Operational reactivity reserve

The operational reactivity reserve a nuclear reactor from the "Chernobyl type" RBMK is a measure of the number of the reactor core retracted control rods .

This key figure is also known as the ORM value ( Operational Reactivity Margin ). It is defined as the reactivity equivalent of those control rods that are wholly or partially located in the reactor core. Their value is given as a multiple of the reactivity equivalent of a medium, fully retracted control rod.

Since control rods influence the performance and criticality of a reactor, the ORM value has operational and safety significance.

• As a reserve, it is required for operational purposes, since control rods have to be extended in the event of load changes, for example to compensate for xenon poisoning , in order to prevent the chain reaction from going out.

• For safety reasons, too many control rods must not be extended. This is because this ORM value, which is too low, increases the positive void effect of the RBMK reactor and reduces the shutdown effectiveness at the beginning of the retraction of the control rods.