Radiation characteristics of graphite removed from operation in uranium-graphite reactors

Abstract

Cores of uranium-graphite reactors use graphite with specified properties produced by high-temperature graphitization of natural materials (coke, pitch, resin, etc.). The graphitization removes impurities because they evaporate faster than the graphite.Nuclear-pure graphite was used in the first industrial reactor (1946), in the Obninsk nuclear power plant (1954), and in many other power units. The total mass of graphite removed from or still in uranium-graphite reactors is approximately 50,000 tons. As it slows down neutrons, graphite has been irradiated, in industrial reactors for example, at an average thermal neutron fhience of up to 3.1022 cm -2.A large number (up to 30) of natural elements exist as impurities in reactor graphite at concentrations of 10"4-10-6%by weight, many of which form long-lived radionuclides as a result of (n, ~,), (n, p), (n, ~), and (n, 2n) reactions. Along with this group of radionuclides there are also actinides and fission products. They are produced for many reasons, but they come mainly from fuel particles that enter the graphite cladding of the fuel elements as a result of accidents and stuck fuel element bundles. The mass and localization of fuel in graphite cladding was determined by nondestructive neutron and x-ray tests [I] after several reactors were shut down and unloaded. Experiments and calculations showed that a total mass of approximately 70 kg of uranium particles with an initial ~ 3% enrichment was in the cladding of the AMB-100 reactor of the Beloyarsk nuclear power plant and 2-10 kg with an initial enrichment of 0.71% in the cladding of industrial reactors. The fuel particles had been irradiated in the open over 20-30 years at a temperature of -550~ and had reacted with purge nitrogen at a rate of -500 m3/hr, as well as with water-steam mixtures when coolant entered the graphite cladding during accidents. As a result, the graphite in several reactors acquired anomalously high radiation properties. However, besides fuel particles from accidents, fission products and actinides that can be explained by natural uranium and thorium impurities are observed in reactor graphite (estimated up to 10-5%), as well as uranium contaminants on the surfaces of the fuel-element cladding. The total of both of these factors gives an upper estimate of about 0.5 kg per 2000 tons of graphite in the cladding of industrial reactors or RBMKs before they are put into operation.Probably radionuclides, 6~ for example, can enter the cladding as a result of corrosion. However, their contribution is negligibly small compared to the background of radionuclides produced from uranium and plutonium fission and from activation of impurities (including 6~ from 59Co at concentrations up to 10-4%). Tables 1-3 show calculated specific activities of long-lived radionuclides in graphite and uranium. For application purposes they are normalized to operational parameters relatively common for all uranium-graphite reactors: 30 years' radiation, 3-1013 sec-t-cm -2 thermal neutron flux, 0.05 spectral hardness...

Investigation of Impurities of RBMK Graphite by Different Methods

“…The documentation on the production graphite impurity concentration is limited as purification was mostly performed to reduce the concentration of neutron absorbing impurities (such as Li, B, Cd, or other rare earth metals) to obtain not higher than the 4.5–4.8 mb of thermal neutron absorption cross-section for graphite (Bushuev et al 2002), without a detailed list of other (not so important for reactor operation) impurities. Several studies on the identification of the impurity concentrations in graphite from commercial and RBMK reactors have been performed by neutron activation and mass spectrometry methods (Virgiliev et al 1994; Bulanienko et al 1996; Virgiliev 1998; Bylkin et al 2004). Concerning the RBMK-1500 graphite due to limited accessibility to the nuclear power plant samples only a few graphite sleeve samples were investigated previously (Ancius et al 2005; Puzas et al 2010).…”

Immobilization of carbon-14 from reactor graphite waste by use of self-sustaining reaction in the C–Al–TiO2 system

“…During the operation period, reactor graphite waste has accumulated in these reactors in the form of fines and crumbs (bulk form). Because of long-term neutron irradiation, this waste contains significant amounts of long-lived radionuclide 14 C [1,2] which has a great potential hazard to human beings. The radionuclide 14 C has a half-life of 5730 years and is a biologically hazardous substance because of its readily assimilation by the human body [3].…”