Comparison of Multi and Average Channel FIREBIRD Circuit Simulation of a 20% RIH Break

Abstract

Most network thermalhydraulic analysis for Point Lepreau and Gentilly-2 has been performed with "single average channel" models. These combine the 95 channels in a core pass into one average channel representation. The approach to modeling specific channels has either been to use "SLAVE" channel models or to couple one channel in parallel with an average channel representation of the remaining 94 channels in the pass. These methods are useful for demonstrating a single channel response to the average channel header conditions but are unable to predict multi-channel effects. Under some accident conditions the header to header pressure drop may become too small to force all channels to behave in a coherent manner, such as flow direction, as is assumed for an average channel representation. Individual channel behaviour, that is dependent upon individual characteristics such as channel elevation, power etc., may feed back on the circuit conditions within and above the headers. This in turn may influence refill behaviour following a loss of coolant event. This paper presents the results of two simulations of a 20% RIH break using different FIREBIRD-III MOD1 circuit models of a CANDU 600 reactor. Both models include two heat transport system loops with both passes in the intact loop and the non-critical pass in the broken loop modelled as single average channels. In both models, the headers are represented by single nodes with homogeneous connections to the feeders (i.e. phase separation is not modelled in the headers). For one simulation, the critical pass downstream of the break is modelled as a single average channel (CIRCUIT-G2-5); for the other, the critical pass is represented by five averaged channel groups (CIRCUIT-G2-6). In the multiple channel model, the five channel groups making up the critical pass have flow, power and feeder geometry averaged from channel groups representing different channel powers and elevations. These channel groupings are the same as those used previously to determine spatial variations in overpower transients. This study extends the use of this multiple channel model to assess the effectiveness of the Emergency Core Cooling System (ECCS) to refill the critical pass following a critical break and to compare the results with those predicted using the average channel model. For the accident scenario considered in this analysis, the use of a multiple channel representation of the critical pass did not have a significant effect on the overall circuit behaviour. The Primary Heat Transport System (PHTS) thermalhydraulic response was very similar in the two simulations. As expected, the critical core pass in the average channel model refilled at sometime between the first and last channel groups to refill in the multi-channel simulation. The use of multiple channels in the critical pass proved to be useful for identifying channel groups yielding later refill times. Minor differences in the critical pass header to header pressure drop and outlet header temperature were noted. These differences affected the ref11 times for subsequent single channel simulations driven from the two sets of header boundary conditions.