Radiological Characteristics and Effects

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

air and saline solutions, can accelerate the corrosion of the material over time and produce greater

possibility for generating airbor ne radioactive material. Stored in a dry environment or coated with an

anti-corrosion surface treatment, the metal may show no visible signs of corrosion for many years.

Uranium metal may be dissolved using nitric acid, which is also used to passivate ("p ickle") the

metal to inhibit oxidation.

2.3 RADIOLOGICAL CHARACTERISTICS AND EFFECTS

Uranium isotopes decay by alpha particle emission and some also emit low-energy gamma rays. For

Classes W and Y material (See last paragraph of Section 2.5 for discussio n of Class D, W and Y), the

inhalation hazard from alpha particle release in the respiratory tract is the predominant radiological hazard

associated with the alpha-emitting uranium isotopes. The primary uranium decay products, listed in Table 2-

2, decay by beta particle emission, most with a small yield of gamma emissions as well. These decay

products increase the shallow dose equivalent and lens of the eye dose equivalent resulting from external

radiation exposures, due mainly to the 2.29 MeV (Emax) beta from 234mPa.The surface exposure rates shown

in Table 2-7 result primarily from beta radiation from decay products. The exposure rates decrease quickly

with distance because of the attenuation of the beta radiation and the small yield of the gamma radiatio n.

Table 2 -7. Beta Surface Exposure Rates from Equilibrium Thickness of Uranium Metal and

Compounds

Beta Surface Exposure

Source

Rate, mrad/h

U-Nat metal slab

233

UO2

207

UF4

179

UO2 (NO3)2 6H20

111

UO3

204

U3O8

203

UO2 F2

176

Na2U2 O7

167

Beta surface exposure rate in air through a polystyrene filter 7

mg/cm2 thick.

Because some uranium decay products have short half-lives (on the order of days), those decay

products will usually be present with uranium during processing. Figure 2-5 illustrates the ingrowth of the

238

U decay products. An assumption of secular equilibrium should not be made until processing is complete

because many routine chemical processing steps separate uranium from its decay products. Both the

inhalation and external exposure hazards associated with the decay products are increased in areas where

the decay products are concentrated. The overall inhalation hazard will typically decrease in those areas as

the uranium is removed. In the case of cast uranium metal, the exposure rates from high

2-18