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

Nuclear Reactors for Thorium
Figure 5.10: Schematic View of the Energy Amplifier System for Electricity Production.
The reason for using a lead-bismuth eutectic as coolant instead of pure lead is to lower the
melting point from that of lead (327°C) to 123°C, which reduces the probability of frozen slugs
clogging the flow channels. It also reduces the heating requirements during reactor shutdowns.
To reduce the induced radioactivity in the coolant, it would be desirable to use pure lead instead
of the eutectic, but this is still under development.
In addition to its subcriticality, the EA is distinguished from most other reactor designs since it
does not use control rods for the power control. The power of the reactor is proportional to the
supplied beam power, which means that power control can be performed by adjusting the
accelerator power. The same method is also used to compensate for reactivity changes caused by
fuel burnup.
When compared with critical reactors, Accelerator Driven Systems have two specific
characteristics:
1. If well designed, they prevent criticality accidents. It has been proposed [107] to take
advantage of this sub-criticality in order to use certain types of fuel with poor neutronic
properties: in particular, because of their small delayed neutron fractions, incineration of
minor actinides appears to be feasible efficiently with ADS.
2. Spallation provides additional neutrons which can be used for increased breeding of U-233 or
Pu-239. Another possible use of the additional neutrons is to transmute long-lived fission
products and transuranics (TRUs).
In Norwegian media it has repeatedly been claimed that an ADS based on thorium cannot melt
down like an ordinary reactor if the cooling is lost, because it is not critical. Unfortunately, this is
not the case. A reactor without cooling will melt no matter how subcritical it is. When the reactor
is shut down, the heat does not come from ongoing fissions in the fuel, it is the decay of the fission
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