Answer to Question #6134 Submitted to "Ask the Experts"

Highly enriched uranium contains the isotope ²³⁵U to the extent of at least 20 percent, the remainder being primarily ²³⁸U. The ²³⁵U decays by alpha decay and also emits a number of conversion electrons and some x rays and gamma rays. The most dominant gamma ray has an energy of 185 keV and occurs in about 54 out of every 100 disintegrations. The major reason that significant masses of ²³⁵U may be handled with relatively little external dose hazard is because the half-life is quite long, namely 704 million years.

The radioactive decay rate of the uranium is inversely proportional to the half-life; thus, for the 3.5 ounces that you mention, if it were 100 percent ²³⁵U, the total ²³⁵U activity would be about 0.21 millicuries, which represents a decay rate of about 8 million disintegrations per second. While this number seems high, it is not a very large amount of activity from a hazards standpoint. Thus, even at a distance of 1 cm from the uranium, if there were no attenuation of the gamma radiation within the uranium itself and the uranium were concentrated in the form of a very small sphere (in reality, a sphere containing this quantity of uranium would have a diameter of about 2 cm), the soft tissue dose rate would be less than 7 mSv/hr. In actuality because of considerable self-shielding of the gamma rays by the uranium material, and because of source geometry effects, the actual gamma dose rate would be appreciably less than this.

While the activity is high enough to be detected at reasonably close distances by most detectors being used for homeland-security purposes, it is not high enough to present an acute radiation hazard to a smuggler who might carry the material in his pocket. Most of the alpha radiations and electrons emitted from the uranium are absorbed within the source material and those that escape from decay events near the uranium surface are attenuated heavily by any material around the uranium and by clothing. The ²³⁸U present has a half-life about six times greater than that of the ²³⁵U and emits very few weak gamma rays, and it would not be an exposure problem.

The low exposure threat of highly enriched uranium is further demonstrated by the fact that such material has been in use as fuel in many research and test reactors throughout the world. When new fuel assemblies are fabricated and sent to such reactors for use, the external radiations present no appreciable threat to workers, who commonly handle new fuel elements directly with no special external radiation protection, generally simply wearing gloves to prevent possible skin contamination and to prevent transferring foreign material to the fuel elements. A shipment of such fuel may contain well over a kilogram of ²³⁵U.

Compared to some other radioactive materials, uranium is not extremely radiotoxic if taken into the human body. If the uranium is present in either a metallic form or in a refractory oxide form (common for uranium), its threat as an internal hazard is rather easy to control. Even keeping the material in a closed plastic bag can prevent any significant loss of the material and internal contamination can be minimized. Again, because of the long half-life, an intake large enough to represent a severe hazard would represent a macroscopic quantity large enough that it would be very unlikely that such an intake could occur.

George Chabot, PhD, CHP