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Isotope Ratio Method Analysis of the Ford Nuclear Reactor

The Isotope Ratio Method (IRM) is a technique for estimating the energy production in a fission reactor by measuring isotope ratios in non-fuel reactor components. The isotope ratios in these components can then be directly related to the cumulative energy production with standard reactor modeling methods. All reactor materials contain trace elemental impurities at parts per million levels, and the isotopes of these elements are transmuted by neutron irradiation in a predictable manner. While measuring the change in a particular isotope’s concentration is possible, it is difficult to correlate to energy production because the initial concentration of that element may not be known. However, if the ratio of two isotopes of the same element can be measured, the energy production can then be determined without knowing the absolute concentration of that impurity since the initial natural ratio is known. This is the fundamental principle underlying the IRM. Extremely sensitive mass-spectrometric methods are currently available that allow accurate measurements of the impurity isotope ratios in a sample. Additionally, indicator elements with stable activation products have been identified so that their post-irradiation isotope ratios remain constant. This method has been successfully demonstrated on graphite-moderated reactors. Graphite reactors are particularly well-suited to such analyses since the graphite moderator is resident in the active core for the entire period of operation. Applying this method to other reactor types is more difficult since the resident portions of the reactor structure available for sampling are typically outside the active core. The goal of this research is to evaluate whether the IRM can produce meaningful results for water-moderated research reactors. In this paper, we use the IRM to estimate the energy production of the University of Michigan’s Ford Nuclear Reactor (FNR) from 1992 until its shutdown in 2003. The FNR is a 2MW, swimming-pool reactor that is currently being decommissioned. In 1992, several ex-core structural components were added to support a large irradiation program. We have taken samples from some of these components, analyzed them and produced an estimate of the total energy production. Our preliminary results are in good agreement with the actual operating history of the reactor during that time period.