For severe accident assessment in a light water reactor (LWR), heat transfer models in a narrow annular gap between the overheated core debris and the reactor pressure vessel (RPV) are important for evaluating RPV integrity and emergency procedures. Some heat transfer models have been proposed as gap cooling CHF (critical heat flux) but local heat fluxes on the hot surface were not taken into account. Therefore, using the existing data, the authors developed heat transfer models on the average CHF restricted by CCFL (counter-current flow limitation) and local boiling heat fluxes, and showed that the average CHF depended on the steam-water flow pattern in the narrow gap and that the local heat fluxes were similar to the pool boiling curve. We evaluated the validity of heat transfer models by simple calculations for an ALPHA/IDC001 experiment performed by JAERI (Japan Atomic Energy Research Institute). Results showed heat fluxes on the crust surface were restricted mainly by its thermal resistance after the crust formation, emissivity on its surface did not have much effect on the heat fluxes, and the calculated vessel temperature during the heat-up process agreed well with the measurements. However, the vessel cooling rate was underestimated mainly due to underestimation of the gap size. The heat fluxes on the vessel inner surface were much higher than the pool film boiling therefore local boiling heat transfer should be studied to improve the heat transfer models.

The loss of RHR during mid-loop operation in PWR is relatively high risk event. More confident analysis of the event is desirable to develop better counter measures and increase plant safety. The analysis methodology with statistical method using a best estimate analysis code to increase confidence of analysis result is under development. The method employs the RELAP5/MOD3.2 code as a best estimate code and is being developed along the CSAU methodology. One of the most important steps in the CSAU methodology is development of PIRT (Phenomena Identification and Ranking Table) for the event. The PIRT is developed for the loss of RHR event during mid-loop operation with mitigation measure of reflux cooling and gravity injection from RWST and important models of the RELAP5/MOD3.2 related to high ranked phenomena are identified. Verification matrix is also developed for the important models. One of the important models identified is void model. This model affects two phase water level of the reactor vessel and how much water is transported with vapor from reactor vessel. Verification of void model is especially focused on low power and low pressure conditions which are characteristics of the loss of RHR event under mid-loop operation. Prediction error of void model was quantified for both heated rod bundle channel and non-heated channels. Experiment with rod bundle core geometry under low power and low pressure conditions used for verification analysis is the THETIS experiment. The experiment was performed under quasi-steady condition. Two phase level under specified collapsed level was measured with varying power and pressure. Analysis results with pressure 0.5 to 1.0 MPa predict two phase level within 10% error. Void prediction analyses with non-heated channels were conducted against both steam-water experiment and air-water experiment with various pressure and hydraulic diameter. Most of data are predicted within 30% error.

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At the moment, there are still more than 20 million people in rural areas of Bangladesh who have no access to safe drinking water. Women and children must take more than one hour every day to fetch water from the pond. Sometimes the pond is located 4 km from their house; water fetching is a very heavy burden. Local people sometimes buy water from a water seller so they don't have to fetch water themselves, but it costs 6 Tk (1USD = 80 Bangladesh Taka) per 20-liter barrel (NEC Corporation, 2013). This is an additional economic burden. Moreover, the water source for local people and water sellers is the same pond which is polluted by water-borne pathogenic microorganisms. Countless people who have drunk pond water suffered from severe diarrhea, in some cases causing death. It has been estimated over 45,000 under-five children die every year in Bangladesh from diarrhea caused by contaminated water (WHO, UN-Water, 2017). Besides, to the diarrhea issue, more than 35 million people are under threat of arsenic contamination from groundwater in 59 of 64 districts. There are currently

The Best Estimate Plus Uncertainty (BEPU) method has been applied by the authors to analysis of the “intentional depressurization of steam generator secondary side” which is an accident management procedure in a small break loss-of-coolant accident with high pressure injection system failure. In the present study, experimental analyses using the RELAP5/MOD3.2 code were carried out for the ROSA/Large Scale Test Facility (LSTF) secondary-side depressurization tests. The two test cases were selected with different break sizes and different depressurization conditions to ensure the reliability for the accident scenario analyses. The uncertainty propagation analyses were performed through the random variations of input parameters whose uncertainty ranges and distributions were determined previously by the PIRT and the separate effects tests. One thousand random calculations were conducted to get the 95% upper limit values of the peak cladding temperature (PCT) by the Monte Carlo method. Furthermore, the 95%/95% tolerance limits for the PCT were obtained according to Wilks formula. It was confirmed that the code predicted well the major event progressions such as rod surface temperature and the 95% uncertainty bands included the measured values. Furthermore, the 95% upper limit values of the PCT are below the 95%/95% tolerance limit values. However, the statistical fluctuation of the tolerance limit values by Wilks first order formula is as large as the uncertainty value itself. The statistical fluctuation decreases with increasing order of Wilk formula. It is desirable to increase the order of Wilks formula to more than the second order to get the reliable safety margin.