Irradiation Performance of Fast Reactor Fuels
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Oktober 1967 KFK 662 EUR 3698 e Institut für Angewandte Reaktorphysik Institut für Material- und Festkörperforschung Irradiation Performance of Fast Reactor Fuels D. Geithoff, G. Karsten, K. Kummerer GESELLSCHAFT FUR KERNF( KARLSRUHE Als Manuskript vervielfältigt Für diesen Bericht behalten wir uns alle Rechte vor Gesellschaft für 'Kernforschung m.b.H. Karlsruhe KERNFORSCHUNGSZENTRUM KARLSRUHE Oktober I967 KFK662 EUR 3698 e Institut für Angewandte Reaktorphysik Institut für Material und Festkörperforschung IRRADIATION PERFORMANCE OF FAST REACTOR FUELS *' D. Geithoff, G. Karsten and Κ. Kümmerer Paper presented at the Symposium on Plutonium Fuels Technology, Nuclear Metallurgy Committee IMDAIME, Oct.46,1967, Phoenix, Ariz., USA. Gesellschaft für Kernforschung mbH., Karlsruhe *) Work performed within the association in the field of fast reactors between the European Atomic Energy Community and Gesellschaft für Kernforschung mbH., Karlsruhe ABSTRACT The requirements for uranium-plutonium dioxide fuels are the basis for a program of irradiation tests which is outlined in the introduction. Some hypothetical considerations supply a rational background for the design of specimens and the chosen operation conditions. The experimental facilities and irradiation devices are shortly described and discussed with respect to the irradiation characteristics. The available hot cell equipment provides for the standard examination technique including X-raying by betatron and fission gas chromatography. One group of experiments refers to the short term behaviour of the oxide type fuel. In another group the long time behaviour of oxide fuel pins is studied. Different pins have been irradiated in capsules and in a helium loop. The present results indicate, that the basic assumptions for the pin design are principally con firmed, the specimen failures only arising from malfunctions of the irradiation devices. The now available irradiation experience is used for the fabrication of further irradiation specimens of more realistic fast reactor dimensions and also for the concept of fast breeder pin specifications. Under these aspects the forth coming main experiments are discussed. Finally the status of knowl edge and experimental experience is composed in a set of conclusive remarks. INTRODUCTION The operational requirements for oxide type fuel, which are 1 described in full details in the related paper in the first session of this symposium, are the principal guidelines for all irradiation test work in the Karlsruhe fast breeder project. The most important technical features are the proper irradiation conditions with res pect to linear rod power, to cladding temperature and to the ex pected total burnup performance. The typical external and internal geometry is characterized by a small fuel diameter (compared to thermal reactor fuel) and a length distribution which includes an ι active fuel zone, at least one axial blanket region and a fission gas plenum. The typical ranges of requirements both for sodium and · steam cooled pins are compiled in Table I. The whole irradiation program is oriented towards these target data. As in the first period only a thermal reactor with a limited neutron flux, the Karlsruhe research reactor FR2, was available, significantly larger fuel diameters were necessary in order to reach the linear rod power wanted. Above the intention of a mere perform• ance test up to high burnups - a goal which will be achieved by tests currently underway - the here described work refers partly to parameter studies. The short term tests try to evaluate a system• atic scheme of parameter variations. The quoted long term tests are only a first step, which has to demonstrate the feasibility of the irradiation equipment and furthermore to experience the post irradia• tion examination routine. The next step with much higher target burnups and parameter constellations more similar to fast reactor conditions are already in operation, on the one side with improved irradiation devices in the FR2 again, on the other side with fast flux irradiations in the Dounreay Fast Reactor. The present paper describes the status of our experimental potential and experience and also some introductory theoretical considerations. We feel it necessary, to establish simple model assumptions concerning the expected irradiation behaviour as a rough guide. IRRADIATION HYPOTHESES The target burnup for a fast breeder fuel pin is 100 000 MWd/t. In order to make predictions about the pin behaviour with• in an equivalent irradiation time, the pin design is based on a theoretical fuel pin behaviour analysis. In this context two limi• tating assumptions are made, namely - the fuel operation time will come to an and, after the avail• able void volume of the most critical fuel cross section has been filled by fission-induced fuel swelling to the highest possible extent, the fuel operation might end earlier by mechanical contacts between cladding and nonplastic fuel. As a consequence of these assumptions three separate questions can be discussed with theoretical methods: 1. What is the maximum achievable burnup locally, if only the volume swelling due to fission products is considered? 