Generations and safety

Generation I reactors were the prototypes of the 1950s-60s (the last, Wylfa 1 in the UK, closed at the end of 2015); Generation II is typified by the present US fleet and most reactors operating elsewhere; Generation III are the advanced designs evolved from them; Generation IV designs are still on the drawing board.

Generation III/III+ designs feature a standardised design to speed licensing and construction, a typical 60-year operating life, passive or inherent safety features, further reduced core-melt probability (about ten times better than current US plants) and a grace period in which the plant needs no active intervention for typically 72 hours after shutdown.

The Generation IV International Forum set eight technology goals in four areas - sustainability, economics, safety and reliability, and proliferation resistance and physical protection - and selected six systems: GFR (helium, 850 C), LFR (lead, 480-570 C), MSR (fluoride or chloride salts, 700-800 C), SCWR (water, 510-625 C), SFR (sodium, 500-550 C) and VHTR (helium, 900-1000 C).

Defence in depth is an approach to designing and operating nuclear facilities that prevents and mitigates accidents through multiple independent and redundant layers of defence, so that no single layer, no matter how robust, is exclusively relied upon; it includes access controls, physical barriers, redundant and diverse key safety functions and emergency response measures.

Successive physical barriers hold radioactivity in: ceramic fuel pellets sealed in zirconium-alloy tubes, a steel pressure vessel with walls up to 30 cm thick, and a reinforced-concrete containment with walls at least one metre thick.

The containment is the metre-thick concrete-and-steel structure around the reactor and steam generators designed to protect them from outside intrusion and to protect those outside from the effects of radiation in a malfunction.

The US NRC requires new reactor designs to meet a theoretical core damage frequency of 1 in 10,000 reactor-years; US utility requirements are 1 in 100,000, the best operating plants are about 1 in a million and European authorities require 1 in a million for new designs.

The AP1000 relies on natural forces - gravity, natural circulation and compressed gases - to keep the core and containment from overheating with no operator action and no on-site or off-site AC power; Westinghouse states its passive systems maintain core cooling and containment integrity indefinitely following design-basis events and that its core damage frequency is 1/100 that of currently operating plants.

The NRC issued the AP1000 design certification rule on 27 January 2006 (71 FR 4464) and the amended certification on 30 December 2011; the design is 1,250 MWe gross and 1,110-1,117 MWe net in the USA.

The EPR (1,750 MWe gross / 1,630 MWe net, double containment, four redundant safety trains) and the VVER-1200 both have core catchers beneath the reactor pressure vessel to contain molten core material in a severe accident; the VVER-1200's long-term decay heat removal does not rely on electrical power.

On the IAEA's International Nuclear and Radiological Event Scale (INES, levels 0-7) Chernobyl 1986 and Fukushima Daiichi 1-3 (2011) are rated level 7 'major accident', Kyshtym/Mayak 1957 level 6, Three Mile Island 1979 and Windscale 1957 level 5, and Saint-Laurent 1969/1980 and Tokai-mura 1999 level 4.

The Three Mile Island unit 2 accident began at about 4 a.m. on 28 March 1979 when the main feedwater pumps failed, a pilot-operated relief valve stuck open while instruments showed it closed, operators cut back emergency cooling, and the core overheated and partially melted.

The roughly 2 million people around Three Mile Island received an average dose of about 1 millirem above background, with a maximum site-boundary dose below 100 millirem, compared with about 6 millirem for a chest X-ray and 100-125 millirem per year of natural background; investigations found negligible effects on health or the environment.

The Three Mile Island accident gave the surrounding 2 million people an average of about 1 millirem - about a sixth of the chest X-ray dose (6 mrem) and about 1% of the annual natural background (100-125 mrem) quoted by the NRC.

The average American receives about 620 millirem (6.2 mSv) a year from natural background and medical sources; a chest X-ray is about 10 millirem (0.1 mSv) and a full-body CT scan about 1 rem (10 mSv).

The Chernobyl accident destroyed the RBMK-1000 unit 4 early on 26 April 1986 during a test; it was the product of a flawed Soviet reactor design (including a positive void coefficient) coupled with serious operator mistakes, and at least 5% of the core's radioactive inventory was released.

Two plant workers died on the night of the Chernobyl accident and a further 28 people died within a few weeks from acute radiation poisoning; UNSCEAR later reported some 5,000 thyroid cancers resulting in 15 fatalities, and found no evidence of a major public health impact attributable to radiation 20 years on.

On 11 March 2011 the magnitude 9.0 Great East Japan Earthquake and a tsunami about 15 m high (against a design-basis tsunami of 5.7 m set in 2002, raised to 6.1 m in 2009) disabled cooling at Fukushima Daiichi; the cores of units 1-3 largely melted within three days and the accident was rated INES level 7.

There have been no deaths or cases of radiation sickness from the Fukushima Daiichi nuclear accident (in 2018 the government recognised one former worker's lung-cancer death as radiation-linked for compensation), whereas the earthquake and tsunami killed about 19,500 people and 2,313 disaster-related deaths have been identified among evacuees from Fukushima prefecture.

No deaths or cases of radiation sickness have resulted from radiation at Fukushima Daiichi; the human toll came from the earthquake and tsunami (about 19,500 deaths) and from the evacuation (2,313 disaster-related deaths among evacuees).

Civil nuclear power has accumulated over 20,000 reactor-years of commercial operation in 36 countries, with three significant accidents at power plants: Three Mile Island (1979), Chernobyl (1986) and Fukushima Daiichi (2011).

Over 20,000 reactor-years of commercial operation in 36 countries have produced three significant power-plant accidents, of which only Chernobyl caused radiation deaths.

Deaths per terawatt-hour of electricity (accidents plus air pollution) are 32.72 for brown coal, 24.62 for coal, 18.43 for oil, 4.63 for biomass, 2.821 for gas, 1.3 for hydropower, 0.035 for wind, 0.03 for nuclear and 0.019 for solar.

Hydropower's 1.3 deaths per TWh is dominated by the 1975 Banqiao dam failure in China; excluding it, hydropower's rate is 0.04 deaths per TWh.

OWID notes the coal figure is conservative and estimates a global average of 93 to 224 deaths per TWh for coal when wider air-pollution evidence is used.

The IPCC's 2014 Fifth Assessment Report (Annex III) gives a median lifecycle emission of 12 g CO2-equivalent per kWh for nuclear power - similar to wind and lower than all types of solar.

The UNECE lifecycle assessment (published March 2022, often cited as UNECE 2021) estimated 5.1-6.4 g CO2-equivalent per kWh for nuclear, the lowest of all technologies assessed.

OWID (using UNECE data) puts solar PV at 23-82 g CO2eq/kWh depending on where panels are manufactured and oil at about 715 g CO2eq/kWh (range 530-900); coal emits more than a hundred times more than nuclear per kWh.

US utility-scale capacity factors in 2024 were 90.8% for nuclear (93.0% in 2023), 64.6% geothermal, 59.5% other biomass, 34.6% conventional hydro, 34.3% wind, 25.0% solar thermal and 23.2% solar photovoltaic.