Analysis of a hot leg SBLOCA in a three-loop Westinghouse PWR plant

A best-estimate analysis was performed for a postulated three-inch-diameter hot leg break in a three-loop Westinghouse PWR using RETRAN-02. The analysis was done to support EPRI's reactor vessel pressurized thermal shock (PTS) program. A three-inch-diameter break was selected as the limiting overcooling transient for a Westinghouse design plant. The results of this analysis were used to provide input to other computer codes to assess the nuclear reactor vessel integrity. This small break loss-of-coolant accident (SBLOCA) analysis proved to be a challenging analysis for RETRAN-02. The break size selected produced a net primary coolant mass depletion in the early part of the transient. The charging and HPSI flow did not fully compensate for the mass lost through the break. The primary system started to refill only after the pressure decreased to the point the accumulator valves opened. As the primary system refilled, there were extreme temperature differentials around the system with the cold, denser fluid collecting at lower elevations and two-phase fluid at higher elevations. This transient proved to be very difficult to analyze due to the extreme temperature stratification and loop flow stagnation. The PTS evaluation using the results of this analysis showed that this vessel has a significant safety margin against crack initiation from an SBLOCA throughout the plant lifetime