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Pressure vessel integrity - the key to nuclear fusion

16 May 2018

In a battle to create a world that no longer relies on fossil fuels, Tokamak Energy is aiming to realise the first-ever reactor that could produce clean power from fusion, the process that happens in the centre of the Sun. 

The requirement for pressure vessel integrity is never more acute than in the nuclear fusion sector, where tolerances and integrity of seals leave absolutely no room for error.
This article, from high integrity welded structure specialist LTi Metaltech (LTi), which is working with Tokamak Energy on the project, describes some of the challenges of helping develop the ST40 fusion reactor, a crucial step towards a device to produce electricity for the first time from 2025.

The globe’s over reliance on fossil fuels is unsustainable and solutions for meeting our ever-growing energy demands have thus far proved limited. One more promising technology could well be nuclear fusion, where, unlike existing nuclear generation, radioactive waste is minimal and meltdown is physically impossible.

Fusion is what happens in the Sun – when ions in a plasma collide, they fuse together to form larger atoms and release huge amounts of energy. Every nuclear reactor ever made so far has been a fission reactor whereby heavy atoms such as uranium decay into smaller atoms when energy is released. Fusion is a different, safer and cleaner process, but the trouble is that it has not yet been proven as a technology.

Tokamak Energy’s solution, combines two emerging technologies – spherical tokamaks (the most advanced fusion concept in the world) and high-temperature superconductors. The concept behind this is to harness heat right here on Earth within ring shaped chambers that produce cheaper and greener results, hence ‘tokamaks’, derived from the Russian word meaning ‘ring shaped chamber’.

The fusion reaction happens between two light hydrogen isotopes, deuterium and tritium, within a plasma hotter than the centre of the sun. The fusion process creates energy which can then be used to power electric generators.

Culham Science Centre has led fusion research for the past 50 years in the UK, and Tokamak Energy, based nearby, has developed a fusion energy plan focused on building smaller reactors faster. Key to the success of this is the development of a containment vessel for the reactor, and the company approached high integrity welded structure specialist LTi for assistance with this.

The most challenging path to fusion energy has been to effectively, consistently and safely heat plasma to the required temperature to sustain fusion in a stable way, whilst doing so within a realistic time-frame and budget.

Tokamak Energy’s basic premise is that smaller reactors can confidently manage the task of creating safe fusion energy. If initial tests prove successful, the new technology will enable production lines to be established manufacturing hundreds of these smaller reactors, similar to making jet engines.

LTi, the UK-based lead manufacturer of the cryogenic pressure vessels used in Siemens MRI Scanners, has used its fabrication expertise to support Tokamak Energy to create the world's first high-field spherical tokamak, the ST40 reactor. Manufacturing of this revolutionary reactor began late last year at the heart of Milton Park, Oxfordshire.

Conceptualising the IVC Angled Support fabrication structure, LTi envisaged the formation of single-sided welds including the gap requirements for a backing at a minimum of 4mm, extending to 10mm from the edge of the joint to prevent burn through. This would be engineered by a raised surface around the flanges to act as location and weld backing. The company proposed solutions that reduced welding times – eliminating the requirement to grind back on some double-sided welds.

The structure was simplified whilst maintaining function by placing prep angles and creating square edges on the coned features. Furthermore, the design was enhanced so that the IVC outer wall is cylindrical and the resulting voids would not affect the cone sections.

The reactor vessel is produced using stainless steel, although the companies are considering alternatives to better withstand erosion. Maintaining the plasma within the magnetic field and preventing it from hitting the walls by retaining a small gap requires accurate control of magnetic coils and the current that passes through.

The ST25 reactor, which combines high-temperature superconducting (HTS) magnets with a smaller than standard spherical tokamak, demonstrated a world record of 29 hours of continuous plasma in 2015. It is expected that the new ST40 reactor will eventually produce plasma temperatures of 100 million°C – several times hotter than the centre of the Sun, relying on magnetic coils which trap hot plasma within a field, keeping it away from the walls of the vessel. This will not only prove that fusion power is a viable alternative to environmentally damaging fossil fuels, but that the technology is achievable.

The world needs abundant, clean energy. Nuclear fusion - with no CO2 emissions, no risk of meltdown and no long-lived radioactive waste – seems like the optimum solution. It is an ambitious challenge that requires serious investment, government backing, R&D initiatives and creative scientists. But if we are successful in commercialising tokamak technologies, widespread adoption of fusion energy could well become a sustainable reality.

About the author

Edgar Rayner is a Chartered Mechanical Engineer and currently holds the role of Technical Director at LTi Metaltech, specialising in manufacturing, engineering and technology management. Prior to his roles at LTi Metaltech, he worked for Siemens where he oversaw a team of physicists, engineers and technicians who developed solutions for pressure vessels and suspension systems.

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