Transient processes in PWR fuel element

The programmes are described for the calculation of thermal and hydraulic behaviour of individual fuel clusters of the reactor core in steady-state and transient processes which may occur in case of a possible accident. The computation programme COBRA-III copes with a detailed subchannel analysis of the fuel element, this both for the stationary and transient states. The basic idea is the fictitious division of the channel formed by a rod cluster into parallel subchannels. Mass, energy and momentum transport occurs between neighbouring subchannels, this owing to turbulent mixing and to transversal flow caused by pressure differences. Heat transfer by conduction may also be considered. Programme TRANS calculates non-stationary processes in the fuel element, not considering detailed subchannel analysis of the cluster but considering the concentrated parameters of coolant along the element cross-section, i.e., the one-dimensional flow of coolant through the cluster (perfect mixing) and a homogeneous distribution of heat sources through the transverse cross-section. (B.S.)