JPH073474B2 - Radioactive waste extinction treatment method - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a method for promptly eliminating radioactivity of radioactive waste by converting radioactive waste containing a long-lived radioactive nuclide into a short-lived or stable nuclide by transmutation.

[Prior art]

Among the high-level radioactive waste generated by the reprocessing of spent nuclear fuel from the nuclear reactor, fission products (FP) such as strontium, actinides (Np, Am, Cm, etc.) produced by nuclear reactions in the reactor are included. ), And long-lived radionuclides such as unrecovered uranium and plutonium. At present, as a method of final disposal of such high-level radioactive waste, it is considered to store it strictly in a vitrified body and store it, and wait until its radioactivity decreases due to collapse. There is. However, the vitrified solid of radioactive waste, which has a long average life, must be safely managed for many years, and as the amount of storage increases, it becomes difficult to select and secure a storage location. On the other hand, if long-lived radionuclides are extinguished using transmutation and converted into short-lived or stable nuclides, the storage period and storage location will be shortened, and the safety and economic efficiency of waste disposal will be improved. In terms of The most common method of annihilation treatment using transmutation is to irradiate a radionuclide with neutrons. Irradiated neutrons are absorbed by atomic nuclei and converted into short-lived or stable nuclides. As neutrons that can be used for such annihilation treatment by neutron irradiation, low energy neutrons such as thermal neutrons obtained from a nuclear reactor are considered.

[Problems to be Solved by the Invention]

The main absorption process of low energy neutrons by nuclei is the radiative capture reaction [(n, γ) reaction]. This reaction exhibits a sharp resonance, as can be seen from the graph in FIG. 5, which shows the radiative capture cross section of neutrons, for example about ⁹⁹ Tc. Such resonance phenomena can be explained by the formation of complex nuclei,
Some of the resonance level capture cross sections have a large value, and the larger the capture cross section, the easier the capture reaction occurs. However, the energy of neutrons obtained from a nuclear reactor or the like is continuously distributed, and it is difficult to efficiently obtain a neutron flux with a specific energy that matches the resonance level energy of atomic nuclei. Therefore, the present invention can cause a complex nuclear resonance reaction even by thermal neutrons that do not have the energy of a specific resonance level, whereby the nuclear transmutation of radionuclides, and thus the annihilation process of radioactive waste, can be performed efficiently. The purpose is to provide a method that can be performed.

[Means for Solving the Problems]

The present inventors considered that instead of controlling the energy of the neutrons that irradiate the atomic nuclei, the compound nuclear resonance reaction is caused by accelerating the atomic nuclei and driving them into the neutron field. That is, when the resonance level energy E of the neutron obtained in a system in which the nucleus is fixed and the accelerated neutron is irradiated to this is obtained by fixing the neutron and accelerating the nucleus, a compound nuclear resonance reaction occurs. So the kinetic energy of the nucleus needed is (M / m) E. Where M is the mass of the nucleus and m is the mass of the neutron. Therefore, if the kinetic energy of (M / m) times the resonance level energy of neutrons is given to the nucleus, the complex nuclear resonance reaction can be caused without controlling the energy of the neutrons. The method for extinguishing radioactive waste according to the present invention was made based on the above-mentioned principle, and in the method for extinguishing radioactive nuclides in radioactive waste by transmutation, a thermal neutron field with a magnetic field was applied. , A complex nuclear resonance reaction is caused by implanting a radionuclide to be processed which has been accelerated to an energy equivalent to the complex nuclear resonance level energy.

[Work]

According to this invention, by implanting the radionuclide accelerated to an energy equivalent to the complex nuclear resonance level energy into the thermal neutron field, even if the thermal neutron does not have the resonance level energy, the accelerated nuclide and thermal It is possible to cause a complex nuclear resonance reaction with neutrons, whereby a long-lived radionuclide can be efficiently converted into a stable or short-lived nuclide.

