Does anybody know what kind of reactors Iran is trying to build?

[AngryAmish]

Is it the usual control rod-boiling water type or the Chinese style pebble bed system?

[punpirate]

1. The one being built now...

... is a Russian designed, pressurized-water reactor of about 950mW. There are plans to sell Iran three more smaller designs of the same type, capable of 650 mW.

Iran, however, is also trying to get approval for a 40mW research reactor, which is more problematic, since it's of a heavy water type.

[HysteryDiagnosis]

2. The kind that is bombed by a neighboring country? n/t

[AngryAmish]

3. Another technical question

Since you seem to know what you are talking about and I do not.

Is the advantage of a heavy water design is the spare neutron to speed the reaction along and that helps enrich the fuel? Or am I an idiot?

[punpirate]

4. This link explains it, in part.

http://www.janes.com/security/international_security/news/jir/jir031114_1_n.shtml

In short, the neutron cross-section (capture ratio) of a material is the chance that a neutron will interact with the nucleus in a given area.

For a reactor moderator, a low neutron cross-section indicates that a larger number of neutrons are available to change U-238 to Pu-239, a bomb-grade material.

This means that fuel of lower enrichment of the uranium is required (there are some small research reactors capable of running natural uranium). And, research reactors are often designed to change fuel on the fly without having to burn up the entire fuel load (this is necessary because the longer the Pu-239 continues to be bombarded by neutrons, more of it is changed to Pu-240, which is not suitable for bombmaking, and which is extremely difficult to separate out).

Hope that helps.

Cheers.

[hunter]

5. Good to see you around, punpirate...

It seems to me that Iran knows a lot about the Bush Administration that we don't.

My prediction: The Bush Administration will do nothing, or they will do something stupid.

Duh. I don't even need a graduate degree in international relations or nuclear physics to figure that one out.

Anyways, there are a few large scale reactor designs that will run on "natural" uranaium. CANDU, which uses heavy water as a moderator is one. I'm certain the creative people in Iran could come up with others.

It's all about "mastery" of the nuclear fuel cycle, as has been reported elsewhere.

[punpirate]

6. Recent news...

... about U.S. selling (well, not really selling... we're paying for `em) bunker busters to Israel could be a sign that the neo-cons have been busier than we think regarding Iran.

I would guess that almost any scenario involving those and Iran would probably fall into your latter category regarding administration actions.

BTW, didn't know CANDU was of that type (not a reactor maven--just did okay years ago in nuclear chemistry in college).

Cheers.

[hunter]

8. India used CANDU technology to make plutonium for bombs.

If you can be trusted to make only electricity CANDU is a better fuel cycle than once-through pebble bed reactors. The problem with CANDU has always been the weapons proliferation issue.

[punpirate]

9. With heavy water as a moderator...

... it's possible to "blanket" the reactor core with U-238 and produce a pretty high-quality plutonium for bomb use, so I've read, and make that production cycle faster, too, since only the blanket needs to be removed at the proper time while the core continues on through burn-up.

Now, if only nations weren't just as clever at finding new ways to threaten each other.

Cheers.

[NNadir]

11. This is true. CANDU reactors are continuously fueled reactors.

Any reactor that can be continuously fueled can make weapons grade plutonium. The separation of the dilute weapons grade plutonium is very expensive however, and given that high speed centrifuge technology now exists, plutonium is probably no longer the cheapest and easiest way to make nuclear weapons.

Even so, the CANDU reactor is nonetheless essential to the world's future. This technology should be expanded to the maximum possible extent. Once through fuel cycles are simply not sustainable on environmental grounds. They are wasteful.

It is possible to minimize (though not eliminate) the proliferation value of CANDU reactors by switching to a Thorium based fuel cycle and using plutonium seeds rather than the U-235 in natural Uranium. Under these circumstances, one achieves very high fuel burn-up, operating the reactor as a near Thermal breeder. This process denatures both the plutonium AND (interestingly) the Uranium rendering much less suitable for nuclear weapons diversion.

Still, even with these somewhat reassuring facts in mind, in the future one hopes that an IAEA with teeth will exist and that all CANDU type reactors will be subject to close international inspection and frequent international auditing of both its incoming and outgoing fuel.

