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http://vorort.bund.net/suedlicher-oberrhein/pressurized-water-reactor-nuclear.html
Nuclear Reactor: Function and Risks - Pressurized Water Reactor
How does a pressurized water reactor work?
Many nuclear power plants operate with pressurized water reactors
(PWRs). The PWR is the most common reactor type. Nuclear fission of the
fuel elements heats up the water in the reactor to approximately 315°C
(600°F). The water is constantly kept under pressure (about 2200 psig
[15 Mpa]). To control or shut down a nuclear reactor, control rods are
used. When a control rod is inserted in the reactor core, it absorbs a
part of the neutrons released by the nuclear fission events. Thus, the
neutrons are not available to trigger off further nuclear fissions. In
this way uncontrolled increase of chain-reactions in the reactor is
prevented. The 315°C warm water heats up the second coolant loop by
means of a heat exchanger (here called steam generator). In the
secondary coolant loop the steam generator forms steam which drives the
turbine.
Every nuclear power plant (PWRs as well) produces the radioactivity of a
hiroshima bomb per megawatt of electric power in one year?s operating
time. This means that in a nuclear power plant with an annual power of
1200 MW the radioactivity of approximately 1200 hiroshima bombs comes
into existence. Certain parts of the radioactive material decay very
quickly. Others, however, have half-lives longer than 24,000 years (e.g.
plutonium) and therefore will factually exist permanently. Ageing
nuclear power plants with extended operating times and embrittled
reactor vessels heighten the risk of an accident.
This type of reactor is not technically safe, as could be seen from the
nuclear meltdown in the almost new PWR in 1979 in Harrisburg, U.S.
(Three Mile Island accident). In case of an accident or a terrorist
attack the nuclear power plant is shut down by means of the control
rods. However, for a period of several days after the shutdown the
radioactive decay of the fission products continues to generate heat.
All nuclear power plants are provided with redundant emergency cooling
systems to safely remove the decay heat also in the event of a disaster.
When the cooling systems break down (as it occurred during the Three
Mile Island accident), the rising temperature can cause a nuclear meltdown.
If the fuel assemblies in the core melt, the rate of chain reactions
will increase and lead to an extreme and uncontrolled heating-up
process. When the reactor vessel does not resist or when a huge amount
of radiation emerges, the accident is called 'maximum assumed accident'.
PWR -- Not safe from terrorist attacks and plane crashes
The Oekoinstitut [ecological institute] Darmstadt has calculated the
areal impact of a disaster of the kind described above, using the PWR of
the French nuclear power plant Fessenheim as an example. The study was
based on an assumed major accident in the French EDF/EnBW- nuclear power
plant Fessenheim:
/In case of south-west wind and rain, the area of damage would have a
spread of up to 370 km from Fessenheim to the region of
Würzburg-Nürnberg. If the Chernobyl guidelines were adopted, in this
zone all inhabitants would have to be evacuated for a period of 50 years./
Klaus Traube (nuclear expert; former director of AEG?s nuclear reactor
department at General Dynamics, San Diego; after this, head of
Interatom) wrote:
/The analysis of numerous hazardous incidents shows that they usually
are triggered off by an unexpected concurrence of malfunctions and
operating errors (like for example in the Harrisburg/Three Mile Island
accident and, by the way, in Chernobyl as well). Individually, the
malfunctions and errors seem to be trivial. Elaborate risk studies
dealing with the possibilities and probabilities of a nuclear reactor?s
disastrous breakdown arrive at the same conclusion. They confirm: Every
running reactor at any time renders possible accidents leading to
nuclear meltdowns with a subsequent disastrous release of radioactivity.
Not the truth of this statement, but solely the probability of
occurrence is debated among experts. Furthermore, it is a fact that acts
of terrorism (like for example plane crashes) can cause a nuclear
disaster. Again, solely the probability is debatable./
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