When to Replace a Reactor Coolant Pump Seal

Abstract

The prime objectives of a seal maintenance strategy for the main reactor coolant pumps of nuclear plants are to avoid spills and forced outages, while minimizing maintenance costs and radiation exposure. Reactor coolant pump seals in early nuclear plants were replaced every fuelling outage, if not sooner! With steadily improving reliability it has become more difficult to decide when to replace a seal. This paper describes tools to help these decisions. Seals can be defined as operating either normally, deteriorated, or failed. These "conditions" can be defined in terms of measured temperatures and leakages (and/or interseal pressures). Recent guidelines and charts based on these parameters, and the ways they are adapted for two- and three-stage cartridges, are described. A further consideration is the type of failure. The most common modes are presented, with their causes and their impact (sudden or gradual) on seal performance. Based on identification of these failure modes and an assessment of the percentage remaining life, a replacement strategy is described. A final issue is service life of the elastomers used in pump seals. These degrade with age and exposure to heat, radiation, stress, and other environmental factors or contaminants (in service, or simply sitting on the shelf). Recommendations are presented based on hardness and compression set aging data for elastomers used in CAN-seals for reactor coolant pumps in the US and in the Bruce and Darlington nuclear plants.