GB2227599A

GB2227599A - Method of treatment of high-level radioactive waste

Info

Publication number: GB2227599A
Application number: GB9001722A
Authority: GB
Inventors: Misato Horie
Original assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan
Current assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan
Priority date: 1989-01-28
Filing date: 1990-01-25
Publication date: 1990-08-01
Anticipated expiration: 2010-01-25
Also published as: GB9001722D0; JPH02201199A; DE4002316A1; FR2642565B1; AU628468B2; GB2227599B; JP2633000B2; DE4002316C2; FR2642565A1; AU4798090A

Description

t c METHOD OF TREATMENT OF HIGH-LEVEL RADIOACTIVE WASTE The present

invention relates to a method of treatment of a high-level radioactive waste generated, for example, from reprocessing of spent nuclear fuels. In particlar, it relates to a method for treating a high- level radioactive waste which comprises heating the radioactive waste at a high temperature, separating part of elements contained in the radioactive waste by utilizing sublimation or boiling of each element in its various chemical forms during the heating step, and recovering a solidified material. The hich-level resultant residue as a radioactive waste generated from reprocessing of spent fuels r contain transuranium elements and Tc (technetium) having long half-lives; Cs (cesium) and Sr (strontium) that are noteworthy elements from the aspect of treatment, storage and disposal because they are responsible for the major proportion of heat generation; and valuable platinum group metals such as Ru(_-uthenium), Rh(rhodium) and Pd(palladium). It is therefore very important to separate and recover them prior to solidification of the waste, and to utilize them as a radiation source, a heat generation membe- and a noble metal, from the point of view of effectively utilizing 1 reso.urces.

The following three methods are heretofore known as prior art techniques for recovering these elements from the high-level radioactive waste:

1) A solvent extraction method wherein the intended nuclides are separated by using a special solvent from the high-level radioactive waste generated from the reprocessing steps; 2) An ion-exchange method wherein the intended nuclides are separated by using an ion-exchange resin from the high-level radioactive waste generated from the reprocessing steps; and 3) A lead extraction method for platinum group elements wherein lead is added to glass at the time of glass melting step of a vitrification process to thereby move platinum group elements to molten lead and separate them with the molten lead.

However, these prior art techniques described above are not free from the following disadvantages, respectively:

1) Since a new-type solvent is introduced to the reprocessing step in the additional solvent extraction method, the solvent treatment step becomes complicated and effeciency of the main solvent extraction step lowers conseqently.

2) Flammable materials are produced when the ionexchange resin comes into contact with nitric acid solution of the radioactive waste. Therefore, the ion-exchange 2 method involves safety problems.

3) The lead extraction method for platinum group elements in the vitrification process can separate the platinum group elements but secondary treatment is necessary in order to extract them from lead.

Furthermore, none of these prior art methods can reduce the volume of the high-level radioactive waste at a high rate, whichever method may be employed.

It is therefore an object of the present invention to provide a method for treatment of a high-level radioactive waste which solves the problems with the above-described prior art techniques and can separate and recover valuable elements in the radioactive waste in an extremely simple manner.

It is another object of the present invention to provide a method of treatment of a high-level radioactive waste which does not generate a secondary waste and can obtain a highly volume-reduced solidified material.

According to the present invention, in order to accomplish the abovedescribed objects, there is provided a method of treatment of a highlevel radioactive waste comprising heating the radioactive waste at a high temperautre to vaporize part of elements contained in the radioactive waste, and cooling the resultant vapor to collect the elements.

I- ( S In one embodiment of the present invention, the radioactive waste is reduction-heated at a high temperature to vaporize part of elements contained in the radioactive waste, and the resultant vapor is then cooled to collect the elements.

The high-level radioactive waste is ordinarily a nitric acid soultion obtained as an extraction residue in the reprocessing step of spent nuclear fuels, and contains almost all the nuclear fission products and actinides in the spent nuclear fuels. In the present invention, the nitric acid solution is heat-treated so as to evaporate the moisture and nitric acid in the solution and to obtain a calcined material, which is further heated at a temperature ranging from about 500 to about 3,OOOOC and more preferably, from about 1,000 to about 2,5000C.

