Fuel Pin and Assembly Design

Abstract

This chapter deals with the mechanical designs of fuel pins and assemblies. These core components must be designed to withstand the high temperature, high flux environment of a fast spectrum reactor for a long irradiation exposure time. In this chapter we will describe many of the factors that influence this design, and we will examine in some detail the stress analysis of the fuel pin.We begin in Section 8.2 with the basic geometric and heat transfer relationships for the fuel pin, and then discuss some topics related to fuel and fission gas that must be considered in analysis of steady-state fuel-pin performance. The discussion of fuel-pin design is continued in Section 8.3, in which failure criteria and stress analysis are presented. Discussion is then shifted in Section 8.4 to grouping the pins into a fuel assembly. This will include discussion of mechanical design problems such as fuel-pin spacing and duct swelling. Limited attention is then directed to the design of other assemblies, including blanket, control and shielding assemblies. The final section of the chapter will provide a description of methods to restrain the assemblies and to control duct bowing. Fuel Pin Design ConsiderationsFuel pin design is a complex process that involves an integration of a wide range of phenomena. The design procedure must integrate the thermal analysis of the pin with an assessment of the characteristics of the fuel and cladding as a function of temperature and irradiation history and with the stress analysis of the fuel-cladding system. Many of the phenomena that affect fuel pin performance are illustrated schematically in Fig. 8.1. Each of these will be treated in Chapters 8 and 9, with additional discussions in Chapter 11. It should be readily apparent that there is no single way to order the examination of so many interacting phenomena. Each sequence selected to cover these tightly coupled processes will inherently suffer from the need for input from processes not yet discussed. In actual design practice, all of the governing processes are integrated in large time-dependent pin analysis codes such as LIFE [1].Our presentation will begin with the geometrical arrangement of the various sections of a fuel pin, including the function of each section. We will then extend the discussion of pin geometry to the selection of pin diameter. This will involve the concept of linear power. Fuel restructuring is then introduced, followed by fission gas release and the associated length of the gas plenum to establish A. Waltar (B) Pacific Northwest