Fuel and fission product behaviour in early phases of a severe accident. Part II: Interpretation of the experimental results of the PHEBUS FPT2 test

One objective of the FPT2 test of the PHEBUS FP Program was to study the degradation of an irradiated UO2 fuel bundle and the fission product behaviour under conditions of low steam flow. The results of the post-irradiation examinations (PIE) at the upper levels (823 mm and 900 mm) of the test section previously reported are interpreted in the present paper. Solid state interactions between fuel and cladding have been compared with the characteristics of interaction identified in the previous separate-effect tests. Corium resulting from the interaction between fuel and cladding was formed. The uranium concentration in the corium is compared to analytical tests and a scenario for the corium formation is proposed. The analysis showed that, despite the rather low fuel burn up, the conditions of temperature and oxygen potential reached during the starvation phase are able to give an early very significant release fraction of caesium. A significant part (but not all) of the molybdenum was segregated at grain boundaries and trapped in metallic inclusions from which they were totally removed in the final part of the experiment. During the steam starvation phase, the conditions of oxygen potential were favourable for the formation of simple Ba and BaO chemical forms but the temperature was too low to provoke their volatility. This is one important difference with out-of-pile experiments such as VERCORS for which only a combination of high temperature and low oxygen potential induced a significant barium release. Finally another significant difference with analytical out-of-pile experiments comes from the formation of foamy zones due to the fission gas presence in FPT2-type experiments which give an additional possibility for the formation of stable fission product compounds. © 2014 Elsevier B.V. All rights reserved.

[1]  D. Manara,et al.  Severe accident research at the Transuranium Institute Karlsruhe: A review of past experience and its application to future challenges , 2014 .

[2]  Donald R. Olander,et al.  Re-solution of fission gas : A review: Part. I. Intragranular bubbles , 2006 .

[3]  R. Grimes,et al.  The diffusion of iodine and caesium in the UO2±x lattice , 2000 .

[4]  Marc Barrachin,et al.  New modelling of the U–O–Zr phase diagram in the hyper-stoichiometric region and consequences for the fuel rod liquefaction in oxidising conditions , 2008 .

[5]  Pekka Lösönen,et al.  On the behaviour of intragranular fission gas in UO , 2000 .

[6]  A. Pasturel,et al.  Fission products stability in uranium dioxide , 2011 .

[7]  M. S. Veshchunov,et al.  Modelling of grain face bubbles coalescence in irradiated UO2 fuel , 2008 .

[8]  M. Akabori,et al.  Release behavior of cesium in irradiated (Th, U)O2 , 1991 .

[9]  D. R. Olander,et al.  Dissolution of uranium dioxide by molten zircaloy: II. Convection-controlled reaction , 1988 .

[10]  B. Uberuaga,et al.  Cooperativity among defect sites in AO 2+x and A 4 O 9 (A=U,Np,Pu) : Density functional calculations , 2009 .

[11]  A. V. Berdyshev,et al.  Critical evaluation of uranium oxide dissolution by molten Zircaloy in different crucible tests , 1996 .

[13]  P. Hayward,et al.  Dissolution of UO2 in molten Zircaloy-4 Part 2: Phase evolution during dissolution and cooling , 1994 .

[14]  Ying Chen,et al.  Point defects and clustering in uranium dioxide by LSDA+U calculations , 2008, 0806.1790.

[15]  A. Pasturel,et al.  Study of Ba and Zr stability inUO2±xby density functional calculations , 2008 .

[16]  A. V. Boldyrev,et al.  Application of mechanistic criteria of cladding oxide shell failure to the analysis of core degradation simulated in bundle meltdown tests , 2008 .

[17]  P. Garcia,et al.  In situ TEM study of temperature-induced fission product precipitation in UO2 , 2008 .

[18]  F. Johnson,et al.  The characteristics of fission gas release from monocrystalline uranium dioxide during irradiation , 1977 .

[19]  P. Hayward,et al.  Dissolution of UO2 in molten Zircaloy-4 Part 1: Solubility from 2000 to 2200°C , 1994 .

[20]  S. Dash,et al.  Regular paperDetermination of standard molar Gibbs energy of formation of SrMoO4(s) , 1994 .

[21]  Daniel Schwen,et al.  Molecular dynamics simulation of intragranular Xe bubble re-solution in UO2 , 2009 .

[22]  H. Matzke,et al.  Gas release mechanisms in UO2—a critical review , 1980 .

[23]  L. René Corrales,et al.  Molecular dynamics simulation of Xe bubble nucleation in nanocrystalline UO2 nuclear fuel , 2011 .

[24]  Peter M. Oppeneer,et al.  Defect energetics and Xe diffusion in UO2 and ThO2 , 2009 .

[25]  C. Mathews,et al.  Thermodynamic properties of ternary oxides of fission products from calorimetric measurements , 1989 .

[26]  S. Dash,et al.  Enthalpy increment measurements of SrMoO4(s) and BaMoO4(s) , 1998 .

[27]  D. Olander.  Interpretation of laboratory crucible experiments on UO2 dissolution by liquid zirconium , 1995 .

[28]  C. Politis,et al.  The kinetics of the uranium dioxide—Zircaloy reactions at high temperatures , 1979 .

[29]  R. S. Nelson.  The stability of gas bubbles in an irradiation environment , 1969 .

