Investigation of intranodal depletion effects

The modeling of depletion induced intranodal e€ects on important neutron physical para- meters in nodal di€usion theory is addressed. Consideration is given to two situations where these aspects are of particular interest, namely, in mixed oxide cores where strong interaction between uranium and plutonium mixed oxide assemblies occur, and in boiling water reactor cores where signi®cant control rod history e€ects are encountered. A model based on a low order polynomial representation of intranodal cross-section spatial behaviour is considered. Two approaches for determining the constraints for the polynomial ®tting procedure are applied. The ®rst one is a conventional method employing intranodal exposure values, whereas the second model combines intranodal exposure and isotopic inventory information. Numerical studies are performed in order to evaluate the relative merits of the di€erent models. It is demonstrated that pin power predictions are signi®cantly in¯uenced by intrano- dal e€ects. It is also found that the combined use of intranodal isotopic inventory and expo- sure distributions for estimating intranodal cross-section behaviour signi®cantly improves the accuracy in pin powers over the more traditional approach of utilizing exposure distributions only.