The large lead volume allows fuel cooling by natural convection. To this aim, the mass of lead is as big as 10000 tons contained in a vessel 30 m high. Natural convection is initiated by the mass difference between the hot lead at the output of the core and the colder lead at the output of the heat exchan­gers. The height of the lead column determines the flow velocity. The 30 m height is needed for a flow velocity sufficient to extract the operating heat. Thanks to natural convection no primary tubing, as a potential cause of serious problems in the case of leaks, is needed. Because of the large lead mass, the thermal inertia is very large, minimizing the effects of sudden variations of the accelerator beam intensity.

Passive safety. In the improbable case where the lead temperature would exceed by more than 100 °C the normal operating value (for example if the

Thermal insulating wall Plenum region

Fuel region
Spallation region

Figure 12.2. Detail of the CERN system [76]. Note the heat evacuation by natural convec­tion. The molten lead is contained in a kind of Dewar vessel. In the case of significant increase (around 100 °C) of the lead temperature it will overflow into the beam tube and into the space separating the inner and outer walls of the Dewar, thus allowing residual heat evacuation by air convection. The large structure above the Dewar helps air cooling.

primary heat exchangers break down while the beam stays in) the lead over­flows into the beam tube, stopping neutron production in the fuel zone. At the same time, the molten lead flows into the void which normally isolates the molten lead pool from air. Thermal contact with air ensures extraction of the residual heat produced by radioactive decay of the irradiated fuel.

Finally the lead overflow positions a neutron absorber which brings the multiplication factor down to 0.9. The system may then stay in a safe state as long as necessary. Note that the core is placed in a deep well and is protected by a lead thickness of 20 m.

Fuel reprocessing [138]. In the original proposal, based on the Th-U cycle, fuel reprocessing was either Thorex for oxide fuels [153], or pyropro- cessing for metallic fuels as developed at Argonne National Laboratory [22].