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 Ministry of Education, Culture, Sports, Science and Technology - MEXT publishes information on the accident in English. 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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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).
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.
