Determination of Core Burnup, Isotopic Inventory, and Conversion Ratio from Selective Sampling of Spent Reactor Fuel

The determination of the isotopic composition and the fission-product inventories of a spent reactor core is demonstrated through the proper selection of sampling points and analytical treatment of data using Core I of the Yankee reactor as an example. This core is found to contain 172 kg of ²³⁵U less than initially loaded, plus 97.0 kg of freshly generated Pu. Mass balances of U and Pu isotopes and the fission products are used to demonstrate the various possible ways of defining the end-of-life conversion ratio, with the preferred definition giving a value of 0.50 for the Yankee core. Methods of determining the total burnup from U and Pu concentrations, from ¹³⁷Cs activity, and from plant calorimetrics are discussed and applied to the Yankee data and give 8.40 ± 0.21 GWD/MTU for the core average burnup.