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

The Back End of the Thorium Fuel Cycle
Th/U-233 cycle waste, with the same reprocessing losses, should lead to a factor 10 lower
radiotoxicity than that of the U/Pu cycle waste.
To conclude, there are advantages in using thorium in thermal reactors in a once-through mode,
particularly if U-233 or U-235 (HEU) can be used as topping fuel. If the burnup of thorium fuel
can be extended, the waste is less radiotoxic up to 20 000 years, and perhaps more importantly,
has a smaller volume compared with that from (enriched) uranium. The closed Th-cycle offers the
best performance with respect to long-lived waste radiotoxicity compared with U/Pu scenarios, in
particular if fast reactors are used.
A recent study has shown that the radioactive waste from a thorium cycle will be significantly
less than for the standard U/Pu cycle, in the same conditions. This is an advantage for thorium
based fuels and has been confirmed in several studies; recently in the EU-supported study
“Thorium as a waste management option” [138].One result of the study is illustrated in Figure 6.2
where the overall radiotoxic inventories (Sv/TWhe) are compared for U/Pu and Th/U cycles as a
function of elapsed time. The study is for a case where all actinides in a fast neutron reactor are
recycled, assuming 0.1 % losses. It has been pointed out by Dominique Greneche et al [139] that
there is a gain of a factor of 5 to 20 up to 10 000 years with thorium based fuels compared with
U/Pu fuels. After about 20 000 years, the radiotoxic inventory of thorium based fuels becomes
greater than that of U/Pu fuels. This is of less concern since in both cases the inventory is much
lower than that of natural uranium itself. The comparison is based on the equivalent amounts of
natural uranium that are needed to feed an open uranium cycle.
Figure 6.2 Reduction of Radiotoxicity as a Function of Time Elapsed after Disposal of the Waste in an
Inventory for the Cases of Uranium and Thorium Cycles and with Recycling of all Actinides.
6.2 Thorium Cycle in Accelerator-Driven Systems
Critical reactors using thorium fuel have been operated in the past, motivated by the prospect of a
high neutron yield per neutron absorbed, which U-233 offers over the whole neutron energy
range, only slightly surpassed by Pu-239 for fast neutrons (see Table 5.1 in Chapter 5.2 for
details). However, there is an associated disadvantage in breeding U-233: i.e. the production of
Pa-233, which has a large neutron capture cross-section, and which must be compensated for by a
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