Reactor 1

Historical information and statistics about Fukushima Daiichi Reactor 1.

Unit 1 Timeline: Launch Now


Fukushima I – 1

INES Disaster Rating: 7
: BWR-3
Containment: Mark I

Date of order: 1966
Construction: July 25, 1967 (also cited as 04-67)
Concrete placing: 1967-07-25
Criticality: October 10, 1970 (also cited as July o5, 1970)
Commercial Operation: March 26, 1971
Grid connection: 1970-11-17
Closed down: 2012-04-19
Electric Power: 460 MW
Main contractor: GE
Reactor System supplier: GE/GETSCO
Reactor Vessel supplier: GE/GETSCO/Toshiba/IHI
In core Structure supplier: GE/GETSCO
Steam raising supplier: GE/GETSCO
Turbine generator supplier: GE/GETSCO
Fuel supplier: GNF-J/NFI
Architecture/ Engineer : Ebasco (GE subsidiary)
Construction: Kajima
Civil works: various

Reactor building dimensions:
Width (north-south): 
40 meters
Height (grade level to roof): 45 meters
Outer wall thickness: .5 to 1.5 meters
Material: Ferroconcrete

Material: ASTM A201 Gr.B or A212 Gr.B
(M) Height:   32
Diameter cylindrical section (m):  10 (inner diameter: 9.6 m per FDADA)
The sphere diameter (m):  18 (inner diameter: 17.7 m per FDADA)
Maximum pressure: 0.43MPa
Design outer pressure (gauge): 13.7 kPa
Design inner pressure (gauge): 0.427 MPa
Maximum temp:  140℃
Design temp: 138 °C
Containment model: Appendix 1 Figure 5
Containment space volume: D / W space: 3410m3
PCV-size-300x2861: approx.32m
2: approx.10m
3: approx.18m
4: approx.8m
5: approx.2m
6: min.15mm
Material: carbon steel (outer concrete & rebar)





Pressure vessel (RPV reactor vessel):
Applicable standard: 
Inner diameter: 
(m)   4.8
 (M) 20 (18.86 m by FDADA)
Thickness of base metal: 160 mm
Thickness of stainless liner: 5 mm
Material of base metal: ASME SA 302B | ASME SA 336
Total weight:
(t)  440
Maximum pressure:
Design pressure (gauge): 8.62 MPa
Maximum temp: 
Design temp: 302 °C
Core temperature: 285  °C
Initial reactor power: 
1380 MWt (rated output)
Initial reactor pressure : 
7.03MPa [abs] (Normal operating pressure)
Operation pressure in reactor pressure vessel (gauge): 6.89 MPa
Initial reactor water level :
4187mm (usually water level: TAF standard)
Total flow rate of coolant: 21.8 x 103 t/h
Steam flow rate: 2480 t/h
RPV node split:  
Attachment 1 4
Effective core node split number:
radial direction: 5 node
Axis direction:
10 node
Cladding temperature damage:
727 ℃ (1000K)
Fuel melting:  
Appendix 1 Table 2
Decay heat:
ANSI/ANS5.1-1979 model ORIGEN2 that reflects the (fuel loading history Adjust the parameters so that it is equivalent to decay heat Integer)

1: approx.19m
2: approx.4.8m
3: approx.16cm
4: approx.18.4m
Material: Carbon steel (with stainless steel lining)




Primary loop recirculation System:
Number of loop:
Inner diameter of pipe: 532 mm
Number of jet pump: 20
Flow rate of pumps: 5600 t/h
Pump head: 103.6 m
Output power of pump motor: 2000 kW
Number of rotation of pump motor: 1380 rpm

Main steam system:
Number of pipe:
Inner diameter of pipe: 364 mm
Number of main steam isolation valve: 8
Number of safety relief valve: 4
Number of safety valve: 3

Fuel: LEU
Weight of UO2: 77t
Number of fuel assemblies (body)  400
Fuel assembly length (m)   4.35
Length of active fuel: 3660 mm
The amount of uranium loading (t)  69 (68 ton according to FDADA)
Averaged uranium enrichment (wt%): STEP2 fuel : 3.4 | 9×9(B) fuel: 3.6
Burnup (STEP2): Averaged 39.5 GWD/t | Maximum 50.0 GWD/t
Burnup (9×9(B)): Averaged 45.0 GWD/t | Maximum 55.0 GWD/t
Fuel (STEP2):
Diameter of pellet: 
10.4 mm
Outer diameter of cladding tube: 12.3 mm
Thickness of cladding(Thickness of zirconium liner): 0.86(0.1) mm
Fuel assembly, Number of FA(Total number): 68(400)
Number of fuel rod in one fuel assembly: 8 x 8 – 4 (Water rod)
Material of Channel Box: Zircaloy-4
Fuel (9x9(B)):
Diameter of pellet: 9.4 mm
Outer diameter of cladding tube:  11 mm
Thickness of cladding(Thickness of zirconium liner):  0.70(0.1) mm
Fuel assembly, Number of FA(Total number):  332(400)
Number of fuel rod in one fuel assembly:  9 x 9 – 9 (Water channel)
Material of Channel Box:  Zircaloy-4

Control Rods:
Number of control rod  
Control material: B4C
Configuration: Cross shape
Pitch: 305 mm

Reactor output:
Electrical output (kW million)  
Heat output (kW million)   138

Suppression Chamber (Torus):
Diameter of torus (torus): 29.6 m
Minor inner diameter of torus: 8.08 m
Number of vent pipe/downcomers: 8
Inner diameter of vent pipe/downcomers: 1.75 m
Inner diameter of header: 1.25 m
Number of downcomer (inside torus tube): 80
Pressure suppression pool water (t)  1750
S / C space: 2620m3
Suppression pool water: 1750m3

Spent fuel pool:
Spent fuel pool current inventory 392
Spent fuel pool damaged assemblies: 70

Technical Specs sources: *link has reactor temps/rads etc.


