Fission product retention in design basis faults

An assessment of the retention of primary liquid and fission products in design basis faults taking place in the auxiliary building of the Sizewell 'B' PWR results in the following conclusions: (I) A large amount of retention can be expected in the compartment in which the fault occurs (as much as 99%). (II) For faults in which flashing takes place, if it is assumed that all liquid enters the ventilation system, then removal by deposition in the ducts is dependent on drop sizes. The extent of removal will be between 50% and 99%. Drops which are not deposited will be removed by demisters. (III) For faults in which flashing does not take place, drops which enter the ventilation system will evaporate to dryness and the final retention depends on the performance of the HEPA filters. Results are presented to show the effect on the amount of coolant retention of the three major parameters (i.e. drop sizes, flow velocity and duct diameter). The analysis is also applied to a simple representation of the building ventilation ducting which shows that, depending on drop sizes, the fraction of liquid discharged from the rupture which reaches the filtration plant may be as small as 1%. (orig.)