Korea pursues LLW vitrification30 June 2000
Korea is following a policy of concentrating and containing its radioactive waste, and as a result plans to vitrify low level waste. A pilot plant is being constructed.
An innovative approach to waste management was developed in the early 1990s known as minimum additive waste stabilisation (MAWS), whose goal was to achieve the maximum volume reduction and maximum cost saving. In this approach low level waste is turned into glass because the waste stream commonly contains glass formers. Because the waste contains these constituents of glass, the amount of waste that can be incorporated into the glass (the waste loading) can be very high. In the US up to 93% waste loading has been achieved.
The vitrification of LLW is mainly concerned with organic products such as ion exchange resins and combustible solid waste. Scrap metal and concrete are not considered suitable, as the volume reduction factor is not high enough for the process to be cost-effective.
Other types of waste (including oil, sludges, decontamination solutions and borated concentrates) are also suitable for vitrification. For borated concentrates the maximum boron oxide content in the glass (20wt%) does not improve on the volume reduction factor offered by conventional encapsulation processes. The volume reduction factor is higher for other waste forms, but the amount of waste to be treated is usually small. In both cases, the vitrification process is cost-effective when the waste can be treated in an existing facility.
The development of LLW vitrification is therefore based on treating organic materials and some liquid waste. Scrap metals and concrete are disregarded .
In Korea it is planned to incinerate and vitrify low level waste in a single operation.
The low level waste produced by Korea’s nuclear power plants mostly comprises dry active waste (DAW), spent ion-exchange resin and borate liquid waste. Much of the DAW - including cotton, PVC, polyester, rubber and cellulose - is combustible, and this material represents around 60% of the total waste volume. In order to achieve maximum possible reduction this material will first be combusted and the ash will then be vitrified with other waste.
In the context of vitrification it is the PVC in the DAW that offers the greatest processing challenge. Typically PVC has a high CaO and TiO2 content, while glass can only tolerate a low concentration of TiO2.
Several other difficulties arise during the combustion process. Combusting PVC and other plastic wastes generally produces chlorinated off-gases, while combusting resins produces suphonated gases. These can cause corrosion and can also form a yellow phase above the formed glass that traps, but does not immobilise, the radioactivity. Secondly the dust in the off-gases may contain radioactive particles and these must be trapped and recycled into the vitrification process.
FORMING THE GLASS
In forming the glass its properties must be considered in several phases. It must be robust and durable in the solid phase, and in the liquid phase its viscosity must be controlled so it can be poured. These properties are affected by the materials that form the glass and can be controlled with various additives.
Studies show that the LLW from Korean power plants contains very small amounts of radioactivity and this therefore does not affect the properties of the glass. A study in Korea assessed the composition of the waste and considered different additives and glass formers that would produce waste of a suitable composition and viscosity.
The glass formers are the major constituents of all waste glasses. If the inorganic oxides in the waste have insufficient glass former, more must be added during the vitrification process. The durability of the resulting glass may be affected by ‘modifiers’ such as alkali metals which affect its structure, but the modifiers have an important role in controlling the viscosity of the molten glass, as well as properties such as electrical conductivity. Fluxes may also be added to the melt; these generally lower the glass melting temperature, the glass viscosity and the leaching resistance.
As the constituents of PVC contribute to weakening the glass, formers must be added to correct the balance. SiO2 is the most important ingredient in glass making and it is used as the major ingredient in most glasses because it is durable and inexpensive. B2O3 is the next most important, but the glass it produces is not chemically durable. Adding CaO will improve the chemical durability, but in large amounts it increases the tendency of glass to devitrify. The high content of CaO and TiO2 in the PVC ash must therefore be balanced with other additives to produce glass with acceptable characteristics.
The viscosity of the glass melt is the most important variable affecting the melt rate and pourability of the glass. It determines the rate of melting of the raw feed, the rate of glass bubble release, the rate of homogenisation and therefore the quality of the glass product. At a typical melter temperature (1000-1200°C) the viscosity must be in the range 20-100Pa/sec.
