Civil Engineering Reference
In-Depth Information
1 connection primary line
2 connection primary line
3 cavity built by melt down
4 loose fragments of the core
5 crust of the melt
6m lt n m t i l
7
fragments at bottom of spherical calotte
8
uranium depleted region
9
destroyed duct
10
perforated shield
11
layer of molten material on the surfaces
of the bypass channels
12
damages at the upper grid
Fig. 9.2
Molten reactor core of the Three Mile Island accident [
1
,
2
]
fission gas plena of the fuel rods had been destroyed, and gaseous fission products
had been released. Parts of the reactor core had molten through at the edge of the
grid plate and collected at the bottom of the reactor pressure vessel. However, the
hemispherical bottom of the pressure vessel did not melt through.
The reactor plant was decontaminated in many years of work, and the reactor top
lid was opened. The partly molten reactor core had to be disposed of. Costs
amounted to approximately $1 billion [
2
].
Lessons Learned
The non-availability of the emergency feed system, the errone-
ous signals produced by the instrumentation with regard to the “open” valves of the
pressure vessel, and the lack of knowledge of the real status of the plant as well as
the shutdown of the primary pumps led to a partial core meltdown.
Present-day pressurized water reactors have better instruments in the reactor
pressure vessel and displays in the control room. In addition they have fourfold
redundancy of primary cooling and emergency cooling systems, and the possibility
to reduce primary and secondary pressures with subsequent possibilities to feed
water.
9.2 The Chernobyl Accident
The Chernobyl accident occurred on April 26, 1986 in unit 4 of the reactor plant of
four units of 1,000 W(e) or 3,200 MW(th) power each. The four reactor plants had
been build some 130 km north of Kiev and comprised four Russian graphite-
moderated, boiling-water cooled so-called RBMK1000 reactors. The UO
2
fuel
was enriched with 2 % U-235. The core of these RBMK1000 reactors is about
7 m high and about 12 m in diameter. The RBMK1000 has two coolant loops with
