Iran's Underground UF4 Plant

Last week, Dafna Linzer had a long, detailed story (and online chat) on intelligence found by German intelligence on a laptop computer stolen by an Iranian citizen in 2004. I am writing a four-part series looking at the key pieces of information: a schematic of a shaft that might be for a nuclear test, plans for an underground facility to produce uranium tetrafluoride (UF4), modifications proposed for the Shahab 3 ballistic missile and the acqisition of relatively advanced P2 centrifuges from Pakistan.

Dafna Linzer’s article on Iranian nuclear activities contained the claim that Iranian engineers in 2002 “completed a set of technical drawings for a small uranium-conversion facility” that would produce uranium tetrafluoride (UF4), although not uranium hexalfuoride (UF6).

Why?

I should note, as Linzer does, that “Nowhere are there construction orders, payment invoices, or more than a handful of names and locations possibly connected to the projects.”

That is in comparison to the very real above-ground Uranium Conversion Facility that Iran is building near Esfahan, based on Chinese designs transferred in the mid-1990s.

Only six countries operate commercial scale uranium conversion facilities.

Source: IAEA Country Nuclear Fuel Cycle Prolifes 2002, 6.

Esfahan is relatively small facility (285 t U/a). Four other countries can convert smaller amounts with pilot plants, including Argentina (Pilcaniyeu, 62), Brazil (Ipero, 40), India (Trombay, 100), Pakistan (Dera Ghazi Khan, 200). South Africa had a medium sized facility at Valindaba (1400) that has been shut down.

Of these facilities, only the French have a commerical facility that just produces UF4 (Malvesi). The North Koreans have some capacity to produce UF4 at Yongbyon.

Overall, the decision to produce just UF4 baffles me. Why hide a large scale production facility under a mountain if you just plan to truck the UF4 to a different facility some place? Doesn’t exposing shipments to interdiction undermine the security created by burying the facility?

Linzer quotes an analyst offering some hypotheses:

A second facility for uranium gas could have been envisioned as a replacement in the event the United States or Israel bombed the existing one in the city of Isfahan. “It was either their fallback in case we take out Isfahan,” one U.S. analyst said. “Or maybe they considered an alternative indigenous plan but they realized it wasn’t as good as what they already have, and so they shelved it.

Understanding the purpose of the plant is difficult—there are at least two additional pieces of evidence that I think I need.

What is the capacity of the plant?

It would be interesting to understand the size of the clandestine facility. Linzer described it as “small”—although that could mean anything smaller than a commercial-scale facility.

Is the “small” plant a copy of Esfahan (with an output of 285 t UF6/a)? Or is it even smaller, such as Brazil’s facility at Ipero (40 t U/a)?

Size, no matter what you’ve been told, does matter: 100 metric tons of uranium, just for comparison, can be enriched into about 165 kilograms of HEU—enough for 6-8 implosion-type fission devices. A duplicate of Esfahan would makes sense as, well, a replacement for Esfahan. A smaller facility would seem suspicious.

What sort of technology is used to purify the U3O8?

A UF4 plant must remove impurities, such as molybdenum, from the U308 before converting it into UF4.

The Iranian UCF originally called for Chinese-designed “mixer-settlers.” After China cut off assistance, Iran experienced technical trouble with the mixer-settlers and switched to indigenously developed “pulse columns” (one and
two).

It would be interesting to see if the underground UF4 plant was based on Chinese mixer-settlers, the TNRC pulse-columns.

Or maybe indigenous Iranian mixer-settlers, which Iran continues to develop.

Mixer-Settlers v. Pulse Columns

I’ll admit, I find the engineering challenges associated with solvent extraction a little tedious (although I did learn that “crud” is a technical term).

I was looking for a nice, readable description of how to use solvents to extract impurities from uranium. Robert Treybal has a nice entry in Access Science on the various methods:

Extractors bring about direct contact of the feed (solution to be separated) and extracting solvent in order to permit diffusional transfer of the constituents from the feed to the solvent. The rate of transfer depends upon the contact area of the two liquids and the degree of turbulence developed within them. The extractor disperses one of the liquids in the other to produce large surface area, and relative motion to produce turbulence. The extractor must also provide for the subsequent mechanical separation of the dispersion, based upon the different densities of the liquids, to permit withdrawal of the two effluent products, the extract (solvent containing the extracted constituents) and the raffinate (unextracted residue).

