Nuclear data assessment based on the analysis of benchmark criticals

Abstract

The recent trend in the nuclear industry has been toward the standardization of nuclear reactor design. Directly coupled to this is the standardization of the nuclear data which is used in the calculations in the reactor design codes. Making an assessment of the data which is now available has been the objective of this work.

The results and trends observed are consistent within a group of lattices, as well as for lattices having similar physical characteristics. In most cases criticality was underpredicted. This is partly due to the assumptions employed in the calculational technique which was used. Overall, the ENDF/B-IV data gave better criticality predictions and was more sensitive to changes in the lattice parameters than was the ANL set of cross sections.

The four groups of light water lattices with varying enrichments in the U-235 isotope all showed similar trends. As the pitch or moderator-to-fuel volume ratio was increased a decrease was observed in the calculated value of keff. The largest error observed was for the lattice with the smallest pitch in each group, about 21. This is due among other reasons to the larger percent error in determining the buckling for these lattices. The ANL data showed less variation from lattice to lattice but underpredicted all of the lattices which were analyzed. The results of the natural uranium heavy water lattices were in agreement with results established at other laboratories. In this instance, the ANL data gave slightly better results than the ENDF/B-IV cross section set. The VOZ group had their buckling determined by three experimental methods. As in the light water cases described above, the lattice with the smallest pitch gave results slightly higher than those of the other lattices. The value of keff varies very little in the four remaining lattices. All five of the lattices in this group were underpredicted by HAMMER. The most consistent results were those obtained by using the bucklings derived by using the variable loading analysis technique. The ²³³U0₂/ThO₂ light water lattices had their bucklings determined by two different techniques. The ANL data gave slightly better results for the lattices with the smaller pitches and the ENDF/B-IV data was slightly better for the oth.er cases. The bucklings determined by using the variable loading technique resulted in the best predictions of keff for all the lattices in this group. The ²³³U0₂/ThO₂ heavy water lattices were all overpredicted by both sets of cross section data. The ENDF/B-IV data gave more consistent values of keff for this group of lattices.

The HAMMER computer code was used to calculate keff for each of the lattices. Two sets of cross section data were used: the most recent version of ENDF/B, ENDF/B-IV and a much earlier set from ANL. The resulting values of keff which were determined from using both sets of data were then compared.

A total of sixty-nine lattices were analyzed. They consisted of eight groups: four groups of light water lattices with varying enrichments in the U-Z35 isotope (1.016%, 1.027%, 1.143% and 1.29%), a group of natural uranium, heavy water lattices, a group utilizing UO₂ as the fuel material, and two groups of ²³³U0₂/ThO₂fueled lattices, one with light water and one with heavy water as the moderator. Each group included lattices with different pitches, moderator-to-fuel volume ratios, fuel rod diameters, and experimental methods of determining the buckling