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4.6 Criticality safety
Typical LWR fuel contains some 3 – 4% of fissile material ( 235 U) before irradiation and about
1.5% fissile material ( 235 U plus 239 Pu and 241 Pu) after a burn-up of 40 GWd/tHM. Fuel from other
thermal power reactors has similar fissile contents. One exception is natural uranium fuel (e.g. from
HWR) where the fissile content of spent fuel is of the order 0.5% or less.
The used fuel is proposed to be encapsulated in canisters with a content of a few tonnes heavy
metal before disposal. This means that one canister with spent enriched nuclear fuel will contain more
fissile material than the theoretical minimum critical mass. A whole repository will contain many times
more. The disposal of spent nuclear fuel thus means that the potential for an unintended critical
configuration of the fissile material has to be addressed in the safety analysis.
Two main cases have to be considered:
• potential risk for criticality of the fuel as deposited in the canister;
• reconfiguration of the fissile material to a critical configuration, e.g. by selective leakage
and precipitation of fissile elements.
The fuel will (in many proposed cases) be encapsulated in the same geometrical configuration
as used in the reactor, (i.e. close to optimum from the reactor physics point of view), the temperature in
the repository is ambient (much lower than in a reactor), and the short-lived strongly neutron-absorbing
radionuclides such as 135 Xe have decayed. This means that criticality is conceivable with only a limited
number of fuel assemblies if a moderating material is added. A spent fuel canister must therefore be
designed in such a way that criticality is not achieved even if the canister is filled with fuel that for some
reason has not reached full burn-up and starts taking in groundwater from the surrounding repository
formation. In practice this is achieved by rearranging the fuel in a non-critical configuration, by having
strict administrative control of the fuel burn-up and other important parameters in the encapsulation
plant, and/or by mixing the fuel with some neutron-absorbing material, e.g. an insert with walls between
each fuel assembly [195]. In the design and safety analysis several real or hypothetical phenomena must
be accounted for, such as long-term reactivity changes, any dissolution of neutron absorbing material or
any reconfiguration of fuel rods.
The issue of reconfiguration of the fissile material was addressed in the 1970s [196] and has
recently been revisited [197,198]. The issue has two aspects: (1) the probability that some process will
rearrange the fissile material into a configuration that might develop and sustain a neutron chain
reaction, and (2) the probable or possible consequences of such a chain reaction if it should occur. The
early studies of a repository in granitic bedrock [196] concluded that the reconfiguration of plutonium
from several canisters is an extremely unlikely event owing to the very slow chemical processes in the
prevailing geochemical environment as compared with the half-lives of fissile plutonium.
Reconfiguration of uranium from LWR-fuel is also very unlikely because (owing to the low 235 U
content) several tonnes of uranium must be assembled in proper concentration and configuration.
Criticality with uranium could, for geometrical reasons, only occur in the backfilled excavations of the
repository, where re-concentration by absorption is possible, and would require the movement of
uranium from several canisters to the same spot during a very long time. The consequence of a
hypothetical criticality in plutonium or uranium from spent LWR fuel was furthermore judged to be
very limited.
Some analyses [197] point out that fissile material in certain concentrations and
configurations could pose a risk of divergent neutron chain reactions (self-sustained criticality) with
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