[en] Carbon deposits formed by the pyrolysis of hydrocarbons or disproportionation of CO over transition metal surfaces consist of crystalline graphite and are formed at temperatures appreciably lower than that normally required for the graphitisation of carbon in the absence of a metal. Two reaction mechanisms have been proposed, (i) that carbon dissolves in the metal and because of its high mobility it nucleates at suitable sites and then precipitates with graphitic ordering, and (ii) that a carbide is formed on the metal surface which on thermal decomposition yields graphite. The first mechanism was regarded as particularly applicable to metals with a high solubility for carbon. Further support for the solution/precipitation theory is the observation that amorphous carbon deposited on Ni and Co films and dissolved by raising the metal temperature, can be precipitated as well-oriented graphite films by cooling. Several different deposit morphologies have been reported but the relative importance of physical and chemical contributions to their growth is not understood. The metallurgical structure of the specimen can also influence the reaction, and further aspects will be discussed below. Carbides, oxides and the metal itself have been regarded by various workers as active catalysts for deposition but this problem remains unresolved because of the difficulty of identifying intermediates in high temperature gas-solid reactions. The object of this paper is to present further results on the morphological, metallurgical and catalytic aspects of carbon deposition. (author)

[en] Measurements of sulphur (present as carbonyl sulphide COS) in the Hinkley Point 'B' AGR coolant gas circuits following commissioning, indicated concentrations up to 400 vpb. This was substantially higher than that found in WAGR and Magnox coolants and, in view of the known susceptibility of nickel and chromium to attack by sulphur, the possibility of enhanced corrosion of the high nickel and chromium steels within the reactor circuit was suggested. Based largely on WAGR experience, a maximum tolerable COS concentration of 35 vpb was recommended by corrosion specialists. This paper deals with the work carried out to determine, and remove, the major source of sulphur and the methods adopted to reach the recommended maximum level. In addition the interaction of sulphur and sulphur control with the operation of the catalytic carbon monoxide converter (recombination unit) is discussed. (author)

[en] Graphite is used as a bearing and seal material at gas outlet temperatures in CAGRs under which condition it is subject to thermal corrosion. During thermal oxidation in CO₂-based gas mixtures most graphites exhibit an increase in reactivity. However the relation between structure and changes in thermal reactivity has received little attention, particularly in the nuclear context where the major oxidation reactions are radiolytic. The reactivity of a sample of graphite is a function of the amount and nature of the surface accessible to the gas. The variation in reactivity with weight loss therefore reflects changes in surface area and/or surface nature. The development of surface area is governed by changes in pore structure. It could be expected that the surface nature may be a function of material source and preparation. The purpose of this paper is to describe briefly an investigation of the relationship between reactivity, weight loss and structural changes for two Gilso-carbon based graphites. (author)

[en] Hydrogenous compounds are present in the coolant gas of all CEGB nuclear reactors. They are useful inhibitors of radiation-induced corrosion of the moderator. Hydrogen and methane deactivate the oxidising species in the gas phase, although their contribution to this process at their normal concentrations is largely insignificant in the presence of approximately 1% carbon monoxide which arises as the corrosion product. It is from the formation of protective deposits from methane and from the combination of water and carbon monoxide that the principal benefits are derived. Thus the methane concentration in the coolant of the highly rated CAGRs is presently maintained at 165 vpm with water controlled at 300 vpm (a higher concentration is undesirable because of the possibility of condensation of corrosive solutions in cool parts of the circuit). There are further constraints on the permissible concentrations of hydrogenous compounds. Too much methane can lead to the deposition of carbonaceous material on the heat-transfer surfaces of the fuel elements, whilst excessive hydrogen or water can exacerbate steel corrosion. There is also a lower limit on hydrogen concentration of approximately 25 vpm because it inhibits radiolytic polymerisation of carbon monoxide which can otherwise give deposits on the cooler fuel elements in Magnox reactors. Details of these various processes have been published; this paper is concerned only with the interactions between hydrogenous compounds. (author)

[en] Prior to start up of the Hunterston AGRs, concern was expressed, following laboratory tests, at the risk of breakaway oxidation in the 9% Cr steel used in the evaporator section of the boilers. To reduce the risk, it was recommended that the carburising potential of the (CO₂) coolant during the critical stage of initial operation should be minimised. A coolant of 0.5% CO, 300 v.p.m. moisture, naturally occurring methane was, therefore, specified. Later changes in the coolant composition are discussed under the headings: target coolants; gas composition control; operating experience; monitoring (gas composition; graphite moderator and sleeves; fuel pin deposition; instrumented fuel stringers). (U.K.)

