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

Thorium as an Energy Source - Opportunities for Norway
The heavy metal was used in form of coated BISO particles (particles with two layers of pyrolytic
carbon).
Figure 5.6: Balance of Fissile Material during the Passage through the Reactor in the Thorium/Denatured
Uranium Cycle (MEU Cycle).
(Source: “Use of Thorium in the nuclear technology – experiences in Germany” [48])
It is of great importance in this context that the isotope Pa-233 not only may decay to the fissile
material U-233, but may also be converted to Pa-234 by a (n, γ)-reaction with a high reaction cross
section (110 barn). This leads to a significant loss with regard to the breeding of new fissile
material. The neutron flux level and the fuel element design are additional parameters of great
importance with regard to the utilisation of thorium in a HTR. The neutron flux should be as
small as possible to realise high breeding gains.
During the open uranium cycle of the HTR, about 90 % of the plutonium bred during operation of
the reactor is burned directly in–situ in the reactor. Therefore the plutonium content of the spent
fuel elements is very low and additionally is denatured by higher isotopes.
Closed cycles, which can be operated with enriched uranium (20wt%) and thorium and which
show a very good proliferation resistance, may show high conversion rates and therefore a
reduced uranium demand. For the realisation of closed fuel cycles for HTRs as well as for other
reactor concepts, economic reprocessing capacities have to be available [54], [55], [56], [57], [58]
and [59].
5.3.2.3 HTR as Converter and Breeder Systems
Thorium allows in principle the realisation of breeding in helium cooled High Temperature
Reactors (HTRs), in heavy water (D2O) cooled systems and in Molten Salt Reactors (MSRs). These
three options have been studied extensively in Germany in the past but only the HTR fuelled
with thorium and U-233 is commented here. For high conversion or breeding in a HTR, the
thorium cycle is favoured. However, a HTR Th-U breeder requires a low moderation ratio and a
low fuel burnup. In the report summarizing the German experiences [48] a symbiotic HTR system
consisting of two pebble bed HTRs are discussed, where one acts as a “pre-breeder”, producing U-
233 for the second HTR. Both reactors need reprocessing, and a breeding ratio of 1.026 (produces
2.6 % more fissile material than it consumes) was estimated. However, this design was never
realized as a prototype.
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