Void coefficient analysis of an LWR lattice experiment with high-enrichment MOX fuel Original Research Article

A series of physics experiments performed between 1985 and 1990 in the PROTEUS reactor at the Paul Scherrer Institute in Switzerland included the investigation of a Pu-fuelled light water reactor (LWR) lattice with a moderator-to-fuel volume ratio of 2.07 and an effective enrichment of about 8%. The analysis of the measurements in this test lattice and in the tighter light water high conversion reactor (LWHCR) lattices investigated previously permits the determination of the k∞ void coefficient of the LWR lattice for cases of partial and total voiding. A comparison of the measured changes of k∞ with values calculated using the cell codes WIMS and KAPER4 shows a satisfactory prediction of the partial void coefficient in the range from 0–54% voidage. Discrepancies increase up to twice the estimated experimental error in cases of further voiding to 100% void. The total void coefficient (0–100% void) results from large compensating effects from individual reaction rate ratios. Its accurate prediction by cell calculations appears to be fortuitous. Improved nuclear data and refined calculational methods are thus required for a more accurate calculation of the void coefficient in high-enrichment MOX-LWRs.