U‐232 and the proliferation‐resistance of U‐233 in spent fuel

Abstract

The factors influencing the level of U-232 contamination in U-233 are examined for heavy-water-moderated, light-water-moderated and liquid-metal cooled fast breeder reactors fueled with natural or low-enriched uranium and containing thorium mixed with the uranium or in separate target channels. U-232 decays with a 69-year half-life through 1.9-year half-life Th-228 to T1-208, which emits a 2.6 MeV gamma ray upon decay.We find that pressurized light-water-reactors fueled with LEU-thorium fuel at high burnup (70 MWd/kg) produce U-233 with U-232 contamination levels of about 0.4 percent. At this contamination level, a 5 kg sphere of U-233 would produce a gammaray dose rate of 13 and 38 rem/hr at 1 meter one and ten years after chemical purification respectively. The associated plutonium contains 7.5 percent of the undesirable heat-generating 88-year half-life isotope Pu-238.However, just as it is possible to produce weapon-grade plutonium in low-burnup fuel, it is also practical to use heavy-water reactors to produce U-233 containing only a few ppm of U-232 if the thorium is segregated in "target" channels and discharged a few times more frequently than the natural-uranium "driver" fuel. The dose rate from a 5-kg solid sphere of U-233 containing 5 ppm U-232 could be reduced by a further factor of 30, to about 2 mrem/hr, with a close-fitting lead sphere weighing about 100 kg.Thus the proliferation resistance of thorium fuel cycles depends very much upon how they are implemented.

Production of Pa-U Fuel with Proliferation Resistance by 14 MeV Neutron for Long-life Core

“…Since 232 U, which can be produced by the capture reaction in 231 Pa, causes high decay heat and strong gamma rays through the eight successive decays to the stable nuclide 208 Pb, it has been considered that 233 U is isotopically denatured by 232 U to enhance the proliferation resistance. 14,15) It is difficult to make it clear how much isotopic fraction of 232 U is required for the sufficient proliferation resistance of 233 U. However, concerning a dose rate, DOE, NRC, and the IAEA consider materials emitting more than 100 rad per hour at 1 m to be sufficiently self-protecting to require a lower level of safeguarding.…”

“…Since 232 U and its decay products bring high decay heat and strong gamma rays, these factors are considered to make the nuclear material difficult to handle and thereby enhance the proliferation resistance. 14,15) It is worth saying that the present thorium fuel cycle is completely independent from the uranium fuel cycle, because both the fissile and fertile nuclides with the proliferation resistance are produced from only thorium.…”

Is nuclear fission a sustainable source of energy?

“…However, accumulation of other radioactive isotopes, notably 212 Bi, which is a daughter product of 232 U, and gammaemitting 208 Tl, which accumulates during irradiation of 232 Th, increase proliferation resistance. 43 …”

Recent Research of Thorium Molten-Salt Reactor from a Sustainability Viewpoint

“…Note that uranium-233 generated from thorium is a fissile isotope and its significant quantity is defined as 8 kg by the IAEA (International Atomic Energy Agency) [13]. In this regard, there remains the possibility of nuclear proliferation, theoretically if pure uranium-233 is obtained from thorium fuel cycle [14]. On the other hand, it should be pointed out that the accompanying strong gamma rays from thallium-208, being the daughter nuclide of thorium, makes it practically difficult to steal uranium-233 for the use of making nuclear weapons.…”

“…Characteristics of helium depend strongly on pressure and temperature. Density of helium can be obtained by Equations (12)(13)(14). Thermal conductivity of helium can be obtained by Equations (15,16).…”