LESSONS FROM FUKUSHIMA ACCIDENT

1 ATS-jäsentilaisuus 26.4.2011 LESSONS FROM FUKUSHIMA ACCIDENT A way towards more safe use of nuclear power? Jukka Laaksonen SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY 26.4.2011

What can we conclude from frequency of severe accidents at NPP s? The PSA studies say consistently that the frequency of severe accidents at older NPP s is in the range 10-4 10-5 /year, or even less. The experience indicates a frequency of more than 3 x 10-4. Can PSA results be that wrong? SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 2

What is the message of PSA results? PSA gives us information only on scenarios that we can model, and it is definitely a good tool for its purpose when it is used right! but None of the scenarios we have seen in severe accidents, TMI Chernobyl 3 x Fukushima was modelled and studied with PSA! SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 3

TMI 1979 Immediate cause: Operators did not handle right a relatively simple incident scenario. Root cause: Lack of safety culture: designers & regulators In 1979, the safety research was focusing only on large break loss of coolant accident. However, in 1974 the Reactor safety study ( Rasmussen study ) had shown the dominant risk importance of small leaks and transients The behaviour of a PWR primary circuit had not been thoroughly studied and was not understood. The operators had no instructions for the event they met. Lessons: Preparation of proper symptom oriented emergency operating procedures started in 1980 SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 4

Chernobyl 1986 Immediate cause: The reactor was not made inherently safe as was required in the countries following US regulations developed in 1960 s. Root cause: Lack of safety culture: designers & operators. The designers were aware of the possibility of explosive reactivity increase and this had been seen in precursory events. Operators were not clearly warned of the danger, and they did not follow the instructions written by reactor designers. Instead they took orders from the grid control centre. Lessons: Develop safety culture. More attention to reactivity transients and severe accident management. SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 5

Fukushima 2011 Immediate cause: Large earthquake followed by tsunami. Root cause: Lack of safety culture: designers & regulators Tsunamis are well known in the Japanese history and large tsunamis have been recorded in the Pacific Ocean several times in a century. Tsunamis were not used as a design basis for Fukushima plants and they were mentioned in the Japanese nuclear safety regulations only about five years ago. Lessons: Learn from experience. Evaluate the external hazards in a conservative manner. SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 6

Next accident???? Immediate cause:??? Root cause: Lack of safety culture? We must not tolerate any more accidents with this root cause. It is time to take strong actions to eliminate severe nuclear accidents in the foreseeable future. This target is achievable but it requires new attitudes among those who decide on the use of nuclear power and those who s work can influence its safety. SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 7

What can we do in practice? Continuous learning, tangible measures to enhance safety. International nuclear safety regime Common safety objectives and safety standards Transparent regulatory decisions, accountability to society International transparency, peer reviews (WANO, IAEA) Responsibility of national leaders Right to use nuclear power should not override consideration of safety Adherence to Convention on Nuclear Safety Recognition of national responsibility Technical infrastructure, competent professionals Site specific conditions, no plants in areas with hazards SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN 8

Laitospaikka ilmasta katsottuna 1. 4. yksiköillä suunnitteluperusteena 5,7 metrin aallonkorkeus (merivesipumput), turpiinihallit 10 m korkeudella merestä. Dieselgeneraattorit turpiinihallien päädyissä, polttoainesäiliöt ulkona. Kytkinlaitokset (ilmeisesti maanjäristyssyistä) kellarissa. SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY

Jäähdytys ilman vaihtosähköä: Eristyslauhdutin (yksiköllä 1) Eristyslauhdutin on passiivinen primääripiirin jäähdytysjärjestelmä, joka oli vanhimmilla BWRlaitoksilla ja joka tulee takaisin uusiin laitoksiin. Fukushima ykkösen eristyslauhdutin lakkasi toimimasta jo 54 minuutin kuluttua tsunamista, kun sen sekundaaripuoli oli kiehunut kuivaksi. Merivesipumppaus alkoi 29 tunnin kuluttua: ykkösen reaktori on pahasti vaurioitunut! SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY

Jäähdytys ilman vaihtosähköä: RCICS (yksiköillä 2 ja 3) RCICS on 1x100 % höyryturpiinikäyttöinen apusyöttövesijärjestelmä, jolla saadaan vettä wet wellistä reaktoriin. Järjestelmä vaatii akkusähköä venttiilien auki pitämiseen, ja muutaman tunnin kuluessa mahdollisuuden siirtää jälkilämpöä wet wellistä mereen. Lisäksi järjestelmän käyttö edellyttää höyryn paineen ylläpitoa reaktorissa. RCICS toimi kakkosella 70 tuntia tsunamin jälkeen, kolmosella 38 tuntia. SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY

Säteilytaso laitosalueella Merkittävimmät ilmapäästöt näyttävät syntyneen 15.-17.3. kakkos- ja kolmosyksikön räjähdysten yhteydessä. Säteily 19.-23.3. lienee suoraa säteilyä polttoainealtaasta. SÄTEILYTURVAKESKUS STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY