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The Fukushima Daiichi AccidentBy Sergio Peraza

Fukushima Daiichi Nuclear Power PlantA brief description 1)

Fukushima Daiichi Units 1-4

Fukushima Daiichi Nuclear Power Plant construction started July 25, 1967

First reached criticality on Oc-tober 10, 1970On March 26, 1971, Unit 1 started its first commercial op-eration In 2011, there were six BWR units8 units were planned to be finished by 2017. Units 7 & 8 ABWR projects were cancelled in April 2011

1) Fukushima Daiichi. IAEA/PRIS. 2013

Fukushima Daiichi Acci-dent

A brief description

1) The Fukushima Daiichi Accident Report by General Direc-tor.. IAEA, 2013

Fukushima Daiichi Nu-clear Power Plant is lo-cated 260 km north of Tokyo.

14:46 earthquake 9.0 Richter Scale15:26 tsunami wave ar-rived

The epicenter of the Great East Japan earthquake and the nuclear power

plants nearby 1)

Friday March 11, 2011

Fukushima Daiichi AccidentUnits Status

1) The Fukushima Daiichi Accident Report by General Direc-tor. IAEA, 2013

2) Technical Volume 1/5 description and Context of the Acci-dent. IAEA, 2013

Units 1, 2, and 3 operating at full powerUnit 4 unloaded

Units 5 and 6 scheduled for main-tenance and refuel-ing

An aerial view of the Fukushima Daiichi NPP 1)

Unit

Power (MWe) Status2)

1 460 Operating

2 784 Operating

3 784 Operating

4 784 Outage

5 784 Outage

6 1,100 Outage

Fukushima Daiichi ReactorsBWR Characteristics

1) Public domain image by U.S. NRC

Water is used as both moderator and coolant

Water is allowed to boil in the core pro-ducing steam

Steam is directly fed to the turbine to gen-erate electricity (steam cycle)Control rods are in-serted from the bottom of the core

LWR: BWR Animated Diagram of Boiling Water Reac-

tor (BWR)1)

Fukushima Daiichi ReactorsBWR Evolution 1)

ECCS: Emergency Core Cooling SystemESBWR: Economic Simplified BWR

BWR Version

First Com-mercial

Operation Date

Representative PlantCharacteristics

BWR/1 1960Dresden 1

Initial commercial size BWRDual Cycle

BWR/2 1969

Oyster CreekPlants purchased solely on economicsLarge direct cycle Forced circulation Variable speed pumps for circulation flow control

BWR/3 1971

Dresden 2Internal jet pumps applicationImproved ECCS: spray and flood ca-pability

BWR/4 1972 Vermont YankeeIncrease power density

1) Boiling Water Reactor Simulator with Passive Sate Systems. User Manual, IAEA. October 2009

Fukushima Daiichi ReactorsBWR Evolution Cont. 1)

BWR Version

First Com-mercial

Operation Date

Representative PlantCharacteristics

BWR/5 1977Tokai 2Improved ECCSValve flow control

BWR/6 1978

CofrentesCompacts control roomSolid-state nuclear system protection system

ABWR 1996

Kashiwazaki-Kariva 6Reactor internal pumps Fine-motion control rod drives Advanced control room, digital and fiber optic technology Improved ECCS: high/low pressure flooders

ESBWR Under Review Natural circulationPassive ECCS

1) Boiling Water Reactor Simulator with Passive Sate Systems. User Manual, IAEA. October 2009

ECCS: Emergency Core Cooling SystemESBWR: Economic Simplified BWR

Fukushima Daiichi ReactorsBWR Evolution Cont. 1)

1) Fukushima Dai-ichi Units 1-6 U.S.NRC May, 2012

Drywell

Suppression Chamber

Drywell Head

Typical Volumes

Drywell: 1.2 million galTorus Air: 0.9 million galTorus Water: 0.8 million gal

Fukushima Daiichi ReactorsBWR Construction

Reactor Pressure Vessel Dimensions

70 feet tall18 feet inside diameter 7 inch thick carbon steel

1) Fukushima Dai-ichi Units 1-6 U.S.NRC May, 2012

Fukushima Daiichi ReactorsBWR Reactor Assembly

Two phases:

Steam generation occurs in a direct cycle with steam separators and dryers in-side the reactor pressure vessel. Operating tempera-ture is 288°C; steam pres-sure ~ 7 MPaThe reactor (steam dome) pressure is controlled by turbine inlet valves and turbine bypass valves

BWR/6 Reactor Assembly 1)

1) U.S. NRC, General Electric, Sandia Laboratories

a sub-cooled liquid phase in the non-boiling region

saturated steam-water mixture in the boiling region

https://www.youtube.com/watch?v=i92VHLRUGeE

https://www.youtube.com/watch?v=i92VHLRUGeE

1) Battle to stabilize the Fukushima Daiichi reactors. Joseph Miller. March 30, 2011

The core: fuel bundles (assemblies) contain fuel rods ar-ranged in a N × N square lattice, with enriched uranium fuel ~ 2% to 5% U- 235 2).

