Radiation and Chemistry in Nuclear Waste: The NO_x System and Organic Aging

Abstract

We describe results that advance the understanding of radiation effects in high level waste (HLW) stored at DOE sites. The scientific issues on which we focus include: a) reactions of primary radicals (e -, OH, and H) of water radiolysis with NO3-/ NO2-b) redox chemistry of NO x radicals and ions, c) degradation mechanisms and kinetics of organic components of HLW, and d) interfacial radiolysis effects in aqueous suspensions and at crystalline NaNO 3 interfaces. Understanding these effects and the chemical reactions they induce have contributed to resolving safety issues and setting waste management guidelines at Hanford. Several safety issues concerning stored high level waste (HLW) at the Hanford Site have been identified prior to the start of the EMSP. The aging of organics in the waste had been shown to lead to several safety concerns such as flammable and noxious gases generation (H 2 , N 2 0, NH 3} volatile organic chemicals). Possible runaway reactions of organics (largely chelates) mixed with 342

Identification of the Non-Pertechnetate Species in Hanford Waste Tanks, Tc(I)−Carbonyl Complexes

“…be generated in high level waste as a result of the radiolytic decomposition of nitrate and nitrite. 22,23 The XANES spectra of Tc(CO) 2 (NO) and fac-Tc(CO) 3 would be similar. Given the similarity of the XANES spectra of the Tc(I) carbonyl complexes, assignment of the non-pertechnetate species to a particular species is not conclusive although Tc(CO) 3 (gluconate) 2-provides an excellent fit to the observed XANES spectrum of the non-pertechnetate species.…”

The Chemistry of Flammable Gas Generation

“…These reactions, of course, have high activation energies. Meisel et al (1991aMeisel et al ( , 1991bMeisel et al ( , 1993Meisel et al ( ,2000 observed that gas generation continued after the radiolysis of the waste had been stopped and concluded that chemical reactions continued as the unstable organic and inorganic intermediates formed in the radiolytic reaction decomposed. Work in other laboratories has confirmed this concept and demonstrated that the radiolytic processes are synergistically coupled with other chemical reactions to fragment and oxidize the original complexants and solvents.…”

“…The quantitative kinetic reaction model has been extended to include evaluation of the rate of oxidation of formate and glycinate ions Meisel et al 2000). The chemical reactions are shown in Figure 3-6.…”

“…However, the yields of hydrogen atoms from the radiolysis of the sodium nitrate-and sodium nitrite-rich Hanford Site wastes are small because hydrogen atoms are very rapidly scavenged in reactions with the inorganic constituents, as shown by the kinetic reaction model shown in Section 2.2 and by empirical expressions of Tabata et al (1991) and Hu (2000). Consequently, Camaioni et al (1998) and Meisel et al (1997Meisel et al ( ,2000 have concluded that little hydrogen is formed by this reaction path. Rather, they attribute hydrogen gas formation in alkaline aqueous solution to base-catalyzed ionic reactions of formaldehyde and other aldehydes as originally proposed by the group at the Georgia Institute of Technology (Ashby et al 1993).…”

“…Ammonia is produced from the nitrogen-containing complexants and from the inorganic oxidants. These conclusions are based upon studies of the chemical behavior of the complexants in well-defined reactions conducted by university and national laboratory teams and upon targeted investigations of the processes governing gas formation in complex simulants and very heterogeneous wastes (Ashby et al 1993(Ashby et al , 1994Barefield et al 1995Barefield et al , 1996Bryan et al 1996Bryan et al , 1997Camaioni and Autrey 2000;Delegard 1980Delegard , 1987Johnson et al 1997;King et al 1997;King and Bryan 1999;Meisel et al 1991aMeisel et al , 1991bMeisel et al , 1993Meisel et al , 1997Meisel et al ,2000Person 1996Person , 1998. Observations that provide insight into the factors governing gas-forming reactions during interim storage and operations are examined in this section.…”