What is the link between nuclear weapons and nuclear power stations?

[strongmotive]

Hey,

Quite an interesting topic and I've been bits and bats on the internet, but what do you think is the main links or what are your views?

 

[D H]

Both use nuclear fission. What point are you trying to make here?

 

[strongmotive]

Not trying to make a point. Just wondering what everyone thought the link was and is, I'm doing some little research myself and thought some interesting views from other people could help me (and its not homework).

 

[mgb_phys]

Nuclear weapons are bad therefore nuclear power is bad - m'kay!

You need a reactor to create the plutonium needed for most weapons. Many of the 'civil' reactors built in the 50s were largely aimed at producing pu and so weren't as good at power generation or as safe as they might have been - in the early cold war days there were other priorities!
What little pu that is produced today for weapons or research (most is recylced from old weapons) is made at specific reactors at weapons plants, material from power generation reactors isn't used for weapons - at least in the west.

Obviously a lot of the technology and expertise to run a civil reactor and a military reactor overlaps but the knowledge to run a reactor and build a weapon are very different.

 

[mathman]

One major difference (Iran problem) is that Uranium for reactors is somewhat enriched (U-235% is higher) from natural Uranium. To make Uranium for weapons the enrichment has to be much higher.

 

[Astronuc]

What is the link between nuclear weapons and nuclear power stations?

In general, there is no link between commercial nuclear power stations and nuclear weapons. The commercial nuclear industry has nothing to do with nuclear weapons, except for one special case where tritium is manufactured.

The U-bearing fuel in commercial plants is in the form of UO₂, and enriched up to 4.95%. In the reactor, it is irradiated to exposure of up to 50-60 GWd/tU. The Pu-vector has Pu-239, 240, 241, 242, as well as some Am/Cm (depending how long after it has been discharged), and is not really suitable for weapons.

The pit of modern nuclear weapons is Pu metal with the a different isotopic vector, although it is principally Pu-239.

 

[mgb_phys]

edit - in reply to mathman.

Thats exactly what makes Iran probably not a problem.
The technology to enrich NU to the 3% needed for a reactor is a lot easier than to enrich it to 90% needed for a weapon.

Having nuclear weapons (or the capability) is as much about convincing your own citizens you are protecting them as it is about convincing the enemy.
Of course the enemy also knows this!

 

[russ_watters]

The link between nuclear weapons and nuclear explosives is roughly the same as the link between cars and B-52 bombers (they use roughly the same fuel). B-52 bombers are weapons, therefore we should ban cars.

 

[quadraphonics]

The technology to enrich NU to the 3% needed for a reactor is a lot easier than to enrich it to 90% needed for a weapon.

I had the impression that weapons-level enrichment isn't necessarily more difficult, just more costly? I.e., you can use the same type of centrifuge systems, but you just need to run them for a lot longer (and/or build a lot more of them). I'd assume that's not the most efficient way to go about it, and so typically higher-tech methods are used, but isn't it possible to simply run a lot of centrifuges for a long time?

 

[mgb_phys]

Yes - the principles are the same but the degree of refinement, level of process control and materials handling are much more difficult. Although I suppose you could just start with a huge feedstock and waste most of it if you aren't concerned about making a profit.

 

[Astronuc]

I had the impression that weapons-level enrichment isn't necessarily more difficult, just more costly? I.e., you can use the same type of centrifuge systems, but you just need to run them for a lot longer (and/or build a lot more of them). I'd assume that's not the most efficient way to go about it, and so typically higher-tech methods are used, but isn't it possible to simply run a lot of centrifuges for a long time?

That's pretty much it for U-235 enrichment. One just adds more stages to the enrichment process, or one uses fewer centrifuge stages, which means that one would have to collect the enriched output, then start it back through the existing centifuges.

Basically natural U is %0.71 U-235 (0.0071), and as the enrichment decreases to about 0.2% U-235 it becomes much less efficient (longer) to continue to extract U-235. Extracting roughly 70% of the U-235 from natural U (or the equivalent of 0.5% of U which is U-235) means processing 1 MT of natural U for 5 kg of U-235. It just takes time.

