Belgian Nuclear Research Centre Research Articles

Investigation of neutron irradiated W/CuCrZr joints

This study investigates the effects of neutron irradiation on tungsten (W) and copper-chromium-zirconium (CuCrZr) joints under conditions mimicking the high neutron flux environment of a tokamak fusion reactor. Samples of W/CuCrZr joints were subjected to irradiation in the Belgian Reactor 2 (BR2) nuclear reactor at SCK CEN (Belgian Nuclear Research Centre) to simulate the intense neutron exposure characteristic for International Thermonuclear Experimental Reactor (ITER) and DEMOnstration power plant reactor (DEMO) operations. The primary objective was to evaluate changes in the mechanical properties and microstructure of these materials, which are critical for their potential use in plasma-facing components.It is revealed that a significant reduction in tensile elongation of the joint, indicating some degree of embrittlement, is observed after the irradiation. Importantly, this effect is independent of the irradiation temperature. Possible physical reasons for the observed phenomenon are discussed.

A novel irradiation module for ANICCA fuel cycle code based on multi-task learning

This paper introduces an alternative approach to irradiation modelling within the context of nuclear fuel cycle codes like ANICCA, the fuel cycle code developed by the Belgian Nuclear Research Centre (SCK CEN). The focus lies on upgrading the irradiation module by substituting the CRAM-based depletion calculations for a more flexible and innovative approach based on Multi-Task Learning (MTL). Utilizing data generated with SERPENT2 Monte Carlo simulations, two MTL neural networks are developed to surrogate irradiation processes of Uranium Oxide (UOX) and Mixed Oxide (MOX) fuels, respectively. MTL enables simultaneous learning of the evolution of the inventories for different observables – i.e., transuranium elements, fission products, minor actinides and fertile materials – offering improved predictive capabilities compared to non-MTL neural networks, as demonstrated in the cross-validation tests.Hyperparameters of the models were found using Bayesian optimization, resulting in superior model performance compared to the old CRAM-based model. Verification against SERPENT2, used as a reference code for comparison, demonstrates the stability and accuracy of the MTL-based models, outperforming the original CRAM method for the majority of predicted isotopes.

Performance assessment and further optimization of the activation based criticality dosimetry system at the Belgian nuclear research centre SCK CEN

In places where fissile materials are being handled, the risk of a criticality accident can never be absolutely eliminated. Criticality accidents require specialized dosimetry techniques, which differ markedly from those used in routine radiological protection. This is caused by the fact that one should be able to measure very high doses up to 10 Gy in mixed gamma and neutron fields with very high instantaneous dose rates up to 105 Gy/s and complex energy spectra. The criticality dosimetry system at the Belgian Nuclear Research Centre SCK CEN is accredited according to ISO 17025 by the Belgian accreditation body BELAC and is used to monitor about 4000 nuclear workers. The system is based on a dedicated criticality dosemeter with four activation detectors (gold foils with and without cadmium shielding, an indium foil and sulphur pellets) and an iterative unfolding algorithm starting from a guess neutron fluence energy spectrum. This work describes the design of the SCK CEN criticality dosemeters and the iterative unfolding algorithm. Furthermore, it also discusses two new developments. Firstly, effective neutron reaction cross sections were calculated by MCNP6.2 Monte Carlo radiation transport simulations to be used in the unfolding algorithm. These effective cross sections take into account the effects of the dosemeter, the potential presence of the body and the neutron angle of incidence. These effects were demonstrated to be important, especially for the gold foils and if the dosemeter is worn against the body. Secondly, a Bayesian unfolding algorithm based on a parametric description of the neutron fluence energy spectrum was developed and tested. The results of this new algorithm were in good agreement with the iterative algorithm. The uncertainty assessment was more realistic with the Bayesian algorithm because it includes also the uncertainty associated with the unfolding process. However, for spectra that cannot be well described by the parametric function, the Bayesian algorithm can lead to significant under- or overestimations. In case sufficient information is available, it is better to make use of the iterative algorithm with a realistic guess spectrum to start from. Finally, this work also demonstrated the compliance of the SCK CEN criticality dosimetry system with the IAEA criteria for criticality dosimetry systems.

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.

