Transuranics cont'd

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

Several DOE facilities have adopted specifications on recycled uranium that limit the amount of

transuranic alpha activity to 0.1% of the total uranium alpha activity, thus limiting the potential inhalation

dose from transuranics to a small fraction of the total. Facilities that handle recycled uranium with higher

levels of transuranics should establish a regular program of analyzing feeds, products, and by-products for

transuranics, and then modifying control limits and action levels as appropriate to reflect the transuranic

content of those materials. This monitoring of the TRU content is essential when the analytical technique

used to identify the level of radiological control needed is based on gross alpha counting (such as for air

sampling), which does not distinguish the plutoniu m from the uranium fraction, or chemical analysis for

uranium (such as photofluorometric urinalysis) which does not detect plutonium.

Raffinate from refinery operations, MgF2 from metal production operations, and chemical traps from

UF6 operations have all been observed to have higher TRU-to-U ratios than either reactants/feeds or

uranium products. Frequently, reaction by-products are not discarded as wastes but are processed further to

recover the remaining uranium. When this occurs, a portion of the impurities is recovered along with the

uranium and can become a perpetual radiological control problem. All facilities that process recycled

uranium should periodically analyze feeds, products, and by- products for transuranics to ensure that

radiological controls are adequate for the mixtures of uranium and transuranic elements that are present.

The uranium isotopes (viewed as contaminants) that will increase due to the recycled uranium feed are

232

U, 234U, and 236U. The health and safety risks of 236U are similar to those of natural uranium because its

specific activity and radiation emissions are similar (See Table 2-2). Its presence in uranium fuel requires

slightly higher enrichments for the same reactor applications, however, because it absorbs neutrons. The

increased concentration of the 234U increases the specific activity of any enrichment of 235U. It is expected

that the specific activity for a given enrichment would be about double that obtained from enrichment of

non-recycled uranium.

The isotope in recycled uranium presenting the greatest potential radiological hazard from external

sources is 232U. 232U a daughter product of neutron activation of 231Pa. The health hazards of 232U are

primarily due to the rapid buildup of gamma activity of its decay products, particularly from 228Th. The

gamma activity buildup is both time- and process-dependent. The 232U decay products form nonvolatile

fluorides and will concentrate in cylinders when UF6 is vapor-fed. The gamma activity in equipment that

processes gaseous UF6 is a function of the mass fraction of 232U present in the gas phase. Estimates indicate

that the level of gamma activity within the enrichment cascade equipment would increase by about a factor

of 3 due to the presence of 232U. The exposure rates on internal surfaces would increase from 10-20 mrad/h

to 30-60 mrad/h; those on external surfaces would increase to about 3-4 mrad/h. The major exposure

increase from the 232U occurs in the handling of UF6 cylinders. Currently, the exposure rate at the external

surface of empty UF6 cylinders is about 50-100 mrad/h. Assuming a 232U concentration of 0.5 ppm based on

235

U and a feed enrichment of 1%, a full 10-ton feed cylinder would have a surface exposure rate of about

80 mrad/h. The exposure rate at 30 cm from the surface of an emptied cylinder would be about 500 mrad/h

without the shielding provided by material in the cylinder. These values are based on the 232U being in

secular equilibrium with its decay products; in reality, it is unlikely that the decay products would reach

much more than 50% of equilibrium values.

Product cylinders produced from processing of recycled uranium typically have higher gamma

radiation fields than the feed cylinders. At 4% 235U enrichment, the contribution from 232U over time could

increase the radiation field at the surface from 80 mrad/h to 300 mrad/h from a full 10-ton cylinder and

from 500 mrad/h at 30 cm to 2 rad/h from an empty cylinder. About half of this increase would be

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