JP3264568B2 - Gadolinium isotope separation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for isolating gadolinium contained as a burnable poison in a part of a fuel rod of a nuclear reactor, and more particularly to a method for separating gadolinium vapor containing a plurality of isotopes. Gadolinium isotope separation method and apparatus for selectively irradiating a plurality of pulsed laser beams to selectively excite and ionize Gd156, Gd158, and the like, thereby lowering (deteriorating) their contents below the natural abundance ratio About.

[0002]

2. Description of the Related Art Some nuclear fuel rods loaded into a nuclear reactor contain gadolinia (gadolinium oxide) mixed with uranium oxide as a burnable poison to control the initial reactivity of the reactor. I have. The number of gadolinia-containing fuel rods used was determined to keep the excess reactivity at the beginning of the reactor operation cycle within an appropriate range, and the gadolinia concentration did not cause loss of reactivity due to gadolinia burning out at the end of the combustion cycle Is determined as follows.

The gadolinium element contains seven types of isotopes, and its natural abundance is 2% for Gd154, 15% for Gd155, 20% for Gd156, and 20% for Gd156.
157 is 16%, Gd158 is 25%, and Gd160 is 2
2%. Among these, Gd155 and Gd157, whose neutron absorption cross sections are extremely large, play the role of the above-mentioned surplus reactivity control, and these decrease as the combustion proceeds. Further, gadolinium combustion is approximately proportional to the atomic number density, but the combustion ratio depends on the atomic number density and the thermal neutron absorption cross-section, so that Gd155 and Gd157 having large cross-sections rapidly increase at the same time as the combustion starts. Decrease. However, after that, the atomic number density reaches an almost constant value when the reduction amount and the production amount are balanced in the nuclide conversion chain. At this point, gadolinium was burned out.

On the other hand, the atomic number density of other isotopes is slightly reduced or increased, so that gadolinium as a whole maintains neutron absorption, which causes a loss of reactivity. The extent of the reactivity loss greatly depends on the content of gadolinium isotope in gadolinia, and the reactivity loss is reduced by concentrating Gd157 or depleting Gd156 or Gd158 with the natural gadolinium currently used. As a result, the combustion efficiency of the fuel can be improved.

[0005] Such concentration of Gd157 or Gd1
Isotope separation technology can be used to achieve 56 or Gd158 impairment. In the field of uranium isotope separation, gas diffusion and centrifugation have been established in the past,
In recent years, it has been found that higher isotope separation efficiency can be achieved by using the atomic laser method. In addition, this method is attracting attention as being capable of obtaining high separation efficiency even when applied to gadolinium isotope separation.

In the gadolinium isotope separation method based on the atomic laser method, a gadolinium metal material is irradiated with an electron beam and heated in a vacuum to generate an atomic vapor flow, and the atomic vapor flow is irradiated with laser light. In this method, a specific isotope component is selectively excited to be ionized, and the ionized specific isotope component is collected on an electrode. That is, by utilizing the energy level difference between a plurality of isotopes (isotope shift), it is possible to selectively excite only a specific isotope, and further perform ionization recovery to concentrate or deplete.

[0007] There are two types of isotopes: those having an even number and those having an odd mass number. Nuclear spin is zero in an even nuclear isotope, whereas this is not zero in an odd nuclear isotope.
Due to this nuclear spin effect, the energy levels degenerated in even nuclear isotopes are slightly split in odd nuclear isotopes.
For this reason, the light absorption line which was one in the even nuclear isotope has a plurality of split structures called hyperfine structure in the odd nuclear isotope.

[0008]

Therefore, even if an attempt is made to increase the content of Gd157 by the conventional atomic laser method of tuning the laser light to the resonance light absorption line,
Since the 157 resonance light absorption line has an ultrafine structure,
It is difficult to efficiently perform excitation and ionization recovery.

Further, Gd156 is formed by a conventional atomic laser method.
In order to lower the content of Gd157, although the excitation and ionization are easy because there is only one resonance light absorption line, the resonance light absorption line of Gd156 that can be used for the first-stage excitation is Gd157 as shown in FIG. , It is difficult to selectively excite and ionize only Gd156 by irradiating laser light, and Gd156 and Gd157, which is preferably concentrated, may be depleted simultaneously.

[0010] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to reduce the off-resonance excitation effect of a non-target isotope so that Gd156, which is an isotope to be impaired, is reduced.
Another object of the present invention is to provide a gadolinium isotope separation method and apparatus capable of separating Gd158 and the like with sufficiently high efficiency.

