Verification of a bug in MCNP code and relative solutions

When using SDEF card in MCNP code to describe source information of fixed sources, it is necessary to input the distribution of the source. MCNP code offers SP card to input the distribution probability. The V option of SP card is for cell distributions only. Recent research discovers that there is a bug when using this option, which may lead to wrong calculation results. Making up a typical model, a comparison between the results using V option and not using V option was made. The comparison shows that the bug really exists. The effectiveness of the solutions proposed was also validated.