Enrichment - doe-std-1136-20040021

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

2.1.3 Enrichment

Uranium-235 enrichment processes selectively increase the 235U concentration by separating it from

the 238U. The method used for many years in the U.S. is the gaseous diffusion process. Laser separation

has also been demonstrated in this country, but a facility built to accommodate the process has not yet

been brought on- line. Centrifugation is a third separation method used by foreign sources. Uranium

feed for the enrichment process is derived from virgin ore or from "very clean" recycled material.

Although some uranium is still mill-derived, much of the feed is recycled material from other countries,

including Canada (where natural uranium is the reactor feed material). Specifications on acceptable

contamination levels limit the feed that may be processed in the U.S. gaseous diffusion plants.

Recycling of reprocessed (irradiated uranium) material from DOE's reactors years ago contaminated

the diffusion process equipment with transuranics, a portion of which remains in the equipment.

The specific activity of essentially pure uranium depends on its degree of enrichment and normally

describes only alpha activity. The beta activity from associated decay products is not included in the

uranium specific activity values, but is expressed separately. Consequently, two specific activities (one

for alpha and one for beta) are frequently calculated for uranium-bearing materials. Some typical alpha

specific activity va lues are given in Table 2-5 and Figures 2-1, 2-2, 2-3, and 2-4.

Table 2 -5. Uranium Specific Activities

Wt. % 235U

Type

Specific Activity (Ci/g) of Mixture

Natural

0.72

7 x 10-7

Depleted

0.20

4 x 10-7

Enriched

2.0

1 x 10-6

Enriched

9 x 10-6

For gaseous diffusion enriched uranium, the approximate alpha specific activity of a given

uranium enrichment can be calculated from the following formula:

Specific Activity of Enriched Uranium = (0.4 + 0.38E + 0.0034E2) x

10-6 Ci/g where E = % 235U by weight, enrichment > or = 0.72

Gaseous diffusion, the predominant existing enrichment technology, causes a greater increase in

234

U concentration than in 235U concentration. For example, when 235U content is increased from 0.72%

(natural) to 2.96%, (an increase of approximately a factor of four),234U content increases from 0.006%

to 0.03%, (a five-fold increase). As a result, the specific activity increases with enrichment, not just

because of the replacement of some 238U with 235U, but more significantly because of the increase in the

amount of 234U present.

Laser isotopic separation (under research) selectively enriches only the 235U, leaving the 234U with

the "tails," or depleted uranium. Therefore, the radiological characteristics of both enriched and

depleted uranium will change when compared to conventional separation techniques. Figures 2-3 and 2-

4 illustrate this effect.

The specific activity of recycled irradiated uranium varies from the value calculated from the

equation given above because that equation is not applicable to recycled material with its added

contaminants. For these reasons, specific activities that are calculated from the formula should be

2-7