[0011]

The present invention is configured as follows to solve the above-mentioned problems.

[0012] The invention described in claim 1 of the present application (hereinafter referred to as the first invention)
The invention discloses a method of heating a gadolinium raw material containing a plurality of isotopes to generate a vapor stream, and irradiating the vapor stream with pulsed laser light for selectively exciting and isolating specific isotopes. Then, in the gadolinium isotope separation method for recovering the ionized specific isotope component, in the vapor stream, the pulse swept in frequency around the resonance light absorption line of the isotope to be depleted at the ground and metastable levels Adiabatic inversion excitation by irradiating laser light, followed by selective excitation and ionization by irradiating pulse laser light for excitation and ionization, and recovering the same, the content of the isotope to be depleted is naturally present. Lower than the ratio.

The invention described in claim 2 of the present application (hereinafter referred to as the second invention) is characterized in that the isotope to be impaired is Gd1
56 and at least one of Gd158.

Further, the invention described in claim 3 of the present application (hereinafter, referred to as a third invention) is characterized in that all of the first and second metastable levels whose occupancy is increased by the heat distribution simultaneously with the ground level. A state in which the total angular momentum is 3 is set as a first excitation level so as to allow an optical transition from, and this is set as a resonance light absorption line.

Further, in the invention according to claim 4 of the present application (hereinafter referred to as a fourth invention), a gadolinium raw material containing a plurality of isotopes is heated to generate a steam flow, and the steam flow is specified. In a gadolinium isotope separation apparatus that selectively excites and ionizes isotopes by irradiating with a pulsed laser beam and recovers the ionized specific isotope component to an electrode, a base stream and a metastable Adiabatic inversion excitation is performed by irradiating pulsed laser light whose frequency is swept around the resonance light absorption line of the isotope to be depleted at the level, and then selectively excited by irradiating pulsed laser light for excitation and ionization A laser system for performing ionization by means of ionization and isotope recovery means for recovering the ionized isotope.

The invention described in claim 5 of the present application (hereinafter referred to as a fifth invention) is characterized in that the isotope to be impaired is Gd1
56 and at least one of Gd158.

Further, according to the invention described in claim 6 of the present application (hereinafter, referred to as a sixth invention), the laser system is characterized in that a frequency sweep of a pulse laser required to cause adiabatic inversion excitation of an isotope to be depleted is performed. And an electro-optical element for performing pulse waveform control.

[0018]

The gadolinium raw material containing a plurality of isotopes is heated by electron beam irradiation or the like to generate a vapor flow.
The gadolinium vapor is irradiated with pulsed laser light whose frequency has been swept around the resonance light absorption line of the isotope to be depleted at the ground or metastable level. Then, during one pulse irradiation of the laser light, these are selectively adiabatically inverted and excited (almost 100% excited) to the first excitation level.
At this time, as shown in FIG. 4, within the frequency sweep width of the pulse laser or in the vicinity thereof, there is a light absorption line of an odd-numbered gadolinium isotope expanded due to the hyperfine structure. These odd nuclear isotopes are also excited to the first excited level.

However, since the odd nuclear isotope has an ultrafine structure, the adiabatic reaction conditions are not sufficiently satisfied for all absorption lines, and the amount of excitation is kept low. Even in the case where the resonance light absorption lines overlap between a plurality of isotopes in this way, the use of adiabatic inversion excitation suppresses the excitation of the asymmetric nucleus, which is an asymmetric nucleus, while suppressing the excitation of the asymmetric isotope, which is an even nucleus. Almost 100% of
Since the vicinity can be excited to a higher level, high selective excitation can be realized.

Further, the isotope to be depleted which has been excited to the first excitation level is successively irradiated with the excitation pulse laser beam and the ionization pulse laser beam to be excited to the ionization level. Since the energy level density of is high, a level having a large isotope shift is selected as a candidate of a second excitation level capable of optically transitioning from the first excitation level,
If this is used as a resonance light absorption line, the selectivity is further enhanced, and a sufficiently high separation performance can be obtained for the isotope to be depleted.

An electro-optical element is an optical effect called a Pockels effect or a Kerr effect in which a refractive index changes in proportion to the intensity of a voltage when a voltage is applied to a certain kind of optical crystal. And is used as an optical switch or an optical frequency modulator.

This electro-optical element is used as an optical switch for controlling the timing and pulse waveform of irradiating a laser beam to a gadolinium vapor stream, and as a device for modulating the optical frequency within one pulse of the laser beam at high speed. By using this, adiabatic inversion excitation of the isotope to be depleted is enabled.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. 1 to 7, the same or corresponding parts are denoted by the same reference numerals.

