Development of anti-permeation and corrosion barrier coatings for the WCLL breeding blanket of the European DEMO

doi:10.1016/j.fusengdes.2021.112453

Fusion Engineering and Design

Volume 170, September 2021, 112453

https://doi.org/10.1016/j.fusengdes.2021.112453 Get rights and content

Abstract

Tritium permeation from breeder material to the Water Coolant System (WCS) in Water Cooled Lithium Lead (WCLL) Breeding Blanket (BB) is one of the technological issues to be solved in the design of the European DEMO. Since the tritium extraction from the Water Coolant System is more challenging and expensive than the extraction from the eutectic alloy PbLi, it is mandatory to use of a protective coating on the blanket wall to minimize the permeation rate. Moreover, a protective coating can prevent the corrosion of EUROFER steel by the action of PbLi.

alumina-based coatings are considered as reference for barriers thanks to their good chemical compatibility with the PbLi alloy and their capability to reduce permeation. Three coating technologies were selected in the frame of the EUROfusion project: electrochemical ECX (chemical deposition) process, Pulsed Laser Deposition (PLD) and Atomic Layer Deposition (ALD) coating. The coatings were developed and optimized in order to satisfy the design requirements of good mechanical compatibility with steels, strong adhesion, corrosion compatibility in PbLi at relevant BB design conditions and a Permeation Reduction Factor at least of 200 under neutron irradiation. The present paper aims to describe the status of the technologies and the main results obtained.

The final objectives of the R&D activities are to demonstrate the applicability of the coating to WCLL BB and therefore the scale-up of the technologies from laboratory scale to the BB scale.

Introduction

In the Water Cooled Lithium Lead (WCLL) Breeder Blanket (BB) the permeation of tritium from the eutectic alloy PbLi, the breeder, to the water coolant system is one of the critical issues to be managed. The possibility to realise a coating on the water pipes, manufactured in EUROFER, at the PbLi interface can mitigate several major risks and improve considerably the WCLL performances. An effective barrier should decrease the permeation of a factor at least of 200, reducing the total inventory of T in the water cooling that at the moment is estimated of around 150 g [1]. The current analyses and calculations are indicating that the use of coating layers (at least at lower performance, a Permeation Reduction Factor higher than 200), will be mandatory for the operation of the WCLL concept in DEMO. The availability of these barriers at higher Permeation Reduction Factor, defined as PRF the ratio of the flux through an uncoated pipe over the flux through a coated one, can largely increase the performances and safety margins of this concept so that the development of this technology is posed at a high level of priority in the R&D plan. A protective coating can act also as anticorrosion barrier, the results about the corrosion behaviour of the EUROFER in PbLi have shown a strong dissolution of the steel by the liquid metal [1]. The corrosion phenomenology is linked to the dissolution of alloying elements in PbLi, where the separation of atoms from the solid matrix into the liquid occurs due to the difference between the chemical activity (chemical potential) in the solid and in the liquid of a particular steel component. The corrosion attack is (besides impurity levels) a function of the temperature and the velocity of the PbLi at the interface. In WCLL BB the PbLi corrosion rate is estimated lower than 1.0 μm/yr [2] due to low PbLi velocity and temperature profile at the interface with EUROFER [3]. However, due to the total PbLi and EUROFER interface area, about 40,000 m², the total amount of dissolved (and activated) corrosion products in all BBs modules is higher than 100 kg, high enough to create issues to the correct operation of the system [4]. This considerable amount of material can be transported by the PbLi and set down in the coldest parts of the blanket and/or of the PbLi loop, thus causing blockage issues.

The developed coating technology can be applied on EUROFER water pipes and PbLi pipes in order to reduce, not only the tritium permeation and corrosion rate, but also the large pressure drops due to Magneto-Hydro Dynamics (MHD) effects generated by electrically coupling of the steel wall to the PbLi flow under relevant magnetic field. A strong mitigation of the MHD effects could be achieved with a very good insulating coating. In particular, the MHD pressure losses can achieve very high values in the inlet and outlet pipes of the inboard and outboard of the BB segment, where the magnetic field reaches up to 8.64 T together with a PbLi velocity up to 0.5 m/s. This combination generates a total pressure drop of 2.43 MPa in the inboard and 1.609 MPa in the outboard [5]. Indeed, the use of coatings will allow to increase the PbLi velocity in the reactor and to reduce the manifold and pipe diameters. Coating technologies are investigated in the BB programme as potentially able to avoid the three mentioned risks.

Section snippets

Coating operation in DEMO reactor

Coatings are passive barriers that can be applied inside the BB modules in order to reduce tritium permeation from the breeder to the coolant and the corrosion rate of the structural material (EUROFER), Fig. 1, and inside PbLi pipes in order to reduce the tritium permeation from the breeder to the environment, the corrosion rate of PbLi pipes and the MHD effects, Fig. 2.

