RD&D-Programme 2004 - SKB

0.5 MPa (H 2 +0.03% CO 2 )
0.5 MPa Ar
100 Mo
0.1
129 I
Released fraction
0.01
10 —3
99 Tc
137 Cs
90 Sr
10 —4
10 —5
0 100 200 300 400 500
Leaching time, days
Figure 15-4. Released fraction of the fission products Sr, Cs, I, Tc and Mo as a function of the
time for leaching of spent fuel in powder form in a 10 mM NaCl, 2 mM NaHCO 3 solution with
either 0.5 MPa hydrogen mixed with 0.03 percent carbon dioxide or only 0.5 MPa argon.
It is difficult to determine the relative importance of these factors or their combined effects
solely on the basis of tests with spent fuel in hydrogen-saturated solutions. Conclusions can
only be drawn regarding consumption of the oxidants produced by radiolysis of water by
reaction with dissolved hydrogen or with the canister surface. In order to separate and quantify
the contributions to the chemical activation of the hydrogen, it is necessary to study separately
the effect of clean surfaces of depleted uranium dioxide or metallic particles in hydrogen
solutions, and – in separate studies – the effect of different types of radiation in hydrogen
solutions. Some new results from such studies in the literature and from the experimental
programme are discussed below.
Recently conducted studies of water radiolysis by gamma radiation show that at hydrogen
concentrations above a given level, no measurable quantities of oxidants such as hydrogen
peroxide occur in the solution /15-22, 15-23/. This is explained by the fact that hydrogen
molecules participate in reactions with strongly oxidizing radicals such as OH. The effect of
alpha radiation on hydrogen activation in solution is expected to be less, but almost no effect
of hydrogen is noted in experimental data from /15-23/.
With the aid of radiolysis models, it is possible to calculate a decrease in oxidant production
in hydrogen-saturated solutions caused by mixed radiation (α, β, γ) /15-24, 15-25, 15-26/.
Even without taking into account the influence of surfaces, very low fuel dissolution rates are
obtained. A modelling study where the presence of iron and the influence of fuel surfaces on
uranium reduction are taken into account shows a very low fuel dissolution /15-27/.
The effect of hydrogen which is noted in the experimental studies is difficult to explain solely
by reference to a reduction of the concentrations of radiolysis oxidants due to reactions with
dissolved hydrogen. In order to explain e.g. reduction of radionuclides during fuel dissolution,
oxygen concentrations below the detection limit after long periods of time and a reduction of the
uranium dioxide surface after tests with gamma radiolysis /15-28/ or alpha radiolysis /15-29/ in
hydrogen solutions, it may also be necessary to take into account the influence of surfaces. The
results of a study of the effect of different types of radiation on water sorbed on uranium dioxide
surfaces /15-30/ indicate that the surfaces greatly influence the radiolysis of sorbed water. The
new knowledge regarding the combined effect of surfaces and radiation may contribute to a
better understanding of the processes and a better modelling of experimental data.
RD&D-Programme 2004 175