Reactions Between Sodium and Various Carbon Bearing
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REACTIONS BETWEEN SODIUM AND VARIOUS A number of investigations have found that lamellar compounds are formed CARBON BEARING COMPOUNDS by the reaction of the heavy alkali metals, potassium, rubidium and caesium 13 with graphite, but there have been doubts about whether lithium and sodium can do so. Dzurus et al(3) on the basis of calculated energies of formation A.C. RAINE, A.W. THORLEY have indicated that neither of these elements should form concentrated lamellar compounds with graphite. Similar considerations for dilute UKAEA, Risley, Warrington, Cheshire, lamellar compounds show that a compound of concentration stage 5 or 6 United Kingdom should be stable. (The concentration stage is defined as the ratio of carbon layers to reactant layers.) Asher and Wilson (4) have prepared a 1. INTRODUCTION compound containing one sodium atom in approximately sixty-four carbon atoms, which X-ray analysis indicates is an eighth stage compound. The presence of carbon bearing materials in liquid sodium is undesirable Dzurus et al(3)were unable to prepare this compound by reaction between because of their ability to carburise stainless steel components. It ultra pure sodium and graphite, and were only able to make a similar has been demonstrated for example that carbon taken up by stainless (but not identical) compound in the presence of contaminants such as water, steels can affect their mechanical properties and that thinner sectioned hydrogen or oxygen. It is not clear whether these impurities act merely as material such as fuel cladding and the tubing of intermediate heat catalysts, or actually enter into ternary compound formations. exchanger may be more sensitive to such effects. (SEE REP 1) Asher has reached the following conclusions on the compatibility of Generally speaking, there are a number of potential carbon sources in sodium and graphite. Initially where sodium is brought into contact with reactor systems. Some of the sources such as the graphite in neutron graphite between 200°C and 300°C, the receding contact angle is greater than shield rods, boron carbide in control rods and carbide fuels are part of 90°. The sodium and graphite (and oxygen?) react when in contact, with a the reactor designs while others such as oil in mechanical pumps and speed depending on the temperature, and there is a slow diffusion of •coupling-fluids' used to inspect plant components are associated with the the sodium along with interlayer gaps into the graphite lattice. The respective operation and inspection of the plant. < interaction will result in a local decrease in contact angle leading to progressive penetration of the pores, and this brings about chemical In this paper it is intended to discuss in general terms the way these interaction within the body of the material. This interaction is various compounds behave in liquid sodium and to assess what effect their accompanied by dilation, and since the penetration is non uniform, it gives presence will have on the materials of construction in fast reactor rise to stress and possible disruption. systems. The paper also reviews the chemistry of the environment in relation to the types of carburising species which may exist in sodium Ad hoc experiments have been carried out at RNL in order to try and get a systems. rough idea of what factors are involved in the behaviour of carbon in sodium loops. In the first experiment, spectrographically pure graphite rods were exposed to sodium at 600 C in a stainless steel dispenser (5 U pore 2. CftRBON IN AS-RECEIVED SODIUM - SODIUM MANUFACTURE size) for three days. Sampling of the sodium after this period indicated no increment of carbon level in the sodium which remained at about 1 ppm. Commercial sodium is manufactured by the Down's process, in which fused sodium chloride is electrolysed, using graphite electrodes. As a result Direct additions of graphite particles and of carbon black (particle size of erosion and to a lesser extent dissolution of the graphite, carbon 0.05 microns) have also been made on to the comparatively static sodium contamination of the sodium is inevitable. Figures of 30ppm carbon in surface in the specimen well of the loops. The carbon black disappeared as-received sodium are quoted in the Liquid Metals Handbook. The experi- quite rapidly at 400 c, while the graphite particles remained on the ence at RNL suggests that settlement in a dump tank followed by filtration surface of the liquid metal for 5 hours at this temperature without disap- through a stainless steel sinter, with a pore size of 5-10 ym, will reduce pearing. The temperature was then raised to 6O0°C for a period of 24 the level to 