The estimated overall length of the PbLi pipework is about 3.7 km, the PbLi velocity at 280−330 °C inside the pipes is lower

Coating technologies

Al-based coatings are presently considered as reference materials for anti-corrosion and anti-permeation barriers on the basis of years of studies about coatings compatibility with PbLi and tritium permeation reduction [6]. As aluminum is activated during the operation of the fusion reactor, with resulting issues at the end of component lifetime, the thickness deposited must be minimised. The target thickness of deposited aluminum is less than 10 μm in order to reduce the amount of activated

R&D activities for coatings characterization

In the period 2014–2020 an optimization of the three technologies for the application to fusion technologies was carried out in order to satisfy the design requirements of good mechanical compatibility with steels, strong adhesion, corrosion compatibility in PbLi at relevant BB design conditions and a Permeation Reduction Factor of at least of 200 under irradiation. A preliminary scaling-up of ECX, PLD and ALD technologies were completed with the final objective to select the reference process

Anti-corrosion barriers

In order to determine the compatibility of the coating processes with PbLi, samples have been exposed in stagnant conditions, relevant for BB application where liquid metal velocity is lower than 5 mm/s, and in flowing conditions in PICOLO and IELLLO loops.

ECX coating was characterized in PICCOLO up to 12,000 h at 550 °C and 0.1 m/s, and evaluated in comparison with bare Eurofer. Material loss has been measured as the difference of the diameters before and after contact to the liquid metal.

Permeation characterization of ECX, PLD and ALD coatings under irradiations

Permeation tests were carried out with hydrogen and deuterium instead of tritium in order to determine the Permeation Reduction Factor (PRF) of the coating technologies. Deuterium permeation was also studied before and after being subjected to 1.8 MeV electron irradiation making use of the RIPER facility at CIEMAT for PLD and ECX coating. Instead ALD coating permeation was characterized in 2019 with hydrogen with the support of PERI-II facility at ENEA; the characterization under thermal cycles

Electrical conductivity of the coatings

ECX and PLD coatings electrical conductivity were measured in the temperature range between 20 °C and 750 °C under vacuum and inert gas, Fig. 18, Fig. 19.

The average electrical conductivity of a metal is in the range between 10⁶-10⁸ S/m, while the one of an electrical insulator is lower than 10⁻⁸ S/m. In Fig. 18 it is possible to observe that the conductivity value in ECX coating, constituted by an external thin layer of Al₂O₃ and an intermediate layer of Fe-Al, is higher than the value of the

Coatings characterization under neutron irradiation

The final test planned in 2020 for the coating characterization under neutron irradiation was carried out in the LVR-15 reactor in CVREZ. The scope of the test was to characterize the PRF of the coating in PbLi under neutron irradiation. For the evaluation of the neutronic conditions, the radiation transport code MCNP6.1 with the nuclear data library ENDF/B-VII.0 was used. In order to perform the experiment, an irradiation capsule filled with PbLi and mounted with coated and uncoated cylinders,

Investigation methods to check the quality of the coating performed at industrial scale

One critical point in the development of coating process at DEMO scale is the identification of a methodology to check the quality of the coating.

Two methodologies were developed in ENEA/IIT laboratories to qualify the coating at industrial scale. The first, called Thermal Quality Test (TQT), exploits the very high oxidation tendency of EUROFER at temperature higher than 20 °C in presence of oxygen and the barrier properties of the alumina coatings, when flawless. After deposition of the

Conclusion

Within the EUROfusion programme, three aluminum coating techniques were investigated: ECX, PLD and ALD process for the application in DEMO reactor as Anti-permeation, corrosion barrier and to mitigate pressure drops due to MHD effect. In order to demonstrate the capability to scale-up the process from laboratory scale to industrial scale to coat about 40,000 m² and 3700 m of PbLi pipes with 10” diameter, a preliminary scaling-up of the processes was carried out in order to coat EUROFER tubes

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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Development of anti-permeation and corrosion barrier coatings for the WCLL breeding blanket of the European DEMO

Abstract

Introduction

Section snippets

Coating operation in DEMO reactor

Coating technologies

R&D activities for coatings characterization

Anti-corrosion barriers

Permeation characterization of ECX, PLD and ALD coatings under irradiations

Electrical conductivity of the coatings

Coatings characterization under neutron irradiation

Investigation methods to check the quality of the coating performed at industrial scale

Conclusion

Declaration of Competing Interest

Acknowledgments

Fusion Eng. Des.

Fusion Eng. Des.

Fusion Eng. Des.

Fusion Eng. Des.

Fusion Eng. Des.

Surface Coat. Technol.

Thin Solid Films