If the seawater could not have been used for treatment, the operators would have continued to vent the water steam to avoid pressure buildup. The third containment would then have been completely sealed to allow the core meltdown to happen without releasing radioactive material. After the meltdown, there would have been a waiting period for the intermediate radioactive materials to decay inside the reactor, and all radioactive particles to settle on a surface inside the containment. The cooling system would have been restored eventually, and the molten core cooled to a manageable temperature. The containment would have been cleaned up on the inside. Then a messy job of removing the molten core from the containment would have begun, packing the (now solid again) fuel bit by bit into transportation containers to be shipped to processing plants. Depending on the damage, the block of the plant would then either be repaired or dismantled.

Now, where does that leave us?

The plant is safe now and will stay safe.

Japan is looking at an INES Level 4 Accident: Nuclear accident with local consequences. That is bad for the company that owns the plant, but not for anyone else.

Some radiation was released when the pressure vessel was vented. All radioactive isotopes from the activated steam have gone (decayed). A very small amount of Cesium was released, as well as Iodine. If you were sitting on top of the plants’ chimney when they were venting, you should probably give up smoking to return to your former life expectancy. The Cesium and Iodine isotopes were carried out to the sea and will never be seen again.

There was some limited damage to the first containment. That means that some amounts of radioactive Cesium and Iodine will also be released into the cooling water, but no Uranium or other nasty stuff (the Uranium oxide does not “dissolve” in the water). There are facilities for treating the cooling water inside the third containment. The radioactive Cesium and Iodine will be removed there and eventually stored as radioactive waste in terminal storage.

The seawater used as cooling water will be activated to some degree. Because the control rods are fully inserted, the Uranium chain reaction is not happening. That means the “main” nuclear reaction is not happening, thus not contributing to the activation. The intermediate radioactive materials (Cesium and Iodine) are also almost gone at this stage, because the Uranium decay was stopped a long time ago. This further reduces the activation. The bottom line is that there will be some low level of activation of the seawater, which will also be removed by the treatment facilities.

The seawater will then be replaced over time with the “normal” cooling water
The reactor core will then be dismantled and transported to a processing facility, just like during a regular fuel change.

Fuel rods and the entire plant will be checked for potential damage. This will take about 4-5 years.

The safety systems on all Japanese plants will be upgraded to withstand a 9.0 earthquake and tsunami (or worse).

I believe the most significant problem will be a prolonged power shortage. About half of Japan ’s nuclear reactors will probably have to be inspected, reducing the nation’s power generating capacity by 15%. This will probably be covered by running gas power plants that are usually only used for peak loads to cover some of the base load as well. That will increase your electricity bill, as well as lead to potential power shortages during peak demand, in Japan .

 

[JohanSjoman]

How come that they did not in some way let the hydrogen gas out of the building before it exploded. Could they not simply make holes in the reactor building?

 

[Maclomer]

I'm not a nuclear engineer and I am mystified by some of the technical information given off by 'experts' on BBC, CNN etc. Focusing on reactor #2 at Fukushima: the steam pouring from it looks like a 'steady state' situation - meaning that the water going in is sufficient to maintain a status-quo (whatever hell that is) inside. Now, is the heat generated JUST the result of radioactive decay (in which case, according to one expert yesterday, things should settle down in about 48 hours) or is there fission taking place (in which case I am guessing it will take a lot longer to 'burn out' with really bad consequences)?
Also if we imagine the rods melt and fall to whatever happens to be the bottom (presumably not China) and form there a nasty glob, could we expect fission to happen within such a glob (especially as it might not be homogeneous and could offer moderation paths for neutrons)? Please forgive me if I have used any technically offensive terms ...

 

[Homer Simpson]

U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko told members of Congress today that there is no water remaining in the fuel pool at reactor 4 at Fukushima Daiichi nuclear power plant. Jaczko told members of the House Energy and Commerce Committee that "we believe that secondary containment has been destroyed and there is no water in the spent fuel pool�radiation levels are extremely high, which could impact the ability to take corrective measures."

http://nei.cachefly.net/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-that-region/


So how does this happen? wasn't #4 the de-fuelled reactor? Would an entry to set up a flow into the fuel bay not have been possible (when the fuel was still covered) if they had detected the level falling? For this side-issue to be causing such a problem seems maddening. Now with the fuel uncovered I would imagine getting in there to address the situation would cause a massive dose to someone.

 

[kloptok]

So according to Reno Denos post things appear to have calmed down at reactors 1-3, am I right in concluding that?

What about the exposed fuel rods in the reactor 4 building? I hear that the water could be completely evaporated ( http://www.nytimes.com/2011/03/17/world/asia/17nuclear.html?_r=1&hp ), something denied by TEPCO though it seems. In case this is true, would it possible that the fuel rods could ignite or melt in some way? Is cooling of these rods in progress?

 

[Drakkith]

How come that they did not in some way let the hydrogen gas out of the building before it exploded. Could they not simply make holes in the reactor building?

COULD you? Yes. But that's generally a bad idea, If they KNEW hydrogen was building up they might have, but I'm guessing they had no idea it would explode like it did. In something like this you don't want to do anything to reduce the integrity of the reactor and surrounding structure. Don't want any radioactive materials released of course.

 

[Homer Simpson]

However, if they had the design (someone said earlier they do) to vent via stacks to the outside, it would seem ridiculous to CHOOSE to vent inside, as the H2 Zr/steam reaction is well known. If it was chosen to vent inside just to allow decay of radiation before emitting outside, you might question the motives behind that choice, no?

