Materials for fusion

The large amount of radiation that will be produced in future fusion reactors will activate the materials that form it, degrading their macroscopic properties. It is essential to know the effect of radiation on materials in order to minimize such effects to guarantee a longer useful life, as well as safety in fusion machines.

The LNF – its Fusion Materials Unit – has been focused on the study and characterization of insulating and structural materials for more than 30 years, emphasizing the effects of radiation. It has carried out works on insulating materials for diagnostics;  development of new materials for application in fusion, microelectronics and telecommunications; studies of the behavior of materials of interest for fusion under the effects of irradiation (combining light ions and radiation); fluid dynamics of liquid metals with application in the IFMIF and TechnoFusión facilities; as well as simulation of the effect of neutron radiation under ionic irradiation.

The main lines of research that the Fusion Materials Unit is currently developing are focused on:

Structural Materials

Stress-strain test in a fusión steel sample.

Radiation resistant steels are going to form the structure of the reactor’s vessel and their specific parts (such as the breeding blankets or the divertor). Concerning structural materials the most damaging elements coming from the plasma will be the neutrons; these materials will be placed just behind the plasma facing components and thus will barely feel interactions with charged particles which would collide directly to the walls of the reactor’s chamber. The impurities and defects, created by neutrons through inelastic and elastic collisions, will evolve with dose and temperature and will cause damage in the microstructure leading to embrittlement, swelling, modification of its phase stability, reduction of its resistance and enhancement of corrosion.

Fusion Materials Unit is devoted to microstructural characterization –new phase precipitation, grain size, secondary phase precipitation, TEM studies –and mechanical—not only macro- but also micro-mechanical (by in-situ essays in SEM microscope) of new steels candidates for DEMO in both conditions: as-received and after irradiation (light and heavy ion implantations). Also corrosion and hydrogen isotopes permeation studies are performed. LNF-CIEMAT coordinates European tasks related to development and characterization of structural materials and enhancing the participation of other research centers such as CEIT or KIT, and the universities C3M, URJC and UPM.

Functional Materials – Insulators

Damaged insulator surface after He implantation.

Functional materials will be used in the diagnostics and in the heating and current drive systems (H&CD) of the reactors and will act as insulators, optical fibers, windows, mirrors or lenses. These materials will feel the effect of both the neutrons –which will cause mainly displacement damage and transmutation—and the gamma radiation –ionizing radiation which will be present both during operation as well as after shutdown. The gamma radiation produces excitation of the electronic charge states in the material which can lead to defect formation. The accumulated damage together with the presence of ionizing radiation affects the optical and dielectric properties of functional materials. On one hand it will induce light emission (radioluminescence) and loss of optical transmission in windows, fibre materials or mirrors. On the other hand it will affect the quality of dielectrics and mineral insulated coaxial cables by generating radiation induced conductivity (RIC), radiation induced electrical degradation (RIED), surface degradation or radiation induced voltages (RIEMF).

The Fusion Materials Unit has expertise in the study of such phenomena and has helped to identify which materials are most suitable to be used in a fusion reactor. The work is made in active partnership with the industry to reach quality standards in the manufacturing of insulators and windows, supporting the design of ITER and DEMO diagnostics. All activities of characterization and radiation damage (gamma, electrons, neutrons) are in collaboration with other European laboratories.

Functional materials – Li ceramics

Li ceramic pebbles.

Lithium ceramics are one of the potential candidates for the breeder blanket, which will be in charge of the self-production of tritium in the future fusion reactors. As well as the other potential candidate for the breeder (the LiPb), they will be placed just behind the reactor’s first wall and therefore the fusion neutrons will be able to transmute the lithium in tritium. These materials because of its position and function will be affected by the neutron damage as well as by the ionizing radiation. The neutrons will not only be responsible of the Li transmutation but also will produce high amount of defects in the microstructure of the ceramics. Also the gamma radiation will be able to vary its electronic structure. The study of the effects of radiation damage which is conducted in our laboratories with different techniques is very important in order to establish the tritium desorption rate in the ceramic matrixes.

