Criticality - DOE-HDBK-1145-2001

Radiological Safety Training for Plutonium Facilities

DOEHDBK11452001

Student's Guide

Plutonium emits one or more of the following types

of radiation: alpha, beta, neutron, gamma, and x-ray.

The half-lives of nuclides range from 21 minutes for

Pu-233 to 80 million years for Pu-244.

Alpha radiation and low-energy (less than 70 keV)

beta radiation will not penetrate the dead layer of skin

and present no external hazard. These become

hazardous when they are introduced inside the body,

where they have direct contact with living cells.

With plutonium, neutron radiation is the most

penetrating and can be a significant biological hazard.

Most x-ray and/or gamma radiation from plutonium

is of lower energy, ranging from 17-20 keV, and is

moderately penetrating. This radiation becomes a

concern mostly with those handling the material.

There is an additional radiological problem with Pu-

241. It is impossible to separate plutonium nuclides to

100% purity; there is always some Pu-241 present.

The Pu-241 decays to americium-241 (Am-241). Am-

241 emits a higher-energy gamma ray, which is a

concern. This americium "in growth" results in

increasing radiation levels for many years. This can

contribute to significant doses, since some

applications may have 30%-50% Pu-241.

D. Criticality

While several plutonium nuclides can fission, only

Pu-239 is of practical importance as a criticality

concern. A criticality accident involves an uncontrolled

chain reaction that releases large quantities of heat,

neutrons, and gamma radiation. It does not create an

atomic explosion. Facilities that possess fissile

material have criticality detection systems to warn

workers of an event. Remember, if a criticality

accident does occur, exit as quickly and safely as

possible. JUST GET OUT!