Validating Geant4 with Monte Carlo simulations in the context of nuclear disarmament verification

In this paper neutron transport simulation results of experimental configurations used during the 2019 IPNDV measurement campaign at the Belgian Nuclear Research Centre SCK CEN in Mol are presented. As these are a good basis for benchmarking simulations, results of the Monte-Carlo simulation codes Geant4, SCALE/KENO-VI, MCNP and openMC are compared with the aim of validating Geant4. With fissile material present as plutonium–uranium mixed oxide, (α,n) emissions are included in the simulations. A Geant4 extension to calculate neutron multiplication factors keff was developed and applied to the IPNDV configurations and a set of OECD/NEA benchmark experiments. Neutron fluxes of the IPNDV configurations through a reference volume are simulated. The Geant4 (α,n) simulation toolkit SaG4n is compared to the SCALE code ORIGEN and small deviations are observed. Geant4 calculations of keff for criticality benchmark experiments deviate in the mean by ＋0.4%, for MCNP and openMC deviations are ≤6%, for KENO-VI ≤8%. Geant4’s total neutron fluxes through a reference volume next to the IPNDV configurations agree within a margin of ±5% with MCNP and openMC and −14 % with KENO-VI. Considerable differences between all codes were observed for thermal neutron scattering but Geant4 results are still within the other codes’ variability. Differences to Geant4 for epithermal energies can be addressed to a not yet included “Doppler Broadening Rejection Correction”. By comparing Geant4 simulations of a complex set-up with various reference codes and demonstrating its good performance as well as some discrepancies, this study contributes to the validation of Geant4 neutron physics in fissile material systems and for nuclear disarmament verification simulations.

Loading Rate of Fracture Toughness Tests in Transition Regime

Abstract ASTM E1921-21, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, contains a limitation on the stress intensity factor loading rate (K˙) for the quasistatic loading. The specimens should be loaded at a rate such that K˙ during the initial elastic portion is between 0.1 and 2 MPa√m/s. However, based on recent improvements in the test technique, it should be possible to narrow the loading rate range in order to reduce variability and have more comparable results between laboratories. For all users to be able to achieve the reduced range, clear guidelines are needed, which are not yet available. The Belgian Nuclear Research Centre (SCK CEN) has extensive experience in fracture toughness testing and has constructed a large database on which statistical studies were performed. This paper illustrates, using the SCK CEN database, that Wallin’s theoretical displacement formulas can only be used in the case in which the compliance of the testing setup is taken into account. An improved equation is proposed, guidelines for implementing the equation are drawn up, and these are verified using the database. It is clear that the methodology is sufficiently robust to provide an average loading rate within the suggested range of 0.5 and 2 MPa√m/s. This methodology could thus be integrated into the testing standard ASTM E1921-21.

Changing face-to-face to online radiation protection training: experiences of trainers and trainees.

The SCK CEN Academy for Nuclear Science and Technology functions as the umbrella structure coordinating the education and training activities of the Belgian Nuclear Research Centre (SCK CEN). One of the main activities of the SCK CEN Academy is to provide customised training to professionals working in the nuclear industry, healthcare, research or governmental institutions. The courses and practical sessions are usually given in a face-to-face (FTF) format. This has drastically changed in the last 2y because of the COVID-19 pandemic when it was necessary to switch course delivery from FTF to an online format. Feedback was collected from trainees and trainers in different training courses in radiation protection provided in FTF and online format. The analysis of this feedback allows training providers to make better-informed decisions on the best-suited training format depending on the content, target audience and the duration of the learning activity.

Fusion-dedicated material irradiation facilities at MYRRHA: Conceptual design and damage equivalence studies

The Belgian Nuclear Research Centre is currently developing MYRRHA - a subcritical accelerator driven system ignited by a 600 MeV superconducting linear proton accelerator, cooled with an eutectic lead bismuth. The implementation of MYRRHA will take place in phases, where phase I, covering 2019–2027, includes the deployment of the 100 MeV proton accelerator and beam-added research facilities. Development of the dedicated facilities to enable irradiation of fusion materials and components is in the portfolio of this R&D block. The baseline concept of the 100 MeV fusion-dedicated facilities has been presented for the first time in 2019 and in this contribution, we extend its description and provide main elements of the conceptual design of the irradiation installations, preliminary configuration of the hot cell facilities (required to load/unload the irradiation modules), information on the irradiation envelope. In addition, we summarize the results of the computational study performed to investigate the equivalence (in terms of damage rate, nuclear transmutation, temperatures and expected irradiation-induced microstructure) of the irradiation conditions at MYRRHA facilities in comparison with other material test facilities such as IFMIF and ESS, and in DEMO as well as in the mixed-spectrum material test reactors. The aspects related to miniaturization of the sample geometries are also discussed to substantiate the feasibility and relevance of this facility for fusion material's R&D programme.

