October 2011 – Review of Accident at Fukushima Daiichi – Proposed Countermeasures

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 Review of Accident at Tokyo Electric Power Company Incorporated’s Fukushima Daiichi Nuclear Power Station and Proposed Countermeasures (Summary)

October 2011

Japan Nuclear Technology Institute

Fukushima Daiichi Nuclear Power Station Accident Investigation and Review Committee

Introduction

This report is a compilation of the aggregate efforts of the nuclear power industry comprising electric power operators and manufacturers in Japan as concerns the accident at Tokyo Electric Power Company Incorporated’s Fukushima Daiichi Nuclear Power Station (“Fukushima Daiichi”) which resulted from the Great East Japan Earthquake that occurred on March 11, 2011.

This report analyzes the accident based on published facts, knowledge grounded in operational experience accumulated to date, and knowledge developed through plant design, and formulates measures and proposals to be addressed in the future. We believe that implementing these measures and proposals will ensure that nuclear power stations are even safer.

Overview of Fukushima Daiichi Nuclear Power Station and the Accident

Fukushima Daiichi is located in northeastern Japan (latitude 37 degrees north and longitude 141 degrees east), and looks out over the Pacific Ocean at a distance approximately 225km north of Tokyo.

The first unit commenced operation in 1971, and has a total electrical output of 4,696MW.

At Fukushima Daiichi, as Units 1 thru 3 were operating and Units 4 thru 6 were in the refueling outage, an earthquake occurred off the Pacific coast in the Tohoku [Northeastern] region of Japan at 14:46 on March 11, 2011, and Units 1 thru 3 automatically shut down in accordance with the signal indicating “increasing seismic acceleration.”

Following the reactor shutdown, generators shut down and, moreover, functionality of any external power source system equipment was also lost.

However, AC power for each unit was supplied by emergency diesel generators (“emergency DG”). Subsequently, due to the tsunami which struck the station, all emergency DG except for one air-cooled unit at Unit 6 shut down, and all AC power sources at Units 1 thru 5 were lost.

Furthermore, DC power sources were also lost due to inundation at Units 1, 2 and 4, and such function was also lost at Unit 3 as a result of batteries being depleted, resulting in a situation where all power being supplied to Units 1 thru 4 was lost.

As a result of the loss of AC power sources, existing cooling function for reactors and spent fuel pools was lost, and cooling was attempted using temporary power sources and alternative coolant injection, but the situation escalated into one where fuel in the core was damaged and radioactive material released into the environment.

In Closing

In this report, the nuclear power industry, which is comprised of Japan’s electric power companies and manufacturers conducted an examination focused mainly on analyzing factors as to why a core meltdown was not able to be prevented after tsunami struck the Fukushima Daiichi Nuclear Power Station and on formulating measures to counter such a situation.

The causes of the accident were that tsunami strikes inundated station electric installations, particularly the emergency AC and DC power supply panels, which caused cooling, control and monitoring functions to be lost, that time was required to establish means for cooling the core which were not dependent on permanent electric power sources, and that the core was not able to be maintained in a submerged state.

In this review, in keeping with the progression of the accident, we focused on whether the functions expected at each stage worked or not, analyzed causes and factors, and deduced issues and countermeasures.

We looked at all measures in combination with existing equipment and countermeasures, arranged the measures from the standpoint of deep protection having the capability to ensure safety, and deduced those with high priority to be multitiered measures. In addition, we have confirmed that safety has been substantially improved by the emergency safety measures already implemented by each company.

As new information about the accident is obtained, we plan to conduct further studies and review this report.

Source: Gengikyo.jp

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