When you hear the words “nuclear safety”, your mind might picture images of mushroom clouds and scary ghost towns. Whether you realise it or not, media and pop culture engrave these types of conceptions into our subconscious minds, creating sometimes intense reactions to the topic. At Greenfish, we believe it is important to rationally challenge our emotions on this topic in order to understand and analyse them. Two underlying concepts determine our perception of nuclear energy and how safe it is: its enormous energetic content and its poisonous potential due to ionizing radiation.
This paper aims to shed light on the underlying disagreements in the academic and political discussions surrounding human safety in nuclear power in order to challenge our beliefs and emotions. Because of the depth of the topic, it deliberately focuses on the human health dimension, leaving apart the radiation effect on the environment and biodiversity, which could be the emphasis of another White Paper. Indeed, this paper is part of a series on the different elements of the nuclear energy debate, previously presented by Greenfish in this paper. Below, we start by explaining the safety design choices in nuclear reactors to then take a risk approach to the nuclear technology. As accidents still happen, the core of the paper focuses on the scientific debate about the effect of radiations at low-dose, illustrating it with examples. To finish, we explain the difficulty of communicating about nuclear safety in the current context.
Thoughtful design choices
Before we go any further on nuclear energy safety, it is important to debunk a pervasive myth that both informed sides of the debate agree on: equating a nuclear reactor to a nuclear bomb. Such a misconception is widely anchored in the public collective knowledge because of the fear of the nuclear destructive potential. Even though nuclear power plants and nuclear bombs rely on the same principle, a chain reaction of atoms splitting apart, they are inherently different by design: one is made to explode and the other is not. To properly understand the difference, please check a brief explanation on the basic principles in the insert on the right.
In reactors, additional design provisions work against uncontrollable thermal runaway reactions. They are called negative reactivity coefficients. A great example of this principle is the addition of water to a nuclear reactor. In many reactor designs, water is used as both a coolant and a moderator to slow down neutrons. ‘Slow’ neutrons are essential to achieve fission in most conventional nuclear reactors. If the temperature in a reactor reaches levels that are too high, the moderator (in this case, the water) will start to boil.
A bomb is a bomb, a reactor a reactor:
The energy liberated in bombs or reactors is based on nuclear fission, which is a reaction sustained thanks to a fissile material serving as the nuclear fuel. Examples of common fissile materials are Uranium (such as U233 and U235 ) and Plutonium (Pu239 and Pu241). On one hand, the bomb design works by simply placing together enough fissile material into a small enough volume (therefore in high concentration). It then creates a chain reaction which burns up most of this material in a matter of seconds. On the other hand, a nuclear reactor does not solely exist out of fissile material. The fuel (in general, uranium) used in nuclear reactors is of low enrichment. This means that it is, for example, composed by 5% of U235 and 95% U238. While U235 is fissile, U238 is not, meaning it does not contribute to the chain reaction in the same manner. This choice ensures that an explosion like the one of a bomb is not possible in the current design of the reactors.
By changing to the vapour phase, which is much less dense than the liquid phase, less moderation is possible, which reduces the number of ‘slow neutrons’ and thus the number of fission reactions occurring. As a result, the temperature is brought down. This negative feedback loop acts as safeguard which keeps the temperature under control.
Safety progress VS “zero risk”
Design choices have allowed nuclear companies and engineers over the world to reach a relatively high level of control of the chain reactions happening in nuclear power plants. Historically, national (e.g. ASN (FR), NRC (USA), AFCN (BE), etc.), European (e.g. ENSREG) and international (e.g. IAEA) independent nuclear safety authorities have worked together to reduce the risks and push the implementation of stringent safety norms. Over the last 30 years, these norms have become stricter  and this has resulted in a substantial decrease of the number of incidents/accidents at civilian nuclear power plants, as shown in Figure 1 (I & II) . It is worth mentioning that, considering the health effects of air pollution and accidents, civilian nuclear power is one of the safest forms of energy to operate (not including radiation from plants and waste, see sections below), especially compared to brown or black coal  (see Figure 2).
Figure 1 – Evolution of incidents per reactor per year (plots I and II) and cost damages (plot IV) due to incidents in civilian nuclear power plants .
On plot II, the drop in events occurence after Chernobyl shows it played a major role of catalyst for change in safety practices. On plot IV, the increase in cost engaged per events due to the severity of the accident can be seen. INES is the scale used to compare nuclear incidents (level 1 to level 7 – each level representing and accident ten times more severe than the previous).
Figure 2 – Deaths per TWh of energy generation based on IPCC data Markandya and Wilkinson research (more details in )
However, even though nuclear reactors are not weapons and their safety has improved over the years, accidents still happen. Fukushima may have happened under extraordinary circumstances, but it still happened. No matter how strict the regulations are or will be, the release of radioactive material due to nuclear power generation seems inevitable. “Zero risk” is unreachable in face of the law of large numbers: the more nuclear power plants that are operating, the more likely it is for an incident to happen, even with low probabilities. In the age of terrorism, we cannot neglect known unknowns. What’s more, unlikely but serious events lead to explosively high costs as shown in Figure 1 (IV) . All these parameters bring into question the boundaries of safety and how much we accept its probabilities.
