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.

Inner

Core

Outer

Core

Breeder

Reflector

Control

Rods

REFERENCES FOR CHAPTER 3

Allen, T. R. and D. C. Crawford (2007) Lead-Cooled Fast Reactor Systems and the Fuels and Materials Challenges, Science and Technology of Nuclear Installations, article ID 97486

Bagley, K. Q., J. W. Barnaby and A. S. Fraser (1973) Irradiation Embrittle­ment of Austenitic Stainless Steels, pp 143-153 in Irradiation Embrittlement and Creep in Fuel Cladding and Core Components, British Nuclear Energy Society, London

Bramman, J. I., C. Brown, J. S. Watkin, C. Cawthorne, E. J. Fulton, P. J. Burton and E. A. Little (1978) Void Swelling and Microstructural Changes in Fuel Pin Cladding and Unstressed Specimens irradiated in DFR, pp 479-508 in Radiation Effects in Breeder Reactor Structural Materials, American Society of Mining Engineers, New York

Dwyer, O. E. (1968) Heat Transfer to Liquid Metals flowing In-line through Unbaffled Rod Bundles, pp 139-168 in Heat Transfer in Rod Bundles, Amer­ican Society of Mechanical Engineers, New York

Etherington, E. W., J. I. Bramman, R. S. Nelson and M. J. Norgett (1975) A UKAEA Evaluation of Displacement Damage Models for Iron, Nuclear Engineering and Design, 33 82-90

Friedland, A. J. and C. F. Bonilla (1961) Analytical Study of Heat Transfer Rates for Parallel Flow of Liquid Metals through Tube Bundles, Journal of the American Institute of Chemical Engineering, 7, 107-112

Hoffman, H. and D. Weinberg (1978) Thermodynamic and Fluiddynamic Aspects in Optimizing the Design of Fast Reactor Subassemblies, pp 133­139 in Optimisation of Sodium-Cooled Fast Reactors, British Nuclear Energy Society, London

Hsiung, L., M. Fluss and A. Kimura (2010) Structure of Oxide Nanoparticles in Fe-16Cr MA/ODS Ferritic Steel Lawrence Livermore National Laboratory report LLNL-JRNL-427350

Mosedale, D. and G. W. Lewthwaite (1974) Irradiation Creep in Some Aus­tenitic Stainless Steels, Nimonic PE16 Alloy, and Nickel, pp 169-188 in Creep Strength in Steel and High-Temperature Alloys, London, The Metals Society, London

Nettley, P. T., I. P. Bell, K. Q. Bagley, D. R. Harries, A. W. Thorley and C. Tyzack (1967) Problems in the Selection and Utilization of Materials in Sodium Cooled Fast Reactors, pp 825-849 in Fast Breeder Reactors (BNES Conference proceedings), Pergamon, Oxford

Subbotin, V. I., A. K. Papovyants, P. L. Kirillov and N. N. Ivanovskii (1963) A Study of Heat Transfer to Molten Sodium in Tubes, Soviet Journal of Atomic Energy, 13, 991-994

Tang, Y. S., R. D. Coffield and R. A. Markley (1978) Thermal Analysis of Liquid-Metal Fast Reactors, American Nuclear Society, Hinsdale, Illinois, USA

Thorley, A. W. and C. Tyzack (1973) Corrosion and Mass Transport of Steel and Nickel Alloys in Sodium Systems, pp 257-273 in Liquid Alkali Metals, British Nuclear Energy Society, London

Zhang, J. and N. Li (2007) Review of the Studies on Fundamental Issues in LBE corrosion, Journal of Nuclear Materials, 373, 351-377