A dissertation submitted to the Department of Physics, University of Surrey, in partial fulfilment of the degree of Master of Science in Radiation and Environmental Protection

ModulOx is a revolutionary process designed to treat some difficult waste streams which contain high ¹⁴C content and which are also chemically contaminated, including chelating agents. The findings of ¹⁴C in some resin streams lead to investigation at Winfrith into the likely impact of the ModulOx wet oxidation process on the levels of ¹⁴C in spent resin. Results indicated that this wet oxidation process can be used to remove >98% of the ¹⁴C present in the original waste, as well as destroying the chelating agents.

This process therefore could allow LLW that has initially high levels of ¹⁴C to be accepted by Drigg, in addition to its existing role of processing Primary Circuit Decontamination (PCD) resins containing chemical contaminants.

It is a processing plant that has been designed to have a simple modular structure and is transportable which makes it very simple to set-up. The plant is the size of a full height ISO container and would fit into some of the smaller areas and can be individually scaled in order to deal with a certain volume of waste. The plant has a predicted lifetime of 25 years. It would need to be able to process 10 RSV’s per year until 2015 and 5 RSV’s per year until 2030. This may extend beyond 2030, but greatly depends on the MoD submarine operating profile. Winfrith currently have a prototype plant and there is much work being done to investigate the possibility of using this as an every day process for treating some of the ILW that is accumulating.

Resins containing concentrations of chelating agents greater than 0.1% by weight are pre-processed using the ModulOx wet oxidation process, destroying chelating agents in PWR2 and PCD resins and therefore preventing them from mobilising radionuclides within LLWR after final disposal.
Figure 9 is an example of the design of a typical ModulOx plant.
There are 4 potential sites at which the ModulOx plant could be situated on Devonport NLS.

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  the scrap pound (off the licensed site)
  
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  next to 8 Dock
  
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  transport and storage area (off the licensed site)
  
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  area of the now demolished N008 building
  
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  on another nuclear licensed site i.e. Winfrith
  

There may also be some potential sites on MoD property close to where the RSV’s are stored in the NUB. This could be a possibility but it may be difficult with regulations as the MoD run under an Authorised Site and not a Nuclear Licensed Site (NLS). Devonport does however have authorisation to take waste from the MoD and process it, so with those regulations already in place, it may be easier to attain the paperwork for additional activities.

• 
  The scrap pound could be a sensible site as there is a good deal of space which would be able to accommodate the plant, but it is ~1200m distance from the NUB which carries its own difficulties in waste handling and management. This site is also very close to the boundary of the premises and the public are able to see over the perimeter wall- obvious public concern issues.
  
• 
  Putting the plant next to 8 Dock would be convenient but there is the possibility of submarine decommissioning in the near future which may happen in this area. If this becomes the case, it may be necessary to keep the dockside clear for moving pieces of the submarine as they are dismantled. Therefore, it would be necessary to move the plant in the future which would not be an easy task as there would be a lot of associated paperwork and cost in order to apply for relocation.
  
• 
  Within the NLS existing boundary is an area where a building has been demolished and so far has not been used for anything else. This site is very close to where the RSV’s are stored and there is only a short distance to travel in order to deliver the radioactive waste.
  

The ModulOx process works on the basis of a catalyst being added to the waste as necessary, with H₂O₂ added continuously throughout the reaction until the target material is effectively destroyed. In basic terms, the waste is added to the reactor, it is heated to a temperature which will sustain the reaction, a catalyst and H₂O₂ is then also added and completes the process. See figure 12.

This flow diagram describes the basics on how the ModulOx process works.

Devonport would probably need to apply for authorisation to discharge an increased amount of ¹⁴C into the atmosphere as ¹⁴CO₂ or to dispose of additional solid waste to LLWR. These processes can be costly and studies would need to be carried out on the environmental impact of the potential discharge route. The timescale for these authorisations could be anything between 2-4 years by the time initial approval has been granted, there is then a preliminary authorisation which is a work in progress and providing there is not any opposition from the public, the Secretary of State looks over the final document and if the document is successful, then full authorisation is granted.

The ModulOx process has excelled and proved itself as a very efficient process in the treatment of contaminated tank waste, organically contaminated liquid effluents, chemical weapons agents, ion exchange resins and wastes containing organic chelating agents. _[10]

The benefits of the ModulOx process are that;

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  It is a low temperature and atmospheric pressure process,
  
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  It is a simple, single stage modular design.
  
