The core components in fast reactors include the following: clad (cylindrical tubes which house the fuel pellets) for the fuel and wrapper (a container which houses fuel elements, in between which the coolant flows) for fuel subassemblies. Figure 3 shows a schematic of clad and wrapper in a typical fuel subassembly. The necessity to develop robust tech­nology for core component materials arises from the fact that the ‘burn-up’ (energy production from unit

Head

Adaptor

B&C

shielding

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shielding

Top _
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Fuel —

Bottom

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Coolant
entry tube

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Section-XX

Figure 3 Schematic of a typical fuel subassembly.

quantity of the fuel) of the fuel depends on the performance of the clad materials. The higher burn — up of the fuel increases the ‘residence time’ of the subassembly in the core, eventually lowering the cost.

The core component materials in fast breeder reactors are exposed to severe environmental service conditions. The differences in the exposure condi­tions of the clad and wrapper in a fast reactor core are listed in Table 1. Under such exposure condi­tions, materials in the fast reactor fuel assemblies exhibit many phenomena (Figure 4), specific to fast reactor core: Void swelling, irradiation growth, irra­diation hardening, irradiation creep, irradiation, and helium embrittlement.

Another selection criterion, namely the compati­bility of the core component materials with the cool­ant, the liquid sodium, has already been established. Presently, methods are known to avoid interaction of the clad material with the coolant.

Detailed books and reviews19,20,21,22,23 are avail­able on all the degradation mechanisms mentioned above, which are related to the production, diffu­sion, and interaction of point defects in the specific lattice of the material. Hence, a brief introduction is presented below (see also Chapter 1.03, Radiation — Induced Effects on Microstructure; Chapter 1.11, Primary Radiation Damage Formation; and Chapter

1. 04, Effect of Radiation on Strength and Ductility of Metals and Alloys).

Void swelling in a fast reactor core can change a cube of nickel to increase (20%) its side from 1 cm to 1.06 cm, after an exposure to irradiation of 1022ncm~ . Void swelling is caused by the conden­sation of ‘excess vacancies’ left behind in the lattice after ‘recombination’ of point defects produced dur­ing irradiation. Void swelling is measured using the change in volume (V V/V) of bulk components of the reactor or image analysis of voids observed using transmission electron microscope (TEM).

The ‘irradiation growth’ (fluence ^102°ncm~ ) can increase the length of a cylindrical rod of uranium three times and reduce its diameter by 50%, retaining the same volume. This occurs mainly in anisotropic crystals, introducing severe distortion in core compo­nents. It is caused by the preferential condensation of interstitials as dislocation loops on prism planes of type (110) of hcp structures and vacancies as loops on the basal planes (0001), which is equivalent to transfer of atoms from the basal planes to prism planes, via irradiation-induced point defects.

Irradiation hardening refers to the increase in the yield strength of the material with a

Table 1 Comparison of exposure conditions of clad and wrapper of fast reactor core

Criterion Clad tube Wrapper tube

concomitant reduction in ductility, under irradiation at temperatures <0.3 Tm. The large number density of defect loops, voids, and precipitates generated during irradiation pins the mobile dislocations and acts as an obstacle to their further movement, requiring addi­tional stress to unpin the immobile dislocations.

The irradiation creep, the most important param­eter for design consideration, is the augmentation of thermal creep of the material, under irradiation. This leads to premature failure of the material and restricts the service life. The mechanisms responsible for irradiation creep are identified as the ‘stress — induced preferential absorption’ (SIPA) and the ‘stress-induced preferential nucleation (SIPN)’ of point defects by dislocations, which revolve around the interaction of excess point defects generated dur­ing irradiation with dislocations.

Irradiation embrittlement, another frequent observation in ferritic steels exposed to irradiation, refers to the increase in the ductile to brittle transi­tion temperature (DBTT) during irradiation. Drastic loss in ductility at low temperatures results from a lower sensitivity of the fracture stress, sf, due to irradiation and less dependence on temperature than the yield strength sy. Materials with a high
value of the Hall-Petch constant are more prone to brittle failure. Such materials like ferritics release more dislocations into the system when a source is unlocked, causing hardening and loss of ductility.

Some of the engineering materials contain nickel, an element which undergoes an (n, a) reaction, produc­ing high concentration of helium. The binding energy of helium with a vacancy being very high ^2 eV, the helium atoms stabilize the voids, enhancing their growth rate. Incorporation of helium during irradia­tion into voids along the grain boundaries assists grain boundary crack growth by linking voids causing ‘helium embrittlement.’

Of these many degradation mechanisms, the alloy development programmes have focused mainly on the void swelling, irradiation hardening, embrittle­ment, and the irradiation creep, since these are the major life limiting factors.