The most common types of commercial power reactor use water for both moderator and coolant. Criticality may only be achieved with a water moderator if the fuel is enriched. Enrichment increases the proportion of the fissile isotope U-235 about five- or six-fold from the 0.7% of U-235 found in natural uranium. Enrichment is a physical process, usually relying on the small mass difference between atoms of the two isotopes U-238 and U-235. The enrichment processes in commercial use today require the uranium to be in a gaseous form and hence use the compound uranium hexafluoride (UF6). This becomes a gas Translate the following article from English into Russian. at only 56oC under atmospheric pressure, but is readily contained in steel cylinders as a liquid or solid under pressure.
The two main enrichment (or isotope separation) processes are diffusion (gas diffusing under pressure through a membrane containing microscopic pores) and centrifugation. In each case, a very small amount of isotope separation takes place in one pass through the process. Hence repeated separations are undertaken in successive stages, arranged in a cascade. The product from each stage becomes feed for the next stage above, and the depleted material is added to the feed for the next stage Translate the following article from English into Russian. below. The stages above the initial feed point thus become the enriching section and those below are the stripping section. Each stage thus has a double feed (enriched product from below and depleted product from above). Ultimately, the enriched product is about one sixth or one seventh the amount of depleted material, so that the product end of the cascade tends to have more stages. The depleted material, drawn off at the bottom of the stripping section, is commonly called tails and the residual U-235 concentration in the tails is the tails assay.
An extract from Bluebells and Nuclear Translate the following article from English into Russian. Energy ,by Albert Reynolds, 1996,CogitoPress updated September 2010