The Fusion Materials Unit makes detailed studies on the effect of radiation (ions, electrons and gammas) on the thermal desorption of light ions in the solid breeder blanket candidates for ITER. Such activities are performed in collaboration with KIT and within the framework of EUROfusion project.

Functional materials – Simulations

Modeling of the damage due to a displacement cascade in Fe.

Computational modelling methods can supply an alternative system for testing neutron damage on materials. Modelling of materials would help to understand damage mechanisms in materials under fusion conditions, being able to predict the evolution of these materials in different conditions and thus significantly reducing the number of time-consuming laboratory experiments required to test a new component, resulting in faster development of new materials. In the short-term it is unrealistic to expect that the physical approach can produce fully predictive models, quantitatively valid for direct technological applications, i.e. for real materials such as steels. Nonetheless, advances in multiscale modelling can help in the medium-term.

The Fusion Materials Unit use and develop different modelling tools, such as MD, BCA codes (MARLOWE), OKMC and Rate Theory to study fundamental effects of radiation in fusion materials from the primary damage to its evolution with time, dose and temperature. To support the modelling the Unit has developed experimental techniques –Resistivity Recovery and He desorption– to provide experimental validation of the modelling. These activities take part of the EUROfusion materials program and are also co-supported by the National Plan and the regional funding project TechnoFusión.

The main equipment and facilities that the Materials Unit has are:

Electron Accelerator

Van de Graaff accelerator and experimental beam line at LNF.

This 2 MeV electron Van de Graaff Accelerator allows material irradiation either by electron beam or by Bremsstrahlung induced by stopping the electron beam. In this way, radiation testing that is normally carried out using a Co-60 source can be undertaken more rapidly (producing a larger and better controlled dose rate) while allowing in-situ measurements. Irradiation parameters (temperature, vacuum pressure, gas environment, dose rate and beam energy) are well controlled. Moreover, irradiation of relatively large components or material samples is possible. The accelerator staff can design and develop different irradiation chambers and experimental set-ups depending on irradiation requirements. Such experimental systems permit performing optical, electrical and dielectrical measurements during irradiation (“in-beam”). This makes it a unique experimental radiation facility in which simultaneous optical, electrical and dielectrical measurements can be made in the range of Hz to GHz. For this, systems to measure optical absorption and radioluminescence, electrical conductivity and dielectric properties during irradiation (in-situ) are mounted on the accelerator beam line.

Beam characteristics;

Energy: 0.25 to 2.0 MeV

  • Current: 10 pA to 150 µA
  • Samples from ≈ 3 mm2 to about 20×20 cm2
  • At target area unfocussed beam is ≈ 1 cm diameter
  • Beam can be focussed up to ≈ 1 mm diameter (for small samples)
  • Beam can be defocussed up to ≈ 3 cm diameter
  • Beam can be scanned over 20×20 cm2 (for large samples)

The facility allows in-beam testing at a controlled temperature of the electrical, dielectric (RF), and optical properties of solid and gas insulators. Irradiation can be performed in high vacuum, air, or controlled atmospheres (such as N or He).

Flexibility. The facility is extremely flexible and has several unique in-beam systems for measuring electrical conductivity, dielectric loss and permittivity (Hz to GHz), and optical absorption and emission during irradiation over a wide range of dose rates and temperatures. Irradiations can be performed in high vacuum, air, or controlled atmosphere such as N or He. Simulation in electron accelerators offers important advantages, namely easy experimental parameter control and high dose rates available up to ≈105 Gy/s.

In-situ measurement capability and expertise. To date a range of studies have been carried out on fusion candidate insulators for which low displacement per atom (dpa) rates are required. For these studies typical dpa rates range from about 10-12 to 10-8 dpa/s while ionization rates (Bremsstrahlung or direct electron irradiation) up to ≈ 104 Gy/s. For instance, electrical, optical as well as hydrogen and helium diffusion properties are measured during irradiation at a controlled temperature, from liquid nitrogen up to 1000 C. Special irradiation chambers and sample holders are designed by the accelerator staff and they are fabricated at the CIEMAT workshops.

