Irradiation Damage in Nuclear Power Plants

Abstract

The chapter gives an introduction into the main processes occurring in metals and alloys under neutron irradiation. Displacement damage, phase reactions, swelling, irradiation creep, and transmutation are the main physical effects changing mechanical properties and microstructure of materials used in nuclear power plants. As results radiation hardening/embrittlement, enhanced stress corrosion cracking, changes in geometry, degradation of creep properties, and other damage can occur. Consequences of such material degradation are discussed for current nuclear plants where a 50-year operation experience exists. Advanced, future nuclear plants are expected to consist also almost exclusively of metals and alloys, and they are expected to undergo in principle the same types of damage. However, changes in other operational parameters (higher temperatures, fast neutrons, other coolants) might change also the degree of radiation damage (e.g., thermal creep in addition to radiation creep). Advanced modeling and testing techniques can be considered as a tool to balance the missing long-term experience with next-generation nuclear plants. Introductory Remarks Central components of nuclear plants are usually exposed to the coolant, radiation, and elevated temperatures. These conditions lead to degradation of components during service and limit therefore the lifetime of plants. In current light water reactors (LWRs), embrittlement of the reactor pressure vessel, irradiation-assisted stress corrosion cracking of reactor internals, and irradiation creep of claddings are typical degradation mechanisms caused by neutrons. Advanced reactors, like Generation IV plants, are expected to be exposed to more damaging fast neutrons (higher energy), higher temperatures, and coolants different from water. Although the physics behind radiation damage is expected to remain the same, some differences in damage development between LWRs and advanced reactors can be expected. In the first part of this chapter, the basic phenomena of irradiation damage of reactor materials will be discussed. The focus will be on metals and alloys which are not only the key materials in current reactors but which will stay also key materials for future reactors. Considerations for ceramics will remain limited to graphite (as moderator for British advanced gas reactors and future hightemperature gas-cooled reactors) and to SiC/SiC composites which are considered for control rod parts or claddings in advanced plants. In the second part of this chapter, radiation damage occurring in current nuclear plants will be highlighted with examples from reactor pressure vessels, reactor internals, and fuel claddings.