Spent Fuel Dissolution: An Examination of the Impacts of Alpha-Radiolysis
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29 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
iii)
the corrosion of steel surfaces inside the canister to produce iron oxide phases, dissolved Fe(II) and H2 (dissolved and gaseous). The bentonite surrounding the canister ensures that all transport is diffusion controlled.
EFFECTIVE YIELDS FROM SPENT-FUEL DISSOLUTION, ALPHA-RADIOLYSIS AND NATURAL-ANALOGUE STUDIES Spent Fuel Studies One approach to estimating Geff is to assume that alpha-radiolysis is the principal oxidant source in spent fuel dissolution experiments. This is likely to result in an overestimate of Geff because other sources of oxidants exist in such experiments, e.g., dissolved oxygen and 03-and yradiolysis of water. We begin by considering the experiments of Forsyth and Werme [2], performed under aerated conditions. The alpha dose rate at the fuel surface, d [rad s-1], together with the effective G value, gives the rate of production of radiolytic H20 2 per gram of irradiated water, P [mol a' g-1], via the expression: d x 6.25 x 10"3 eV rad-' x 3.15 x 107 s a' x 0.01 x Geff 6.02 x 1023 molecules mol' where 0.01 is the assumed maximum yield of H20 2 in molecules per eV. Assuming all of the H20 2 produced reacts with the fuel, the fractional dissolution rate that results is:
D = 238 g mol'x 10-9 tHg g-1p
M
(2)
.M
MUM
where M [g] is the mass of irradiated water and M/-M [tNM] is the mass of uranium in a sample. The experiments involved 16 g samples, which, assuming a typical specific surface area for spent fuel of 2 x 10-4 m2 g-1 [3], would have a geometric surface area of 3.2 x 10-3 m2. Although there are uncertainties in surface area, because water may penetrate narrow (1 to 2 nm) grain boundaries to some degree, this does not change the results significantly because the volume of irradiated water enclosed by the grain boundaries is so small that the total yield of oxidants is unaffected. Further assuming a 30 [tm c-particle range in water, the mass of irradiated water would be: M = 3.2 x 10-3 m2 x 30 x 10- 6 m x 10 6 g
m
3
=
0.096 g
16 g of fuel corresponds to a mass of uranium, M
MHM,
16gx238gmol-' x10- 9 tl g9-1.=4lX0-8 lM 270 g mol-'
(3) of: t
(4)
Substituting Eq. 1, Eq. 3 and Eq. 4 in Eq. 2, and rearranging:
= 1.89 x 104 D
(5) d The measured fractional dissolution rate, D, was 1.1 x 10-4 a-' for the aerated fuel dissolution experiment. The dose rate, d, for the fuel used in this experiment, at 15 years following Geff
30
unloading from a reactor (the age of the fuel in the original dissolution experiments of Forsyth and Werme [2]) is - 40 rad s-1 . Thus, according to this approach, the effective G value is: Geff=
".89x104)X x1. 10-4) 40
=0.05,
(6)
It is, however, known that the dissolution rate of U0 2 and spent fuel are very similar in such experiments, and therefore radiolysis cannot be the most important factor controlling the rate (the slightly higher dissolution rate of spent fuel may, however, be attributable to radiolysis, although 03-and y-radiolysis are likely to be more important t
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