Centre Etudes Nucléaires de Bordeaux Gradignan

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The ACEN group’s research activities are dedicated to fission and capture reaction studies on nuclides of interest for present or future fuel cycle and for nuclear waste transmutation.

The group is involved in national and EU programs dedicated to nuclear data measurement, i.e. precise determination of neutron induced reaction cross section fission and capture cross section measurement, in the fast neutron energy range (MeV) or in the resolved resonance region (1-100 eV). Experiment are performed at AIFIRA local facility, and at several national and European facilities (IRMM Geel, IPN Orsay, CEA/DAM Bruyères-le-Châtel, Oslo University, GSI Darmstadt).

We are also interested in fission fragment experimental studies (fission fragment charge, mass and energy distribution) and modelisation.

The ACEN group is engaged in the systematic measurement of neutron cross-sections for actinides: these are nuclides involved in the present U/Pu fuel cycle (^235,238U, Pu) or in the innovative thorium fuel cycle (²³³U, Th and Pa) and minor actinides that are the most radioactive part of the nuclear waste on the thousand year range (Np, Am and Cm isotopes).

The measurements of neutron cross-sections for these heavy nuclei very often represent a challenge for experimentalists due to their high α-activity, their scarcity and technical difficulties arising from the fabrication of isotopically-pure targets. According to the nature and availability of the studied elements, we use two complementary approaches to measure their cross-sections:

• Direct approach with the irradiation by neutrons
  This is the method that we applied for the measurement of fast neutrons induced fission cross-section for ²³³U, which is the fissile nuclide of the thorium fuel cycle, then ²⁴³Am, a long lived minor actinide, and more recently for the non fissile Pu isotopes ^240,242Pu. According to our requirements, the experiments were carried out with mono-energetic neutron beams produced by Van de Graaff and AIFIRA facilities in CENBG, at the Van de Graaff in IRMM (Geel, Belgium) or at CEA/DAM (Bruyères-le-Châtel) from the following reactions ⁷Li(p,n)⁷Be, D(d,n)³He or ³H(p,n)³He.
  In order to acheive the required accuracy, we have developped an experimental setting using the neutron on proton elastic scattering cross-section as a standard reference for the precise determination of the incident monoenergetic neutron flux on target.
  Our group has also been involved in time-of-flight technique measurement in the resolved resonance region at the GELINA facility of IRMM, Geel.
  
• Indirect approach through transfer reactions (surrogate method).
  One can use transfer reactions induced by a beam of charged particles (³He) to reach the fissioning systems that are inaccessible through neutron induced reactions due to their short half life and their high α-activity. The technique was put into operation at Tandem accelerator of IPN Orsay. This method known as the surrogate method was first used in the measurements of fission cross-sections for ²³³Pa nucleus (using a ²³²Th target), then for Am and Cm isotopes (using a ²⁴³Am target). The extension of the surrogate method to the determination of radiative capture cross-sections has been studied for nuclides in the rare earth region and for actinides. Our results have generated a renewal of interest for this indirect approch.

We are also involved in experiments on fission fragment properties and distribution studies and in fission process modelling, via the development of a performant code (GEF).

Our activities have been within the successive interdisciplinary programmes of CNRS (PACEN/GEDEPEON) and/or form a part of integrated European projects of the 6th and 7th Framework Programme.