Investigation of irradiated soil byproducts

The high dose irradiation of windblown soil deposited onto the surface of s pent nuclear fuel is of concern to long-term fuel storage stability. Such s oils could be exposed to radiation fields as great as 1.08 x 10(-3) C/kg-s (15,000 R/hr) during the 40-year anticipated period of interim dry storage prior to placement at the proposed national repository. The total absorbed dose in these cases could be as high as 5 x 10(7) Gy (5 x 10(9) rads). This investigation evaluated the potential generation of explosive or combustib le irradiation byproducts during this irradiation. It focuses on the produc tion of radiolytic byproducts generated within the pore water of surrogate clays that are consistent with those found on the Idaho National Engineerin g and Environmental Laboratory. Synthesized surrogates of localized soils c ontaining combinations of clay, water, and aluminum samples, enclosed withi n a stainless steel vessel were irradiated and the quantities of the byprod ucts generated measured. Two types of clays, varying primarily in the prese nce of iron oxide, were investigated. Two treatment levels of irradiation a nd a control were investigated. An 18-Mev linear accelerator was used to ir radiate samples. The first irradiation level provided an absorbed dose of 3 .9x10(5)+/-1.4x10(5)Gy (3.9x10(7)+/-1.4x10(7) rads) in a 3-h period. At the second irradiation level, 4.8x10(5)+/-2.0x10(5)Gy (4.8x10(7)+/-2.0x10(7) r ads) were delivered in a 6-h period. When averaged over all treatment param eters, irradiated clay samples with and without iron (III) oxide (moisture content=40%) had a production rate of hydrogen gas that was a strong functi on of radiation-dose. A g-value of 5.61x10(-9)+/-1.56x10(-9) mol/J (0.054 /-0.015 molecules/100-eV) per mass of pore water was observed in the clay s amples without iron (III) oxide for hydrogen gas production. A g-value of 1 .07 x 10-8f2.91x10(-9) mol/J (0.103 +/-0.028 molecules/100-eV) per mass of pore water was observed in the iron (III) oxide containing clay samples for hydrogen gas production. This value was noticeably larger when the samples were spiked with both KCl and KNO3 salts. The ratio of oxygen to nitrogen gas was observed to increase as a function of absorbed dose particularly in the presence of both KCI and KNO3 salts. The creation of radiolytic byprod ucts produced an observable but small increase in headspace pressure. Tempe rature increases during irradiation were not observed. Additionally, KCl an d KNO3 salts added to the clays enhanced nitrite production as a function o f radiation-dose and the type of clay considered. The addition of aluminum to these samples had no statistically discernable impact at the alpha = 0.0 5 level. Generation of the irradiation products, hydrogen peroxide and hydr ogen gas also depended upon the type of clay irradiated and the presence of both KCI and KNO3 salts and the total dose received. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.