ABSTRACT
The steady state solution of the coolant continuity, momentum and energy transport equations for a vertical, parallel array of heated channels connected only at plena is illustrated. The solution is applicable to a fuel assembly modeled as an array of laterally isolated subchannels, similar to a BWR core. The physical basis for channel behavior is stressed since the problem formulation allows multiple solutions for the flow in individual channels. The low flow rate situation requires the inclusion of gravity effects that are neglected at high flow rates. This first regime is applicable to decay heat conditions with impaired circulation through the primary system in which substantial internal circulation occurs within the reactor vessel. These internal circulation flows involve both upflow and downflow zones and are instrumental in minimizing peak core temperatures. The solution procedure for decoupled conservation equations applicable to high flow rate conditions is also presented. It is assumed that one-dimensional mass, momentum and energy transport equations are applicable. For transients in which time-dependent boundary conditions vary slowly compared with the fluid channel transit time, a quasi-steady state approach is taken as adequate. The relevant conservation equations are rewritten to specifically allow for downflow as well as upflow within the channel array.
