GB2246467A

GB2246467A - Transmutation treatment of radioactive wastes

Info

Publication number: GB2246467A
Application number: GB9115002A
Authority: GB
Inventors: Kenji Konashi; Nobuyuki Sasao
Original assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan
Current assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan
Priority date: 1990-07-13
Filing date: 1991-07-11
Publication date: 1992-01-29
Anticipated expiration: 2011-07-11
Also published as: GB2246467B; FR2665570B1; DE4123145A1; CA2045809C; FR2665570A1; CA2045809A1; JPH0472598A; DE4123145C2; GB9115002D0; JPH073474B2

Abstract

A method for the transmutation treatment of radioactive wastes comprises: accelerating radioactive nuclides contained in the radioactive wastes to be treated to an energy level corresponding to a compound nucleus resonance level; and bombarding the accelerated nuclides into a thermal neutron field, which is under a magnetic field, to cause the compound nucleus resonance reaction to occur; thereby transforming the radioactive nuclides into those which are more stable or have shorter life. The nuclides are accelerated in accelerator 9 then passed into an annular-cylindrical chamber 3 positioned about a reactor 1 where they are treated with neutrons. <IMAGE>

Description

:2 --7 "3= e,.4 6s 7 TRANSMUTATION TREATMENT OF RADIOACTIVE WASTES The

present invention relates to a method of quickly transmutating or reducing the level of radiations from radioactive wastes by transforming radioactive nuclides with long lifetime contained in radioactive wastes into those with shorter life or stable ones.

Contained in high-level radioactive wastes generated after reprocessing spent nuclear fuels from nuclear reactors are various kinds of long-lived nuclides, which include fission products (Cs, Sr, Tc, etc.), actinoids (Np, Am, Cm, etc.) produced as a result of nuclear reactions and unrecovered uranium and plutonium.

The currently available method for final disposal of such high-level radioactive wastes is to seal them in vitrified solids under rigid control and store them in a controlled area until the radiation level decreases to allowable levels by the decaying process. However, the vitrification of radioactive wastes with long average lifetime requires many years of stringent supervision of the stored wastes and as the amount of wastes increases, it will become increasingly difficult to select and secure the storage site.

If it is possible to transform the long-lived radioactive nuclides into those with shorter life or stable ones, the period over which the stored radioactive waste materials must be supervised can be reduced and so also the need for finding storage sites, offering significant advantages in terms of safety and economy involved in the waste disposal process.

Among the transmutation processing of the radioactive material utilizing the nuclear transformation, the most radiates neutrons against radioactive nuclides. The radiated neutrons are absorbed in nuclei, transforming the nuclei into those with shorter life and more stable ones.

Possible candidate neutrons that can be utilized for the trans mutation treatment include low-energy neutrons such as thermal neutrons obtained from the nuclear reactor.

The absorption of low-energy neutrons into nuclei occurs mainly during a radiative capture rea.ction [(n,() reaction). This reaction shows an acute resonance, as can be seen from a graph of Figure 5 that shows the neutron radiative capture cross section for 99Tc. Such a resonance phenomenon can be explained by the formation of compound nuclei. Some of the capture cross sections at the resonance level have large values, and the greater the capture cross section, the more likely the capture reaction will occur.

However, the neutron energy from the nuclear reactor is continuously distributed and hence it is difficult to common is considered to be one that 1 i f 1 W 1 1 i- i i i 1 J i i i 1 1 i 1 i i 1 efficiently obtain a neutron flux of a particular energy that agrees with the resonance level of the nuclei.

An object of the present invention is therefore to provide a method which can trigger a compound nucleus resonance reaction even with thermal neutrons that do not have an energy equal to a particular resonance level, and thereby carry out the nuclear transformation of radioactive nuclides to efficiently perform the transmutation treatment on the radioactive wastes.

The inventors have come to an idea that instead of controlling the energy of neutrons to be radiated against nuclei, the nuclei are accelerated and bombarded into a neutron field to trigger the compound nucleus resonance reaction. That is, if the resonance level energy E of neutrons in a system where the accelerated neutrons are radiated against the fixed nuclei is to be produced by accelerating the nuclei with the neutrons fixed, the kinetic energy of the nuclei required to cause the compound nucleus resonance reaction will be (M/m)E, where M is a mass of nuclei and m is a mass of neutrons. Thus, the compound nucleus resonance reaction can be made to initiate without controlling the neutron energy, only by giving the nuclei the kinetic energy which is (M/m) times the resonance level energy of the neutrons.

