Technical Consideration of the Use of Nuclear Fuel Spikants for Proliferation Deterrence

The use of a gamma active radionuclide with nuclear fuel has been proposed as a way to inhibit unauthorized diversion of the fuel and thus provide proliferation deterrence. Proposed dose rate ranges have varied from small additions to increase detectability of diverted material up to large additions to provide lethal doses in a relatively short exposure time. Some of the practical aspects of incorporating spikants into nuclear fuel are examined in an attempt to identify any technically adverse consequences of their use. Selection of potential spikants was made by the application of some somewhat arbitrary radiation criteria to 64 candidate spikants followed by an analysis of the chemical and physical state of each potential spikant. As a result of this analysis, the list of candidates was narrowed to ⁶⁰Co, ¹⁰⁶Ru, and ¹⁴⁴Ce. Following this, we investigated the practical aspects of the use of these three spikants in nuclear fuel. Among the subjects considered are dose rates available from fuel elements, fission product buildup, chemical behavior of spikants during reprocessing, and possible effects of spikants on refabrication and on the fuel properties. Neither ¹⁰⁶Ru nor ¹⁴⁴Ce is present in sufficient quantity to produce the maximum radiation dose rate level considered. Nonradioactive nuclides of ruthenium and cerium dilute the radioactive nuclides to 2 to 4% of the total element in the fission products 2 yr after removal from the reactor. Recycling ruthenium and cerium will result in dilution of the radionuclides even further by a buildup of stable isotopes of each of these elements. Approximately 50% of the fission product ruthenium and 3 to 5% of the cerium can be coprocessed with the fuel, while cobalt cannot be coprocessed at all. No single radionuclide was found to be preferred in all stages of reprocessing and refabrication. To provide deterrence in all stages of reprocessing and refabrication, a duplex spiking process appears necessary, in which two different spikants, ¹⁰⁶Ru and ⁶⁰Co, are used in different portions of reprocessing. The use of nominal amounts of ruthenium or cobalt as spikants is not expected to adversely affect fuel performance.