Ion Implanter

This facility is a 60 keV Danfysik ion implanter that allows ion implantation like helium, hydrogen and deuterium ions between others, for studies related with fusion research. The implanter staff have designed and developed different irradiation chambers and experimental set-ups depending on the study requirements. The developed experimental systems permit in-situ optical, electrical and desorption measurements. For example simultaneous ionoluminescence and surface electrical conductivity measurements can be made thus allowing correlation between macroscopic material degradation and defects produced by implantation.

The facility has been used regularly to implant H, He, D ions in metals and insulators in order to evaluate microstructural surface degradation, ionoluminescence, surface electrical degradation and to implant He and H isotopes to perform diffusion and desorption experiments. These studies can be carried out at controlled temperatures, from liquid nitrogen up to 1000 C. Available special irradiation chambers and sample holders have been designed by the implanter staff and fabricated at the CIEMAT workshops.

Beam characteristics:

  • Beam energy: up to 60 keV.
  • Beam current: up to 150 µA depending on ion.
  • Sample size: from ≈ 3 mm2 to about 4×2 cm2
  • Unfocused beam diameter at target: ≈ 1 cm

In-beam technique

In order to characterize candidate materials with possible applications in nuclear fusion reactors, a new instrument to undertake in-situ magnetometry studies is under development at IMDEA for the CIEMAT ion implanter. This is being done within the framework of a collaboration between the Spanish National Fusion Laboratory and Advanced Magneto-Optics and Permanent  Magnets divisions of the Nanomagnetism Program at IMDEA Nanociencia (this work is partially financed by project ENE2016-76755-R, MINECO/FEDER). The new system is based on a Variable Temperature/Full Angular Range/Vectorial MOKE magnetometer developed previously (J.L.F. Cuñado, et al. Review of Scientific Instruments 86 (4), 046109) at IMDEA. Based on this instrument a portable prototype is now under construction and it will be directly located at CIEMAT ion implanter.

In parallel, a magnetic enclosure, to enhance magnetic flux, has been designed and built. It will allow optical access to the aforementioned magnetometer when carrying out irradiation experiments under strong magnetic fields and at high temperatures on the ion implanter. This development has been undertaken in collaboration together with Ingeniería Magnética Aplicada, (IMA, Ripollés, Barcelona), a company dedicated to research and manufacture of permanent magnets.

Magnetic box designed in collaboration with IMA (Barcelona)

The collaboration also has access to other material characterization techniques at IMDEA Nanociencia, such as Kerr microscopy which is used to explore the effect of damage in magnetic domains of FeCr alloys.

NAYADE: Co60 γ-source

For gamma ray irradiation, CIEMAT has a Co-60 facility with unlimited access. Irradiations can be carried out for long periods. La Nayade is a “pool” type facility with water as a biological shield. It consists of a 1.2 m side by 4.5 m deep pool that provides enough biological shielding for about 100,000 Ci of Co-60. The pool has the necessary equipment and systems to guarantee safety control through water level measurements, radiation detectors, and water purity control through pH and conductivity measurements. The use of water as a biological shield allows a direct view of the bottom of the pool where the radiation is produced, at the same time that it facilitates the movement and positioning of the sources and the extraction of samples in the different devices.

SIMS / SNMS elementary analysis equipment in solids

SIMS / SNMS equipment from Hiden SIMS Foundation SIMS / SNMS Workstation

The Fusion Materials Unit has a SIMS/SNMS equipment from the Hiden SIMS Foundation SIMS / SNMS Workstation. It was acquired in 2012 and financed within the framework of CEI Moncloa and co-financed with Feder funds (MINECO_CIEMAT Agreement). This equipment allows the analysis of the samples by erosion in layers of their surface by the action of a focused ion beam, analyzing the ion gas formed during the beam-surface interaction with a quadrupole mass analyzer. It has the advantage of direct determination of the elemental composition of solids on the surface and in depth (3D), and allows a routine analysis of the elements in the entire Z range (better A) including light elements.

This equipment, together with the Profilometer (Bruker Dektak XT) financed with funds from the TechnoFusión Program, allows to carry out spectral analysis of impurities in materials and depth profiles (such as samples implanted with D and He), analyzing the depth of the craters generated, with very high and promising results.

