Abstract
Containment is the last barrier for release of radioactive materials in the case of an accident in the nuclear power plant (NPP). Its overall integrity is tested during a containment integrated leak rate test (CILRT) at the design pressure, at regular intervals. Due to applied risk based licensing, the test intervals can be increased up to once in 10 years and beyond. Taking that into account it is important to prepare the test properly and to use obtained results to assess the real status of the containment. The test can be used to verify existing containment calculation models. There is a potential benefit of verified computer models usage for the explanation of some test results, too. NPP Krško has performed CILRT during the plant outage in 2016. The paper presents a comparison between measured data and results calculated using a multivolume GOTHIC (Generation Of Thermal Hydraulic Information For Containment) model. The test scenario was reproduced using limited available data up to the end of the pressurization phase. The depressurization phase is calculated by the code and measured leakage rate is implemented in the model. Taking into account the 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 the 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 reactor containment fan cooler (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, relative humidity 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 a new set of weighting factors is proposed too.
Highlights
Containment leakage should be kept at the minimum rate in order to restrict possible radioactive emissions in the environment below acceptable limits
The calculated temperature decrease in the depressurization phase was much faster than the measured data (Figure 5), due to the already mentioned approximate nature of the calculation in that phase
The weighting factor is the ratio between the volume whose temperature is being “measured” by the detector located in the compartment and the total containment free volume
Summary
Containment leakage should be kept at the minimum rate in order to restrict possible radioactive emissions in the environment below acceptable limits. In nuclear power plant (NPP) Krško (NEK) the design rate is 0.2% of air weight per day [1]. Type A test is the containment integrated leak rate test (CILRT) intended to measure the reactor containment overall. In order to independently verify test data, the existing GOTHIC plant model [4] was adapted to comply with the test alignment and conditions, and was used to simulate containment behavior during the CILRT. Both the measured and calculated results are presented and evaluated in paper
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