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  The output can be readily tailored to further conditioning, storage and disposal requirements.
  
• 
  There is the potential for volume reduction of wastes with associated economic benefits.
  
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  It removes organic components to enable compliance with acceptance criteria for disposal.
  
• 
  It can be used to treat a wide range of organic material.
  

This process uses hydrogen peroxide with a catalyst at atmospheric pressure and a temperature of 100’C to oxidise organic materials, forming predominantly CO₂, H₂O and inorganic salts. Development of the process has focused on the destruction of spent ion exchange resins, especially where chemical chelating agents from decontamination (EDTA/Citric Acid) are present.

The following classes of organic material can be treated directly by the ModulOx process.

• 
  Organic material which is water soluble or partially water soluble.
  
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  Organic components containing unsaturated carbon bonds.
  
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  Organic compounds containing functional groups, such as carbonic acids, esters, sulphonate, phosphate, chloro and nitro substituted compounds.
  

The process uses aqueous oxidation process chemistry. H₂0_{2 ,} in the presence of a ferrous ion is used to destroy the organic species (known as Fenton’s reaction). This forms the basis on which the ModulOx process works. This happens because H₂0₂ in aqueous solution acts as a strong oxidising agent and reacts with organic molecules under appropriate conditions. At ambient pressure and under acidic conditions, the hydroxyl radical has a higher reported electrochemical oxidising power than any other species with the exception of Fluorine. The oxidation reaction between organic material and H₂0₂ alone are in fact slow or even non-existent reactions, but, in the presence of a metal catalyst, such as Fe, H₂0₂ degrades to form a powerful radical reaction;

H₂0₂ + Fe²⁺ → Fe³⁺ + OH^- + OH∙

Under acidic conditions, the hydroxyl radical readily reacts with organics through two main processes;

R-H + OH → R∙ + H₂0 (H abstraction)

R=R + OH∙ → ∙R- ROH (O addition)

Both reactions result in an organic free radical which reacts further through continued oxidation, rearrangement and radical-radical combination. Further oxidation leads to production of alcohol carbonyl compounds, carboxylic acids and peroxide groups. Breakdown of the carbon bond structure takes place as part of the oxidation and rearrangement reactions, with aromatic ring cleavage, fragmentation across unsaturated bond sites and decarboxylation being the most important mechanisms. Some of these reactions lead to a reduction in the carbon chain length. Oxidation and breakdown of the bond structure continues until the final products of CO₂ and H₂O are formed. The hydroxyl radical reacts functionally with the organics, but wastefully with the Fe^2+, or H₂0₂, generating hydroxide and oxygen products. If significant concentrations of organics are present, and under acid conditions, the organic destruction mechanism dominates. _[9,10]

Hydroxyl radical production is the precursor to both organic destruction and wasteful degradation of H₂O_2. Although it is a fast reaction, hydroxyl radical production is slow in comparison to the subsequent radical interaction reactions and is usually the rate which determines the speed for the overall reaction.

The overall rate for the ModulOx system is regulated by controlling the hydroxyl radical production rate, through carefully controlling the rate of H₂O₂ addition, temperature, catalyst concentration and pH. _[10]

The extent of the reaction depends on the specific compound involved and the reaction residence time. Organic destruction reaction is highly exothermic. It is therefore important that the factors influencing the rate of heat production are properly understood and controlled within the reaction scheme if safe and efficient operation conditions are to be maintained. _[10]

There is an acid and a base feed system for pH control and the whole reaction is run close to boiling point and atmospheric pressure. Throughout the reaction, steam and other gases produced by the oxidation reaction pass through a heat exchanger where the distillate can be collected or refluxed to the reactor. Non condensed gases pass through a HEPA filter before being discharged to the atmosphere. The final product, of oxidation after neutralisation with calcium oxide (lime) is predominantly calcium sulphate which can be encapsulated prior to disposal at the LLWR.

Option 3; Chemical /Thermal Treatment _[2]

The methods used for this option are not able to deal with waste on a large scale and would not be a viable option for Devonport. Further development however may see these processes improve in their volume handling in the future. However, the details are included in appendix 1 for the interested reader.

