On a significant deceleration of the kinetics of fast transient processes in a fast reactor
The kinetics of nuclear reactors is determined by the average neutron lifetime. When the inserted reactivity is more than the effective fraction of delayed neutrons, the reactor kinetics becomes very rapid. The fast reactor kinetics can be slowed down by increasing the neutron lifetime. The authors consider the possibility of using a lead isotope, 208Pb, as a neutron reflector with specific properties in the lead-cooled fast reactor. To analyze the emerging effects in a fast reactor, a point kinetics model was selected, which takes into account neutrons returning from the 208Pb reflector to the reactor core.
Such specific properties of 208Pb as the high atomic weight and weak neutron absorption allow neutrons from the reactor core to penetrate deeply into 208Pb reflector, slow down in it, and have a noticeable probability to return to the reactor core and affect the chain fission reaction. The neutrons coming back from the 208Pb reflector have a long «dead-time» i.e., the sum of times when neutrons leave the reactor core entering the 208Pb reflector and then diffuse back into the reactor core. During the ‘dead-time’, these neutrons cannot affect the chain fission reaction. In terms of the delay time, the neutrons returning from the deep layers of the 208Pb reflector are close to the delayed neutrons. Moreover, the number of the neutrons coming back from the 208Pb reflector considerably exceeds the number of the delayed neutrons.
As a result, the neutron lifetime formed by the prompt neutron lifetime and the «dead-time» of the neutrons from the 208Pb reflector can be substantially increased. This will lead to a longer reactor runaway period, which will mitigate the effects of prompt supercriticality. Thus, the use of 208Pb as a neutron reflector can significantly improve the safe fast reactor operation.
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