Subcritical Accelerator-driven Systems Research Articles

Contributions of MYRRHA to the European Fusion Energy Roadmap

One of the main objectives of the EUROfusion Roadmap to Fusion Energy during Horizon Europe is to finalize the conceptual design (thus preparing the engineering design phase) and initiate the construction of a fusion spectrum neutron source (IFMIF-DONES). The Belgian Nuclear Research Center is currently developing the MYRRHA research facilities, which include a subcritical accelerator-driven system ignited by a 600 MeV superconducting linear proton accelerator coupled to a 100 MeV proton injector. The implementation of the multipurpose fast spectrum reactor MYRRHA will proceed in phases, with phase I, covering 2019–2027, including the deployment of the 100 MeV proton accelerator and associated fusion material research facilities. The research on fusion materials on the basis of the 100 MeV proton beam and with hot cell facilities will focus on innovative materials and composites and will be fully complementary to IFMIF-DONES as well to the planned research program on using currently available nuclearized facilities such as BR2 (Belgian Research Reactor 2) and Hot Cells for mechanical, microstructural and chemical analysis. A bespoke fusion irradiation module is also foreseen on the MYRRHA reactor, the construction of which is planned in the third phase of the implementation of the project (i.e. after 2035). Provisions have been made in the core of the MYRRHA reactor to include so-called In-Pile Sections around the central core or at the central core position below the spallation target when operating in subcritical mode. In this contribution, we review the role and technical capabilities that MYRRHA facilities may provide in the framework of the European Fusion Electricity Roadmap and specifically for DEMO Engineering Design, Material's Development and enhancement of Technology Readiness Level. Aspects of education and training, being an integral part of the European nuclear fusion program, are also touched upon. Finally, the synchronization of the timelines for the implementation of MYRRHA and the EU fusion facilities are discussed.

Study of the residual nuclei generation in a massive lead target irradiated with 660 MeV protons

A massive lead spallation target was irradiated with 660 MeV protons to generate a secondary neutron field. The field was studied via measurement of the residual nuclei generation in the lead activation samples located inside the target. Experimental results were compared with the Monte Carlo simulations using Geant4 and MCNPX 2.7.0 codes. The comparison shows a general agreement between experiment and simulation and thus serves as a validation of the utilized codes, high-energy nuclear models (ICLXX_HP, BIC_HP, BERT_HP, and INCL4.2) and nuclear data libraries (ENDF/B-VII.1), which can be used for the development of the subcritical accelerator-driven systems in the future.

Thermal-hydraulic study of the LBE-cooled fuel assembly in the MYRRHA reactor: Experiments and simulations

Heavy liquid metals (HLMs), such as lead-bismuth eutectic (LBE) and pure lead are prominent candidate coolants for many advanced systems based on fast neutrons. In particular, LBE is used in the first-of-its-kind MYRRHA fast reactor, to be built in Mol (Belgium), which can be operated either in critical mode or as a sub-critical accelerator-driven system. With a strong focus on safety, key thermal-hydraulic aspects of these systems, such as the proper cooling of fuel assemblies, must be assessed. Considering the complex geometry and low Prandtl number of LBE (Pr∼0.025), this flow scenario is challenging for the models used in Computational Fluid Dynamics (CFD), e.g. for relating the turbulent transport of momentum and heat. Thus, reliable experimental data for the relevant scenario are needed for validation.In this general context, this topic is studied both experimentally and numerically in the framework of the European FP7 project SEARCH (2011–2015). An experimental campaign, including a 19-rod bundle with wire spacers, cooled by LBE is undertaken at KIT. With prototypical geometry and operating conditions, it is intended to evaluate the validity of current empirical correlations for the MYRRHA conditions and, at the same time, to provide validation data for the CFD simulations performed at NRG. The results of one benchmarking case are presented in this work. Moreover, this validated approach is then used for simulating a complete MYRRHA fuel assembly (127 rods).