The construction of the HADES underground research laboratory and its role in the development of the Belgian concept of a deep geological repository

Abstract When the Belgian Nuclear Research Centre (SCK CEN) launched a research, development and demonstration programme on geological disposal in the 1970s, it was not certain if a deep geological repository could be constructed in poorly indurated clay. This was tested by constructing the HADES underground research laboratory (URL) in Boom Clay, 225 m below SCK CEN's site in Mol. The construction history of the URL reflects how the understanding of the Boom Clay increased and how the excavation techniques and design of the gallery lining improved. It demonstrated that shafts, galleries and crossings between galleries can be constructed using industrial techniques. It also allowed characterization of the hydromechanical response of the clay and the clay disturbances induced by the excavation. This increased understanding is also reflected in the evolution of the geological disposal concept considered in Belgium. The current disposal concept foresees the installation of seals in the shafts and galleries. The HADES URL also offered the opportunity to test possible seal designs and develop a better understanding of the behaviour of bentonite, a possible seal material, owing to its swelling capacity, under in situ conditions.

Making Belgian Big Science

This article explores the history of the MYRRHA research reactor currently under construction at the Belgian Nuclear Research Centre (SCK CEN). The article demonstrates that the construction of large instruments has obtained an essential role in the moral economy of modern nuclear laboratories. First, an analysis on the internal discussion within the research center reveals how MYRRHA’s transformation from a commercially oriented project into a transmutation machine was primarily characterized by the wish to upscale the project. Second, a focus on the European research landscape demonstrates how European collaboration led to further upscaling of the project, having to address technological requirements to fit into the European roadmaps. Third, an emphasis on the interaction between MYRRHA and the Belgian political and public sphere shows how MYRRHA was constructed in a narrative that made the project seem essential to the future of the nuclear research center. Based on archival material and background interviews with key players, this article contributes to our understanding of the development of Big Science in the post–Cold War era in western Europe.

Study of Reactor Core Loading Monitoring at the GUINEVERE Facility

The potential use of a pulsed neutron source (PNS) to measure reactivity during nuclear fuel loading as a means to prevent core loading errors has been studied at the GUINEVERE facility. This facility couples the deuteron accelerator GENEPI-3C to the fast neutron subcritical reactor VENUS-F at the Belgian Nuclear Research Center SCK·CEN. The 14-MeV neutrons are produced in the reactor core center via fusion reactions. PNS experiments were performed in five reactor configurations corresponding to the different loading steps of VENUS-F. The evolution of the neutron flux during these PNS experiments was measured by several 235U fission chambers in various positions in the inner and outer reflector and analyzed using the area-ratio method. The results show that, despite strong spatial effects, a strong correlation between the reactivity values given by the area-ratio method and some reference reactivity values remains throughout the reactor unloading. Monte Carlo simulations were first validated by comparison with the data and then used to investigate the sensitivity of the method to a core loading error. First results show that some loading errors could be experimentally detected using a PNS.

Transforming Big Science in Belgium: Management Consultants and the Reorganization of the Belgian Nuclear Research Centre (SCK CEN), 1980–1990

This article analyses the relationship between the Belgian government and the national Nuclear Research Centre (SCK CEN) in the renegotiation of the mission and organization of Big Science in Belgium. While the founding decades of nuclear laboratories are often characterized by ever-increasing budgets and the establishment of large infrastructure, I show that downsizing or transforming Big Science demanded a new form of politics on the organization of science. Drawing on archival material, this article will demonstrate how the mission and management of SCK CEN were critically followed by a variety of stakeholders. Furthermore, the Belgian government explicitly aimed to allow corporate principles to guide the reorganization of the research centre by hiring consultants from Arthur Andersen. This episode in the history of nuclear research invites historians of science to reflect upon both the politics of Big Science in an era in which its original principles were fundamentally questioned and the historical development of new norms and expectations attached to this phenomenon.

