A neutron diffraction investigation of the structure is reported for four glasses in the Na O-2-B2O3 -SiO2 system and one in the Na2O-B2O3 system. Measurements have been made to high values of the scattering vector magnitude, Q, in order to obtain high real space resolution. Of primary interest is the fraction, x(4) , of four -fold coordinated boron atoms present at each composition and the bond lengths within the borate and silicate structural units. These have been extracted using peak fitting techniques and compared both with NMR results and with the predictions of a model based on earlier NMR data. It is concluded that, within the experimental uncertainty, the values of x(4) are in agreement with the NMR results but at variance with those predicted by the model.

NNL and ANSTO on behalf of Sellafield Ltd have developed a process for the immobilisation of a range of Pu containing wastes and residues. Following the inactive demonstration of the technology the project is now focusing on the design of an active pilot plant capable of validating the technology and ultimately immobilising a waste inventory containing around 100kg plutonium. The diverse wastes from which it is uneconomic to recover Pu, require a flexible process with a wide product envelope capable of producing a wasteform suitable for disposal in a UK repository. Ceramics, glass ceramics and metal encapsulated wasteforms can be delivered by the process line which incorporates size reduction and heat treatment techniques with the aim of feeding a hot isostatic pressing process designed to deliver the highly durable wasteforms. Following a demonstration of feasibility, flowsheet development is progressing to support the design which has the aim of a fully flexible facility based in NNL's Central Laboratory on the Sellafield site. Optimisation of the size reduction, mixing and blending operations is being carried out using UO2 as a surrogate for PuO2. This work is supporting the potential of using an enhanced glass ceramic formulation in place of the full ceramic with the aim of simplifying glove box operations. Heat treatment and subsequent HIPing strategies are being explored in order to eliminate any carbon from the feeds without increasing the valence state of the uranium present in some of the inventory which can result in an unwanted increase in wasteform volumes. The HIP and ancillary systems are being specifically designed to meet the requirements of the Sellafield site and within the constraints of the NNL Central Laboratory. The HIP is being configured to produce consolidated product cans consistent with the requirements of ongoing storage and disposal. With the aim of one cycle per day, the facility will deliver its mission of immobilising the identified waste and residues inventory within 3 years. During that period it will also be used to demonstrate the potential of this technology to deliver the immobilisation of a proportion of the UK plutonium stockpile that may not be suitable for use as MOx fuel should that decision be taken.

Glasses related to those used for the vitrification of high-level waste (HLW) have been produced from the sodium-lithium borosilicate system. Their thermal and structural characteristics have been measured for a wide range of boron oxide contents (NBS series) and for a range of alkali oxide to boron oxide ratios, R, for several fixed silica to boron oxide ratios, K (ABS series). The NBS series of glasses was seen to exhibit a maximum in the glass transition temperature, T g , and a minimum in the fraction of 4-coordinated borons, N 4 , at 22 mol% and 29 mol% boron oxide, respectively. With variation of K and R, initial results indicate an increase in T g to a maximum before a gradual decrease. Density measurements show a general increase before remaining within experimental error at larger R. Initial α-particle irradiation tests have been carried out on several base glass samples and show evidence of B(α,n) reactions, the occurrence of which could have important consequences for the future viability of wasteforms designed for reprocessing of higher burn-up fuel.

The effects of the addition of divalent metal oxides on the structure and thermophysical properties of the mixed alkali borosilicate glass system used for high level radioactive waste immobilisation have been studied. Densities, molar volumes, linear thermal expansivities and glass transition temperatures are reported and structural information has been obtained using B-11 and Si-29 magic angle spinning (MAS) NMR. Molar volume is controlled by ion size and by the formation of network polyhedra, whilst thermal expansion reflects the network rigidity. Changes in the glass transition temperature, T-S, are consistent with the different values of the heat of formation of the oxides added. Alkaline earth oxides produce an initial increase in the fraction of 4-coordinated boron, N-4, whereas PbO and ZnO reduce N-4. Si-29 NMR of the alkaline earth oxide glasses shows the presence of significant concentrations of nonbridging oxygens, consistent with a previous study of the aqueous corrosion behaviour of these glasses. PbO and ZnO reduce and ultimately eliminate the nonbridging oxygens. Estimates of the quantities of Q(4)(Pb) have been made and are shown to change with composition in the same manner as Q(4)(Me) in glasses with trivalent additions.

