Operational reactivity margin

The operational reactivity margin ( ORR ) is part of the total reactivity margin of the reactor , compensated by some moving absorbers (rods) of the control and protection system (CPS).

Being, in essence, reactivity, the ORM is a dimensionless quantity , however (for convenience) it is measured in practice in various conditional quantities, for example, the immersion depth of the absorbing rods of the reactor, the effective fraction of delayed neutrons , etc. When operating RBMK reactors, it is customary to measure the ORR in the effective the number of fully immersed rods for manual regulation of the control system OZR, expressed in rods , approximately shows what margin the operator has to increase power, as well as the maximum positive reactivity that can be introduced into the reactor with CPS rods.

In this case, the total length of the immersed parts of the rods is not equal to the product of the length of the rod by the OZR. The reason for this is, in particular, in the following:

• the reactivity introduced by the rod has a nonlinear dependence on the depth of immersion of the rod;
• the number of neutrons absorbed by the CPS rod depends on the neutron flux in the rod region, including the distribution of the neutron flux over the height of the reactor core. Therefore, rods located in different parts of the core introduce different negative reactivity.

From the point of view of the economics of the reactor, as well as its safety, the most favorable is the minimum value of the OZR. In this case, on the one hand, “unproductive” neutron absorption by CPS rods is reduced (which could be spent on fissioning fissile isotopes, that is, energy production), and on the other hand, the risk of introducing significant positive reactivity due to unexpected extraction of the CPS absorbing rod is reduced .

The RBMK reactors mentioned above prior to the Chernobyl accident had a rather low OZR value, which further increased their economic efficiency. However, too low a value of the OZR contributed to a decrease in the stability of the reactor and the deterioration of its dynamic characteristics.

At present, when designing reactors, it is considered advisable to provide for the lowest possible OZR. The neutron-physical characteristics of uranium-graphite reactors and reactors with a heavy water moderator (for example, generation 3+ MCPE or CANDU ) make it possible to achieve a very low ODS. Low RBM is also characteristic of fast-neutron reactor plants, such as BREST .