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

Chemistry and Electrochemistry of Thorium and Uranium
In Fused Solvents at High Temperature
IPNO Participation: S. Delpech, N. Bear, S. Jaskierowicz
Collaboration : LECIME Paris, PCR ANSF PACEN/CNRS, AREVA, RHODIA
Le comportement chimique et électrochimique du thorium et de l’uranium est regardé dans des solvants de
fluorures fondus à haute température. Ces études, basées sur l’analyse de courbes électrochimiques, des
mesures d’analyse thermique différentielle, de potentiométrie, ont montré la stabilité des oxydes d’uranium
en fluorures fondus selon le degré d’oxydation de l’uranium. Des mesures ATD et DSC ont conduit à la détermination
de constantes thermodynamiques de composés tel que K 2 U 2 O 7 .
Les études sur le thorium s’inscrivent dans le cadre plus général des recherches menées sur le réacteur
nucléaire à sels fondus. Ce concept est l’un des six concepts retenus dans le cadre de Generation IV et est
le seul à mettre en œuvre un combustible liquide qui est à la fois combustible et caloporteur. Les objectifs
de nos travaux sont (i) de comprendre la chimie du sel combustible liquide, mélange de LiF et ThF 4 et la
chimie des solutés dans ce sel et (ii) de proposer un schéma de traitement du sel combustible usé.
The Molten Salt Reactor
An innovative molten salt reactor concept, the
MSFR (molten Salt Fast Reactor) is developed by
CNRS since 2004. Based ont he particularity of
using a liquid fuel, this concept is derived from the
American molten salt reactors (included the demonstrator
MSRE) developed in the 1960s. The
major drawbacks of these designs were (i) a short
lifetime of the graphite blocks, (ii) a reactor fuelled
with 233 U, not a natural fissile isotope, (iii) a salt
constituted of a high chemical toxic element: BeF 2
and (iv) a fuel reprocessing flux of 4000 liters per
day required for reaching a high breeding gain.
However, this concept is retained by the Generation
IV initiative, taking advantages of using a liquid
fuel which allows more manageable on-line control
and reprocessing, fuel cycle flexibility (U or Th)
and minimization of radiotoxic nuclear wastes.
In MSFR, MSR concept has been revisited by removing
graphite and BeF 2 . The neutron spectrum
is fast and the reprocessing rate strongly reduced
down to 40 liters per day to get a positive breeding
gain. The reactor is started with 233 U or with a Pu
and minor actinides (MA) mixture from PWR spent
fuel. The MA consumption with burn-up demonstrates
the burner capability of MSFR.
The objective of this work is to study the chemistry
of the fuel salt and to propose a reprocessing
scheme. This work is performed in the frame of the
PACEN program of CNRS dedicated to nuclear
fuel cycle. This work is also a strong contribution of
the european project EVOL of FP7.
Some results
Some molten salts have been studied to establish
a fluoroacidity scale. The fluoroacidity in fluoride
molten salt is a propertie which leads to define the
speciation of elements in the molten salt. An acide
fluoride salt will be able to form complexes with
fluoride ions and a basic salt will give its fluoride
ions. Generally alkaline salts are basic salt and are
totally dissociated.
Potentiometric measurements of redox system Ni
(II)/Ni contained in various molten salts were performed
as described in the figure 1.
Figure 1: Potentiometric cell for potential
measurements
Various composition and nature of molten salts
have been tested and the results are reported on
the figure 2.
Figure 2: Acidity scale of various fluoride
salts
A conept of pF indictaor electrode is currently in
progress based on these potentiometric measurements.
Chemistry of Uranium in fluoride molten salt
The chemistry and electrochemistry behavior of
Uranium is studied in molten salt with the aim of
determining the form and the oxidation states of
uranium in fluoride molten salt LiF-NaF-KF at 550°
C.
Thermodynamic measurements have been performed
using DSC technic.
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