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

Thorium as an Energy Source - Opportunities for Norway
for 15 ms (milli seconds), corresponding to a total insertion of 2.55$ 11 , in a lead-cooled ADS with
an assumed subcriticality of -4$. The subcriticality is conservatively assumed to be relatively
small and the ramp rate and total insertion to be rather high, particularly since an ADS will most
probably have no reactivity control rods but only safety rods.
Figure 5.14: Power Excursion in a Lead-cooled Energy Amplifier (with k = 0.99)
for a Slow Reactivity Ramp Insertion.
(The reactivity increases at a rate of 170 $/s for a period of 15 ms (this corresponds to a control rod
withdrawal speed of 0.55 cm/ms in the case of a reactor). The accelerator is not shut-off [118].
In the case of the Energy Amplifier operated at k = 0.99, the power increases by a factor 2.5 after
15 ms, and after 20 ms the power decreases almost proportionally with the neutron source
strength. If on the other hand the neutron source is maintained (the accelerator is not shut-off),
the power remains almost constant in this time range. The average temperature of the fuel rises
gradually, but at a much lower rate. Note that in this case the Doppler reactivity feedback is
almost negligible and very much delayed (appears only after 23 ms). The long time constant of the
response implies that the heat loss from the fuel cannot be neglected anymore. In fact, there is
sufficient time (of the order of a few seconds, as estimated by the convection studies described in
[118]) for the natural convection mechanism to safely adapt itself to the new operating conditions
without any fuel damage occurring.
The same reactivity accident was also calculated for the corresponding unscrammed critical
sodium-cooled fast reactor [118], [124]. This led to a power pulse of more than 1000 times nominal
and nearly complete core destruction. Two aspects explain the difference. First, the subcriticality
of the ADS and second, the positive void coefficient that came into play in the accident in the
critical reactor, but not in the ADS case. In the proposed ADS design, lead-bismuth is used which
has a negative void coefficient.
To conclude, a lead or lead-bismuth cooled ADS with a good natural coolant circulation capability
and a passive emergency decay heat removal system is very attractive to prevent or mitigate
severe accidents. The critical Loss-Of-Flow and Loss-Of-Heat-Sink accidents in such a system will
11 One dollar (1$) is the reactivity insertion that will make a reactor prompt critical. It is equal to the
effective delayed neutron fraction (β ), and therefore depends on the fuel.
eff
64