Evaluation of the potential for carbonaceous dust explosibility in the decommissioning of the Windscale piles

Full text of publication follows: Concerns have been expressed by nuclear regulatory bodies, and others, about the possibility of initiating an dust explosion during the decommissioning activities in graphite-moderated reactors. The graphite and carbonaceous dusts which may be present inside the reactor vessel are regarded as a potential explosion risk, particularly in the situation where cutting activities such as the flame-cutting of adjacent steel components is taking place. In the Windscale Piles, which were air-cooled, the only carbonaceous dust which should be present is that arising from nuclear-grade graphite, and high-purity graphite has historically been classified as 'non-explosible'. However, more recent work, and particularly the application of modern standards to the classification of dusts, has somewhat changed the perspective. The standard ISO test uses a chemical igniter of 10 kJ in a spherical vessel of approximately 1 m3, and the chemical energy added is sufficient to result in a weak explosion in nuclear graphite when the dust concentration and particle size range are favourable. It is more than ever desirable therefore to understand in more details the characteristics of graphite dust likely to be encountered in the Windscale Piles, and in particular the effects of possible impurities and potential catalysts which might be present particularly in Pile 1 which was the scene of the core fire in 1957. In addition, it was considered prudent to investigate the propagation of pressure waves and the possibility of secondary explosions using the 'connected-enclosure' facilities at The University of Leeds which can be configured to represent approximately the relative volumes of parts of the Windscale Piles gas circuits. This work utilised Pile Grade 'A' graphite characteristic of Calder Hall reactors, the nearest currently available equivalent to that used in the Piles. The work has revealed an unexpected influence of particle size, with only the very finest particles (≤∼ 5μm) participating actively in combustion whilst larger particles behave as heat sinks and thus serve to suppress flame propagation. This results in a rather low reactivity of typical dust samples to the extent that a methane 'driver' has been utilised to produce the explosions necessary to study connectivity and evaluate more detailed kinetics. One impurity likely to present in dusts in Pile 1 is lead and its oxides, since lead was used as a spacer material in certain cartridge irradiations. Whilst lead is a powerful catalyst of graphite oxidation in the Mode 1 ('kinetics controlled') regime, it appears to have little catalytic effect in the present studies suggesting that diffusion-controlled or mass-transfer-controlled oxidation regimes are operating, which is physically reasonable under the conditions of a suspended dust cloud. In common with other studies conducted in support of European decommissioning of graphite reactors, other impurities have an inerting effect. Italian studies have shown that these inerters include the general circuit dusts collected during the operation of Magnox reactors (principally metal oxides from component corrosion). The concrete dusts drawn into the Windscale Piles through the concrete air ducts are likely to have a similar effect. Comparison of the dust-explosion results with the recently re-assessed bulk-graphite reactivity to air confirm that there is an extremely low probability of any hazard from the explosibility of impure carbonaceous dusts arising during the decommissioning of the Windscale Piles. If a risk is still suspected steps can be taken in process design to ensure that the conditions which must be simultaneously achieved to bring about a dust explosion are avoided. References: 1)P. Field, 'Dust Explosions', Vol. 4 of 'Handbook of Powder Technology', ed. J.C. Williams and T. Allen, Elsevier, 1982 2)BS 6713-1 (1986), EN 26184-1 (1991), ISO 6184-1 (1985), 'Explosion Protection Systems - Part 1: Method for Determination of Explosion Indices of Combustible Dusts in Air', British Standards Institution ISBN 0 580 15204 9 3)L. Arnold, 'Windscale 1957: Anatomy of a Nuclear Accident', Macmillan 1992