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Safety Aspects of HTR Technology G Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction The substructures of polycrystalline graphite Extrusion Direction C-C Distance: 1.42 A 1Ox 500ox 150x • •3± Interlayer Spacing: 3.5 Ak Coke particles3 Crystallites partially - Arranglement of aligned in a particle carbon atoms Mean dimension of Mean dimension of particles about crystallites about 0.3 mm 500 A Graphite Body R.E. Nightingale, Academic Press, 1962 •j Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction Radiation Damage in Graphite o C atoms within the graphite lattice * Interstitial C atoms after neutron irradiation Aus E. Fitzer Graphit als Reaktorwerkstoft Haus der Technik, 1967 (?) w Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction The number of C-atoms in graphite crystallites displaced by a single 1 to 2 MeV neutron is of the order of 20 000. Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction Irradiation Induced Dimensional Changes 12 "10 C 008 E 02 Fast Neutro n/ence/ 1 0," cm' (EDN) 0.- 3,0 3,5 -6 12 W r-in a) 0\0 E 4 2 0 -.2 ID Fast M," 2,0 2,5 3,0 cc neutron Fiuence / 10:*/cm' (EDN) - .-.---e. --- - Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction Influence of Coke and Forming Technique on Important Properties of Reactor Graphites Grade ATR-2E ASR-1RS ASR-2RS ASR-1RG Ordinary Coke Special Ordinary Ordinary Pitch Coke Pitch Coke Pitch Coke Pitch Coke Sec. Coke T. Sec. Coke T. Forming Extrusion Vibration Vibration Vibration App. Density 1.80 1.82 1.87 1.78 (g/cm3) 13.0 par. 12.6 18.3 1.9.5 Tens.Strength 11.6 (N/mm 2) perp. 12.4 18.3 18.5 1.15 Anisotropy 1.12 1.05 1.02 Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction Statements I e Nuclear graphite for non exchangeable core components must be nearly isotropic - but not isostatically moulded. 9 Special coke processing and careful vibrational moulding yield the best graphite grades with respect to isotropy, strength, and homogeneity. * The expected lifetime of graphitic core components has to be verified by stress analysis using reliable irradiation data. e Today, none of the former widely tested graphites is still available. - - *- Safety Aspects of HTR Technology G. Haag: Nuclear Graphitefor the HTR Research, Development, and IndustrialProduction Statements II * Graphite for the PBMR reflector should be produced on a best guess basis using still existing procedures and experience. * Data for stress analysis calculations should be deduced from similar materials tested in former irradiation programmes. * Therefore, an international database with data from former nuclear graphite test programmes (US, UK, Japan, Germany, France) should be supported by possible users. e For future HTR projects, development and irradiation testing of new graphites should be resumed as soon as possible. Journal of Nuclear Materials 171 (1990) 41-48 41 North-Holland DEVELOPMENT OF REACTOR GRAPHITE * G. HAAG KernforschunganlageGmbH, Jnsfitut fir Reaktorwerkstoff4 Postfach 1913, D-5170 JAdich Fed Rep. Germany D. MINDERMANN and G. WILHELMI Signi GmbH, Postfach 1160. D-8901 Meitingen, Fed Rep. Germany " H. PERSICKE and W. ULSAMER Ringidorff-Werke-GmbH,-Pasfach210187, D1-5300 Bonn 2, Fed pep. Germany I Received 30 June 1989; accepted 15 July 1989 The German graphite development programme for High Temperature Reactors has been based on the assumption that reactor graphite for core components with lifetime fluences of up to 4X1022 neutrons per cm2 (EDN) at 400*C can be manufactured from regular pitch coke. The use of secondary coke and vibrational moulding techniques have allowed production of materials with very small anisotropy, high strength, and high purity which are the most important properties of reactor graphite. A variety of graphite grades has been tested in fast neutron irradiation experiments. The results show that suitable graphites for modern High Temperature Reactors with spherical fuel elements are available. 1. Introduction crystal structure) on the physical properties of graphite at very high neutron fluences. Graphite is an almost perfect reactor materiaL In Most of the nuclear graphites have been produced by 1942, Enrico Fermi already used graphite as moderator the Acheson process which for several decades had been in the first nuclear reactor fashioned by the hand of used for the production of furnace electrodes (1): A man. In the course of nuclear reactor development, the coke filler is mixed with thermoplastic hydrocarbon physical and chemical properties of graphite have been binder, pressed to form a "green" artifact and then in respons.ible for many important advances in nuclear two steps is subjected to a more or less complex heat reactor technology. treatment. The first step consists of the baking process It is obvious that on the way to modem High