FIG. 2 is a perspective view showing a gadolinium isotope separation apparatus according to an embodiment of the present invention. The gadolinium isotope separation apparatus selects an isotope to be depleted such as at least one of Gd156 and GD158. The metal gadolinium 1 containing a plurality of isotopes is accommodated in an evaporating crucible 2 in a vacuum vessel (not shown).

The metal gadolinium 1 is irradiated with an electron beam 4 emitted from an electron gun 3 after being deflected by an external magnetic field coil (not shown). The metal gadolinium 1 irradiated with the electron beam 4 is heated and evaporated to form a vapor stream 5.

In the middle of the vapor flow 5, a positive electrode 6 and a negative electrode 7, which are strip-shaped ion recovery electrodes, are alternately arranged at a required interval in a direction perpendicular to the vapor flow 5. A photoreactor 8 is provided between each of the electrodes 6 and 7. A pulsed laser beam 10 from a laser system 9 is incident on each of these photoreaction sections 8 from the longitudinal direction, and the gadolinium vapor stream 5 is irradiated with the laser beam 10.

The isotope to be depleted in the gadolinium vapor stream 5 is selectively excited by absorbing the laser beam 10, becomes further ionized cations, and is sucked and collected by the negative electrode 6. The vapor flow 5 that has passed through the photoreactor 8 without being ionized collides with the product recovery plate 11 disposed above the photoreactor 8 in the drawing and is collected.

FIG. 3 shows an energy level (level scheme) used in the steps of exciting and ionizing Gd156 to be depleted according to such an embodiment. That is, FIG. 2A shows the level scheme of Gd156, and FIG. 2B shows the level scheme of odd nuclear isotopes (Gd155, Gd157) having an energy level spread due to the hyperfine structure. ing.

In FIG. 3, the energy levels of Gd 156 include a ground level 12 and first and second metastable levels 13 and 14 which are excited at a high rate due to thermal energy of the environment. Is called a first-stage excitation level, and a step of further exciting the first excitation level 15 to the second excitation level 16 is called a second-stage excitation. The step of exciting from the two-stage excitation level 16 to the third excitation level 17 is called third-stage excitation. In particular, third-stage excitation 1
7 is also called an ionization stage because it is a step excitation to a level at which ionization occurs immediately after excitation (automatic ionization level).

A thick arrow line 1 connecting these levels 12 to 17
8 to 22 schematically show the transition intensities excited to the upper level by the irradiation of the pulse laser beam 10 depending on the thickness.

On the other hand, the odd nuclear isotope (G
The energy level of d157 or Gd155) is also a ground level 23, a first metastable level 24, a second metastable level 25, a first excitation level 26, a second excitation level 27, and a third excitation level. 28
Arrows 29-3 connecting these levels 23-27
Numeral 2 schematically shows the transition intensity excited to the upper level by the irradiation of the pulse laser beam 10.

Regarding the restrictions on the level scheme to be used, generally, atoms have an enormous number of energy levels, and a plurality of candidates can be called even as a level scheme suitable for the impairment of Gd156. The following points must be considered when doing so.

[0033]

[Outside 1]

Moreover, each energy level has its own angular momentum (denoted by J). Between two levels at which optical transition is allowed, the change ΔJ of the J value is ΔJ.
= 0, ± 1 (selection rule). This and
Each of the ground level 12, the first metastable level 13, and the second metastable level 14, which have no degree of freedom as energy levels to be used,
Since the values are J = 2, 3, and 4, respectively, the J value of the first excitation level 15 needs to be 3. Therefore, in this embodiment, a level scheme of 2, 3, 4, 3 is used as the J value of each of the levels 12, 13, 14, and 15, respectively.

Next, the irradiation conditions of the excitation pulse laser at each stage by the laser system 9 shown in FIG. 2 will be described with reference to FIG.

This embodiment utilizes adiabatic inversion excitation for the first stage 33 of the level scheme shown in FIG.
A frequency sweeping type in which the frequency is swept around the resonance light absorption line of the isotope to be depleted for excitation from the ground level 34 and the first and second metastable levels 35 and 36 to the first excitation level 33. Irradiation is performed with pulsed laser beams 10a, 10b, and 10c, followed by irradiation with single-mode (monochrome) pulsed laser beams 10d and 10e for excitation in the second and third stages 37 and 38 and thereafter.

