CONCLUSIONES Y RECOMENDACIONES 1 Conclusiones

87 FZJ has continued working in this area as part of the CARBOWASTE project [147], along with the development of leaching studies according to a number of standards (ANSI 16.1 [USA], NEN 7345 [Netherlands] and the standard ‘semi-dynamic’ methodology as originally proposed to the IAEA [148]). It is the latter work which forms the focus of the present CRP report, concentrating on ¹⁴C and ³H.

The key objectives of the current Manchester University work within this CRP are also to provide analysis on the long-term behaviour and stability assessments of i-graphite.

Leach rates have been determined under representative conditions (the groundwater at LLWR, the UK disposal facility for LLW), along with isotope diffusion coefficients and mechanisms within the graphite. It is shown that the diffusion process for ¹⁴C has two stages:

a surface ‘wash-off’ followed by a slow diffusion-controlled process: in the case of tritium, only the surface wash-off applies.

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FIG. 26.Outline of the THOR process as adapted for i-Graphite.

The Russian molten-salt oxidation process is a development of an earlier process for converting highly contaminated graphites into a more stable vitreous form which has been under investigation for a number of years, both in RADON in Moscow and at The University of Sheffield in the UK. The former process is based on the highly exothermic reaction of graphite with aluminium and titanium dioxide, for which comprehensive studies of the thermodynamics have already been made [149, 150] and has been shown to be moderately successful, although a significant increase in the volume of the material results.

Molten-salt oxidation offers a much more controllable process in which inorganic oxidizing salts are held at high temperature in the liquid phase, under which the graphite again oxidizes to give a product which is vitreous at normal temperatures. A pilot-scale facility is now operating at SIA RADON with a throughput of about one kilogram of graphite per hour. As a result of initial trials, detailed modifications have been made to the process, such as heating the oxidizing gases ahead of the process to improve efficiency of reaction.

Gasifier Reformer (GGR)

Oxidiser (PGO)

Condenser (GGC) Solidification

Moisture Absorber Filter/HEPA Graphite

Boiler Oxygen

Water

Liquefy CO2

Offgas

ILW

S

G

G

L,S G

G

L

G

G Additives

S

Transport L

CO2 to storage G S,L

89 A number of different melts have been tested: generally these are based upon the carbonates of the alkali metals. In some instances, an additional ‘oxidizer’ compound such as sodium sulphate or barium chromate is included in the salt melt.

It is planned to develop this methodology further such that the process might be used for decontamination of i-graphite (rather than entire consumption in the process), in comparison with decontamination procedures utilizing air or water vapour. This latter idea stems from the observation that, in graphite blocks where contamination has occurred in NPPs, the depth of surface contamination by actinides and fission products rarely exceeds two millimetres.

The accompanying annexed report includes extensive numerical data on decontamination efficiencies.

5.3.4. Biological treatments

A final category of ‘processing’ which should be considered here is the deliberate use of microbial agents to foster changes in the graphite (potentially the release, and subsequent capture, of specific radioisotopes). When ‘care and maintenance’ regimes were first considered for the retention of i-graphite within its original irradiation environment, potential issues arising from microbial action were essentially dismissed [151]. Subsequently, consideration has been increasingly given to the potential for such action to release radioactive material (principally ¹⁴C), especially where water ingress is possible to allow the creation of an environment more amenable to the colonization and growth of microbial species. As an example, concern has been expressed by the UK regulators following ingress of rainwater into the core of the Magnox reactors at Trawsfynydd. Finally, the deliberate use of microbial ‘cocktails’ to process graphite has been investigated, led by Necsa and the PBMR Co in South Africa in combination with The University of the Free State (RSA).

Microbial species which are adaptable to the chemical and radiation environment must be identified, and then systematically investigated to identify those in which ¹⁴C (again regarded as the critical isotope in this context) becomes incorporated into the bacterial cell structure as opposed to being released in a gaseous or water-soluble form – thus facilitating removal of the ¹⁴C from the system. Further work on this aspect has been included in the CARBOWASTE project (to be published by the EU in due course) and also resides as intellectual property of the residual PBMR Co following the cancellation of the modular pebble-bed reactor system in South Africa.

There is renewed interest in this phenomenon following the discovery at the Ukraine Institute of Environmental Geochemistry that graphite exposed to the extremely intense radiation of the Chernobyl accident subsequently became rich in organic sugar groupings, attracting fungal spores which attacked the sugar groupings and effectively rendered the graphite soluble, with the release of all contained radioisotopes into either the solution or the ambient atmosphere. Some 2000 strains of 200 species of 98 genera of fungi have been isolated from the region surrounding ChNPP, and many of these have been found associated with ‘hot particles’ of graphite, into which they are growing with consequent decomposition of the graphite. It has been suggested that the fungi may direct their growth towards the sources of

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radioactivity. Clearly such issues are worthy of consideration in respect of repository behaviour of graphite, as well as a potential innovative ‘processing’ tool.

As a consequence of those biological aspects, Spain has initiated similar tests using melanic fungi that are able to live in a harsh UV environment (Atacama Desert in Chile and Deception Isle in the Antarctica). The first tests were performed on virgin graphite, as the only source of carbon, with the aim to determine whether or not some fungi strains grow in a sterilized nitrogen broth along with virgin graphite. Several fungi strains that presented a positive growth were selected for further active graphite tests (Acremonium sp, Gliomastix sp). Ongoing tests on i-graphite show positive results of growing for some of the selected fungi strain, taking into account both the mass and activity balances for i-graphite, fungus and broth. In a parallel way to these ongoing activities, new fungi strain and broths are being investigated in order to have greater variety of possibilities for further developments.

5.3.5 Reverse engineering: graphite as an absorber of radioactivity

To complete the 'processing' picture, it should not be overlooked that graphite is a potential absorber of other radioactive isotopes and could in principle be deployed in this fashion as part of an integrated waste management plan. Although the utilization of carbon in the form of 'activated' amorphous material as an absorbent has been understood for many years, its first usage in the context of the absorption of radioactive material other than in filtration systems appears to have arisen following the Chernobyl accident in 1986. Urgent remedial measures were put in place to inhibit the spread of radioactive material in groundwater and rivers. Entire train loads of clays, sand, granulated carbons and even coal were brought to Kiev where scientists from what would become the Institute for Sorption and Problems of Endoecology devised mixtures which could be placed in sunken barges across the River Pripyat to minimise the passage of radioisotopes into the main Dneipro River.

Few details of this work have been published, but some work on the specific use of carbons has been discussed at International Carbon Conferences. The eight different absorbants used in the Chernobyl recovery programme are not specifically identified in a 1996 publication [152] but it was noted that different combinations of absorbants could be more effective than the sum of the individual constituents. Close to 100% take-up of ⁹⁵Zr, ¹⁰⁶Ru, ¹³⁴Cs, ¹³⁷Cs,

140Ba and ¹⁴⁴Ce was claimed using 5 gram absorber in 200 cm³ water circulating at 100 cm³/h. It had previously been shown that the use of carbons doped with materials such as ferrocyanides would increase the take-up of certain radionuclides thousands of times [153], whilst treatment with simple salts such as sodium carbonate or zinc sulphate also increased the take-up efficiency. Investigations in this area have also explored electro-adsorption [154].

Such lines of work raise the possibility of the use of granulated i-graphite as an overpack or filler material for repository wastes, considerably inhibiting the eventual leaching of radioisotopes into groundwater: however this area requires further research.