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Module 103 - External Dose Control
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Radiological Safety Training for Uranium Facilities - index
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External Dose Measurements - hdbk1113cn10135


DOE-HDBK-1113-98
Radiological Safety Traning for Uranium Facilities
Module 103 - External Dose Control
D.
Gamma and X-Ray External Dose
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.
E.
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.
20


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