Gamma and X-Ray External Dose - doe-hdbk-1113-98_reaffirm_2005

DOE-HDBK-1113-98

Radiological Safety Training for Uranium Facilities

Module 103 External Dose Control

and 231Th (a daughter of 235U). Typically, these are shielded with -inch of

plastic.

Gamma and X-Ray External Dose

Although beta dose from unshielded uranium presents the most common radiation

problem, 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 cylinders. Gamma radiation emitted from residual

materials can result in 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.