Thermal-Hydraulic (T/H) core code prediction of the existence and localization of boiling zones is crucial in the framework of axial offset anomaly (AOA) risk assessment of PWR cores. In this prospect, an experimental program — NESTOR — has been completed by Commissariat a` l’Energie Atomique (CEA, France), Electricite´ de France (EDF, France) and Electric Power Research Institute (EPRI, USA). The aim of the NESTOR program has been to develop an accurate prediction model for the onset of nucleate boiling (ONB) boundary in a nuclear fuel bundle based on an ONB wall superheat criterion associated with a dedicated single-phase heat transfer model. The experimental scope of NESTOR program involved using two loops to measure axial velocities in sub-channels and heater rod surface temperatures, respectively, in identical 5×5 rod bundles. The first set of experimental measurements were devoted to a bundle configuration containing only simple support grids (SSG) in order to resemble, as closely as possible, a bare rod bundle. Test data analyses in this configuration have since been jointly carried out by the three NESTOR partners, each using its own T/H core code (FLICA IV for CEA, THYC-COEUR for EDF and VIPRE-I for US Penn State University on behalf of EPRI). This paper describes the analyses results and conclusions based on SSG configuration data. The data analyses methodology consisted of three successive stages: (i) T/H core code calibration — determination of specific input data related to the bundle configuration and required by further core code simulations of the tests; (ii) Single-phase heat transfer analysis — development of dedicated single-phase heat transfer models using single-phase test data along with sub-channel-averaged temperatures and velocities obtained from T/H code simulations. The heat transfer models were unique for each of the three codes and included both a heat transfer correlation and grid enhancement correction factor; (iii) ONB test analysis — assessment of an ONB wall superheat criterion based on stage (ii) models and, if necessary, development of a new ONB wall superheat criterion. The analyses showed that while the heat transfer models could correctly represent the single-phase test data, the ONB wall superheat is over-predicted by 1–3.5 K when compared to experimental values and open literature wall superheat correlations. However, the actual impact of this inconsistency on prediction of ONB boundary localization in this experimental SSG bundle configuration is low. Similar concurrent data analyses for tests on a mixing vane grid bundle configuration are under progress and results should be available by the end of 2010.

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