TORT, an SN 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 keff 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 keff 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.