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Safety in Fusion

An inherently safe process

Carley Willis, Joanne Liou

One of ITER’s vacuum vessel sectors is installed — a 440-tonne piece that will help contain the device’s plasma. (Photo: ITER)


Safety in Fusion

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While nuclear fission derives energy from splitting atomic nuclei, nuclear fusion does so by joining them, releasing energy in the process. Though both atomic reactions produce energy by modifying atoms, their fundamental differences have broad implications for safety.

The conditions required to start and maintain a fusion reaction make a fission-type accident or nuclear meltdown based on a chain reaction impossible. Nuclear fusion power plants will require out-of-this-world conditions — temperatures exceeding 100 million degrees Celsius to achieve high enough particle density for the reaction to take place. As fusion reactions can only take place under such extreme conditions, a ‘runaway’ chain reaction is impossible, explained Sehila González de Vicente, Nuclear Fusion Physicist at the IAEA.

Fusion reactions depend on the continuous input of fuel, and the process is highly sensitive to any variation in working conditions. Given that a fusion reaction could come to a halt within seconds, the process is inherently safe. “Fusion is a self-limiting process: if you cannot control the reaction, the machine switches itself off,” she added.

Furthermore, fusion does not produce highly radioactive, long lived nuclear waste. “Fusion produces only low level radioactive waste — more than fission does — but this low level waste does not pose any serious danger,” said González de Vicente. Contaminated items, such as protective clothing, cleaning supplies and even medical tubes or swabs, are short lived, low level radioactive waste that can be safely handled with basic precautions.

Most current experimental fusion devices use a mix of deuterium and tritium as fuel. Tritium is a radioactive isotope of hydrogen with a half-life of 12.3 years. As a result of the fusion reaction, neutrons are released, which impact and are absorbed by the wall surrounding the reactor core, said González de Vicente, making it radioactive. “The neutrons react with lithium contained in the wall, generating tritium that can then be reinjected into the machine.”However, fusion and fission facilities do share some similarities, such as in how radioactive material is handled and how cooling systems are used. “Regulatory bodies have vast experience in the realm of safety and security for fission. We are working with them to ensure that all applicable knowledge is transferred to fusion,” González de Vicente said. “Not everything can be translated one-to-one, however, and the differences with fusion, such as the reduced amount and variety of radioactive material, the impossibility of core meltdown conditions and the lack of long lived waste, should be identified and addressed. The IAEA is helping to facilitate these efforts.”

International collaboration

ITER, the world’s largest fusion experiment has gathered experts from 35 countries to work towards making fusion energy sources a reality, while also helping to solve fusion’s safety and security challenges as the project develops.

A high degree of safety can be ensured by applying relevant safety requirements for fission, such as the IAEA safety standards, to fusion. For example, just as with nuclear fission reactors, proposed fusion plants must also consider dose regulations, and installations should be designed so that the minimum dose is ‘as low as reasonably achievable’, or ALARA. However, given the fundamental differences in the risk of accidents, the application of a graded approach is necessary to avoid overregulating the fusion process. “The problem with all existing safety standards is that they are geared towards fission,” said Stéphane Calpena, Deputy Head of the Safety & Quality Department at the ITER Organization. “We need to extract the standards that are relevant to fusion and apply them in a manner commensurate with risk to make sure that the technology is not only feasible, but that it is truly safe. Fusion is a new way to create energy, and is still very much a young technology.”

The IAEA is helping to foster this technology by holding Technical Meetings for experts to share knowledge that can aid in overcoming challenges in fusion and ensure the safety of fusion facilities. The First Joint IAEA–ITER Technical Meeting on Safety and Radiation Protection for Fusion, chaired by Calpena in November 2020, focused on developing a methodology to determine the potential types and amounts of radioactive or hazardous material that could be released into the environment at fusion facilities, as well as on preparing publications equivalent to IAEA Safety Standards Series Nos SSR-4 and SSG-12 for fusion. The meeting covered topics such as risk criteria for, and the design and operation of fusion facilities. The Workshop on Waste Management for Fusion, scheduled for October 2021, will look at how radioactive waste from fusion energy production is classified and disposed of.

May, 2021
Vol. 62-2

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