Since the 2010 Fukushima Daiichi nuclear power plant disaster, which resulted in the meltdown of three of the plants' reactors, nuclear safety has become a polarised issue that is under increased scrutiny. As a result, nuclear power plants designed and built in the future will be safer than those built before.
But what does this mean for the safety standards of older nuclear power plants, some of which are now reaching 40 years of age and are being considered for life extension; can they, then, not be deemed fully safe?
In an attempt to bring existing nuclear power plants up to the same safety standards of new plants, the Swiss nuclear regulator, Swiss Federal Nuclear Safety Inspectorate (ENSI), proposed to amend the Convention on Nuclear Safety (CNS), which represents 77 member countries, so that existing nuclear plants be held to the same safety standards as new ones and a binding backfitting obligation be introduced at the international level.
The proposals were rejected on the grounds that too many countries would be unable to ratify the amendment due to domestic reasons. Instead, a non-legally binding Vienna Declaration, which includes safety principles derived from the original Swiss proposal, was drawn up.
The ENSI has said it is happy with this outcome. However, its proposals have highlighted the issue of safety at ageing nuclear power plants that many are concerned about, as well as the overall effectiveness of international agreements like the CNS.
Safety: an evolving issue
The French nuclear regulator, the Nuclear Safety Authority (ASN), is expecting to start making decisions on whether nuclear power plants over 40 years of age can be extended up to 20 years from 2020. It will then review plants, judging them by today's safety standards, on a case-by-case basis.
The US nuclear industry is struggling to survive in tough US electricity markets flooded.
The US has already begun this process and has so far granted life extension to 76 reactors. In order for life extension to be granted the US Nuclear Regulatory Commission determines whether a currently operating plant will continue to maintain the required level of safety. According to the commission, this level of safety is enhanced through maintenance of the plant and its licensing basis, which is an evolving set of requirements and commitments.
"The issue of life extension needs to be looked at," says Steve Thomas, professor of Energy Policy at the Public Services International Research Unit (PSIRU) at Greenwich University.
"It is a fact of life that yesterday's design is not going to meet today's standards but over the period of 40 years that gap is going to get increasingly wide."
One of the key failures at Fukushima during the 2010 tsunami and earthquake was that all on-site power was lost to drive pumps and coolers. Power from generators was then lost, meaning the nuclear reactor could not continue to be cooled, which needs to happen for several weeks post-shut down.
John Large, chartered engineer, nuclear analyst and chief executive at Large Associates, points out that the French nuclear regulators, when conducting stress tests on French nuclear power plants as required post-Fukushima, discovered that the majority lacked a 'hardened safety core'. Forming part of a hardened safety core is an alternative heat sink backed up by diesel generators - a means of dissipating heat from the reactor should it lose all onsite power - as well as a crisis centre built on-site from where an emergency response would be conducted in the event of an accident. A hardened safety core is now being applied to all plants in France with a deadline of 2024.
He adds that in Europe if you want to build a reactor over 1200MW you have to have a core catcher, which is too costly to be retrofitted into an existing plant.
"I think it is impossible to impose current safety standards on an existing plant," agrees Thomas. "You can't add aircraft protection, core capture and all those other things; it is not feasible."
Referring to ENSI's proposals, he says, "Asking for all old plants to meet current standards is just not realistic."
In response, David Hess from the World Nuclear Association says it is misleading to compare the safety of old plants to new plants because safety at existing nuclear plants has been increased over time through component upgrades and better staff training and culture, and as regulatory requirements have tightened.
"Existing nuclear plants must meet the regulatory requirements. If they don't, they get shut down. In fact plants often shut down for short periods when a minor operational event takes them outside their licence condition. They correct that and aim to restart as quickly as possible."
Is heightened security necessary?
Another issue that concerns many is that ageing nuclear power plants with the potential for life extension may not stand up to modern day hazards, such as terrorism. For example, newly-built nuclear power plants are required to be able to withstand an aeroplane being flown at them; older plants are not. Such events, however, are extremely rare. Therefore, do they compromise nuclear safety on a tangible level?
"That is difficult to say," says Thomas. "It depends what your perception of the risk of someone driving an aircraft into a nuclear reactor is.
"I guess today you are thinking it is probably more likely than you did yesterday."
Nuclear energy must evolve to secure its future, but how does change affect the performance of plant operators?
(Thomas was interviewed on the day it became apparent a Germanwings pilot deliberately flew a commercial aeroplane into the French Alps, killing 150 people.)
Such events, it has to be acknowledged, are extremely rare, almost inconceivable; though Thomas points out that Fukushima and Chernobyl in 1986, where an explosion and fire released large quantities of radioactive particles into the air, also seemed inconceivable before they happened.
However, other industries, such as oil and gas, are not held to such scrutiny over safety or do not need to be designed to withstand the impact of an aeroplane, for example.
But the industry has experienced three core meltdowns. Thomas believes, statistically speaking, a core meltdown should happen no more than once every 100,000 years. Even though the recorded death toll of these three disasters is 31 - relatively small compared to, for example, mining - there is the fear of the unknown, of how many more people will subsequently die from illness due to increased radiation exposure. Although some studies have shown this to be small in the case of the Fukushima nuclear disaster, many believe it is hard to attribute deaths to radiation exposure and that there have not been enough detailed studies, especially in regard to the Chernobyl disaster.
Hess says: "There has, to my knowledge, never been a 'terrorist' attack - excluding green groups - on an operating nuclear power plant, from a plane or otherwise.
"Of course such an attack is possible, but as the September 11 terrorist attack demonstrated other targets are far more accessible and capable of causing more damage.
"At issue now is our collective ability as a species to recognise and respond rationally to different risks. Do we continue to ratchet up requirements for nuclear energy, which is safe even on a bad day, even with vintage tech, or do we focus on, say, climate change with impacts which are growing in magnitude day by day?"
Hess adds that industry is focusing even more on mitigation of accidents caused by a "black swan event", or an event that cannot be anticipated at design stage, as well as the ensuing emergency response. He says this is more worthwhile than trying to reduce the already extremely low odds of such an event happening in the first place.
No simple answers
Nuclear safety standards are not easy to agree on, not least because the industry is very complex in nature and much trust must be put into the hands of nuclear physicists. Large says it's only the people who understand nuclear plants - nuclear physicists - that can ask and answer questions about nuclear safety. "The question is, are they asking the right questions of themselves, or ones that are convenient?"
However, even nuclear physicists, it seems, can't agree on the same safety standards for nuclear power plants.
For example, despite international Conventions and guidelines, such as the CNS, WANO and the International Atomic Energy Agency (IAEA), approaches and requirements for safety vary widely from country to country.
"State regulators in places like France and Germany will not agree to a unified nuclear regulatory framework across Europe," says Large. "Yet the nuclear plants that they are safeguarding are supposed to be very much the same. But they all declare their nuclear plants to be safe. I'm not sure how you can do that?"
It's evident that it would be difficult to enforce umbrella laws on the global industry. Different governments may disagree with certain laws and thus refuse to ratify them, or they may consider it too much of an upheaval to change their current system, which they already deem safe, or they may believe that the process of passing laws through parliament is too lengthy or time-consuming.
Hess acknowledges there are benefits to harmonising regulation internationally, but that this might "just as likely be organisational and economic improvements as risk reduction".
"This obfuscates the fact that nuclear plants are subjected to strict independent regulation everywhere in the world," adds Hess. "What other energy technologies are subject to such?"