The Magnox and AGR reactor designs are unusual in their ability not only to detect the presence of a defective fuel element but also to locate it for discharge. In the case of Magnox reactors, the prime reason was the rapid oxidation of uranium metal under defective fuel cladding leading to swelling, loss of heat transfer and possibly fire. In the case of AGRs, the primary concern was release of fission product to the coolant, leading to not only contamination of the circuit but also releases of radioisotopes (especially I-131) to the environment in the event of a depressurisation incident or from routine coolant leakage (typical AGR leak rates are of the order 1-3% of coolant inventory per day).

The detection systems used on both types of reactor were similar. Samples of gas were drawn from the reactor and their activity measured in the burst can (or cartridge) detection equipment (BCD). Measurements of noble gas fission products, which leak from defective fuel pins and elements, are used to detect the presence of failed fuel: the radioactive isotopes of xenon and krypton. Only short­lived isotopes are of use for locating defective fuel, as long-lived isotopes are soon mixed into the general coolant flow with little enhancement in the channel containing the fuel defect.

A bulk activity measurement is not appropriate as the coolant carries standing levels of beta and gamma emitting isotopes such as N-16, O-19 and Ar-41. In order to discriminate the fission gas isotopes from gaseous activation products, use is made of the fact that the majority of the short half-life (tens of seconds) krypton and xenon fission gas isotopes have short half-life radioactive decay products, isotopes of rubidium and caesium respectively.

The coolant sample flow is directed into a ‘precipitation chamber’ containing a wire held at high voltage. The wire attracts charged decay products of the fission gases to its surface. After a ‘soak time’ of a minute or so, the wire is fed into a counting chamber, isolated from the gas sample flow, and the radioactive progeny of the rare gases are detected. This simple system has proved to be robust and sensitive for the detection of failed fuel.

In order to locate the channel containing the failed fuel, quite complex systems of pipework and valves are used in which large sections of the reactor can be sampled, followed by smaller groups of channels and eventually single channels. By scanning sequentially to the zone with the highest signal, the failure channel can be (in principle) rapidly identified and (where on-load refuelling is possible) the fuel can be discharged promptly. The wire is spooled around the BCD precipitator in a continuous loop.

Various supplementary systems have been used, particularly in AGRs, including continuous sampling of coolant and measurement by a high-resolution gamma ray spectrometer (not available when the Magnox and early AGRs were being constructed), and sampling of bulk coolant through a charcoal pack to determine the levels of radio-iodine.