Acco-rding to another embodiment of the present invention, in a first stage treatment, those elements which sublimate or boil in the form of oxides are heat-t.-eated at a normal or reduced pressure to vaporize those elements. The resultant vapor is then cooled by a plurality of cooling/ collecting units whose temperatures are differently set so as to correspond to sublimation or boiling points of each compound of element, thereby collecting the respective elements separately. In a second stage treatment, the remaining highlevel radioactive waste is heated in the presence of a reducing agent such as hydrogen to reduce the radioactive waste, and those elements which sublimate or 4 I---- C1 j boil in the form of metal are vaporized. The resultant vapor is then cooled, in the same manner as in the first stage treatment, by the cooling/collecting units whose temperatures are set so as to correspond to sublimation or boiling points of the respective elements, thereby Needles to to metals during can be separated by sublimation or boiling without reduction in the second stage treatment.

A voloxidation method is known as a technique for removing radioactive materials from spent fuels but this method is merely directed to nonmetallic elements such as krypton, iodine, tritium and the like. The present invention is directed to metallic elements and not only removes radioactive materials with high boiling points by heating the high-level radioactive waste at a high temperature, but also can remove both Cs and S-, that are high heatgeneration elements and pose problems during disposal, by combining the heat-treatment with the reduction reaction.

The resultant residue comprises metals or a mixture of the metals and oxides, and can be recovered as a volumereduced high-level radioactive solid.

Almost all the elements have boiling points 0" sublimation points different from those of other elements. Some elements contained in the high-level radioactive waste collecting the respective elements separately. say. those elements which are reduced heatinq in the first stace treatment - 5 have a relatively low sublimation point or boiling point in the form of- oxide or metal. For example, the boiling point is 6900C for metallic cesium. 3110C for technetium oxide, for metallic cadmium and 1,3840C for metallic strontium. By utilizing the difference in these boiling points, therefore, each valuable element can be separated and recovered by heat-treating the high-level radioactive waste at a high temperature to obtain the oxides thereof or by reducing them by hydrogen or the like to obtain metals, causing their sublimation or boiling, and cooling stepwise the resulting vapor mixture at the predetermined 7650C temperatures.

After the removal of Cs and Sr, the amount of heat generated from the high-level solid waste is reduced to about 10% and therefore the burying density for disposal can be improved drastically. Incidentally, if Cs alone is removed, the amount of heat generation becomes only 50% and a large effect cannnot be expected. The boiling points of oxides of Sr are at least 2,4300C and that of metallic Sr is 1,3840C as described above. Accordingly, strontium can only be separated by the method of the present invention wherein the heating step is combined with the reduction reaction.

Incidentally, vaporization of each element can be effectd at a lower temperature if the heating step or the reduction-heating step is carried out under a reduced pressure.

6 R "C a cl t Referring to the accompanying illustrative drawings:

Fig. 1 is a conceptual view showing an example of an apparatus suitable for practising the method of the present invention; Fig. 2 is an explanatory view showing a discharge method tor a residual molten material using a bottom flow system; and Fig. 3 is an explanatory view showing another discharge method for the residual molten material using an overflow system.

Fig. 1 is a conceptual view of an apparatus used for practising the method of the present invention. The apparatus is equipped with a heat-treatment unit 10 and a plurality of cooling/collecting units 12a,... 1 12n connected to the former. The heat-treatment unit 10 includes a heating vessel 14 and a heat-generation member 16. A feed port 18 for a reducing agent is provided at the upper part of the heating vessel 14 and a vapor passage 20 is interposed between the vessel 14 and the cooling /collecting unit 12a. A heat-generation and insulating member 22 is fitted around the vapor passage 20.

The heating vessel 14 may be made of a ref.ractory metal such as tungsten or a ce.ramic material such as alumina or 7 - c i high chromium refractory brick, depending on heat-treatment temperatures. Besides external heating by supplying power to the heat generation member 16 shown in Fig. 1, high-frequency heating, microwave heating, heating by directly flowing electric current through the highlevel the like may be employed as the heating method. it is also important to utilize effectively the heating due to the decay heat of the high-level radioactive waste to be treated.

The high-level radioactive waste 24 to be treated is charged into the heating vessel 14 and heated. This radioactive waste 24 is, for example, a calcined material obtained by heating nitric acid solution generated from the reprocessing step of the spent nuclear fuels to evaporate the moisture and nitric acid. The heatt_-eatment in the heating vessel can of course be carried out continuously from the state of the nitric acid solution. The calcined material is heated to about 5000C to about 3,OOOOC, more preferably to about 1,OOOOC to about 2,5000C. The elements contained in the calcined material are vaporized due to heating at their sublimation or boiling points in accordance with their chemical forms and are sent to the cooling/ collecting units 12a,..., 12n through the vapor passage 20. Each of these elements that are vaporized is individually cooled and collected by each of cooling/collecting units 12a,..., 12n whose temperature is controlled so as to correspond to a sublimation or boiling point of each radioacative waste or - 8 Ot C; compound of element.

