BLOWDOWN, a computer program for the calculation of LOCA

The main computation effort of blowdown in LWR LOCA analysis should be concetrated on the phenomenon in the ECCS evaluation codes. The cooling ability and the fuel peak clad temperature is not very sensitive to the accuracy of the break flow rate calculation. The choked flow calculation is just a tool in the calculation of the system pressure and water content history. The BLOWDOWN calculation program is written in Basic and calculates the transient course of the pressure, temperature and water content as a function of time in the pressurized system by means of the calculated choked water flow rate under LOCA conditions. The input data are the weight of the pressure vessel, the temperature distribution of the water in the pressure vessel and the tubing system, the pressure in the vessel, the amount of subcooled and hot water in the system and the flow restrictions down to the opening. The calculation model in the program is based on the concept that choked flow is controlled by the onset of the flow regime change from homogeneous bubble flow to dispersed droplet flow at the choking point. The choked flow correlation is that of the Bernoulli type and calculates the choked flow rate explicitly. This report presents the analysis of a number of the experimental test results obtained at 5 MPa at the Marviken Full Scale Critical Flow Test Facility. The agreement is very good and it verifies the applicability of the ''change-of-flow-regime'' model. It also supports the idea that BLOWDOWN might be used in LWR LOCA safety analysis as concers the ECCS evaluation