Page 49 - Radiochemistry and nuclear chemistry

38 Radiochemistry and Nuclear Chemistry

FIG. 2.11. Section through a separation nozzle arrangement showing stream lines.

stages for a 3 % 235U enrichment (with 0.2 % tails), the nozzle technique will require only
about 500 stages. However, the power consumption is said to be larger, ~ 3000 kWh
SWU-l at a capacity of 10 MSWU/y, as compared to ~ 2500 kWh SWU-l for a diffusion
plant of the same size. The operating cost is said to be higher than for both the diffusion
and the centrifuge methods. A plant based on the original German design is being built in
Brazil (10 kSWU/y). A 0.3 MSWU/y separation plant at Valindaba, South Africa, used its
own version of the nozzle technique (Helikon process), but is now shut down.
A number of photoionization and photoexcitation processes are being investigated for
isotopic separation, especially of uranium. In one such process UF 6 is irradiated by a laser
beam, producing selective vibrational excitation in the 235UF6 molecule (of. w By
irradiation with ultraviolet (possibly, but not necessarily, by laser) light the excited
molecule is caused to dissociate, leaving 238UF 6 undissociated. It is important that the
ultraviolet pulse follows quite rapidly after the laser pulse, so that the vibrationally excited
235UF6 molecule does not lose its excitation energy through collision with surrounding
molecules. It is obvious that this necessitates gas phase reactions. The 235UFn 6-n ion
formed through the dissociation (n < 6) is then collected by the action of electromagnetic
fields. This technique is not limited to UF6; pure uranium metal vapor and plutonium
compounds have been separated into their isotopic constituents by two or three photon
ionization with laser light (e.g. the US AVLIS process and the French SILVA process).
Another alternative is to selectively excite 235UF 6 molecules by laser light in the presence
of a reactive gas (the CRISLA process). The excited molecules then reacts preferentially
with the gas forming molecules with lower vapor pressure than UF 6. Although research in
these areas has indicated a large scale feasibility of several similar processes, no predictions
can yet be made of their technological value. The energy consumption for the quantum
processes discussed above are in the range 10 - 40 kWh SWU-1. However, economic
estimates indicate that their enrichment cost falls in the same price range as for centrifuge
based plants. Hence, the interest in these methods has decreased and development of the
ALVIS and SILVA processes has been terminated. Finally, it should be pointed out that the
general concept of separative work breaks down for very high separation factors leading
to very few units in a cascade (and always for mixtures of more than two isotopes).