TRACE assessment of the ACHILLES ISP-25 reflood transient

The purpose of this paper is to assess the capability of the best estimate thermal-hydraulic code TRACE Version 5.211 to predict the reflood process in a rod bundle test section using ACHILLES experimental data from the ISP-25 reflood transient. For the purpose of this assessment study, two detailed TRACE models representing the entire ACHILLES test section without the downcomer were developed and applied to simulate the ISP-25 transient. The TRACE models differed only in the hydrodynamic components, VESSEL and PIPE, which were used to represent the rod bundle region of the test section. Code predictions were compared against ISP-25 test measurements for both local- and integral-type quantities. These measurements included rod surface temperatures for individual rods at various axial elevations, sub-channel steam temperatures at different axial elevations, test section exit liquid and steam mass flow rates, quench front location, test section collapsed liquid level, test section overall pressure drop, and differential pressure drops across various axial sections of the test bundle. Considering the involvement of a non-uniform axial power profile combined with radial temperature variations among individual rods in the experimental rod surface temperature data, TRACE exhibited reasonable capability in predicting the ACHILLES ISP-25 reflood transient implementing an average-rod test bundle modeling approach. Consistent with other reflood simulations obtained with recent TRACE code versions, major differences between ACHILLES ISP-25 simulation results and experimental data for rod surface temperatures were observed mainly for the upper part of test section, also caused by lack of spacer grid models in TRACE. (author)