Gamma and X-Ray External Dose

DOE-HDBK-1113-98

Radiological Safety Traning for Uranium Facilities

Module 103 - External Dose Control

Although beta dos e from uns hielde d uraniu m present s the most common radi ation pr oblem,

storage of large quantities of uranium can create low-level gamma radiation fields

(less than 5 mrem/hr). Such fields can create external exposure problems, particularly when

significant numbers of people are working in adjacent areas.

In addition to gamma emissions from the uranium decay chains (238U and 235U), recycled fuel

materials introduced back into the enrichment process will result in higher gamma radiation

fields because of 228Th, a gamma-emitting daughter of 232U with a relatively short half-life (1.9

yr).

Larger sources of gamma radiation may exist from specific uranium processes, including

unflushed UF6 cylinder s. Gamma rad iation emitted f rom resid ual mater ials can result i n gamma

radiation fields of several hundred millirem per hour. This problem can be controlled by flushing

empty cylinders to remove residual material.

Neutron External Dose

As uranium is processed in the fuel cycle, it is often chemically bonded to fluorine to create

compounds such as UF4 and UF6. When uranium atoms in these compounds decay, they emit

alpha particles that are sometimes captured by the neighboring fluorine atoms. The resulting

atom is unstable and may emit a neutron to gain back its stability. The neutrons emitted can

result in neutron radiation fields between 0.5 and 4 mrem/h.

The probability of spontaneous fission is small; therefore exposure is not expected. However, if

fission does occur, such as in a reactor or from experiments, the neutron radiation is typically

contained. Neutron radiation that is not contained is usually the result of a criticality accident,

which generates potentially fatal doses of gamma radiation.