As with all reactors the fission process is controlled by balancing the reactivity in the core. The core itself contains structural materials, which absorb neutrons, and the fission process is influenced by the energy of the neutrons and hence the effectiveness of the moderation. Reactivity is controlled by means of absorbers in the form of control rod clusters and by the addition of dissolved absorbers to the water. The control rods contain either a silver/indium/cadmium alloy or in some cases hafnium. Boric acid is used as the dissolved absorber to provide bulk reactivity control.

Since PWRs are periodically shutdown for refuelling the reactivity held in the core at the beginning of the cycle (BOC) must be high enough to last for the complete cycle. This is achieved by enriching the fuel to increase the proportion of 235U, which is about 0.7% in natural uranium, to between 3 and 5%, depending on the cycle length. The fission processes will consume the 2 35U and so reduce reactivity. In addition some of the fission products, referred to as ‘neutron poisons’, act as absorbers, further reducing reactivity. However, neutron capture by 238U will lead to the formation of 2 39Pu, which is fissile and so adds reactivity. A proportion of the plutonium will be consumed later in the cycle.

The core reactivity therefore falls during the cycle and is compensated for by reducing the amount of dissolved boron in the water. This is achieved by the chemical and volume control system (CVCS) (Fig. 10.82 . A small amount of water passes through the CVCS system at all times to clean up the coolant, including the removal of radioactive material. It can also be used to either add borated water or demineralised water to change boron concentration. The advantage of using dissolved boron is that the absorption is uniform and so the neutron flux profile across the core remains undisturbed.

The absorber rods provide rapid control; they are divided into different groups with some designated for control and the others held out of the core for shutdown. The boron concentration is maintained so the plant normally operates with the control rods just inserted into the top of the core. In this position the control is sensitive and rapid shutdown can be achieved by dropping the control and shutdown rods under gravity. Fast power changes will be carried out using the rods but slower variations including compensation for burnup will be achieved by changing the boron concentration.

Changes in boron concentration involve adding more concentrated boric acid or adding water. In both cases fluid must be drained from the circuit and treated by the waste water plant. To avoid the need to treat large quantities of waste water some plants make use of so called ‘grey rods’. When normal (‘black’) rods are inserted into the core this changes the flux shape (see Pouret et al, 2009) because they essentially absorb all the neutrons that impinge on them. This increases peaking factors as well as leading to non-uniform xenon transients. Grey rods are

Containment sump

10.8 Chemical and volume control system (Source: USNRC).

Rod Cluster Control Assemblies (RCCAs), which contain lower densities of absorber material and so do not absorb all the incident neutrons. They can therefore be more deeply inserted into the core without unduly disturbing the flux shape. They are used to change load without the need to change the concentration of the dissolved boron. They do, however, still have local effects on the flux and therefore do affect fuel utilisation.

As was noted above, PWRs are under-moderated, which means that increases in moderator density will increase reactivity. This tends to lead to a negative moderator temperature coefficient (MTC), which is good for control. However, because the moderator generally contains dissolved boron, the extent of the effect will depend on the boron concentration since changes in the moderator density will also change the absorber content. At the end of cycle (EOC) when the boron concentration will be very low the MTC will be strongly negative. However, high boron concentrations at BOC could give a positive MTC. To avoid this, burnable poisons are introduced. Burnable poisons were initially used in the form of discrete rods, which were inserted into fuel assemblies that did not contain RCCAs. These provide negative reactivity early in the cycle but are burnt out during the cycle. It is now more common to use integral burnable poisons in which the burnable poison is incorporated into the fuel rods either by mixing (e. g. gadolinia) with the fuel or as a surface coating (e. g. zirconium diboride) on the fuel pellets.