Architecture and management of a geological repository - Andra

4 – Waste disposal Packages4.3.1.3 Operating requirementsIn a similar way to C waste, Andra has adopted horizontal tunnels for repository architecture for thestudy. Spent fuel packages are able to be handled horizontally for transfer to underground drifts andplacement in disposal cells. Within the framework of reversible repository management, thesepackages should be able to be removed for at least a hundred years.4.3.2 Design principles adopted4.3.2.1 Choice of technical optionsIn the same way as for C waste over-packs, leaktightness is ensured by a metal shell. From amongstthe various categories of metal materials presented in section 4.2, non-alloy steel has been adopted forthe same reasons.In the same way as for C waste, material thickness determines leaktightness duration. Containerclosure with electron beam has also been adopted. It is worth mentioning that this process offers theadvantage of minimising thermally affected zone extension and provides metallurgical qualities closeto those of the base material concerning corrosion phenomena.In order to comply with repository heat transfer design basis criteria, package capacity is restricted tofour PWR fuel assemblies of CU1 type (UOX/URE) and one MOX fuel assembly (CU2).The choice of a low package capacity thus restricts the mass, so that it remains in a scope covered byindustrial experience feedback for shaft transfer.Another important design choice is to limit residual voids within the package. For packages containingmore than one assembly, this leads to a massive object containing compartments adjusted to assemblydimensions.Several solutions for obtaining this object have been envisaged. Manufacture of a single-piececontainer made of cast steel with moulded emplacements, incurs the risk of obtaining a heterogeneousstructure. However, forged steel single-piece manufacturing would provide a more homogeneousstructure but machining of square emplacements for bare assemblies over such lengths is nottechnologically feasible at this stage. It seems too difficult to manufacture a steel shell and separateinsert, which would be obtained from foundry and shrink fitted into the shell, for such dimensions, andthis would result in deformations and stress too difficult to control. Simple mechanical fitting wouldresult in significant clearance which would eventually have to be filled in. Therefore, direct pouring ofa cast iron insert into the shell, after installation of a mould composed of steel tubes to form theemplacements, is the solution adopted.Cast iron is of sufficient mechanical resistance for the insert to withstand long-term strain on thecontainer.It should be pointed out that cast iron was also chosen by SKB in Sweden (see below and [49]) forspent fuel container insert manufacture.4.3.2.2 Solution adoptedThe solution adopted at this stage consists of two models of cylindrical containers made of non-alloysteel composed of a body and a welded lid using the electron beam method. These two models differin their external diameters and internal layout related to the number of fuel assemblies they containand their conditioning mode (bare or in cladding) [54].The first model (Figure 4.3.1) is large in diameter (about 1,250 mm with a thickness of 110 mm ofsteel) and contains four fuel assemblies of CU1 type (UOX/URE) representing total thermal power ofabout 1600 W, after storage of about 60 years once unloaded from the reactor.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM145/495