2. Applying the answer of question 1, where is the most critical fuel cross section along the fuel pin located? 3. Does a socalled "nonplastic" fuel behaviour arise under the planned operation conditions, leading to a mechanical attack on the cladding? In order to produce an answer to the first question a burnup model is established using some schematic considerations: The internal volume of the pin consists of the fuel volume Vp, F' the (hot) gap volume between fuel and cladding V«, and the dishing volume (if applicable) V_. The fuel volume itself can be divided into a plastic zone V with temperatures above 17OO C, a creep zone V with tempe vi ratures between I3OO and I7OO C, where creep and diffusion are of remarkable velocity, and a low temperature zone V.. O TE il "•■t below 1300 C with viscoelastic behaviour mainly ' ' . Furthermore there are defined availability factors £,, m and n in the 3 fuel zones, which give the volume portions of the porosity being actually available for volume expansion due to swelling processes. The porous volume should be available to any expansion with about 80 v/o above I7OO C, with 50 v/o at 13OOI7OO C and according to a numerical evaluation with not more than 30 v/o below 1300°C. The last figure may rise somewhat during long time operation as the fuel becomes more plastic at low temperatures. Hence we apply for numerical evaluations L = 0.8, m = O.5, n = 0.3· Finally two swelling processes are envisaged which need a con tinuously increasing amount of space with burnup. The basic swelling effect is defined here to be caused by solid fission products and a contribution of fission gases, working in all 5 three zones. According to a literature evaluation this basic swelling rate h seems to be 1.6 % volume change in 100 % S dense oxide per 10 MWd/kg heavy metal burnup. That means a figure h =1.6x10 kg/MWd. An additional swelling effect S caused by fission gases mainly is added for the creep zone. Its rate is assumed to be h = 0.4 χ 10— 3 kg/MWd. With these explanations the maximum possible burnup Β (in MWd per kg heavy metal content), after which all available volume is used up by the swelling fuel, is: (gV + mV + IlV ) +V + V Β „ °-V pl cr lt G D (<|) ^Fhs VF + hg Vcr where ,$_ means the fuel density in the pellet fuel. A graphic evaluation of this burnup formula is given in Fig.1 for a linear rod power of 500 W/cm. The second question for the most critical cross section is handled by applying the burnup formula, which represents the fuel swelling capacity. Hence locally the maximum allowable burnup can be evaluated for the axial power distribution of a pin. These local maximum burnup figures have to be compared with the really arising burnup at each axial part of a pin in equal times. For certain parts of a pin, which are not necessarily the absolute maximum burnup ranges, the capacity for burnup is implemented firstly. These constitute the critical fuel cross sections. Such an analysis can be expanded of course on a total reactor core by application of specific thermal design data. Generally the critical zones are in the half of the core with the coolant inlet. To deal with the third question for the existence of non- plastic fuel, the expansion rate of the fuel, which is given by the swelling rate, is compared to a compression rate, which is produced by an evaluation of fuel-cladding interaction, Fig.2. Here irradiation damages and thermal stresses with the cladding are taken into account by a pessimistic assumption for the 0.2 $-creep stress for 10 000 hours of the considered stainless steel. 2 3 If Furthermore, by application of mechanical oxide data ' ' the force of the cladding on the swelling fuel ring with the volume V It is calculated. The result is, that, under the assumption of 950 C average ring temperature, only those parts of the fuel with linear rod powers lower than 250 W/cm behave non-plastic. They are situat• ed beyond the critical zones mentioned above and hence do not re• present limitations for operation time in such designs. EXPERIMENTAL FACILITIES The irradiation program in the Karlsruhe research reactor FR2 a heavy water-moderated reactor with kk MW thermal power - is performed with two types of irradiation equipment. There is a helium loop operating in the central channel and furthermore, there can be introduced irradiation capsules into normal fuel sub• assembly positions. The helium loop can be loaded with two diffe• rent irradiation inserts. For long term irradiation an insert is used containing k fuel pin specimens in a cluster.