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

Thorium as an Energy Source - Opportunities for Norway
cycle should not be considered to reduce radiotoxicity, because a fast reactor requires a much
higher fissile inventory. The problem is the U-233 in the spent fuel.
6.1.2 Closed Thorium Cycle
In a closed cycle, the spent fuel is reprocessed to recycle U-233 and thorium and possibly also the
other produced actinides, in particular protactinium. The ideal is a self-sustaining cycle, i.e. after
a number of cycles there is no further need for topping fuel. Self-sustaining cycles are in principle
possible for thermal as well as for fast reactors. In thermal reactors, this is only possible if there
is an excellent neutron economy, on-line refuelling capability and, preferably, on-line removal of
fission products. The best systems are molten-salt reactors (MSRs), followed by CANDU reactors
and HTRs. However, in the latter two reactors, the maximum burnup that can be achieved is
limited. Accelerators could also provide additional neutrons to facilitate a self-sustaining cycle
(see Chapter 6.2).
Figure 6.1 shows some results for a CANDU reactor with HEU topping. The radiotoxicity of the
waste is shown for the case if only uranium is recycled (curve B). Up to 1000 years, the remaining
toxicity is caused mainly by Pu-238, and later by protactinium-231 (Pa-231). If plutonium and
protactinium are recycled, it is demonstrated [137] that after two recyclings and after 1000 to
2000 years of storage, the radiotoxicity of the waste is already below the level of the uranium ore
needed for fuel fabrication for the once-through PWR.
Figure 6.1: Actinide Radiotoxicity of the Waste if Thorium is used in a CANDU Reactor with
Once-through Operation and HEU as Topping Material (curve A).
Curve B shows the actinide radiotoxicity of a CANDU with HEU topping and recycling
of U in equilibrium (curve B). Further reduction is possible if also Pu and Pa are
recycled. References are the spent fuel of standard once-through CANDU (curve C)
and PWR reactors (curve P) and the necessary U ore for a PWR (curve O).
The closed thorium cycle in a fast reactor breeds U-233. The radiotoxicity of the spent fuel could
be further reduced if the uranium and the actinides are recycled. Due to the fast neutron
spectrum, fissioning of produced minor actinides is more efficient than in a thermal reactor. A
self-sustaining fast reactor based on the Th/U-233 cycle is possible, albeit that the doubling time
of U-233 is quite large (about 250 years). Improved safety feed-back coefficients are obtained. The
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