Spent fuel pool unloading date: 2017 (planned)* exceeded
Unit 1 reactor fuel debris removal: 2020 (planned)

The first of the Fukushima Daiichi reactors to be built, unit one lost power at 3:41pm JST in March 11 after the earthquake and suffered a hydrogen explosion at 3:36pm JST March 12th.  The isolation condenser valve was closed in the initial response to the earthquake, technicians were unable to reopen it after power was lost. The building’s older design featured a sheet metal upper level that blew out during the explosion leaving the building’s roof laying on the refueling floor. It is now estimated that the reactor core melted to the bottom of the reactor vessel within 16 hours of the station black out (power loss). The melted core is also likely melted through the vessel and is somewhere within the containment structure. Updated analysis leaves some concern than the melted fuel is outside containment. Visual inspections of the containment structure in late 2012 showed no traces of the melted fuel. Heavy damage inside the torus room and extremely high radiation levels outside of containment at unit 1 have added to concerns about the fuel being outside containment. Tepco started to inject seawater into the No. 1 reactor at 8:20 p.m. March 12, but at 7:04 p.m. JST that day, stopped it at 7:25 p.m. JST and resumed it at 8:20 p.m. JST

Reactor 1 had the shroud, core spray and feedwater spray rings replaced in 2008

News articles and reports on Unit 1 can be found here:

Radiation Readings In Unit 1 (2011 -2016)


Unit 1 Water Levels 2012-2013 (tepco has revised down for U1 containment water to below the downcomer)


Inspections & Information For Unit 1:

Unit 1 Photo Gallery
Unit 1 Post-disaster Inspection Videos 
Dr. Robert Jacobs Explains The Meltdowns At Fukushima October 2012
Unit 1 Corium (melted fuel) analysis October 2012
Unit 1 Containment Scope Inspection (videos, images, analysis) October 2012
Unit 1 Containment Scope Preparation October 2012 
Unit 1 Containment Additional Analysis September 2012
Unit 1 Debris Found In Containment September 2012
Unit 1 Hydrogen Increases In Pressure Suppression Chamber September 2012 
Unit 1 Peter Melzer’s Analysis Of Events, Fukushima Station Blackout & Delusion July 2012
Unit 1 Robots Inspect TIPS Room July 2012 
Unit 1 Torus Inspection, Analysis & Images June 2012 
Unit 1 Group Findings April 2012
Unit 1 Workers Inspect IC System & Floors 3-4 October 2011 (with video)
Unit 1 512 Sv/h Found In Drywell (Containment) September 2011
Unit 1 Radiation Jumps 17 Sv/h After Earthquake August 2011
Unit 1 Radiation Jumps 57 Sv/h After Earthquake August 2011
Unit 1 Anomalies Tied To Quakes August 2011
Fukushima Daiichi Reactor Specs

Unit 1 Reactor Data Recorder From March 2011 U1_datarecorder_f1_6_Katogensho1


Event Timeline:

March 11, 2011  14:52 – Isolation Condenser Starts (source)

March 11, 2011  15:03 – Unit 1 reactor pressure drops, temperatures decreases 55c per hour, this exceeded technical specs.
Operators closed valves MO-3A & 3B return isolation valves of the isolation condenser.
Operators decided one train of the isolation condenser (IC) was sufficient to keep the reactor pressure at 6-7 MPa
Operators used IC train A by opening valve MO-3A

Date and time Time after scram [hr] Event
3/11 14:46   0.00   Earthquake; Reactor scrammed
3/11 14:47   0.02   MSIVs* close due to loss of instrument power, loss of normal heat sink
3/11 14:52   0.10   Isolation Condensers automatically starts (Train A and B)
3/11 15:03   0.28   Isolation Condensers (Train A and B) manually stopped to control cool down rate
3/11 15:07   0.35   Containment A and B spray systems activated
3/11 15:17   0.52   Isolation condenser train A manually started
3/11 15:19   0.55   Isolation condenser train A manually stopped
3/11 15:24   0.63   Isolation condenser train A manually started
3/11 15:26   0.67   Isolation condenser train A manually stopped
3/11 15:27   0.68   First tsunami wave hits
3/11 15:32   0.77   Isolation condenser train A manually started
3/11 15:34   0.80   Isolation condenser train A manually stopped
3/11 15:35   0.82   Second tsunami wave hits
3/11 15:41   0.92   Station Blackout; containment sprays stopped
3/11 18:18   3.53   Isolation condenser train A manually started (not implemented in the model)
3/11 18:25   3.65   Isolation condenser train A manually stopped (not implemented in the model)
3/11 21:30   6.73   Isolation Condenser train A manually started (not implemented in the model)
3/12 5:46   15.00   Fresh water injection from fire water pump starts, 80,000 liters injected by 14:53
3/12 9:05   18.32   Drywell venting attempted (model assumes no venting)
3/12 11:00  20.23   Isolation condenser train A manually stopped (not implemented in the model)
3/12 14:30  23.73   Wetwell vented using portable generator and air compressor(model assumed vent value re-closure consistent with TEPCO wetwell pressure data)
3/12 14:53  24.12   Fresh water injection stopped due to running out of fresh water. 80 ton total injection
3/12 15:36  24.83   Hydrogen explosion in the reactor building
3/12 19:04  28.30   Seawater injection from the firewater system starts
3/14 0:00  57.20   Seawater injection from the firewater system ends
3/15 0:00  81.20   Seawater injection from the firewater system starts


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