Secondary emission chain reactions

Some studies demonstrate increased emissions at increased ventilation rates, while others fail to do so. Oxidants in the supply air may initiate chains of reactions between the oxidants and otherwise stable chemicals on the inner surfaces or in the indoor air. Partial pressure of VOCs in the air may reduce their emission. When ventilation is increased, the result may be less reduction of air pollution than expected, since the emission rates may increase at the resulting lower VOC concentrations and the increased oxidant concentration may promote secondary emissions. 

The fact that the fission process involves the emission of secondary neutrons leads immediately to the possibility of setting up a chain-reacting system. We start by considering the problem of designing a nuclear reactor in which the fuel is natural uranium. The criterion for a successful chain reaction is the following starting with a certain number of fission events taking place per unit time, it is necessary that the fraction of the secondary neutrons produced in fission which survive to cause further fissions should be sufficient to maintain the fission rate in the system at a constant level. 

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