On the Determination of Bottom Drain Oxygen in Boiling Water Reactors

The viability of an often-employed engineering method of determining bottom drain (lowerplenum) oxygen levels in boiling water reactors is explored, in which bottom drain oxygen is back-calculated from the recirculation system oxygen level and the combined recirculation system/bottom drain value. For a low flow fraction f where 0.16 <f <0.20 is often employed, the back-calculated bottom drain oxygen level can be grossly in error, reflecting the minimal amount of information that is derived from the lower plenum. This finding cautions against using back-calculated lower plenum oxygen levels to specify hydrogen water chemistry conditions for protection of the components in the lower plenum, particularly when f is small. The uncertainty in the bottom drain [O₂I has been characterized by using a Monte Carlo error analysis for both systematic and random errors. Modifications to the sampling system that would greatly reduce these errors are identified.