Giving homes to orphans13 November 2009
Cement encapsulation is not suitable for some types of intermediate level wastes including ion exchange resins, organic liquors and metallic wastes. Polymer encapsulation methods being studied by the UKAEA offer a viable alternative. By Steve Black and Michelle Wise
Geological disposal of nuclear wastes requires that waste packages are designed to prevent the mobilisation of the entrained radionuclides to the local groundwater over an extended time period. This may run to many thousands of years and it is expected that during this timescale groundwater will penetrate the repository and interact with the wastes.
Waste encapsulation using cementitious materials is widely used within the nuclear industry worldwide. For many materials it is a suitable method of encapsulation for storage and disposal. However it is limited in scope with two types of waste: reactive metals which present problems with corrosion and pyrophoric material formation, and wet wastes where unknown water content and the presence of mobile ions can interfere with the curing process. These types of wastes are often referred to by waste strategy developers as WRATs (wastes requiring additional treatment) or orphan wastes (those with no identified disposal route).
The encapsulant of choice for immobilising nuclear wastes has traditionally been a grout matrix based on a mixture of OPC (ordinary portland cement) with PFA (pulverised fly ash) or BFS (blast furnace slag). This formulation provides a good mix of chemical stability, compatibility with most wastes and well established physical properties, as well as being relatively cheap and with a stable supply base. The container most often used for the packaging of intermediate level wastes in the UK is the 500-litre drum.
Whilst grout offers a viable waste encapsulation route for a wide range of wastes there are some streams where it is not suitable, some of these are outlined below:
• The stability of a grout-based wasteform for ion exchange resins and zeolite wastes can be affected by high rates of caesium and strontium leaching during the initial cementation process, resulting in mobile radionuclides being present in the grout matrix.
• Encapsulating reactive metallic wastes with cementitious materials is not straightforward. Wastes containing metallic aluminium, uranium and Magnox cladding for example, prove to be problematic in terms of expansive cracking from corrosion of the metal, hydrogen generation from reaction with the grout and the formation of pyrophoric metal hydrides (in some cases) with cement-based encapsulants.
• There are particular chemical species present in some waste streams that interfere with the cement hydration process, such as borates which can significantly retard setting causing the cement to take longer to cure or to fail completely.
• Absorbents designed to mop up residual organic liquors from the reprocessing process will readily leach organic materials from grouted wasteforms. This has made wastes of this type unacceptable for sentencing to the Low Level Waste Repository (LLWR) at Drigg in Cumbria due to high organic bleed.
As nuclear waste must be immobilised in a passively safe and stable matrix prior to consignment to a geological repository, direct cementation of reactive metal wastes, ion exchange resins or other orphan wastes is proving to be an issue for sentencing of some wastes in the UK.
Polymeric encapsulation is an alternative approach to this problem as the factors causing the corrosion, expansive cracking and generation of hydrogen are absent due to the nature of the polymer.
Polymers are high molecular weight organic compounds (macromolecules) consisting of hundreds or thousands of repeating monomer units. Polymers molecules may be formed from simple compounds, such as ethylene, and polymers may be divided into classes according to the way they are cured; thermoplastic and thermosetting materials.
A thermoplastic is a polymer that melts to a liquid when heated and freezes to a solid state when cooled sufficiently in a reversible fashion. Properties of thermoplastics range from flexible films (e.g. polythene sheet) to brittle glassy materials (e.g. acrylic impact resistant glass substitutes). Thermoplastic polymers differ from thermosetting polymers in that they can be melted and reformed through many cycles without significant degradation. Many thermoplastic materials are addition polymers such as polyethylene and polypropylene.
There has been some work carried out in the USA at Brookhaven National Laboratory on use of low density polyethylene melts to encapsulate a range of wastes via screw extrusion for fine wastes or macroencapsulation for larger components. Investigations in France have also used bituminous materials for the same purposes. The use of thermoplastic materials for waste encapsulation of this nature in the UK is not favoured as it is relatively easy to re-melt the wasteform and recover the waste. This is not viewed as an acceptable approach. The polymeric materials currently under study in the UK are all thermosetting in nature to ensure that once cured they cannot be re-melted to retrieve the waste.
Thermosetting plastics (thermosets) are polymer materials that irreversibly cure. The cure may be done through heat (generally above 200°C), through a chemical reaction or by irradiation of the monomers such as electron beam processing. Thermoset materials are usually liquid or malleable prior to curing and designed to be moulded into their final form where they are used as adhesives, coatings and encapsulants.
The polymers of choice for ILW encapsulation in the UK are epoxy resins. Epoxy resins are prepared from the polymerisation of epichlorohydrin and bisphenol A in the presence of a sodium hydroxide catalyst. The product is a diglycidyl ether of bisphenol A (DGEBA).
Epoxy resins exhibit a high degree of cross-linking on a molecular level. This results in a highly rigid three-dimensional structure with compressive strengths of up to 175MPa. Grouts of the type used for other waste streams typically yield in the region of 10-15MPa after a 28-day cure. The polymer can also demonstrate a non-brittle failure mode where it can retain a good degree of integrity.
