When the time comes to replace fuel in a reactor, either because of loss of reactivity or because of changes in its physical properties, the reactor operator is faced with a number of alternative choices. The operator must decide whether to remove all or part of the fuel in the reactor, and whether to move some of the fuel remaining in the reactor from one location to another, and he or she must choose the composition of new fuel to replace the fuel removed.

The reactor operator may also elect to add neutron-absorbing poisons to the fuel when charged, and may change control-poison concentration or move poison from place to place in the reactor during fuel life. Procedures used in charging, discharging, or moving fuel and control poison are known collectively as fuel and poison management.

2.1 Objectives

The principal objectives of fuel and poison management are as follows:

1. To keep the reactor critical during long-term changes in fuel composition and reactivity

2. To shape power density distribution to maximize power output

3. To maximize heat production from fuel

4. To obtain uniform irradiation of fuel

5. To maximize productive use of neutrons

Not all these objectives can be achieved simultaneously in a given reactor, and some compromises among them are usually necessary. Each objective will be described briefly in turn.

Maintenance of criticality. As each fuel element in a reactor is irradiated, its composition changes, as does its contribution to overall reactivity. To maintain criticality in the face of these composition changes, it is necessary either to move control poison or change its concentration or to move fuel or change its concentration. Because reactivity changes caused by changes in fuel composition occur at low rates, seldom greater than a tenth of a percent per week, movement of fuel or poison to compensate for fuel composition changes may be very slow, in contrast to the rapid movement that may be required to compensate for load changes, operating disturbances, or emergencies.

Shaping power density distribution. A nuclear power reactor and its fuel are so costly that it is very desirable, economically, to obtain the maximum amount of power from a given charge of fuel and a given size of reactor, or conversely, to design a reactor in which a desired power output can be obtained from the minimum size of reactor and the minimum investment in fuel. The optimum use is made of fuel when each element is operating at the maximum allowable condition, i. e., at the maximum allowable cladding temperature, maximum allowable thermal stress, maximum allowable heat flux, and/or maximum allowable linear power density. A uniformly fueled and poisoned reactor is far from this ideal condition because of the wide variation of neutron flux and power density from point to point. In a cylindrical reactor whose fuel and poison distribution is spatially uniform, the neutron flux and power density vary with radius r and axial distance from midplane z as /0(2.405г/Л) cos (nz/H), where R is the effective radius and H the effective height of the fuel-bearing core of the reactor. The power density at the center is more than three times the average and the power density at the outer radius, top and bottom, is nearly zero. In all power reactors designed with economical performance in mind, fuel and/or poison is so managed that the power density distribution is more uniform than this cos J0 distribution. The optimum power density distribution will depend on what factors limit power output, whether it be temperature, thermal stress, heat flux, or linear power, and usually is quite specific to a particular reactor.

Maximum heat production. Before fuel can be charged to a reactor, it is usually necessary to bring it into a closely specified chemical and physical condition and to seal it in pressure-tight cladding fabricated to narrowly specified dimensions. After fuel is discharged from a reactor, it usually still contains enough fissile material to justify its recovery through chemical reprocessing. These operations of fuel preparation and reprocessing often cost $200,000/ton of fuel or more. It is therefore economically desirable to obtain the maximum possible amount of heat from each fuel element before it is discharged from the reactor. Even at the burnup of 30,000 MWd/MT, now obtainable from oxide fuel before physical damage necessitates fuel replacement, fabrication and reprocessing contribute $6.7/MWd or more to the cost of heat, or 0.9 mills/kWh to the cost of electricity in a power plant that is 30 percent efficient. It is thus of considerable economic importance to strive for maximum burnup until limited either by physical damage or by offsetting economic factors such as the higher cost of the richer fuel needed for higher burnup. The economic optimum burnup will be discussed later in this chapter.

Uniform burnup. Because of the high cost of fuel fabrication and reprocessing, it is also important to manage fuel so that every element at discharge has been irradiated to nearly the same burnup. If this is not done, some of the fuel would have generated much less heat than elements that had received the maximum permissible irradiation, and the unit cost of heat from these underirradiated elements would be undesirably high.

Productive use of neutrons. In thermal reactors, the number of neutrons produced per neutron absorbed in fissile material (t)) is of the order of 2.0. One of these neutrons is needed to keep the fission reaction going, but the second neutron, in theory, is available to produce valuable by-products of nuclear power. In practice, of course, some of these extra neutrons are necessarily lost through leakage and absorption in reactor materials, but around 0.6 neutron is available in water-moderated reactors for productive use. Examples of productive uses of neutrons are making plutonium from 238 U, 233 U from thorium, or 60Co from natural cobalt. To maximize production of such by-products, it is desirable to use methods of fuel and poison management that minimize leakage of neutrons and their nonproductive absorption in control materials that upon neutron absorption produce relatively valueless materials. For example, it would be better to use 238 U or thorium to absorb extra neutrons than boron control poison, because plutonium from 238U or 233U from thorium may be worth as much as $20/g as nuclear fuels, whereas boron produces only valueless helium and lithium. We shall see that some methods of fuel management conveniently permit the 238 U remaining in uranium fuel after 233 U is depleted to absorb the extra neutrons produced from fresh fuel of higher 235 U content. Such a method of fuel management is clearly more desirable economically than one that uses boron control poison to absorb extra neutrons produced in fresh fuel.