Avoidance of reactor trip on high PHT system pressure for 220 MWe Indian PHWRs with addition of pressuriser

Full text: As reported from 220 MWe PHWRs operating units, most of the plant outages are attributed to disturbances like single PCP trip, turbine trip etc., which result in tripping of reactor on high PHT system pressure. To avoid these unwanted plant outages the PHT pressure control system capacity needs to be enhanced or increase the PHT inventory in the form of pressuriser. To minimize the capital cost in D2O holdup no pressuriser is provided in the existing units of Indian PHWRs. Since in the primary side the coolant remains in incompressible liquid phase, a small rise in the PHT temperature (swell in the PHT volume) results in pressure surges of larger magnitude. To arrest this pressure rise without jeopardizing the safety limits, different mitigation schemes are being studied to increase the plant availability during such incidents. The various mitigation schemes are like full opening of bleed valves, enhanced cooling from the secondary side through CSDVs/ASDVs under certain conditions. In this respect, incorporation of a pressuriser is being analysed using an in house computer code developed for plant transient studies. Promising results have been obtained with the help of added pressurizer of 10-meter cube, which results in an increase in the net compressibility of the PHT system. The pressuriser rides over the PHT system in a passive manner and provides a steam cushion. This helps in mitigating sudden swell and the steam bleed valves help in taking care of sustained swells. The integrated code incorporates the mathematical models of the thermal hydraulics system of the primary and secondary heat transport systems along with associated control system modules. The fluid models are based on the unsteady state, one dimensional conservation equations of mass, momentum, energy and equation of state of fluid. In the present paper, parametric responses for different transients obtained with addition of the pressuriser will be compared with the responses obtained with the present existing configuration of the plant