Nuclear radiation in fusion: low risks and plenty of benefits

Eurofusion Bianca Giacomelli | October 19th 2016

As Professor Ian Chapman, the new CEO of the UK Atomic Energy Authority (UKAEA), remarks on BBC5 Radio Live, nowadays explaining the concept of nuclear fusion to the public is a hard, but essential, task. Looking towards the future of fusion power appears challenging to most people used to relating nuclear technology to destruction and fear. Worries about the aftermath of a nuclear attack, accidents in nuclear power plants and a lack of education with regard to radiation have fed the distrust in nuclear applications. Although fusion power does come with some radiation risks, its perception remains affected by the general stigma attached to radiation.

Fusion comes with negligible radiation

JET, the Joint European Torus, one of the nuclear fusion devices that has already achieved fusion, producing 16MW in 1997, relies on the principle of bringing together (fusing) deuterium and tritium nuclei to produce helium and highly energetic neutrons (14.1 MeV). Magnetic confinement is used to overcome the mutual electric repulsion between tritium and deuterium, both of which are positive ions. The handling of massive amounts of tritium and the production of highly energetic neutrons are the main radiation protection concerns faced by fusion experiments using tritium. Although tritium has a very low radio-toxicity –it emits radiation which is not able to penetrate the skin – it easily forms organic compounds and diffuses through materials. So, due to the large quantity of tritium in fusion reactors (for example in ITER, about one kilogram per cycle plus a few kilograms stored on site), power plant staff members use protective clothing to avoid inhalation and direct contact with the material. Moreover, pressure systems have been designed to limit tritium spread outside the power plant. High energy neutrons with a high fluency rate induce radioactivity in reactor materials. The radioactivity remains even after the experiment has been closed down. Specifically shielding, optimisation programmes of tritium handling and storage, protective clothing for the staff and a remote maintenance plan will result in negligible radiation risks for both the public and the power plant staff.

Power from fission

Nuclear fission, on the other hand, is the physical phenomenon employed in nuclear reactors and also, when intentionally “uncontrolled”, in nuclear weapons. A large nucleus, for example of uranium-235, when bombarded with neutrons will fission, or split, into two smaller nuclei, called fission products, also emitting a few neutrons and gamma photons. These fast neutrons can themselves induce fissionin other uranium nuclei, creating a chain reaction process. Avoiding a runaway chain reaction and managing radioactive fission products are the main issues of fission power. Uncontrolled chain reactions in reactors may, in fact, causemeltdowns and damages (however not comparable with the effects of nuclear weapons), while radioactive fission products are high-level radioactive waste that contains 95 percent of the radioactivity arising from nuclear power (according to the World Nuclear Association) and requires safe storage for more than a thousand years.

Sievert is an indirect unit that represents the risk of the biological harm on the human body due to radiation exposure: the probability of cancer induction and genetic damage. In ITER the objective is to ensure an annual individual dose limit of 2.5 mSv (about that similar to background radiation) for the reactor workers and an annual maximum public dose of 0.1 mSv. Chart: EUROfusion/IAEA (data)

Realistic perspective on radiation risks

Even though, in most countries, research into the operational safety of fission plants has progressed in ensuring a minimal risk of human and organisational error, the memories of Chernobyl have certainly contributed in increasing the distance between the population and radiation education which has resulted in a fear of radiation, often regardless of the actual circumstances. A realistic perspective on radiation related risks may therefore be crucial in improving fear management.

Towards safer and cleaner energy

Coming back to fusion, in case of the worst possible accident in a future nuclear fusion reactor, there would be no need to evacuate nearby residents. The implementations of radiation protection in fusion reactors, as mentioned earlier, follow the ALARA (As Low As Reasonably Achievable) approach: an ongoing and iterative process designed to keep the likelihood and the magnitude of radiation exposure, as well as the number of people exposed, as low as is reasonably possible. Furthermore, the absence of both long-lived, highly radio-toxic radioactive waste and dangers due to runaway chain reactions tips the nuclear fusion power scales in favour of the higher benefits and away from radiation risks to staff and population.