The frequency-swept pulsed laser beam 10a
-10c are modulated within one pulse so that their frequencies straddle the resonance light frequency of the isotope to be depleted. FIG.
Are the levels 33 to 3 of the level scheme shown in FIG.
8 (b), time waveforms 39 to 43 indicating the irradiation timings of the five-wavelength pulsed lasers 10a to 10e are shown in FIG.
4 to 48 are shown in FIG.

FIG. 5 shows a basic system of a laser system 9 for three-stage excitation and ionization of five wavelengths in this embodiment. The laser system 9 excites five sets of wavelength selecting lasers 50a to 50e and five sets of amplification lasers 51a to 51e by a pumping excitation laser 49, and the five sets of laser beams 10a to 10e are respectively excited. Gd1 in the stage
A required laser for selectively exciting or ionizing at least one of 56 and GD158 is oscillated, and the irradiation timing of laser beams 10a to 10e by five sets of first and second electro-optic modulators 52a to 52e and 53a to 53e. And control the frequency sweep.

Next, the operation of this embodiment will be described.

As shown in FIG. 2, the electron gun 3 is set in a vacuum atmosphere.
An electron beam 4 is radiated from the electron beam 4, and the electron beam 4 is deflected by a deflecting magnetic field coil (not shown) and irradiates the metal gadolinium 1 in the crucible 2.

To this end, the metal gadolinium 1 is heated and evaporated, forming a vapor stream 5. In this steam flow 5,
Pulsed laser beams 10a to 10c whose frequencies are swept around a resonance light absorption line of an isotope such as Gd156 to be depleted at the base or metastable level are emitted.

Then, the pulse laser beams 10a to 10a
During one pulse of c, the isotope to be depleted such as Gd156 is selectively excited, and the first excitation level 33 shown in FIG.
Adiabatic inversion excitation (almost 100% excitation). At the same time, the odd nuclear isotopes Gd157 and Gd155 are also partially excited.

As a result, as shown in FIG.
Is excited about 100% (about 96% in FIG. 6),
Regarding Gd157 and Gd155, the excitation amount of Gd157 is suppressed to about 50% or less as shown in FIG. 7 irrespective of the fact that the light absorption line of Gd156 is located in these ultrafine structures. FIG. 6 shows G in this embodiment.
A calculation example of adiabatic inversion excitation of d156 is shown. In the figure, a curve 54 shows a temporal change of the occupancy of the ground level, and a curve 55 shows a temporal change of the occupancy of the first excited level.
FIG. 7 shows a calculation example of adiabatic inversion excitation of Gd157. In the figure, a curve 56 shows a time change of the occupancy of the ground state, and a curve 57 shows a time change of the occupancy of the first excitation level. Is shown.

Then, the isotopes to be depleted (such as Gd156) excited to the first excitation level are further irradiated with the excitation and ionization pulsed lasers 10d and 10e.
The isotope to be impaired such as d156 is selectively excited and ionized, and is collected and removed by an isotope collecting means such as an ion collection electrode, so that the isotope to be impaired such as Gd156 is impaired from its natural abundance.

As described above, this embodiment has five wavelengths 3
Irradiating gadolinium vapor with pulsed laser light that has been frequency swept around a resonant light absorption line of the isotope to be depleted at the ground or metastable level using a step excitation / ionization scheme and laser system 9 Since the adiabatic reversal is performed to the first excitation level, gadolinium isotopes to be depleted can be separated with high efficiency while suppressing excitation of non-depleted isotopes such as Gd157.

It should be noted that if a level having a large isotope shift is selected as a candidate of a second excitation level capable of optically transitioning from the first excitation level, and this is used as a resonance light absorption line, the selectivity is further improved. As a result, a sufficiently high separation performance can be obtained for the isotope to be depleted. In addition, if adiabatic inversion excitation is also performed for the second-stage excitation, a higher separation performance can be obtained.

[0047]

As described above, according to the gadolinium isotope separation method and apparatus according to the present invention, the frequency sweep pulse is generated in the first stage excitation of Gd156 (and at least one of Gd158), which is the isotope to be depleted. Since adiabatic inversion excitation is performed by the laser beam, almost all of the isotope to be depleted can be excited while the excitation rate of the non-depleted isotope, which is an odd-numbered nuclear isotope, is kept low.

Further, in the excitation process reaching the automatic ionization level, there is a degree of freedom in selecting a level having a large isotope shift. By using the level having a large isotope shift as a resonance light absorption line, the selectivity can be further improved. Sufficiently high separation performance is obtained for the isotope to be depleted.