A preliminary stability analysis of MYRRHA Primary Heat Exchanger two-phase tube bundle

The MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) project, started at SCK·CEN since 1999, aims at the construction of a pool-type sub-critical Accelerator-Driven System (ADS) which could also operate as a critical reactor. The primary system, enclosed in the primary vessel, is filled with Lead Bismuth Eutectic (LBE) which acts as primary coolant. The power is then delivered through four heat exchangers to four secondary loops. The secondary cooling fluid is two-phase water operating at relatively low pressure (16bar). Four aero-condensers act as heat sinks, since MYRRHA design does not foresee any electricity generation.The MYRRHA Primary Heat eXchangers (PHXs) cover a role of fundamental importance in normal operation and accidental conditions, being part of the primary and secondary cooling system and of the Decay Heat Removal (DHR) system. It is thus highly relevant to understand the PHXs behavior under all the potential working conditions.In particular, the stability of the PHXs must be guaranteed under all operating conditions.System code models play an important role in understanding and predicting the behavior of the reactor in all conditions, from steady state to operational and accidental transients, and simulating all the postulated scenarios.A solid PHX design requires a complete assessment of two-phase flow instabilities in the secondary system water tube bundle and the potential implementation of a suitable stabilizing device (orifice) to reduce the impact of the perturbations along the channel. The stability assessment should take in consideration all the possible reactor operational power levels in order to prove the stable behavior under all operational conditions.The tube bundle stability assessment has been carried out by following a similar procedure used for BWR fuel channels, through a specific RELAP5-3D model representing the PHX and able to evaluate the propagation of a density wave in the tube length.A series of suitable boundary conditions, on both primary and secondary side, and perturbation triggers have been foreseen into the model, so to discover all kind of unstable behavior and to dimension the needed orifice to guarantee the flow stability in all operating conditions.The PHX stability analysis is initially performed on the original tube bundle without the adoption of any stabilizing devices, in order to check the natural behavior of the system. The possible adoption and design of an orifice is then conducted on the basis of this preliminary study. The system response against the various types of instabilities, before the introduction of an orifice, is not completely satisfactory: a stable flow is found within certain specific system parameters ranges. After the introduction of a suitable orifice, the system behavior becomes stable under all operating conditions against all types of two-phase flow instabilities.

Comparison of MYRRHA RELAP5 MOD 3.3 and RELAP5-3D Models on Steady State and PLOF Transient

The MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) project aims at the construction of a pool-type subcritical accelerator-driven system that could also operate as a critical reactor. The primary system, enclosed in the primary vessel, is filled with lead-bismuth eutectic (LBE). The secondary cooling fluid is two-phase water operating at relatively low pressure (16 bars). Four aerocondensers act as heat sinks.The code version used for the development of the MYRRHA models is RELAP5 MOD 3.3; this version has been properly modified to allow the use of LBE as a fluid.Since the RELAP5-3D code is already equipped with LBE as working fluid, RELAP5-3D has recently been acquired by SCK•CEN in anticipation of the licensing process.The first important action taken consisted of comparing the two codes by running the existing MYRRHA model input deck, developed for RELAP5 MOD 3.3, on RELAP5-3D.From the steady-state comparative analysis, it appears clear how the two code versions are using different physical models since the steady-state predictions show several differences. Several code issues have been found, mainly about LBE physical properties, initial noncondensable gas computation at LBE free surface level, and LBE heat transfer coefficient correlations.For what concerns the transient analysis, the protected loss-of-flow (PLOF) accident has been taken as reference. Also, in PLOF conditions the mass flow rates and temperature distributions are affected by physical properties and heat transfer model differences.

A method for the continuous monitoring of reactivity in subcritical source-driven systems

The reactivity monitoring in subcritical accelerator-driven systems is a key aspect for the development of this technology. In this work, an inverse method for the determination of the system reactivity from the analysis of flux and power signals, based on the point kinetic approach, is applied to source-driven systems. The features of the algorithm specific to the application to subcritical assemblies are identified, and the sensitivity to the integral parameters characterizing the system is discussed. The technique is applied to different transient situations, simulated by neutronic codes adopting point kinetics and multigroup diffusion, and its accuracy in the presence of localized spatial and spectral phenomena is assessed. Different approaches for the reduction of the uncertainties introduced by the experimental noise are proposed and compared.