The transmutation treatment method for radioactive wastes of the present invention is based on the abovedescribed principle. Thus, according to the present invention, radioactive nuclides to be treated contained in the radioactive wastes are accelerated to an energy level corresponding to a compound nucleus resonance level. The accelerated nuclides are then smashed or bombarded into a thermal neutron field which is under a magnetic field, to cause the compound nucleus resonance reaction to occur, thereby transforming the radioactive nuclides into those which are more stable or have shorter life.

In the present invention, by accelerating the radioactive nuclides to an energy equal to the compound nucleus resonance level and smashing them into the thermal neutron field, it is possible to cause the compound nucleus resonance reaction to occur between the accelerated nuclides and the thermal neutrons, even when the thermal neutrons do not have a resonance level energy. This method, therefore, can efficiently transform the long-lived radioactive nuclides into stable or short-lived nuclides.

Referring to the accompanying illustrative drawings:

Figure 1 is a schematic cross section showing the concept of a preferred embodiment of the transmutation treatment apparatus employed in the present invention; Figure 2 is a schematic illustration showing the operation of the apparatus of Figure 1; Figure 3 is a plan view of Figure 2; i i i i 1 1 1 i i i c i j 1 1 j i i i j 1 Figure 4 is a graph showing the relationship between the acceleration energy and the transmutation rate of 99TC; and Figure 5 is a graph showing the neutron capture cross section of 99Tc.

Figure 1 schematically shows radioactivity transmutation treatment embodies the method of this invention.

one example of apparatus that Around a reactor core 1 of a nuclear reactor such as a fast reactor is installed a moderator layer 2 consisting of heavy water.

The moderator layer 2 is further enclosed by a nuclear transmutation vessel 3. The core 1 and the nuclear transmutation vessel 3 are enclosed by a shielding material 4 to shield radiations. The core 1 is of course equipped with a cooling system 5 such as a coolant circulation system. The nuclear transmutation vessel 3 is under the influence of a magnetic field generated by an electromagnet

7 electrically connected to a power source 6 and is evacuated by a vacuum pump 8. In the nuclear transmutation vessel 3 thermal neutrons emanating from the core 1 produce a thermal neutron field. The magnetic field applied to the nuclear transmutation vessel 3 accumulates the accelerated nuclei thrown into the thermal neutron field. An accelerator 9 accelerates radioactive nuclides to drive them into the nuclear transmutation vessel 3. The accelerated nuclei are introduced through a leading pipe 10 into the nuclear transmutation vessel 3 and discharged from a discharge pipe 11.

The operation of the transmutation is explained by referring.to Figures 2 radioactive liquid waste are chemically treated to concentrate only the elements that are to be processed for transmutation. For example, 99Tc and 107Pd may be extracted as single-color solid materials and 1291 as a stable compound. In the case of Tc, the nuclei are introduced into the ion source portion of the accelerator 9 where they are heated, evaporated and ionized by electrons. After this, the nuclei are sent to the acceleration portion of the accelerator 9 where they are accelerated to a specified energy level, and then introduced through the leading pipe 10 into the nuclear transmutation vessel 3. The accelerated nuclei introduced in the vessel 3, as shown in Figure 2, speed spirally through the thermal neutron field in the nuclear transmutation vessel 3 toward the discharge pipe 11. During this spiral movement, the nuclei react efficiently with the thermal neutrons resulting in the compound nucleus resonance reaction. Then, the long-lived radioactive nuclide is transformed into a stable or short-lived nuclide before being discharged from the discharge pipe 11. The nuclides that have not reacted are separated and recovered by chemical treatment and resupplied to the accelerator 9 as shown by the broken line for further transmutation processing.

treatment apparatus and 3. High-level i 1 i i i 1 1 i 1 i k 1 1 1 i 1 i The result of calculation is shown below for a case where the transmutation treatment method of this invention is applied to the nuclide 99TC. The nuclear transformation of 99Tc proceeds as follows.

