NL90C1051 lNIS-mi— 12739 Experimental studies of neutron irradiated uranium dioxide at high temperatures R.H.J. Tanke EXPERIMENTAL STUDIES OF NEUTRON IRRADIATED URANIUM DIOXIDE AT HIGH TEMPERATURES EXPERIMENTELE STUDIES VAN MET NEUTRONEN BESTRAALD URANIUMDIOXIDE BIJ HOGE TEMPERATUREN (met een samenvatting in het Nederlands) PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR AAN DE RIJKSUNIVERSITEIT TE UTRECHT, OP GEZAG VAN DE RECTOR MAGNIFICUS PROF.DR. J.A. VAN GÏNKEL, VOLGENS BESLUIT VAN HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE VERDEDIGEN OP WOENSDAG 20 JUNI 1990 DES NAMIDDAGS TE 4.15 UUR. door Richardus Hendrikus Johannes Tanke geboren op 19 april 1957 te Hengelo (O) Promotor: Prof.Dr. CD. Andriesse Aan Pien CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK DEN HAAG Tanke, Richardus Hend. ^kus Johannes Experimental studies of neutron irradiated uranium dioxide at high temperatures / Richardus Hendrikus Johannes Tanke. - Arnhem : KEMA. - 111. Thesis Utrecht. - With index, ref. - With summary in Dutch. ISBN 90-353-1023-3 pbk. ISBN 90-353-1024-1 geb. SISO 538 UDC [539.125.06:546.791-31:536.45](043.3) Subject headings: uranium dioxide / gamma tomography. Contents Summary and outline 7 Outline of this thesis 9 Chapter 1: Introduction 11 The safety issue during the development of nuclear power 11 Experimental programmes on the release of fission products 15 from overheated fuel The KEMA source term experiment 17 References 18 Chapter 2: Experimental equipment and laboratory facilities 21 The hot cell 23 Measurement equipment for the evaporation experiment 29 The tomographic equipment 40 Reference 45 Chapter 3: Sample characteristics 47 Uranium dioxide fuel in nuclear reactors 48 Uranium dioxide spheres 50 The fuel fission product inventory after irradiation 57 The fuel characteristics after irradiation 59 Conclusion 69 References 69 Chapter 4: Theory and models 73 The relation between the theory and the measured ion current 73 Evaporation of irradiated fuel 76 The diffusion model 81 The principle of tomography 85 References 90 Chapter 5: Calibration experiments and preparatory studies 93 Calibration experiments 93 Free evaporation tests with powders resembling fission 100 product molecules 100 Tomographic experiments with test objects 102 References 107 Chapter 6: Results from irradiated fuel:evaporation of urania 113 The appearance of the urania species in the mass spectra 113 Results 115 References 123 Chapter 7: Results from irradiated fuel: release of fission products 125 The analysis of the mass spectra 125 The presentation of the results 127 Results 130 Consistency of the mass spectrometer data 158 Results of the diffusion model 162 Results from the tomography 164 References 171 Chapter 8: Discussion and conclusions 173 Discussion of the results per element or group of elements 173 Conclusions 178 References 182 Samenvatting (met indeling) 183 Indeling van dit proefschrift 184 Dankwoord 186 Curriculum vitae 188 Summary and outline In the most common types of nuclear reactors the fuel has the ceramic form of uranium dioxide (UO2). Fission of uranium leads to heat production inside the fuel by collision of energetic fission products with atoms of the U02-matrix. As a result the microstructure of the fuel becomes very complex. The fission products are incorporated into the nuclear fuel. During normal operation of the nuclear reactor only a small part of the gaseous products can escape from the fuel. Most of the fission products are radioactive and they form, when released from the reactor, a health hazard for the population. In case of an accident situation, in which the heat of the nuclear fuel can no longer be transferred to cooling water, the temperature of the nuclear fuel may rise very strongly, so that the other fission products may also be released. In fact, the release of fission products from overheated nuclear fuel is the first step in a long series of steps, which can ultimately lead to the release of radioactive substances from the nuclear power station into the environment. In this respect, it is important to know more about both the release rate of the various fission products and their chemical forms. This thesis deals with this first step. It describes a research which had the following three objectives: (1) to determine the release rate of the fission products from overheated UO2; (2) to determine the chemical form of these fission products; (3) to determine the mechanisms of transport inside the nuclear fuel. The nuclear fuel samples used are representative of the actual nuclear fuel from the reactor. These are UO2 spheres of approximately 1 mm, which are irradiated in the High Flux Reactor in Petten, the Netherlands. Subsequently, each of them is evaporated in a vacuum system at KEMA in Arnhem. To be able to manipulate the radioactive spheres safely many tools have been developed which can be used in a hot cell especially made for this purpose. The chemical forms of the particles which are being released from the spheres during evaporation have been