Enrichment of Uranium

1980? film, with transcript

Catalog description: An explanation of uranium enrichment, particularly focused on the older gaseous diffusion methods.

This film is in our catalog.

Show transcript
00:04 This is the core of a nuclear reactor during refueling. The unloaded assemblies have
00:14 progressively consumed the enormous energy reserves they originally contained in the
00:19 form of uranium. Not natural uranium, but as in nearly all nuclear power stations in the world,
00:27 enriched uranium, the fruit of long preparation. It all begins in the mine, underground, or opencast.
00:39 A relatively poor ore, one to three kilograms of uranium per tonne, which has to be concentrated
00:50 and purified. It is sorted, ground and submitted to a whole sequence of
00:59 chemical treatments in which the uranium changes form and color many times.
01:05 It appears as yellow cake or uranium concentrate, then ammonium urinate, now as
01:15 trioxide, here as tetrafluoride, and finally as uranium hexafluoride, a gas which is crystallised
01:34 for shipment in these containers. It is ready to be submitted to a much deeper transformation
01:40 of quite another nature: enrichment.
02:10 Behind this cooling tower, a huge plant. It can supply a hundred or so nuclear power stations
02:20 of the 1,000 megawatt range. More than a quarter of world enrichment capacity. These buildings
02:29 are so wide and so long that they appear squat.
02:34 Yet they are around 40 meters high.
02:47 In these containers, uranium hexafluoride,
02:50 natural uranium, soon to be introduced into this plant for a kind of sorting process
02:56 on an infinitely small scale, the scale of the atom.
03:03 Natural uranium contains two isotopes, uranium-235
03:09 and uranium-238.
03:13 Uranium-235, which will undergo fission, is far the less abundant, only 0.7%.
03:24 Thus, in one tonne of uranium in the natural state, there are only seven kilograms of uranium-235.
03:33 Light-water reactors require uranium four to five times richer, with up to 3.2 percent of uranium-235.
03:40 Thirty-two kilograms per tonne.
03:44 That is twenty-five more than in natural uranium.
03:47 Such enrichment can only be obtained at the expense of further tonnes of uranium.
03:51 uranium. To enrich this tonne of uranium by adding 25 kilograms of uranium-235,
03:58 other tonnes must be depleted and as much as possible. If we were content to
04:07 extract one kilogram of uranium-235 from every tonne, we'd need 25 tonnes of
04:12 natural uranium. That's a lot because uranium is expensive. In practice, about
04:18 About 5 kilograms of uranium-235 can be extracted from every tonne,
04:23 thus it's enough to deplete only 5 tonnes of uranium in an exchange to modify the 2,3,5 content,
04:29 initially 7 kilograms per tonne.
04:35 On the one hand, we obtain 1 tonne containing 32 kilograms of uranium-235,
04:41 uranium at 3.2 percent.
04:43 And on the other hand, 5 tonnes containing not more than 2 kilograms
04:48 uranium at zero two percent. The enriched uranium on the one side
04:53 and the depleted uranium on the other.
04:58 The goal of the enrichment plant is to effect such an exchange with natural
05:02 uranium as point of departure. This exchange takes
05:07 place progressively due to a series of modules which enrich
05:10 and deplete at the same time.
05:14 The uranium fed in is separated into two equal parts.
05:18 It comes out on one side slightly more enriched, with 2 per 1,000 of uranium 235 more.
05:24 And on the other side, slightly less enriched, with 2 per 1,000 of uranium 235 less.
05:31 These slight variations mean that the operation must be repeated hundreds of times
05:36 in similar modules working in series.
05:39 The enriched uranium from each of these is conveyed to the inlet of the following module.
05:45 While the uranium that comes out depleted is fed back to the preceding module.
05:55 Thus each module is fed both by the uranium enriched in the preceding module
06:00 and by the uranium depleted in the next module up the line.
06:05 Both flows have the same uranium-235 content.
06:10 These modules, thus linked, make up what is known as a cascade.
06:14 Progressive continuous exchange gives us an increase in uranium-235 content in one direction,
06:21 compensated for by a reverse movement of uranium-238.
