The Inversion Point of the Isothermal Reactivity Coefficient of the IPEN/MB-01 Reactor - II: Theoretical Analysis

TORT, an S_N three-dimensional transport code, is employed for the analysis of the inversion point of the isothermal reactivity coefficient of the IPEN/MB-01 reactor. The analyses are performed in companion NJOY, AMPX-II, and TORT systems considering the data libraries ENDF/B-VI.8, JENDL3.3, and JEF3.0. The analyses reveal that for this peculiar problem, there is a need to convert all the computer codes to DOUBLE-PRECISION as well as to increase to seven the number of digits of the ANISN library generated by XSDRNPM. Contrary to the traditional diffusion theory codes, TORT k_eff results are very sensitive to the number of both fine and broad groups. For instance, the traditional and very well known two- and four-group structure, largely utilized in several diffusion codes, produced simply unacceptable k_eff results. The highest deviation between calculated and experimental values found for the inversion point was -4.48°C. At first glance, there appears to be a significant discrepancy. However, in terms of reactivity coefficient, this discrepancy means a deviation of -0.90 ± 0.05 pcm/°C, which indicates that the calculational methodology and related nuclear data libraries meet the desired accuracy (-1.0 pcm/°C) for the determination of this parameter for thermal reactors.