JPS5926326B2 - Isotope separation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for separating a substance into two or more parts, each part enriched with a different isotope of an element contained in the substance. .

Specifically, the present invention involves irradiating gaseous molecules with infrared radiation to excite them using isotopic selectivity, converting these molecules either by the same radiation or by excited state chemical reactions, It relates to a method allowing products to be separated by known techniques.

The present invention is useful, but not exclusively, in the separation of major isotopes of uranium.

The present invention is based on Lyon U.S. Patent No. 3.
No. 937956 and its continuation in part No. 570849
No. (filed April 23, 1975).

1 in the isotope mixture in the first step.
The present invention relates to a method for isotope separation that utilizes infrared photon absorption to selectively excite different isotopes, and in a second step converts said excited isotopes into a form such that they can be recovered from said mixture.

To fully understand the present invention, one must be familiar with conventional photochemical isotope separation techniques.

US Pat. No. 2,713,025 and British Patent No. 1,237,474 are good examples of methods for photochemical separation of mercury isotopes.

To separate isotopes photochemically, conditions must be found such that atoms or molecules of one isotope of a given element absorb light more strongly than atoms or molecules of another isotope of that element. .

Mercury is a volatile metal and easily forms atomic vapor.

These atoms absorb 2537A ultraviolet light.

The absorption line of Hg202 is approximately 0 compared to the absorption line of Hg200.
．． It is deviated by 01A.

Since the absorption line is extremely narrow, either Hg200 or Hg202 can be excited by using light within an extremely narrow wavelength range.

Also, in order to photochemically separate isotopes, atoms or molecules that are excited by the light must be given a small amount of light to which the unexcited atoms or molecules are unaffected or at least not rapidly affected. It is necessary to carry out processing.

A quantum of ultraviolet light from 2537A provides 112.7 Kcal 11 moles of excitation to the mercury atom that absorbs it.

The number of mercury atoms that are thermally excited to this energy at room temperature is extremely small, so atoms excited by light are not diluted by thermally excited atoms.

This highly excited atom readily reacts with H2O (see US Pat. No. 2,713,025, supra) or 02, H2O.
It reacts with CI or butadiene (see the British patent cited above), but these reactions do not occur at room temperature in the case of unexcited mercury.

However, uranium is a highly heat-resistant metal and only boils at extremely high temperatures.

Therefore, it is clearly difficult to apply the above method to uranium atoms rather than mercury.

The most volatile form of uranium is UF6.

Both U235F6 and U2"8F" absorb ultraviolet light,
Since this absorption is exactly the same at all wavelengths of ultraviolet light, ultraviolet light excitation of UF6 cannot satisfy the first requirement of photochemical isotope separation.

However, UF6 also absorbs infrared light in the region around 626crI'L-1 (v3 band) and 189crI'L-1 (v4 band).

The ν3 and ν4 bands of U235F6 are both U238
There is a slight shift toward higher energies relative to the ν3 and ν4 bands of p6, but these shifts are small compared to the bandwidth.

In other words, the infrared absorption spectra of U238F6 and U235 p6 do not exactly match, but they overlap at all wavelengths, so when one isotope absorbs light, the other also absorbs to the same extent.

Infrared excitation of UF6 by absorption of a single infrared photon is therefore a method that takes advantage of limited isotopic selectivity.

Since the means of stepwise isotope separation are known, it is possible to take advantage of this limited selectivity, but in the case of UF6, there are also some difficulties in satisfying the second requirement of isotope separation. There is.

That is, UF6 excited by a single photon of infrared light receives a small amount of energy, but it is almost the same as an unexcited molecule.

Therefore, if one attempts to convert only excited molecules while leaving unexcited molecules, a highly sensitive conversion means is required for the energy content of the molecules.

Another difficulty exists because the energy provided by photon excitation is small and molecules are thermally excited to the same energy level.

(If each molecule in a group absorbs an infrared photon, then those molecules that are photoexcited with isotopic selectivity are It becomes diluted.

Photoexcited molecules quickly disappear, but thermally excited molecules continue to be replenished, so that as time passes after excitation, the selectively excited molecules become non-selectively excited molecules. Dilution by molecules proceeds.

In order to minimize this undesired dilution, it is clearly necessary to reduce the time delay between excitation and conversion.

Irradiation conditions that cause a molecule to absorb one or more infrared photons will excite the molecule to a sufficiently higher energy level than that propagated by thermal means.

In fact, molecules can be excited to the dissociative limit.

In order to perform multiphoton absorption, at least two limiting factors must be overcome.

Firstly, the excitation process must be faster than the various molecular relaxation processes; secondly,
Processes must be devised to help overcome the anharmonic properties of molecular vibrations.

