Monte Carlo calculation has come to be used as reference solutions instead of experiments in nuclear design code validation and verification (V&V), although comparisons with measurements are still indispensable for V&V in nuclear design. MCNP [1] is one of the most famous Monte Carlo codes widely used in the world. Many reference results are given for the analyses of critical experiments. When using the use MCNP calculations for validations of commercial design codes, we will face to a problem of lacking temperature dependent cross-sections. The cross-sections can be generated by the NJOY code [2]. However, if the model has complex temperature distribution, many NJOY calculations are necessary. Besides, if the temperature profile changes with fuel power and so on, many NJOY calculations have to be performed again and again. These back and forth procedures make us give up using MCNP for commercial LWR calculations. In order to solve this problem, we propose an easy approximation to solve the temperature problems using MCNP. Note that our technique does not require any code modifications.

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