G. ROSSITER, NationalNuclearLaboratory(NNL),UK

Abstract: It is essential to ensure that nuclear fuel operates within its design constraints, and therefore safely. This requires an understanding of the fuel behaviour under irradiation. The behaviour is complicated by the effects of the neutron flux, fissioning and fission product generation, and microstructural changes in the fuel as irradiation proceeds. Although there are many differences between the behaviour of different fuel types, there are also many commonalities. This chapter describes: (i) the important phenomena that occur under irradiation;

(ii) how and why the phenomena are modelled using computer codes; and

(iii) the advantages, limitations and future trends in the use of such codes.

Key words: nuclear fuel behaviour, nuclear fuel performance.

14.1 Introduction

It is essential to ensure that nuclear fuel operates within its design constraints, and therefore safely. This requires an understanding of the fuel behaviour under irradiation. The fuel behaviour includes both physical (in particular thermo­mechanical) and chemical aspects. The behaviour is complicated by the effects of the neutron flux, fissioning and fission product generation, high temperatures and large temperature gradients, all of which produce microstructural changes in the fuel as irradiation proceeds. Although there are many differences between the behaviour of different fuel types, there are also many commonalities.

This chapter describes: (i) the important phenomena that occur under irradiation; (ii) how and why the phenomena are modelled using computer codes; and (iii) the advantages, limitations and future trends in the use of such codes. In the interests of brevity, only the fuel pin (or fuel rod) design concept used in the majority of reactor types (LWR, CANDU, Magnox, AGR and fast reactor systems) is considered. The focus is on LWR fuel, since this is the most prevalent fuel type internationally. Information on the behaviour under irradiation of the coated particle fuel used in HTR systems can be found elsewhere (IAEA, 1997).