Impurities in and serviceability of reactor graphite

Abstract

The high purity of nuclear graphite is ensured mainly by choosing pure initial raw materials --coke and pitch. The impurity content is sharply lowered by high-temperature processing --graphitization. It was shown in the early works that as the gravitation temperature increases, the content of both individual impurity elements and the total ash content drop sharply, especially when combined with gaseous purification.Reactor graphite of the I~GP type for the Bilibin nuclear power plant is purified by thermal diffusion of impuriies in the process of high-temperature graphitization without gaseous refinement. For GR-280 RBMK graphite gaseous purification in the process of graphitization at temperatures up to 2800 K is used. At the same time, to obtain matrix graphite for the beaded fuel elements of high-temperature gas-cooled reactors chemical purification alone is inadequate because its maximum temperature is limited to 2100 K, above which fuel microelements rupture and the uranium contained in them becomes contaminated. Therefore, here, the main direction is high purity of raw materials. This path is also more economical than high-temperature refinement.The present paper summarizes the substantial published information about the content of impurities in reactor graphite and their influence on its serviceability.The behavior of constructional graphite during operation is determined by its structure. However, inorganic impurities, even in quantities less than 0.1%, can strongly influence the operational characteristics of graphite. Impurities accelerate corrosion (iron, nickel, aluminum, and others), retard oxidation (boron, silicon), decrease the rate of sputtering and radiationstimulated sublimation (boron), change the rate of radiation-induced change in dimensions (boron), capture neutrons (boron, gadolinium, samarium, cadmium), lead to the formation of artificial radioactive isotopes (lithium, silicon, aluminum, and others), and accelerate mass transfer of carbon. Impurities also include adsorbed gases: H z, CO, Co-z, N 2, H20. Their effect is analyzed in detail [11, and for this reason it is not studied here.To determine the content of ceramic impurities present in low concentration, graphite samples are first burned in air at 1200~ Next, in some cases chemical methods are used to separate one or another group of elements. Chemical, spectral, x-ray spectral, mass-spectrometric, neutron-activation, and atomic-adsorption methods are used for further analysis (Table 1).Corrosion. As is well known, some metallic impurities increase the oxidizability of carbon materials in corrosive media. Mass loss in turn increases open porosity, which decreases strength and thermal conductivity and increases the gaspermeability of graphite. Figure 1 shows a direct proportionality between the oxidation rate at 1000~ of MPG-6 matrix graphite and both the ash content of the graphite and the iron content with constant ash content [4].The effect of the impurity content on the corrosion rate in carbon dioxide at 900-1100~ of dense graph...

Strength of Structural Carbon Materials

“…1, the strength is roughly the same for petroleum and pitch cokes and is lower for schistose coke. For analogous foreign graphites based on highly anisotropic cokes [4], the slope of the lines in Fig. 2 is more gradual than that in Fig.…”

“…** Additional irradiation (with no oxidation) of core samples taken from the reactor wall. *** Evaluated from compressive strength (1,2,9) and dynamic modulus of elasticity (3)(4)(5)(6)(7)(8) data.…”

Modelling of the spatial distribution of the induced activities in the RBMK-1500 reactor graphite blocks and rings/sleeves

“…The chemical compositions and concentrations of the main elements and the impurities of these reactor components were defined based on the data presented in the available literature for the specific material (GR-280 and GRP-2-125 grade graphites), see Table 1 [ Ancius et al, 2005, Bylkin et al, 2001, 2004, Bushuev et al, 2002, Virgilev, 1998]. It was assumed that the isotopic composition of each chemical element is the same as the naturally occurring isotopic content.…”

“…5 b)) and the total (Fig. 5 Table 1 Impurities and their maximum concentrations (weight %) in GR-280 and GRP-2-125 grade graphites [Ancius et al, 2005, Bylkin et al, 2001, 2004, Bushuev et al, 2002, Virgilev, 1998]. …”