The ion exchange resin is a very resistant organic material. It is possible to destroy organic material in the waste by using an oxidative treatment program, allowing a controlled release of ¹⁴CO₂ to the atmosphere.

A process which can destroy the ion exchange resin will also be able to destroy the chelating compounds (EDTA and Citric Acid), used to capture the metals. But, removing the organic component will leave a concentrated inorganic component which will need to be treated as separate waste. It must also be noted that this secondary waste may not necessarily qualify as LLW.

Destruction of the organic material can be achieved in different ways, depending on the material and the degree of destruction required. Oxidative and Thermal treatments are the two main ways in which destruction is achieved and there are several different methods under these two principle methods;

≤400^oC

Destruction of organic matter relies on the addition of an oxidant to a solution of waste in water. The destruction of organic matter depends on temperature, the type of oxidant and the presence of a catalyst. It may therefore, not be possible to achieve total destruction. The process may require subsequent treatment in order to process the inorganic material in the ash.

Thermal treatment

>600^oC

A form or pyrolysis is used and oxygen is added in the process in order to remove the carbonaceous material as CO₂ . The end product is process-dependent but may be incorporated into a glass or solid matrix as the inorganic components melt. This end product is more likely to be more homogenous than that obtained from the above oxidative process.

Oxidation and thermal processes yield a large reduction in volume of the solid material which is good for the waste handling at the repository and also helps to offset some of the capital investment in setting up a plant.

This process relies on the reaction occurring in the presence of water into which an oxidant, (usually a gas) is dissolved.

At higher temperatures the process is run under pressure to ensure that the gas remains dissolved and this also improves the solubility of waste breakdown products. Higher temperatures have the capability to destroy the most resistant carbonaceous material. There are three main types of wet oxidation processes, based on temperature; Cold Wet Oxidation, High Temperature Oxidation and Super-Critical Thermal Oxidation.

High Temperature Oxidation

This process also uses soluble oxidants such as hydrogen peroxide, a catalyst and a range of inorganic oxidants such as nitric acid, permanganate and dichromate ions at elevated temperatures. It is a process conducted at temperatures above 100^oC in an autoclave to allow batch processing. More resilient materials can be treated in this way, including aromatic compounds and some polymers. Carbonaceous material is converted to CO₂ or carbonates but there may be further treatment required to treat the residual ash. An example of this process is used in the ModulOx process, developed by Lawrence Livermore Laboratories in USA, using a different oxide, sodium persulphate. This process is not totally effective at destroying all ion exchange resins and probably needs an autoclave process in addition.

This process involves the method above but involves temperatures above the critical temperature of water (374^oC). There are problems with depressurising and waste disposal, so this process is operated as a batch process. This process has a high probability of destroying all organic material.

These processes function at temperatures greater than 600^oC in one of two ways; Combustion or Pyrolysis.

The dominant process is interaction between a fuel, any organic material and an oxidant, normally oxygen, which then produces a flame. The most obvious example of this process is incineration. This process is self-sustaining as the material is combustible, but the process does not normally operate at temperatures where the entire residue is molten, hence why an ash may remain.

High temperature incinerators burn waste in different ways, usually with the addition of supplementary fuel such as natural gas to maintain burning and pure oxygen may also be added so that total melting can occur and then form a glassy matrix. Incineration is a very good process for reducing waste volume, especially contaminated plastics and clothing. This is important from an environmental sustainability perspective. Incineration of mixed waste, both chemically hazardous and radioactive, has two main aims, firstly to reduce the volume and secondly to reduce the chemical toxicity. Any residual compounds left behind in the ash are able to be cemented and then disposed of, and waste gases are normally scrubbed. Incineration is the dominant treatment process in Europe, mainly because of its capacity for waste volume reduction.

Steam-Reforming Pyrolysis

This process avoids using the two-stage process, where organics react with steam at high temperatures, yielding CO and H₂ which can be burnt separately. Most of the char is also removed, so there is little organic matter left behind as secondary waste. Ion exchange resin in this process is totally destroyed. Gas treatment is necessary as some metals can be volatised due to the high temperatures reached. Any char remaining can be cemented and disposed of (in a similar manner to incineration).