High speed centrifuges have decreased the time required.


To get Pu-239, one needs to convert U-238, extract the Pu-239 chemically and process it to it's metal state. That's only the beginning.

 

[vanesch]

For the OP, there are several ways to make a nuclear weapon, and nuclear power makes, that shouldn't surprise you, use of *similar* technology than in nuclear weapon production. That means, there are similarities, and there are differences.

If you want to learn about nuclear weapons, you can find a lot of information here:
http://nuclearweaponarchive.org/

according to the website author, all this is publicly available information, and I recon that's true, as a lot of the information there I found in other sources.

Point is, in order to make a fission weapon, you need a strongly over-critical mass, because you want to liberate as much fission energy before the weapon destroys itself, which happens on the time scale of a few microseconds. So you only have a few microseconds to do all the energy production, and that means that you need a very fast divergence.
Just barely critical is not going to work, because the energy production rate would increase too slowly: this is what's called a fizzle: your nuke is a dud.

There are a few materials who can potentially serve to make such strongly over-critical masses: there's U-235, there is Pu-239 (and in fact, most mixtures of plutonium isotopes except Pu-238), you can even do it with Neptunium-237 and also with Am-241. You could also do it with U-233.

Happily, none of these materials exist as such, in pure enough form, in nature. Plutonium, neptunium and americium simply don't exist in nature, and U-235 is only 0.7% of natural uranium. U-233 doesn't exist in nature (or if it does, it is in trace amounts). U-238 is an efficient neutron absorber, and if it is too much present, it will stop any supercriticality.

So the first difficulty is to get the right material. And it is here that there is a link with the technology of nuclear power stations.

The second difficulty in a nuclear weapon is the fact that you need to obtain strong supercriticality, but of course before the weapon is used, the material cannot be present in critical form, or it would blow into your face during assembly. So one needs a technique to turn, quickly, a sub-critical set of materials into a strongly supercritical assembly. This part of technology has nothing to do with nuclear power stations, but it is important to know.
Historically, there have been two ways to make the supercritical assembly: by far the simplest one is "gun assembly": with some kind of "gun", two or more sub-critical parts are shot into one another, so that they form a supercritical assembly. The advantage of this technique is that it is robust, relatively easy to make, but it has two strong drawbacks:
- the supercritical mass thus obtained needs a LOT of material
- no matter how fast one thinks a gun can fire, this is, on nuclear scale, a very slow process. In fact, it is so slow, that one cannot use any material that generates a lot of neutrons by radioactive decay, because the assembly "slowly" goes from subcritical to supercritical, and crosses hence the zone of "barely critical", at which point background neutrons can start a chain reaction, and blow the weapon apart with "low energy". We have a fizzle.

It turns out that only pure U-235 and eventually pure U-233 could be used for such a weapon. All other materials have far too high a neutron background.

The other way is "implosion": chemical explosives in a special configuration generate a converging spherical shock wave which compresses a ball of nuclear material. It turns out that criticality rises strongly with material density. So the same amount of material, at normal density, can be sub critical, and at higher (compressed) density, supercritical. It turns out that such a shock wave compression is much much faster than gun assembly. Moreover, one needs much less nuclear material that way (we don't need a supercritical mass at normal densities, while we did with gun assembly). However, this time, the difficulty is that you only have your supercritical assembly during a few microseconds (the time of the implosion). So now you need, exactly at the right time, to generate a pulse of neutrons that will start the chain reaction.
This is much more difficult technology, and this time, the success depends upon the precision and the mastery of the shock wave and the neutron pulse.
Although one can cope with higher neutron backgrounds, nevertheless, also here, the higher the neutron background, the higher the chances to obtain a dud or a low-yield weapon. Implosion works very well with Pu-239 if one masters enough the implosion technology. The higher the contents of Pu-240, the harder it becomes to make it work, although with enough implosion quality, any Pu mixture (except for Pu-238) can be made into a bomb.

