Total Loss of Feedwater Analysis of PWR Using RELAP5

In Europe the design extension conditions (DEC) were introduced after the Fukushima Dai-ichi accident as preferred method for giving due consideration to the complex sequences and severe accidents without including them in the design basis conditions. The objective of the study is to determine available elapsed time before core uncovery and needed DEC safety features for total loss of all feedwater (TLOFW) in a two-loop pressurized water reactor. RELAP5/MOD3.3 computer code has been used for calculations. The initiating event for TLOFW are multiple failures in which, besides the loss of main feedwater also the auxiliary feedwater is lost. The scenarios without DEC safety features and the scenarios with DEC safety features assumed have been simulated. The results showed that after TLOFW event initiation it is very important to trip the reactor as soon as possible. In case of loss of offsite power the reactor coolant pumps stop and the reactor very quickly trips on low reactor coolant pump flow. When normal operation systems are assumed the reactor trip occurs on low-low steam generator narrow level few tens of seconds after accident initiation, resulting in less time available before core uncovery occurence. The results for TLOFW scenarios with normal operation systems and DEC safety featured assumed demonstrated that secondary side bleed and feed can prevent core uncovery in case when no operator actions are credited before 30 minute. When primary side bleed and feed is used, less time is available for operator actions.