To date, the nuclear accidents at the Chernobyl (1986) and Fukushima Daiichi (2011) nuclear power plants, are the only INES level 7 nuclear accidents.[1] [2]
The following table compares the Chernobyl and Fukushima nuclear accidents.
| Plant Name | Chernobyl | Fukushima Daiichi | |
|---|---|---|---|
| Location | Soviet Union (Ukrainian Soviet Socialist Republic) 51.3895°N 30.0991°W | Japan 37.4255°N 141.0216°W | |
| Date of the accident | April 26, 1986 | March 11, 2011 | |
| INES Level | 7 | 7 | |
| Plant commissioning date | 1977 | 1971 | |
| Years of operation before the accident | 9 years (plant) | 40 years (plant) | |
| Electrical output | plant (net): 3700 MWe (4 reactors) reactors (net): 925 MWe (Units 1 - 4) | plant (net): 4546 MWe (6 reactors) reactors (net): 439 MWe (Unit 1), 760 MWe (Units 2 - 5), 1067 MWe (Unit 6) | |
| Type of reactor | RBMK-1000 graphite moderated, 2nd generation reactor without containment | BWR-3 and BWR-4 reactors with Mark I containment vessels | |
| Number of reactors | 4 on site; 1 involved in accident | 6 on site; 4 (and spent fuel pools) involved in accident; one of the four reactors was empty of fuel at the time of the accident. | |
| Amount of nuclear fuel in affected reactors | 1 reactor—190 tonnes (t, metric tons = 210 U.S. short tons): spent fuel pools not involved in incident[3] | 4 reactors—854 tonnes (t, metric tons): 81 t in Unit 1 reactor, 111 t in Unit 2 reactor, 111 t in Unit 3 reactor, 0 t in Unit 4 reactor (defueled), 59 t in Unit 1 spent fuel pool (SFP), 119 t in Unit 2 SFP, 104 t in Unit 3 SFP, and 269 t in Unit 4 SFP | |
| Cause of the accident | Proximate cause was human error and violation of procedures. The unsafe reactor design caused instability at low power due to a positive void coefficient and steam formation. When an improper test was conducted at 1:00 am at low power, the reactor became prompt critical. This was followed by a steam explosion that exposed the fuel, a raging fire, and a core meltdown. The fire lasted for days to weeks, and there is controversy over whether it was the fuel burning, nuclear decay heating or whether the graphite moderator that made up most of the core was involved. See Chernobyl Disaster, Note 1, for more discussion. | The plants were not designed with consideration of such a large tsunami concurrently occurring with the ground sinking. Subsequent review did not lead to mitigation. A major earthquake and tsunami caused the destruction of power lines and backup generators. Once the plants were without external power and the generators were flooded, a catastrophic decay heat casualty ensued, leading to major reactor plant damage including meltdowns and explosive loss of reactor containment. | |
| Maximum level of radiation detected | 300 Sv/h shortly after the explosion in vicinity of the reactor core.[4] | 530 Sv/h inside Unit 2 containment vessel in 2017 according to Japan Times.[5] | |
| Radioactivity released | According to IAEA, total release was 14EBq.[6] 5.2EBq in iodine-131 equivalent [7] [8] | As of 2014, a peer reviewed estimate of the total was 340 - 780 PBq, with 80% falling into the Pacific Ocean.[9] Radiation continues to be released into the Pacific via groundwater. | |
| Area affected | An area up to 500km (300miles) away contaminated, according to the United Nations.[10] [11] [12] | Radiation levels exceeding annual limits seen over 60km (40miles) to northwest and 40km (30miles) to south-southwest, according to officials. | |
