The fission products present in the reactor coolant give valuable information on the integrity of the cladding of the uranium fuel.

In CC>2-cooled reactors, a system of on-load fuel monitoring and failed fuel location called ‘burst can detection’ (BCD) was developed for the Calder Hall reactors. Gas samples are piped to a special detector and the fission products have to be detected against a background of neutron induced activity. Discrimina-

tion against this is achieved by a method of electro­static precipitation and subsequent measurement of the beta-emitting daughter products. Originally de­veloped for magnox reactors, this precipitator method was subsequently adapted for AGRs.

A recent development is to record the signals from two equal successive periods. The first signal is ap­proximately proportional to the short-lived and long — lived fission products in the gas sample. The second signal is approximately proportional to the compo­nent from the longer-lived fission products, so that the difference of the two signals gives a measure of the shorter-lived components. This ‘split-count’ tech­nique gives an improved indication of early failure of AGR fuel pins.

In magnox stations, the channel containing the failed fuel is located by a fixed-pipe sampling system with automatic selection valves. In AGRs there is also a fixed sampling system, but selection of the channel being sampled is by a flexible hose that is manually attached to each channel in turn. A method of auto­matic selection is being investigated.