Abstract
Discrete burnable poisons like the Wet Annular Burnable Absorber and Pyrex have been used in PWR core to improve core power distribution and provide more negative moderator temperature coefficient. The burnable absorber in the burnable poison rods burns fast and is completely gone after one cycle exposure in the core. Traditionally, the burnable poison rods are designed to be part of the fuel assembly. They are burned together with the fuel in the first loading cycle and discharged after one cycle. In recent years, different insertion scenarios of the burnable poison rods have been introduced in the PWR plants operation to improve the fuel performance, for instance, fresh burnable poison rods are inserted into a burned assembly; burned burnable poison rods stay in the original assembly or are replaced in a different assembly. A Generic Insert Methodology [1] was developed in Westinghouse and implemented in NEXUS/ANC9 code system. With this new methodology, ANC9 is able to follow the assembly history and model all types of absorber insert components for all kinds of insertion scenarios. An extensive methods validation and qualification effort has been completed by modeling different insertion cases. This paper provides details of the qualification cases along with the analysis of the results.
Highlights
Different absorber components have been designed and used in Pressurized Water Reactors (PWRs) to compensate for core excess reactivity, to maintain the desired state of core reactivity and to reduce neutron leakage
Others are separable from the fuel, e.g. control rods and discrete burnable absorbers like the Wet Annular Burnable Absorber (WABA) and PYREX
ANC9 adopted Pseudo Pin-by-Pin Calculation (P3C) methodology in pin power calculations, it can follow the true history of each individual fuel pin and capture the history effect of burnable absorber insertion
Summary
Different absorber components have been designed and used in Pressurized Water Reactors (PWRs) to compensate for core excess reactivity, to maintain the desired state of core reactivity and to reduce neutron leakage. Based on the type of the absorber components and the usage, Westinghouse PWR core design code system NEXUS/ANC9 [2][3] adopts two different methodologies to model the different inserts, i.e. control rod (CR) method and BP method. This special design brings challenges to any code system with once through cross-section methodology since the required assembly cross-section data cannot be generated upfront without knowing the insert types and fuel history when the insertion occurs In view of these special discrete BA insertion scenarios, a Generic Inserts Method (GIM) has been developed in Westinghouse Electric Company and implemented in NEXUS/ANC9 code system. Like CR insertion, the insertion of BP rods brings local spectrum and heterogeneity changes, which impact the local pin power distribution and the impact accumulates with fuel depletion This was a significant challenge to the conventional pin power methodology [4]. This issue has been resolved in ANC9 with new pin power methodology [5] by following individual fuel rod history explicitly along with the fuel rod history correction to both pin cell cross-sections and flux form factors
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