As is the case in all nuclear technology, the issue needs to be examined not using a "what can we imagine happening" standpoint but from a "what IS happening" standpoint. In other words we have to balance risks. Very clearly the biggest risk to the planet now is NOT that nations lead by dangerous religious psychopaths like those ruling Iran and the United States will have nuclear weapons. The biggest risk is that the planetary environment will begin a precipitous and irreversible collapse. This collapse at this point can only be prevented by switching to proven immediately scalable technologies that have low GHG impact. This is of course nuclear energy. Moreover, we are likely to need nuclear technology merely to survive for several centuries, thus we cannot afford to be wasteful of fissile fuels.

The weapons risk in Iran has nothing at all to do with their reactor technology and everything to do with their enrichment capacity. It is probably true that the entire enrichment program is weapons oriented, since the price of SEU (slightly enriched Uranium) is very low and the industry is way over capacity. Thus the only reason for building such a plant is to escape international restrictions on the uses to which enrichment can be put. On the plus side, nuclear weapons building and maintenance is very expensive and difficult. It is much more expensive to enrich Uranium to 95% U-235 than it is to enrich it to 3-5% as is now done for fuel purposes. This puts real limits on the number of weapons a nation like Iran or North Korea can build.

I note that in a Thorium based fuel cycle, enrichment is probably completely unnecessary.

We ought not to ask ourselves whether Iran (or other nations) CAN build nuclear weapons but rather WHY they build nuclear weapons. It's the old "Charles Atlas" ad with the 97 pound weakling getting sand kicked in his face repeatedly by a bully and deciding to work out until he can punch the bully out. Given the history of Iran and its relationship to the United States since the 1953 CIA coup that installed the Shah, I can understand, even if I don't applaud, Iran's decision to build nuclear weapons. If a person with the equivalent to George Bush conquered Mexico under false pretenses using extreme violence to steal its resources, we might have extreme reactions. The Iranians are no less human than we are. If anything gutted the NNPT, it would be the dangerous psychotic bully installed in the White House.

This is the problem, not the the type of nuclear technology available in Iran and elsewhere.

[NNadir]

7. Good. Mostly correct.

A cross section is not a capture ratio. A capture ratio is actually a ratio of two cross sections, which, as you defined represents a probability that a neutron will interact with a nucleus. The two particular cross sections that define the capture ratio, an important parameter in nuclear engineering, are the capture cross section and the fission cross section. There are also other cross sections, such as the scattering cross section, or cross sections associated with particular types of reactions for instance. Although cross sections can be thought of as "target areas" and in fact their unit, named whimsically "barns" are in fact units of area, they do not represent areas per se.

The cross section of a particular nucleus is not constant for all neutrons, but actually varies with the speed of the neutrons in question, which can be thought of as a "neutron temperature." The connection between speed and temperature is given by the Boltzman relation <E> = 1/2 m<v>^2= 3/2 kT. Since neutrons interact weakly with atoms, their "temperature" does not necessarily correspond to the thermal temperature. Moderating is a means by making the correspondence occur between thermal molecular speeds and neutron molecular speeds. We refer to neutrons that are moderated to the same speed as the atoms in a liquid or gas as "thermal neutrons." The lighter an element is, the better it is as a moderator. The best moderators are hydrogen (in "light water") deuterium (in "heavy" water), beryllium and carbon. Beryllium has never been used in commercial reactors because it is too expensive.

Neutrons that are partially, but not completely slowed down are "epithermal" neutrons. Neutrons that have not been slowed much from the energy at which they are emitted, roughly 1 to 2 Mev (Million electron volts) are referred to as "fast" neutrons.

Reactors have been built that utilize all three types of neutrons, thermal, epithermal and fast, but in general, thermal neutrons are most efficient and allow for the largest safety margins in reactor operation.

[punpirate]

10. Quite right....

... as I said, I'm not a reactor maven. :)

[NNadir]

12. I am a reactor maven, but I skipped an important moderator.

Helium is also sometimes used as a moderator.

Helium-4 is interesting because it is the only nucleon in the table of nuclides that has a capture cross section of zero. Therefore 100% of the neutrons that bounce off a Helium will bounce off, meaning that Helium moderation offers excellent neutron economy, no neutrons are wasted by absorption into the moderator. (Deuterium is not quite as good, but it's close.)

It is also an excellent moderator because of its low atomic weight. Unfortunately it is very expensive and has a very low molar density when compared to water, carbon and the (unused) Beryllium.