2.1 Concept of reactor control

Two characteristics of CAREM reactor core design will be discussed:

— strong negative temperature coefficient

— no soluble boron for bumup compensation

The stron negative temperature coefficient enhances the self-controlling features of the PWR: the reactor is practically self-controlled and need for control rod movement is minimal. In order to keep a strong negative temperature coefficient during the

/1/ Chapter 6 oflAEA’s TECDOC on "Status of Small and Medium Reactors" In print.

whole operational cycle, it is necessary to do without soluble boron for bumup compensation. Burnup reactivity compensation is obtained with burnable poisons, i. e. gadolinium oxide dispersed in the uranium oxide fuel. Nonetheless, soluble boron is used to compensate cold-hot reactivity difference (the strong negative temperature coefficient means that a large difference in reactivity must be compensated between cold and hot states). Soluble boron is also used as a back-up of the safety shutdown rods: if the protection system of the plant orders to shut reactor down, and safety rods fail to do so, the reactor is shutdown by boron injection.

The effect the two core design features have on burnup, tend to cancel each other. A strong negative temperature coefficient means that fuel arrangement is not optimum from the reactivity point of view, but this charecteristic is compensated by the fact that power density in the core is lower than normal for PWRs. As a consequence, fuel temperature is lower, and the reactivity at hot state is higher.