Investigation by a group from Kyoto University and the Nuclear Environment Technology Institute showed that in dealing with combusted PVC other components should be added to the glass in the following proportions: 35% SiO2, 10% B2O3, 15% Na2O, 4% Al2O3, 2% Fe2O3. These additives produced a good-quality glass with a loading of 30-50wt%.
When this composition had been assessed for PVC waste the group considered whether it would be suitable for the mixed waste that would be generated in practice, whose composition would vary unpredictably. The group produced a variety of types of combustible DAW and tested them using the PVC glass formers. The other types of waste (such as cellulose and cotton) produced less ash than the PVC and had less effect on the composition of the waste ash. The varied forms of DAW had no particular effect on the viscosity of chemical durability of the glass and the group concluded that the PVC-frit was tolerant of variations in waste composition.
The disposal of organic ion exchange resins can pose a difficult packaging problem. In the past this has been dealt with by conditioning them in cement or polymer matrices, but the result has been that disposal is required for a significant volume of waste. Combusting and vitrifying the resins along with other waste has been pursued in Korea. Cold crucible melting allows for thermal decomposition of the organic structure on the glass bed, simultaneous combustion and incorporation of the radionuclides and noncombustible constituents into glass. The procedure was investigated by a joint team from Commisariat a l’Energie Atomique and SGN in France, and Korea’s Nuclear Environment Technology Institute.
Laboratory-scale tests were carried out successfully at Marcoule. ‘Technology scale’ tests followed, using a direct induction-heated cold crucible 300mm in diameter, an electronically heated afterburning chamber and a system designed to ensure cooling, particle separation and chemical scrubbing of the process off-gases.
The tests provided valuable information on the physical and chemical operating conditions and on the resin processing capacity of the cold crucible. The process atmosphere must be oxidising, to avoid glass reduction by carbon from the resins and combustible waste - all the tests were carried out with 25% excess oxygen. The glass surface temperature was limited to 1200°C to limit radionuclude volatility during combustion on the glass melt. Continuous analysis of the off-gases (CO, CO2, O2, NOx, SO2) from the cold crucible and afterburner confirmed the advantages of afterburning and allowed the off-gas treatment system to be optimised.
With the success of these lines of research it was decided to construct a pilot plant in Korea for combustion and vitrification.
The pilot plant will have a melter 550mm in diameter, with a capacity of around 30kg/hr. An advanced off-gas system, rated for 150N3/hr, will allow research to be carried out on future applications.
The main process line will have the following components:
•Two melter feed lines. Waste will be supplied to the melter by a metering screw conveyor, while glass frit will be fed via a transfer system comprising a hopper, a vibrating table to convey the frit to he bucket, and a weighed bucket used to dump the desired mass of glass frit into the melter via a transfer lock consisting of two valves.
•The cold crucible melter, a water-cooled assembly of stainless steel segments forming a cylindrical structure 550mm in diameter. The cooling temperature is about 110°C at a pressure of 2bar.
•The off-gas treatment system. The off-gas includes a large particle fraction when burning resins, high SOx content when burning cation resins, high water content when burning wet resins and high HCl content when burning waste containing PVC. To prevent dust from reaching the afterburning chamber the off-gas stream will be prefiltered across a cleanable high-temperature filter designed to trap volatile radionuclides (mainly caesium). The afterburner then heats the off-gas to 1100°C for a minimum residence time of 2sec to burn any residual carbon in the form of CO or complex hydrocarbons. The exhaust gas is quickly cooled by full quenching and scrubbed with a sodium hydroxide solution to eliminate SOx and HCl. The scrubber outflow, laden with sodium and caesium salts, is sent to a final conditioning process.
Construction of the pilot plant is now under way in Korea and completion is expected this year.