RD&D-Programme 2004 - SKB

15 Fuel
The spent fuel that will be emplaced (deposited) in the repository comes from nuclear power
plants. For an alternative with 40 years of reactor operation, the quantity of BWR fuel is
estimated to be 7,200 tonnes and the quantity of PWR fuel 2,300 tonnes /15-1/. In addition,
23 tonnes of Mox fuel, 20 tonnes of fuel from the Ågesta reactor and some residues from fuel
investigations at Studsvik will be deposited. The fuel’s burnup can vary from approximately
15 to 60 MWd/kgU. Differences in radionuclide content between PWR and BWR fuel are
marginal viewed from a safety assessment perspective. Mox fuel has a higher decay heat than
uranium fuel, which means that less fuel can be deposited in each canister. The differences
between different fuel types are more important when it comes to assessing criticality. The
assessments are therefore made with the fuel types that are least favourable with respect to
criticality.
15.1 Initial state in fuel/cavity
15.1.1 Variables
For the safety assessment, the fuel is described by means of a set of variables which together
characterize the fuel in a suitable manner for the assessment. The description applies not only
to the fuel itself, but also the cavities in the canister, into which water can penetrate if there is
a defect in the copper canister. Processes such as fuel dissolution and corrosion of the cast iron
insert will then take place in the cavity. The cavities could thus be included in either the fuel
or the canister subsystem, and have been included in the fuel here. The variables are defined in
Table 15-1.
The initial state, i.e. the value these variables were assumed to have at the time of deposition,
was described in the main report of SR 97, section 9.2 /15-2/. The research programme around
the initial state for the different variables in the fuel is described in the following.
Table 15-1. Variables in the fuel.
Variable
Geometry
Radiation intensity
Temperature
Hydrovariables
Mechanical stresses
Total radionuclide inventory
Gap inventory
Material composition
Water composition
Gas composition
Definition
Geometric dimensions of all components of the fuel assembly, such as fuel pellets and
Zircaloy cladding. Also includes the detailed geometry, including cracking, of the fuel
pellets.
Intensity of α, β, γ and neutron radiation as a function of time and space in the fuel
assembly.
Temperature as a function of time and space in the fuel assembly.
Flows and pressures for water and gas as a function of time and space in the cavities in
the fuel and the canister.
Mechanical stresses as a function of time and space in the fuel assembly.
Total occurrence of radionuclides as a function of time and space in the different parts
of the fuel assembly.
Occurrence of radionuclides as a function of time and space in the gap and grain
boundaries.
The materials of which the different components in the fuel assembly are composed,
excluding radionuclides.
Composition of water (including any radionuclides and dissolved gases)in the cavities in
the fuel and canister.
Composition of gas (including any radionuclides) in the cavities in the fuel and canister.
RD&D-Programme 2004 163