A radiating scientific debate: radiation
If incidents happen or will happen, then the largest discussion point remains the poisonous potential of ionizing radiation. You probably already know the story: radiation, both from natural and artificial sources, submits the cells in our body to several DNA-breaks per day. Luckily, our bodies are equipped with mechanisms to fix such DNA deteriorations. However, in cases where too many breaks are incurred, radiation (along with other factors) leads to genetic mutations and cancer.
Radiation protection science is based on 3 letters: LNT, or Linear-No-Threshold theory. It implies that there is no healthy dose of radiation. Consequently, each received dose increases your probability of contracting cancer in a linear fashion all the way from zero exposure (which doesn’t exist due to natural background radiation present in the environment). It is this methodology, measuring the harm caused by radiation, that creates the current discrepancy between the safety track record of civilian nuclear power and the large clean-up costs of nuclear events. At high dose, LNT has been confirmed by epidemiological studies, and scientists agree upon radiation consequences. However, at low doses*, the theory is hard to prove and just as hard to refute, creating a significant scientific disagreement. In this range, according to LNT, absorbed radiation would cause only a small increase in cancer probability. However, these small probabilities are very difficult to confirm or refute because of the influence of all the other factors that contribute to cancer prevalence. The statistical significance of findings from studies on nuclear radiations in the low-dose area are therefore easily called to question.
LNT is the core of the safety debate on radiation stemming from nuclear power plants. It is part of the reason why the Japanese government evacuated a large area during Fukushima: to prevent citizens from receiving an elevated dose of radiation. However, in many of the regions that were evacuated, the dose-equivalent was inferior to the dose received by natural background radiation in other populated regions on earth (such as the Kerala in India) , , , . When modeling is done through the LNT theory (WHO data), the deaths caused by the release of radiation are lower than the fatalities due to the stress of the Fukushima zone evacuation, which affected principally elderly people , . Numerous scientific papers have been published criticizing the LNT methodology, showing that it overestimates instances of cancer due to radiation (such as the Chernobyl accident, Hiroshima & Nagasaki bombings, and animal tests with chronic low-dose radiation ). However, in the absence of a scientific consensus, safety regulations are based on LNT, in application of the precautionary principle.
What are deemed as acceptable levels of radiation? Those who oppose LNT state that an acute dose of around 100 mSv** and a chronic accumulated dose of over 500 Sv in a lifetime does not cause significant increases in cancer . On the other hand, LNT-based regulations in Europe call for a yearly (non-background radiation) dose of no more than 1 mSv (0.1 nSv/hr) limit for the public, 20 mSv by 12 consecutive months sliding for radiation workers and an acute dose limit of 50 mSv over a year . This divergence on the assumptions of radiation damage causes enormous differences in the estimations of fatalities (including in the studies considering the LNT methodology only). As an illustration, Figure 3 shows the difference that these low-dose assumptions create on the estimated death toll from the Chernobyl accident compared to the primary estimates*** , , , , .
Figure 3 – Estimated number of deaths due to Chernobyl, comparison of different studies 
Finding a balance in communication
How to properly communicate with the public about safety and radioactivity while there is still so much uncertainty at the scientific level? Low probabilities and the difficulty of pinpointing the origins of cancer make it very difficult to create statistically certain statements. Even if those statements would become certain in the future, polls show that Europeans get most of their information on nuclear safety from the media, but they do not trust it . It is also very challenging to find objective and reliable information on the internet , . and since no assessment of the public opinion has been done since Fukushima , it is hard to understand were Europe stands on safety.
Pro-nuclear advocates communicate by pointing out the relative risks and damages of the radioactivity release on health, the comparatively good track record of nuclear power plants, and its potential role in a low-carbon future. On the other hand, anti-nuclear advocates highlight the potentially vast costs associated to major accidents, the humanitarian and environmental disaster which is created by regional evacuations and contamination, and the promotion of renewable energy as a sufficient solution toward a low-carbon future.
Both sides have some valid points but the assumptions at the core of these talking points lead to such divergent assessments of risk that any attempt to find common ground in the discussion proves to be arduous. To get the full picture of the risks posed by nuclear power generation, we must be aware of our prejudices, be eager to learn by following closely track with the scientific progress and listen to both sides of the debate with a weary but thoughtful mindset. Finally, any opinion on the matter will come down to the personal cost-benefit analysis of the risks: deciding if the risks outweigh the benefits would eventually remain an individual choice.
In line with the evolution that had characterised the consensus on climate science over the last 30 years, at Greenfish, we believe that the creation of a worldwide multi-disciplinary intergovernmental panel on nuclear power would be an appropriate vector to put light on the debate and characterize those risks properly. It would represent divergent and the best in-class scientific opinions, and strive to coordinate and communicate the emergence of a consensus on nuclear safety. This could allow to understand if nuclear energy is a safe enough alternative for a sustainable future. To pursue our thoughts and analysis on nuclear, in the following white paper, Greenfish will delve and try to clarify the current debate on the cost of nuclear power and its consequences.
* Low dose = less than 200 mSv
** Millisieverts (mSv) are units of absorbed dose weighted for the different types of radiation. 100 mSv is in the order of the yearly dose that one would get from spending a year in the most radioactive measurement station of Fukushima  which is currently at 114 mSv/year. The other regions of Fukushima are much less effected as shown in the animation of .
*** Those shown in (2005a) in Figure 3.
Djef Brak – Junior Consultant, Energy Transition at Greenfish
Quentin Lancrenon – Project Analyst, Green Solutions at Greenfish
Nassim Daoudi – CEO at Greenfish