Decay of Fission Products and Burnout Poisons

The purpose of the present section is to examine some of the problems associated with the estimation of the concentration of poisons in the reactor as a function of time for periods of operation which are compa- 

We will consider first the concentration of and determine its value as a function of time. Iodine 135 is a first-stage decay product of Te which is a direct fission product. The tellurium which appears from fission reactions decays by negative beta emission to form which in turn decays to Xe again by negative beta emission. The decay process is indicated below, along with the appropriate half-lives  

The yield of Te is 0.056 atom per fission. We note from the above relation that the decay rate of the Te is relatively fast so a reasonable approximation to this reaction would be to assume that appears as a direct fission product of yield 0.056 and to ignore entirely the presence of the tellurium. This approximation has some merit since the half-life of Te is short, relative to the time periods of interest here. 

On the basis of this assumption we can write the following balance equation for 1(0, the concentration  

2 0 = fissions per unit volume per unit time and 1(0 is given in nuclei per unit volume. It should be recognized that the cross section indicated in this equation must correspond to the neutron energy implied in the flux 4 . Inasmuch as the largest fission-fragment cross sections occur in the vicinity of thermal energies, we will confine our attention to the effects of these poisons on the thermal-neutron flux. Thus the cross section and those which follow refer to 

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