This is technical background information on North Korea’s indigenous Graphite Moderated Reactors (GMR), why those were and are problematic in regard to proliferation, and why Light-Water Reactors (LWR) are much less so. Not an item related to current news, but an interesting tidbit and piece of the DPRK nuclear big picture.

Nuclear weapons generally use one of two nuclear isotopes – Plutonium (Pu-239) or Uranium (U-235). Both have advantages and disadvantaged; Pu-239 is much cheaper and easier to reprocess, but requires a complicated detonation mechanism, while U-235 is relatively difficult and expensive to reprocess, but is extremely easy to ‘set off’ (scientists have been killed by accidentally dropping pieces of U-235 weighing only a few kilograms on each other and releasing large amounts of radiation).

North Korea chose GMR for the simple reason that low-grade deposits of uranium exist in the northern half of the peninsula. Natural uranium, with a very rudimentary sort of processing for use, contains about 0.7 percent U-235, the rest being U-238 which is not suitable for nuclear weapons.

LWR, as opposed to GMR, use uranium containing between 2 and 4 percent U-235, which falls into the slightly enriched uranium (SEU) range (0.9 to 20 percent). The technical expertise and expense required to enrich to this level only reinforced North Korea’s decision to go with GMR.

U-235 is the key uranium isotope to the nuclear process of reactors as well as uranium based weapons, and the percentage present in the reactor determines how long the fuel can be used. Because LWR have higher percentages of U-235, the fuel used in LWR stays in use much longer than fuel used in GMR.

For proliferation concerns, what happens to the U-238 during this process is critical. Although U-238 cannot be used for a nuclear weapon, or even be enriched for such use, the change it undergoes in the nuclear reactor produces Pu-239.

Plutonium does not exist in nature. While in an active nuclear reactor – GMR or LWR – U-238 absorbs a neutron and transmutes into Neptinium-239, which after a half-life of 2.3 days becomes Pu-239 by emitting an electron. Any nuclear reactor can produce a certain amount of Pu-239 that is suitable for nuclear weapons.

Like U-238, Pu-239 can and does absorb neutrons, and becomes Pu-240 when this happens. The longer the U-238 is in an active reactor, the more Pu-240 is produced. Pu-240, like U-238, is not suitable for nuclear weapons.

Because the Pu-239 produced in LWR remains in the reactor for a longer period of use than in GMR, much more of it is transmuted into Pu-240, useless for reprocessing into weapons grade material. For this reason LWR are considered very much less prone to aiding the proliferation efforts of rouge nations. It is also why LWR were considered technically preferable for North Korea to have than GMR.

The U.S. first became aware of the seriousness of North Korea’s nuclear weapons program in 1986, when U.S. intelligence satellite photos revealed cylindrical craters in the sand of the riverbank near Yongbyon, which appeared to be consistent with experimental high-explosive detonations used for detonating plutonium nuclear devices.

At the same time, an approximately 200 meter long building was also under construction at the site. The long series of thick-walled cells taking shape were typical of reprocessing plants capable of extracting nuclear grade weapons material. As construction slowly progressed, the U.S. State Department began to discuss the North Korean nuclear situation with Soviet, Chinese, South Korean, and Japanese counterparts in 1989. The rest is history.

This is somewhat moot at this point in time, but is useful in understanding the overall issue.

Technical information regarding the suitability of isotopes for nuclear reactors, nuclear weapons, and the transmutation process was gleaned from a small, but excellent, book published by The Sejong Institute, “US-DPRK Agreed Framework and Implementation,” by Yo Taik Song, 1998.
—–