Void effect

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The formation of steam or the ingress of gas bubbles in the moderator and/or the coolant influence the criticality of the reactor. The void effect can be decisively influenced by the design of the reactor core. Since e.g. in the case of a thermal reactor there is an optimum ratio of moderator to fuel volume, there is a void effect with positive coefficient in an excessively moderated reactor. An increase in the steam bubble portion increases the neutron multiplication factor and thus the reactor power. In a reactor undermoderated due to the core design the conditions are the other way around; here the increase of the steam bubble portion reduces the neutron multiplication factor and thus the reactor power. Therefore, an inherently safe reactor regarding steam bubbles and gas ingress must always be slightly undermoderated; it has a negative void coefficient. See 'steam bubble coefficient'.

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