Mixer-settlers (Fig. 1) provide for these requirements in separate vessels. The feed and solvent flow continuously through the mixer, in which the rotating agitator disperses one of the liquids into small droplets immersed in the other. The size of this vessel must provide sufficient residence time for the liquids that the desired diffusional transfer occurs. The degree of agitation must be intense without, however, producing so fine a dispersion that subsequent settling is difficult. The dispersion flows to the settler, most simply a drum, in which low velocity and lack of agitation promote gravity settling and coalescence of the drops to provide clear effluents.

Fig. 1 Single-stage mixer-settler extractor.

Since in such single-stage apparatus the extractable substance approaches a concentration equilibrium in the effluents, nearly complete extraction requires a multiplicity of stages. An arrangement for countercurrent interstage flow of the liquids reduces the amount of solvent needed (Fig. 2). The compact modification of Fig. 3 has found particular favor in extraction of radioactive metals from aqueous solutions in processes associated with atomic energy operations.

Fig. 2 Diagram of a three-stage countercurrent mixer-settler extractor. (After R. E. Treybal, Mass Transfer Operations, 2d ed., McGraw-Hill, 1968)

Fig. 3 Three-stage, box-type, mixer-settler extractor.

To reduce the floor space and pump requirements for multistage extractors, a variety of vertical towers are also used. These involve countercurrent vertical flow, under gravity, of one of the liquids in dispersed form through a continuum of the other by virtue of the different liquid densities. A packed tower (Fig. 4a) is a cylindrical shell, the bulk of which is filled with manufactured packing, such as rings or saddles, randomly arranged. The more dense liquid, introduced at the top, flows downward as a continuum. The less dense liquid enters at the bottom through small nozzles. The resulting small droplets rise through the heavy liquid, during which time extraction occurs, and then coalesce into a bulk and leave at the top. The packing serves to maintain the dispersion and provide moderate turbulence. The dispersed liquid may be either feed or solvent, light or heavy. If it is heavy, the droplets settle downward. Although the liquids are not repeatedly dispersed and settled as in the multistage mixer-settler, nevertheless multistage effects are obtained. Spray towers contain no packing and are not as effective. See also: Gas absorption operations

Fig. 4 Vertical tower extractors. (a) Packed-tower extractor. (b) Perforated-tray extractor. (After R. E. Treybal, Mass Transfer Operations, 2d ed., McGraw-Hill, 1968)

In perforated-tray towers (Fig. 4b) the light liquid collects in a layer under each tray and is dispersed into droplets by the small perforations. The drops rise through the heavy liquid, which flows across each tray and through the downspouts. The frequent redispersion achieved makes these towers very effective. Alternatively, by turning the tower upside down, the heavy liquid may be dispersed.

Mechanical agitation, provided by rotating impellers as in the towers of Fig.5 a, b, and c, is used to obtain finer dispersions and increased turbulence. The pulsed tower (Fig.5 d) provides the mechanical agitation by rapid (20-100 cycles/min), small-amplitude (0.25-2 in. or 0.64-5.0 cm), reciprocating motion superimposed upon the natural flow of liquids as they alternately pass through small perforations in the plates. This is particularly useful for handling radioactive liquids, since moving parts may be located in a place of safety.

In all these designs, the tower diameter is governed by the quantity of liquids to be handled, the height by the number of stages of extraction required. Towers up to 15 ft (4.8 m) in diameter and 125 ft (38 m) tall have been built. Auxiliary equipment may include pumps for movement of the liquids, motor drives for agitators, valves and flowmeters for control of flow rates, and liquid-level control instruments.

Robert E. Treybal, “Solvent extraction”, in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.636100, last modified: February 26, 2001.

The whole article is worth a read, really (subscription only).