The Fukushima accident
BfS report on the accident’s development and causes
Emergency response provisions for nuclear power plants
The radiological situation in Japan
Fallout im Vergleich
Fragen und Antworten zu Strahlenschutz-Aspekten in Japan
Fragen und Antworten zu Strahlenschutz-Aspekten in Deutschland und Europa
FAQs on nuclear aspects of the Fukushima accident
FAQs on nuclear safety in Germany
Stellungnahme zur Jodblockade
Interview of the German Press Agency (dpa) with BfS president König about Fukushima

Nuclear Safety > ... > ... > FAQs on nuclear aspects of the Fukushima accident

FAQs on nuclear aspects of the Fukushima accident
FAQs on nuclear aspects of what happened in Japan

What happens during a core meltdown in a nuclear power plant?

What radioactive substances may be released?

In Fukushima Daiichi, units 1 to 5 are designed with a Mark I containment. What is a Mark I containment?

Who is gathering information on the Fukushima accident?

What does the categorisation of the Fukushima Daiichi accident as a INES Level 7 event mean?

What is the difference betweeen a core meltdown in a reactor with MOX fuel and a reactor with uranium fuel?

Based on current information, core damage occurred and core meltdowns took place in units 1, 2 and 3 at the Fukushima Daiichi nuclear power plant.

What happens during a core meltdown in a nuclear power plant?

In a nuclear power plant the reactor core is located in a thick-walled reactor pressure vessel. In case the reactor cooling systems fail, decay heat causes the water in the reactor pressure vessel to gradually turn to steam; at the same time the fuel heats up to its melting point resulting in a core meltdown. The molten mass from the destroyed fuel elements collects at the base of the reactor pressure vessel.

If enough mass collects, the core meltdown melts through the wall of the reactor pressure vessel and leaks into the containment. This vessel is part of the reactor building; its task is to ensure that the radioactive inventory is securely contained even in the event of an accident. If the containment is not designed to cope with a core meltdown, it also fails and enables the release of radioactive substances from the molten core into the environment.

What radioactive substances may be released?

During a core meltdown, the radioactive substances contained in the destroyed fuel elements (uranium, plutonium, and fission products such as krypton, strontium and caesium) are initially released into the reactor pressure vessel; if this is damaged (by the molten core, for instance), they leak into the containment, and if the latter is breached, they escape into the environment of the plant.

If released, the substances behave differently - depending on their chemical characteristics, as well as the ambient temperature and atmospheric pressure conditions.
  • Generally, if the containment is breached, all gaseous substances (e.g. noble gases such as krypton and xenon) are released more or less. This is also the case for highly volatile substances such as iodine and caesium.
  • Less volatile substances such as strontium, uranium and plutonium are either present as dust particles (aerosols) or are bonded to dust particles. It depends on the specific circumstances of the core meltdown, whether the entire inventory of these substances contained by the reactor is released, or only a proportion thereof, and how far they are transported.
Weather conditions such as wind speed, wind direction and precipitation therefore have a huge influence on the protection measures for the public.

The published radiation measurement data (local gamma dose rate) taken in Fukushima on 12 and 13 March 2011 indicate that both noble gases and longer-lived radionuclides were released into the environment during venting measures designed to relief pressure.
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In Fukushima Daiichi, units 1 to 5 are designed with a Mark I containment. What is a Mark I containment?

Mark I is the name of a containment construction line for boiling water reactors produced by the American manufacturer General Electric.

A total of 32 reactor blocks around the world feature a Mark I containment, 23 of them are located in the USA. This containment is used for reactors of different construction lines and performance levels. The Mark I containment was designed in the 1960s and was the first containment for boiling water reactors to be produced commercially in larger numbers.

The containment encloses the nuclear components of the primary cooling system (reactor pressure vessel, coolant pumps and related pipes, and in the case of pressurised water reactors, the steam generator and storage pools for spent fuel elements) and is, thus, the final technical barrier preventing the release of the reactor's radioactive inventory. In the event of loss of coolant accidents, it fulfils a special function: the leaking coolant is collected in the containment sump for recirculation in the cooling system.

