A formal mathematical description of the time dependence of activity in the maln coolant loop of a pressurized water reactor resulting from the discontinuous release of core contalned fission products is developed. The description accounts for the deposition, re-solution, and demineralization of activities due to one, two, and three member mass chnins- A description of the IBM-650 WOLONTIS Code No. S0168, which numerically evaluates the various activities discussed, is given in the appendix along with the input data and results for a pair of exemplifying problems. (auth).