[30]  Robin W. Grimes,et al.  Predicting the probability for fission gas resolution into uranium dioxide , 2009 .

[31]  R. J. White,et al.  The development of grain-face porosity in irradiated oxide fuel , 2004 .

[32]  P. Chatelard,et al.  Assessment of ICARE/CATHARE V1 Severe Accident Code , 2006 .

[33]  M. S. Veshchunov,et al.  An advanced model for intragranular bubble diffusivity in irradiated UO2 fuel , 2008 .

[34]  P. J. Hayward,et al.  Dissolution of UO2 in molten Zircaloy-4 Part 4: Phase evolution during dissolution and cooling of 2000 to 2500°C specimens , 1996 .

[35]  B. Cheynet,et al.  Progress in the thermodynamic modelling of the O–U–Zr ternary system , 2004 .

[36]  Christopher R. Stanek,et al.  Segregation of xenon to dislocations and grain boundaries in uranium dioxide , 2011 .

[37]  D. Olander,et al.  Release of fission products (Xe, I, Te, Cs, Mo and Tc) from polycrystalline UO2 , 1988 .

[38]  S. Yoda,et al.  Non-contact thermophysical property measurements of refractory metals using an electrostatic levitator , 2005 .

[39]  M. E. Gulden,et al.  Migration of gas bubbles in irradiated uranium dioxide , 1967 .

[40]  Kwangheon Park,et al.  Atomic diffusion mechanism of Xe in UO2 , 2008 .

[41]  P. Hofmann,et al.  UO2/zircaloy-4 chemical interactions from 1000 to 1700°C under isothermal and transient temperature conditions , 1984 .

[42]  B. Dorado,et al.  GGA+U study of the incorporation of iodine in uranium dioxide , 2009 .

[43]  V. D. Ozrin,et al.  A model for evolution of oxygen potential and stoichiometry deviation in irradiated UO2 fuel , 2011 .

[44]  Hansjoachim Matzke,et al.  Atomic transport properties in UO2 and mixed oxides (U, Pu)O2 , 1987 .

[45]  B. Uberuaga,et al.  Solubility and clustering of ruthenium fission products in uranium dioxide as determined by density functional theory , 2012 .

[46]  M. Veshchunov.  On the kinetics of UO2 interaction with Zircaloy at high temperatures , 1990 .

[47]  J. A. Turnbull,et al.  The distribution of intragranular fission gas bubbles in UO2 during irradiation , 1971 .

[48]  Blas P. Uberuaga,et al.  U and Xe transport in UO2±x: Density functional theory calculations , 2011 .

[49]  M. S. Veshchunov,et al.  Dissolution of solid UO2 by molten Zircaloy , 1994 .

[50]  R. Grimes,et al.  The solution and diffusion of ruthenium in UO2±x , 2003 .

[51]  R. Dubourg,et al.  Development of the mechanistic code MFPR for modelling fission-product release from irradiated UO2 fuel , 2006 .

[52]  D. Jain,et al.  Characterization and thermo physical property investigations on Ba1−xSrxMoO4 (x = 0, 0.18, 0.38, 0.60, 0.81, 1) solid-solutions , 2012 .

[53]  Ying Chen,et al.  Stability mechanism of cuboctahedral clusters inUO2+x: First-principles calculations , 2008, 0806.1792.

[54]  Robin W. Grimes,et al.  Molecular dynamics study of Xe bubble re-solution in UO2 , 2012 .

[55]  R. Dubourg,et al.  Mechanistic modelling of urania fuel evolution and fission product migration during irradiation and heating , 2007 .

[56]  M. Verwerft,et al.  On the solution and migration of single Xe atoms in uranium dioxide – An interatomic potentials study , 2010 .

[57]  É. Mikhlin.  The mobility of intragranular gas bubbles in uranium dioxide , 1979 .

[58]  G. Ducros,et al.  Fission product release under severe accidental conditions ; General presentation of the program and synthesis of VERCORS 1-6 results , 2001 .

[59]  C. R. A. Catlow,et al.  The stability of fission products in uranium dioxide , 1991, Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences.

[60]  Y. Mok,et al.  Effects of UO2/Zircaloy-4 Mole Ratios on Reaction of UO2 Fuel with Molten Zircaloy-4. , 1994 .

[61]  Alain Pasturel,et al.  Location of krypton atoms in uranium dioxide , 1999 .

[62]  A. Pasturel,et al.  Investigation of molybdenum and caesium behaviour in urania by ab initio calculations , 2009, Journal of physics. Condensed matter : an Institute of Physics journal.

[63]  R. J. White,et al.  A new fission-gas release model , 1983 .

[64]  P. Garcia,et al.  Thermal diffusion of iodine in UO2 and UO2+x , 2008 .

[65]  J. A. Turnbull,et al.  The characteristics of fission gas release from uranium dioxide during irradiation , 1979 .

[66]  D. Olander,et al.  Measurement of 2000–2100°C oxygen diffusion coefficients in hypostoichiometric UO2 , 1997 .

[67]  J. K. Fink,et al.  Thermophysical properties of uranium dioxide , 2000 .

[68]  J. Evans.  Post-irradiation fission gas release from high burn-up UO2 fuel annealed under oxidising conditions , 1997 .