Heat transfer effects in severe accident integral analyses

How much energy is removed from the core and where it is deposited are important considerations in severe accidents. The core heatup rate will affect the timing of the damage progression and the nature of the core debris. Heat transferred from the core to other structures in the reactor coolant system can affect the structural integrity of the piping and the fission product transport and retention. The SCDAP/RELAP5 computer code has been used to simulate a hypothetical station blackout transient in the Surry nuclear power plant. The natural circulation flows present during this high pressure sequence transferred energy from the core to other structures in the reactor coolant system. Heating of the piping in the coolant loops could lead to the creep rupture failure of the pressurizer surge line prior to melt-through of the reactor vessel lower head. Studies of insulation performance indicate that the piping temperatures are not very sensitive to the boundary conditions on the outer surface.