[Brief description of the drawings]

FIG. 1A shows a pulse waveform of a laser beam for excitation ionization in one embodiment of the gadolinium isotope separation method according to the present invention, FIG. 1B shows an irradiation timing of the laser beam, and FIG.
3 is a diagram showing control examples of the frequency sweep of the laser light in correspondence with each other.

FIG. 2 is a perspective view of a main part of one embodiment of the gadolinium isotope separation apparatus according to the present invention.

FIGS. 3 (a) and 3 (b) show G in the embodiment shown in FIG.
The schematic diagram which shows the energy level of d156 and Gd157 (or Gd155), and the optical transition intensity between levels, respectively.

FIG. 4 is a spectrum diagram of an example of a gadolinium isotope resonance light absorption line.

FIG. 5 is a diagram showing a basic configuration of the laser system shown in FIG. 3;

FIG. 6 is a diagram showing a calculation example of adiabatic inversion excitation of Gd156 in the gadolinium isotope separation method according to the present invention.

FIG. 7 is a diagram showing a calculation example of adiabatic inversion excitation of Gd157 in the gadolinium isotope separation method according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Metal gadolinium 2 Crucible 3 Electron gun 4 Electron beam 5 Vapor flow 6 Positive electrode 7 Negative electrode 8 Photoreaction part 9 Laser system 10 Laser light 10a Pulse laser for excitation between ground and first excitation level 10b First metastable-first 1 Excitation level excitation pulse laser 10c 2nd metastable-first excitation level excitation pulse laser 10d 1st excitation-second excitation level excitation pulse laser 10e 2nd excitation-third excitation level Interpulse excitation pulse laser 11 product recovery plate 33 first excitation level 34 ground level 35 first metastable level 36 second metastable level 37 second excitation level 38 third excitation level (automatic ionization level) 39 Time waveform of first-stage pulse laser 10a 40 Time waveform of first-stage pulse laser 10b 41 Time waveform of first-stage pulse laser 10c 42 Time waveform of second-stage pulse laser 10d 43 Time waveform of three-stage pulse laser 10e 44 Frequency sweep of first-stage pulse laser 10a 45 Frequency sweep of first-stage pulse laser 10b 46 Frequency sweep of first-stage pulse laser 10c 47 Frequency of second-stage pulse laser 10d (fixed) 48 Third Stage Pulsed Laser 10e Frequency (Fixed)

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-244219 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B01D 59/34

Claims (6)

(57) [Claims] 1. A method for heating a gadolinium raw material containing a plurality of isotopes to generate a vapor stream, and irradiating the vapor stream with a pulse laser beam for selectively exciting and isolating a specific isotope. In the gadolinium isotope separation method for recovering the ionized specific isotope component,
Adiabatic inversion excitation by irradiating pulsed laser light frequency-swept around the resonance light absorption line of the isotope to be depleted at the ground and metastable levels, and then irradiating pulsed laser light for excitation and ionization Selectively excited and ionized,
A method for separating gadolinium isotopes, wherein the content of the isotope to be impaired is reduced by recovering the content from the natural abundance. 2. The isotopes to be impaired are Gd156 and Gd1.
2. The semiconductor device according to claim 1, wherein at least one of the first and second 58 is provided.
A method for separating a gadolinium isotope as described above. 3. A state in which the total angular momentum becomes 3 so as to allow optical transitions from all of the first and second metastable levels whose occupancy is high due to heat distribution simultaneously with the ground level.
4. The gadolinium isotope separation method according to claim 2, wherein the excitation level is a resonance light absorption line. 4. A method for heating a gadolinium raw material containing a plurality of isotopes to generate a vapor stream, and irradiating the vapor stream with a pulse laser beam for selectively exciting and isolating a specific isotope. In the gadolinium isotope separation device for recovering the ionized specific isotope component to the electrode, the vapor stream was swept around the resonance light absorption line of the isotope to be depleted at the ground and metastable levels. A laser system for irradiating pulsed laser light to perform adiabatic inversion excitation, and subsequently irradiating with pulsed laser light for excitation and ionization to selectively excite and ionize, and an isotope recovery means for recovering ionized isotopes An apparatus for separating gadolinium isotopes, comprising: 5. The isotope to be impaired is Gd156 and Gd1.
5. At least one of 58.
A gadolinium isotope separation apparatus as described in the above. 6. The laser system according to claim 1, further comprising an electro-optical element for performing frequency sweep and pulse waveform control of a pulse laser required to cause adiabatic inversion excitation of the isotope to be depleted. The gadolinium isotope separation device according to claim 4.