Page 139 - Dynamics and Control of Nuclear Reactors

11.5 Accidents in Generation-II power reactors 135

because insertion introduced graphite at the tip of the rods, introducing reactivity. In
any case, a runaway power transient occurred and destroyed the reactor. Explosions
occurred, exposing the damaged reactor to the atmosphere and releasing
radionuclides.
Radioactive debris littered the reactor site and fires erupted. Heroic workers
risked their lives to bring things under control. Thirty-two people died immediately
or shortly after the accident. The radiation contamination in surrounding regions cre-
ated the likelihood of eventual health problems for residents.

11.5.3 Fukushima Dai-ichi [16]
On March 11, 2011 a major accident occurred at the Fukushima Dai-ichi power plant
on the eastern shore of Japan near the city of Sendai. The plant has six boiling water
reactors built by General Electric.
Unlike the prior accidents at Three Mile Island and Chernobyl, neither operator
errors nor reactor design flaws caused the accident. The cause was a huge
earthquake-caused Tsunami and unsafe location of emergency power systems and
heat exchangers.
At the time of the earthquake, units 1, 2 and 3 were operating and units 4, 5 and 6
were shut down. The operating reactors were scrammed immediately after the earth-
quake struck. Fifty minutes after the earthquake a 13m (42.6ft) high tsunami struck.
The reactor buildings survived the earthquake and tsunami, but the offsite power was
disrupted and the backup power sources (diesel generators and batteries were dis-
abled) and heat exchangers used to transfer heat to seawater were destroyed.
The unsafe positioning of crucial safety-related backup systems close to the
ocean was clearly a mistake. The potential problem with the location of these sys-
tems was recognized and some work had started on relocating these systems to higher
ground, but little had been done prior to the accident.
Without cooling, decay heat caused fuel melting in units 1, 2 and 3. Additionally,
a chemical reaction between Zirconium and water produced hydrogen gas, which
eventually exploded in units 1, 2, 3 and 4 (which received hydrogen leaked from
unit 3).
Units 1–4 were destroyed and radionuclides were released into the air and water,
creating long-term health concerns for residents in nearby areas. This increased the
already large impact on the human population (almost 19,000 died immediately due
to the tsunami).
After the initial chaos subsided, a long process of stabilization and cleanup began.
As mentioned above, the Fukushima event is a Beyond-Design-Basis Accident. A
near-term task force established by the U.S. Nuclear Regulatory Commission pub-
lished a report on the lessons learned from the Fukushima Dai-ichi accident [17]. The
NRC recommendations included twelve major points encompassing regulatory
framework, ensuring protection, enhancing mitigation, strengthening emergency
preparedness, and improving the efficiency of NRC programs. These must be applied
to both operating reactors and new reactors.