Assessment and reduction of proliferation risk of reactor-grade plutonium regarding construction of ‘fizzle bombs’ by terrorists

The approximately 23.7 wt% 240Pu in reactor-grade plutonium denatures the 239Pu to the extent that it cannot fuel high yield nuclear weapons. 240Pu has a high spontaneous fission rate, which increases the spontaneous neutron flux within the fuel. When such a nuclear weapon is triggered, these neutrons cause the nuclear fission chain reaction to pre-detonate which blows the imploding fuel shell apart before the designed level of compression and reactivity could be attained, thereby greatly reducing the average energy yield of such “fizzle” bombs. Therefore reactor-grade plutonium is normally viewed as highly proliferation resistant. In this article the literature on the proliferation resistance of reactor-grade plutonium and on the mechanism and effect of fizzle bombs is reviewed in order to test this view. It is shown that even very low yield fizzle bombs, exploded in urban areas, would still cause serious blast damage as well as radioactive contamination. Combined with the high levels of induced terror, fizzle bombs might thus be attractive psychological weapons for terrorists. Therefore reactor-grade plutonium may not be sufficiently proliferation resistant against nuclear terrorism. However, denaturisation with more than 9% 238Pu produces high levels of decay heat which will melt or explode the high explosives around uncooled implosion type weapons, rendering them useless. Unfortunately, reactor-grade Pu contains only 2.7% 238Pu and is thus not sufficiently proliferation resistant in this respect. It is also shown that the associated neptunium poses a substantial proliferation risk. present study strong improvement of the proliferation resistance was demonstrated by simulation of incineration of reactor-grade plutonium in the 400 MWth Pebble Bed Modular Reactor Demonstration Power Plant. Results for modified fuel cycles, aimed at transmutating 237Np to 238Pu are also reported. However, these modifications increased the disloaded heavy metal mass, thereby substantially increasing the radiotoxicity of the spent fuel. Therefore this intervention is not recommended. 237NP should thus rather be incinerated it in fast reactors, light-water reactors or CANDU reactors.