W. WIESENACK, InstituttforEnergiteknikk, Norway

Abstract: Fuel assemblies and their components are subjected to the most harsh conditions existing in a nuclear reactor. They are designed and manufactured to satisfy stringent functional and safety requirements for normal operation and transient conditions. With an emphasis on boiling and pressurised light water reactors and going from general principles to details, the chapter describes the main components of a fuel assembly and how design and functionality are related to operating conditions, safety criteria and reactor physics. Phenomena affecting fuel rod endurance are addressed and illustrated.

Key words: fuel assembly, fuel rod, pellet, cladding, water rod, spacer grid, debris filter, nozzle plate, tie plate, power distribution, fuel failure, safety criteria, fuel fabrication, cladding fabrication.

8.3 Introduction

A reactor core is composed of fuel assemblies, also called elements or bundles. The fuel assembly is the basic fuelling unit manufactured by fuel vendors, transported to the reactor site, inserted into and removed from the core, and stored on site or, later, reprocessed or disposed of to a waste repository. It consists of individual fuel rods arranged in a square (BWR/PWR), hexagonal (VVER) or circular matrix (CANDU reactor and AGR). The rods are kept in position by grid plates at the ends of the fuel assembly and by spacer grids in between. They are filled with cylindrical pellets consisting of fissile and fertile atoms (U, Pu, Th), most often as oxides.

Fuel assemblies and their components are subjected to the most harsh conditions existing in a reactor: high neutron and gamma flux, high temperatures in a corrosive environment, high pressure and thermo-mechanical loads. Their design and manufacture must satisfy a number of functional and safety requirements for normal operation and transient conditions and guarantee many years of reliable and economical in-core service. Fuel assemblies must: [18]

• be long lived without undue deterioration and thus permit high fuel burn-up, minimising handling and storage needs

• be suitable for intermediate and final storage or reprocessing

• allow removal of decay heat in accident conditions

Today’s fuel assemblies and the materials employed in them are the result of many years of development and efforts to perfect the fuel designs for the different types of water-cooled and moderated reactors. Their optimisation contributes to reducing electricity generating costs while adhering to high safety and reliability standards. Ideally, fuel should not fail under normal operation conditions, but a more realistic goal reachable with present products is less than one failed fuel rod in ten reactor operating years, which corresponds to about one failed fuel rod in a million.

Typical reliability issues leading to failure are debris fretting (caused by foreign objects stuck in the spacer grid) and pellet-clad interaction assisted by pellet chipping. Such failures are avoided, respectively, by enhanced spacer and debris filter designs and improved pellet production and quality control. Another reliability issue is the bowing of long structural parts in connection with long operation cycles and neutron flux gradients. Such effects may impede the insertion of control rods and influence the local thermo-hydraulic and nuclear conditions in a fuel element. Finally, phenomena associated with corrosion of the outer surface of the fuel rod have an impact on safety and reliability. Among them are hydrogen pick-up and embrittlement, the thermal resistance of the corrosion layer and oxide spalling leading to temperature differences and increased hydrogen concentration reducing the cladding ductility.

The fuel assembly design and the choice of materials are governed by safety criteria (NEA, 2003). The assembly as a whole must:

• be able to withstand the mechanical loads and accelerations occurring during transport and handling

• provide sufficient stability margins against buckling under axial loads and bear the hydraulic lift forces arising during normal operation and accident conditions

• accommodate differential axial expansion and stay sufficiently straight under the influence of neutron-induced material growth such that incomplete control rod insertion (IRI) is prevented

• ensure safe reactor shut-down for events as severe as loss-of-coolant accidents (LOCA) and reactivity insertion accidents (RIA)

The fuel rod component must:

• provide sufficient internal free volume to accommodate fission gases released during operation without exceeding the pressure limit defined by the associated safety criterion

• withstand the elastic and plastic strains due to pellet-clad mechanical interaction

• be sufficiently resistant against cladding corrosion and hydrogen pick-up for the envisaged discharge fuel burn-up

The typical components of a fuel assembly and their functions will be treated in detail in the next sections.