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Calculation of NEK CILR Test Using GOTHIC Code

Containment is last barrier for release of radioactive materials in case on accident in NPP. Its overall integrity is tested during Containment Integrated Leak Rate Test (CILRT) at design pressure, in regular intervals. Thanks to applied risk based licensing the test intervals are increased up to once in 10 years and beyond. Taking that into account it is important to prepare test properly and to use obtained results to assess real status of containment. The test can be used to verify existing containment calculation models with potential benefit to use that verified models for explanation of some test results. NPP Krsko has performed CILRT during plant outage in 2016. The paper presents comparison between measured and results calculated using multivolume GOTHIC model. The test scenario was reproduced using limited available data up to the end of pressurization phase. The depressurization phase is calculated by the code and measured leakage rate is implemented in the model. Taking into account necessary adjustments in the model overall prediction of the measured results (in terms of pressure, temperature and humidity) is very good. In the last phase of the test some non-physical behavior is noticed (without influence on overall test results), probably caused by combination of air redistribution within the containment and influence of heat transfer to plant systems that were in the operation during the test. Gothic model was used to check sensitivity of the predicted pressure (leak rate) to different heat inputs and to investigate the influence that operation of only one RCFC train during pressurization can have on the mixing of air within the containment. In addition, the influence of currently used weighting factors (weighting of measured temperature, RH and pressure data) on the used test methodology is investigated. The possible non-conservative direction of the influence (currently used weighting factors are giving lower leakage rate) was demonstrated and new set of weighting factors is proposed too.

NEK, CILRT, GOTHIC code, containment

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