Nuclear Reactor Rod Replacement Time

[David Berger]

If an enemy had a secret nuclear reactor and they were retrieving the rods, or whatever it is that they retrieve to make secret nuclear bombs, how much time (how fast) would it take for them to retrieve their needed material? Would just a part of the reactor be shut down, and if so, what percentage of the reactor? If in the future a person had a small antineutrino detector, what amount of time period would that antineutrino detector be showing that nuclear material had been retrieved? What could be expected to be seen, using such a device in order to catch a rogue operation?

 

[anorlunda]

There is no simple answer to your questions. Production of fissile weapons materials (we call it diversion) is much easier done in research reactors than in power reactors.

If you would like to learn more, including detection, this Wikipedia article is a good place to start.
https://en.wikipedia.org/wiki/Nuclear_proliferation#Scope_of_safeguards

 

[David Berger]

Its pretty slim pickings in that wiki article. They didn't say anything about how long it would take a reactor to be down.

I found another article and it stated that it took five days of shut down for replacement time, that seems like a very long time. I would have thought it would have taken only hours. A five day shut down period would be easy to detect from quite a distance using an anti-neutrino detector.

 

[anorlunda]

Five days is not bad for a power reactor. The Canadian CANDU reactor is fueled with natural uranium (no enrichment needed). and it can refuel online with no shutdown.

But remember what I said in #2 about research reactors. Research reactors are not intended to run all the time. They run for a while, then shut down to examine the results. That's how they accomplish the research. So merely detecting shutdown of a research reactor is not enough to arise suspicion.

There are more subtle reasons involving neutron spectrum and fuel composition that make it possible for a research reactor to produce more fissile material than a power reactor. That shouldn't be surprising; power reactors are optimized to produce power, not other things. I recall a quote (sorry can't find the link) that said, "Only a fool would try to use a power reactor to divert fissile material. There are much better ways."

This article https://www.iaea.org/topics/basics-of-iaea-safeguards is more in-depth than the WIkipedia article. An analogy is that IAEA has as many methods of catching diversion as police detectives have at catching criminals. They use every method at their disposal and don't rely on any one.

 

[jim hardy]

See if you can find a copy of "Plutonium Handbook" .
IIRC it discussed production. But it was over twenty years ago i perused it so memory isn't very detailed.

John McPhee's "Curve of Binding Energy" is about proliferation. And a very enjoyable read.

old jim

 

[David Berger]

Anorlunda it would be nice if it was five days for a shut down even for the smaller research reactors. I can see where the larger commercial reactors wouldn't be interested in shutting down at all due to loss of income. On the other hand, I can see why smaller nuclear reactors would be easier to shut down and no one would notice them producing material to be used in weapons. The CANDU reactors seem like they operate better since they really don't need to shut down as you stated. Robots can shove in new fuel rods, while other robots at the other end can collect the spent fuel and/or leaking fuel rods. The safeguards used in known power plants seem to be pretty simple, but in detecting a rogue power plant it might be much harder. That was a very interesting read, Thanks.

Jim Hardy, I read in the second book that commercial reactors are incapable of creating weapons grade material since they don't use fully enriched uranium, which the small reactors can use. That book is truly an over kill on information about the entire matter and is a very good read, Thanks

 

[rpp]

As anorlunda stated, there is no simple answer to your question.

If you were trying to produce plutonium for a weapon, you usually want short irradiation times. The reason for this is that you want to build up the "good" plutonium isotopes, but not the "bad" ones. The longer the fuel is irradiated, the more "bad" isotopes are produced. Because of this, most plutonium production reactors have had continuous fuel refueling, so there won't be a shutdown to detect. At least a shutdown to "retrieve" the fuel.

The good plutonium is often called "weapons grade plutonium". The bad plutonium is "reactor grade plutonium". Commercial reactors produce reactor grade plutonium and it would be very difficult to convert the reactor grade plutonium to a bomb. You would need to enrich the plutonium.

 

[David Berger]

Thanks rpp. If its a very short period of time for shut down for them to retrieve their fuel, then it would be absolutely worthless to monitor such a nuclear power plant.

 

[girts]

David, I think one factor here to consider is the specific reactor design, because as others have pointed out modern commercial power gen III or IV reactors are pretty much useless for bomb material production because their design is such that refueling takes quite some time and effort, if I recall from what I read for a LWR you have to take the top of the pressure vessel off and then use special apparatus to take out the old fuel assemblies and put new ones in so normally they plan the life cycle of the fuel and change it only once per 2 or so years, I'm sure some people with more knowledge will be able to say more precisely.

If you look at the first experimental reactors built for the Manhattan project and afterwards they were specifically designed for plutonium production and so they were refueled without shutdown with people using long rods to physically push the irradiated fuel out the other end, see Hanford etc.

As far as I know there are only few commercial large scale power reactors that could be used or have been used for this purpose, one is CANDU and the other that comes to mind is the soviet RBMK 1000/1500 reactor, the soviets had another reactor that was more safe called VVER but they also built the RBMK models up until the Chernobyl accident, from what I have read the main reasons for that were it's simplicity and large output power combined with some other aspects like the online refueling capability, the RBMK has a large machine in the reactor hall that rides above two rails mounted on each side of the reactor hall support pillars,an operator sits behind a thick wall and operates the machine and so fuel can be taken out and new fuel put in in each of the channels without shutdown.

I don't have any information and I doubt someone here has, about whether the soviets used the RBMK power reactors for bomb material production but they were perfectly capable of such a thing.Even though as others have pointed out I believe that those countries that already have nuclear weapons and are considered nuclear superpowers like USA, Russia, UK, France, China have special purpose built enrichment facilities and plutonium production reactors and don't mess around with making bomb material in commercial power reactors while rogue nations like North Korea and possibly some others that have tried or have gotten nukes like Pakistan probably just built a special reactor for weapons material production and have no civil nuclear power industry whatsoever like I believe is the case with N Korea

 

[David Berger]

Thanks girts, that was some very interesting research that you have done there. I think North Korea is going to be trouble, and the whole world needs to de-esculate on the nuclear weapons. I hope you have a Merry Christmas and a Happy New Year!

 

[Eric Bretschneider]

As anorlunda stated, there is no simple answer to your question.

If you were trying to produce plutonium for a weapon, you usually want short irradiation times. The reason for this is that you want to build up the "good" plutonium isotopes, but not the "bad" ones. The longer the fuel is irradiated, the more "bad" isotopes are produced. Because of this, most plutonium production reactors have had continuous fuel refueling, so there won't be a shutdown to detect. At least a shutdown to "retrieve" the fuel.

The good plutonium is often called "weapons grade plutonium". The bad plutonium is "reactor grade plutonium". Commercial reactors produce reactor grade plutonium and it would be very difficult to convert the reactor grade plutonium to a bomb. You would need to enrich the plutonium.

Plutonium production during the Manhattan project used a reactor design that didn't require any shutdown. The reactor was designed as a pass through. Fuel canisters were pushed in one end. Every time a new canister was added, it pushed an irradiated canister out the opposite side. The "entrance" side was basically a wall full of holes. Irradiated canisters fell into a deep pool of water to "cool down". This was figurative and literal as the irradiated canisters produced a lot of heat from short lived isotopes that had to decay so radiation levels dropped to much lower, but still significant levels before the plutonium extraction.

From memory the canisters had a residence time of about a month.