Reactor 2

Historical information and statistics about Fukushima Daiichi Reactor 2.
Scroll down for more date-specific events at Unit 2. We are in the process of adding to this.
Historical graphs of reactor data, click here.

Unit 2 Interactive Timeline: Launch Now 

Unit 2 Extended Timeline (detail-heavy)
Launch Now

Fukushima I – 2

INES disaster rating: 7
Type: BWR-4
Containment: Mark I
Status during earthquake: operating

Date of order: 1968
Construction: 1969-01
Concrete placing: 1969-06-09
Initial criticality: 1973-05-10
Grid connection: 1973-12-24
Commercial operation: 1974-07-18
Closed down: 2012-04-19
Electric Power: 784 MW
Main contractor: GE/Toshiba
Architect Engineer: EBASCO
Reactor System supplier: GE/Toshiba
Reactor Vessel supplier: GE/GETSCO/Toshiba/IHI
In-core Structure supplier: GE/Toshiba also listed as Kajima/Kumagai
Fuel supplier: NFI
Steam raising supplier: GE/Toshiba/GETSCO
Turbine generator supplier: GE/Toshiba/GETSCO
Civil works: Kajima/Kumagai

Reactor building dimensions:
Width (north-south): 
 46 meters
Height (grade level to roof):  46 meters
Outer wall thickness:  .5 to 1.5 meters
Material: Ferroconcrete

Material: ASME SA 516 Gr.70
(M) Height:   33
Diameter cylindrical section (m):   10.9 m
The sphere diameter (m):  20 m
Maximum pressure: 0.38MPa
Design outer pressure (gauge): 0.14 kg/cm2g
Design inner pressure (gauge): 3.92 kg/cm2g
Maximum temp:  140℃
Design temp: 138 °C
Containment model: Appendix 1 Figure 5

Containment space volume: D / W space: 4240 m3 , S / C space: 3160 m3



1: approx. 34m
2: approx. 11m
3: approx. 20m
4: approx. 9m
5: approx. 2m
6: approx. 17-34mm
Material: Carbon steel





Pressure vessel (RPV reactor vessel):
Applicable standard:

Inner diameter: 
 5.57 m
Hydraulic equivalent diameter:  4.03 m
22.0 m
Thickness of base metal: 138 mm
Thickness of stainless liner: 5 mm
Material of base metal:  SA533 -, SA-508
Total weight: 
(t)  500t
Maximum pressure: 
Design pressure (gauge): 87.9 kg/cm2g
Maximum temp: 
300 °C
Design temp: 302 °C
Core temperature:  286 °C
Initial reactor power: 
2381 MWt (rated output)
Initial reactor pressure:
7.03 MPa [abs] (Normal operating pressure)
Operation pressure in reactor pressure vessel (gauge):  6.93 MPa
Initial reactor water level:
about 5274 mm (normal water level: TAF standard)
Total flow rate of coolant: 33.3 x 103 t/h
Steam flow rate:  4440 t/h
RPV node split:  
Attachment 1 4
Effective core node split number: 
radial direction: 5 nodes, Axis direction: 10 node
Axis direction: 

Cladding temperature damage: 
727 ℃ (1000K)
Fuel melting:  
Appendix 1 Table 2
Decay heat: 
ANSI/ANS5.1-1979 model ORIGEN2 collapse, which reflects (fuel loading history Adjust the parameters so that it is equivalent 壊熱)


1:  approx. 21m
2:  approx. 5.5m
3:  approx. 14cm
4:  approx. 18.4m
Material:  Carbon steel
(with stainless steel lining)




Primary loop recirculation System:
Number of loops:
Inner diameter of pipe: 627 mm
Number of jet pumps: 20
Flow rate of pumps: 7800 t/h
Pump head: 152 m
Output power of pump motor: 3750 kW
Number of rotation of pump motor: 1380 rpm

Main steam system:
Number of pipes:
Inner diameter of pipe: 610 mm
Number of main steam isolation valves: 8
Number of safety relief valves: 8
Number of safety valves:  3

Fuel: LEU
Weight of UO2: 107 t
Number of fuel assemblies (body) 548
Fuel assembly length (m)   4.47
Length of active fuel: 3710 mm

