RD&D-Programme 2004 - SKB

15.1.11 Gas composition
Conclusions in RD&D 2001 and its review
RD&D 2001 did not include any research programme for this area, and no direct viewpoints
were offered in the review.
Newfound knowledge since RD&D 2001
The largest quantity of water in a canister reported in previous research programmes was
50 grams. This may be an optimistic figure, and 600 grams has now been given as a realistic
upper limit.
Programme
The consequences of more water entering the canister must be studied. By filling the canister
with argon (section 8.1.1), the gas composition inside the canister can be changed from air and
water vapour to an inert gas and water vapour, see also section 15.2.9.
15.2 Processes in fuel/cavity
A number of processes will with time alter the state in the fuel and in the canister’s cavity.
Some take place in any circumstances, while many others only occur if the isolation of the
copper canister is breached and water enters the canister.
15.2.1 Overview of processes
The radionuclides in the fuel will eventually be transformed into non-radioactive substances by
radioactive decay. This process gives rise to alpha, beta, gamma and neutron radiation which,
by interaction with the fuel itself and with surrounding materials, is attenuated and converted to
thermal energy. The temperature in the fuel is changed by heat transport in the form of conduction
and radiation, and heat is removed to the surroundings. The temperature change will lead
to some thermal expansion of the fuel’s constituents. This can, in combination with the helium
formation caused by the alpha radiation, lead to rupture of the cladding tubes in the fuel.
In an intact canister, radiolysis of residual gases in the cavity will lead to the formation of small
quantities of corrosive gases, which could contribute to stress corrosion cracking (SCC) of the
cast iron insert.
If the copper canister is not intact, water may enter the canister cavity, radically altering the
chemical environment. Radiolysis of the water in the cavity will further alter the chemical
environment. The water in the canister causes corrosion of cladding tubes and other metal parts
in the fuel. If the cladding tubes’ isolation should be breached initially or later by corrosion
or mechanical stresses, the fuel will come into contact with water. This leads to dissolution
of radionuclides that have collected on the surface of the fuel matrix, and to dissolution or
transformation of the fuel matrix and release of radionuclides. The radionuclides may either
be dissolved in the water, rendering them accessible for transport, or precipitate in solid phases
in the canister void. This is determined by the chemical conditions in the canister cavity. On
dissolution of the fuel, colloids with radionuclides may also form.
Radionuclides dissolved in water can be transported with mobile water in the canister (advection)
or by diffusion in stagnant water. Colloids carrying radionuclides can be transported in the
same way. Nuclides dissolved in water can be sorbed to the different materials in the canister.
Certain nuclides can also be transported in the gas phase.
Finally, water can attenuate the energy of neutrons in the canister cavity. Low-energy neutrons
can subsequently cause fission of certain nuclides in the fuel, releasing more neutrons. If conditions
are unfavourable, criticality may be achieved, i.e. the process becomes self-sustaining.
168 RD&D-Programme 2004