Fukushima Nuclear Accident Interim Reportgrouper.ieee.org/groups/npec/N12-01/TEPCO_Fukushima...Fukushima Nuclear Accident Interim Report ~ Lessons Learned and Countermeasures~ IEEE

Fukushima Nuclear Accident Interim Report ~ Lessons Learned and Countermeasures~

IEEE NPEC N12-01 San Antonio, TX January 25, 2012

Yasushi Nakagawa

Chief Assistant, Nuclear Corporate Planning Group Nuclear Power & Plant Siting Administrative Department

Tokyo Electric Power Company

All Rights Reserved ©2012 The Tokyo Electric Power Company, Inc. 1

Lessons Learned from the accident


Issues based on plant behavior

If the cooling and water injection functions of the high-pressure systems are lost at an early stage after reactor shutdown, the reactor water level decreases rapidly, and event progression is very quick.

⇒It is necessary for high pressure water injection

systems to be initiated immediately after an accident.

It is important to utilize originally installed equipment to cope with this.

Issues concerning plant behavior at the time of the accident


Issues based on plant behavior Dry well (D/W) pressure increases gradually while the high

pressure systems are in operation. However, once core damage begins, the D/W pressure increases rapidly. It was also observed that the D/W pressure started to

increase rapidly after the depressurization of the reactor. It is considered that core cooling further degraded due to the rapid decrease in the amount of water retained in the reactor caused by the flashing, leading to core damage. ⇒It is important to prepare reliable low-pressure

systems before reactor depressurization and to smoothly switch over to the low-pressure systems while maintaining a balance between the decrease in water level due to depressurization and the amount of water injection.


（At Fukushima Daini Unit 1, switching over could be executed）


Issues based on plant behavior

At Fukushima Daini Unit 1, preparations had been made to remove heat from the PCV through low pressure water injection and venting (feed and bleed) in case if the D/W pressure became higher, although this was not implemented in the end. ⇒It is important that such response can be realized

even under adverse conditions. Monitoring functions are also important for switching the

water injection systems, in addition to the understanding of the plant status. ⇒It is important to maintain monitoring function for

parameters such as reactor water level.



1) Promptly initiate core injection methods using high-pressure cooling water injection equipment

2) Initiate depressurization methods before losing of high-pressure cooling water injection function

3) Stable low-pressure cooling water injection methods should be available during the depressurization stage

4) Prepare reliable PCV venting methods (heat removal through the atmospheric discharge of heat)

5) Prepare measures to restore the cooling function using sea water

6) Prepare measures which enable necessary monitoring for those operation and plant conditions.


Lessons Learned




Issues on facilities and functions


Issues on facilities and functions

The accident was caused by the simultaneous loss of multiple safety functions due to the tsunami flooding. The main factors of the accident are “the simultaneous loss of total AC power and DC power for a extended period of time” and “the loss of the heat removal function of the emergency seawater system for a extended period of time.”

Preparations had been previously made to receive power from neighboring units in the event that AC power and DC power were not available. During the accident, direct tsunami damage was so widespread that the neighboring units were all in the same condition.

Summary of equipment and functional issues


Future responses based on the causes of the accident


Response policies to prevent core damage

The necessary measures have been identified to contribute to enhancing the safety of existing NPSs based on the accident at the Fukushima Daiichi NPS.

The presented countermeasures are centered on

responses to technical issues to prevent core damage, in view of the fact that multiple severe events occurred that resulted in core damage and to prevent similar circumstances from occurring again.






Specific countermeasures in consideration of the Fukushima accident

(1)Flooding Protection Countermeasures for site and building Installation of tidal embankment, barrier, and wall and flood protection of door and penetration.



(2) High Pressure Water Injection Facilities (Required within 1-hour)

Concepts -High pressure injection is initially required due to high reactor pressure in case of an abnormal shutdown.

-During the Fukushima accident, some motor-driven equipments were inoperable due to the station black out (SBO). Hence, a steam-driven high pressure system is essential.

SBO RCIC steam-driven ○

SLC or CRD motor-driven ×

HPCS



(2) High Pressure Water Injection Facilities (Required within 1-hour) SBO

RCIC steam-driven ○ SLC or CRD

motor-driven × HPCS

Concepts -Furthermore, when choosing motor-driven high-pressure water injection systems, it is important to select systems with minimum related system and equipment.


(3) Depressurizing Equipment (Within 4-8 hours) Concepts -Depressurization of the rector pressure vessel is essential in order to remove heat and bring it to a cooling stage -During the Fukushima accident, the DC power for the SRV was insufficient to depressurize RPV. In addition to N2 spare gas for valve operations, the spare DC power source is necessary.



(4) Low Pressure Water injection Facilities (Within 4-8 hours) Concepts -Low pressure cooling water injection equipment consists of an emergency system, a make-up water condensate system (MUWC) and a fire protection system. In the case of the SBO, only the diesel-driven fire pumps (DDFP) of the FP will be operable. -Preparing reliable low pressure injection equipment is important including the fire-engine used.

