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

Nuclear Reactors for Thorium
removal of the nominal power by pure natural circulation for a certain time. However, this would
still require a complete heat removal by the secondary loops.
In a Loss-Of-Heat-Sink (LOHS) Accident, the core would slowly heat up (by about 200ºC in 1000
seconds). Switching off the accelerator would get the power rapidly down to decay heat level. In
case this shutting off is not done, one can rely on a simple passive means of interrupting the
proton beam by using a melt-rupture disk at the side of the vacuum pipe through which the
protons are streaming into the subcritical core [118], [123]. When this melt rupture disk is pushed
inside the pipe by the coolant pressure, the pipe will be unconditionally filled up by the liquid
metal coolant that will block off the proton beam.
An important initiator for Loss-Of-Flow and Loss-Of-Heat-Sink accidents is a station blackout.
However, in an ADS this would also lead to the shutting off of the accelerator and the decrease of
the power to decay heat level. The remaining problem would be the passive decay heat removal
that can be achieved by natural air circulation cooling.
Loss-Of-Coolant (LOCA) Accidents, which are a concern in LWR safety, should pose no significant
problem. First, a lead-bismuth (Pb-Bi) cooled system is at a low pressure; second, a pool design
has been proposed in which all the primary coolant is in one vessel and there is no primary piping
which might develop leaks; and third, a guard vessel surrounds the main vessel in case the latter
would leak. Another important safety aspect of a lead-bismuth or lead coolant is that it is
chemically rather inert and does not react strongly with air or water. This is an advantage
relative to sodium-cooled fast systems.
Beam Power Accidents. Since the proton beam intensity can be increased to compensate for burnup,
one can also imagine that the beam power is increased accidentally. Figure 5.13 shows the
power increase with an assumed beam power increase by a factor 2 in one second [118], [124].
According to accelerator specialists, this large increase is not credible. It can be seen that the
power does not increase too much and levels off at about 1.5 times nominal. The elevated power
leads to some pin failures after about 16.5 seconds. This would not have happened if the beam
power was shut off actively or passively.
Figure 5.13: Assumed Beam Power Increase by a Factor of 2 in One Second.
Reactivity Accidents: It has also been realised that fast reactivity insertions do not lead to rapid
power increases due to the subcriticality of an ADS which acts as a large delayed neutron group
[124]. Figure 5.14 shows the power and reactivity histories due to a reactivity insertion of 170$/s,
63