NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Poster 4.3: Characterisation of Essential Corium Subsystems Up To Melting

Sara Mastromarino1,2, A. Seibert1, E. Kassim1, D. Prieur1, M. Naji1, N. Magnani1, A. Ciccioli2, D. Manara1
  • 1: European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany
  • 2: Università “La Sapienza”, Department of Chemical Physics, Piazzale Aldo Moro 5, 00185, Rome, Italy

Abstract

In the unfortunate event of a Nuclear Power Plant (NPP) core meltdown accident (cf. Fukushima, Chernobyl, Three Mile Island) a liquid radioactive mass containing nuclear fuel, cladding and containment materials, called “corium”, is formed. In-depth investigation of such phenomena is paramount for the a priori determination of the fuel’s safe performance limits and the prevention of such mishaps both in current reactors and in future even safer ones. In addition, the occurrence of segregation in the re-crystallised corium can lead to the formation of zones richer in fissile materials. This would result in additional safety and safeguard issues.

The present research project is focussed on the experimental study of solid/liquid equilibria and phase segregation in crucial material systems simulating the real corium. In particular, the system UO2-ZrO2 is revisited in this work by laser heating under controlled (reducing, oxidising) atmospheres coupled with fast optical thermometry. Newly measured phase transition points measured in an inert or reducing atmosphere are in fair agreement with the early measurements performed by Masset in 1953, the only study available in the literature on the whole pseudo-binary system. However, a minimum melting point is more clearly identified here for the composition (U0.6Zr0.4)O2, at 2815 K. The solidus line is rather flat on a broad range of compositions around the minimum. It increases for compositions closer to the end members, up to the melting point of pure UO2 (3130 K) on one side and pure ZrO2 (2970 K) on the other. UO2-rich compositions are the most challenging because of the complex chemistry of uranium dioxide. In fact, uranium can assume different oxidation states (+3, +4, +5, +6) in its dioxide form, this feature allowing the formation of both hypo-stoichiometric and hyper- stoichiometric dioxides. The behaviour of uranium dioxide is therefore strongly atmosphere-dependent and differences can be noticed between the thermograms recorded when the sample is melted in an inert or reducing atmosphere (argon or Ar+H2) compared to those recorded in an oxidising atmosphere (compressed air). In the last case, solid/liquid phase boundaries can decrease by several hundreds of K.

Solid state phase transitions (cubic-tetragonal-monoclinic) have also been observed in ZrO2-rich compositions with the help of XRD and Raman spectroscopy. The well-known Raman spectra of ZrO2 are used to identify the different crystal lattice structures. The U and Zr valence states of samples melted in different atmospheres have been assessed with the help of X-ray absorption spectroscopy (XAS).

Additional systems have been explored, for example by substituting Th to U in the mixed dioxide, or by adding small amounts of PuO2 to the pseudo-binary UO2–ZrO2 in order to better simulate the reaction of a real MOX fuel with its oxidised cladding. Raman spectroscopy has been employed, in these particular cases, also for the identification of the hypothetically segregated PuO2 hotspots upon fast cooling.

The present results are important for assessing the thermal stability of the system fuel – cladding in an oxide based nuclear reactor, and for simulating the system behaviour during a hypothetical severe accident.