Fukushima Daiichi ReactorsBWR Fuel assemblies 1)

Unit 1: 400 as-sembliesUnits 2-5: 548 as-sembliesUnit 6: 740 as-sembliesEach assembly has 72 fuel rods con-taining the ura-nium oxide fuel within zirconium alloy cladding

2)http://www.tepco.co.jp/en/nu/fukushima-np/outline_f1/index-e.html

https://www.youtube.com/watch?v=6LIu7bhRDXE

BWR Control Design 1) Fukushima Daiichi Reactors

1) ABWR Seminar-Reactor, Core & Neutronics LE Fen-nern April 13, 2007

Fukushima Daiichi Reactors

1) Dynamic Behavior of BWR Massachusetts Institute of Technology Department of Nuclear Science and Engineer-ing. Class of Engineer-ing of Nuclear Systems

BWR Power Control 1)

Power controls: control rods and recirculation flow controlControl rods control the

power level using com-pacted boron carbide

The recirculation flow is con-trolled by a change in water density using a pump

The recirculation flow is controlled by a change in wa-ter density using a pump

BWR Control Rods 1) Fukushima Daiichi Reactors

1) Dynamic Behavior of BWR Massachusetts Institute of Technology Department of Nuclear Science and Engi-neering. Class of Engineering of Nuclear Systems

Fukushima Daiichi ReactorsBWR Power Control

Control rods control power level using compacted boron carbide

1) BWR Description. Jacopo Buongiorno. CANES.

Control Blade 1) Four Bundle Array 2)

2) ABWR Seminar-Reactor, Core & Neutronics LE Fennern, 2007

Power in fresh fuel assembly of conventional de-sign as adjacent control rod is withdrawn toward

bottom1)

1) BWR/6 General Description of a Boiling Water Reactor. GE Nu-clear Energy

Fukushima Daiichi ReactorsBWR Power Control

Control rods control power level using compacted boron carbide

Systems for cooling the coreUnit 1

Isolation condenser 1)


Unit 2-6 Systems for cooling the core

Reactor core isolation cooling 1)


Fukushima Daiichi ReactorsBWR

Boiling water reactors at Fukushima Daiichi NPP 1)


Earthquake Scenario The development (History)

Earthquake at 14:47

When the earthquake hit the coast, control rods were in-serted into the reactors, but temperature and pressure in-creased due to the decay heat.Off-site power was lost

Emergency Diesel Generator (EDG) turned on automatically. On-site power was activated

1) The Fukushima Daiichi Accident Report by General Di-rector.. IAEA, 2013

Unit 1 at Fukushima Daiichi NPP 1)

Tsunami Scenario The development (History)

Tsunami at 15:27

The EDG was dis-abled; on-site power was lost

Waves went over the sea wall (designed for 4.5m), flooding the lower part of the plant.

Unit 1: Operators unable to operate the IC (cooling rate)

Units 2 and 3: RCIC designed to op-erate for 8 h. It just cooled down some of the steam.

Units 1-5 experi-enced station

blackout during the first 10-15 minutes after the flooding

Variation of tsunami wave impact, runup1)

1) Coastal Engineering Committee Of Japan Society Of Civil Engi-neers, The 2011 off the Pacific coast of Tohoku Earthquake In-formation (30 March 2012), Coastal Engineering Committee, JSCE

https://www.youtube.com/watch?v=vggzl9OngaM

Meltdown Scenario The development (History)

Meltdown Scenario Units 1, 2, and 3

Water evaporated

Temperature increased to 2300°C

Fuel was exposed

Radioactive steam

Core melted to the bottom of the vessel

1) https://www.youtube.com/watch?v=YBNFvZ6Vr2U

Meltdown Scenario for Units 1-3 1)

Estimated conditions of the RPVs and PCVs of Units 1–3 February 2014

Meltdown Scenario The development (History)

Meltdown Scenario Units 1, 2, and 3

RPV: Reactor Pressure VesselPCV: Primary Contain-ment Vessel

ExplosionsThe development (History)

Operators decided to release gas into the atmos-phere Hydrogen leakage occurred through pass-ways and accumulated into the reactor building Reactor buildings for Units 1, 3, and 4 exploded due to the con-centration of hydrogen

H formation 2)

ExplosionsThe development (History)