The amount of uranium loading (t): 94 t
Averaged uranium enrichment (wt%):   3.80 wt%
Burnup (9×9(B)): averaged 45.0 GWD/t, maximum 55.0 GWD/

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): 548(548)
Number of fuel rods in one fuel assembly:  9 x 9 – 9 (Water channel)
Material of Channel Box:  Zircaloy-4

Control Rods:
Number of control rods: 
Control material:
Configuration:  cross shape
Pitch: 304.8 mm

Reactor output:
Electrical output (kW million):  
Heat output (kW million):    238.1
Thermal output:  2381 MWt

Suppression Chamber (Torus):
Diameter of torus (torus): 33.5 m
Minor inner diameter of torus: 8.9 m
Number of vent pipe/downcomers: 8
Inner diameter of vent pipe/downcomers: 2.06 m
Inner diameter of the header: 1.46 m
Number of downcomers (inside torus tube): 96
Pressure suppression pool water (t)  2980
S / C space: 
Suppression pool water: 2980 m ^3

Spent fuel pool:
Spent fuel pool current inventory: 615
Spent fuel pool damaged assemblies: 3

Source documents:


Spent fuel pool fuel removal: 2017 (planned) – Rescheduled for 2024-2026
Removal of reactor fuel debris: 2020 (planned) – Rescheduled for 2024-2028

The second of the Fukushima Daiichi reactors to be built, unit two lost the emergency cooling function of the ECCS at 4:36 pm on March 11 after the earthquake. The pressure control function was lost at 5:32 am JST on March 12th. The reactor water level was found to be low at 1:24 pm JST leading to the conclusion that reactor cooling function had been lost. On the morning of March 13, an attempt was made to manually open the venting valve. At Midnight March 14th, another attempt to open the venting valve was tried. Seawater was injected into unit 2 in an attempt to establish cooling functions at 4:34 pm JST on March 14th. It suffered a hydrogen explosion at 6:20 am JST on March 15th. The hydrogen explosion blew holes in the suppression chamber. (**this initial reporting and assumption from TEPCO have now proven to be untrue, inspections of the suppression chamber found no damage). After this incident radiation levels at the main gate spiked to doses reaching 11,930 microsieverts per hour from 73 microsieverts 3 hours earlier. The damage led to leakage of the contaminated water into the rest of the building, then into the ocean. What may be the likely failure point is the containment cap gaskets. Radiation levels are quite high on the refueling floor near the reactor well and a visible steam leak under pressure was cited in the edge of the reactor well near the blowout panel.

It is now estimated that the reactor core melted to the bottom of the reactor vessel within 16 hours of the station blackout (power loss). The melted core is also likely melted through the vessel and is somewhere within the containment structure. It is also likely that the lower suppression chamber is damaged and causing radioactive water to leak out of the containment. This initial assumption about the suppression chamber is being rethought after multiple inspections of the suppression chamber showed no visible damage. Radiation levels in the suppression chamber are also fairly low. An inspection of the MSIV room (main steam isolation valve), represented on building diagrams as the area of pipes that go towards the turbine building found low levels of radiation and no visible damage. These newer findings challenge the earlier idea that unit 2 had a massive lower fuel leak that caused the high radiation found in the unit 2 intake canal near the port.

Reactor 2 had the shroud replaced in 2001

*please excuse our mess as we update the reactor pages. We are adding factual data of the ongoing events to build a larger record of each unit.

Radiation Readings In Unit 2 (mostly 2012 readings, readings up to 2016 added)



March 2012: Unit 2 was found by scope to have 60cm of water in the drywell that had a temperature of about 48.5 C to 50 C

Read all articles and reports related to unit 2 here.

Spent Fuel Pool Data & Issues:

July 2012 TEPCO states spent fuel pool has been desalinated and releases this document with a timeline of activity & testing of the SFP water chemistry.

  • It shows the elevated chlorine levels in ppm and is proof of the fact that the levels were extremely higher than normal which can accelerate chloride stress corrosion on all the piping and fuel.
  • The normal chloride concentration was about 100 ppm per TSR they were at 1500 ppm.
  • These are critical units for chemistry; chlorides, ph, conductivity, and filterable solids.
  • As we all knew they are completely out of control from a chemistry standpoint, they looked at the boron concentration and pumped that level up long ago.

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