Module 103 External Dose Control
Discuss all other beta
emitters of importance at
Several uranium radioactive decay products are beta
emitters. Normally, most of these betas are shielded
by the surrounding material or material worn as
personal protective clothing (such as Tyvek). A
primary radionuclide of concern is protactinium-234
in its metastable state (234mPa), a daughter of 238U
which produces a very high energy beta particle that
can travel up to 20 feet in air. Significant beta
radiation is also emitted from 234Th (also a
daughter of 238U) and 231Th (a daughter of 235U).
Typically, these are shielded with ½-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.