This process does not need a separate hydrolysis step. It was originally designed to handle material containing high levels of nitrate but has been further developed and is now suitable for dealing with carbonaceous material.
The principal combustion techniques and their operational temperatures used for incineration of radioactive waste are as follows;

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  Excess Air Incineration 800-1100^oC
  
• 
  Controlled Air Incineration 800-1000^oC (a primary process)
  
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  Pyrolysis 500-600^oC (a primary process)
  
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  High Temperature Slagging Incineration 1400-1600^oC
  
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  Fluidised Bed Incineration 800^oC _[11]
  

This diagram shows how the pyrolysis process works.

The initial part of the process does not require an oxidant, but is performed in the presence of an inert gas without the need for a flame. This helps to reduce runaway temperatures which could occur due to direct burning of organic material, forming a char and the volatiles are lost. The process operates at a reduced pressure to ensure the removal of the volatiles. Adding oxygen in the second step of the process oxidises the char left behind. The initial products can be combustible hydrocarbons such as methane or aromatics. CO₂ is only produced if there is a sufficient amount of oxygenated compounds in the waste. Most thermal processes need extensive off-gas treatment to meet environmental regulations, both for CO₂, SO₂, NO_X and any particulate material carried over. Depending on the temperature and the process itself, volatile inorganic materials and powders may be emitted and require treatment, for example electrostatic precipitation or scrubbing, which is critical for controlling radioactive emissions. Gas scrubbing is necessary as the gases produced during this process are not emitted as CO₂ which is especially important if the waste contains halides because these can form dioxins. Commercial examples of this process include Thermal Organic reduction (THOR). _[11]

The very nature of this process results in massive oxidation of the feed material which produces a volume reduction of 60:1. Residual ashes can be managed as LLW and are packaged appropriately for final disposal at LLWR. The ash is non-flammable, chemically inert and more homogenous than the initial waste feed. Radioactive wastes need to be separated into incinerable and non-incinerable categories ideally at the site of waste generation. Pyrophoric, corrosive, halogenated plastics, metal, glass and other non-combustible materials will need to be dealt with separately unless the incinerator is designed to deal with them. Resins have a tendency to cluster and there may be inefficiency in the combustion process of all material. To avoid this problem, a balance of resin and combustible material is found to sustain the combustion process when fed into the incinerator in equal balance. Some incineration processes are able to process the waste in the same form as it was generated, but other processes require pre-treatment, for example reduction, homogenisation, moisture content reduction and adsorption of combustible liquids onto sorbent material. _[11] The ash needs to be monitored to prevent an ILW ash from forming. The NII disapprove of the accumulation of ILW or even converting this waste to a lower level of ILW, so it is of paramount importance that the radioactivity levels are checked regularly. An exhaust stack would discharge the filtered waste gases, which are a product of the incineration process and include CO₂, N₂O, N, O, CO, steam, halogenated compounds, sulphurous compounds, hydrocarbons, organic and mineral particles. See appendix 3 for details of the effects on the body that may occur due to the presence of these gases. Caesium becomes a volatile component of the off gas at 1000’C. ¹⁴CO_2, ³H₂O and ³⁵SO₂ are permanently within the gas streams.

The stacks provide elevated release of the off-gas with a velocity in the region of >12m/s. Stack design must meet air pollution control laws and regulations. See appendix 5 for details of stack discharges which show the different doses emitted from a 1m and 10m stack height.

The product of incineration would yield a higher activity concentration than the input feed, so it may be preferred to use this option for the initially higher activity resins. This may also be favoured by the regulators as the resins are converted into a passive form until final disposal at LLWR. _[10]

The design and build of the plant would be influenced by the type of waste to be dealt with and so too would the combustion technique. At the very beginning of the design process, it is necessary to decide if the plant is to process wastes involving acid gases, such as halogenated or sulphur compounds as the plant will need to incorporate the specialised facilities to deal with the off-gases. Gaseous effluents are a by-product of the incineration process, these include CO₂, N₂O, N, O, CO, steam, halogenated compounds, sulphurous compounds, hydrocarbons, organic and mineral particles. Combustion is easily sustainable with homogenous material, but not with bulky, non-homogenous material. _[11]

In very basic terms, filtration, separation and wet scrubbing are used to remove particulate matter, wet scrubbing and absorption for the removal of corrosive acid gases and adsorption is used for the removal of radioactive iodine. _[11]

Off-gases may contain radioactivity in the form of solid aerosols (dust containing ⁶⁰CO), in chemical gases (CO₂ containing ¹⁴CO₂) and in liquid aerosols (H₂O condensed in cold sections of the system containing ³H). Off-gas treatment is a process which ensures minimal chemical and radiological environmental release is in compliance with regulatory standards for each specific pollutant. The types of off-gases discharged will depend on the initial waste feed characteristics.

