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When a certain void fraction value is reached in the two-phase flow regime, a problem occurs in the COBRA-EN code. This problem was observed in the drift-flux model option and interrupts code execution. Two solutions are proposed to solve the problem.

The computer codes presently utilized for spectrum average cross section calculations are discussed. A cost/benefit analysis of the NJOY system, the PREPRO2000 codes and the INTER code is presented. Considering the criteria of cost, availability, versatility and completeness the PREPRO2000 codes are considered the best option for researchers in developing countries involved with spectrum averaged cross section calculations.

It was verified after a fuel burnup calculation with the WIMSD-5b code using the IAEA.LIB library that the computed average number of neutrons released per fission of Pu94242 shows up as a Not-a-Number (NaN) for some energy groups. As this problem does not permit the use of the generated multigroup microscopic cross sections by a reactor calculation code, the value of 1.0E−38 barns was attributed to all energy groups of the IAEA.LIB library that have null values of multigroup microscopic fission cross sections for this material.