|
| DOE-STD-1136-2004
Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities
apparent within 2 years of initial usage and the highest levels could occur in 20 years without mitigating
actions. Frequent cylinder cleaning can prevent this significant exposure rate buildup. The presence of 232U
may also require other changes in processes used to handle cleaning solutions due to the higher gamma
radiation present.
2.1.4.2 Technetium
In facilities with significant quantities of 99Tc, radiation monitoring techniques must be able to
detect the low-energy beta radiation from this isotope. Individual and area monitoring equipment
and techniques selected to measure the 2.29 MeV (Emax) beta from 234mPa may not measure the
99
Tc 0.292 MeV (Emax) beta effectively. If a mixture of uranium and 99Tc is suspected to be present,
the monitoring technique selected must be based on 99Tc or on the actual mixture, rather than on
234m
Pa. The 99Tc levels have not been the controlling factor in many situations to date. However, it
is important to ensure that monitoring instruments and techniques are adequate to detect 99Tc.
Technetium-99 tends to deposit within enrichment equipment and will "pocket" in the higher
enrichment sections of the gaseous diffusion cascade. Special precautions must be taken when evacuating
and purging or performing other maintenance work on this equipment. In equipment with accumulations of
99
Tc, low energy beta radiation fields of a few rad per hour may be encountered. This radiation is
effectively attenuated by the protective clothing required for contamination control (one pair of industrial
cloth coveralls, one pair of impermeable (Tyvek) coveralls and heavy neoprene gloves). While the 99Tc
should be effectively removed from the Gaseous Diffusion Plant (GDP) product, it will be present in
uranium used by other DOE facilities. Because the ALI for 99Tc is higher than that of uranium, inhalation is
the controlling concern only in situations where the technetium activity greatly exceeds that of the uranium
that is present. Technetium as pertechnetate is also difficult to remove from skin and can therefore cause
significant skin doses from skin contamination.
The tendency of technetium to become airborne more readily than uranium can lead to beta
contamination in areas where it is not otherwise expected and environmental emissions even when the
uranium is effectively confined in the work place. Residues in ventilation systems from high-temperature
operations, such as uranium remelting/casting, or uranium chip burning, tend to have higher Tc-to-U ratios
than either feed or product material in uranium metal processing facilities. Because of its low atomic
weight and relative volatility, technetium also tends to concentrate at the top of the gaseous diffusion
cascade, where it becomes an inhalation and effluent concern when the cascade is opened for maintenance.
Facilities that handle recycled uranium should 1) analyze feeds, products, and by-products to determine the
fate of 99Tc within their processes, then 2) modify monitoring equipment, control limits, and action levels
as needed to properly evaluate and control 99Tc hazards.
Environment, safety and heath personnel should also evaluate the presence of and radiological
consequences from other fission products impurities in recycled uranium.
2.2 PHYSICAL AND CHEMICAL PROPERTIES
Uranium fuels vary with reactor type. Some reactors use the natural isotopic composition in the fuel.
Others use enrichment varying from 2% to > 90%. Because of the radiation- induced growth of uranium
metal used in the early reactors, alloys were developed to stabilize dimensional changes. Many of the
alloys with favorable dimensional stability characteristics had sizeable neutron absorption cross-sections,
resulting in poisoning of the nuclear reaction. Zirconium-clad ceramic uranium dioxide and uranium
carbide fuels were found to have acceptable characteristics and are in common use.
2-16
|
Privacy Statement - Press Release - Copyright Information. - Contact Us |