REACTOR PHYSICS
The “Nuclear waste management” group of the laboratory carries out its work within the framework of European projects which follow one another since 2007 and within the framework of a collaboration contract between the CNRS and the SCK-CEN. This work concerns the on-line measurement of the reactivity of an ADS, in support of the MYRRHA project. These experiments are carried out at the GUINEVERE facility of SCK-CEN.
Neutronics experiments for critical reactors
Particle accelerator-driven subcritical nuclear reactors (or ADSs) could be advantageously used to incinerate highly radiotoxic nuclear waste such as minor actinides. Indeed, ADSs can operate with fuels heavily loaded with such waste, although these nuclei have very low delayed neutron fractions (β). However, this advantage only remains if it is possible to permanently guarantee that the ADS reactor remains subcritical (its reactivity remains negative).
In order to study and validate the methodology of online monitoring of ADS subcriticality, the group is strongly involved in various European projects (Euratom FP7 FREYA project (2011-2016), H2020 MYRTE project (2015-2019)), and in the MYRACL collaboration contract between CNRS/IN2P3 and SCK-CEN (since 2016). All these projects aim at contributing significantly to the design, R&D and licensing activities of MYRRHA. This very first industrial-scale ADS prototype in the world will be built in Mol, on the SCK-CEN site, and is expected to be commissioned in 2033.
For the research and feasibility studies of an ADS such as the MYRRHA, it is necessary to carry out experiments on models. The one on which our group is working is the GUINEVERE facility, of almost zero power, also hosted at SCK-CEN.
GUINEVERE is composed of the GENEPI-3C gas pedal (built by IN2P3) which delivers a 220 keV deuteron beam bombarding a tritiated titanium (TiT) target placed in the center of the Venus-F reactor. The fusion of deuterons with tritium provides the external source of 14 MeV neutrons necessary to continuously initiate new fission chains in the subcritical reactor. GENEPI-3C delivers a pulsed or continuous beam, with or without periodic beam interruptions. Originally, the core of the VENUS-F reactor was loaded with fuel rods of 30% enriched uranium metal (supplied by the CEA) inserted between solid lead strips (simulating the Pb-Bi coolant of a power ADS), and surrounded axially and radially by a lead reflector. Other configurations of VENUS-F, more representative of the most recent MYRRHA design and instrumentation, were investigated with the introduction of Al2O3, graphite, bismuth strips and devices to mimic MYRRHA irradiation assemblies.
The modular nature of VENUS-F has allowed a wide range of subcriticality to be explored. VENUS-F is equipped with about ten fission chambers (FCs) distributed in the core and with different deposits that allow the study of the neutron flux and its temporal variations during the experiments.
The “Nuclear waste management” group has been involved in the project since its inception and works in close collaboration with the Reactor Physics group of the LPSC in Grenoble. During the construction of the GUINEVERE facility and since its commissioning in 2011, the group has been involved in the monitoring of the external neutron source and in the proposal, preparation and execution of the experiments, as well as in the optimization of the experimental setup, using advanced Monte Carlo simulations of the facility. The group analyzes the collected data and develops numerical simulation tools and complementary theoretical approaches to interpret the experimental results.
The MSM method, widely studied, tested and used by the group is applied to experimentally determine the absolute reactivity of all VENUS-F configurations. These reactivity values serve as a reference for comparisons with reactivities estimated with other dynamic techniques such as the “beam interruption” technique. The latter will be used during the start-up of an ADS and, periodically, for a few minutes during the normal operation of a power ADS to determine its absolute reactivity. The group is also experimentally testing the “current-flux” method which is envisaged for monitoring the evolution of the reactivity of an ADS during its normal operation, or possibly during accidental situations.
Monitoring of core loading with a pulsed neutron source
The LPC Caen and LPSC groups have proposed to the SCK-CEN a new agreement project named SALMON which concerns the monitoring of reactivity during the loading of a reactor core by means of dynamic measurements with an external pulsed neutron source. The objective is to increase safety during the loading phase of industrial reactors (destined to be critical) by adapting the techniques used for ADS. Ideally, this project should be divided into three phases. The first two phases, which are exploratory, have already been supported by the SCK-CEN. The experiments of phase 1 were carried out in 2019 with measurements performed during the different stages of the unloading of the VENUS-F reactor and indeed, for highly asymmetric cores associated with a keff ranging from 0.86 to 0.55, GENEPI-3C operating in pulsed mode. This mode was chosen because it is the standard operating mode of commercial neutron generators.
The fast spectrum of VENUS-F, a drawback in terms of representativeness of current power reactors, offered the possibility to study the monitoring of the loading of a future fast power reactor. The preliminary results are very promising.
Phase 2 can be carried out when the VENUS-F reactor becomes a pool-type thermal water reactor with low-enriched fuel. The experiments carried out in phase 1 will be repeated when loading this thermal version of VENUS, which is obviously much more representative of PWRs. Phase 2 of SALMON is part of the SUCRE integrated project accepted by the CNRS multi-partner NEEDS research program in February 2020.
The analysis of the results of the first two phases in terms of the impact of the reactivity measurement on the monitoring of the core loading and the transposition to a power reactor will allow to consider a third phase on a larger scale. This phase should involve both the research (or development) of a miniature neutron generator meeting the specifications (performance, size) established during the previous phases, and new experiments, more representative of fuel loading in a PWR.