| Exclusion Zone Area | 30 km | 20 km (30 km voluntary) extending north-west to 45 km in the downwind direction to Iitate, Fukushima[13] | |
| Population relocated | 335,000 (About 115,000 from areas surrounding the reactor in 1986; about 220,000 people from Belarus, the Russian Federation and Ukraine after 1986) | 154,000[14] | |
| Population returned | None | 122,000[15] | |
| Direct fatalities from the accident | Two immediate trauma deaths; 28 deaths from Acute Radiation Syndrome out of 134 showing symptoms; four from an industrial accident (helicopter crash); 15 deaths from radiation-genic thyroid cancers (as of 2005);[16] as many as 4000 to 90000 cancer related deaths.[17] | 1 confirmed cancer death attributed to radiation exposure by the government for the purpose of compensation following opinions from a panel of radiologists and other experts, medical sources pending for long-term fatalities due to the radiation. | |
| Current status | All reactors were shut down by 2000. The damaged reactor was covered by a hastily built steel and concrete structure called the sarcophagus. A New Safe Confinement structure was installed in November 2016, from which the plant will be cleaned up and decommissioned. | Cold shutdown declared on 16 December 2011, but decommissioning is likely to take 30 to 40 years.[18] [19] All fuel rods in reactor 4 pool removed. Fukushima disaster cleanup is ongoing. |
| Report date | Place | Period | Iodine-131 (TBq) | Caesium-137 (TBq) | Source | |||
|---|---|---|---|---|---|---|---|---|
| from | to | from | to | |||||
| 2002 | Chernobyl | 25 April – June 1986 | 1,600,000 | 1,940,000 | 59,000 | 111,000 | NEA[20] | |
| 22 March 2011 | Fukushima | 12 – 15 March 2011 | 400,000 | 3,000 | 30,000 | ZAMG[21] | ||
| 2 April 2011 | Fukushima | 12 – 19 March 2011 | 10,000 | 700,001 | 1,000 | 70,000 | ZAMG[22] | |
| 12 April 2011 | Fukushima | 11 March – 5 April[23] | 150,000 | 12,000 | NSC[24] | |||
| 12 April 2011 | Fukushima | 11 – 17 March 2011 | 130,000 | 6,100 | NISA | |||
| 7 June 2011 | Fukushima | 11 – 17 March 2011 | 160,000 | 15,000 | NISA[25] | |||
| 24 Aug. 2011 | Fukushima | 11 March – 5 April | 130,000 | 11,000 | NSC[26] | |||
| 15 Sept. 2011 | Fukushima | March – September | 100,000 | 200,000 | 10,000 | 20,000 | Kantei[27] | |
| Report date | Place | Period | Amount (TBq) | Source | |
|---|---|---|---|---|---|
| 12 April 2011 | Chernobyl | 25 April – June 1986 | 5,200,000 | NISA | |
| 12 April 2011 | Fukushima | 11 March – 5 April 2011 | 630,000 | NSC | |
| 12 April 2011 | Fukushima | 11 – 17 March 2011 | 370,000 | NISA | |
| April 2011 | Fukushima | 4 April 2011 | 154 | NSC | |
| 25 April 2011 | Fukushima | 24 April 2011 | 24 | NSC | |
| 6–7 June 2011 | Fukushima | 11 – 17 March 2011 | 770,000 | NISA[28] > | |
| 7 June 2011 | Fukushima | 11 – 17 March 2011 | 840,000 | NISA,[29] press printing | |
| 17 August 2011 | Fukushima | 3–16 August 2011 | 0.07 | Government[30] | |
| 23 August 2011 | Fukushima | 12 March - 5 April 2011 | 630,000 | NISA[31] |
| Report date | Period | Into the sea (TBq) | Source | ||
|---|---|---|---|---|---|
| direct | indirect | ||||
| 21 May 2011 | 1 – 6 April 2011 | 4,700 | Tepco[32] | ||
| End of August 2011 | March – August 2011 | 3,500 | 16,000 | JMA[33] | |
| 8 September 2011 | March – April 2011 | align=center colspan=2 | 15,000 | Scientist Group[34] | |
| 29 October 2011 | 21 March – 15 July 2011 | align=center colspan=2 | 27,100 | IRSN[35] | |