A solution to the first problem is to make the excitation pulse width shorter than the associated relaxation time.

There are many solutions to the second problem. U.S. Patent No. 39379
No. 56 discloses a means to overcome the anharmonicity.

This means uses a second gas to promote rotational relaxation during absorption of infrared photons.

Another method is to use a laser that emits not a single precise wavelength but a finite range of wavelengths that overcome n-level anharmonicity.

Finally, multiphoton absorption can be achieved by increasing the power, the power enhancement limit being determined by the breakdown threshold induced by the laser in a particular gas species or mixture of gas species.

The present invention enables multiphoton absorption by breaking anharmonicity,
Alternative methods of achieving greater isotope enrichment relative to single photon absorption are also taught.

The present invention is a two-step process, where the first step is a combination of selective excitation and conversion of isotopes, and the second step is the collection and separation of converted molecules from unexcited molecules by known techniques. be.

This combined excitation and conversion step is accomplished by simultaneously irradiating gaseous molecules of the element containing the isotopes to be separated with infrared radiation of two different wavelengths.

One of these radiations can be called resonant radiation, since its wavelength must match the absorption band of these molecules (which corresponds to the mode of molecular motion due to the participation of the atoms of that element) .

If the above gaseous molecule is UF6, 1888-185
2crrL-1,1300-1280cIIL-1,1
Preference is given to using resonant radiation with wavelengths in the range 170-1143°, 636-613crL-1 and 196-186crIL'.

Resonant radiation does not need to be very powerful.

The second radiation can be called non-resonant radiation and requires high power.

That is, the power density of the resonant radiation imparted to the molecules is 106 Watts/crtl, preferably 107 Watts/crA,
More preferably, the ratio should be 108 watts)/cr7 or more.

There is no need for a relationship between the wavelength of the non-resonant radiation and the absorption band of the molecule, so any high power laser can be used.

However, the frequency of this high power radiation is preferably brought close to the resonance of the molecules excited by the low power radiation.

Due to its high efficiency, CO2, CO, HF and DF
Lasers are preferred.

The time for irradiating the molecules is less than 10-5 seconds, preferably less than 10-6 seconds, and most preferably less than 10-7 seconds.

The preferred mode of transformation is unimolecular decomposition, in which the irradiated molecules receive sufficient energy to decompose.

However, it is also an option within the scope of the present invention if the energy received by the irradiated molecules is insufficient for decomposition, but for reaction with other gaseous molecules present in the irradiated volume. Examples of processes for chemically converting physically excited molecules are shown in the aforementioned US Pat. No. 3,937,956 and others.

The simplest form of excitation/conversion step is a sequential process.

Resonant radiation selectively excites molecules isotopically.

The density of vibrational states available for further excitation of an excited molecule is greater than that of an unexcited molecule, and thus is more likely to absorb and decompose high power non-resonant radiation.

Incidentally, high power non-resonant radiation refers to radiation that is not absorbed by the fundamental vibrations of molecules.

However, this description assumes that high power non-resonant radiation does not affect the absorption of resonant radiation.

Although it is within the scope of the present invention to operate under conditions where this is effective, high power non-resonant radiation can significantly interact with the absorption of resonant radiation without compromising isotope selectivity. It is also within the scope of the present invention to utilize such conditions.

Furthermore, it is understood that it also falls within the scope of the present invention to inelastically scatter high-power laser non-resonant radiation according to the Raman process, so that the same result as multiphoton vibrational excitation can be obtained when it comes to direct absorption of radiation. sea bream.

If large amounts of photons are produced, this approaches a stimulated scattering process with a larger cross section than observed at low production.

Due to the high density of molecular states, resonance Raman effect processes dominate.

From the above description, the differences between the present invention and the prior art will be easily understood.

That is, US Pat. No. 3,443,087 discloses that an infrared laser is used to selectively excite one of the molecules, the excited molecules are then ionized with ultraviolet light, and the ions are recovered by an electric field and/or a magnetic field, or by a chemical reaction. It is disclosed that U235F6 can be separated from U238F6 by.

Union Carbide Corporation of America
Farrar and Smith's paper ``Photochemical Isotope Separation'' published in Nuclear Tivision's Oak Ridge Gaseous Diffusion Plant Review (March 15, 1972 - L-3054, revised p. 1.29). According to ``Application to Uranium'', the second-stage photoionization method in the above patent is practically inconvenient.

Instead, it suggests photodissociation by a single ultraviolet photon.

British Patent No. 1284620, German Patent No. 195
No. 9767 and No. 2150232 describe the use of infrared radiation to selectively excite molecules and cause them to react chemically (
A method is disclosed in which unexcited molecules react more slowly.