SBO

DDFP diesel-driven ○ MUWC motor-driven ×



(4) Low Pressure Water injection Facilities (Within 4-8 hours) SBO

DDFP diesel-driven ○ MUWC motor-driven ×



1) PCV venting (Within 1-2 days)

Concepts -In the case that seawater cannot be used as a cooling source, suppression chamber venting that utilizes air as a cooling source is necessary. -In order to conduct suppression chamber venting, opening motor-operated (MO) valves as well as air-operated (AO) valves are necessary.

Necessary Equipment Flooding Countermeasure Flexible Countermeasure

AC power (MO-valve, solenoid valve for AO-valve)

Flood protection for power supply equipment including EDG or considering rearrangement

Preparing power-supply cars, portable AC generator or portable batteries

Compressed air (For AO-valve operation)

Portable air compressor (or tank preparation)

Remodeling AO-valve so that it can be operated manually

(5) Heat Removal / Cooling Facilities



2) Heat removal via Shutdown Cooling Mode (Within 3-7 days)

Concepts -Shutdown cooling mode procedures by residual heat removal system (RHR) that utilizes sea water as a cooling source is necessary. -Thus, in addition to ensuring a power source, restoring the seawater system utilized as the ultimate heat sink for preparing alternative pumps, or motor repairs is necessary.




3) Heat removal from spent fuel pool (Within 7-10 days: Depending on decay heat from spent fuels)

Necessary equipment Flooding Countermeasure Flexible Countermeasure

FPC pump

Flooding protection for the pump room -Preparing fire engines

-To establish redundancy with fire protection piping

Installation of water level detection instruments or a thermometer inside the pool

AC power Flooding protection for power supply equipment or considering rearrangement

Preparing Power-Supply cars

Concepts -Spent fuel pool cooling and cleanup system (FPC) is basically tsunami-resistant since it is located inside the reactor building. Hence it is important to maintain the power source. -Furthermore, in light of having a sufficient a amount of time to respond, monitoring utilizing the instruments is important.




(6) Ensuring power supply to the monitoring instruments (Required within 1 hour) Concepts -During the Fukushima accident, the monitoring instruments were rendered inoperable and restoring power to the instruments took time. -Thus ensuring immediate power supply for instruments is important.



(7) Mitigation measures following reactor core damage Concepts -During the accident, not only was the containment function lost, but also restoration efforts were seriously hampered due to the hydrogen explosion caused by the possible leak of hydrogen from the primary containment vessel to the building. -In light of defense in depth, it is important to establish countermeasures in the case of the reactor core damage, which happened at Fukushima Daiichi

Items Countermeasure

Hydrogen Accumulation Prevention

Installing equipment or establishing procedures for drilling holes through the roof or opening the blow-out panels in order to improve reactor building ventilation.

Mitigation of Radioactive Material release

Establishing the water injection procedures to the PCV via fire engines etc. as is being done with the suppression chamber venting. (Established for smooth venting via water filtering)



(8) Common Countermeasures -In addition to implementing each countermeasure, it is important to reinforce the supporting work and auxiliary equipment for safe and efficient activity in order to achieve the aforementioned countermeasures effectively.


1) Off-site Power (Substation equipment ) -A lot of damage to substation equipment such as circuit breakers and line switches by earthquake. ⇒An analytical evaluation of the cause of damage is being conducted for substation equipment. ⇒Future measures to enhance seismic resistance should be considered.


(8) Common Countermeasures


1) Off-site Power (Transmission Line Towers ) -In terms of the transmission line towers, the Yonomori Line No. 27 tower collapsed due to the large-scale collapse of the adjacent embankment by earthquake. ⇒Evaluations are being conducted regarding following three issues that caused secondary damage to off-site power transmission lines of the NPS :

- Collapse of embankments, - Landslides - Mudslides on steep terrain




1) Off-site Power (Off-site power line design ) Off-site power line design will be reviewed from the perspective of maintaining the reliability of off-site power sources of NPSs in an earthquake, in order to provide a reliable power supply even in a severe case of a total black out in one nearest substation.

⇒The concept of design improvement will be reviewed as follows, -To receive power from two different substations

Or

-To prepare system that enables switching transmission systems in order to restore off-site power, although power stations receive power from one substation.




2) Communication methods -Communication methods such as wireless phone could not be used. This affected the smooth exchange of plant information and response operations. ⇒The establishment of communication methods that are appropriate for the situation will be considered.

Example: preparation of mobile radios, satellite telephones, batteries and etc., for use as a power source.

3) Lighting equipment -Lighting that was invaluable for response operations due to the loss of power. ⇒In order to conduct safe, prompt, and reliable response, preparation of headlight-type lighting that enables the use of both hands as well as lighting that light up a wider area is required.

