Abstract
This study investigates the effects of various treatments of temperature-dependent cross sections on a hot HTTR criticality calculation, which involves many nuclides in the core configuration and a detailed temperature distribution. By using MCNP, the authors considered and compared five popular cross-section treatments: (1) approximate temperatures by rounding up or down to the nearest available temperature in data libraries, (2) a pseudo material method based on interpolation through mixing of nuclides at two temperatures, (3) the makxsf utility with Doppler broadening and interpolation to create customized libraries, (4) an on-the-fly methodology to create the Doppler broadened data sets, and (5) the fundamental NJOY nuclear data processing system to generate data libraries at problem-specific temperatures. The MCNP results with the NJOY-processed cross sections was taken as a reference base on which the accuracies of various cross-section treatments were evaluated. The eigenvalue comparisons show that both the on-the-fly and makxsf treatments gave satisfactory results with small differences of approximately 20-70 pcm; the methods of approximate temperatures and pseudo materials led to slightly larger discrepancies of approximately 100-300 pcm. Looking into the details of axial flux distribution, the comparisons indicate that the makxsf treatment provided the best consistent result (<0.5%) with NJOY. The makxsf creates nuclide datasets at new temperatures by considering Doppler broadening of resolved resonances and interpolations of both unresolved resonance probability tables and S(α,β) thermal scattering data. The on-the-fly method showed a slightly larger discrepancy of approximately 1.5% in flux distribution because of no specific treatment for temperature-dependent unresolved probability tables and S(α,β), corresponding data at approximate temperatures were used instead in this case. The flux discrepancies caused by cross sections generated using the pseudo material and approximate temperature methods increased to certain extent, approximately 2.5% and 3.5%, respectively. Most of these differences were identified resulting from approximations of temperature-dependent S(α,β) data for the huge amount of graphite in the HTTR core.