Soluble neutron poisons assure criticality control in two of the headend fuel reprocessing systems at the Idaho Chemical Processing Plant. Soluble poisons have been used successfully since 1964 and will be employed in the projected new headend processes. The use of soluble poisons (1) greatly increases the process output (2) allows versatility in the size of fuel assemblies processed and (3) allows the practical reprocessing of some fuels. The safety limit for all fluids entering the U-Zr alloy dissolver is 3.6 g/liter boron. To allow for possible deviations in the measurement systems and drift between analytical sampling periods, the standard practice is to use 3.85 g/liter boron as the lower limit. This dissolver has had 4000 successful hours of operation using soluble poisons. The electrolytic dissolution process depends on soluble gadolinium for criticality safety. This system is used to process high enriched uranium clad in stainless steel. Electrolytic dissolution takes advantage of the anodic corrosion that occurs when a large electrical current is passed through the fuel elements in a corrosive environment. Three control methods are used on each headend system. First, the poison is mixed according to standard operating procedures and the measurements are affirmed by the operator's supervisor. Second, the poisoned solution is stirred, sampled, analyzed, and the analysis reported while still in the mix tank. Finally, a Nuclear Poison Detection System (NPDS) must show an acceptable poison concentration before the solution can be transferred. The major disadvantage of using soluble poisons is the need for very sophisticated control systems and procedures, which require extensive checkout. The need for a poisoned primary heating and cooling system means a secondary system is needed as well. Experience has shown, however, that production enhancement more than makes up for the problems.

Studies indicated that the use of soluble poisons as a primary criticality control offers economic and other advantages in that it permits the factors of vessel size and shape and solution concentrations to be dictated by considerations other than those of criticality. It is believed that soluble poison criticality control can be made as reliable as other methods of coaditional control if the application is preceded by adequate development work and is monitored by multiple. independent safeguards. The studies included multigroup machine calculations of the required content of poisons in solutions of fissile and fertile material, a compilation of data on the detection, stability, decontamination, and costs of soluble poisons, and an assessment of the possible effects of a nuclear excursion. (auth).

Soluble neutron absorbers are proposed for criticality safety control in future processes at the Idaho Chemical Processing Plant. Solutions of neutron poisons have been used in the past for criticality control in processing various reactor fuels. No problems were encountered in the safe use of the neutron poisons although dissolution of different types of fuel occasionally required reevaluation of the poison concentrations. Proposed plans include the uses of soluble neutron poisons in the Rover fuel dissolver, the Fluorinel dissolver, and in increased concentrations in the electrolytic dissolver. These proposals are presented and the criticality safety aspects are discussed. The criticality safety of the Rover Fuels Processing Facility is assured by means of engineering design, soluble nuclear poison (boron), and administrative controls. Accumulation of a critical mass in the Fluorinel dissolver is prevented by positive identification of fuel units, administrative controls, procedures, and design of equipment to preclude double batching. The electrolytic dissolution facility is an existing facility at the ICPP for dissolution of stainless steel fuels. Gadolinium is used as a soluble neutron absorber in the nitric acid dissolving reagent and the cooling system. Stainless steel fuels planned for processing in the future will require reevaluation and adjustment of the gadolinium concentration to retain adequate criticality safety. Equipment design, administrative controls, sampling, and procedures are used to assure criticality safety.

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