(The following press release from Tepco was received by e-mail. The sender
verified the statement.)
Findings help explain progression of the accident
TOKYO, August 6, 2014- Answers to a series of open technical questions about
exactly how the Fukushima Daiichi nuclear accident progressed are addressed in
a report that the Tokyo Electric Power Co. released today.
The report, the second in a series of progress reports issued on the inquiry,
was praised by Dr. Dale Klein, the former U.S. Nuclear Regulatory Commission
chairman who now chairs TEPCO’s Nuclear Reform Monitoring Committee: “I am
pleased that TEPCO is continuing the investigation and study of what exactly
happened regarding the Fukushima nuclear accident. Lessons learned from these
inquiries will benefit the safety of nuclear power plants around the world and
the people who depend upon them. I hope that TEPCO will continue to make
efforts to share these findings with the world as they discover new information
during the decontamination and decommissioning process. This information will
assist in making nuclear plants more safe. ”
Four High-Priority Questions reported
The report addresses four issues that had been deemed “high priority” by the
investigators. Although the answers are in some cases more in the nature of
hypotheses than definitive resolutions, they bring additional data and a higher
degree of confidence to those hypotheses.
Unit 3 RCIC System
In the Fukushima Accident, Units 1, 2, and 3 all ultimately suffered the
melting of nuclear fuel in the reactor cores as the result of the loss of
cooling power following the tsunami and the failure of both the off-site AC
electrical power and the backup power that was being produced by diesel
generators. But not all reactors behaved identically, and cooling power was
lost at different times in different reactors.
In Unit 2, a component of the cooling system known as “reactor core isolation
cooling” (RCIC), which is designed to maintain its function for basically four
hours continued to operate for several days. But in Unit 3, the same RCIC
system failed after 20 hours. Determining the reason for this earlier shutdown
was deemed important for its potential to provide information that would
improve the design of similar systems elsewhere.
According to the report, the Unit 3 RCIC system failed before its batteries
were drained because a safety device designed to prevent the RCIC system from
being damaged by high turbine exhaust pressure seems to have operated exactly
as it was designed to: it shut RCIC down.
Under normal reactor operations, this safety device prevents the RCIC system
from being damaged. But in an emergency setting where all other cooling systems
have failed, it may be better to let the RCIC continue operating even at the
risk of damage. Therefore, the report says, steps will be taken to allow this
“interlock” between the RCIC and the turbine exhaust pressure to be released in
emergencies. This will enable the RCIC system to continue working in a
situation where it is the only system available to protect the reactor core.
The finding should not be construed to suggest that the premature RCIC failure
caused Unit 3’s meltdown. As was the case with the other two reactors, given
the circumstances at Fukushima Daiichi after the earthquake and tsunami, all
cooling systems would have eventually been lost. Its significance, rather, lies
in understanding how to prevent premature shutdown of the RCIC system in future
emergencies in reactors of similar design, both in Japan and elsewhere.
Extent of Unit 3 core damage
The report helped resolve a discrepancy between what engineers believed had
happened inside Unit 3, based on various findings, and the analysis provided by
computer programs. Based on the findings, it has been reevaluated that most of
the melted core fuel had been dropped from the “Reactor Pressure Vessel” (RPV)
to the “Primary Containment Vessel” (PCV) at Unit 3. The reevaluation was made
after surveys revealed that the emergency water injection system known as “High
Pressure Coolant Injection” (HPCI) had not supplied the initially estimated
amount of water into the reactor. However, according to the analysis, even if
all the melted core fuel had dropped to the PCV, the estimated maximum erosion
of concrete mat did not lead to the breach of the PCV boundary. Further precise
evaluation is currently being made with government support, to determine how
deeply the fuel has eroded the concrete mat.
Unit 2 Pressure Increase and Fuel Melting
In an inquiry into the sequence of events leading to the melting of fuel in the
Unit 2 reactor core, the report concludes that the insufficient injection of
cooling water into the reactor core by fire engines actually accelerated the
According to the report, steam generation from the injection of water at a time
when the reactor’s core had been exposed caused a zirconium-water reaction, in
turn generating hydrogen and large amounts of heat, causing pressure in the
reactor to rise. The report also determined that the detected presence of
neutrons near the facility’s main gate on March 13-14 was the result of
spontaneous nuclear fission of easy- to-decay (short-life) actinides in Units 2
and 3 as the result of the fuel melting, and not a return of the reactor core
to recriticality (sustained nuclear fission).
Estimate of Amount of Cooling Water Provided by Fire Engines
The report estimates the total amount of water poured on Unit 1 by fire engines
in the immediate aftermath of the loss of cooling systems. Determining how much
cooling water actually reached the cores is important for engineers to
understand the speed and progression of the accident.
The report estimates that only 20-50 percent of the cooling water sprayed from
the trucks actually reached the reactor core. But the matter requires further
study, the report says, in part because data on the water pressure and flow
rate from the fire trucks is limited, preventing accurate estimates of the
Measures to inject poured water with certainty into the reactor are already
implemented not only at Fukushima Daiichi but also at TEPCO’s
Kashiwazaki-Kariwa Nuclear Power Station, the company reported.
The summary report, Report on the Investigation and Study of
Unconfirmed/Unclear Matters in the Fukushima Nuclear Accident, Progress Report
No. 2, can be accessed at
Dr. Klein’s comments on the report can be seen at
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