MYRRHA: A multipurpose nuclear research facility

MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) is a multipurpose research facility currently being developed at SCK•CEN. MYRRHA is based on the ADS (Accelerator Driven System) concept where a proton accelerator, a spallation target and a subcritical reactor are coupled. MYRRHA will demonstrate the ADS full concept by coupling these three components at a reasonable power level to allow operation feedback.As a flexible irradiation facility, the MYRRHA research facility will be able to work in both critical as subcritical modes. In this way, MYRRHA will allow fuel developments for innovative reactor systems, material developments for GEN IV and fusion reactors, and radioisotope production for medical and industrial applications. MYRRHA will be cooled by lead-bismuth eutectic and will play an important role in the development of the Pb-alloys technology needed for the LFR (Lead Fast Reactor) GEN IV concept. MYRRHA will also contribute to the study of partitioning and transmutation of high-level waste. Transmutation of minor actinides (MA) can be completed in an efficient way in fast neutron spectrum facilities, so both critical reactors and subcritical ADS are potential candidates as dedicated transmutation systems. However critical reactors heavily loaded with fuel containing large amounts of MA pose reactivity control problems, and thus safety problems. A subcritical ADS operates in a flexible and safe manner, even with a core loading containing a high amount of MA leading to a high transmutation rate. In this paper, the most recent developments in the design of the MYRRHA facility are presented.

The accelerator-driven thorium reactor power station

Aker Solutions conceptually designed the accelerator-driven thorium reactor 600 MWe power station, an accelerator-driven, thorium-fuelled, lead-cooled fast reactor. Project objectives were to demonstrate the technical feasibility of the design to ensure a viable product. Aims were to apply established technology where possible, minimising research and development requirements, develop and protect intellectual property and align with Generation IV strategy. A business case demonstrates economic and market potential to stakeholders, and partners are being pursued to take the project through to successful completion. Thorium is an attractive alternative to uranium fuel, being more abundant and avoiding the need for enrichment. Additionally the accelerator-driven thorium reactor can burn waste actinides generated in uranium-fuelled reactors, providing sustainable energy for future civilisation. Choosing a sub-critical accelerator-driven system provides safe operating margins for the thorium fuel cycle. The proposed reactivity coefficient of 0·995 allows selection of an industrial-scale accelerator with commercial benefits which led to a novel solution for measurement and control of reactivity.

Design study of a fast spectrum zero-power reactor dedicated to source driven sub-critical experiments

In the framework of the European P&T program (IFP6-EUROTRANS), the Generation of Uninterrupted Intense NEutrons pulses at the lead VEnus REactor (GUINEVERE) project consists of an Accelerator Driven System (ADS) that is composed by a fast lead simulated-cooled reactor operated in sub-critical conditions, coupled with an updated version of the GENEPI neutron generator previously used for the MUSE experiments. The GUINEVERE facility aims at developing and improving different techniques for the reactivity monitoring of sub-critical ADS’s. As such, the GUINEVERE project will comprise a series of major experiments that will be performed in the near future. The GUINEVERE facility will be located at the VENUS light water moderated research reactor at the SCK-CEN site of Mol (Belgium), which needs to be modified in order to accommodate a completely different and new type of core. A series of constraints were taken into account in the technical design of the GUINEVERE core, in order to properly conjugate the technical feasibility of this facility and the necessity to comply with the envisioned experimental program and its associated scientific outcome. The complete design study of the GUINEVERE core is the subject of this paper. The final design of the fuel assemblies, safety and control rods is provided. Also, the critical core configuration, to be used as reference for absolute reactivity measurements, is presented along with its associated reactor physics parameters, calculated by means of Monte Carlo methodologies. Finally, for licensing purposes, the GUINEVERE facility must satisfy the required nuclear safety criteria of the Belgian safety authorities, and in this paper, an overview of the safety analysis that has been performed with regard to the core physics, thermal assessment and shielding issues is also provided.

New Nuclear Data Libraries for Lead and Bismuth and Their Impact on Accelerator-Driven Systems Design

New ENDF-6 formatted nuclear data libraries are presented for 204,206,207,208Pb and 209Bi, for incident neutrons and protons. Apart from the resonance range, which we have adopted from the best available source in existing libraries, the nuclear data evaluations are completely revised in the 0 to 20 MeV energy range and moreover extend up to 200 MeV. This collection of isotopic evaluations is created by using the nuclear model code TALYS with a consistent set of input parameters for all isotopes. The most important nuclear reaction models and parameters needed for our data files are described. We have intended to make these evaluations complete in their description of reaction channels, and use a consistent method to store the data in ENDF-6 format, which includes cross sections, angular distributions, double-differential spectra, discrete and continuum photon production cross sections, and residual production (activation) cross sections including isomers. It is shown that the data present in our libraries give an improved agreement with existing basic experimental data. Moreover, we have validated the new libraries with criticality and shielding benchmarks, where available. We present the results of neutronics calculations on subcritical accelerator-driven systems to show the impact of our new nuclear data on critical reactor parameters, such as keff, when compared with the existing ENDF/B-VI, JENDL, and JEFF libraries.

