Analysis of Manual Depressurization of a Boiling Water Reactor Under Transient Condition

Analyses of depressurization rates other than full Automatic Depressurization System (ADS) blowdown are performed to study the effect on the core cooling capability and to minimize thermal stress on the reactor vessel. Four different arbitrary conditions are demonstrated in order to verify the possibility of defining a range over which the slow depressurization can be performed without uncovering the core.The first two cases demonstrate the sensitivity to the depressurization starting time by manual blowdown at 25% capacity at 5 and 8 min into the transient. Two other cases are chosen to show the possible effects of the rate of depressurization. This is done by blowing down at 50 and 75% steam relief capacity at 5 min after the initiation of the transient. For all these cases it is assumed that an additional 25% flow discharge exists due to a stuck open relief valve.The last case demonstrated that early full ADS blowdown might flash water mixed with steam through the relief valves. This is due to the sudden expansion in the reactor vessel inventory upon pressure relief and high water level at the time of depressurization. This, however, is of paramount importance in the integrity and design operating conditions of the ADS valves and steam piping system.