Effects of alpha and gamma radiation on glass reaction in an unsaturated environment Page: 3 of 9
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At the time of glass production, the dose rate of gamma radiation from
a container of glass will be approximately 8 x 103 rad/h . The gamma
field will decrease by more than three orders of magnitude during the first
1000 years of storage. Thus, the effects of gamma radiation on glass
performance will be important only under unanticipated conditions of
premature canister breach and water ingress. The glass will also contain
long-lived alpha-producing actinides such as Pu-239 and Pu-240. The
initial concentrations of these two radionuclides are 3.48 x 10-3 and
2.34 x 10-3 Ci/lb glass, and the half lives are 2400 and 6580 years,
respectively . Due to their longer half lives, the radiation field
produced by alpha-emitters is expected to dominate during the later stages
of the service life of the repository.
Only alpha particles produced within approximately 10-4 cm of the
glass surface may escape from the waste and contribute to radiolysis
reactions. Using the actinide concentration data of Aines for a
1000-year-old glass waste form , an alpha dose rate of 20 rad/h is
expected to be emitted and absorbed within a 4 cm air layer above the
As indicated earlier, the greatest potential for radiation to affect
glass reaction is through the formation of radiolytic products in the gas
phase followed by dissolution of these species in a thin film of liquid
water contacting the glass. Experimental evidence obtained during gamma
irradiation of a two-phase air/water system indicates a reaction efficiency
(G value) of -2 molecules of NO3- produced for each 100 eV of energy
absorbed . Using this G value, the G/L ratio, the dose rate, and the
concentration of N2 in the gas phase, the amount of nitric acid produced in
the system can be calculated by using an equation presented by Burns et al.
Under long term storage conditions, after most of the gamma-producing
nuclides have decayed, alpha radiolysis may result in continued production
of nitric acid. Any alpha particles escaping the glass would also escape
into the air if the thin film of water adsorbed on the glass is less than
4 x 10-3 cm in thickness. The HNO3 production by alpha particles in moist
air is expected to be the same as for gamma radiation , although there
is a lack of high G/L ratio experimental data to support this theory.
EXPERIMENTAL CONFIGURATION, RESULTS, AND DISCUSSION
Experiments were performed to demonstrate that under anticipated long-
term unsaturated conditions, where dose rates are less than those examined
by Burns but the G/L ratios are far greater, whether measurable concentra-
tions of radiolytic products can be detected in solution. These initial
tests were conducted with no glass present. If radiolytic products are not
concentrated in the small volumes of liquid present, then it is unlikely
that radiation will affect glass performance and further tests involving
glass will be unnecessary.
Alpha Blank Tests
Blank tests (no glass present) were performed to study the alpha
radiolysis of a moist air system at room temperature. Each test was
performed in duplicate using two sample assembly rod configurations. For
both configurations the source of alpha particles was a 1500 pCi 241Am foil
with 4.6 MeV energy/alpha decay. In initial tests, the foil was cemented
to a lucite rod (Figure 1), while in later experiments it was attached to a
type 304 stainless steel rod to eliminate potential sources of organic
carbon. The support rod was attached to the underside of a thin metal
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Wronkiewicz, D.J.; Young, J.E. & Bates, J.K. Effects of alpha and gamma radiation on glass reaction in an unsaturated environment, article, December 31, 1990; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc621952/m1/3/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.