Assessment of Dancoff adjusted Wigner-Seitz cells for self-shielding LWR lattices

Abstract

The objective of this thesis was to assess the effectiveness of using a Wigner-Seitz (WS) cell with an adjusted moderator thickness to produce more accurate resonance self-shielded cross sections for light water reactor (LWR) lattices. The WS approximation has been commonly used in lattice physics calculations for many decades regardless that it has been shown to underestimate k-eff for an infinite LWR lattice by several hundred pcm. The WS cell moderator thickness was adjusted in order make the WS cell Dancoff correction match that for the square unit cell. It was shown that the effectiveness of this method is sensitive to the Dancoff correction which was being calculated from the real three-dimensional geometry because in practice users commonly employ unconverged values for the Dancoff correction. For an infinite lattice the Dancoff adjusted Wigner-Seitz cell (DAWSC) resulted in small improvements in k-eff (-20 pcm) and reaction rates when using converged Dancoff corrections, however much larger improvements in values (up to 220 pcm) were seen for unconverged values of Dancoff corrections. When the DAWSC method was applied to a boiling water reactor (BWR) bundle, k-eff was worse for the DAWSC cases then for the normal WS cell treatment relative to continuous energy results. Improvements were seen in U238 absorption reaction rates for DAWSC cases in the inner fuel pins of the bundle; however the results were the opposite for fuel pins on the outer edges of the bundle. These results showed that the DAWSC method failed to account for irregularities in the bundle for the Dancoff corrections that were calculated. The Dancoff correction calculation sequence was evaluated against CASMO4e. Good agreement (-0.34% difference) was seen for infinite lattices, however large variations (+5% to - 4%) were seen among neighboring pins in a BWR lattice. These results for Dancoff correction prediction along with the significant improvements seen in k-eff for infinite lattices using unconverged Dancoff corrections implies that the DAWSC method may work if given the correct values for Dancoff corrections. The originally intended use for a Dancoff correction was to adjust the fuel escape probability for a particular energy group. Conversely, the application of DAWSC uses a single Dancoff correction to effectively change the fuel escape probability for all energy groups. A method for calculating an appropriate Dancoff correction for use in the DAWSC method should be investigated.