Civil Engineering Reference
In-Depth Information
Within the time period of about 1 min until the diesel generators reached full power,
the running down turbine generators systems was to support the diesel generators.
As the coolant flow rate decreased (the four coolant pumps were also running down;
only one coolant pump was to obtain its power supply from the diesel generator and
the turbine generator) this lead to increased steam formation (coolant near boiling
temperature) in the core. With its positive coolant temperature coefficient and
increasing steam formation the reactor theoretically, was now on its way to
power runaway.
At this point in time the emergency shut down (SCRAM) bottom was pressed
manually. All control rods started immediately to be fully inserted. The control rods
moved with a speed of 0.4 m/s into the 7 m high core. The control rods contained a
graphite section in their bottom parts followed by absorber sections with boron
carbide. During insertion into the upper core neutron-absorbing water was
displaced by non-neutron absorbing graphite. This led to an additional increase of
the effective neutron multiplication factor k
eff
. A steep power increase occurred
causing the core to overheat. The fuel rods ruptured under the pressure of over-
heated fuel and fission product gases. The finely dispersed fuel abruptly mixed with
the cooling water causing of steam explosion (Sect.
10.2.1
). According to theoret-
ical analysis the reactor power jumped to about 30,000 MW(th), ten times the
normal operational output. The last reading on the control panel showed
33,000 MW(th). The steam generated caused the destruction of the steam boiler
and of core structures and lifted the 2,000 tons top shield together with the refueling
machine upwards. Fuel elements and red glowing (not burning) graphite parts were
ejected from the core. The reactor building was heavily damaged [
8
]. A second
explosion occurred some seconds later terminating the nuclear reaction and
destroying the reactor core and building structures even more, dispersing damaged
fuel elements and red glowing graphite parts. There are hypotheses that this second
explosion was a second steam explosion or a hydrogen explosion (hydrogen
generated from a chemical reaction between the zirconium fuel rod cladding [
8
]).
As bitumen had been used for the construction of the reactor building floor and the
turbine hall ejected material ignited fires. The remains of the overheated reactor
core were now open to the atmosphere. The fission product gases as well as fission
product aerosols and fuel aerosols released were driven by the heat release to an
altitude of roughly 2,000 m, in some cases even 10,000 m. Strong winds at these
altitude distributed the aerosols over the Ukraine, Belarus, western parts of Russia
and Europe. The damaged RBMK1000 (unit 4 of the Chernobyl reactor plant) is
shown by Fig.
9.4
.
Shortly after the accident firemen arrived to extinguish the fires. Many firemen
received very high doses of radiation. The fire was finally extinguished by a
combined effort with helicopters dropping 5,000 tons of sand, lead, clay and
boron carbide onto the burning reactor. However none of the neutron absorbing
boron carbide reached the core [
8
]. Remotely controlled cranes and bulldozers were
used to push back the radioactive material into the reactor. Radioactive debris was
shoveled by liquidators wearing heavy protective gears. These workers could only
spend a maximum of 40 s working because of the high radioactive doses [
9
]. There
