Description du sujet de thèse

Sensitivity analysis studies [1, 2, 3] have been performed for different types of advanced nuclear systems, in particular for Generation-IV fast reactors. These studies indicate that an important reduction of the uncertainties on nuclear data is needed for many actinides, such as heavier mass Pu isotopes.

The 240Pu isotope is produced in the nuclear fuel of thermal or fast reactors by successive neutron captures and  or  decays. This isotope is the second most present Pu isotope (behind 239Pu), but the first non-fissile one. The 240Pu isotope is particularly unsuited for recycling in a thermal reactor, due to its non-fissile property. A more efficient burning via the fission process would occur in a fast reactor, where the harder fission neutron spectrum would better match the fission threshold. Sensitivity studies performed in recent years have shown the need to reduce the uncertainties in the fission cross-section for 240Pu in the fast region. Existing nuclear data on 240Pu(n,f) show discrepancies between databases and experiments up to 10% between 1 and 2 MeV. It is then very important to reduce this uncertainty to around 3-4%.

A fission cross section measurement is usually performed relative to a well known cross section like 235U(n,f) or 238U(n,f). This process induces very strong correlation with these cross section, and with all actinides cross sections measured in the same way. On the contrary, the proton recoil technic consists in performing measurement relatively to the 1H(n,n) cross section. This cross section is a primary standard, very well known, structureless, and which can be calculated with ab-initio calculations. In this technic the 235U or 238U target used as reference is replaced by a H-film (usually a plastic foil). The neutrons will scatter on H nuclei, and recoil protons will be emitted and easily detected. This technic has been used several times in the past by the ACEN group of the LP2i lab [4, 5]. It led to totally uncorrelated results with uncertainties around 3-4% (more or less the same order of magnitude than standard fission measurement).

One of the Challenge of such experiment is the availability of the fissile target, as actinide target with suitable properties are quite rare. Hopefully, several 240Pu targets have been manufactured in the JRC-Geel lab (Belgium) in 2010. One of these targets is still available in this lab. Preliminary calculations will have to be made to adapt the experiment to the properties of this specific target.
The experiment will be carried out in the MONNET facility in the JRC-Geel lab, which presents several advantages:
- can produce neutron in the 1-2 MeV energy range thanks to the use of tritium targets
- availability of the 240Pu target in the same lab, negating the paperwork and delays of radioactive target transportation
- expertise of the MONNET team on cross section measurement

[1] NEA Nuclear Data High Priority Request List, http://www.nea.fr/html/dbdata/hprl/
[2] OECD/NEA Working Party on Evaluation and Co-operation (WPEC) Subgroup 26 Final Report
http://www.nea.fr/html/science/wpec/volume26/volume26.pdf
[3] E. Gonzalez-Romero (ed.), Deliverable D5.11 from IP-EUROTRANS
[4] G. Kessedjian et al., Phys. Rev. C 85, 044613 (2012)
[5] P. Marini et al., Phys. Rev. C 96, 054604 (2017)

Contexte de travail

The student will be based at LP2i. A first part of the work will consist in the appropriation of the simulation and analysis codes, as well as the preparation of the experiment. He/she will also participate in the activities of the ACEN group: other experiments, instrumental developments, etc.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

A background in nuclear physics is mandatory, as well as knowledge of common programming languages such as C++. MCNP code will be used, after registration and local training.
The ability to speak French is not a prerequisite, but the candidate is expected to learn French during the PhD period.