PROPUESTA DE INTERVENCIÓN

6.1. OVERVIEW

Issues relating to temporary storage and packaging of i-graphite were discussed in Reference [2]: this present review is concerned with immobilization of radioisotopes and packaging matrices for permanent disposal to a repository. Packaging requirements are the responsibility of individual radwaste authorities, and the subject has been raised already in the context of waste acceptance criteria (Section 2). The generic decisions applicable to all will concern container size and weight, package surface dose rate, heat output and consideration of accident scenarios in handling, transport and disposal. In addition, the specific properties of the graphite need to be considered, which will include gas evolution (chemical form and radioisotopic nature) and the potential for leaching of radioisotopes in the event of package penetration through corrosion or major earth movements.

The major surface radwaste facilities in the Aube district ('CSA', France) and El Cabril (Spain) involve immobilization of the waste by cementation. This is also the solution studied in a number of other Member States.

Investigations of suitable immobilization matrix material for employment in drums and boxes have been conducted for a number of years, principally by CEA in France and BNFL in the UK. Currently an alkaline cementitious grout based upon Portland cement has found favour for use with graphite: a recent IAEA CRP on the topic has pooled the experience of a number of additional Member States [157]. Previous investigations [158, 159] assessed not only these varied cement mixes but also polymer-modified cement, polymers, resin sand, glass, low melting-point metal, ceramics and bitumen, this last being the subject of extensive investigation at CEA before finally being abandoned [160, 161]. Further work continued in the UK which identified a mix of three parts ‘blast-furnace slag’ to one part Portland cement as the preferred matrix material for graphite from UK AGRs [162].

The present project has focussed on alternative immobilization procedures either for general utilization or for addressing specific problem i-graphite. Six of the contributing projects come into this category and are described below: full reports for each project are available in the attached CD ROM.

6.2. PACKAGING SOLUTIONS FOR GRAPHITE WASTE

The CRP researches have currently or previously considered following conditioning and immobilization routes for i-graphite waste:

• Grouting of i-graphite blocks (all CRP participants);

• Epoxy resin impregnation of i-graphite blocks to decrease its porosity (Russia);

• Crushing of i-graphite followed by mixing with cement grout (Switzerland);

• Use of geopolymers for i-graphite encapsulation (Germany);

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• Self-propagating high temperature synthesis for fuel-contaminated i-graphite (SHS, Russia);

• Glass-graphite composite materials obtained both via sintering and melting routes (Germany and UK).

Cementation remains the favoured technical option considered everywhere at present.

At this point, we focus upon the recent investigations. Reference has already been made (Section 5.3.3) to the transformation of graphite into alternative chemical forms (e.g.

carbonates and more complex salts) and their incorporation in to a impermeable vitreous medium, especially useful where the graphite is highly contaminated with fuel debris. A significant disadvantage with this type of process, as usually described, is the overall increase in volume of the waste form, with its attendant costs in regard to GDF disposal.

6.2.1. Vitreous immobilization

FNAG has developed a process for the production of a graphite/glass composite material which, when used for i-graphite, exhibits no overall volume increase. Essentially the graphite is crushed and mixed with an amount of glass which is equal in volume to the combined open and closed porosities of the original graphite, followed by hot pressing and compression under vacuum. The product has negligible porosity and is largely impermeable to water, thereby having the advantage of ‘fixing’ the radioisotopic content.

The technique therefore allows disposal of i-graphite with package densities greater than 1.5 tonne.m^-3. It may also be utilised as the embedding material for other forms of radwaste, thereby creating a larger volume saving overall compared with the use of standard cementitious matrix.

A pilot plant has been installed and the initial products, using a range of glass compositions, subjected to careful analysis to determine the ideal manufacturing procedures. This detail is provided in the full report which is annexed to this TECDOC.

The University of Sheffield (UK) has focussed on the production of base glasses of differing compositions, which are subsequently sintered with powdered graphite or simulant TRISO particles. A microwave forming technology has also been investigated. It was found that the products with TRISO particles were generally more successful since wasteforms containing larger amounts of graphite were resistant to densification and the porosity of the graphite was, in general, poorly penetrated. Up to now, these materials have been investigated at atmospheric pressure: it is planned to use higher pressures in order to move the work forward.

An annexed report provides further information.

Development of the flameless molten-salt oxidation process has been the principal contribution to this CRP from RADON. This follows on from earlier work conducted in association with The University of Sheffield (UK) on direct thermal vitrification processes [149, 163]. These projects have already been discussed under the ‘Processing’ heading (Section 5.3.3).

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6.2.2. Mineral/cementation processing

The Institute of Environmental Geochemistry in Kiev has conducted new research into cementitious matrices. This work has been conducted after consideration of ASTM standard methodologies to determine basic properties and hydraulic data of candidate grouts. Portland cement is the basic material, with the focus of this work on the water/cement ratio and its influence on the resulting porosity of the grout.

In addition work has been conducted on the incorporation of crumbled graphite and clay into the mix in various proportions up to 30% graphite. The longer term intention is to investigate the leaching of important isotopes from these experimental grouts: so far, leaching of ¹⁴C has proven to be below the limit of detection, so no representative data are presently available.

Whilst the main thrust of the PSI contribution to this CRP has been the determination of ³⁶Cl in the Swiss graphite waste, the underlying process which was adopted for the i-graphite from the reactor DIORIT with the nuclide inventory well above the exemption limits was to incorporate the material directly into a cement matrix for disposal [43]. Such a technique might be considered for i-graphite which is LLW and where other radwaste immobilization with cement-based materials is being carried out: this again minimises the volume occupied by the graphite within the disposal containers.

A different approach has been to look at the potential of geopolymers, which are solid aluminosilicate materials usually formed by alkali hydroxide or alkali silicate activation of solid precursors such as coal fly ash, calcined clay, and metallurgical slag. Current investigations in the field of radwaste immobilization are the utilization of such materials as an alternative to Portland cement. In respect of immobilization of i-graphite, they are fire, freeze/thaw and acid resistant, demonstrate a low leach rate for isotopes of concern and have a high initial strength with low shrinkage: their postulated use in the building of ancient pyramids suggests an appropriate level of stability for radwaste. The current investigations within FZJ have included direct mixing of graphite with geopolymers (with and without sand for mechanical stability) [164], production of cement-graphite granulates as intermediate products and embedding these in geopolymer, and the coating of intact graphite components with geopolymer.

Full reports on each of these studies are annexed to this TECDOC.

It should also be noted that consideration has been given to immobilization of isotopes within graphite wastes through conversion to silicon carbide [165]. The so-called mechano-chemical process, developed at The University of Sheffield in the UK after original work elsewhere [166], involves prolonged milling of silicon and graphite powders followed by a heat treatment at up to 900°C. Silicon carbide is an extremely inert material which could itself be appropriate for a matrix material in immobilizing other wastes: however, further research is required in this area

95 6.3. IMMOBILIZATION SUMMARY

The work described under the ‘immobilization’ heading falls into two categories:

incorporation of the graphite into cementitious matrix material, and incorporation into a glass.

The second of these processes may be further subdivided; formation of a glass or vitreous material in which the graphite chemical form is changed to a form such as carbonate, and incorporation into an existing glass, for which a number of potential processes are available.

All of these variants are potential embedding materials for other forms of radioactive waste.

The commencement of leaching studies from embedded graphite in cementitious grouts in Ukraine may enable an increase in confidence in predictions of long-term release rates from repositories,

This work has widened the scope of investigations of vitrification which, hitherto, had been considered only in the context of treated highly contaminated graphite in order to prevent the leaching of isotopes from fuel debris.

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