13.1. Control-rod requirement

The control system of a reactor must be able to shut down the reactor and keep it in undercritical condition at any moment of its life, compensating also the reactivity introduced by the worst possible accident.

Beside that, enough excess reactivity must be provided to regulate the system and to allow for Xe override and burn-up compensation.

The above-mentioned requirements must be met also if one or more control rods fail to enter the core.

In this way we get then the following list of requirements:

1. Temperature effect between cold and operating condition.

2. Decay of fission products after shutdown (135Xe^> 135Cs).

3. Decay of 233Pa into 233U in Th cycles, of 239Np in 239Pu in 238U cycles (this effect is considerably smaller than that of 233Pa)

4. Compensation of worst accident.

Points 1 to 4 give the reactivity which has to be compensated by the control rods after a shut-down. To this the so-called excess reactivity must be added. This reactivity is invested in control rods which are initially inserted in the critical reactor in order to be able to compensate criticality losses due to:

5. Fine regulation.

6. Xe override.

7. Burn-up compensation in case of batch or discontinuous loading.

8. Damping of spatial oscillation.

The excess reactivity should be reduced to the minimum compatible with the operational requirements. The neutrons absorbed in control rods are lost for conversion and increase the fuel cycle cost.

The increase in number of control rods due to excess reactivity means a considerable increase in plant cost.

The absorption in control rods tends also to disturb the power distribution, and the uniformity of the burn-up, so that hot spots can result.

The requirement of point 5 is very small.

Point 6 depends on the operation requirement of the power station.

Point 7 can be strongly reduced by frequent or continuous reloading, and by the use of burnable poisons. With proper lumping, the rate of destruction of the burnable poison can be made similar to the change in reactivity due to the burn-up.

Point 8 is only important for very big systems.

The build-up of 149Sm after shut-down could be considered to reduce the control-rod requirement, but this contribution is rather small, and not always available: if the reactor has run at reduced power in the last days before shut-down, also the 149Sm build-up is reduced.

The control-rod requirement is dependent on the reactor composition and therefore has to be calculated as a function of burn-up. In reactors with continuous reloading the control requirement has to be calculated during the running-in period and in the equilibrium condition.

The reactivity requirement of the control system Akr is normally obtained as a difference of two calculations, one corresponding to the highest reactivity (cold reactor without Xe and Sm, at the beginning of a refuelling cycle, after decay of 233Pa and 239Np) and one to the lowest (hot reactor with maximum Xe concentration, equilibrium 233Pa and 239Np, at the end of a refuelling cycle). To this value is added the reactivity introduced for the requirement of regulation and by the worst accident.