Though heating may be carried out at the normal pressure, it is preferably carried out under a reduced pressure from the aspect of energy efficiency because the sublimation or boiling point drops and heattreatment can be made at a lower temperature.

In a preferred embodiment of the present invention, those elements which sublimate or boil in the form of oxides are heattreated under a normal or reduced pressure and separated in the first stage treatment. The remaining high-level radioactive material is then heated in the second stage treatment while a reducing agent is being introduced through the feed port 18 to reduce the radioactive material and to separate those elements which sublimate or boil in the form of metal. Finally, the resultant residue inside the heating vessel 14 is recovered. Hydrogen gas, carbon, carbon monoxide or the like may be used as the reducing agent to be introduced through the feed port 18.

The discharge method of the residual molten material 25 from the heating vessel 14 may be of a bottom flow system such as shown in Fig. 2 or of an overflow system such as shown in Fig. 3. In either case, the residual molten material 25 is discharged into a vessel 26 for solidification and is left for cooling to obtain a highly volume--educed solidified material.

Example 1

0' A simulated nitric acid solution of a high-level radioactive waste in which radioactive nuclides were simulated by stable elements was prepared and was subjected to evaportion treatment to obtain a calcined matetial. The calcined material was then heated and reduced at a high temperature of 1,OOOOC for 4 hours in a mixed gas stream of H2-HeO:4). In the interim, Te, Cd. Se, Cs and Na were deposited in the cooling/collecting units and could be collected. The respective temperatures in the cooling/ collecting units with respect to these elements were 200 to 6000C for Te, 200 to 3000C for Cd, about 6000C for Se, 900 to 1,OOOOC for Cs and 600 to 1,OOOOC for Na.

Example 2

The calcined material obtained after the heating and reducing treatment at the high temperature in Example 1 was further heat-treated at 850 to 1, 0500C in a vacuum. It was confirmed that Pd and Ru were deposited in the cooling/ collecting units.

As being apparent from the foregoing, according to the method of the present invention, the high-level radioactive waste is heated, or reduction-heated, at a temperature to vaporize part of elements contained in the radioactive waste and the resultant vapor was separated and collected. Therefore, in comparison with the prior art methods described he--einbefo.re, the method of the present 'invention has simplified treating steps, and does not need to add t 0 afresh any special reagent or ion-exchange resin in the subsequent reprocessing or solidification step. Furthermore, since the collected elements are solids in the form of oxides or metals,they can be used as radiation sources or valuable metals, and can be subjected to transmutation without the need for complicated secondary treatment.

In addition, the solidified material obtained by the present invention hardly contains additives other than the nuclear fission products and actinides and has an extremely smaller occupying volume for strage and disposal than the conventional solidified materials and can drastically reduce the costs for st.rage and disposal. The solidified material can preferably be used as a radiation source for nuclear transformation by neutron ir.radiation, since its volume is small and the irradiation efficiency is high.

11 el-

Claims

Claims:

1. A method of treatment of a high-level radioactive waste comprising heating the radioactive waste at a high temperature to vaporize part of elements contained in the radioactive waste, and cooling the resultant vapor to collect the elements.

2. A method of treatment of a high-level radioactive waste comprising reduction-heating the radioactive waste at a high temperature to vaporize part of elements contained in the radioactive waste, and cooling the resultant vapor to collect the elements.

3. A method of teatment of a high-level radioactive waste comprising heating the radioactive waste at a high temperature to vaporize a first part of elements contained in the radioactive waste, cooling the resultant vapor of the first part of the elements to collect the first part of the elements, reduction-heating the remaining radioactive material to vaporize radioactive material, cooling the resultant vapor of the second part of the elements r to collect the second part of the elements.

4. The method acco-rding to claim 2 or 3, wherein said reduction-heating step is car.ried out in the presence of hydrogen, carbon or carbon monoxide.

- 12 0

5. The method according to claim 1. 2 or 3, wherein said cooling step comprises cooling stepwise the vapor at different temperatures each corresponding to sublimation or boiling point of each compound of element to separately collect the respective elements.

6. The method accordiing_ to claim 1, 2 or 3, wherein said high temperature is from about 5000C to about 3f0000C.

7. The invention substantially as herein described.

- 13 Publ 1990atThePatentOffice, State House, 66171 High Holborn, London WC1R4TP. Purther copies maybe obtained from The Patent=ce.