How does one get "weapon-grade" materials ?
U-235 is obtained by isotopic separation. From 20% U-235 onward, one can in principle make a bomb, but one needs high purities in order for the needed mass of material to remain reasonable (at 20% one would need several tons, which is impractical). Usually one wants over 80% U-235, and preferentially, more.

There are 2 known "large scale" isotopic separation technologies: diffusion and centrifuges. Centrifuges are better at making small quantities of high-purity material, while diffusion is better at making large quantities of low-enriched uranium, but both can be used for both purposes. It is difficult and large-scale technology.

However, as PWR need low-enriched U-235, this technology is present in the nuclear power sector, although not directly in the form needed for highly enriched U for a bomb. So the technology is present, but the configuration of a plant for power production is different than the configuration for making weapon-grade U-235.

There exist however, other technologies to separate U-235 with high purity, which are not used in the power sector, but which do exist. The most promising example is probably laser based isotope separation (AVLIS http://en.wikipedia.org/wiki/AVLIS ).

In order to make plutonium, one has to place U-238 in a neutron flux, such as is present in a reactor. A PWR has a certain amount of plutonium in its spend fuel ; however, the isotopic composition of that plutonium is rather bad: it contains a lot (~ 30%) of Pu-240, which gives it a huge neutron background.
In order for this plutonium (which is present on percent-level in spend fuel) to be extracted, one needs a chemical treatment of the spend fuel, in what's called the PUREX process. This chemistry is known now, but the difficulty resides in handling the highly radioactive spend fuel. The high Pu-240 content in spend fuel comes from the fact that fuel elements remain a long time in a PWR. In order to obtain much more pure Pu-239, one needs to irradiate U-238 only slightly, and reprocess it. That's better done in a reactor in which one can change continuously some fuel elements. A PWR is not suited for that, but a graphite reactor is, most research reactors are, and Candu reactors are.

So, "good-quality" weapon plutonium needs fiddling with non-power reactors, and need in any case a reprocessing facility. "poor-grade" plutonium can be obtained from spend fuel of a PWR. One can make an implosion weapon from both, but it is harder with the second kind of material.

In conclusion: if one goes for simple weapon design (gun type), one needs enrichment to a high degree. If one thinks one can handle an implosion weapon design (much harder), one can do with a reactor and some chemistry in a reprocessing facility. Using spend fuel from a PWR gives plutonium of a worse quality than making special-purpose reactors.

One can also irradiate thorium, which converts to U-233. Now, this is an interesting combination for a bomb builder, because he doesn't need isotopic separation (the U-233 can be separated chemically from the thorium in a reprocessing plant, using the THOREX process). Moreover, the activity of this irradiated thorium is lower than that of spend fuel for Pu production. And U-233 can in principle also be used in a gun-type weapon. So this combines the two "easy" approaches in both cases: chemistry instead of isotopic separation, and gun type instead of implosion. Only, this is not the path nuclear nations have followed, and hence more of a "development risk" than proven technologies.

So we see the link between the technology of nuclear power on one hand, and nuclear weapons on the other:
- enrichment, needed for low-enriched uranium for PWR, is a technology which can be modified into making highly enriched uranium for a weapon. However, we're talking about very large installations here, which have to be seriously modified to switch them from producing lowly enriched uranium into highly enriched uranium. However, it is the same technology, applied differently.
- fuel reprocessing which extracts plutonium. The difficulty here is that these are also large facilities, because they have to handle chemistry of very active materials.
- reactors. PWR can only make "bad quality" plutonium (which can be used in a bomb, but with more difficulties). Other reactors can be turned into specific weapon-grade plutonium production reactors.

Clearly, the technological knowhow of nuclear power is interesting for a bomb builder, although it doesn't contain everything. Also, existing facilities for nuclear power have to be seriously modified in order for them to make weapon material ; so as long as they are regularly inspected, this can be verified. Finally, there exist technologies totally outside of the nuclear power sector, which can also lead to nuclear weapons. I mainly think of laser isotope separation.

