It has been suggested that crud deposits on a number of adjacent fuel rods might
reduce coolant flow rates in associated sub-channels. Such reduced flow rates could
then worsen thermal-hydraulic conditions, such as margin to saturated boiling, fuel
surface temperature, and the DNB ratio. We report the results of a detailed
computational fluid dynamics study of the flow pattern in a partially-crudded rod
bundle. Values obviously depend on, for example, the thickness of crud assumed,
but sub-channel flow rate reductions of ~10% were predicted by this analysis.
However, this mass flow rate reduction was found to be more than offset by
improved heat transfer induced by the relatively rough surface of the crud. Cladding
temperatures were predicted to be essentially unchanged, and the DNBR was
similarly little altered. We conclude that such flow reduction and diversion is not likely
to be of concern.