Reactor Shielding Development

Paul Theodosis
March 19, 2012

Submitted as coursework for PH241, Stanford University, Winter 2012


The viability of nuclear energy has been a controversial topic for decades. [1] A significant hazard that limits the application of nuclear reactors is the ionizing radiation that is produced during a reaction. Ionizing radiation is a particle that can free an electron from an atom or molecule. If these particles come in contact with living tissue, radiation sickness, mutation, cancer, and death can occur. [2] In order to make nuclear power plants a realistic option for energy production, an understanding of how to shield workers from this hazard was developed. This report is intended for the general public in order to learn some fundamental principles engineers and scientists use when building a reactor shield.

Topics of Consideration for Building a Reactor Shield

A detailed list by Theodore Rockwell III when building the rector shield is summarized as follows: [3]

  1. Decide which type shield best suits the purpose. This decision encompasses decisions like building the shield around the reactor and all its components or compartment shielding where the reactor is partially shielded and the radioactive coolant system is separately enclosed in a shielded room.

  2. Allowable radiation levels must be established for both power operation and shutdown conditions.

  3. Location, energy, and intensity of the sources of radiation.

  4. Calculate and design the required shield thicknesses.

  5. Arrangement of piping and heat-exchange system in and outside the shield.

  6. The selection of materials.

  7. Design and fabrication details including bonding lead to steel and treating water and other shielding fluids, and selecting and handling heavy concretes.

  8. Inspecting and testing materials prior to and after installation.

  9. The consideration of radioactivity when the system is shut down.

  10. The process of monitoring the potential for streaming radiation through gaps, thermal insulation, structural member, and shield penetrations.

  11. How the geometry of the radiation sources can effect the shielding design.

  12. Calculate radiation heating in the structure near the reactor to avoid excessive material weakening.

The two major components that will be discussed from required shield thickness.

Scattering and Absorption

The first parameter is the cross-section for scattering and absorption which can be roughly approximated as how much material is between the radiation source and subject. [4] This quantity can be measured in g/cm2 and is usually expressed as the amount of material required to reduce the intensity of radiation by half. Below is a list of common materials:

Material Halving Thickness (cm) Halving Mass (g/cm2)
Lead 1 12
Concrete 6.1 20
Steel 2.5 20
Water 18 18
Air 15000 18
Fig. 1: Scattering and absorption of various common materials.

When building a shield for a nuclear reactor, cost plays an important role in addition to safety considerations. An example is that if the engineers are deciding between using lead and steel, if steel is significantly cheaper, an argument could me made to use make a thicker, cheaper shield that worry about how much space the shield is occupying. Other considerations might include creep, a process where metals weaken or deform over a period of time when exposed to a high enough temperature. Lead is the most susceptible to creep in spite of being one of the most effective shielding materials, while concrete is more moderate.

In 1996, a new technology was developed called grade-Z shielding. This new material has been shown to reduce penetration with the same halving mass by over 60%. [5] However, in spite of this significant improvement in weight, this technology is most commonly seen in satellite applications rather than nuclear reactors.

The Inverse Squared Law

The second factor that will be discussed in shield design is the separation distance the shield creates between the source and subject. The exposure one experiences from a radio active source is related by the inverse squared law:

I = S
4 π r2

where S is the source strength, r is the distance between the source and subject and I is the intensity the subject experiences. With this understanding, the farther shields keep people away from the radiation source, the better. When the two factors discussed are considered together, it leads to another engineering balance between shield thickness and distance from the source. The thicker the shield and the farther away it keeps a person, the more it reduces the radiation exposure. However, when these two parameters are taken to an extreme, cost as well as other engineering considerations make the design unrealistic.


Having a well designed shield significantly reduces the risks involved in harvesting energy from nuclear power. Without careful planning, the risks outweigh the benefits. So how much radiation exposure is tolerable? The British government has a policy called ALARP (As Low As Reasonably Practicable). The United States and Canadian governments have similar policies called ALARA (As Low As Reasonable Achievable). The policy is based on using the hazards of working in a job that is considered safe as a benchmark for hazards present in nuclear power plants. These standards help define the parameters discussed. How thick should the shield be if it were made of a specific material? How far away should it restrict a worker coming to reduce exposure. In addition to these precautions, strict policies of maintenance and radiation sensors are required. Even with these safety precautions, the justification for nuclear power will still be a subject of debate and probably will be until a safer and equally abundant energy technology surfaces.

© Paul Theodosis. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] J. J. MacKenzie, "Review of The Nuclear Power Controversy by Arthur W. Murphy," Quarterly Rev. Biol. 52, 467 (1977).

[2] "Radiation Injury" in Goldman's Cecil Medicine, ed. by L. Goldman et al. (Saunders, 2007).

[3] T. Rockwell III, Ed., Reactor Shielding Design Manual (Van Nostrand, 1956).

[4] R. G. Newton, Scattering Theory of Waves and Particles, 2nd Ed. (Dover, 2002).

[5] W. C. Fan et al., "Shielding Considerations for Satellite Microelectronics," IEEE Trans. Nucl. Sci. 43, 2790 (1996).