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4.1 The processing of abandoned ion-exchange resin
The system to process abandoned ion-exchange resins with oxygen-plasma is being developed ( at 2011 ).
This is a way to reduce the volume and stabilize of ion-exchange resins which are generated in reactors for cleaning radionuclides, through oxygen-plasma.
A part of metallic materials composing a reactor which are corroded under high temperature reaches the reactor core as ion through coolant, and another part of those materials reaches the core as suspended matter respectively.
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And, corroded products radio-activated by the irradiations of neutrons circulate in the system, adhere to pipings as clads.
Most of elements composing those clads are Fe, meanwhile the ratio of 60CO and 54Mn for contributing to radio-activity is high.
These nuclides are originated in CO , Ni and Fe composing stainless steel, stellite and inconel, etc which are materials to compose a nuclear power plant.
In particular, the radioactive half-life of 60CO is long compared with other nuclides, its energy of γ-ray is high.
So, it's difficult to handle this element.
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Steams generated in a reactor are condensed through a device for returning water after driving the turbine.
Corroded products generated in the upper stream of the device for returning water are removed through ion-exchange resins in a purifier returning water.
Accordingly, powdered ion-exchange resins clean up radioisotopes activated in a reactor, so that those resins adsorb radio-activated CO and others.
Abandoned ion-exchange resins are generally stored in facilities. But, as it's hard to keep places for storing those matters, it's desired to reduce the volume and stabilize of those matters.
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Because abandoned ion-exchange resins are in-combustible, it's hard to usually burn-reduce the volume of those.
Accordingly, the volume of low-level abandoned resins are reduced by burning those resins together with miscellaneous wastes, and they are stored as uniform solid matters through cement-slugging in facilities or are disposed.
Meanwhile, most of high level abandoned resins are stored at half-processed in the tank of the facility.
The same as that, ways to process abandoned resins generated during the disposition have never been found out ( about 2011 ).
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A method with RF-plasma at decompression ( 0.25 〜 0.9 torr ) also is one of the efficient ways to process abandoned ion-exchange resins.
This method can decrease secondary wastes such as tar and soot through complete oxidation by oxygen-atoms and radicals.
These active particles directly interact with ion-exchange resins and decomposed gases from those resins to pull hydrogens out of an alkyl group, and they oxidize hydro-carbons through cutting into c-c coupling.
So that, those resins are transformed into gases such as CO, CO2, H2O and the volume of them are reduced.
And, metallic ions such as CO sticking to exchange bases of resins are left as oxides in reduced remnants.
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The method to process abandoned ion-exchange resins by this decompression way has following advantages because that is at low temperature compared with burning.
・ It's possible to decrease the shift of radioactive nuclides into gas-phase.
・ It's possible to reduce the quantity of issued gas because this method is a direct oxidative process by oxygen-plasma.
In a demonstrative test-device, processing ion-exchange resins of 4L for three hours,
the ratio of reducing volume was about 90 % .
Then, supplying oxygen into the system at 50 L/min , the RF-plasma was realized under the condition at 450 Hz , 10 kW .
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And, a method to process abandoned ion-exchange resins by oxygen-thermal plasma under atomospheric pressure also is being considered to increase the speed of processing.
It has been reported that abandoned ion-exchange resins were processed at 4 kg/h through the oxygen-plasma generated by a transition arc-discharge ( 42 kW ).
In this case, the mixture ratio of positive ion-exchange resins and negative ion-exchange resins was 50 : 50 .
This processing by plasma was carried out together with glasses of 25 kg ( Na2O - Al2O3 - 2SiO2 ) to trap radioactive nuclides sticking to ion-exchange resins.
In this method to process, ion-exchange resins are supplied from the upper part of the system and they are dropped into the glass molten through arc-discharge.
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And, a comparison of batch-processing with continuous processing has been carried out.
Batch-processing is a method to process ion-exchange resins put in a melting pot by an arc-discharge of 24 kW.
In this case, ion-exchange resins of 100 cm＾3 were processed, the ratio of reduced volume of 65 % on positive ion-exchange resins ( IRN 77 ) and the ratio of reduced volume of 90 % on negative ion-exchange resins ( IRN 78 ) were obtained respectively.
The condition of continuous processing was set up same as that of batch processing, IRN 77 was processed at 200 g/min , IRN 78 was processed at 63 g/min respectively.
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The ratio of reduced volume in this case didn't depend on the speed to supply ion-exchange resins so much.
That ( the ratio of reduced volume ) of IRN 77 was equal to about 60 % , and that of IRN 78 was equal to about 90 % .
This result was approximately equal to that of batch processing.
But, as the heating of batch processing can't be uniform, it's hard to control issued gases in this case.
Meanwhile, in the case of continuous processing, it's comparatively easy to control the composition of issued gas.
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As the method to process abandoned ion-exchange resins by thermal plasma is basically a kind of burning process by oxygen in plasma, oxygen or air is used as a plasma-gas for processing.
※ When using air, we should pay attention to the generation of NOx .
And, as COs sticking to ion-exchange resins are roughly gathered up to resins after processing, they are not shifted into gas-phase almostly.
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The method to process ion-exchange resins under atmospheric pressure with blowing oxygen into plasma-jet also is being considered ( about 2011 ).
As time for processing goes, the weight of resins substantially decreases.
So, the ratio of reduced mass of 90 〜 95 % can be obtained for about 20 〜 40 minutes on the process.
And, from the result of infrared absorption spectrum, it has been confirmed that strong acid ion-exchange resin has a character that as time goes, the peak on sulfonic acid group decreases and that peak completely vanishes by the processing of 20 minutes.
Furthermore, it also has been confirmed that CO sticking to ion-exchange resin is divided into CO 0 and CO3O4 , Cs turns to be Cs2SO4 .
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