Methodology for CPR estimations of BWR cycle specific transient reload analyses using CTF sub-channel code

Abstract

For BWRs, computations of the minimum critical power ratio (CPR) during transients constitute one of the main components for core design and reload licensing. The primary objective of this is to establish the operating limit minimum CPR (OLMCPR) that will prevent occurrence of dryout during both normal operation and for the most limiting transients. To this aim, state-of-the-art industrial methods combine plant system transient simulations with hot-channel analyses using fuel-design specific CPR correlations in order to compute the margins to boiling crisis. To overcome the complexity of CPR correlations, an alternative approach based on combining the full core transient simulator SIMULATE-3 K (S3K) with the COBRA-TF (CTF) sub-channel code along with generic CHF correlations was established at PSI. The objective of this paper is to present the concepts of this computational scheme and to provide an assessment of its applicability for independent audits of core reload licensing analyses. A part of this, experimental validation of each component of the scheme is presented, including a first-of-a-kind validation of CTF against available CPR tests for a modern BWR fuel design employed in Switzerland. For steady-state conditions, a numerical verification of the CTF performance is then performed by comparing the results to those obtained by the SIMULATE 3-D core simulator with implemented CPR correlations. For transient evaluations, an assessment of the S3K/CTF scheme is presented by comparing the estimated OLMPCRs against results from a real reload licensing application submitted by a vendor for a recent Swiss BWR cycle. The qualitative as well as quantitative agreement between S3K/CTF and the vendor results using CPR correlations is presented for the complete matrix of transients including in the reload licensing submittal. Some systematic trends observed in the CTF CPR results are pointed out, e.g. the fact that CTF tends to underpredict the CPRs by around 10%, and discussed in the perspective of further enhancements of the methodology.