Civil Engineering Reference
In-Depth Information
high-pressure safety feed system because the water level in the pressure vessel
became too high, and opened the valve to a dump pipe. As a consequence of these
steps, the water level in the reactor pressure vessel dropped so far that the water
coolant began to boil. Although the operating team now succeeded in opening the
valves in the feed water line, which had been shut at the beginning of the sequence
of accidents, this changed nothing in the further course of the accident.
Roughly 15-30 min after the start of the accident, also the storage tanks filled
with radioactive water in the auxiliary systems building started to spill over.
Radioactive gases and aerosols entered the atmosphere of the auxiliary systems
building. The filters of the auxiliary systems building were able to retain some
99.9 % of the aerosols. But the radioactive noble gases, escaped through to filters to
the environment.
As the primary pumps began to vibrate under the impact of steam in the cooling
water, the operators first shut down the primary pump of primary cooling circuit B
and, slightly later, also that of cooling circuit A. As a consequence, the cooling
water in the core began to boil even more violently. The fuel elements, in particular
the fuel rod claddings, heated up. At temperatures of the fuel rod claddings above
1,200
C, steam began to react chemically with the zirconium of the zircaloy
cladding, and hydrogen was produced.
Zr
þ
2H
2
O
!
ZrO
2
þ
2H
2
:
This situation changed only gradually after the operators had created a feed-and-
bleed procedure by again feeding water through the high-pressure safety feed
systems and bleeding the steam through the open pressure relief system. As late
as 15 h after the start of the accident it became possible to restart a primary pump
and transfer the reactor into a stable residual heat cooling mode.
The hydrogen produced in the reactor core during the accident entered the
reactor building together with the steam, initiating an explosion as a result of
self-ignition. As iodine and cesium combined chemically to produce, e.g., CsI,
and occurred as aerosols, they were largely retained by the filters in the auxiliary
systems building.
The small amounts of aerosols, the shortlived radioactive noble gases, and the
gaseous I-131 (halflife 8 days) gave rise to only a relatively low mean radioactive
exposure of the population of 0.015 mSv [
2
] (The world wide annual effective dose
caused by natural radiation is about 2.4 mSv/year with a typical range of 1-10 mSv/
appointed by the U.S. President to investigate the Three Mile Island accident
arrived at this finding: “The Three Mile Island accident would cause so few cases
of cancer, if any, that they would not be detectable statistically” [
3
].
Analysis of the accident (Fig.
9.2
) indicated that roughly one third of the zircaloy
fuel rods had reacted with steam and produced hydrogen. When the water of the
emergency cooling feed systems contacted the hot fuel rods, this resulted in
fragmentation of the zircaloy fuel rod claddings and the UO
2
pellets. The silver-
indium-cadmium control rods had molten almost completely. Nearly the entire
