Civil Engineering Reference
In-Depth Information
30 MW(th) because of operator error. This led to additional buildup of Xe-135
(neutron poison). As a consequence the operators had to withdraw the control rods
manually to their upper limits after they had shut off the automatic control system.
The RBMK1000 was known to have a positive coolant temperature coefficient.
This gave rise to instabilities in power production, coolant flow and temperatures in
the low power range.
Then the experiment began at the power level of 200 MW(th). Steam to the
turbine was shut off. The diesel generators started and picked up loads. The primary
coolant pumps also run down. However this led to increased steam formation as the
coolant temperature was close to its boiling temperature. With its positive coolant
temperature coefficient the RBMK1000 reactor now was on its way to power
runaway. When the SCRAM button was pushed the control elements started to
run down into the reactor core. However, due to a wrong design of the lower part of
the control elements (graphite sections) the displacement of the water by graphite
led to an increase of criticality. A steep power increase occurred, the core over-
heated causing the fuel rods to burst, leading to a large scale steam explosion and
hydrogen formation. The reactor core was destroyed and the top shield cover and
the fuel refueling machine were lifted up. Fuel elements and graphite blocks were
dispersed outside the reactor core. The reactor core was now open to the atmo-
sphere. Fission products and fuel aerosols were distributed over the Ukraine,
Belarus, Russia and Europe. Very high radiation doses were received by fire
fighters, operators, helicopter pilots and members of the emergency team. Approx-
imately 800,000 military people were involved in rescue teams receiving various
levels of high radiation doses. About 135,000 people were evacuated rather late. In
total about 3,000 km
2
of land were contaminated with more than 1,500 Bq/m
2
,
roughly 7,200 km
2
with 600-1,500 Bq/m
2
and about 103,000 km
2
with 40-200 Bq/
m
2
of Cs-137. The Chernobyl accident was classified a level 7 accident on the
International Nuclear Event Scale (INES).
The severe reactor accidents at Fukushima occurred in 2011 after a severe
earthquake with intensity 9 (Richter scale) close to the northeastern coast of
Japan. The earthquake was followed by a tsunami wave which hit the six BWRs
of the Fukushima-Daiichi plant with a water level up to 14 m. Unfortunately the
Fukushima-Daiichi plant was only protected up to a tsunami wave level of 5.7 m.
Only three BWRs of the six BWRs of the Fukushima-Daiichi plant were in
operation when the earthquake and the tsunami wave hit the reactor site. All
BWRs were duly shut down by the seismic instrumentation and changed into the
residual heat removal mode. However, the tsunami wave flooded the two diesel
generators of each of the three reactor units 1-3, located in the lowest part of the
turbine building. The diesel generators and the battery systems failed. The external
grid power and heat exchangers transferring afterheat to the ocean water had
already been destroyed by the earthquake. In unit 1 due to the lack of electrical
power the high pressure coolant injection system did not work. The steam driven
isolation condenser system worked only partly in time and failed. The primary
coolant system could not be depressurized due to lack of electrical power and
pressurized nitrogen. Low pressure emergency pumps, therefore, could not feed
