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Plastic ceramic films to improve safety of modern nuclear energy

Periodic Reporting for period 1 - PLASTICERA (Plastic ceramic films to improve safety of modern nuclear energy)

Período documentado: 2019-04-16 hasta 2020-04-15

Aim of the project PLASTICERA is to prevent nuclear accidents similar to Fukushima Daiichi from happening in Europe. Primary objective of PLASTICERA is to develop a new accident tolerant fuel (ATF) concept for modern nuclear light water reactors (LWR). Today, nuclear energy is an essential environmental issue as it is one of the key scalable technologies to battle climate change. Promoting the use of nuclear energy is largely based on public opinion and therefore creating safer and more sustainable ways to produce nuclear energy is more important than ever. The concept of PLACTICERA relies on amorphous oxide thin films to protect the primary fuel cladding from catastrophic damage during nuclear accident conditions. The oxide thin film can provide unique combination of a strong oxygen diffusion barrier with the capability to accommodate the plastic strain originating from the fuel bar thermal expansion. This functional coating could significantly delay the onset of uncontrollable degradation of the primary fuel cladding, allowing timely emergency cooling, and preventing the release of radioactive substances. The primary objective will be achieved by training Dr. Erkka J. Frankberg (fellow) with new skills in disruptive material manufacturing technologies capable of producing ceramic materials, especially amorphous oxides, with prerequisites for low temperature plasticity. These materials will then be tested for mechanical and corrosion properties in relevant environment resembling LWR normal operating conditions and conditions occurring during “loss of cooling water” (LOCA) -type accident.

Pulsed laser deposition of chromium oxide (Cr2O3) was optimized to produce dense films. The films were tested in operational and accidental conditions and we found that the material is not sufficiently corrosion resistant during simulated accidental conditions in LWR. Results on room temperature plasticity of oxide ceramics were published in the Science Magazine 15.11.2019.
The water-cooled environment in LWR is a complex corrosive medium. The materials designed for the LOCA -type accidental conditions in LWR must withstand both the normal operating conditions for the whole life-time of the nuclear fuel spanning several years, and in addition the harsh accidental conditions for as long as possible. It is thought that even some hours of extra time to set up emergency cooling for the nuclear fuel could prevent the catastrophic consequences of a full-scale nuclear fuel core meltdown.

First, we tested a larger set of sintered oxide bulk materials to screen the best candidates to be produced as pulsed laser deposited thin films. The screening was done under normal operating conditions in water at 360 °C and 18.6 MPa and characterized after 3, 10 and 24 days of exposure. The overall best performing material, Chromium oxide (Cr2O3), was selected to be studied further in the project. The PLD process for amorphous Cr2O3 was then optimized to produce dense and flaw free microstructure (Figure 1).

We measured an average corrosion rate of + 1.21 ± 0.82 µm/year for the optimized PLD Cr2O3 coatings on steel. In practice, the coating gained mass during corrosion in 360 °C water up to 24 days, most likely due to healing of oxygen vacancies in the deposited non-stoichiometric material. The substrate material was fully covered with the chromia PLD coating, and therefore the mass gain is connected purely to the chromia coating. This signals that the material could be used a protective layer for the primary cladding under nominal operating conditions. In the second phase, we performed accidental condition tests for the PLD Cr2O3 on steel in steam (water)/Argon mixture at 1200 °C for 60 minutes and we found that the material corroded severely exposing the substrate material for further corrosion. The results regarding the Cr2O3 coatings are disseminated at the 2020 TMS Annual Meeting & Exhibition 26.2.2020 with the title “Ceramic Oxide Coatings for Water Reactors: Corrosion Protection in High Temperature Pressurized Water”.

In PLASTICERA, the feasibility of the oxide coatings as new type of ATF coating was tested successfully. The nominal operating conditions can be sustained by the coating, however, the selected material, amorphous chromium oxide (Cr2O3), underperformed in the accidental condition tests. Therefore, it is not certain whether the possible plasticity of these coatings show any practical use under LOCA conditions as the dissolution of the coating is very fast.
The parallel studies regarding the room temperature plastic deformation mechanisms of glass materials led to breakthrough results showing room temperature ductility for amorphous aluminum oxide (Al2O3). The results were published in the Science Magazine 15.11.2019. The published results are in line with the preliminary results of the PLASTICERA project, however the results also highlight that with current manufacturing technology it is still difficult to obtain large quantities/areas of ductile glass, needed for example in exploitation of the material in large scale application, e.g. to prevent nuclear accidents.

We were able to show that an oxide glass can be ductile at room temperature. If found applicable on large scale, this can completely change our view on inorganic glass materials and substantially increases the potential applications of this material group. We found that aluminum oxide glass can ductile, while it is also several times stronger than steel and much lighter material compared to steel.
Figure 1. Cross section of an optimized pulsed laser deposited Cr2O3 film. Thickness 530 nm.