Novel oils and paints for the Energy Sector in the 21st Century

Richard Seggie Memorial Lecture

By Dr Amanda Quadling (nee Seggie), FIMMM, CSci

Introduction

In the 21st century, innovation is taking place both at the ‘engine’ side of the electricity grid – with new nuclear powerplant technologies being developed for fusion and Generation III and IV fission – as well as the distribution end – where traditional substations are being upgraded with digital hardware and alternative transformer insulation. This briefing lifts a few key aspects of the modern dielectric ‘oils’ being introduced in transformers, and provides some introductory pointers on paints and coatings for new nuclear powerplant builds.

Dielectric oils

Mineral oils, derived from light hydrocarbon processing routes (hydrocracking etc.) and classified to fire points around 170°C, are the traditional dielectric liquids in the transformers which step voltage up and down, across our electricity grids. An alternative chemistry – reacting mono acids with poly alcohols to produce polyol esters – provides dielectric capability with:

Improved thermal performance – natural (canola and soy derived), and synthetic, esters have fire points over 300°C;
Improved oxidation stability – synthetic esters do not oxidise in breathing transformers;
Improved green credentials – esters are more than 80% biodegraded within 28 days of exposure to bio-organisms;
Improved transformer health – esters exhibit robust water saturation characteristics, enabling the cellulose insulation used to protect transformer windings to last longer at higher operating temperatures (by effectively ‘wicking’ some of the moisture released through pyrolysis and/or hydrolysis of the cellulose);
Improved cold temperature performance – unsaturated bonds in the ester molecule produce bend shapes conducive to pour points as low as ~-40°C and low viscosities under extreme climates.

While these new properties are attractive, the introduction of esters to transformers also has costs: it has meant development of new Dissolved Gas Analysis (DGA) schemes to replace those long established for mineral oils. The latter relate the generation of specific gases within the transformer, in response to arcing, partial discharge and other ‘fails’ and are based on empirical evidence built over decades. DGA schemes are used to measure
‘transformer health’ by analysis of dielectric oil tapped off working transformers according to fixed maintenance / monitoring schedules.
Ester manufacturing also requires careful control of acidity and tan delta, the metric used to quantify dielectric dissipation. For quality assurance purposes, it is useful – alongside low variability in the production process – to have a margin for error well away from the customer tan delta acceptance threshold, to allow for some product degradation during transport and storage.

Nuclear powerplant paints and coatings

Fusion – the joining of hydrogen isotopes under high temperature / pressure confinement in devices such as tokamaks – is becoming an engineered reality in this century. Like fission (the splitting of atoms into isotopes), it presents the opportunity for clean baseload energy to the grid. In nuclear powerplant new builds, paints and coatings must have good adhesion to their substrates so that in the event of a ‘loss of coolant accident’ (a LOCA), these coatings do not strip off under high heat, creating debris that blocks sumps in the emergency cooling systems. In addition, coatings are used:

To prevent corrosion – and thereby, possible leakage of radioactive substances;
To prevent wear;
To protect against the ingress of radionuclides and active dust into porous substrates;
To enable easy wipe / wash protocols for decontamination of surfaces. (For fission, this would mainly involve removing radionuclide particles; in fusion, this would include wiping off tritiated contaminants)

Multiple lines of defense (surface coating, primer and topcoat) are used in applications which extend from reactor walls and ceilings to unlagged pipes and structural supports. Quality standards are well established from America to Scandinavia, and commercial offerings are available in mature product ranges internationally.

Epoxy, alkyd and inorganic zinc dominated chemistries provide a variety of trade-offs between advantages such as good moisture resilience (epoxies) and galvanized steel protection (zinc) and disadvantages such as brittleness (epoxies) and poor acidity resilience (zinc).

Paints may react with their substrates and in particular, concretes provide challenges – releasing moisture, alkali’s and carbonate build-up at interfaces. Paints on nuclear concretes must accommodate cracking as the latter ‘settles’, and new products are put through rigorous tests including recirculating fluid loops, air blowdowns and air- and steam-gamma irradiation.

Innovation in coatings for nuclear powerplants is a growing field:

In 2019 at Argonne National Lab, researchers used a combination of Atomic Layer Deposition (providing slow but high density cover) and Electrophoretic Deposition (providing faster but less dense cover) to generate a sticky ceramic coating for fuel cladding metals that shows high irradiation resistance.

In 2023, at the UK Atomic Energy Authority, the Materials Division is exploring strain resilience in alumina-based coatings for ferritic martensitic steels; the latter developed for reduced activation under fusion neutron exposure. The coatings must prevent damage by water-based coolants. Early tests show Electroplating generating fewer challenges than Chemical Vapour Deposition as the latter leads to development of an interface layer between the alumina and steel. During strains up to 1%, the formation of intermetallics in the coating layer appears to exacerbate crack-based coating failure.