exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Studies and synthesis of specific materials based on uranium.
Applications on nuclear fuel and radioactive ion beam production targets
IPNO Participation: A. Ozgumus, N. Barré-Boscher, V. Sladkov, B. Fourest, M. Cheikh Mhamed, E.
Cottereau, S. Essabaa, B. Hy, C. Lau, B. Roussière
Collaboration : Sciences Chimiques de Rennes (Univ. Rennes 1), GANIL
Les études sur les carbures d’uranium dans le groupe de Radiochimie ont commencé avec l’aspect retraitement
du combustible usé il y a quelques années. En effet, ce matériau est préconisé comme combustible
des réacteurs de génération IV refroidis à l’He. S’en est suivie une étroite collaboration avec une équipe de
l’Université de Rennes. Depuis 3 ans, nos objectifs sont d’optimiser les méthodes de synthèse pour pouvoir
contrôler la densité, la stœchiométrie, la taille des grains et pores, mais aussi de comprendre les mécanismes
de frittage et d’étudier le comportement de ce matériau en conditions d’usages (haute température,
haute dose d’irradiation). Depuis plus récemment, nous travaillons avec l’équipe du pôle ALTO sur la même
problématique mais le matériau est alors utilisé comme cible de production d’isotopes radioactifs.
Introduction
For three years the Radiochemistry group of the
IPNO has been working on the synthesis of uranium
carbides in the framework of a project on new
generation (IV) nuclear plants [1]. This project has
proceeded from an international sustainable and
safe nuclear fission initiative [2, 3]. Uranium carbide
pellets exhibit high melting temperature and
thermal conductivities about eight times larger than
uranium dioxide. The uranium carbide pellet microstructure
and density have a large impact on the
fuel performance. The required characteristic of
the uranium carbide for this use is the capability to
retain the fission products inside the material for
safety and waste reprocessing. The first studies
concerning this material were made by Blandine
Fourest and Vladimir Sladkov at IPNO in the
framework of the reprocessing stage [4]. They
worked on the electrochemical dissolution and the
redox properties of the uranium carbides. Very recently
studies have been started at IPNO with the
ALTO group within the framework of the SPIRAL2
project to optimize the properties of uranium carbide
used as a radioactive isotope production target.
The required characteristics of this target are
both the high fission production yield and the capability
to release the fission products outside the
material as fast as possible. In spite of some opposite
characteristics of the expected material, these
two applications are directly linked by the synthesis
requirement: the production of this material must
be well-controlled by a simple and reproducible
method. This allowed us to undertake our studies
to an in depth understanding of the properties of
the uranium compounds as a function of the synthesis
parameters and also of their behaviour under
operating conditions: high temperature and
high irradiation dose.
Some results
For these two applications, the methods must allow
to control various properties such as the
stoichiometry, the density, the size of the pore and
their connectivity, the mechanical resistance to
swelling by irradiation and the thermal stability.
Most of the important characteristics detailed
above are controlled by dependant parameters like
synthesis and sintering temperatures and durations.
Our studies concern three ways of synthesis: the
a
c
10 µm
10 µm
10 µm
first two methods are
the carbo-thermic reductions of a mixture of uranium
oxide and uranium oxalate and graphite at
high temperature [5-8]. The third way is the synthesis
by fusing a stoichiometric mixture of metallic
uranium and graphite into an electrical arc [5]. Because
of the drastic conditions of use, all these
characteristics could change.
High temperature (1000°C or 2000°C) and high
irradiation dose will modify considerably the initial
properties of the material and could lead to the
collapse of the material by swelling, polygonization
or amorphization [9-15]. Our first studies show that
no drastic amorphization has been observed for an
irradiated pellet at the ALTO facility (results to be
published). Now, we are able to well-control the
stoichiometry of the material (monocarbide UC or
dicarbide UC 2 ) determined by the X-Ray diffraction
b
SEM micrograph from
sintered (a) uranium dicarbide
(97% of the d th ).
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