Thermophysical Properties of PuO2 and AmO2 Solid Solutions Simulated by Molecular Dynamics

AbstractPuO2 and AmO2 solid solutions: (Pu, Am)O2 are one of the candidates of fuels in a sub-critical accelerator-driven system (ADS). To understand the fuel performance, the thermophysical properties such as thermal conductivity and heat capacity are quite important. However, it is very hard to experimentally determine the physical properties of plutonium as well as americium compounds due to their handling-difficulties. Molecular dynamics (MD) would be a specific method to describe the physical properties of such materials. In the present study, we have investigated thermophysical properties of PuO2, AmO2, and their solid solutions in the temperature range from 300 to 2,500 K. The lattice parameter, compressibility, heat capacity, linear thermal expansion coefficient, and thermal conductivity were evaluated. A Morse-type potential function added to the Busing-Ida type potential was employed as the potential for interatomic interactions. The calculated lattice parameters of (Pu, Am)O2 obeyed Vegard's law, and the values increased with temperature. The heat capacities of (Pu, Am)O2 were similar in any compositions. The thermal conductivities of (Pu, Am)O2 were lower than those of PuO2 and AmO2, indicating that a point-defect scattering effect of phonons could be realized in the MD calculations.

On using alternative perturbation theory methodologies in the evaluation of reactivity effects in ADS cores

Core-average Doppler and coolant void reactivity coefficients, as well as the kinetic parameters ( β eff and Λ), have been determined for sub-critical accelerator-driven systems employing lead–bismuth eutectic (LBE) and helium gas coolants. To determine these parameters use is made of the standard procedure for analyzing critical reactors, which is based on “perturbation-theory” (PT), while in addition two dedicated methodologies for sub-critical systems, i.e. “inhomogeneous perturbation-theory” (IPT) and “heuristically based generalized perturbation-theory” (HGPT), have been employed to compute these parameters in a more rigorous manner. The two methods (PT and IPT/HGPT) are found to give similar results for each application and despite a smaller target k eff-value, the sensitivity of the method is small in the case of the gas-cooled system, thus confirming the adequacy of the standard procedure. As regards the coolant void reactivity coefficient in the gas-cooled ADS, this finding can mostly be attributed to the fact that the core is always transparent with respect to the source neutrons, irrespective of the specific helium content. The sensitivity of the Doppler coefficient is also rather low in the case of the LBE cooled system. However, the dedicated methods are needed for the correct prediction of the coolant void reactivity coefficient, especially if minor actinides are introduced into the core. More important, in this case, is the fact that the PT-approach does not produce conservative results. Finally the sensitivity of the reactivity and kinetic parameters to the different methods is of the same order as that due to uncertainties in nuclear data and therefore these will need to be included in any overall evaluation of the impact of uncertainties on steady-state and transient ADS performance.

Assessment of americium and curium transmutation in magnesia based targets in different spectral zones of an experimental accelerator driven system

The potential to incinerate minor actinides (MA) in a sub-critical accelerator-driven system (ADS) is a subject of study in several countries where nuclear power plants are present. The performance of the MYRRHA experimental ADS, as to the transmutation of Am and Cm in the inert matrix fuel (IMF) samples consisting of 40vol.% (Cm0.1Am0.5Pu0.4)O1.88 fuel and 60vol.% MgO matrix with a density of 6.077gcm−3 in three various spectrum regions, were analysed at the belgian nuclear research centre SCK·CEN. The irradiation period of 810 effective full power days (EFPD) followed by a storage period of 2 years was considered. The ALEPH code system currently under development at SCK·CEN was used to carry out this study. The total amount of MA is shown to decrease in all three considered cases. For Am, the decrease is the largest in the reflector (89% decrease) but at the cost of a net Cm production (92% increase). In the two other positions (inside the core region), 20–30% of Am has disappeared but with a lower production of Cm (between 7% and 11%). In the reflector, a significant build-up of long-lived 245Cm, 246Cm, 247Cm and 248Cm was also observed while the production of these isotopes is 10–1000 times smaller in the core. The reduction of the Pu content is also the highest in the reflector position (41%). In the other positions the incinerated amount of Pu is much smaller: 1–5%.