Examples: preparation of headlights, LED lights, and floodlight balloons



Items Countermeasure Debris Removal Equipment

Preparing equipment to remove debris hampering restoration activity.

Health Protection Equipment

An abundant supply of protective gear, masks, APDs, portable air refreshers, etc. should be on hand along with the deployment of the power supply car in order to ensure that workers will be able to restore the main control room ventilation system promptly.


4) Others


Other mid and long-term Technical Issues -In this study, the aforementioned core damage countermeasures have been established. In addition, mid and long-term technical issues such as those listed in the right-hand table should be considered -These technical issues will be considered separately.


1) Reviews to enhance reliability of high-pressure cooling water injection equipment

Unit 1 Isolation Condenser lost DC power due to the impacts of the tsunami and was isolated. Consequently, the unit lost the cooling function.

⇒It is necessary to consider the measure that will enhance reliability of high-pressure cooling water injection equipment, including the IC valve isolation logic.

In addition, it is necessary to carefully consider whether more flexible operation is possible.


Other mid and long-term Technical Issues

The indication of the reactor water level instrument differed greatly from the actual water level after core damage. ⇒It is necessary to start R&D in order to diversify monitoring instruments that meets the needs in the accident response, rather than simply enhancing the accuracy of the water level instrument.


2) Research and development of measurement devices for accidents


Other mid and long-term Technical Issues

Items Action Plan

Venting line improvement

In order to improve venting that is able to significantly filter out radioactive materials, measures such as the aggressive activation of the Rupture Disk will be looked into while taking the accidental release of radioactive materials into consideration

Mitigation measures for radioactive material release during venting

The design of a filter vent to mitigate the release of radioactive materials will be considered


3) Others


Countermeasures for Kashiwazaki-Kariwa NPS


Immediate Safety Measures at Kashiwazaki-Kariwa NPS Countermeasures for Kashiwazaki-Kariwa Power Station


Further Safety Measures at Kashiwazaki-Kariwa NPS Countermeasures for Kashiwazaki-Kariwa Power Station


Conclusion

• This presentation intended to identify lessons as the central player of the accident based on what we have experienced data that have been collected and etc.

• As a first step, it describes the facts of the

investigations that have been verified so far and identified countermeasures to prevent core damage.

• These countermeasures will be incorporated in

TEPCO’s nuclear power plants, but we hope that many people in the nuclear power industry will read through the TEPCO Fukushima Accident Interim Report and use it to enhance safety in BWR plants both in Japan and abroad.


References

TEPCO English website http://www.tepco.co.jp/en/nu/fukushima-np/index-e.html

TEPCO Internal Investigation Committee Interim Report (Dec. 2nd, 2011) http://www.tepco.co.jp/en/press/corp-com/release/11120205-e.html

Mid- to Long-Term Road Map Towards Decommissioning of 1F Units 1-4 (Dec. 21st, 2011) http://www.tepco.co.jp/en/press/corp-com/release/11122107-e.html

NISA (Nuclear and Industrial Safety Agency) http://www.nisa.meti.go.jp/english/

Japanese Government Investigation Committee Interim Report (Dec. 26th, 2011) http://icanps.go.jp/eng/interim-report.html

JAIF (Japan Atomic Industrial Forum) http://www.jaif.or.jp/english/

INPO—Special Report on Fukushima Daiichi Nuclear Power Station http://www.nei.org/resourcesandstats/documentlibrary/safetyandsecurity/reports/special-report-on-the-nuclear-accident-at-the-

fukushima-daiichi-nuclear-power-station

EPRI—Fukushima Daini Independent Review and Walkdown http://my.epri.com/portal/server.pt?Abstract_id=000000000001023422

NEI—Article on Fukushima Daini http://safetyfirst.nei.org/safety-and-security/fukushima-daini-model-of-a-safe-shutdown/

http://www.tepco.co.jp/en/nu/fukushima-np/index-e.html

http://www.tepco.co.jp/en/press/corp-com/release/11120205-e.html

http://www.tepco.co.jp/en/press/corp-com/release/11122107-e.html

http://www.nisa.meti.go.jp/english/

http://icanps.go.jp/eng/interim-report.html

http://www.jaif.or.jp/english/

http://www.nei.org/resourcesandstats/documentlibrary/safetyandsecurity/reports/special-report-on-the-nuclear-accident-at-the-fukushima-daiichi-nuclear-power-station

http://www.nei.org/resourcesandstats/documentlibrary/safetyandsecurity/reports/special-report-on-the-nuclear-accident-at-the-fukushima-daiichi-nuclear-power-station

http://my.epri.com/portal/server.pt?Abstract_id=000000000001023422

http://safetyfirst.nei.org/safety-and-security/fukushima-daini-model-of-a-safe-shutdown/

Documents

Fukushima Nuclear Accident Interim Reportgrouper.ieee.org/groups/npec/N12-01/TEPCO_Fukushima...Fukushima Nuclear Accident Interim Report ~ Lessons Learned and Countermeasures~ IEEE