Epoxy resin-based wasteforms are effectively impermeable to water and display excellent leach resistance both for organic components and for retention of encapsulated radionuclides. Epoxy resins are highly resistant to attack by aqueous alkalis (making them suitable for the ILW repository concept) and organic solvents but are decomposed by strong acids. Figure 2 shows the water barrier properties where silica gel beads have been encapsulated in an epoxy polymer and immersed in water over an extended time period with no indication of any moisture penetration as shown by the fact the beads have retained their blue colour.
Epoxies have low flammability and are self-extinguishing. Most epoxy resins begin to thermally decompose at around 300°C with decomposition becoming rapid at greater than 400°C.
Radiolytic damage has been observed at doses greater than 1MGy where material hardening results in embrittlement (with a benefit in terms of an increase in compressive strength). Irradiation of epoxy resin based wasteforms also results in the generation of modest amounts of gas with G(H2), G(CO2) and G(Total Gas) of 0.32, 0.08 and 0.36 molecules / 100 eV reported. This low gas generation rate and high tolerance towards radiation indicate that this polymer will meet requirements for the ILW repository concept.
As an integral part of this work towards advancing waste encapsulation technology UKAEA have been involved in the development and testing of a series of formulations designed to achieve high cure strengths without generating excessive heat. Commercial off the shelf epoxies can generate significant temperatures when cured on a large scale and this would be unacceptable in an encapsulation plant on a nuclear licensed site. These low temperature cure materials prevent this high heat buildup occurring and allow cure to occur to a high strength material in a controlled manner even on a bulk scale.
Pros and cons
Polymeric materials offer several advantages over grout in that they are non-aqueous systems so direct corrosion of the metals by water is eliminated. Other advantages include:
• A good barrier to moisture transport.
• Gaseous species can diffuse within the polymer matrix.
• Highly efficient at infiltrating round a diverse range of shapes.
• High compressive strength.
• Good radiation resistance in bespoke materials.
• Polymers can entrain hydrophobic materials like graphite readily.
• Polymers can achieve higher waste loadings by volume than grout with a resultant drop in the number of packages required.
In the past, UKAEA has successfully encapsulated wet waste simulants in a polymeric system using vinyl ester styrene (VES). However, styrene polymers contain a volatile precursor material with a very low flashpoint, presenting a fire hazard that may be considered too high for certain applications. Additionally VES formulations are very sensitive to the ratio of ingredients used in the polymer to effect proper curing, and the presence of an indeterminate amount of water would cause problems in this respect.
Despite the problems with low flashpoint there is an active VES plant in operation in the UK – the Trawsfynydd plant which is used for encapsulation of ion exchange resin wastes. The first successful campaign this plant undertook sentenced 656m3 of ion exchange resins contaminated with fission products and actinides. A further two campaigns to encapsulate a further 1400m3 are planned currently.
Polymeric encapsulants are capable of entraining a wide range of waste media including materials that are difficult to dispose of in grout due to their hydrophobic nature.
These wastes are typical of radium-contaminated materials found at the Harwell site. One significant advantage polymeric encapsulation has over cement for these waste streams is that the permeability of the radon gas generated during the decay train of the wastes through the polymer is very low preventing the gas from being released from the wasteform.
UKAEA have developed polymer technology to entrain some of the more challenging waste streams present in the UK inventory over the past five years. Figure 5 demonstrates the difference in integrity of the wasteforms for graphite powder with cement showing a poor waste monolith which was not self-supporting outside of the plastic bottle. With polymer encapsulation by simple tumble mixing, a highly stable fully entrained material with a regular wasteform is produced.
Polymeric encapsulation is ideally suited to the treatment of active metal wastes such as the fuel rods from the GLEEP (Graphite Low Energy Experimental Pile) reactor at Harwell. The fuel is made up of uranium bars coated with aluminium which are not compatible with grout encapsulant formulations. UKAEA have successfully carried out a series of trials to encapsulate waste simulants and this data is being used to gain approval for this encapsulation technology to be used for disposing of the pile fuel itself. The efficiency of the polymer in entraining the wastes and the high level of waste loading is shown in Figure 6.
This paper has discussed the issues facing many nuclear waste management organisations when dealing with problematic waste streams. Although cement encapsulation of wastes is the commonly followed method for immobilising these wastes, polymer encapsulation38-40ave been developed for this work. UKAEA have the in house expertise that have allowed the Radioactive Waste Management Directive Letter of Compliance procedure to be followed for the use of polymers as encapsulation media for UK orphan wastes throughout the length of this work.
Dr Steve Black, section manager, waste characterisation and conditioning, technical services group (UKAEA, The Innovation Centre, Westlakes Science Park, Cumbria, CA24 3TP) and Michelle Wise, head of UKAEA technical services group. UKAEA wish to acknowledge the support given by the NDA and RSRL, Harwell. The work of Dr Stephen Farris, Lesley Oliphant, Andrew Green, Barrie Williams and Jonathan Cox at UKAEA laboratories has helped the project considerably.