This section of the literature review with give a brief overview of the currently available thermal treatment technology platforms, for a more in-depth review of the application of thermal technologies for processing radioactive waste the author is referred to IAEA
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(2006), Chapman et al. (1986) and Ojovan (2005). No single waste form or process is suitable to economically handle the total clean-up of the UK’s nuclear legacy waste, with which the NDA faces. It is important to examine the key factors influencing the selection of thermal technologies as part of the NDA national waste management strategy. This section will concentrate on joule and plasma heating, as these two thermal treatment processes have been utilised in full scale thermal treatment plants processing radioactive waste.
2.5.1 Joule heating
Joule heating depends on the transfer of heat energy to a material from the resistance of an electric current flowing through that material. This dissipated power is predicted by Joules law:
P=I2R
P = Dissipated power (watt W), I = Current through the material (amps A), R = Resistance (ohms Ω)
This shows that if the current can be maintained then an increase in electrical resistance will result in additional power being dissipated in the form of thermal energy resulting in the material heating more rapidly. Importantly to the operator, unless the voltage is increased, an increase in resistance will also decrease current, as predicted by Ohm’s Law:
R = V/I or V = IR
V = Voltage (volts V), I = Current and R = Resistance
Several properties of glass impact the joule heating process, with Jantzen (1995) providing a detailed account. Among these properties is glass has poor electrical conductivity (high resistivity) as a solid, however as temperatures pass the liquidus temperature the resistance falls. This is due to the breakup of the glass structure allowing ions in the silica framework to be much more mobile (Shelby 2005), increasing the ability to carry the electrical charge. In terms of processing parameters, melt viscosity is considered the most important. Viscosity controls processing rate, glass homogeneity and heat transfer within the molten glass by controlling the strength of convection currents within the melt (Orfeuil 1987). Viscosity is modified by changing the feed composition or process temperature. This is an important point for operators of melter technology such as the GeoMelt system, as an increase in temperature can increase the performance of the
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vitrified product through increased homogeneity due to an increase in convection currents, however it is known that an increase in operating temperature can also volatilise radionuclides resulting in poor retention within the glass matrix. There are a number of variations to induction melters including cold crucible induction melting (CCIM) (Sugilal 2008).
2.5.2 Plasma Vitrification
Plasma vitrification uses extremely high temperatures in an oxygen starved environment to completely decompose waste material enabling plasma facilities to treat a large number of waste streams safely. Radioactive waste vitrification exploits the plasma’s ability to rapidly initiate a variety of chemical reactions including decomposition and oxidation (Moustakas et al. 2005). The author is refereed to Gomez et al. (2009) for a comprehensive review of plasma treatment of waste. This section provides relevant operational experience of plasma treatment of PCM and LLW with particular attention to radionuclide partitioning and off gas considerations.
Tetronics have demonstrated the use of plasma vitrification to treat simulant PCM waste (Deegan 2004). Full oxidation of the PCM feed was attempted with an operating temperature was 1600 ºC, with the aims to volatilise the organic component whilst oxidising the metallic fraction. The Hyatt et al. (2006) study characterises the resultant vitrified product which had a glassy appearance with noticeable porosity. Porosity is a disadvantageous characteristic of a vitrified waste form as a porous structure increases the area from which chemical dissolution can occur, potentially increasing the rate at which radionuclides are released. The porosity and fraction of crystalline components was observed to increase with increased waste loading of PCM simulants. CeO2 (as a Pu surrogate) was found to be incorporated in the slag phase at 90 % of the initial inventory.
PCT experiments were used to determine the dissolution of the vitrified slag, with results showing no Ce detected in solution. Despite the relevant success of the vitrification trials in terms of incorporation of Ce and the apparent durability of the vitrified product, there were concerns relating to the off gas emission, expressed by Deegan (2007) study. These concerns arise from the high PVC content which, upon volatilisation, result in the possibility of enhanced cerium volatility through the formation of oxychlorides.
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2.5.3 Hot isostatic pressing
A brief overview of hot isostatic pressing is presented here. HIP essentially applies high temperature and pressure simultaneously to densify and remove porosity from materials.
Atkinson (2000) provides an overview of HIPing, which the reader is referred to for more information. Figure 2.7 shows that the HIP utilises a furnace (maximum temperature up to 2200 ºC) contained within a pressure vessel (maximum pressure 300 MPa). The HIP process densifies the material through sintering of the bulk material. The added pressure to the system improves densification further by forcing the particles together which promotes the formation of grain boundaries by providing an opposing force to the internal pressure of the porosity Kinger et al. (1976).
Figure 2.7 Schematic of HIP unit for nuclear application. Figure adapted from Maddrell (2013)
There are a number of distinct advantages to HIPing radioactive waste, these include (Maddrell 2013):
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HIP has a wide processing window allowing suitability for processing a number of waste streams generating minimal secondary waste
Unlike JHCM, HIPing is insensitive to the physical, electrical and thermal properties of the waste form.
The major benefit as opposed to other thermal treatment systems is that the process is consolidated meaning no volatile losses.
For these reason there are a number of active HIPing facilities currently under development for processing a number of waste streams including, ILW liquid waste resulting from 99Mo production at ANSTO (Carstens et al. 2014).