FORMAL REPORT GERHTR-32 UNITED STATES-GERMAN HIGH TEMPERATURE REACTOR RESEARCH EXCHANGE PROGRAM Original report number_______________________ _________ Title .. Far i.at ion a. flnd Annealing in the. Pyrolytic. Cjarbo.n------------- .:..... ------------------------------------------------- --- Author(s)K» Koizlik, E» Baltiiesen Originating Installation Kernforschungaanlage Julich Inaitute for Reactor Materials Date of original report issuance................ ... Reporting period covered____ :_______________ ;_______ Translated from the original German This report, translated wholly or in part from the original language, has been reproduced directly from copy pre­ pared by the United States Mission to the European Atomic Energy Community THIS REPORT MAY BE GIVEN UNLIMITED DISTRIBUTION U5AEC Division of Technical Information Extension, Oak Ridge, Tennessee r GERBTR -32 Variations in Anisotropy duo to Irradiation and Annealing in the Pyrolytic Carbon Cladding Layers of Fuel Particles by K, Koizlik, '£» Baltiissen Kernforschungsanlage Jtllicb,# Institute for Reactor Material® ------------------- —notice ---------------------- This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com ­ pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. The usability and service life of the fuel elements of gas-cooled high-temperature reactors depend essentially, according to current views, on tha retention, behavior of th& coated fuel particles* especially in relation, to the gasaous fission, products. The cladding layer of pyrolytic carbon, the mechanical Integrity of which must he preserved throughout tho operating period of the reactor, is of primary importance in this connaction. The cladding layer is exposed during reactor operation to stresses, resulting primarily from two causes i (l) The fission gases formed and. released in the fuel core with increasing burnup exercise a progressively increasing pressure on the cladding layer. (2) The high-energy neutrons cause radiation damage in the pyrolytic carbon* leading to dimensional and structural changes. If the stresses * caused by these two processes, are not reduced by thermal or neutron-shock-indueed creep, the cladding layer will burst as soon as its breaking load, which is of the order of 10 kp/iam * is exceeded. Comprehensive Irradiation experiments have shown that the irradiation-resistance of the particle cladding layer depends on the material properties of the pyrolytic carbon after deposition. These initial properties can be influenced, within wide limits by variation of the deposition parameters and of the pyrolytic gases. « 1 The small dimensions and geometry of the particles have been mainly responsible for the fact that the only properties te have received systematic investigation so far ar© the density and orientation anisotropy of the elatMing layers* A large number of papers have been published in the literature t reporting on the irradiation behavior of flat pyrolytic carbon layars (Hof, !# 2), The results described are, however, only capable of limitod application to the behavior of* particle cladding layers, in view of the unrepresentative stress states in flat layers, The attempt has therefore been made to study the changes in the properties of pyrolytic carbon under different stresses by under­ taking irradiation and annealing experiments directly on the particle eiadditig layers. The material property of orientation anisotropy has proved particularly -suitable for this purpose, since it is both easily accessible to raesasurement anti can also be used in production control to establish the viability of the pyrolytic carbon. It also provides a valuable criterion for assessing the irradiation resistance of pyrolytic carbon, since it has been shown in irradiation experiments that the lower the orientation anisotropy of the cladding layers, the longer the mechanical integrity of the particles will be maintained. The orientation anisotropy has been directly measured by means of an optical technique on metallographic sections of the particle layers (Kef. 3, 4), The optical anisotropy factor OJPTAF, determined in this way, has been correlated by an empirically proved formula with the Bacon anisotropy factor BAF, measured by X-ray analysis,{ 2 this means that th<a optically iaea«ured surfao® effect of the orientation anisotropy includea the volume effect* measured by X-ray analysis. The measurem®nts have been given as BAP values, using this correlation curve. Our investigation of the variation in the orientation anisotropy of particle cladding layers due to irradiation was carried out on different particle varieties from several experiments f in which the particles were irradiated with, ^eutrgns at dose values of up to 16 x (E > 0,1 MeV) or at burnup levels up to 15 ^ firaa. The measurements show in all cases an increase in the orientation anisotropy (Fig. 1). The increase in the anisotropy is progressively greater, the higher the neutron dose. Particles, which have been exposed not only to fast neutrons but also to a considerable burnup of their fuel cores, exhibit at the same dose values substantially higher increases in anisotropy than particles without burnup,"' The irradiation causes an increase not only in the mean value of the orientation anisotropy of a particle cladding layer, but also in the half-life width of the distribution curves of the individual measurements (Fig. 2}. Plotting the increases in the anisotropy of the particles with high burnup levels as a function of the burnup, we obtain the graph in Pig. 3* A dependence of the increases in anisotropy on burnup is apparent in the particles, which, in addition to burnup of their fuel cores, 3 hava als© reached, dose values in ©xcass 21 **2 ' ' ■ of 1 x 10 cm , The increases in anisotropy of particles, exposed to dose values ?1 «-2 lower than 0,5 x 10* ora '% are very low even at high burnup values. The changes appearing in the orientation anisotropy of the particle cladding layers reveal the decisive influence of the stresses« built up in she pyrolic carbon. This appears plausible, .sine© stress- enhanced grsphitization has been repeatedly obsoarvea in pyrolytic carbon (Ref. 5, h, 7), It has been, found that a greater Increase in the graphitization and the oriontation anisotropy, associated with an extension parallel to and. a contraction perpendicular to the plane of deposition, occurred, in the pyrolytic carbon mides* external stresses at temperatures between 2Ji)0°€ and 3200°C. In this temperature rang© the vacancy concentration, is increased by thermal stress to such an extent that large-scale atomic rearrangement processes can occur, Tli# structural changes described above, on the other hand, were obtained at irradiation tempera cures between bOO°G and 1400°G f afe which thermally Induced vacancy concentra­ tions ar® not present to the extent required for rearrangement processes. High vacancy concentrations are, however, formed by neutron-induced shock processes during the irradiation. The results have shown that the migration and annealing of vacancies give rise to rearrangement processes, which reduce the stresses and lead to an increased orientation, anisotropy. la tide ease of* neutron irradiation without - 4 - appreciable burnup• as already mentioned, the pyrolytic carbon cladding layer undergoes dimensional changes, leading to marked, faitreaching tangential tensile stresses. These stresses and the resulting creep processes in the layer cause an increase in the orientation anisotropy with increasing dos®a, i,e.* with increasing damage rates. The fission gas release from the cores of particles with burnup creates in the cladding layer additional stresses, which increase with the lavel of burnup. In consequence, the particles with burnup exhibit at identical doses higher increases in the anisotropy than the particles without burnup. This hypothesis explains also the dependence of the increase in anisotropy on burnup, indicated in Pig. 3* Too few measurements are available, however, for any clear conclusions to be drawn; there is also considerable dispersion of the measurements, since this plot did not take into account the fact that the measurements were obtained on different particle varieties, in which not only did the pyrolytic carbon cladding layers possess different initial properties, but different fission product release rates from the cores have to be assumed. The low increases in anisotropy in the particles with high burnup levels but low dose values are obviously due to the low vacancy formation races (Fig. 3). This interpretation of the irradiation results presented above made it desirable to check the described model hypothesis by means of annealing experiments. The CO pressure in coated fuel particles with a heavy metal-oxide core increases with temperature and at sufficiently high temperatures can exceed the fission gas pressure of burnt particles, leading to bursting of the cladding layer. The 0 - 5 - teaall® stresses, created by the CO pressure in the cladding layer could be expected on the basis of" the model hypothesis to produce under the influence of the stresses occurring