06:26 We enrich in one direction and deplete in the other.
06:33 Within the plant there are 1,400 modules, 1,400 stages and while each stage enriches
06:40 and depletes at the same time we may say that 800 are enriching uranium and 600 depleting
06:47 it.
06:49 For natural uranium is supplied at about the middle of the cascade.
06:57 from this uranium at 0.71%. On the one hand we have the 800 stages needed to obtain the
07:04 desired degree of enrichment 3.25%. But this uranium can only be enriched through the uranium
07:14 235 extracted from the other side of the cascade and if we need 600 stages on the depletion
07:19 side it is to get the most out of the uranium and finally reject uranium depleted down to
07:25 point two percent. Don't forget that the more we deplete the less natural uranium
07:32 we need at the entry to the process. If we only need to feed in six tons of
07:38 natural uranium to produce one ton of enriched uranium it is because we reject
07:43 the uranium depleted two point two percent five tons of it. This is the
07:51 principle of the cascade. But what exactly are these stages? There are large elements
08:00 about 20 metres high linked by condites, some going in the enrichment direction and others
08:10 in the depletion direction. Isotope separation takes place in the upper part, the diffuser.
08:20 In these tubes, where the uranium circulates in the gaseous state.
08:24 Thus the process is known as gaseous diffusion.
08:29 This is why uranium hexafluoride, UF6, is used.
08:34 It remains in the gaseous state at low temperatures, above 55 degrees centigrade.
08:39 In each gas molecule, the uranium atom is surrounded by six fluorine atoms.
08:47 All of mass 19.
08:48 But there are two possible masses for the uranium atom, 238 or 235.
08:59 Total molecular mass is thus 352 or 349, a difference of three and about 350.
09:08 This is very little to differentiate between the two isotopes.
09:10 Now let's see how we go about it.
09:15 In a gas, the constantly colliding molecules have the same kinetic energy.
09:21 This means that the heavier molecules, containing uranium-238, move slower than the lighter
09:27 molecules, containing uranium-235.
09:34 The faster molecules strike the wall of the containing vessel more frequently.
09:41 If micropores are put in the wall, the lighter molecules will have more chance
09:46 to go through than the heavier ones.
09:57 This leads to slight enrichment of the gas after passage through the wall.
10:03 These porous walls, known as diffusion barriers,
10:06 take the form of long tubes with billions holes per square centimetre. The secret of this technique lies in the manufacture of
10:16 these porous barriers. In each diffuser there are several thousand such barriers between
10:26 two circular plaques seven metres apart. The gas inlet is at the same level as the outlet.
10:33 The gas rises in the up-going tubes, reaching an upper ring shaped area where it is channelled
10:42 to the down-going tubes. There are fewer down-going than up-going tubes, since gas flow progressively
10:49 diminishes over the journey. Flow is half as much at the outlet. This is the depleted
10:55 fraction of the gas. The other half is diffused through the barriers becoming enriched. The
11:02 the enriched gas appears in the low pressure central it is sucked down by a compressor and
11:12 sent to the next diffuser at slightly under atmospheric pressure this has the effect of
11:17 heating it a few dozen degrees each stage is composed of three distinct parts the upper
11:26 part, the diffuser, with the hexafluoride inlet conduit, and the depleted gas evacuation
11:32 conduit. The enriched gas goes down inside the stage. At the bottom is the compressor
11:41 to send the gas to the next stage. Between these two units, a heat exchanger compensates
11:49 for gas heating at each compression.
11:55 The gas is cooled by contact with the pipe network of a special water cooling circuit.
12:00 But let's return to gas circulation in the cascade stages.
12:12 Each diffuser is associated with the compressor at the base of the preceding diffuser because
12:18 it is the one which supplies it with the uranium hexafluoride. The stage is completed with
12:23 the heat exchanger. Such is the complete stage. The diffuser is fed both by the enriched gas
12:34 from the preceding diffuser and by the depleted gas from the following diffuser. A conduit
12:43 It feeds it back to the same compressor.
12:50 This is how the cascade's various enrichment stages are hooked up.
12:54 The size of the elements is impressive.
12:58 This is just the upper part of a stage, the diffuser.
