Quantifying the impact of PWR primary side chemistry regime utilizing surface activity data

The collection of gamma spectroscopic data, known as gamma scanning in the US, allows for identification and quantification of radionuclides contributing to shutdown dose rates. Based on such data, plants can quantitatively assess the impact of operational and chemistry changes and optimize their source term reduction programs.. To quantify the effect of PWR primary side chemistry on activity incorporation including zinc addition, a simplistic activity balance assessment of Co-58 activity incorporation and release rate constants was developed. Under steady state conditions, the sum of the net release rates of corrosion product radionuclides to the primary coolant from fuel cladding/crud and steam generator tubing and stainless steel surfaces must be equal to the net removal rates from the coolant. Release and re-incorporation of activated corrosion products can occur from each of the referenced primary surfaces, and isotopic (and mass) balances are developed based on the net value of these processes. Net deposition is expected locally in high duty boiling regions, but the reverse is expected in regions where the boiling rate has decreased as fuel burnup occurs during the cycle, i.e., there could be a net release of nickel and iron from such regions. Although transport rates of non-radioactive species such as iron and nickel can be modelled, it is difficult to validate the models since significant transport occurs during shutdowns, surface analysis results are currently only performed after shutdown. However, release of radionuclides from out-of-core surfaces has been shown to be minimal during the shutdown evolution; and non-destructive surface analyses performed following shutdown can be used to reasonably approximate surface activity during power operation prior to the shutdown. From these measurements, net deposition or release rates from out-of-core surfaces and the core during power operation can be estimated. The observed reductions in RCS piping surface activity as a result of zinc addition have been significant. Although the increase in the reactor coolant Co-58 concentration following zinc addition was consistent with the reductions in the steam generator tubing and stainless steel surface incorporation rates, the data also suggest that zinc addition is reducing the release rates of Co-58 from the fuel surfaces. Such behaviour has been observed in BWRs. (authors)