Unit

Amount, Kg

1 890

2 460

3 810

Amount of H gen-erated1)

1) Yanez J, et al., An analysis of the hydrogen explosion in the Fukushima-Daiichi accident,International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.154

The main sources of hydrogen are the oxidation of Zircaloy by steam, the radiolysis of water, the reaction between water and boron car-bide and the interaction of the molten core with the concrete of the contain-ment

2) https://www.youtube.com/watch?v=YBNFvZ6Vr2U

ExplosionsUnits 1-4

https://www.youtube.com/watch?v=YBNFvZ6Vr2U

https://www.youtube.com/watch?v=YarjI1FwsuA

http://skeptics.stackexchange.com/questions/9522/is-fukushima-reactor-no-4-on-the-verge-of-catastrophic-failure-that-will-destro

Spent fuel poolUnits 1-4

All units had lost their cooling system.

Unit 1 contained very little spent fuel

Unit 4 contained the amount of 3 reactor cores: 1535 spent fuel assemblies Helicopters dropped sea water and fresh water was sprayed by water cannon trucks

A visual inspection by remote controlled camera has shown no significant damage to the used fuel pond of Fukushima Daiichi unit 4http://www.world-nuclear-news.org/RS_No_significant_damage_to_fuel_at_Unit_4_3004111.html

Surveying of the Area

Radiation dose rate mea-surement of nearly 12 mSv/h was recorded at the main gate at 09:00 on 15 MarchResidents within a 20–30 km radius of the Fukushima Daiichi NPP were required to take shel-ter indoors.Atmospheric release of I-131 was estimated to be 100-500 PBq Cs-137 was in the range of 6-20 PBq

The Fukushima Nuclear Accident and Crisis Management, 2012

Post Fukushima Daiichi NPP Accident

Lessons Learned

Mitigation Strategies: to enhance the capability to maintain plant safety during a prolonged loss of electrical power

http://www.nrc.gov/reactors/operating/ops-experience/japan-dashboard/mitiga-tion-strategies.html

The mitigation strategies are ex-pected to use a combination of cur-rently installed equipment (e.g., steam-powered pumps), additional portable equipment that is stored on-site, and equipment that can be flown in or shipped in from support centers


Lessons Learned

Containment Venting System: to provide a re-liable hardened containment vent system for boiling water reactors (BWRs) with Mark I or Mark II containment designsThe NRC issued an order on March 12, 2012 requiring all U.S. nuclear power plants with the Fukushima-style con-tainment design to install a reliable, hardened vent that can remove heat and pressure before potential damage to a re-actor core occurs. This not only helps preserve the integrity of the containment building, but can also help delay reactor core damage or melting

The NRC issued an order on March 12, 2012 requiring all U.S. nuclear power plants to install water level instrumenta-tion in their spent fuel pools. The instrumentation must re-motely report at least three distinct water levels: 1) normal level; 2) low level but still enough to shield workers above the pools from radiation; and 3) a level near the top of the spent fuel rods where more water should be added without delay

Spent Fuel Instrumentation: to provide a reli-able wide-range indication of water level in spent fuel storage pools



Lessons Learned

Flooding Hazard Reevaluation: to reanalyze potential flooding effects using present-day information to determine if safety upgrades are neededSeismic Reevaluation: to reanalyze potential seismic effects using present-day information to determine if safety upgrades are needed

Plant Walkdowns of Seismic and Flooding Protection Features: to inspect existing plant protection features against seismic and flood-ing events and correct any degraded condi-tionsEmergency Preparedness – Staffing and Communications: to assess staffing needs and communications capabilities to effectively re-spond to an event affecting multiple reactors at a sitehttp://www.nrc.gov/reactors/operating/ops-experience/japan-dashboard/mitiga-tion-strategies.html


Lessons Learned

Station Blackout Mitigation Strategies: the eventual rule will ensures that if a plant loses power, it will have sufficient procedures, strategies, and equipment to cope with the loss of power for an indefinite amount of timeOnsite Emergency Response Capabilities: The NRC is rewriting its rules to strengthen and integrate the various emergency response ca-pabilities. The Mitigation of Beyond Design Basis Events rulemaking will establish stan-dards that ensure the plants can smoothly transition between the various procedures, keeping the plants' overall strategies coher-ent and comprehensive


Post Fukushima Daiichi NPP AccidentMore Lessons Learned

Design and Equipment

Equipment required to respond to a long-term loss of all AC and DC power and loss of the ultimate heat sink should be conveniently staged, protected, and maintained such that it is always ready for use if neededPlant modifications may be needed to ensure critical safety functions can be maintained during a multi-unit event that in-volves extended loss of AC power, DC power, and the ultimate heat sink


Lessons Learned Sum-mary


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