The subassemblies rest on a support structure, often known as a “diagrid”, which also serves to distribute the coolant from the cir­culating pumps. A typical arrangement is shown in Figure 3.22: each subassembly terminates in a hollow spike that is located in a hole in the diagrid, and coolant enters the subassembly through slots in the spike. If the coolant is a liquid metal, when it is flowing the pressure inside the diagrid is some 200-400 kPa above the hydrostatic pressure of the coolant outside, and this causes a substantial lifting force on a subassembly, which can exceed its weight. To prevent it floating away a “hold-down” mechanism is required. In the case of sodium coolant this can be done by arranging for the spike to pass right through the diagrid, so that its lower end is exposed to coolant at low pressure.

Alternatively, for sodium, hold-down can be provided by a rigid structure above the core that prevents upward movement. In the case of lead coolant a hold-down structure above the core is

essential because otherwise the subassemblies would float away when the coolant flow-rate and the pressure in the diagrid are low. The hold­down structure has to make provision for the coolant flow to leave the subassemblies and for the control-rod drive mechanisms to operated through it, and also to allow for the subassemblies to be removed and replaced. Figure 3.23 shows, in diagrammatic form, how diagrid, subassemblies and hold-down are commonly arranged.

The wrappers contain high-pressure coolant from the diagrid but the space between the wrappers is open at the top to low-pressure coolant, so that there is a pressure difference across the wrappers of the order of 100 kPa. This induces stresses, and in the core region, where the neutron flux is significant so that irradiation creep takes place, the flat faces of the hexagonal wrappers bulge outwards. If this brings neighbouring wrappers into contact self-welding may occur and withdrawal of irradiated subassemblies may be difficult. The size of the gap between wrappers, which is determined by the thickness of the hard pads where contact between subassemblies is allowed, must be great enough to prevent contact occurring anywhere else. Typically a gap of 6-8 mm is required.