Fukushima Daiichi NPS Prompt Report 2014

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 melting.

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 water volume.

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 (PDF 544KB)

Dr. Klein's comments on the report can be seen at
http://www.nrmc.jp/en/report/detail/1240092_5233.html

About TEPCO

Tokyo Electric Power Company, Inc. (TEPCO) is Japan's largest power company, supplying energy to the greater Kanto area, including Japan's two most populous cities, Tokyo and Yokohama. Its 35,000 employees are committed to providing safe, reliable power to its 28.8 million customers, diversifying energy resources to ensure sustainability, and contributing to economic growth while fully meeting its responsibilities after the Fukushima Daiichi accident.