National Nuclear Laboratory, in collaboration with the Australian Nuclear Science and Technology Organisation, is developing hot isostatic press (HIP) based ceramic technology for the immobilisation of a diverse range of wastes arising from nuclear fuel processing activities on the Sellafield site. Wasteform compositions have been identified and validated for the immobilisation of these plutonium containing wastes and residues in glass-ceramic and ceramic forms. A full scale inactive facility has been constructed at NNL’s Workington Laboratory to support the demonstration of the technology. Validation of the inactive wasteform development using plutonium has been carried out at ANSTO’s Lucas Heights facility. A feasibility study has been conducted to evaluate the construction and operation of a plutonium active pilot facility which would demonstrate the immobilisation of actual residues in the NNL Central Lab. This could form the basis of a facility to treat the plutonium wastes and residues in their entirety. The technology is being explored for the immobilisation of additional wastes arising on the Sellafield site taking advantage of the investment already made in skills and facilities.

Raman spectroscopy has been used to measure the fraction of tetrahedral silicate units connected at three corners into the network (Q3) in binary lithium silicate glasses and also in the more complex borosilicate glasses used for waste immobilization. Agreement within experimental error was obtained with 29Si MAS NMR measurements of the same samples. Raman provides an alternative method of structural determination for silicon-containing glasses with a high content of paramagnetic species where NMR loses resolution. Analysis was performed on borosilicate glasses containing up to 11.98mol% Fe2O3 and the Q3 values obtained by Raman spectroscopy agree within error with the published 29Si NMR results from borosilicate glasses containing the equivalent quantity of Al2O3.

In 1987 samples of Pu238 and Cm244 doped Synroc C were prepared in the UKAEA Laboratories at Harwell. They were studied for five years before being archived. During decommissioning of the Harwell laboratories the samples were transferred to Sellafield and the opportunity was taken to conduct further studies. Given the age of the samples, they offer a unique insight into the long term radiation stability of Synroc. To date, the three Pu238 samples have been examined. The alpha decay dose experienced by the samples is estimated to be 3 × 1019 alphas per gram. The sample allowed to accumulate alpha decay damage (10587) was slightly heterogeneous, with an apparent grain size in the range 3–15νm. EDX analysis confirmed the phases present to be those expected in Synroc C and that the Pu had partitioned predominantly into the perovskite and zirconolite. Microcracking was observed in the hollandite and rutile but cracks arrested once they reached a zirconolite or perovskite grain. Sample 10588 was annealed after five years, at temperatures up to 1200°C, this sample was microstructurally similar to 10587 at the 20-50νm scale but differs at small scales. A major difference is the presence of both intergranular and intragranular porosity. Sample 10589 was annealed at the same time as 10588 but differed significantly probably due to actual fabrication temperatures being higher than those recorded. Information on these samples will be presented, along with a discussion of the implications for the expected long term stability of titanate ceramic wasteforms.

Glasses of nominal composition xMeO.(100-x)MW have been prepared, where MW is the mixed alkali borosilicate glass system used for high level waste immobilisation; Me=the divalent cations Ca, Sr, Ba, Mn, Zn and Pb; and 2.4 <= x <= 15 mol%. Their chemical durability has been determined using an international standard soxhlet leach test procedure and ionic conductivity has been measured using ac impedance. The durability tests showed that the alkaline earth oxides BaO, SrO and CaO decrease the durability, whilst ZnO and PbO improve the corrosion resistance. The presence of a surface Mn7SiO12 crystalline phase was identified on corroded MnO containing glasses. The ionic conductivity of the base glass (MW) at 300 degrees C was found to be 0.93 +/- 0.01 mu S cm(-1) and this decreased on addition of the oxides, with 12 mol% PbO reducing conductivity by a factor >20. The activation energy for dc conductivity is in the range 0.40 to 0.59 +/- 0.03 eV, typical of alkali ion motion, and shows some dependence on the fraction of 4-coordinated borons, N-4, in the glasses.