determined using a mass spectrometer. At the same time, the activity of the fission products has been measured using a gamma spectrometer. A gamma tomographer has been developed for determining the three-dimensional distribution of the concentration of radioactive fission products in the sphere. This gamma tomographer could be used for determining the change of this distribution as a function of temperature. For interpretation of the results two models have been developed: a model of the evaporation of the non-stoichiometric LJO2, and a model of the diffusion of fission products in UO2. The first model was used to determine the stoichiometry of the sphere (that is the ratio of the number of oxygen atoms to the number of uranium atoms). There appeared to be no clear relation between the stoichiometry and the uranium consumption. This does not correspond with the existing model which predicts an increase of the amount of free oxygen (the oxygen potential) by the consumption of uranium. This thesis ascribes this absence of any relation to the, still unclear, influence of the microstructure of the nuclear fuel. The fact that a low oxygen potential can reduce the release of the fission products cesium and xenon, indicates the presence of a mechanism in which the vacancies (open places) in the UO2 matrix are occupied by chemically neutral fission products. The newly generated vacancies are then able to accommodate oxygen atoms. The second model has been used to determine the activation energy for the diffusion of the fission products. The values found correspond well with those found for the nuclear reactor fuel, but are considerably higher than those found for UO2 samples which have only been irradiated to a small extent. The main conclusion is that the microstructure of the nuclear fuel has a great effect on both the amount of free oxygen atoms, the release rate and the chemical form of fission products. Since this microstructure has not been investigated in greater detail, all other conclusions are of a qualitative nature. In addition to the above-mentioned conclusion the following may also be concluded: (1) above 2300 K, evaporation of the UO2 matrix is the prevailing release mechanism for all fission products; (2) the element barium can be as volatile as iodine; (3) the elements niobium and lanthanum can be volatile; (4) the presence of molecular combinations of the fission products iodine, cesium and tellurium is very improbable; (5) the elements barium and niobium may form compounds with oxygen and will then be released as simple oxides; (6) fission products are released from overheated UO2 as simple atoms or as oxides. Finally, a new model will be proposed for describing the oxygen behaviour in irradiated nuclear fuel. Outline of"this thesis Chapter 1 starts with a brief description of the history of the commercial application of nuclear energy with respect to safety and concludes with a survey of the experimental activities in the field of the release of fission products from overheated nuclear fuel. Chapter 2 describes the experimental equipment used in this research. Chapter 3 describes the irradiation facilities and the samples used: irradiated spheres of uranium dioxide. Several aspects of these samples are compared. The chapter concludes with a survey of the chemical form of fission products in nuclear fuel. Chapter 4 deals with the theory of free and Knudsen effusion into a vacuum. In addition, the theory of three-dimensional gamma tomography is described. Furthermore, this chapter describes the two models which have been developed. First, it deals with the model of free evaporation of uranium dioxide and the way this model is used for estimating the stoichiometry of the nuclear fuel and second, with the model of diffusion of fission products and the estimation of the activation energy and the diffusion constant. Chapter 5 describes the calibration of the mass spectrometer. In addition, the results of the evaporation of some powders are discussed. These powders are non-radioactive forms of fission product molecules. Finally, the results of the test experiments with the tomographer are presented. Chapter 6 is the first chapter containing results of the evaporation of irradiated UO2 spheres. This chapter deals with the results of the evaporation of the urania species. The stoichiometry of the spheres is estimated with the evaporation model. Chapter 7 is the second chapter with results of the evaporation of irradiated UO2 spheres. The results of the release of fission products are presented and analyzed. Chapter 8 includes a discussion of the results and contains the final conclusions. Finally, a model is proposed which deals with the uptake of oxygen atoms in irradiated uranium dioxide. •/* Chapter 1 Introduction The safety issue during the development of nuclear power Since the early days of nuclear reactor power plant development, attempts have been made to calculate the magnitudes and the probabilities of source terms and consequences associated vith 'hypothetical* nuclear reactor accidents.