 

[daveb]

One other aspect of using U-233 is that you would need to separate out the U-232 from the U-233 (since there is a potential for U-232 in any sample). U-232 has in its decay chain Tl-208 which has a very penetrating gamma in high abundance (see http://www.nti.org/e_research/cnwm/overview/technical2.asp).

 

[vanesch]

One other aspect of using U-233 is that you would need to separate out the U-232 from the U-233 (since there is a potential for U-232 in any sample). U-232 has in its decay chain Tl-208 which has a very penetrating gamma in high abundance (see http://www.nti.org/e_research/cnwm/overview/technical2.asp).

Yes, I know. But that's only a radiation hazard, which can be solved with a good glove box, I'd say. U-233 combines the advantages for making a bomb:
- can use gun-type assembly (far far easier than implosion), and no initiator strictly necessary
- can be chemically separated, no isotopic separation necessary
- has no significant heat generation

As stated in the excellent article you linked to, the only reason to prefer plutonium production over U-233 production initially was 1) that one ignored the problems with gun assembly with plutonium initially 2) that in a reactor with *natural uranium*, the plutonium production is higher than the potential U-233 production. But I keep wondering why, right now, those people intending to make a bomb don't follow the "new" U-233 route, as it seems to avoid the two main obstacles to make an "easy bomb": isotopic separation, and implosion. I find the gamma radiation from Tl-208 a minor problem in that respect.

 

[oldsloguy]

As stated in the excellent article you linked to, the only reason to prefer plutonium production over U-233 production initially was 1) that one ignored the problems with gun assembly with plutonium initially 2) that in a reactor with *natural uranium*, the plutonium production is higher than the potential U-233 production. But I keep wondering why, right now, those people intending to make a bomb don't follow the "new" U-233 route, as it seems to avoid the two main obstacles to make an "easy bomb": isotopic separation, and implosion. I find the gamma radiation from Tl-208 a minor problem in that respect.

Your last two posts were very interesting, but then it finally came down to "Every day there is always one thing that comes to remind me how ignorant I am"! When you say "initially", I think of the WWII bomb crew and a couple of questions I never really considered.
1) Were the Hanford reactors natural uranium (I know Dodo, the thought never even entered my mind)?? Was that because they didn't want to waste enriched uranium?
2) Is the prodcuction of U233 in a thorium reactor really much less than Pu239 in a Uranium reactor?
3) Were the physics of the Thorium reactor and U233 fully understood at the time?

 

[vanesch]

I will try to answer to the best of my knowledge, which is probably (certainly) incomplete.

Your last two posts were very interesting, but then it finally came down to "Every day there is always one thing that comes to remind me how ignorant I am"! When you say "initially", I think of the WWII bomb crew and a couple of questions I never really considered.
1) Were the Hanford reactors natural uranium (I know Dodo, the thought never even entered my mind)?? Was that because they didn't want to waste enriched uranium?

Yes, the initial Hanford reactors were natural uranium reactors. One didn't yet know how to make enriched uranium, that was the other track that was being explored right at that moment. If one had the technology to fill a reactor with LEU, then one also had the technology to make enough HEU for a bomb! It was because of the presumed difficulty of enriching uranium in sufficient quantities that one took also the road (as a kind of backup) of plutonium.

In fact, the Hanford reactors were initially barely critical, and one ignored a lot about fission products (things which never had been produced! Isotopes which had never been made, and of which one didn't know any physical properties). They were based upon Fermi's initial graphite reactor, CP1. And it turned out that initially, the Hanford reactors could only work for a few hours (or days ?), and then had to stop, because they became sub-critical due to Xenon poisoning (due to an unknown fission product at that time, Xe-135, which has a monstrous absorption cross section for thermal neutrons http://en.wikipedia.org/wiki/Xe-135 ). People could solve it initially by adding more uranium than foreseen to the reactor. I don't know if later on, LEU was used, but I would guess not, as all enrichment during the war was in order to make enough for one single bomb (the Hiroshima bomb).