The alternatives for the disposition of the UK’s civil plutonium stocks are currently being investigated by Nexia Solutions Ltd. on behalf of the Nuclear Decommissioning Authority (NDA). A number of scenarios are currently being considered depending on the strategic requirements of the UK. The two main disposition options are: re-use as MOX (Mixed Oxide) fuel in reactors, or immobilisation in the event of any material being declared surplus to requirements. The amount of Pu which will require immobilisation will depend on future UK nuclear strategy, along with the extent of any stocks deemed unsuitable for re-use. However, it is likely that some portion will have to be immobilised and therefore three credible wasteforms are under consideration; ceramic, glass and ‘immobilisation’ MOX. These are currently being developed and assessed in a systematic programme that involves periodic evaluation against a range of criteria. In this way, by down-selecting on the basis of robust and technical review, the most appropriate option for immobilising surplus civil plutonium in the UK can be recommended. The latest results from the immobilisation experimental programme are presented following the de-selection of the least favourable glass and ceramic candidates. The main criteria for this decision were waste loading, durability, processability, criticality and proliferation resistance. In addition, the durability of unirradiated MOX fuel is being examined to determine its potential as a wasteform for Pu, and recent leach test data is discussed. The current evaluation comprises not only a comparison of the relevant physical properties of the various wasteforms, but also key processing parameters, e.g. glass viscosity and melter technology, ceramic fabrication routes, and criticality issues. Other important aspects of the long-term behaviour of the wasteforms under consideration in a potential repository environment, such as radiation damage, criticality control and the properties of any neutron poisons present, are also included. INTRODUCTION The options for disposition of the UK’s separated civil plutonium stocks are currently being assessed by the NDA. Nexia Solutions Ltd is carrying out a package of work on behalf of the NDA considering technical aspects of both the options of re-use as fuel and immobilisation as waste [1]. This will allow robust decisions to be made on future management options for UK separated civil plutonium. The majority of the Pu-inventory could be used in the manufacture of MOX fuel in potential power generation scenarios, but immobilisation is being investigated for any surplus or unsuitable material. Immobilisation would be achieved by assimilation of the 1 ‘Immobilisation’ MOX would comprise MOX fuel pellets manufactured ‘as normal’, with the exception that the dimensional tolerances required of reactor operations could be relaxed. WM2008 Conference, February 24 -28, 2008, Phoenix, AZ plutonium into a durable host matrix followed by long term storage and subsequent final repository disposal. The immobilisation component of the Civil Plutonium Disposition Project is intended to develop and subsequently recommend to the NDA a technically underpinned option or options for the immobilisation of any, or all, of the UK plutonium stockpile. This will entail the evaluation of a range of candidate wasteforms using a variety of criteria and will result in a fully qualified wasteform. These evaluation criteria include: • waste loading, • durability, • criticality protection, • proliferation resistance, • radiation damage tolerance, • processability, and • chemical flexibility to accommodate impurities. It is important that the UK programme on plutonium disposition is targeted at UK needs and requirements while taking account of the technical efforts that have been carried out elsewhere [2]. To that end, ceramic, vitreous, and ‘immobilisation’ MOX wasteforms along with the possibility of cementation have been identified as the most appropriate for study as plutonium host matrices. For the vitrification option, following an initial set of trials [3], three borosilicate glasses were selected for inclusion in a more detailed experimental programme: lanthanide borosilicate (LaBS), alkali tin silicate (ATS) and high-lanthanide alkali borosilicate, or ‘modified-MW’ (MMW). The minimum acceptable waste loading of 10 wt% Pu was applied, and the temperature dependence of the solubility of inactive surrogates in these leading candidate glass hosts was studied. The durability of the glasses was also investigated in detail, and powder-based static leach test data is now available for a range of waste loadings at different pHs. Similar data is being generated through the systematic examination of a number of ceramic host phases based on zirconolites, pyrochlores and related phases where the capability of including neutron poisons alongside plutonium while maintaining durability and tolerance of radiation damage is being explored [4]. In addition to vitreous and tailored ceramic hosts, the suitability of unirradiated MOX fuel pellets as a potential wasteform for plutonium is being investigated via a series of durability tests. These are being carried out at the Institute for Transuranium Elements (ITU), Karlsruhe, Germany, and preliminary data from short-term leaching is presented below. WORK TO DATE Ceramic Wasteforms A systematic study of ceramic phases suitable for plutonium immobilisation has been performed in collaboration with the Immobilisation Science Laboratory (ISL), Sheffield, UK. Cerium was 2 MW is the glass used for the immobilisation of UK high-level radioactive waste, and has a composition (in wt%) 62 SiO2, 22 B2O3, 11 Na2O and 5 Li2O. WM2008 Conference, February 24 -28, 2008, Phoenix, AZ used as the surrogate for plutonium in the initial work on the examination of phase development and waste loading. This has now been extended to the use of uranium. Several ceramic phases have been investigated and a number of these have been confirmed as having adequate capacity to accommodate PuO2 and neutron poisons such as Gd and Hf. Surrogate loading levels of over 10 wt% have been shown to be possible, and work is being carried out to evaluate higher incorporation rates. The availability of the glove box at the ISL has allowed an investigation of the capacity of these phases to accommodate uranium, which is known to be a better surrogate for plutonium and unlike cerium has no tendency to auto-reduce to the 3+ state. This work has established that while some phases can accommodate over 10 w% Pu surrogate in the oxidised state, a very different result is seen with the reduced species. Table I summarises the ceramic phases currently under investigation along with a synopsis of me of their properties. currently under consideration for e immobilisation of the UK’s surplus civil plutonium.

