Changes in the temperature of the fuel or core which happen from a change in power can cause changes in reactivity, which in turn result in effects on power. These feedback effects have an important consequence on the safety of reactors. For example, if an increase in power and therefore temperature leads to an increase in reactivity, this will result in a further increase in power (and temperature), and if this is not controlled, the unstable condition could lead to an accident; this is known as a positive reactivity feedback effect. However, if an increase in power and temperature leads to a decrease in reactivity, then the initial power level will be reduced, along with the temperature, and the core will be stable; this is known as a negative reactivity feedback effect. Clearly, the latter condition is how the specific iPWR core should be designed.

The term ‘temperature coefficient’ is used to express the effects of changes in temperature on reactivity, and is defined as the change in reactivity per degree change in temperature. The two major reactivity temperature coefficients of interest in nuclear design of PWRs and iPWRs are fuel temperature and moderator temperature coefficients. They are considered separately as they are caused by different conditions that have to be analyzed, but also occur at different rates.

The fuel temperature coefficient is particularly important as it acts with little or no delay in a power rise, and as such it is important that is negative. The use of slightly enriched uranium fuels (as currently used in all light-water reactors (LWRs) and planned for iPWRs) ensures that the fuel temperature coefficient is negative due to ‘Doppler broadening’. As the fuel temperature increases, the thermal vibration of the atoms in the fuel also increases, which results in a wider range of neutron energies, and the resonance peaks in the U-238 absorption cross-section are broadened, increasing the probability of a neutron capture in the U-238, and not producing a fission; this is ‘Doppler broadening’. In general, the fuel temperature coefficient only becomes of concern for those fuels with lower U-238 content that seen in traditional fuels, e. g., by increasing the proportion of U-235, or adding another material such as plutonium. Therefore, there is relatively little nuclear design in the control of this reactivity coefficient for PWRs, including iPWRs.

The moderator temperature will rise more slowly in the event of a power rise in a PWR core due to the time to transfer the heat from the fuel to the water moderator/ coolant. In PWRs (including iPWRs) the increase in temperature results in a reduction in the moderator density, and also reduces the moderator to fuel ratio. These effects generally reduce the reactivity of the core and therefore result in a negative moderator temperature coefficient.

However, for all PWRs (including iPWRs) that have boron present in the coolant, the moderator temperature coefficient becomes less negative (or more positive) if the boron concentration in the moderator is increased. The reduction in the moderator density caused by the temperature rise reduces the density of the soluble boron that is present, therefore reducing the absorption in the boron, and this effect is clearly increased for higher boron concentrations. Therefore, the nuclear design of the fuel and the core addresses this by using burnable poison (BP) rods (see Section 4.3.2 for further details) to limit the amount of soluble boron needed in the coolant to control the excess reactivity, and ensure a negative moderator temperature coefficient in the core throughout the cycle of operation.