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Feasibility study to develop densification processing of a ceramic matrix composite material for nuclear waste containment

FIBERSTONE is a (commercially-available) metallic fibre-reinforced ceramic composite (MFC). It has high toughness, relatively low density and an economically attractive cost. It is "cast" by infiltrating a ceramic slurry into an assembly of fibres within a mould. The ceramic matrix sets via chemical reactions at ambient temperatures. The casting process is highly versatile and complex shapes of various size can be created very cost-effectively. Features such as mounting screw threads, sensors and additional steelwork can be incorporated into components, which is helpful for integrating into larger assemblies, increasing load-bearing capacity and monitoring performance in service. Because of these attributes, Fiberstone is already deployed in industrial environments that are aggressive (thermally, mechanically and chemically), but there is scope for penetration into the civil nuclear market. In particular, it has potential for the containment of (low level) nuclear waste. In its standard form, its permeability to aqueous liquids is probably too high, but a study will be made of the potential for reducing this dramatically, using low cost modifications to the production procedures.

The low magnification micrographs of Fiberstone produced under different conditions are shown in Fig 1.

Optical micrograph of the free surface of a Fiberstone sample produced at normal condition.      Optical micrograph of the free surface of a Fiberstone sample produced at 10MPa    

Fig.1 Optical micrograph of the free surface of a Fiberstone sample produced at (a) normal condtion, and (b) 10MPa.

The porosity level is currently being measured by the water infiltration technique. Firstly, sample weights were monitored while submersed in water. As water penetration into the sample occurs, the apparent sample weight will rise as the buoyancy effect of the entrapped air is lost. The porosity being monitored in this way is, of course, surface-connected. After a period of immersion, the weight tends to stabilise as equilibrium is attained. Preliminary investigation shows that the “accessible” (surface-connected and penetrable) porosity level is about 20% for the conventionally-produced Fiberstone, but only about 7-8% for the pressurised sample. Data are shown in Fig.2 for sample weighing in water. This does at least confirm that the pressurisation is having the type of effect that was anticipated. 

Fiberstone water infiltration data

Fig.2 Water penetration data for Fiberstone composites produced with and without an applied pressure of 2 MPa.

There is limited information in the open literature concerning production of Fiberstone, or indeed about its microstructure or properties, although there has, of course, been fairly extensive study of MFCs as a class of material. However, two recent publications from the Gordon Laboratory [1, 2] do give some details about optimisation of the toughness of Fiberstone and about its performance after prolonged exposure to high temperature.

The project will lead to improved understanding of the factors dictating whether MFC material of the type manufactured by Fiberstone is potentially suitable for this important application. This will benefit both the firm and the authorities responsible for designing and commissioning nuclear waste containment vessels. 

The work will be of interest to a wide range of people in the academic community, concerning the fluid permeation through complex (metal fibre reinforced ceramic) composite materials. In particular, the use of X-ray tomography data to construct a mesh for use in a numerical model for simulation of fluid flow through such a complex structure is novel and likely to be of widespread interest.

 

1. Pemberton, S.R., E.K. Oberg, J. Dean, D. Tsarouchas, A.E. Markaki, L. Marston, and T.W. Clyne, The fracture energy of metal fibrereinforced ceramic composites (MFCs). Composites Science and Technology, 2011. 71(3): p. 266-275.

2. Lam, S.K. and T.W. Clyne, Toughness of metal fibre/ceramic matrix composites (MFCs) after severe heat treatments. Mat. Sci. & Techn., 2014. 30(10): p. 1135-1141.

 

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