1-2 ppm carbon in loop sodium. Providing that this procedure hours after which all the graphite had disappeared. Is followed .in PFR and contamination from other carbon sources in the reactor is avoided the resulting conditions should be satisfactory. Subsequent analysis of the sodium showed a level of about 30 ppm carbon in the carbon black case and about 2 ppm in the graphite case. After a further 24 hours the carbon levels, by analysis, were VLppm. o 3. REACTIONS BETWEEN SODIUM AND GRAPHITE In general, it has been found that raising the cold trap temperature ro produces a burst of carbon in the circuit, but that the level falls to o According to Guernsey and Sherman writing in 1925(2) sodium vapour will the 1 ppm level in a matter of hours. o react with carbon at 80O-90O°C yielding a carbide Na C . If this compound CD exists it would presumably be an ionic carbide similar to calcium carbide This suggests that we are not dealing primarily with a temperature dependent 6 but no further substantiating evidence is available. solubility process, but that oxide in the cold trap is acting as a mechanical trap for particles of carbon less than 5 microns in diameter, 42 hours. If it is assumed that the cold trap is 30% efficient, the minimum and that when the temperature is raised and oxide dissolves, carbon is clean-up time is of the order of 140 hours. 14 released into the circuit. Earlier studies have shown that oil in sodium is a powerful carburising agent. Subsequent tests (SEE TABLE 1) to assess the thermal stability of oil have SOLUBILITY OP CARBON IN SODIUM 4. shown that a greater degree of decomposition occurred when oil was heated in air, compared with heating it in argon. The addition of sodium caused Earlier attempts at measuring the solubility of carbon in sodium as a decomposition at lower temperature and accelrated the rate of reaction. function of temperature were made by Gratton'6) He claims to have The products were initially, a black coke-like residue (Fig 2), tar and measured the solubility of carbon in the temperature range 147 c to gasses, but at higher temperatures or longer times, the tar disappeared. 7oO°C, at two fixed oxygen levels of 0.004% and 0.026%. Analysis of the solid product, by X-ray and chemical analysis, showed it to be mainly sodium hydride with some free carbon. Analysis of the off- This was determined by introducing a stick of spectroscopically pure gases showed them to be chiefly hydrogen with 6% methane and also several carbon into a sodium filled vessel, sampling the sodium at various tempera- other hydrocarbons (Table 2); which agrees closely with the findings of ture levels through a 5y filter, and then analysing for carbon by the method of Pepkovitz and Porter"' . Hobdell at BNL^ ' . carburisation tests carried out on type 321, stainless steel foils by heating them in contact with clean sodium and the reaction products from other oil-sodium tests, showed that the foils contained Very little credence can be given to the work for at least two reasons: 2.0% carbon after 21 days at 65O°C and 1.1% carbon after 28 days at 400°C first, the explicit assumption was made that all carbon which passed compared with a starting level of <0.1%. through a 5y filter was soluble. Second, as the sodium was filtered at difference temperatures, the oxygen content of the sodium could not have been constant as suggested. In order to establish how much carbon can be taken up by the surface of stain- less steels when oil leakages occur in the reactor system; tests have It does however, appear from Gratton's work that the quantity of carbon been mounted where type 316 stainless steel foils (50 ym thick) and M316 tubing ( <\, 500 pm thick) , have been immersed in sodium-oil mixtures over the tempera- passing through a $\l filter, whether it is in solution or not, increases ture range 4OO to 650 C for various periods of time. with increasing temperature. In recent years, three determinations of carbon solubility, plus an The results from the experiments are illustrated in Figs 3 and 4. Fig 3 estimate derived from low temperature measurements using Na2C2 and thermo- shows the amount of carbon picked up by the foil materials, as a function of chemical data for Na2C2 , have been carried out. While differing in the time, at various temperatures, while Fig 4 illustrates the effect of these absolute value of carbon solubility these determinations agree well in experimental parameters upon the depth of penetration of carbon into thicker sectioned material. The depths of penetration were obtained for us, by the derived heat of solutions, with values of 105, 105, 114 and 110 kJ.g- 13 atom~l for the data of Longson and Thorley, Gehri, Ainsley et al and AERE using the nuclear microprobe analyser.