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About North Korea's Nuclear Program
& Plutonium Production

(If this title doesn't trigger the "watchers", nothing will ;-))

"What One Photo Tells Us About North Korea's Nuclear Program"
https://www.nytimes.com/interactive/2017/02/24/world/asia/north-korea-propaganda-photo.html

It remember that the honored Pres. Bill Clinton
gave N. Korea two nuclear reactors (and apparently 5 billion dollars)
     - if they would only be used for power generation
     - if inspection teams would have full access anytime.
     - if N. Korea would [promise to quit seeking nukes].

About two weeks after the nuclear systems went "on line"
     - all nuclear inspectors were stopped and sent back at the airport -
     - and no westerner has seen the reactors since.

http://nypost.com/2016/01/06/you-can-thank-jimmy-carter-and-bill-clinton-for-north-koreas-nukes/

from Tim Coslet
collected from several e-mails.

"Do you know the main difference between a reactor operated to produce power and a reactor operated to produce plutonium for nuclear explosives? (regardless of its original design purpose)"

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You can still generate steam to spin a turbine and produce power while using the reactor mostly to produce plutonium for nuclear explosives. It actually worked very well at Hanford's N Reactor (although this was deliberately designed for both, and was one of the safest power reactors ever built). Although the Soviets never admitted it, several features of the RBMK (e.g Chernobyl) although it was designed as a power reactor, make it obvious that it allowed for very easy conversion to produce plutonium for nuclear explosives if needed (however it was one of the most unsafe power reactor designs ever built).

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It is the time the fuel remains in the reactor. The longer the fuel stays in the high neutron flux of the reactor the more of the undesirable Pu-240 and Pu-241 are produced. Both have high spontaneous fission rates (the reason the MK-II "Thinman" Plutonium gun assembly bomb would fizzle instead of exploding and the MK-III "Fatman" Plutonium implosion assembly bomb had to be developed instead) and Pu-241 is a strong gamma emitter potentially making a bomb with too much of it dangerous to even be around (so called "submarine grade" plutonium is produced by reducing the time the fuel is in the reactor even more, to limit the radiation exposure of submarine crew members that would be near the missiles or torpedoes having nuclear warheads in the close quarters of a submarine). In a reactor operated to produce power you usually just leave the fuel in until it begins to "wear out" and is no longer producing energy efficiently. Depending on the reactor design this is usually many months to even many years before fuel changes.

In a reactor operated to produce plutonium for nuclear explosives you remove the fuel as soon as possible after a reasonable amount of Pu-239 has been produced. This could be only a few days, rarely more than a few weeks before fuel changes.

So one could fairly easily take a reactor originally designed only to produce power and change out the fuel very very frequently and use it to produce plutonium for nuclear explosives. Many reactor designs could permit the reactor to continue to be used to produce power during a fuel change out, but others could require a shutdown during each fuel change out. Someone that wanted to be sneaky could switch back to longer fuel cycle times before an inspection, but this could be checked for by inspectors by random sampling of fuel rods to test for unusually low Pu-240 and Pu-241 levels and surprise inspections.

Many people are not aware of this, but this is also the reason that plutonium reprocessed from "normally operated" power reactors is not a realistic weapons proliferation risk! The very high levels of both Pu-240 and Pu-241 make it unsuitable for any nuclear explosive (the spontaneous fission rate is so high that even implosion assembly bombs would have a very high probability of a fizzle instead of an explosion) and the high level of Pu-241 would make any handling of such a bomb or the assembly of its plutonium components hazardous to the workers.

Plutonium production reactors are typically designed with horizontal tubes that cooling water and fuel pass through. Fresh fuel pellets are just pushed in at one end, with used fuel pellets falling out the other into a cooling pond where they sit as the neptunium decays to plutonium (that will not be exposed to more neutron flux and won't make Pu-240 and Pu-241). Power reactors usually (but don't have to) use fuel rods containing multiple fuel pellets and are inserted and removed vertically, changing many fuel pellets frequently in short periods of time in this design is less convenient but not entirely impossible if one is determined to convert such a reactor to making plutonium for making explosives that way.

BTW, at the time of Chernobyl the Hanford N Reactor was in a shutdown for maintenance but was expected to be back online in a few months. Although it had none of the design safety issues of an RBMK reactor N Reactor was never restarted and was eventually demolished due to protests and false concerns due to the facts that it was also graphite moderated and was a dual use (power and plutonium for nuclear explosives) design reactor. N Reactor was capable of a passive self shutdown when the core got too hot (few modern reactors can do this) due to an extremely high negative temperature coefficient of reactivity (produced by using a combination of graphite and water as the moderator, expansion of the water at higher temperature reduced its part of the neutron moderation enough to fully shutdown the reactor and fission product decay heat would keep the reactor in shutdown) that could not be manually overcome by removing control rods. Ordinary water moderated reactors cannot take advantage of the expansion of the water as removal of control rods is deliberately designed to overcome this so that the reactor can operate normally.

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I asked Tim Coslet (above) if I could post these e-mail fragments - knowing he was an electrical type - never having worked on "government nuclear projects".

Absolutely. Of course. It is from several publicly available sources that anyone can find. Most of which I read long ago (probably the N Reactor material was the most recent I found). Also it helps to take a tour of the Hanford B Reactor (which I did in 2014 IIRC) to see how the first plutonium production reactor was setup (and the Oak Ridge X-10 Reactor a miniature prototype of the Hanford B, D, and F reactors which I saw in 1996).

So it is very very easy to believe that we naively gave "power reactors" to North Korea that they used to make plutonium for use in nuclear explosives by frequently changing the fuel (even if the design of the reactor required it to be shutdown for more time than it was operated in each fueling cycle). This would of course require they prevent any surprise inspections so that they could not be caught. However by now they must have built their own dedicated plutonium production reactors, and don't need these anymore.

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