Significance of Beta and Gamma Dose on Environmental Qualification of Components

Abstract

Safety-related systems and components that are required to perform safety functions during accident conditions must be designed to withstand the harsh environmental conditions that occur as a consequence of the accident. Where these conditions are harsh, and equipment operability can potentially be affected by the post-accident environment, environmental qualification of the equipment must be conducted to demonstrate that the required safety function can be maintained. It is also understood that non-safetyrelated equipment that affects, or prevents, the satisfactory operation of a safety-related system should also withstand the harsh environmental conditions caused by an appropriate design-basis accident. There are essentially two types of requirements that must be satisfied to qualify equipment or components to withstand radiation damage, namely economic requirements and safety requirements. The general objective of the economic requirement is to reduce maintenance cost and to maximize component life during reactor operation. The general objective of the safety requirement is that the equipment should be qualified to withstand the harsh post-accident environmental conditions and should function properly for the appropriate length of time after a design-basis accident has occurred. To address the economic factors - i.e., to reduce maintenance costs and to maximize component life - the radiation dose rates to equipment are calculated throughout the reactor building and the service building during reactor operation. These are also used for the safety requirement purpose, to assess radiation ageing of safety-related components caused by degradation of material properties with time at radiation exposure. To address the safety requirement, the dose-rate estimates and accumulated doses after a LOCA coincident with loss-of-emergency-core cooling (LOECC) are provided. The harsh post-accident environmental conditions defined for environmental qualification of components include conditions that deviate from those established for normal operation and those that could adversely affect the components, such as temperature, pressure, radiation, humidity etc. This paper deals with the radiation aspect for environmental qualification, and thus presents methods used to calculate dose rates received by various components for a typical CANDU 6 reactor during normal reactor operation and after a LOCA-LOECC scenario.