Neutron transport in boiling media

The formulation of an adequate description of neutron populations inthe presence of boiling moderator or coolant is attempted. Starting froma framework of comprehensive stochastic descriptions we concentrate on thespecial requirements of the boiling system such as the use of transporttheory and the retention of heterogeneity in our model. Working equationsare derived which show how adequate corrections for boiling can be made tothe approximate models currently used.The detailed distribution of bubbles in a boiling system should beaccounted for in accurate evaluations of criticality, especially where thebubbles are large. Evaluations of leakage, resonance escape probabilityand thermal absorption are modified by the detailed bubble distribution.Explicit corrections are given for simple examples, while inclusion of theeffects into a practical criticality code, such as WIMS, is discussed.The second moments of the neutron population in the presence ofboiling media are also described. The power spectral density arising fromthe fission process is evaluated, showing how this spectrum is affected bythe boiling. Usually more important sources of neutron noise in a boilingreactor are the fluctuations in cross-sections and collision probabilitiesarising directly from random local void ages. This 'bubble noise' is evaluated with particular emphasis on the detection of onset of boiling in achannel or change in boiling regime.The use of Monte Carlo techniques in calculating neutron transportin boiling media is discussed in some detail. The applicability of theanalyses of neutron means and variances to fast reactor situations isconsidered.The bulk of the work treats critical reactors. However, a treatmentof transients and feedback effects is also included. A new analysis issuggested whereby means and variances throughout the transient can be evaluated without extreme difficulty. The concepts of stochastic stability are discussed