The subassembly wrappers become distorted due to the effects of thermal expansion, irradiation swelling and irradiation creep. The resulting movements have to be checked; otherwise they may cause

Flow restrictor

Coolant inlet

Figure 3.20 A typical fuel subassembly for a breeder reactor.

unwanted reactivity changes, interfere with movement of the control rods, and make withdrawal of irradiated subassemblies difficult.

As explained in section 3.2.5 there may be a considerable temper­ature difference across a subassembly at the side of the core due to the radial variation of power density. The side of the wrapper nearer the core centre is hotter and expands more, causing the subassembly to become curved or “bowed” with the convex side facing the core centre. The effect on reactivity of the resulting displacement, if it were unconstrained, is explained in section 1.6.3. A similar effect can arise from radiation-induced swelling of the wrapper, which is greater on the side nearer the core centre where the neutron flux is higher.

The effect of temperature bowing in an unconstrained core would in fact be small. Temperature differences of 10 K across subassemblies would cause radial displacements at the core centre plane of the order of 0.2 mm and reactivity changes of the order of 10-4, which are of
minor importance. The effect of irradiation swelling could be much greater. A rough estimate can be made as follows.

If the difference in linear expansion between the two sides of the subassembly is As its radius of curvature is w/Ає and its outward dis­placement at the core centre plane is d & H2As/8w if As is uniform along its length, where H is the height of the core and w is the width of the subassembly. As actually varies along the subassembly in a com­plicated way because it depends on temperature as well as fluence, but a mean value of 0.003, corresponding to a 1% difference in volumetric strain, is typical. For a subassembly 0.15 m wide this would cause a displacement of about 3 mm at the level of the core centre and about 10 mm at the top of the core, and the displacement of the top of the subassembly could be 20-40 mm, depending on how long it is. This would create many problems, and in particular it would make it dif­ficult to maintain the alignment of control rods with their operating mechanisms.

In most reactors radial movement is prevented by some sort of restraining or clamping system. Figure 3.21 shows such a system, where radial movement of the outer periphery of the radial breeder is pre­vented at two restraint planes, both above the core. According to where the restraints are placed small movements of the fuel due both to thermal expansion and to irradiation swelling still occur, but because the subassemblies are in contact with each other across the core any tendency to positive reactivity feedback can be eliminated.

A restraint system may be “active”, meaning that after the sub­assemblies have been assembled the restraints are tightened so that the whole array is clamped together, or “passive”, meaning that the restraints are fixed and prevent outward movements only after clear­ances between subassemblies have been taken up as the wrappers swell. The disadvantage of an active system is that the clamping mech­anism has to be operated remotely and reliably under sodium.

To avoid self-welding between the wrappers they can be provided with hard pads at the points where they are in contact. This is one

of the few places where neutronics affects the choice of materials: the cobalt alloys that are usually used for hard-faced surfaces are not acceptable in a reactor core because the radioactive 60Co that would be generated would create severe problems in handling the irradiated fuel and disposing of radioactive waste.

The radial loads between the restrained subassemblies vary with time and depend on the design of the restraint system, and predicting them is a very complex task. The order of magnitude can be estimated fairly simply, however. If a simple cantilever of length L is subjected to a transverse force F at its free end, the displacement d is given by d = FL3/3EI, where I is the second moment of area of the cross­section and E is Young’s modulus. For a hexagonal wrapper 0.14 m across flats made of sheet 3 mm thick, I ~ 4 x 10-6 m4. If the length of the subassembly, L, is 3 m and the displacement d is 0.01 m, typ­ical of bowing due to irradiation swelling, then with E = 2 x 1011 Pa

we obtain F и 900 N. If the restraint system is passive and the coeffi­cient of friction between subassemblies is about 1, similar forces, of the order of 1 kN, may be needed to overcome friction as a subassembly is withdrawn from the core.