Magic-angle spinning nuclear magnetic resonance of 11B, 29Si and 27Al has been used to study the distribution of nonbridging oxygen atoms (NBO) in an alkali borosilicate glass to which surrogate oxides for high-level radioactive waste have been added. The properties of such glasses are shown to depend on the fraction N 4 of four-coordinated boron atoms (B4) and on the fraction of silicate tetrahedra possessing one NBO, Q3. The aqueous corrosion rate increases with Q3 content, as does weight loss due to evaporation from the melt. The activation energy for direct current conduction scales with N 4. Values of N 4 obtained for these glasses deviate from those predicted by the currently accepted model and are strongly affected by the modifier or intermediate nature of the surrogate oxide and also by its effect on the distribution of NBO between the silicate and borate polyhedra.

Structural and thermal properties are reported for a range of caesium oxide-containing alkali borosilicate glasses, of the form xCs2O(100−x)ZMW (0<x<10), where ZMW represents a variety of simulated base-glasses. Glass densities increase and glass transition temperatures decrease with increase in caesium oxide concentration. Mass-loss from the melt is found to depend on composition in the same manner as the fraction of silicon Q3 units, resolved from 29Si MAS NMR, and is related to the presence of danburite medium-range order units, resolved from 11B MAS NMR. Volatilization is shown to occur even in the absence of caesium oxide and the mixed alkali borosilicate composition of the volatile species, evolved from the melt at high temperature, is independent of the starting composition of the glass.

Residues and waste streams containing plutonium present unique technical, safety, regulatory, security, and sociopolitical challenges. In the UK these streams range from lightly plutonium contaminated materials (PCM) through to residues resulting directly from Pu processing operations. In addition there are potentially stocks of Pu oxide powders whose future designation may be either a waste or an asset, due to their levels of contamination making their reuse uneconomic, or to changes in nuclear policy. While waste management routes exist for PCM, an immobilisation process is required for streams containing higher levels of Pu. Such a process is being developed by Nexia Solutions and ANSTO to treat and immobilise Pu waste and residues currently stored on the Sellafield site. The characteristics of these Pu waste streams are highly variable. The physical form of the Pu waste ranges from liquids, sludges, powders/granules, to solid components (e.g., test fuels), with the Pu present as an ion in solution, as a salt, metal, oxide or other compound. The chemistry of the Pu waste streams also varies considerably with a variety of impurities present in many waste streams. Furthermore, with fissile isotopes present, criticality is an issue during operations and in the store or repository. Safeguards and security concerns must be assessed and controlled. The process under development, by using a combination of tailored waste form chemistry combined with flexible process technology aims to develop a process line to handle a broad range of Pu waste streams. It aims to be capable of dealing with not only current arisings but those anticipated to arise as a result of future operations or policy changes.

Boron-11 nuclear magnetic resonance (MAS NMR) and Raman spectroscopies have been used to study four families of glasses: xCs(2)O(100-x) ZMW(0 < x < 10), where ZMW represents the borosilicate glass MW to which oxides Z (Al2O3, La2O3 and MgO) are added such that, overall, 1.0 <= R (=[modifier]/[B2O3]) <= 2.5; K (=[SiO2]/[B2O3]) = 3.2. These glasses are related to the system used for the vitrification of high-level nuclear waste. The spectra reveal the presence of reedmergnerite and danburite medium-range order structural units in the glasses. The fraction of danburite units increases with the addition of caesium oxide when Al2O3 or La2O3 are present, and decreases when MgO is present.

