THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

Non-proliferation
Nations with secure nuclear capabilities (fuel cycle states) would provide fresh fuel and recover
used fuel to nations that agree to use nuclear energy for peaceful purposes. If fast breeder
reactors with a closed fuel cycle are developed and with natural uranium as a fuel, many of the
perceived problems with present Light-Water Reactors can be avoided as far as proliferation is
concerned. There has not been a single example of proliferation with Light Water Reactors. The
elimination of uranium enrichment and recycled actinide (not pure Pu-239) fuel precludes
unauthorized handling.
The military development of bombs has been based on fissile U-235 and Pu-239. With an
emerging civil nuclear energy technology based on thorium it could not be excluded that also
uranium-233 (U-233) produced from thorium could be used for the weapons sector. The fissile
weapons quality is evaluated in terms of:
1. The critical mass of an isotope (or different isotopic composition),
2. the weapon yield degradation due to the pre-initiation caused by spontaneous fission neutrons
and
3. the weapon stability degradation caused by heat emission.
U-233 has been determined to be at least as efficient as U-235 as a weapon material, e.g. the
critical mass of U-233 is approximately 5 - 8 kg (with neutron reflector) [48], [148] and U-233 has
to be diluted by uranium-238 (U-238) below a level of 12 wt% in order to reach the proliferation
limit [148].
Reprocessing of thorium-based fuel yields almost pure U-233 and therefore weapon-grade
material. E.g. for the equilibrium fuel cycle of a uranium-thorium fuelled light-water reactor the
recycled uranium typically contains about 55 wt% U-233 and 10 wt% U-235, which is a fissile
content sufficient for nuclear explosives [152]. However, traces of U-232 are always present in
fissile U-233. In the reactor, thorium is irradiated by neutrons leading not only to the isotope U-
233 but also to a minute fraction of U-232. The appearance of gamma emitting nuclides (e.g. Tl-
208) in the decay chain after U-232 (life time 68.9 years) would make an extracted material
highly radioactive. The contact dose rate of 30 kg of U-233, with the small content of U-232,
would after a few years be about 72 Sv/hour, which corresponds to a 50 % lethal dose after 5
minutes exposure to the bare mass [153]. The fractional quantity of U-232 depends on the
neutron spectrum and on the burn-up and can reach 5000 ppm [148]. The presence of U-232 will
thus create a radiation hazard which is sufficiently large to require remote handling within a
short time after chemical separation. This problem rules out the military use of U-233 by the
Nuclear Weapons States – they appear to have enough of the more conventional weapons
material. On the other hand, terrorist groups or rogue states might want fissile material for
immediate use rather than for stockpiling. U-233 could be decontaminated from decay products
for a few days [154] or even weeks [155] allowing the fabrication of a weapon.
In conclusion, it seems that there is consensus amongst the experts that thorium based fuel cycles
do not produce (weapon-grade) plutonium. However, the proliferation resistance of U-233 depends
on the reactor and reprocessing technologies. The judgements range from that the risk of nuclear
proliferation is “negligible” [156] to “very small (Radkowski LWR), medium (gas-cooled hightemperature
reactor) or small (EA)” [157] and to statements that “U-233 requires the same level
of safeguards oversight and physical protection as does plutonium” [154]. It is obvious that the
thorium fuel cycle in general has advantages concerning the proliferation resistance, advantages
that can be exploited in the design of a reactor technology and a fuel cycle merely for civil
purposes and without connections to the military sector. However, due to the lack of experience
with industrial-scale thorium fuel cycle facilities we adopt the view that similar safeguard
measures as for plutonium are mandatory.
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