During operation of a nuclear reactor facility an equilibrium level of fission products is established in the reactor coolant. These fission products are the result of trace impurities of fuel material contained in the cladding surfaces as either natural impurities or a result of the fuel fabrication process. The mechanism by which the fission products enter the coolant is normally by fission recoil. Weld porosity is another potential path for the fuel, but generally quality control prevents this from occurring. During normal facility operation, these fission products are minor contributors to the overall radioactivity of the reactor coolant system.

If a defect were present or a failure of a fuel element occurred, large amounts of fission products would potentially have a path to the coolant system. If this happened, significant changes would occur within the reactor coolant chemistry parameters. Because most facilities analyze for gross coolant radioactivity either continuously or periodically, the analysis would be likely to detect all but the most minute failures.

When routine gaseous radioactive levels are monitored, an increase in this parameter's value would be seen. This is because many of the fission products are gaseous, and these gases are more mobile than particles of exposed fuel (the exposed fuel generally undergoes a process of erosion that washes the fuel into the coolant stream). The other parameter that may change is the ion exchange efficiency (where utilized), because many of the fission products released have a lower affinity for the exchange sites on the resin beads than the exchange anion or cation. Accordingly, the ion exchanger would not effectively remove these fission products and effluent radioactivity levels would increase significantly. Fission gases would also pass on through the ion exchanger and contribute to effluent activity. In addition, because some of the fission gases have relatively short half-lives, the amount of time they are held up in the ion exchanger is sufficient for some of these gases to decay to a radioactive solid.

These solid particles would contribute to effluent samples that may be concentrated prior to analysis. Some facilities monitor for specific fission product inventories in the reactor coolant to provide base level information. If a defect or failure were to occur, these levels would obviously increase to indicate the failure.