ICONE11-36020 COUPLED FLUID-STRUCTURE INTERACTION ANALYSIS OF LIGHT WATER REACTOR INTERNALS WITH PHASE CHANGE

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In the case of Boiling Water Reactors, Inter Granular Stress Corrosion Cracking (IGSCC) near the critical circumferential welds of SS 304 core shroud has been reported worldwide. Potential safety concerns were raised by regulatory bodies for the 360 degrees circumferential separation of core shroud following the postulated pipe break in the re-circulation line. Such separation of the shroud might either prevent full insertion of the control rods or open a gap in the shroud large enough to preclude adequate core cooling. Based on Electrical Power Research Institute (EPRI) co-ordinated Vessel and Internals (VIP) project, NRC through its generic letter 94-03 issued guidelines for the assessment of core shroud response for design basis blow down accidents due to re-circulation line break. The in-service inspection carried out at TAPS-BWR has demonstrated that there is no indication of flaw like defects. However, a major safety evaluation programme was initiated to assess the integrity of TAPS-BWR core shroud in Reactor Safety Division, BARC, Trombay. One of the important issues after the initiation of the blow down is determination of acoustic load and the associated fluid-structure interaction response evaluation of the core shroud. The present paper focuses on this problem and the coupled fluid-structure interaction analysis results are reported for the core shroud of TAPS-BWR with an in-house three-dimensional finite element code FLUSHEL. For the safety evaluation of the TAPS-BWR core shroud the performance of code FLUSHEL was evaluated with the analysis of standard benchmark example of HDR-PWR core barrel blow down experimental results. The blow down induced depressurisation wave is traced within the two phase fluid medium of downcomer and lower plenum and is found to be consistent with the experimental results (within an accuracy of 11%) of reported blow down test results on a full scale reactor vessel of PWR design. Modifications were made in code FLUSHEL to account for the blow down induced phase change and the influence of acoustic speed variation in the dispersive fluid medium was appropriately accounted. The estimation of the critical flow for blow down due to LOCA was carried out with systematic review of Burnell's model, Moody's homogeneous equilibrium model and Leung's generalised equilibrium model. The adequacy of Leung's generalised model was established for the prediction of sub-cooled and two-phase blow down induced critical discharge for HDR-PWR and TAPS-BWR problems respectively. After the validation of the code the coupled analysis of core shroud and downcomer annulus fluid for TAPS-BWR was undertaken for the postulated recirculation line break. It has been demonstrated that the acoustic Helmholtz modes of the downcomer annulus and the shroud shell multi-lobe modes of TAPS-BWR are well separated. The transient dynamic response of the core shroud shows that the acoustic load induced stresses are within service level D limits of Section III NB of ASME Boiler and Pressure vessel Code.

ICONE11-36020 COUPLED FLUID-STRUCTURE INTERACTION ANALYSIS OF LIGHT WATER REACTOR INTERNALS WITH PHASE CHANGE

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