Only one example of this reaction is shown, and that is the thermal decomposition of U(BH4)4.

In all the above references it is clear that the energy imparted to the molecule in the photoexcitation step is that of one infrared photon, whereas in the present invention the molecule is imparted with the energy of multiple infrared photons.

Some literature uses multiple infrared photons.

U.S. Pat. No. 3,937,956 discloses a method for isotope-selectively exciting molecules of a gaseous compound containing the element whose isotopes are to be separated by resonant infrared radiation at a required high power density of at least 104 W/crti torr pressure. is disclosed.

Ambartumian et al. (Nveet Physics, JE
TP21.375.1975), and Lyman et al. (Applied Physics, Letter 27.87.1975
) reports that isotope-selective decomposition of SF6 is carried out at a power density of 109W)/CWt.

These references require the user to use high power lasers operating at resonant wavelengths.

The present invention is clearly different from these documents,
It's advantageous.

That is, in the present invention, the high-power laser may operate at any wavelength, and it is possible to perform the most efficient laser operation.

It is known that the efficiency and cost of producing high power infrared radiation is sensitive to the wavelength at which the laser is operated.

In the present invention, resonant radiation, that is, radiation to be matched to the molecules containing the isotopes to be separated, need only be generated with low power.

EXAMPLE Uranium ore, in which the isotope is naturally distributed, is converted to UF6 by known techniques.

This UF6 has an infrared low power resonant emission in the wavelength range of 636-613 cm,' and at least 108 Watts/cnf.

Simultaneous irradiation with infrared radiation from a Co2 laser of power density greater than or equal to 10-5 seconds for a time period of less than 10-5 seconds, thereby isotope-selectively decomposing UF6.

The low-power resonant radiation should be strong enough to excite the molecule and absorb the non-resonant high-power radiation, but the strength must not exceed a value that would cause level expansion such that isotope selectivity is impaired. must not.

ν3 transition of UF6, i.e. 636-613cIfL-1
For, this strength is 100 watts 104~106
Limited to the range of Watts/cm.

The high power radiation should be of such frequency and intensity that it does not cause level expansion and compromise the isotope selectivity obtained with the low power radiation alone.

Preferably, the frequency of the high power radiation approaches the resonance of the molecules excited by the low power radiation.

Wavelengths, bandwidths, energies of both small and high power radiation,
Pulse width and pulse repetition characteristics should be adjusted to obtain maximum production with optimal isotope separation conditions.

The decomposed molecules are then separated from the undecomposed molecules to form isotopically enriched and depleted uranium.

This separation and recovery is performed by known techniques.

Stepwise isotope separation techniques are known, and if it is desired to further deplete depleted uranium or further enrich enriched uranium, the separation process can be repeated using known techniques.

Claims (1)

[Scope of Claims] 1. A method for separating isotopes of an element, which is applied to a gaseous compound of the element, wherein the molecules of the compound are simultaneously exposed to two infrared radiations of different wavelengths at 10-5
emitting for a time subsecond, the wavelength of the first radiation matching an absorption band of said compound (which absorption band itself corresponds to a mode of molecular motion involving the atoms of said element); and a second radiation. at a power density of 106 Watts/cst or higher, thereby selectively exciting and converting isotopes of the compound, provided that the conversion is either by decomposition or reaction with a second gas, and the converted A method consisting of steps in which molecules are separated and recovered from unconverted molecules by known techniques. 2. The first radiation is of sufficient intensity to excite molecules that cause them to absorb the second radiation, but less than the intensity that would result in level expansion that would inhibit isotope selectivity. A method according to claim 1, characterized in: 3. The method according to claim 1 or 2, characterized in that the gaseous compound is a uranium compound. 4. A method according to claim 3, characterized in that the power density is greater than 107 Watts/crA and the irradiation time is less than 10-7 seconds. 5. A method according to claim 3, characterized in that the power density is greater than 108 watts/lst and the irradiation time is less than 10-6 seconds. 6 The above second radiation is CO2, CO1HF or DF.
The method according to claim 4, characterized in that the generation is performed by a laser. 7. A method according to claim 6, characterized in that said second radiation is generated by a CO2 laser. 8. The method according to claim 3, characterized in that the uranium compound is UF6. 9. The wavelength of the first radiation is 1880-1852c.
rrL-1, 1300-1280cIn-1, 1170
1143cIn-1, 636-613cTl-1, and 196-186C111''. 10. The method according to claim 8, wherein the gaseous compound is 11. A method according to claim 1, characterized in that the wavelength of said first radiation is in the range 636-513 m'.11. /cr
11. A method according to claim 2 or claim 10, characterized in that it is in the range of A to 106 watts/crlt.