SEM+ FIB double beam equipment (electrons and ions)

SEM + FIB double beam equipment Zeiss Auriga
SEM image of a lamella obtained in this equipment

The Fusion Materials Unit has a SEM + FIB double beam equipment from Zeiss Auriga, financed by CIEMAT through Resolution 11/274 of MINECO. This equipment allows the processing and study of materials at the micro and nanoscale. The double beam microscope integrates the capabilities of a field emission scanning electron microscope (FESEM) with a focused gallium ion microscope (FIB). A FIB has a very high degree of analogy with an SEM, however, instead of electrons it uses an ion beam that can also be focused and controlled and this effect that can be used to modify the structure of the specimen on a nanometric scale. With this equipment, high resolution SEM images can be obtained at the same time as making modifications to the sample with the ion beam. To date, the following activities have been carried out with this equipment: i) microstructural or topographic studies on the surface, using SEM, as well as the evaluation of the microstructure in cross-section (using the FIB tool); ii) STEM analysis of thin samples, iii) Preparation of lamella by FIB for microstructural analysis by TEM and iv) Composition studies using the EDX technique (in SiC subjected to heat treatments, of corrosion layers by Li in Eurofer steel, …) .

Mechanical testing machine

Mechanical testing machine

The Fusion Materials Unit is equipped with a 100KN MTS brand servo-hydraulic mechanical testing machine, model 310. It consists of two load cells of 100 and 15KN, which gives it great versatility since it allows testing a wide range of materials with different sizes of specimen geometry. As a servo-hydraulic machine, it has the option of conducting tests under static and dynamic conditions. In it, the properties of traction, fatigue at low and high number of cycles, fracture mechanics, creep-fatigue, crack growth and compression can be determined. The properties can be determined in a wide range of temperatures, since a climatic chamber is available to carry out tests between -130ºC and 315ºC, and an oven that allows the study of properties up to 1000ºC. In addition, it has accessories such as a high temperature extensometer for axial deformation control, two COD-type extensometers for fracture mechanics tests or a video extensometer.

All the control of the machine is carried out by means of digital control in whose testing software all the ASTM standards are included.

As a complement to this equipment, there is also a resonance machine for conducting tests under dynamic loads of the RUMUL brand, to carry out the pre-cracking of specimens that will later be tested in fracture mechanics. In this equipment it is also possible to study the growth rate of fatigue cracks using the potential drop technique.

Sample Preparation Laboratory

The Fusion Materials Unit has a Sample Preparation Laboratory (LPM), which includes a complete set of equipment for the manufacture, processing and preparation of ceramic and metallic samples of materials with applications in Fusion. They stand out among them:

Metallographic cutting equipment:

  • 2 manual cutters (Marce Buehler, Isomet; sample size 25×65 mm)
  • 1 automatic cutter (Marce Struers, Accutom-50; sample size 55×42 mm)

Roughing / polishing equipment

  • 3 automatic polishers (Logitech polisher for plane parallelism (± 10 microns) Logitech; one Buehler brand AutoMet 250; one Struers brand RotoPol-35 RotoForce-4)
  • 1 manual polisher
  • Electrolytic polishing machine (Electrolytic polishing machine, Struers brand model LetroPol-5)
  • Machine for the preparation of fine samples suitable to be observed by TEM (Transmission Electron Microscopy) Struers model TenuPol-5

 General equipment

  • 2 Carbolite brand tube furnaces with adjustable temperature (up to 1000º and up to 1200ºC)
  • 1 Carbolite brand controlled atmosphere furnace (up to 1700º)
  • 1 Eurotherm brand high temperature furnace (up to 1700ºC)
  • 1 Muffle up to 1200º brand Thermoconcept
  • 1 stove (300ºC) Binder brand
  • 1 Sartorius brand precision balance (4 decimal places)
  • 1 granate
  • Extractor hood
  • Heating plate

Auxiliary equipment

  • An optical microscope for polishing control

In addition, the Sample Preparation Laboratory has implemented the 9001 quality system (PT-DTF-15)