Time, Distance and Shielding: Radiation Safety at B Reactor

The Health Physics Exhibit at Hanford's B Reactor, part of Manhattan Project National Historical Park, explores the development of health physics, the science of protecting people and the environment from radiation's harmful effects. One of its panels explains the three basics of radiation protection.

The basics

Herbert Parker stressed time, distance and shielding, backed by administrative controls:

• Time. Health physicists worked out how long a task could safely take, to limit how long workers were exposed to a radioactive source.
• Distance. They found ways to keep people farther from a source, using ropes, long handles and similar tools. When control rods were pulled from B Reactor, for example, workers used ropes to operate at a distance from the radioactive rods.
• Shielding. Thick concrete walls and layers of Masonite provided protection.

For each job, health physicists identified the type of radiation involved and issued Special Work Permits spelling out the time, distance and shielding required.

Different radiation, different protection

Gamma, beta, alpha and neutron radiation carry different risks, so workers combine time, distance and shielding differently depending on the source:

• Alpha radiation cannot get through a sheet of paper, and even the outer layer of skin stops it — but it is very dangerous inside the body.
• Gamma radiation penetrates most deeply and is the biggest external hazard. Stopping it takes thick shielding: six feet or more of concrete, or more than a foot of lead.

Visiting

B Reactor can be visited only on guided tours run by the Department of Energy.

Sources

• National Park Service, Manhattan Project National Historical Park: "B Reactor Health Physics Exhibit: Time, Distance & Shielding," describing the exhibit panel's text. The photograph of the panel, whose images carry separate credits, is not reproduced.