A wide range of plutonium containing wastes and residues are currently stored on the Sellafield site. These wastes and residues arising from early fuel development activities at Sellafield are diverse in nature and in the absence of a long-term disposition strategy, ongoing storage represents a major financial commitment. An immobilisation technology is being developed, producing a glass ceramic wasteform that will be suitable for extended periods of storage and ultimate disposal. Hot isostatic pressing [HIP] has been chosen as the preferred consolidation route. Following demonstration of the chemical feasibility of the approach at ANSTO's Lucas Heights facility, the project will demonstrate the technology by designing and building a fullscale inactive facility at Nexia Solutions' Workington site supported by a small scale active facility in the Technology Centre (BTC) at Sellafield. Delivery of this technology will allow the project sponsors, British Nuclear Group, the option of engineering a full scale plant in order to immobilise the actual waste arisings and discharge the liability to site.

An acoustic emission (AE) technique has been employed for nondestructive monitoring of the performance of a laboratory scale cementitious wasteform encapsulating Al. Recorded AE activity revealed the possibility for classification and differentiation of detected AE signals with a potential relationship between the corrosion rate of encapsulated Al and the accumulated structural damage within the cement matrix.

Physical parameters and structural characteristics are reported for the mixed alkali borosilicate glass system, xCs2O(100−x)(MW) (0⩽x⩽9.66) where MW is the BNFL wasteform glass (10.29mol% Li2O, 10.53mol% Na2O, 18.57mol% B2O3, 60.61mol% SiO2). Glass density was found to increase with x whilst the glass transition temperature (Tg) decreased. The fraction of four-coordinated boron (N4) determined from 11B magic angle spinning nuclear magnetic resonance (MAS NMR) increased with x in a manner consistent with previous reports on the sodium–lithium–borosilicate system and contrary to the Dell model. 29Si MAS NMR was used to identify the fraction of Q4 (B) and Q3 units as a function of x. Raman spectra indicated the presence of reedmergnerite and danburite superstructural units. Mass loss measurements showed a non-linear variation with x, having a maximum between 7 and 8mol% Cs2O and a correlation was found between the amount of Q3 units in the solid and the volatilization loss from the melt.

The phenomenon of spin freezing at low temperatures in iron-containing oxide glasses resulting from antiferromagnetic interactions has been previously reported for several oxide glass systems. The temperature dependence of the DC magnetic susceptibility has been measured with a SQUID magnetometer for a series of four iron oxide–phosphorus pentoxide glasses, containing between 30 and 44mol% Fe2O3, and prepared so as to have an Fe3+ fraction of ∼0.8. Well defined cusps are observed in the susceptibilities at temperatures between 5K and 8K for the four samples, cooled in zero field and measured at 0.025T. As for classical metallic spin glass alloys, the susceptibilities measured after cooling from high temperature in this field are almost constant below the cusp temperatures, corresponding to spins freezing into a complicated configuration. Similar measurements at 0.5T also demonstrate the freezing transition but the cusps in the susceptibility after zero-field cooling are broadened. The distribution and environments of iron ions within the samples are discussed in the light of the temperature-dependent magnetic structure factors and spin correlation functions, observed in neutron diffraction experiments on the same samples, along with the nuclear real space total correlation functions.

The mixed alkali modified borosilicate base glass used in the vitrification of high level radioactive waste (HLW) has been mixed with various trivalent oxides to give a range of glass transition temperatures (T-g). The ultrasonic wave velocities in these samples have been measured from room temperature to 20 degreesC above T-g using noncontact, laser based ultrasound. A pulsed Nd: YAG was used to generate ultrasound in the samples and ultrasonic detection was performed using a wide band (>80 MHz) Michelson interferometer. A sharp decrease in velocity is observed at temperatures in the region of the onset of glass transition as indicated by differential thermal analysis and dilatometry This technique should allow the remote determination of the elastic properties of the glass and supercooled liquid at temperatures up to and above the liquidus.