TEPCO ISSUES PROGRESS REPORT ON INQUIRY INTO UNANSWERED DETAILS

     (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  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, can be accessed at  http://www.tepco.co.jp/en/press/corp-com/release/betu14_e/images/140806e0101.pdf  Dr. Klein’s comments on the report can be seen at  http://www.nrmc.jp/en/report/detail/1240092_5233.html    
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