13:13 This is the base, housing the compressor, and the intermediate part, the heat exchanger,
13:28 containing the cooling tubes.
13:37 In the plant, the stages are set out in vast adjacent halls, each with a group of 20 stages.
13:43 The uranium hexafluoride from the preceding group is moved by an initial compressor, the
13:51 only one not topped by a diffuser.
13:53 The gas becomes more and more enriched through the two ranks of stages until reaching the
13:59 last diffuser which has no compressor.
14:03 The gas which comes out is fed to the compressor on the next group.
14:08 Feedback of depleted gas is in the opposite direction.
14:12 time conduit link is between the two ranks of stages and not on the outside.
14:18 Let's take a closer look at gas circulation.
14:24 There are two flows in opposite directions, one towards enrichment, the
14:33 other towards depletion, progressing from group to group. A group can be
14:42 isolated for maintenance from the rest of the plant. This makes for great
14:46 subtleness of operation.
14:55 It takes 70 of these 20 stage groups to make up the 1,400 stages of the cascade.
15:02 Groups are located on both sides of a central aisle in each of the plant
15:07 buildings. Buildings are designed on the scale of the huge units they house.
15:23 This diffuser when set up with its companions in a group of 20 will be streamed through
15:29 by uranium hexafluoride at around 100 degrees centigrade. Almost throughout, gas is at lower
15:36 than atmospheric pressure.
15:40 But not all stages are the same size.
15:48 The complex theory of enrichment requires that throughout the cascade, from the natural
15:54 uranium input to both ends, hexafluoride flow, instead of being even in all stages, must
16:05 be greater near input and diminish progressively towards the extremities
16:10 according to this ideal curve characteristic of enrichment plants.
16:17 In theory each of the 1400 stages should be different. In practice to facilitate
16:28 stage mass production only three stage sizes have been selected.
16:34 Stages differ in height, weight, and compressor power.
16:46 Each size is set up in groups of 20.
16:50 Given these three stage sizes, the number of groups of each size was selected for optimum approximation to the theoretical curve.
16:58 This accounts for the layout of the plant's 70 groups, 40 on the enrichment side and 30
17:08 on the depletion side.
17:13 All these groups are set up in the plant's four buildings.
17:18 The largest houses the large size depletion groups.
17:25 The next houses the large size enrichment groups.
17:31 Then the same building houses the medium size enrichment
17:39 and depletion groups.
17:44 And the smallest building houses the small groups at both ends of the cascade.
17:54 So throughout the plant, all these adjacent stages with natural uranium as the starting
17:58 point lead on the one side to uranium enriched to more than 3% and on the other to uranium
18:04 depleted to around 0.2%.
18:09 The total making up the 70 groups and the 1400 stages of the cascade.
18:23 While in these four buildings the uranium hexafluoride is being diffused and re-diffused
18:27 ceaselessly through thousands of kilometres of barriers, the heat transmitted to the gas
18:32 by compression must be removed.
18:36 This is the role of the cooling towers.
18:38 The water heated in the heat exchangers loses its calories here before being recycled to
18:44 the plant in closed circuit.
18:50 enrichment requires electricity. The power and number of the 1400 compressors
18:56 permanently absorbs the energy provided by three of the four reactors of the
19:01 nearby nuclear power station. Three reactors to run a plant which provides
19:07 enriched uranium for more than a hundred others. A plant which can assure
19:12 long-term nuclear fuel supply for Belgium, Italy and Spain, France's
19:17 associates in this enterprise and for many other customers.
19:26 Every year, these containers bring more than 25,000 tons
19:29 of uranium hexafluoride to the plant.
19:37 At the plant exit, in smaller containers, the same chemical product comes out,
19:42 but with a far greater wealth of energy.
19:46 Soon, this low-enriched uranium will be fuel for a reactor
19:50 after final, more traditional modifications and various industrial processes.
20:00 Now, it is in the form of tiny uranium oxide pellets,
20:05 soon to be placed into fuel rod assemblies.
20:16 Every year, thanks to the Trikasta enrichment plant, about 5,000 such assemblies can be
20:42 produced, the equivalent of 150 million tonnes of oil.