Local temperature and heat transfer changes in the LMFBR core will be treated in detail in Section 4.4. One possible perturbation not treated is the addition of cold sodium to the core. This might be presumed pessi­mistically to occur if an inactive loop is activated in some sudden fashion resulting in a reduction of the inlet temperature. The result would be a slight addition of reactivity if the overall coolant temperature coefficient were negative, which might be followed by a further reactivity addition as the fuel temperature was reduced. However, even on an instantaneous basis assuming very pessimistic inlet temperature changes, this reactivity change cannot be larger than that which the protective system can handle. In practice the system would be surveyed for possible ways in which the inlet could experience colder sodium flow, and the reactivity change would be calculated. It could then be shown to be small compared to the largest acceptable step of reactivity as defined in Section 2.3.2.3.

One further local temperature change not yet noted concerns the steam — cooled fast reactor system and the gas-cooled system. In reactivity or flow transients which result in coolant temperature variations, the coolant density is very sensitive to these temperature changes, and additional heat transfer variations will occur in addition to the usual reactivity feedbacks. Such effects will be included in the modeling by ensuring that the density of the coolant is allowed a pressure and temperature dependence.

mc = Aoc = Aoc0 T0P/TP0 (2.7)