OSART Good Practices

The principal leak detection system for primary to secondary leaks is monitoring of the Nitrogen-16 activity in the live steam at the exit of the steam generators (SG). This is supplemented by tritium measurement in the condensate or feedwater, Xenon measurement in the incondensible gases exhausted from the condenser and radioelement detection in the steam generator blowdowns. Tritium is especially useful in determining the presence of a leak because of the low detection threshold (< 0.5 L/h) and the precision of the method. However, it cannot be used to determine which SG is leaking. Similarly, Xenon detection is normally performed on the incondensible gases from the condenser and cannot be used for individual SG leak detection (it is possible to measure Xe in the live steam but it is difficult to sample). The principle means of monitoring an individual SG has been the detection of radioelements in the SG blowdown. The detection threshold with this method is variable, as is the precision (30 to 300 percent). The plant has developed the lithium detection method as an alternative means of determining the leak rate in a specific SG. Lithium is, of course added to the primary circuit as the pH adjusting agent and the method has the advantage that it does not require counting equipment or any additional chemistry instrumentation and the analysis time is short. It is thus a very rapid assessment method. The detection threshold, as reported by the station, is good (< 1L/h) as is the precision (20 percent). A relatively short equilibration period of about 8 hours is required.