New approaches for interlaboratory comparisons analysis using dark uncertainty applied to radioactive materials

In order to further improve the management of contaminated materials in nuclear facilities subject to a decommissioning programme, as well as during post-accidental site remediation and clearance, the definition and selection of the most appropriate intervention scenarios producing well-characterized radioactive waste for which storage and disposal routes are clearly identified is needed.As a step towards this accomplishment, we propose a methodology for the organization and analysis of coordinated interlaboratory comparisons (ILC) for the performance assessment and the uncertainty evaluation of available measurement techniques (methods and tools) of radioactive materials. This methodology is new for this type of comparison and demonstrated on the BR3 (Belgian Reactor 3, Belgian Nuclear Research Centre, Mol) case study from the H2020 INSIDER project (2017–2021), for which barium 133, cobalt 60 and europium 152 are analysed with gamma spectroscopy in ILC, based either on irradiated concrete from the BR3 bioshield or from spiked concrete certified reference material (CRM).On one hand, we show the advantage of organizing ILC on CRM for a more reliable uncertainty evaluation taking bias into account following ISO 21748:2017. But using CRM may be impossible due to their scarcity or too costly for performance assessment thus limiting the use of CRM in ILC in practice.On the other hand, we show that for performance evaluation and monitoring, ILC can be alternately performed on reference materials provided that laboratories’ uncertainties are reported and the most appropriate analysis of data is performed using dark uncertainty (excess variance) in the presence of inconsistent data.

Hunting for the correct pressure drop in a scaled reactor pool: Effect of geometry, mesh resolution, turbulence model and mass flow

The E-SCAPE (European SCAled Pool Experiment) facility at the Belgian nuclear research center SCK•CEN is a thermal hydraulic 1/6th-scale model of the MYRRHA reactor with an electrical core simulator and cooled by LBE. The E-SCAPE facility provides experimental feedback to the designers on the forced and natural flow patterns, flow mixing and stratification during operational and accidental conditions. In addition, it provides valuable data for validation of the computational techniques for their use with LBE and in the design of MYRRHA. In terms of momentum field, one of the most important integral quantity to reproduce is the pressure drop in the system which has a direct consequence on the evolution of the mass flows and determines the free-surface levels in transient scenarios. In this paper, a detailed CFD study is performed on the flow in the E-SCAPE core region with the aim on predicting the correct pressure drop. Different numerical representation of the core geometry have been modeled and subjected to validation against experimental results from E-SCAPE. Special attention has been addressed to the choice of geometric simplifications and numerical models for accurate representation of the core of the E-SCAPE pool facility.

Comparing the effectiveness of learning formats in radiation protection

At the Belgian Nuclear Research Centre (SCK CEN) more than 850 employees advance the peaceful applications of ionising radiation through research, services and education and training. Several initiatives are taken to guarantee a safe working environment, one of them being a compulsory introductory safety training for all new employees, followed by refresher courses. With the objective to increase the flexibility for the participants and to optimise the teaching time of lecturers, the training format is adapted from face-to-face training to blended learning in which a significant amount of online learning is introduced. To appraise the perception and effectiveness of the original face-to-face training and the newly developed e-learning modules, and to compare both, information is gathered on two levels. Firstly, to monitor the perception of the participants regarding the face-to-face and e-learning formats an online questionnaire was launched. Secondly to evaluate the knowledge gain (difference between pre- and post-test results), the participants were requested to answer the same set of technical/scientific questions before and after the training. Results of the perception study show that the e-learning format was well received by the learners. For both learning formats an overall knowledge gain was noticed. The highest knowledge gain was noticed for the e-learning format. The results on perception and effectivity of the e-learning format confirm the further use of this format.

Microstructural Changes and Chemical Analysis of Fission Products in Irradiated Uranium-7 wt.% Molybdenum Metallic Fuel Using Atom Probe Tomography

Understanding the microstructural and phase changes occurring during irradiation and their impact on metallic fuel behavior is integral to research and development of nuclear fuel programs. This paper reports systematic analysis of as-fabricated and irradiated low-enriched U-Mo (uranium-molybdenum metal alloy) fuel using atom probe tomography (APT). This study is carried out on U-7 wt.% Mo fuel particles coated with a ZrN layer contained within an Al matrix during irradiation. The dispersion fuel plates from which the fuel samples were extracted are irradiated at Belgian Nuclear Research Centre (SCK CEN) with burn-up of 52% and 66% in the framework of the SELENIUM (Surface Engineering of Low ENrIched Uranium-Molybdenum) project. The APT studies on U-Mo particles from as-fabricated fuel plates enriched to 19.8% revealed predominantly γ-phase U-Mo, along with a network of the cell boundary decorated with α-U, γ’-U2Mo, and UC precipitates along the grain boundaries. The corresponding APT characterization of irradiated fuel samples showed formation of fission gas bubbles enriched with solid fission products. The intermediate burnup sample showed a uniform distribution of the typical bubble superlattice with a radius of 2 nm arranged in a regular lattice, while the high burnup sample showed a non-uniform distribution of bubbles in grain-refined regions. There was no evidence of remnant α-U, γ’-U2Mo, and UC phases in the irradiated U-7 wt.% Mo samples.

