Facebook Instagram Youtube Twitter

Heavy Reflector

WWER-1000 nuclear core. The lower support structure, the neutron reflector and 11 fuel assemblies.
Top view of the WWER-1000 nuclear core. The lower support structure, the neutron reflector and 11 fuel assemblies.
Source: gidropress.podolsk.ru
used with permission of АО ОКБ “ГИДРОПРЕСС”

There are a variety of materials that are used as a reflecting medium for neutrons in nuclear reactors. In general, reflector materials are normally not fissionable and have high scattering cross-section and low absorption cross-section. Since main differences among reactor types arise from differences in their neutron energy spectra, we must specify whether we mean reflectors in fast reactors or thermal reactors. Since the moderation of neutrons is undesirable in fast reactors, they use only high Z materials.

Its material must possess the following properties to be an effective reflector.

  • Low absorption cross-section
  • High reflection coefficient
  • Radiation stability – Radiation stability is a very important property of reflectors because the reflector material will be exposed to high levels of radiation (especially gamma radiation and fast neutrons)
  • Resistance to Oxidation – The reflector is often situated in a chemically aggressive environment (especially in fast reactors). Therefore the material should not get oxidized.
Elastic scattering cross-sections for light and heavy elements
Elastic scattering cross-sections for light elements
Elastic scattering cross-sections for light elements are more or less independent of neutron energy up to 1 MeV. Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
Elastic scattering cross-sections for heavy elements
For intermediate and heavy elements, the elastic cross-section is constant at low energy with some specifics at higher energy.
Core Baffle
Core Baffle

Essentially, for thermal reactors, a good moderator is also a good reflector because most of the moderators also possess the properties mentioned above of a good reflector.  Water, heavy water, beryllium, or graphite are commonly used as reflectors. In pressurized water reactors, water serves as an axial reflector. This axial reflector does not form any special device, and the neutrons are simply reflected by the core inlet and outlet coolant.

On the other hand, common water volume cannot be used as a reflector in the radial direction because it is of the highest importance to maintain high flow rates in the core and not bypass fuel assemblies. Therefore the neutron reflectors are installed in PWR and BWR reactor cores. The design of neutron reflectors may vary, but we can distinguish between two basic types:

  • Core Baffle. Core baffle consists of the baffle and former assembly consisting of vertical plates called baffles, and horizontal support plates called formers. This assembly forms the interface between the core and the core barrel, and there is water between the baffle and the core barrel. Since the coolant flow in the former region is significantly reduced by the former, the high flow rate through fuel assemblies is maintained. A secondary benefit is that water in the former region reduces the neutron flux on the pressure vessel, which causes irradiation embrittlement of pressure vessel material.
  • heavy reflector
    Visualization of a heavy reflector. It is only an illustrative example.

    Heavy Reflector. The heavy reflector is a structure that is installed inside a core barrel (similarly to a core baffle). But the heavy reflector is a wall made of stainless steel slabs stacked vertically surrounding the reactor core. Due to higher atomic number density, heavy reflectors reduce neutron leakage (especially of fast neutrons) from the core more efficiently than the core baffle. This provides additional protection of the reactor vessel from irradiation embrittlement, caused especially by fast neutrons. While acting as a neutron shield, the heavy reflector is heated due to the absorption of gamma radiation. The heat in the reflector is removed by water flowing through cooling channels drilled through the reflector to avoid overheating.

In fast reactors moderation of neutrons is undesirable, therefore reflectors are not composed of moderating materials. In fast breeder reactors the core is surrounded by a radial core reflector. The radial core reflector is usually composed of fuel assemblies with natural uranium (so called the radial blanket zone), for which the diffusion coefficient is about the same as that of the core. This reflector  improves the radial flux distribution, reduces the fissile inventory, and increases the internal breeding ratio.

Nuclear and Reactor Physics:
  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. Kenneth S. Krane. Introductory Nuclear Physics, 3rd Edition, Wiley, 1987, ISBN: 978-0471805533
  7. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  8. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  9. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

  1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
  3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Neutron Reflector