4.11 Fuels

4.11.1 Introduction

The following sections, which deal with nuclear fuel, are concerned almost entirely with those commercial reactor systems which are currently operating, or are in the course of design and construction in the UK. This gives a natural division between the indigenous gas cooled systems (magnox and the commercial ad­vanced gas cooled reactor CAGR) and the USA devel­oped pressurised water reactor (PWR) which evolved from the small reactors originally designed for opera­tion in nuclear submarines.

Harris and Duckworth (1982) [1] have shown how the British magnox reactors grew from the weapons programme and how the choice of natural uranium fuel was dictated largely by circumstances, rather than a systematic search for the best solution to an engi­neering problem. Nonetheless, natural uranium metal has been found to be a highly satisfactory nuclear fuel with several advantages over other types. It is the densest form of uranium and, with a graphite mod­erator, can be used without enrichment. It is readily available in a fairly pure form and it has a high co­efficient of thermal conductivity. This last factor is important since a major problem with metallic ura­nium is that it cannot be operated at temperatures much above 660°C; the high thermal conductivity al­lows this constraint to be met fairly easily whilst still producing gas outlet temperatures which allow rea­sonable turbine thermal efficiency. Provided that there is a sufficient concentration of fissile atoms to achieve criticality, the thermal conductivity of the fuel is pro­bably the most important factor in deciding the fuel shape. This is because the thermal conductivity has a direct bearing upon the temperatures which, as al­ready observed for uranium metal, generally sets the operating limits for the fuel.

In the case of uranium dioxide, the coefficient of thermal conductivity is low (between 4 and 17 times lower than for pure uranium) so that, although the allowable operating temperature for the oxide is much higher (melting temperature 2800°C) than for the metal, the fuel diameter has to be considerably smaller (approximately halved) in order that it can be ade­quately cooled. This separation of the fuel to allow cooling, together with the lower density of the oxide, usually dictates the need for enrichment of the U-235 (in the CAGR the use of stainless steel clad makes this essential). The balance of the fuel comprises the fertile isotope U-238 which can be converted by neu­tron capture and beta decay to (fissionable) Pu 239.

By the use of enriched fuel, smaller pins and more efficient cooling, the rate of heat generation per unit mass of fuel, or rating has been progressively in­creased: magnox reactors and CAGRs have peak ele­ment ratings of about 5 and 20 MW/t respectively; in PWR the maximum fuel rod rating is about 60 MW/t. The increase in rating between magnox and CAGR is largely attributable to the higher fuel tem­peratures possible with oxide fuel. The difference be­tween CAGR and PWR is mainly the result of the increased effectiveness of pressurised water cooling (compared to gas cooling) which allows these ratings to be reached whilst maintaining the fuel at accept­able temperatures.

In this section frequent reference will be made to the fuel burn-up. This can be expressed in three ways (e. g,, see Olander, 1976 [2]); firstly, the fission density, or the total number of fissions/unit volume; secondly, as a fractional burn-up, or the total number of fissions divided by the initial number of heavy metal (not necessarily fissile) atoms; or, thirdly, as the thermal energy released by one tonne of heavy metal atoms (noting that 210 MeV/fission is equivalent to 0.95 MWd/g fissioned) — this is the measure adopted here. For conversion purposes, 1% fractional heavy metal burn-up is equal to 8.6 GWd/t. It is important to remember that the fuel rating, and hence the burn — up, will vary from point to point in the reactor. Thus in CAGRs, for example, the peak stringer burn — up will be less than the peak element burn-up, which will be less than the peak pin burn-up and the peak point burn-up.

In general, the lifetime of the fuel will be deter­mined by its endurance which we may loosely define as the maximum burn-up attainable by the fuel be­fore significant numbers of failures begin to occur; this is the main subject of this section. In addition to this, however, there is also a limit on the maximum achievable burn-up which is dictated by the reactor physics; this is the point at which the reactor runs out of reactivity. In a reactor using enriched fuel the burn-up limit can be increased, at least in theory, by increasing the fuel enrichment, although in some cases it may also be necessary to use burnable poisons to maintain a more uniform reactivity throughout the life of the fuel. In reactors which are fuelled with natural uranium, however, this option is unavailable and in the magnox reactors, where the endurance limit has been steadily increased over the years, the reacti­vity limit is now being approached.