Guidelines for first responders based on results from deploying a mockup radiological dispersal device

During the past 7th Security Framework Program the European Commission funded a research project called CATO (CBRN Crisis management, Architectures, Technologies and Operational procedures) to develop a prototype decision support system for crisis management in addition to providing a suite of guidelines for first responders and incident commanders when dealing with chemical, biological, radiological or nuclear incidents. In order to derive these guidelines a proof-of-concept experiment was setup during which several passive agent (Stable CsCl) dispersions with improvised explosive devices and vehicle-borne improvised explosive devices were carried out. Each dispersion was thoroughly characterised by a number of monitoring devices, including high-volume air samplers and size-segregated air samplers. All environmental and forensic samples were collected by the UK counter terrorism police, following strict labelling and chain-of-custody protocols. The samples were analysed at the Belgian Nuclear Research Center suing the k0 method for instrumental neutron activation technique. A full consequence assessment analysis was carried out assuming that the observed concentration of Cs-133 in samples was Cs-137 instead and use was made of the specific activity of Cs-137. Due to the sensitivity of the information the European Commission classified this research. The resulted reported on in this work have been unclassified and are released to assist emergency planners and first responders to take the necessary precautions. The results indicate that, up to distances of 50 m from ground zero radiation levels will be considerable and therefore live-saving actions must be performed by fire/rescue wearing full protective gear. In addition, low-wind conditions will favor a long airborne residence time and therefore the use of full-face protective gear is a must. In order to protect first responders, a radiation protection specialist is to determine how long people can enter and remain in the contaminated area. The recovery of evidence in the case of a car-bomb will be hard or even impossible due to the high level of radioactive material remaining inside the vehicle.

Validation of CFD analyses against pool experiments ESCAPE

MYRRHA is a pool-type research reactor under design at the Belgian nuclear research center SCK·CEN. The multi-purpose fast-spectrum MYRRHA reactor is cooled with lead–bismuth eutectic (LBE). ESCAPE is a thermal hydraulic 1/6th-scale model of the MYRRHA reactor developed within the European framework programs THINS and MYRTE.The experiments from ESCAPE are crucial for the design and licensing of MYRRHA. First of all, these experiments can provide information on the flow patterns and heat transport in the different compartments of the reactor. Secondly, the data from the ESCAPE experiments can be used for qualification of thermal hydraulic computer codes that will be applied for analyzing the flow and heat transport in the MYRRHA reactor. Considering the rare and typical conditions in heavy liquid cooled nuclear reactors, thorough validation of these codes against relevant experimental data is a prerequisite before application.This paper presents the approach followed by NRG to model the flow and heat transport in ESCAPE using the Computational Fluid Dynamics (CFD) code STAR-CCM+. CFD analyses for different forced flow conditions are presented and compared with experimental results. The predicted temperature distribution in the pool, core and plena agrees well with the temperature measurements from ESCAPE.

Fusion target station on MYRRHA facility: Baseline concept

The Belgian Nuclear Research Centre SCK CEN launched the 1st phase implementation of the MYRRHA reactor. This will deploy the construction of 100 MeV proton accelerator and complimentary research facilities including a dedicated station for irradiation of fusion materials (Fusion Target Station - FTS). This paper presents a baseline pre-conceptual design and analysis of the irradiation conditions for the most common in-vessel fusion materials i.e. tungsten and Eurofer97 steel. FTS will allow a direct irradiation by protons and/or irradiation via spallation neutron source. The former concept requires application of small specimen test techniques (SSTT), while the latter enable large irradiation volume with a limit damage rates.

Nondestructive Investigation of Neutron Irradiation Generated Structural Changes of Reactor Steel Material by Magnetic Hysteresis Method

The neutron irradiation embrittlement of four different types of nuclear pressure vessel materials (three base metals and one weld material) were investigated by a magnetic nondestructive testing method, magnetic adaptive testing (MAT). The method is based on the measurement of minor magnetic hysteresis loops on Charpy specimens irradiated by neutrons in the BR2 reactor. Due to the neutron irradiation, the structure of the material was modified. The Charpy impact method is suitable for destructive characterization of material embrittlement. The results of Charpy impact test measurements at SCK CEN Belgian Nuclear Research Centre were compared with the nondestructively measured magnetic parameters. A definite correlation was found between magnetic descriptors and the ductile-to-brittle transition temperature (DBTT), regardless of the type of material or irradiation condition. The results suggest that this “calibration curve“ can be used to estimate the DBTT from non-destructive measurements.