Tem at temperatures of about 10000 C, converting the binder perature Reactors (HTR) providing temperatures of into almost pure carbon, while in the second step the about 1000 0 C severe technical problems had to be baked material is graplbitized by electric resistance-heat solved, but in no case was graphite with its good mecha ing to a final temperature of 2600 ° C or higher. In some nical and excellent thermal properties the reason for cases where higher density is required, the porosity these'problems. Only the future reactor concepts HTR resulting from the raw materials and the baking process 500, HTR-100 and to some extent the Modular HTR is decreased by impregnation with pitch or tar prior to make use of graphite in a way that limiting factors have graphitizatior. to be considered. This was the reason for the develop The fast neutron irradiation behaviour of polycrys ment of new isotropic graphites and for irradiation talline graphite had been studied for many years in the programmes to investigate the influence of the Wigner frame of the Advanced Gas-cooled Reactor programme effect (i.e. fast neutron radiation damage in the graphite and the OECD Dragon Reactor Project in Britain, the Experimental Gas-Cooled Reactor (EGCR) program and the High-Temperature Gas-cooled Reactor (HTGR) Dedicated to Prof. H. Nickel on the occasion of his 60th program in the USA and the AVR and THTR Pebble birthday. Bed Reactor programmes in the Federal Republic of 0022-3115/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland) 42 G. Haag et aL /De/ dLopment of retorSraphite Germany. During the early 1970s it became clear that only isotropic graphites might be able to meet the needs of future pebble bed High Temperature Reactors. The most promising way of manufacturing isotropic "di~ graphites had been based on Gilsonite coke as filler, a naturally occurring bitumen mined in Utah, USA, which consists of almost spherical grains. However, for some reasons the longterm availability of Gilsonite coke had to be called into question and the first oil crisis in 1973 led to the decision to use only domestic raw materials for German reactor graphites. 6!: 2. HTR requirements The German development programme for reactor graphite is based on the cooperation of the companies Hochtemperatur-Reaktorbau GmbH (HRB) and Gesell schaft fuir Hochtemperaturreaktor-Technik GmbH (GHT)/ Interatom GmbH as nuclear reactor construct ing industry, Sigri GmbH and Ringsdorff-Werke GmbH HORgh-fluence region • High-stressed manufacturing graphite and the Kernforschungsanlage large blocks Jhilich GmbH (KFA). 0E Low-fluence region Medlum-stressed As an example for future High Temperature Reac large blocks tors, fig. 1 shows schematically the core construction of a HTR-500 pebble-bed reactor. The different require sCaore Low-stressed support columns large blocks ments with respect to neutron fluence, mechanical load, chemical impact or block size led to different goals of Fig. 1. Vertical section of a pebble-bed HTR (schematic di the graphite development programme. agrm). 2.1L Reflector components stresses ae created in the components which might be damaged severely if the stresses exceed the strength. Unexchangeable components for the high- and low Therefore, some other physical properties are supposed fluence zones are called reflector components. Their to satisfý limiting conditions such as dynamic Young's main purpose is to reflect and moderate the neutrons modulus not to exceed 12 kN/mm2 , thermal conductiv escaping from the reactor core. ity higher than 90 W/mK at room temperature, and Because the reflecting power depends on the number coefficient of linear thermal expansion (CTE) smaller 3 of carbon atoms per cm , the density of reflector gra than 6 x 10-6/K from 20 to 500 0 C. phite should not be less than 1.70 g/cmr. The anisotropy factor of graphite is defined as the With respect to neutron physics, the overall ratio of the linear thermal expansion coefficients in the neutron-capture cross section is required to be smaller two main crystallographic directions. For the construc than 5 mbarn. Consequently, the content of neutron-ab tion of reactor components only isotropic graphites with sorbing chemical elements such as Gd, B, Sm, and Eu, anisotropy factors from I to 1.05 are stable enough has to be kept smalL With respect to corrosion by against fast neutron irradiation damage. A compromise impurities from the cooling gas such as 02, H 20 and had to be found between small CTE-values to be CO2 , catalytically acting chemical elements such as Fe, achieved using anisotropic coke and isotropic products Ca, Sr and Ba are important. Experience shows that all based on coke with high CT17 this can be summarized by an upper limit for the ash content of nuclear graphite of about 600 ppm.
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