Interfacial radiolysis effects in tank waste speciation. 1997 annual progress report
'The purpose of this program is to deliver pertinent, fundamental information that can be used to make technically defensible decisions on safety issues and processing strategies associated with mixed chemical and radioactive waste cleanup. In particular, an understanding of radiolysis in mixed-phase systems typical of U. Department of Energy (DOE) heterogeneous, radioactive/chemical wastes will be established. This is an important scientific concern with respect to understanding tank waste chemistry issues; it has received relatively little attention. The importance of understanding solid-state radiolysis, secondary electron interactions, charge-transfer dynamics, and the general effect of heterogeneous solids (interface and particulate surface chemistry) on tank waste radiation processes will be demonstrated. In particular, the author will investigate (i) the role of solid-state and interfacial radiolysis in the generation of gases, (ii) the mechanisms of organic compound degradation, (iii) scientific issues underlying safe interim storage, and (iv) the effects of colloid surface-chemical properties on waste chemistry. Controlled radiolysis studies of NaNO{sub 3} solids and SiO{sub 2} particles were carried out using pulsed, low- (5--150 eV) and high- (3 MeV) energy electron-beams at Pacific Northwest National Laboratory (PNNL) and at Argonne National Laboratory (ANL), respectively. The pulsed, low-energy electron beams probe the inelastic scattering and secondary cascading effects produced by high-energy beta and gamma particles. Pulsed radiolysis allows time-resolved measurements of the high-energy processes induced by these particles. Using low-energy (10--75 eV) electron-beam irradiation of nominally dry NaNO{sub 3} solution-grown and melt-grown single crystals, they observed H{sup +}, Na{sup +}, O{sup +}, NO{sup +}, NO, NO{sub 2}, O{sub 2}, and O({sup 3}P) desorption signals. The threshold measurements and yields indicate that the degradation proceeds mainly via destruction of the nitrate moiety. The H{sup +} and Na{sup +} yields are primarily related to the presence of water and Na metal, Na hydrides and oxides, or other defect sites on the salt surface. The water is due to diffusion from the bulk of solution-grown crystals and controlled water adsorption on melt-grown crystals. The metallization and/or metal hydride/oxide build-up is a result of the very large electron-beam degradation cross-section (> 10--16 cm{sup 2}) of NaNO{sub 3}. The build-up of alkali-metal colloids during the irradiation of alkali-halide materials is well known and is expected for other alkalai salts such as NaNO{sub 3} . Figure 1 shows the Na{sup +} desorption yield as a function of incident electron energy. The signal below the 33 eV is all due to Na buildup and the break seems to be associated with charge build-up, band-bending, and then charge release. Charge trapping and metallization is reduced at temperatures above 420 K, 1,3 a temperature higher than typically found in high-level liquid waste (HLLW) tanks.'
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE Office of Environmental Management (EM), Office of Science and Risk Policy
- OSTI ID:
- 13628
- Report Number(s):
- EMSP-54646-97; ON: DE00013628
- Country of Publication:
- United States
- Language:
- English
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