Ash removal should ideally be done under gravity and it is also necessary to prevent blockages arising from slag formation. Incineration concentrates the ash, so it should be contained inside a pressure retaining enclosure during cooling and removal from the combustion chamber to prevent aerosol dispersion.

To meet ALARP principles, remote handling, shielding and double lid systems and other containment methods should be used, depending on the level of radioactivity. Glove boxes should also be available for use when the ash has cooled. Negative pressure in regards to ambient pressure should be maintained in the ash container, especially during removal and cool down. For the purposes of transfer, the ash must be maintained in sealed metal components with a design which prevents air leaks or the spread of loose contamination. Expensive, high quality components should be used to minimise maintenance requirements. Ash sampling is necessary and this process needs forethought in the design of the plant. There is the possibility of combustible gases being generated in the process of immobilising the ash with cement so special precautions are therefore necessary.

To reduce the levels of gaseous discharges scrub solutions are used in the off-gas cooling/cleanup process and should be treated by a radioactive liquid treatment system. Wet scrubbers are used primarily in the treatment of radioactive waste and are used to remove the acidic gases, HCl, HF and SO₂ etc from the off-gas, along with particulate matter. Non-commercial methods such as pyrohydrolysis, acid digestion, molten salt and molten glass oxidation have been developed and demonstrated for the oxidation of radioactive waste, but, as yet, have not been utilised. _[11]

Aqueous off gas treatments use wet scrubbing techniques and have the capacity to remove corrosive gases, and yields the highest decontamination factors for total radionuclides. This type of system would be used for processing wastes containing large amounts of corrosive acid forming gases. _[11]

Operators may receive radiation dose when they are removing wastes from packaging/containers, but all necessary safeguards would be in place to keep doses ALARP. Remote handling procedures would be used for any further packaging or conditioning of the waste. _[10]

This option poses more of a problem in the fact that it requires further treatment of the waste products of combustion. The volatile parts of the resin would be liberated along with a tiny amount of other radionuclides in particulate form. Filtration and abatement systems would substantially reduce atmospheric discharges. ³H and ¹⁴C are assumed to be atmospherically discharged in their entirety. According to appendix 3 and 5, the height of the stack on any facility could make a considerable difference on the doses received by both members of the public and operators alike. A 10m stack provides the lowest doses and quite possibly lower doses still with a higher stack.

Gas scrubbing techniques are efficient ways of removing potentially harmful chemicals from the waste plume as well as HEPA filters reducing the amount of radioactivity released by upto 99.99%.

If a wet gas scrubber is used, it would give rise to a secondary waste stream, but in-line with BPEO, due to the very low environmental impact, this waste stream would be able to be discharged to the Hamoaze.

DRDL may experience considerable problems when this option went forward for public consultation. There have been situations where incineration processes have been turned down purely based on non-technological issues, when the public have a major concern with the incineration of radioactive waste.

The levels of atmospherically discharged ß/γ could pose a problem for the existing authorisation which imposes a limit of 0.3MBq per year. There are other activities already in progress at Devonport which give rise to aerial discharges, so a variation in the discharge authorisation would need to be sought.

It should be noted that the final product of this process results in an ash which is compacted and sent to LLWR as pucks. The ash has not undergone any pre-treatment and because of the mobilisation and concentration properties of chelating agents, there are concerns about accepting this form of waste. If the pucks become hydrated, due to the compaction process they have an increased possibility of swelling and causing complications at the LLWR. Therefore, it is necessary for Devonport to do some further investigative work in order to prove that the resin ashes are safe to be stored in this form. This could take some time and so considerable delays could be encountered before full scale implementation of the process. _[9]

Atmospheric discharge levels of ³H and ¹⁴C are unlikely to be higher than equivalent amounts arising through the process of curing of cementitious grout.
The above includes some of the activities that would be involved in each of the options, detailing the predicted doses and risk of death from each activity. This represents normal operations doses and for current activities and does not include additional activities ie Modulox or incineration. _{[16, 17, 18, 19, 20, 21]}

Option 5; Deep Geological Repository