COMPLETE DOCUMENT (1862 kb) - OECD Nuclear Energy Agency

Figure II.30 Overview of the canister
The steel insert is cast with thick steel walls between fuel assemblies. This gives a good
mechanical stability besides providing adequate protection against criticality in the unlikely event that
the canister at some unspecified future time should be filled with water.
The fabrication of copper canisters of the size needed is by no means an industrially available
technology. The seal welding technology has been demonstrated on a laboratory scale in work sponsored
by SKB at The Welding Institute in UK. Full-size canisters have also been fabricated on a trial scale. In
order to develop the key technology, SKB operates a laboratory for encapsulation technology at
Oskarshamn. This laboratory will be ready in 1998 and will primarily be devoted to further
development of the seal-welding process and of the NDE-methods.
4.5.1.3 Performance assessment of spent fuel disposal in clay [190]
Taking into account the uncertainty about the choice of reprocessing as a fuel cycle step, a
study was recently (1996) undertaken by the nuclear sector in Belgium to determine the impact of
5000 tHM spent fuel (consisting of a mixture of UO 2 and MOX fuel with burn-ups ranging from 33
to 45 GWd/tHM) on the Boom Clay repository environment (see next paragraphs describing the RFC
scenarios) in Belgium. The result of the calculation shows that 129 I is the most important contaminant
giving rise to about 10 µSv/year from 20 000 to 200 000 years after disposal. The actinide dose is
several orders of magnitude below that figure during this period (see Figure II.31), and crosses the 237 Np
curve at around 3 000 000 years. Beyond that “geologic” period the decay products of U and Np
become predominant. The very long-term dose on a geological time scale is determined by 226 Ra and
231 Pa. This dose is of the same order of magnitude as the initial 129 I dose, i.e. 10 µSv/year. As a
conclusion, 129 I dominates the dose rate between 10 000 and 2 000 000 years; later, the actinides and
their decay products determine the ultimate dose.
224