2) Is the prodcuction of U233 in a thorium reactor really much less than Pu239 in a Uranium reactor?

No, that's not the point. The point is that the AMOUNT of thorium that one can put in a NATURAL uranium reactor before it becomes sub-critical is quite low, as compared to the amount of U-238 that one has. So one cannot irradiate much thorium at a given time in a natural uranium reactor. Don't forget that a natural uranium/graphite reactor is BARELY critical, and the least bit of neutron-absorbing material that one adds, will make it subcritical. From the moment that you work with LEU, that's no issue anymore. The advantage of U-Pu is that the fertile material is already present in large quantities - so one doesn't have to ADD any fertile (and hence, neutron-absorbing) material. Making U-233 in a natural uranium/graphite reactor requires you to ADD extra thorium, which will kill the chain reaction if in too big quantities.

3) Were the physics of the Thorium reactor and U233 fully understood at the time?

I think that the conversion was known, and I also think that one knew, or could guess, that U-233 would be fissile, as it is an uneven mass number isotope. But I don't know the details.

 

[BarkingMad]

The link between nuclear weapons and nuclear explosives is roughly the same as the link between cars and B-52 bombers (they use roughly the same fuel). B-52 bombers are weapons, therefore we should ban cars.

They also use the same pilot. Maybe we should ban them...;) after all, that is where the true difference between a weapon and a tool comes from.

 

[Utwig]

Hi

The link really centres around a kind of reactor called a breeder or fast breeder reactor. In general, we use U235 to produce power, and also to make weapons with. U235 is refined from an ore called yellow cake via gas centrifuges usually linked together in series, so the output of one is the input of the next. The problem is that there is a finite amount of U235 available, and its a pain to refine.

There is another material U238, that is far more abundant but can’t really be used in its native or refined state to produce power or weapons, but, and its a big but, if you place staves of the stuff around a U235 fission pile the effect of the neutron bombardment it is subjected to turns it into something else, plutonium.

We can make power from plutonium, but its a hideously toxic material and to be honest, you don’t really want to have anything to do with it. The sad fact though is that its really really good at making unbelievably lethal bombs. As you have probably guessed, the kind of reactor space used for this purpose is a breeder reactor, where staves of U238 are arranged near the U235 core for precisely the purpose of manufacturing plutonium, and that is your link.

Hope this helps.

Utwig

 

[Astronuc]

Yellow cake (oxide) is converted to UF₆, which is used in centrifuges where is is a gas. Enriched UF₆ (that is up to 4.95% U-235) is sent to commercial nuclear fuel plants where it is converted into an oxide, ultimately in the form of ceramic pellets which are then encapsulated in Zr-alloy cylindrical tubing, and hermetically sealed with Zr-alloy endplugs (Zr-alloy barstock).

For the naval program, the enrichment is much higher, and that value is classified. Naval reactor fuel is quite different from commercial fuel.

Nuclear weapons are predominantly based on Pu-239 which is produced from U-238 in special production reactors, which are not necessarily fast breeder reactors.

U-235 based weapons have been phased out.

 

[Utwig]

Hi,

Yep, I have no doubt plutonium can be made in other kinds of reactor, but I've only really ever had experience with the breeder type, and pressurised water kind. I could have put down a lot more detail, but was really only trying to highlight the link between commercial power generation and instant sunshine machines in layman's terms.

My personal fave was he pressurised water reactor, during the time I was involved with them we refuelled 2 boats and pulled the solid relief's out of 4 more. Its amazing how small the reactor actually is, once you pull all the boron impregnated polythene off one its not much longer than a family car and maybe about half a meter in diameter. Stunning how something so small can put out so much heat. It wasn't often we had to enter a reactor space while the pile was in plant state A but the odd time we did, if the blowers were not on it was uber hot. Oh, incidentally the exp with breeders didn't involve working on them but was more along the lines of getting the monitoring certification completed.

Utwig

Utwig