Internal reactor instrumentation

To operate and ensure the safety of nuclear reactors of any type, it is necessary to measure the energy supplied by the fission of uranium 235 nuclei, i.e. nuclear power.

In all cases, measuring this power involves measuring the radiation emitted by the reactor core, and more specifically the neutron flux. The following paragraphs will describe how neutron flux is measured inside the core, but before going into detail, it's worth dwelling on a fundamental feature of nuclear reactors.

The laws of neutron physics dictate that neutron power or flux is not uniformly distributed within the reactor volume. There are places where the power is higher than in others, typically in the center of the reactor compared with the periphery. These are known as hot spots.

Of course, it is at the hot spots that the power delivered is closest to the design or even safety limits, hence the need for perfect knowledge of the power value, and therefore for neutron flux measurements at these points. This phenomenon of non-uniform power distribution must be understood as a physical phenomenon occurring throughout the reactor volume. This is known as three-dimensional power distribution or, more simply, 3D power distribution. Another limiting physical phenomenon associated with power distribution, but more specifically linked to the average power of each rod (integral of the power of each rod in the axial direction) rather than to local power, is the appearance of calefaction. By creating an insulating vapour film that reduces heat exchange between part of the fuel rod and the primary fluid, this phenomenon could, if it were to occur, cause an increase in cladding temperature, leading to a deterioration in the cladding's mechanical properties and a hydriding reaction in the metal alloy (Zircalloy) that makes it up.

Power distribution is a parameter that evolves over time, with a short-term time constant essentially linked to the power variations performed daily by the operator during reactor operation, and a long-term time constant dependent on fuel depletion. It can therefore distort more or less rapidly. During these deformations, the hot spot(s) move(s) within the core, changing both amplitude and location.

The function of neutron instrumentation is not only to measure the power level, but also the 3D neutron flux or power distribution and, in particular, the value of the local power delivered to hot spots. In the remainder of this presentation, flux distribution and power distribution will be equated as a first approximation.