 

[Maclomer]

Snippet from BBC:

16 March 2011 Last updated at 16:16 ET
By Richard Black Environment correspondent, BBC NewsReactors

"Once a reactor is turned off, radioactivity and heat generation in the rods die away quickly; down to 7% of the original power within a second of switch-off, 5% within a minute, 0.5% within a day."

If this is true, where is all the heat coming from?

 

[snoopies622]

radiation leak is a very simple and misleading description

Indeed! It's been frustrating to hear so many TV anchors and reporters speak of "radiation leaks", "radiation exposure", "radiation levels", etc. without bothering to be specific about it.

 

[promecheng]

Snippet from BBC:

16 March 2011 Last updated at 16:16 ET
By Richard Black Environment correspondent, BBC NewsReactors

"Once a reactor is turned off, radioactivity and heat generation in the rods die away quickly; down to 7% of the original power within a second of switch-off, 5% within a minute, 0.5% within a day."

If this is true, where is all the heat coming from?

Let's take unit 2 for example. It is rated at 784 MWe (MegaWatt electric). Let's say that it is 30% efficient, that would mean it is rated at just over 2,600 MWt (MegaWatt thermal). 0.5% of 2,600 is 13 MWt, or 13 million Watts of thermal energy. This is still a considerable amount of energy that needs to be removed via heat transfer.

Ideally water is present, as water is superior to air or steam when it comes to removing heat from an object it comes in contact with. If water is not present, the fuel cannot remove the heat as fast as it produces it, which causes its temperature to rise until it finds equilibrium of heat produced to heat transferred, (energy balance), or it reaches melting point of the material.

 

[Bob S]

Let's take unit 2 for example. It is rated at 784 MWe (MegaWatt electric). Let's say that it is 30% efficient, that would mean it is rated at just over 2,600 MWt (MegaWatt thermal). 0.5% of 2,600 is 13 MWt, or 13 million Watts of thermal energy. This is still a considerable amount of energy that needs to be removed via heat transfer.

The heat of vaporization of boiling water is 2260 joules per gram, or 8.55 MJ (megajoules) per gallon. To remove 13 MWt would require boiling about 1.5 gallons of water per second.

Bob S

 

[promecheng]

The heat of vaporization of boiling water is 2260 joules per gram, or 8.55 MJ (megajoules) per gallon. To remove 13 MWt would require boiling about 1.5 gallons of water per second.

Bob S

I think that makes sense. I'm rough guessing here as I don't have any tech. specs. in front of me, and no calculator, (sorry for any gross errors). Assuming a 10 ft. diameter x 25' tall vessel, the volume is approximately 15,000 gallons. 1.5 gallons/sec = 5,400 gallons/hr. I think I read earlier on that they thought the RPV would boil down in a few of hours if no water was added. 15,000 / 5,400 = < 3hrs.

 

[SredniVashtar]

What about the exposed fuel rods in the reactor 4 building? I hear that the water could be completely evaporated ( http://www.nytimes.com/2011/03/17/world/asia/17nuclear.html?_r=1&hp ), something denied by TEPCO though it seems. In case this is true, would it possible that the fuel rods could ignite or melt in some way? Is cooling of these rods in progress?

This comes from http://news.blogs.cnn.com/2011/03/16/japan-quake-live-blog-death-toll-expected-to-rise-as-crews-reach-more-areas/" , quoting the IAEA

Temperatures recorded at spent fuel pools at the Fukushima plant Tuesday reached 84.0 degrees Celsius (183 Fahrenheit) at Unit 4; 60.4 degrees C (141 F) at Unit 5 and 58.5 degrees C (137 F) at Unit 6, the International Atomic Energy Agency said.

The agency said on Wednesday that "no data" registered for Unit 4, and Unit 5 had risen to 62.7 degrees C (145 F) and Unit 6 had risen to 60.0 degrees C (140 F). The temperature of these pools is normally kept below 25 degrees Celsius (77 degrees F)

"no data"...

 

[Thermalne]

Here is a link to the original IAEA article.

http://www.iaea.org/newscenter/news/tsunamiupdate01.html

Also,

Injuries or Contamination at Fukushima Daiichi Nuclear Power Plant

Based on a press release from the Japanese Chief Cabinet Secretary dated 16 March 2011, the IAEA can confirm the following information about human injuries or contamination at the Fukushima Daiichi nuclear power plant.

Please note that this list provides a snapshot of the latest information made available to the IAEA by Japanese authorities. Given the fluid situation at the plant, this information is subject to change.

Injuries
2 TEPCO employees have minor injuries
2 subcontractor employees are injured, one person suffered broken legs and one person whose condition is unknown was transported to the hospital
2 people are missing
2 people were ‘suddenly taken ill’
2 TEPCO employees were transported to hospital during the time of donning respiratory protection in the control centre
4 people (2 TEPCO employees, 2 subcontractor employees) sustained minor injuries due to the explosion at unit 1 on 11 March and were transported to the hospital
11 people (4 TEPCO employees, 3 subcontractor employees and 4 Japanese civil defense workers) were injured due to the explosion at unit 3 on 14 March

Radiological Contamination
17 people (9 TEPCO employees, 8 subcontractor employees) suffered from deposition of radioactive material to their faces, but were not taken to the hospital because of low levels of exposure
One worker suffered from significant exposure during ‘vent work,’ and was transported to an offsite center
2 policemen who were exposed to radiation were decontaminated
Firemen who were exposed to radiation are under investigation

The IAEA continues to seek information from Japanese authorities about all aspects of the Fukushima Daiichi nuclear plant.

Taken from the IAEA facebook page.

http://www.facebook.com/notes/inter...uake-update-17-march-0115-utc/202364423126685