The main component of a Mark I containment is a pear-shaped pressure vessel made of reinforced concrete. Its cylindrical section houses the reactor pressure vessel, the latter's concrete biological shield, and the recirculation pumps; the round section below contains the concrete load-bearing structure which supports the reactor pressure vessel and the control rod drive mechanisms.

The Mark I containment features a pressure release system in the form of a condensation chamber (so-called wet well). This is a torus-shaped steel container (a shape reminiscent of a donut), located below the pear-shaped pressure vessel and connected to it by a series of pipes. Open at both ends, these pipes are submerged below the water seal in the condensation chamber. Should a leak occur and steam be released into the pressure vessel, the water vapour is channelled through the pipes into the water held in the condensation chamber for immediate condensation.

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Who is gathering information on the Fukushima accident?

Japan's former NISA - Nuclear and Industrial Safety Agency supplied information on the damage to the Japanese nuclear power plants. This information can be found on the website of NSR - Nuclear Regulation Authority, Japan. Another source of information about Fukushima accident is provided by JAIF - Japan Atomic Industrial Forum.

At international level the IAEA – International Atomic Energy Agency in Vienna is collacting information on the Fukushima accident; it publishes topical material on the Internet (English only). The IAEA is an independent scientific organisation linked by special agreement to the United Nations.

Das Fukushima information site by the GRS – Gesellschaft für Anlagen- und Reaktorsicherheit mbH - also offers inforamtion about the current sitiuation in Japan (German only).

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What does the categorisation of the Fukushima Daiichi accident as a INES Level 7 event mean?

INES (International Nuclear and Radiological Event Scale) is the assessment scale used by the International Atomic Energy Agency (IAEA) in Vienna. Member States which have signed up to the international contracts (Japan is one such signatory) are obliged to report nuclear power plant incidents or reportable events (accidents) of INES level 2 or more to the IAEA.

According to this international standard, the INES Level 7 categorisation indicates the severest possible release of radioactivity having an impact on health and the environment over an extended period of time and over a considerable area

The INES Level 7 categorisation does not, however, give any indication as to how many people will be affected, and what the long-term consequences for the environment might be. What might still happen in Japan and how much radioactivity will be released in total cannot yet be assessed. To help the local population and to keep environmental contamination to a minimum, extensive measures must be initiated, based on the specific readings, findings, and facts established to date.

The accident in the Fukushima Daiichi nuclear power plant has been categorised as INES Level 7. This is the first accident which has to be assigned to this level since Chernobyl. However, technically speaking the accident in Fukushima developed in quite a different way compared to the event at Chernobyl:
  • in Chernobyl radioactivity was expelled to a great height and distributed over a huge area.
  • in the case of Fukushima very high levels of radioactivity have generally been limited to the region around the nuclear power plant and specific locations, some of which are outside the evacuation zone.
A comparison of the two accidents can be found in the feature Fallout compared (in German only).

What is the difference between a core meltdown in a  reactor with MOX fuel and in a reactor with uranium fuel?

Unit 3 of the Fukushima Daiichi nuclear power plant used mixed oxide fuels (MOX fuel elements). MOX fuel elements not only contain a uranium fuel source, but a small amount of plutonium as well. Since plutonium is constantly being created from uranium during reactor operations, even pure uranium fuel elements end up containing plutonium after a while. In the process, plutonium is not only created, but also split, thus contributing to the production of energy. As a result the composition of the reactor core gradually changes. In a reactor loaded with mixed oxide fuels, the core contains approximately two to five times more plutonium than a uranium reactor core which has been in operation for an extended period. Moreover, once operations have been going for some time, the core contains considerably higher levels of so-called transuranium elements such as neptunium, americium and curium.

According to the increased amounts of plutonium, neptunium, americium, and curium in the fuel elements, larger quantities of these substances are released during a core meltdown and may, therefore, escape into the environment. In such cases plutonium, neptunium, americium, and curium are present, like uranium, either in the form of dust particles (aerosols) or bonded to dust particles.

Temperatures must be considerably above 2,000° Celsius for these substances to be released in any quantity from the molten mass. The differences between a uranium core and a mixed oxide core are not of any real significance when it comes to the environmental impact of a core meltdown.
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