Architecture and management of a geological repository - Andra

11 – Operational Safety- The selection of design options of the equipment to ensure good stability (cf. Section 9.3) with theload in all positions and, on the lifting system, provisions such as brakes and safety sensors,redundancy of various components of the lifting system, double electric power supply, device forlowering the load in the event an anomaly is detected, etc. should prevent the risk of a packagebeing dropped. These various systems should be regularly inspected;- In the event a package were to fall, the disposal packages provide protection from the primarypackages contained.To quantify the deformations of the B waste disposal packages in the event of a fall and to examinewhat the consequences would be, one initial approach has been to carry out a simulation study toverify the resistance of the package when dropped (cf.Section 11.8). These results are expected to bevalidated by full-scale B2 and B5 waste disposal package drop tests during 2005.• Uncontrolled cage displacement or drop when loaded with the B, C or spent fueldisposal package transfer cask in the shaft 158Experience acquired in mines with this type of transport and the combined preventive measures andinspections are such that the likelihood of the cage falling is extremely low 159 .The preventive measures that serve to counter this risk involve both the design of equipment(independent braking systems on the driving pulley, bundle of independent cage suspension cables 160 ,etc.) and maintenance, control and operating procedures. A cage anti-drop system could foreseeably beadded to these measures to provide an additional safety system which would be independent of thecage's command and control system. The principle of this system would be to use cables suspended inthe shaft as braking cables which would stop the cage in the event of overspeed. In order to limit thestress to which these cables would be subjected in the event a load were to be exerted upon them, theywould be connected to shock absorbers that would dissipate a large part of the kinetic energyassociated with the movement of the cage.Two kinds of measures would be employed to limit the mechanical consequences of the uncontrolleddisplacement of a cage or of a cage falling down the shaft:- An end of travel braking system, similar to the kind installed by law in mines, some metres belowthe bottom station would stop a cage passing its stopping point if its speed does not exceedapproximately 10 m/s. This would deal with a case of cage displacement associated with themalfunction of the braking system when it reaches the stopping point.- In the case of speeds in excess of 10 m/s, a shock absorber composed of “honeycomb” typematerial and installed at the bottom of the shaft would have the advantage of being a passivesystem capable of absorbing substantial amounts of energy. Laboratory tests have provided thecharacterization of the behaviour of this material for high speed impacts (cf. Figure 11.2.2).158 Other drop cases have been envisaged involving the direct drop of the transfer cask transporter down the shaft (cage not in position) orthe transfer cask falling through the floor of the cage (subsequent to impact or as a result of structural weakness). The identification ofthese two cases, which would appear even more improbable than the preceding one [111], serves, above all, to justify the reinforcementof preventive measures associated with access to the cage and the regular inspection of the condition of the shaft and the cage.159 In Germany, a study carried out for the Gorleben radioactive waste repository project [111] estimates that for a comparable facility, theprobability of a cage falling down the shaft is 5.10 -7 /year (for 5000 hours of operation per year).160 The study envisages a system with 10 independent suspension cables which allows loads of around one hundred tonnes to betransported. Shutdown is triggered in the event a single cable breaks which makes the successive breakage of all multi-cables unlikely.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM448/495