The pinching effect in a co-decontamination extraction process is investigated with much concern for criticality safety control. To predict the pinching effect, computer codes, such as PULCO, are used to make numerical simulations. Using computer codes for criticality safety control seems to be impractical, however, because some uncertainties are inevitably associated with the calculation due to the assumptions that are included in a simulation code; thus, a safety margin must be taken into account in designing extraction equipment. A new model for inferring pinching effects is proposed. It is based on knowledge that represents the intrinsic nature of the pinching effect and a co-decontamination process holding independent of process conditions. The predictions obtained from this model are conservative, but practical from the standpoint of criticality safety control. The margin in designing equipment can be reduced if the overall reliability of a measurement system in which this model is to be incorporated is high enough to predict pinching effects. The program of this model is written in logic programming language, C-Prolog.