URANIUM HEXAFLUORIDE INDUSTRIAL DEFLUORINATION
P. Boissin
Deputy general manager of COGEMA Pierrelate site
B. Duperret
Project manager, COGEMA Pierrelatte
W. Bramy
Director Fuel Cycle Front-end promotion, SGN (FRANCE)
A.M. Jouan
Process engineer - Enrichment Division, SGN (FRANCE)
ABSTRACT
Huge quantities of depleted uranium hexafluoride UF6 are generated as a by-product of the uranium enrichment industry.
This compound creates some risks for long term storage :
Total transformation to sesquioxide U3O8 gives a higher specific mass and chemically stable form, allowing economic and simple storage such as using thin cubic steel containers piled up into light buildings.
The main steps in the uranium hexafluoride defluorination process are hydrolysis, followed by pyrohydrolysis using a combination of steam and hydrogen.
The complete process, however, involves other upstream and downstream operations. The sequence is :
It should be noted that this process, as such, generates absolutely no unwanted effluent, acidic or other: all the fluorine is reduced to hydrofluoric acid, without uranium, which allows its sale to chemical industry.
If needed, U3O8 can be recovered and transformed into oxide or fluoride then metal.
An important industrial experience has been gained on this process in COGEMA's plant, located at the Tricastin Nuclear Complex Site in France. The Defluorination Facility has been in operation since 1984. With the commissioning of a second facility in 1993, the plant can process 14,000 metric tons of uranium per year as depleted UF6 (» 21,000 mt of DUF6), yielding » 17,000 metric tons of U3O8 and » 10,000 mt of 70 % hydrofluoric acid.
The acquired experience is based on roughly 150,000 mt of U3O8, with safe and economic results in a large scope of technologies.
In case of damaged UF6 containers, the process can be started through direct sublimation. The damaged container is fixed inside a leaktight heated containment so as to avoid UF6 liquefaction. UF6 emission is made without risks.
INTRODUCTION
Huge quantities of depleted uranium hexafluoride, DUF6, are generated as a by-product of the uranium enrichment industry. Storing them in standard metal containers would tie up considerable store space for many years and involve some risks : potential chemical reactions with water may give rise to solid uranium compounds and dangerous gaseous hydrofluoric acid releases; underground water might thus be contaminated for a long time.
COGEMA therefore sought out, and came up with, a better solution to the storage problem: basically, this consists of defluorinating the by-product and converting it to a fluorine-free oxide : uranium sesquioxide U3O8, and a marketable product: hydrofluoric acid, HF.
PROCESS PRESENTATION
The main steps in defluorination of uranium hexafluoride are hydrolysis followed by pyrohydrolysis using a combination of steam and hydrogen. The complete process, however, involves other upstream and downstream operations.
The sequence is (see Figure 1)
It should be noted that this process, as such, generates absolutely no unwanted effluent, whether acid or other: all the fluorine is reduced to hydrofluoric acid, without uranium.
Fig. 1.
FACILITY DESCRIPTION
DUF6 Emission (see figure 2)
DUF6 is transported in 48" standard containers (48G or 48 Y types) from the enrichment plant store to the defluorination facility. Each container has a 12.5 mt DUF6 capacity.
These containers are then transferred to the DUF6 Emission unit and put into hermetically sealed autoclaves. Wet steam (100°C) is admitted to heat, liquefy and evaporate the DUF6. Water is collected, controlled and then flushed away.
The implementation of several batchwise autoclaves leads to the continuous production of gaseous UF6 fed to the next step: Defluorination.
Emission absolute pressure : ~ 2 bars
In case of damaged containers, UF6 can be sublimated to avoid UF6 liquefaction and reduce risks. The requirement is to have no UF6 leak, no HF release and no contamination.
Containers are placed in hermetically sealed autoclaves in which air is stirred and heated.
Air temperature : between 80 and 100°C
UF6 absolute pressure : below 1.5 bar
In case of very damaged UF6 containers, the autoclaves are externally heated by electrical heat tracing.
Autoclave surface temperature : ~ 60°C
UF6 absolute pressure : below atmospheric pressure
Gaseous UF6 is then compressed and transferred to Defluorination.
Fig. 2.
Defluorination (see figure 2)
Defluorination is carried out in a continuous conversion kiln consisting of a reactor with rotary tube.
Gaseous UF6 is fed into the reactor where it comes into contact and reacts with overheated steam.
The hydrolysis reaction is :
This exothermic reaction requires controlling and maintaining the temperature.
Solid uranyl fluoride falls to the reactor bottom and is transferred by a screw conveyor to the externally heated rotary tube. UO2F2 reacts at 750°C with hydrogen and overheated steam. This temperature is needed to obtain free fluorine oxide.
The endothermic pyrohydrolysis reaction is :
Pyrohydrolysis heat is supplied by electric kiln surrounding the rotary tube.
Solid uranium sesquioxide falls into a hopper and leaves the conversion kiln through a lock chamber to a compacting station.
Hydrogen fluoride and steam go to HF processing system through a filtration chamber and then through a safety filtration stage to remove UO2F2 and/or U3O8 entrained by the gas. The filtration chamber is made up of a candle bundle and installed vertically above the reactor.
U3O8 Compacting and Packaging (see figure 2)
U3O8 is continuously transferred from Defluorination to the compaction station hopper. Compaction aims at minimizing storage spaces before packaging .
Compacting is carried out by a rolling press. Compacted U3O8 (density 3 to 4) falls in the 3 m3 cubic storage metal container placed on a vibrating table.
U3O8 containers are stacked on three levels in modular sheds.
HF Processing and 70% HF Storage (see figure 3)
Defluorination gases contain HF and steam in the following proportions :
HF : 70 % + x %
H2O : 30 % - x %
HF is processed in two steps.
In the first step most of the gases are condensed and the condensate containing slightly more than 70% HF flows from the demister to a tank.
In the second step the gases from the demister are scrubbed first in a water jet and then in a finishing column. The scrubbing solution consists of water fed at the top of the scrubber and circulated in the reverse direction to the gases so that it is progressively enriched with HF. The solution is finally mixed with condensates.
The water feed flowrate is such as to obtain aqueous HF with 70 % HF content at the end of HF processing.
This uranium free product is pumped to the "70 % HF store".
Non-condensable products released to the atmosphere at the top of the column contain no more than 3 ppm HF. 70 % HF is stored in 20 m3 tanks pending transfer to road or rail tanks and final shipment to companies whichrepack it for sale or otherwise employ it.
Tight drip-trays are installed under the tanks and there is provision, in the event of leakage or spillage, for spreading film of oil to prevent any escape of HF into the atmosphere.
Fig. 3.
Cogema W Plant
The above described process is implemented by COGEMA in its W Plant located on the Tricastin site, France.
The W Plant features two similar units commissioned in 1984 and 1993, and is consisting in :
The annual material balance of the plant is as follows :
|
14,000 metric tons U as DUF6 (» 21,000 mt DUF6) |
|
» 17,000 mt U3O8» 10,000 mt aqueous HF (HF content : 70 weight %) |