[en] AGR coolant (CO₂ with controlled minor quantities of CO, CH₄ and H₂O) is chemically quite stable at reactor temperatures until it is irradiated. Of the many reactions which then occur only a few are likely to be important in deposition. The first step in analysing this is to group the reactions into chemical 'streams' originating in the destruction of a coolant component. Two streams involve intermediates known to be capable of forming carbonaceous deposit. The first is the oxidation of methane to CO and H₂O via a number of hydrocarbon and oxygenated organic compounds. Although the proportion of CH₄ in AGR coolant is < 0.05%, it is rapidly destroyed because it reacts with products of CO₂ radiolysis. Diversion of even a small fraction of this stream therefore produces significant deposition. The second stream is the direct radiolysis of carbon monoxide to give carbon suboxide (C₃O₂), which in turn can polymerise and degrade to deposit on hot surfaces. This stream is much smaller than the first because the radiation energy must be dissipated directly in the minor component; moreover C₃O₂-derived deposit is normally readily oxidised (though it does become less reactive as its formation temperature increases). Nevertheless, in principle sufficient CO is radiolysed to produce significant deposition in an AGR, and this stream cannot therefore be ignored. (author)

[en] The Advanced Gas Cooled Reactor (AGR) was introduced for the second generation of British nuclear power stations. It was recognised that problems of compatibility between the carbon dioxide coolant and the moderator graphite would arise because of the increased power rating of the reactor compared with the first generation MAGNOX system. This led to the realisation that it would be necessary to reduce the rate of oxidation of the moderator to acceptable levels by the addition of inhibitors to the coolant and to this end carbon monoxide and methane were chosen. This paper describes experiments which have been made in a materials testing reactor at AERE Harwell in which moderator graphite reaction rates have been measured in carbon dioxide containing carbon monoxide at concentrations between 0.03% and 2% and methane concentrations up to 600 vpm. The effect of impressing a flow of coolant through the graphite structure, the so-called ventilation effect, and the role of coolant temperature and pressure have also been assessed. The results confirm the inhibiting power of methane and carbon monoxide on the graphite/CO₂ reaction and demonstrate that the application of ventilation in the presence of these inhibitors enhances their effect. A minimum or 'terminal' oxidation rate may be achieved by the CAGR Gilso carbon graphites when irradiated in the presence of 200 vpm methane, or more, under appropriate conditions. (author)

[en] Measurements of the rate of CH₄ destruction in Hinkley Point 'B' AGR at different coolant compositions have indicated a higher CO concentration dependence than would have been expected from calculations based on previously used empirical expressions. The present results suggest much higher methane destruction rates at low CO concentrations. This has important implications on the sizing of the coolant control plant for future CAGRs and further tests may therefore be required to confirm the observed trend. If the comparatively simple expression for the CH₄ destruction rate as a function of coolant composition derived in this paper is confirmed, it should be possible to derive reasonable values for full power CH₄ destruction rates from measured data at lower powers, by a nuclear flux correction alone. (author)

[en] In graphite moderated, carbon dioxide cooled, reactors, radiolytic graphite oxidation arises from the production of short lived energetic species and is confined to the internal porous structure. Exponential weight loss is possible as the volume of internal porosity, absorbing radiation, increases with time. Inhibitors are added to the coolant to minimise weight and hence strength loss. Carbon monoxide and methane are the principal gas phase inhibitors, competing with the graphite for the oxidising species. Methane has the further effect of producing a sacrificial layer at the pore wall. It follows from the mechanisms of inhibition that oxidation is reduced in the larger pores. In the small pores, the probability is high that oxidising species will reach the pore wall and exponential weight loss can occur until such time as the pores become sufficiently large for the inhibitors to operate. The results from a high weight loss experiment confirm this behaviour and allow predictions to be made with some confidence for other coolants - for which initial oxidation rate data are available. In this paper the results from an earlier weight loss experiment in an 'uninhibited', nominally pure, carbon dioxide coolant are assessed. Particular attention is drawn to the information which can be obtained from a study of transport properties as they develop with graphite weight loss. The objectives in understanding more exactly the process occurring inside the complex pore structure are to allow extensions in planned life, or greater flexibility in coolant plant operation and compatibility with fuel clad. (author)

[en] The use of methane addition to the CO₂ coolant of CAGRs, as a sacrificial inhibitor of graphite corrosion, and the consequential need for a drier plant, is discussed. Some details are given under the headings: CAGR drier plant; requirements of the desiccant; mechanism controlling the rate of adsorption; selection of a mathematical model; and results (comparison of theoretical and experimental data; prediction of CAGR drier bed behaviour). (U.K.)