Importance of on-line primary coolant gamma activity monitoring for the assessment of the fuel condition during power reactor operation

In comparison to the well-known fuel condition monitoring based on gamma activity measurements of grab samples of the primary coolant, the on-line coolant activity monitoring technique offers several benefits to be discussed in the paper. First, on-line coolant activity monitoring allows for near real-time qualitative or quantitative assessment of cladding defects, eliminating the time delay, personnel exposure and waste associated with the collection of grab samples. The possibility of monitoring the fuel condition during transients is also important for the plant operator. Due to the much greater number of activity measurements performed for a variety of nonvolatile, volatile and gaseous isotopes the accuracy of measured activities is improved. The prediction of defect characteristics such as type and size of the defect and its approximate location in the reactor core is also improved. For failure characterization the isotopic ratios I-131/I-133, I-133/I-134, Xe-133/Xe-135, Kr-87/Xe-135, Xe-138/Xe-135, Cs-134/Cs-137 are important. An on-line fuel condition monitoring system can be incorporated into the Plant Anomaly Detection System, providing important information to the plant operator on defect deterioration and thus assisting in plant operation and refuelling strategy. (author). 30 refs, 1 tab