In general the reactivity of a reactor core decreases with increasing temperature (as explained in the following section) and with burnup. Thus a new, cold reactor core is in its most reactive state and a core at power at the end of its refuelling cycle is least reactive. The control rods, fully inserted in the new core, have to have enough negative reactivity to compensate for these changes so that, when they are fully withdrawn at the end of the cycle, the reactor remains critical. Table 1.3 gives typical values of the reactivity requirements for a breeder reactor.

The “shutdown margin” is a safety allowance to make sure that the reactor remains subcritical during refuelling even if errors are made and excessive fuel is added. It also has to allow for one or more control rods themselves being removed and replaced. The safety rods are there so that the reactor can be shut down from any operating condition even if some of the control rod mechanisms fail to work, including the possibility of a rod being accidentally withdrawn. These points are discussed in more detail in Chapter 5.

In principle there is no difference between the control, shut-off and safety rods: they could be identical items distinguished only by differ­ent names to denote their different functions. However as explained in Chapter 5 the reliability of the shutdown system, which is crucial for the safety of the reactor, is enhanced by diversity. Thus the shut-off rods might utilise a different absorber material from the control rods or different operating mechanisms.

It is important to note that Table 1.3 applies to a typical breeder reactor in which the loss of reactivity by burnup of fissile nuclides is reduced substantially by internal breeding of new fissile material in the core (see section 1.4.3). In the core of a reactor designed to con­sume fissile material the internal breeding is much reduced or possibly eliminated entirely, so that the loss of reactivity with burnup is much greater, possibly three times as much as indicated in Table 1.3. In a typ­ical breeder reactor core some 7-10% of the space is occupied by con­trol rods (including shutdown and safety rods), but in a consumer core the proportion is likely to be greater. Loss of reactivity with burnup may in fact limit the length of the refuelling cycles for such a reactor.