Black rods which spend most of their life substan­tially inserted into a high neutron flux may exhibit a reduction in worth due to partial exhaustion of their neutron capture capability. Consideration of the neutron flux incident on the surface of the rod, the surface area of the rod (to calculate the number of neutrons entering the rod) and the number of boron atoms in the boron steel inserts of a black rod, en­ables an estimate to be made of the time it will take for all the atoms of Boron-10 (the isotope with the high capture cross-section) to each absorb a neutron and thus the boron component to become ineffective. This phenomenon has been experienced at Hunterston

A where, after several years of operation, black rods used for radial flux shaping started to decrease in worth so that they were being progressively further inserted into the core to achieve the desired effect.

This phenomenon does not occur however in the steel component of a black rod or in a grey rod. Consideration of the number of iron atoms in a rod, the capture cross-section of iron and the neutron flux in the rod shows that it will be thousands of years before the neutron capture capability of the steel is exhausted. It is interesting to note that if, for ex­ample, an atom of Fe-56 (the most abundant stabte isotope of iron) absorbs a neutron it becomes Fe-57 which is also stable and equally capable of absorbing a neutron, so that even when all the atoms have each absorbed a neutron the rod is still capable of further neutron absorption. Examination of the Chart of the Nuclides will clearly show this effect.

A control rod may of course become time-expired after a few years for mechanical reasons rather than reactivity reasons.