Tritium Distribution and Capture Requirements for UNF Reprocessing
- Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6152 (United States)
Four radionuclides have been identified as being sufficiently volatile in reprocessing of nuclear fuel that their gaseous release needs to be controlled to meet U.S. regulatory requirements. These radionuclides are {sup 3}H, {sup 14}C, {sup 85}Kr, and {sup 129}I. Previous studies have discussed the need for treating off-gas streams to control {sup 3}H, {sup 14}C, {sup 85}Kr, and {sup 129}I from the reprocessing of used nuclear fuel (UNF) and the regulations that set the limits for releases from such a facility. These regulations ultimately prescribe the decontamination factor (DF) that must be met in the facility. Jubin et al. examined the role of off-gas pathways since the gaseous radionuclides can partition to different reprocessing off-gas streams. Based on this report and subsequent analysis, the required DF for {sup 129}I could be 1000 to 3000. The required DF for {sup 3}H could be as high as 200, depending on the age of the fuel processed. The DF for {sup 85}Kr could be up to ∼35, also depending on fuel age. The required DF for {sup 14}C is 1 in many cases (no treatment required) but could be higher in specific cases. While iodine has been the focus of a number of studies due to the high DF requirements on multiple escape pathways, it is clear from the studies noted above that the control of tritium will also be a significant issue for future reprocessing plants, especially if processing relatively short-cooled fuels (<10 yr post reactor discharge). Tritium contained in a UNF matrix will be released into the liquid and off-gas streams during fuel reprocessing. The amount released into the process streams is a function of the type and burnup of the fuel processed, and the chemical processes employed within the reprocessing plant. Previous studies have examined the tritium capture requirements for the reprocessing facility assuming that all of the tritium is volatilized in the head-end operations, i.e., tritium pretreatment is employed and tritium is completely released from the cladding. This is clearly a worst case scenario from the standpoint of the amount of tritium that could be released, but the scenario tends to limit the release pathway to the head-end off-gas stream. In this analysis, the volume of HTO and the HTO to H{sub 2}O ratio for each of the three plant-wide tritium DFs were determined. The overall amount of HTO that had to be captured from the off-gas streams was virtually the same for the three DF cases evaluated; 98%, 99% and 99.5% of the tritium in the fuel entering the plant needed to be recovered for overall plant DFs of 50, 100 and 200, respectively. The use of the 'tritium barrier' to limit the spread of HTO from the head-end systems to the balance of the plant results in an increase in tritiated waste concentration in the head-end recycle acid systems which results in a decrease in the total amount of HTO + H{sub 2}O that must recovered from the gaseous effluent from the plant. The use of the 'tritium barrier' decreases the waste volume due to the higher HTO:H{sub 2}O ratio in the treated off-gas stream. The results also show that increasing the acid recycle percentage increases the concentration of HTO within the UNF reprocessing plant. This is then manifested in less HTO + H{sub 2}O that must be recovered from the gaseous effluents. Finally, the application of tritium recovery on the recycle acid reduces the HTO concentration in the recycle acid stream and generates a separate HTO + H{sub 2}O stream that must be managed. However, the resulting volume of tritium-containing waste generated plant-wide is a function of the selectivity of the tritium recovery system and the ability of the tritium recovery system to limit the accumulation of HTO in the recycle acid such that the HTO + H{sub 2}O recovered from gaseous effluents does not contribute significantly to the overall tritium waste volume. (authors)
- OSTI ID:
- 22991827
- Journal Information:
- Transactions of the American Nuclear Society, Journal Name: Transactions of the American Nuclear Society Journal Issue: 1 Vol. 114; ISSN 0003-018X
- Country of Publication:
- United States
- Language:
- English
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