Long-Term Performance of Atmospheric-Detritiation Dryers

Tritiated heavy water (DTO) accumulates in the process systems of a CANDU^®a reactor due to neutron capture by the heavy-water moderator and coolant. After twelve years of service, the moderator in the Point Lepreau Generating Station has reached approximately 1.6 TBq/kg, and the total inventory exceeds 300 PBq. Point Lepreau uses nine desiccant dryers to control airborne heavy water and tritium. Ranging in size from 1 000 m³/h to 6 800 m³/h, the majority are single-bed, cocurrent-regenerated units filled with 13X or 4A molecular sieve. These dryers have operated almost continuously for twelve years without a significant breakdown. During the last thirteen years, their availability has exceeded 99% and they have routinely dried air to a dew-point temperature of −60°C or below. Tritium emissions from the dried areas in the reactor building remain a small fraction of the tritium released into the reactor building. The keys to the success of this detritiation system are the mechanical simplicity of the dryers, the versatility of the ventilation system, a comprehensive preventative-maintenance program, and an advanced control system unique to Point Lepreau. In this paper the layout of the Point Lepreau vapour-recovery system is described and operating performance discussed. This includes a comparison of the Point Lepreau dryers to earlier designs, a description of the advanced control system, and a discussion of the operating experience and philosophy. Performance data for the system under various operating conditions are presented, including a description of operating problems caused by volatile organics released during maintenance operations. Recommendations for how this experience might be applied to a fusion power reactor are made.