Page 81 - Neutrons for Sciences and Society

the better neutron properties of deuterium compared to hydrogen. It has a capture cross-section about one thousand times smaller. The notion of reactivity helps explain this advantage. During a fission event about two neutrons are emitted from each nucleus
of uranium-235. In order for the chain reaction to continue at least one of these neutrons must induce a new fission process. Neutrons are lost because they are absorbed by the other matter in the reactor, or by escaping from the confines of the reactor. This leads to the idea of a critical mass or size which must be attained to reduce these losses and allow the chain reaction. This critical mass can be reduced by placing a reflector around the core which scatters back a fraction of the neutrons which would otherwise be lost. When the chain reaction is being established the reactivity excess is greater than 1 for neutrons inducing a new fission. In continuous operation this reactivity excess is zero and the reactor is controlled by introducing or withdrawing neutron absorbing material.
Replacing light water by heavy water for cooling the core obviously gives a greater safety factor in operation. When restarting after an interruption there is a xenon64 build-up. With light water about 36 hours must elapse for this xenon poisoning to decay, or have negligible effect on the reactivity. In a reactor cooled by heavy water the greater reactivity margin allows a restart at any time. This is of some importance in the case of unplanned stoppages (for example an electricity supply failure) As fewer neutrons are captured by the cooling water, for a given power there are more neutrons in the reflector where
the experiment beam tubes end, hence more neutrons for the
64 Amongst fission products is xenon-135 which absorbs neutrons very strongly and has a half life of 9.2 hours.
Chapter 4 - The negotiations
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