Analysis of break size effects on SBLOCA scenario in a 4 loop PWR using Relap5/mod 3.3

Small-Break loss of coolant issue was highlighted in 1979, after the Three Mile Island accident. The reactor system response to a small break is characterized by a small rate of coolant discharges and slow pressure variations with time. The depressurization may be slow enough to delay the accumulator's intervention for some time. The accident scenarios may change drastically due to many factors; the break size is one of these factors. In this study, RELAP5/MOD 3.3 thermal hydraulic computer code is used to simulate the effect of break size on the consequences of SBLOCA in a 4-loop PWR Westinghouse design. Plant nodalization consisting of two loops, the first one represents the broken loop and the second one represents the other three intact loops, is considered. All the plant main components in addition to the emergency core cooling system (ECCS) trains are modeled. To investigate the worst break size in the cold leg, a spectrum of five different break sizes with diameters 1 inch, 2 inch, 4 inch, 6 inch and 8 inch are considered. Results show that for break sizes 1 and 2 inches, the charging system and the high pressure safety injection overcome and limit the consequences. The worst consequences occur at break size 6-inch where most of the core uncovered for a period of time accompanied with a sharp increase in fuel rod cladding temperature. A maximum cladding temperature of approximately 1037 °F occurs before the accumulator’s intervention.