Safety analysis code 'COOLTMP' for assessment of PHT cooling under reactor shutdown conditions

The thermal energy generated by the reactor core is removed by the Primary Heat Transport (PHT) System when the reactor is under normal operation, by operation of the primary circulation pumps and steam generators. However, when the reactor is shutdown, the decay heat removal is done by the Shut Down (S/D) Cooling heat exchangers and pumps of lower capacity. In the event of loss/stoppage of circulation of PHT under such a situation, the bulk of the decay heat generated will be distributed to the moderator system, end shield system and through the feeders to the feeder cabinet/FM vault environment. However, the PHT inventory in the channel will be heated up because of loss of flow in the channel. The code COOLTMP has been developed to estimate the temperature of PHT following a loss/stoppage of circulation, when the reactor is under shutdown condition. It predicts the increase in the PHT temperature with time for hot channel, average channel or a specific channel under such a condition. It also calculates the apportionment of the decay heat to different heat sinks, viz. moderator, end shield and FM Vault. This computation is required when the plant is required to be under shutdown for doing some maintenance job on the PHT system, feeders or channels where the S/D cooling system has to be stopped and in some cases the headers have to be drained. At that time such a calculation gives whether the peak PHT temperature, or the time available to reach such a temperature, as obtained, is acceptable to carry out such a job. Hence, the schedule of the maintenance job can be decided. This code has been validated for RAPS and MAPS and used extensively for predicting PHT temperature after reactor shutdown to obtain regulatory clearances to stop forced circulation with and without header filled. (author)