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.
A detailed list by Theodore Rockwell III when building the rector shield is summarized as follows: [3]
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.
Allowable radiation levels must be established for both power operation and shutdown conditions.
Location, energy, and intensity of the sources of radiation.
Calculate and design the required shield thicknesses.
Arrangement of piping and heat-exchange system in and outside the shield.
The selection of materials.
Design and fabrication details including bonding lead to steel and treating water and other shielding fluids, and selecting and handling heavy concretes.
Inspecting and testing materials prior to and after installation.
The consideration of radioactivity when the system is shut down.
The process of monitoring the potential for streaming radiation through gaps, thermal insulation, structural member, and shield penetrations.
How the geometry of the radiation sources can effect the shielding design.
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.
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:
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| 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 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 |