Configuration of the Reactor Core

A reactor is made up of an array of subassemblies of various types. The core subassemblies may contain fuel of several different enrichments arranged to give annular enrichment zones, as explained in section 1.3.3. The core may be surrounded by a radial breeder consisting of two or three rows of subassemblies consisting of fat fuel elements con­taining fertile material. Around this there may be a neutron reflector consisting of subassemblies similar to those of the core and breeder but containing mainly steel. Around this there may be additional sub­assemblies containing neutron-shielding material, or the shield may be a separate structure as indicated in Figure 3.23. Control rods occupy subassembly positions in the core region and are usually inserted into the core from above. They are operated by mechanisms that are situated on top of the reactor vessel so that they are available for maintenance.

Figure 3.24 shows a core pattern for a 3000 MW(heat) sodium — cooled breeder reactor. The core has two fuel enrichment zones and is surrounded by a radial breeder, which in turn is surrounded by a reflector. The neutron shielding is not shown. The effective diameter of the core is 3.6 m and it is 1 m high. Figure 3.25 shows a much smaller 600 MW(heat) sodium-cooled core that is 1.5 m in diameter and 0.9 m high. As explained in section 3.2.3 the height of the core of a sodium — cooled core is constrained by coolant flow considerations to be about 1 m, and the core diameter is adjusted to accommodate the required power output.

The configuration is different if the reactor is intended to consume rather than breed fissile material. Figure 3.26 shows how the core shown in Figure 3.24 could be modified for this purpose. There is no
breeder, and in addition some of the 238U is removed from the core (by increasing the fuel enrichment). This would make the core excessively reactive so some of the fuel subassemblies have to be removed and replaced by diluent subassemblies containing inert material.

Подпись: Figure 3.25 The configuration of the core of a 600 MW (heat) sodium-cooled breeder reactor.











Figure 3.26 The configuration of the core of a 3000 MW (heat) sodium-cooled consumer reactor.


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