【Example】

FIG. 1 schematically shows an example of an extinction treatment device for carrying out the method of the present invention, in which a moderator layer 2 such as heavy water is provided around a core 1 of a nuclear reactor such as a fast reactor. A nuclear reaction vessel 3 is arranged, and the reactor core 1 and the nuclear reaction vessel 3 are surrounded by a shield 4 and shielded against radiation. It goes without saying that the core 1 is equipped with a cooling device 5 such as a coolant circulating device. On the other hand, the nuclear reaction container 3 has a magnetic field applied by an electromagnet 7 electrically connected to a power source 6 and is evacuated by a vacuum pump 8. As a result, a thermal neutron field is formed in the nuclear reaction vessel 3 by the thermal neutrons emitted from the core 1, and the accelerating nuclei driven into this thermal neutron field can be stored by the magnetic field. Further, the accelerator 9 is for accelerating the radionuclide and driving it into the nuclear reaction container 3, and the accelerated nucleus is introduced into the nuclear reaction container 3 through the introduction pipe 10.
It is discharged from the discharge pipe 11. The operation of the disappearance processing device will be described with reference to FIGS. 2 and 3. First, the high-level waste liquid is chemically separated, and only the elements to be eliminated are taken out. For example ⁹⁹ Tc or ¹⁰⁷ P
d is a monochromatic solid and ¹²⁹ I is considered to be a stable compound. For example, in the case of Tc, these nuclides are introduced into the ion source part of the accelerator 9 and ionized by a procedure such as heating evaporation and ionization by an electron beam, and then sent to the accelerating part of the accelerator 9 to have a predetermined energy. It is accelerated and introduced into the nuclear reaction container 3 through the introduction pipe 10. As shown in Fig. 2, the introduced accelerated nuclides are guided to the discharge pipe 11 while making a spiral motion of the thermal neutron field in the nuclear reaction vessel 3, and during this time, they react efficiently with thermal neutrons to cause complex nuclear resonance. Cause a reaction. Thus, the long-lived radionuclide is transmuted into a stable or short-lived nuclide, and then discharged from the discharge pipe 11. The unreacted radionuclide can be separated and recovered by chemical separation or the like, and can be circulated to the accelerator 9 as shown by the dotted line to be processed again. The calculation results when the annihilation processing method of the present invention is applied to ⁹⁹ Tc annihilation processing are shown below. The transmutation process of ⁹⁹ Tc in this case is as follows: ⁹⁹ Tc (half-life 2.1 × 10 ⁵ years) ↓ Neutron absorption ¹⁰⁰ Tc (half-life 15.8 seconds) ↓ β decay ¹⁰⁰ Ru (stable) The parameters used are as follows. Thermal neutron flux obtained in nuclear reactor: 10 ¹⁵ cm ^-2 sec ^-1 Thermal neutron temperature: Resonance parameter of 300 ° K ⁹⁹ Tc: Neutron resonance energy in the center of gravity system; 5.6eV Neutron decay width Γn; 5.00meV decay width Γr; 134.0 meV Resonance level width Γ; 139.0 meV Spin of nuclear nuclei before reaction I; 4.5 Spin of complex nucleus J; 4.0 The relationship between the accelerating energy of ⁹⁹ Tc and the annihilation rate in the thermal neutron field as described above Shown in the figure. As can be seen from this graph, the peak value of the annihilation rate of ⁹⁹ Tc is 1.4 × 10 ^-5 sec.
It becomes ^-1 . For comparison, if we consider the case where the thermal neutron capture reaction is carried out with the nuclei fixed, the thermal neutron capture reaction cross section of ⁹⁹ Tc is 20
Since it is b, the extinction rate is 20 × 10 ^-24 * 10 ¹⁵ = 2 × 10 ^-8 sec ^-1 . Therefore, the present invention using the nuclei accelerated to the resonance level energy can obtain 700 times faster annihilation processing speed. In other words, ⁹⁹ Tc has a natural half-life of 2.
1 × 10 ⁵ years. That is, 2.1 × 1 if left naturally
0 not take ⁵ years if not in half. Meanwhile, 10 ¹⁵ cm ^-2 se
When placed in a thermal neutron field with a flux of c ^-1 , it halves in 1.1 years. Furthermore, when ⁹⁹ Tc is accelerated to the complex nuclear resonance energy and is injected into the above thermal neutron field as in the present invention, 13.8
It will be halved in time. As described above, the method of the present invention provides a high annihilation processing speed, and the effect brought by it can be expressed as follows. That is, the amount of radioactivity of ⁹⁹ Tc
If it is to be 1/1000, it will take 11 years if it is left in a thermal neutron field with a flux of 10 ¹⁵ cm ^-2 sec ^-1 , whereas it can be processed in 5.75 days according to the method of the present invention. . Therefore, when actually designing the annihilation processing apparatus, such a fast erasing processing speed is advantageous in consideration of the durability of the apparatus.

【The invention's effect】

As can be seen from the above, according to the present invention, by accelerating the atomic nuclei to an energy equivalent to the complex nuclear resonance level energy and driving it into the thermal neutron field, specific neutrons such as thermal neutrons obtained from a nuclear reactor can be obtained. Even when using a neutron source where it is difficult to obtain neutrons of high energy, it is possible to cause a complex nuclear resonance reaction, which allows efficient conversion of long-lived radionuclides into stable or short-lived nuclides. it can. As a result, the reduction of the storage period of radioactive waste and the reduction of the storage location can be achieved, which can have a great effect in terms of safety and economic efficiency regarding waste disposal. Further, the advantage of the present invention is that since the complex nuclear resonance energy is unique to each nucleus, even if other nuclides are mixed in the accelerated nuclide, only the target nuclide is selectively and rapidly extinguished. This is the point where as a result,
As the target adjustment, only chemical separation needs to be performed, and isotope separation is not necessary.

[Brief description of drawings]

FIG. 1 is an explanatory view showing the concept of an extinction processing device that can be preferably used for carrying out the present invention, FIG. 2 is an explanatory view showing the operation of the device of FIG. 1, and FIG. 3 is a plan view of FIG. , 4th
Figure is a graph showing the relationship between the extinction process speed energy of accelerating nuclide in ⁹⁹ Tc, FIG. 5 is a graph showing a radiation capture cross section of a neutron in ⁹⁹ Tc. 1 ... Reactor core, 2 ... Moderator layer, 3 ... Nuclear reactor, 4 ... Shield, 7 ... Electromagnet, 9 ... Accelerator.

Claims (1)

[Claims] 1. In a method for transmutation and extinction of a radionuclide in a radioactive waste, the radionuclide to be treated is accelerated in a thermal neutron field applied with a magnetic field to an energy equivalent to a complex nuclear resonance level energy. A method for extinguishing radioactive waste, which comprises causing a complex nuclear resonance reaction by implanting.