99Tc (half-life: 2.1 X 105 years) 4 Neutron absorption 10OTc (half-life: 15.8 seconds) P decay 100Ru (stable) The parameters used in the calculation are as follows. Thermal neutron flux obtained by the nuclear reactor:

101 5cm-2sec-1 Thermal neutron temperature: 3000K Resonance parameters of 99Tc:

Neutron resonance energy in the center-of-mass system: 5.6 eV Neutron decay width T'rj: 5.00 meV ^f decay width f.: 134.0 meV Resonance level width 'F: 139.0 meV Nucleus spin before reaction 1: 4.5 Compound nucleus spin J: 4.0 The relationship between the acceleration energy and the transformation rate of 99Tc in the thermal neutron field is shown in Figure 4. As can be seen from this graph, the peak value of the transmutation rate of 99Tc is 1.4 x 10-5sec-1.

For comparison, let us consider a case where the thermal neutron capture reaction is performed with the nuclei fixed.

t Since the thermal neutron capture reaction cross section of 99Tc is 20 barn, the transmutation rate is given by x 10-24 1015 = 2 x 10-8 sec-1 This means that the transmutation rate is 700 times higher for the method of this invention that uses nuclei accelerated to the resonance level.

If this is interpreted in terms of half-life, 99Tc has the half-life of 2. 1 x 105 years in the natural environment. That is, when left in the natural environment, the 99Tc takes 2.1 x 105 years before the intensity of the radioactivity is reduced to one-half. On the other hand, when the nuclide is placed in the thermal neutron field with a flux of 1015cm-2sec1, the radiation level decreases to one-half in 1.1 years. Further, when 99Tc is accelerated to the compound nucleus resonance energy and smashed into the thermal neutron field as in this invention, the radiation level decreases to one-hallf in 13.8 hours.

As mentioned above, this invention quickens the transmutation rate and the resulting advantages may be summarized as follows.

When it is desired to reduce the radiation level of 99Tc to one thousandth and if the nuclide is simply placed in the thermal neutron field with the flux of 1015cm-2sec-1, it will take 11 years. The method of this invention, however, takes only 5.75 days. Therefore, in the design of actual transmutation treatment apparatus, this high transmutation rate is advantageous in light of durability of the apparatus i i 1 1 j 1 i 1 1 1 1 1 1 As described in the foregoing, the method of this invention accelerates the nuclei to an energy level corresponding to the compound nucleus resonance level corresponding to the compound nucleus resonance level and drives them into the.-thermal neutron field. This allows the compound nucleus resonance reaction to occur even when such a neutron source as nuclear reactors is used, in which it is difficult to obtain neutrons of a specific energy. This invention therefore can tansform long-lived radioactive nuclides into stable or short-lived nuclides with high efficiency.

As a result, the period over which the radioactive wastes are stored and the area of the storage site can be reduced, offering great advantages in terms of safety and economy involved in the waste disposal.

Another advantage of this invention is that since each nuclide has its own unique compound nucleus resonance energy, it is possible to selectively transmutate a particular nuclide at a high rate even when other nuclides coexist. Hence, there is no need to separate the isotopes and the only process required before this transmutation treatment is the chemical separation process.

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Claims

CLAIMS:

1. A method for the transmutation treatment of radioactive wastes which comprises: accelerating radioactive nuclides contained in the radioactive wastes to be treated to an energy level corresponding to a compound nucleus resonance level; and bombarding the accelerated nuclides into a thermal neutron field, which is under a magnetic field, to cause the compound nucleus resonnance reaction to occur; thereby transforming the radioactive nuclides into those which are more stable or have shorter life.

2. A method as claimed in claim 1 wherein the thermal neutron field is formed around a nuclear reactor and thermal neutrons are emanated from the nuclear reactor.

3. A method as claimed in claim 1 or claim 2 wherein the radioactive nuclides to be treated are chemically extracted and separated from the radioactive wastes.

4. A method as claimed in claim 1 substantially as herein described with particular reference to the exemplification and/or the accompanying drawings.

- 10 h Published 1992 at The Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 I RH- Further copies mav be obtained from Sales Branch. Unit 6. Nine Mile Point. DArmfeIinfach. Cross Keys. Newport. NPI 7HZ. Printed bv Muljjpje.x techniques ltd. Si Mary Cray. Kent- i