Effects of Tube Rupture Modeling Methods on MSSV Lift Time Following a MSGTR Event in PWR

Abstract

A multiple steam generator tube rupture (MSGTR) event in APR1400, an advanced pressurized water reactor, is investigated using the best estimate thermal hydraulic system code, MARS1.4. The effects of parameters such as the number of ruptured tubes, rupture location, affected steam generator on analysis of the MSGTR event in APR1400 are taken into account. In particular, the effects of tube rupture modeling are compared. In the present study, single tube (STM) and double tube modeling (DTM) are examined for assessment on the main steam safety valve (MSSV) lift time. Nuclear steam supply system (NSSS) and several safety systems that are relevant to the APR1400 are modeled. Automatic safety systems are assumed to mitigate the MSGTR events including the reactor protection trip, reactor coolant pump trip, the pressurizer heaters, high-pressure safety injection (HPSI) pumps, and the valves for atmospheric dump, main steam safety, main steam isolation, and turbine stop and bypass. When five tubes are ruptured, the STM permits the operator response time of 2085 seconds before lifting of MSSVs. The effects of rupture location on the MSSV lift time is not significant in case of STM, while the MSSV lift time for tube-top rupture is found to be 25.3% larger than that for rupture at hog-leg side tube sheet in case of DTM. The MSSV lift time for the cases that both steam generators are affected (4C5x, 4C23x) are found to be larger than that for the single steam generator cases (4A5x, 4B5x) due to a bifurcation of the primary leak flow. The discharge coefficient of Cd is found to affect the MSSV lift time only for smaller value of Cd below 0.5. For larger values of Cd than 0.5, its effect on the leak flow rates as well as the MSSV lift time become negligible. It is found that the most dominant parameter governing the MSSV lift time is the leak flow rate. Whichever modeling method is used, it gives the similar MSSV lift time if the leak flow rate is similar, except the case of both steam generators are affected. Therefore, the system performance and the MSSV lift time of the APR1400 are strongly dependent on the break flow model used in the best estimate system code.