Reactor core meltdown and radioactivity releaseduring the Fukushima Daiichi accident
12/06/27 Ebisawa T

1. Introduction
2.Structure of “Mark Ⅰ” Boiling Water Reactor
3. Available Core Cooling Systems
in the Fukushima Accident after SBO
4. How the Reactor Cores Now ?

25m掘削
原子炉を設置Fig. 1 Turbine Building Attacked Tsunami

Turbin Building

Vent for D/G

Basement of Turbine Building

Deigel generator

Electric Board

Water pump

2 Structure of MARK Ⅰ BWR

Fig. 2 Reactor Building②Spent Fuel and Pool②③ Reactor③Primary Containment
Vessel(PCV)④ Dry Well(D/W)④⑤⑤ Suppression chanber(S/C)

Fig. 3 Structure of Reactor System1 Core5 Fuel storage pool
8 Reactor pressure vessel
9 Biological shield
10 Concrete shield
11 Free standing steel dry well
13 Concrete embedment
27 Spent Fuel
30 Steam pipe
31 Control rods
32 Blow down line
PCV Radiation Monitor
When the radio activities in the core flow
into PCV, the monitor detects the radiations.

Basement concrete ofabout 7 meters thickness

Fig. 4 BWR Reactor

Fig. 5 BWR Cooling System3. Available Core Cooling Systems
in the Fukushima Accident after SBO
3‐1 The 1 st Reactor has two Isolation Condensers (IC)
3‐2 The 2 nd and 3 rd Reactor have A Reactor Core Isolation
Cooling (RCIC) System.
3‐3 Fire Engines were used to cool the Molten Cores.

Fig. 5 Isolation Condenser(IC)1. HT Steam goes out
from the reactor
Reactor
VesselSteam is released
In the atmosphere
Heat
Exchanger

3. Condensed
Water goes down
to the reactor.

CoolingWater
2. The heat exchangerchanges steam towater.

Fig 6 Reactor Core Isolation System (RCIC)
RCIC is normally used to cool the reactor core
after the normal stop. The pump is driven bythe steam from the reactor.
Steam Supply

WaterLevel

RCICTurbine

Fig. 7 The Cooling of the molten Core using the Fire Engine
The Cooling Water from The FireEngine was injected to theReactor along the Red Line.

Water filler was installedin June of 2010
Fire protection
Fire Engine
facility

Condensate
Storage Tank
PCVRPVFig. 8 Simulation results by TEPCO on the PCV pressure.

Measured values showa pressure peak, but
The pressure drop shows
Radio activities was released from P
This pressure rise shows
the reactor vessel was broken

Tepco simulateswith the smooth curve.

The accident sequence of the 1st Reactor
In the 1st Reactor, any cooling system did not operate after SBO, so that theevent sequences were very simple. IC were not available by the valve problems .
1)11/03/11/14:46 : Earth Quake occurred : Reactor scrummed
2)
11/15:35 : Power station was attacked by Tsunami
3)
11/15:42 : Station Black Out (SBO)4)
11/18:30 : Reactor water level went down to TAF(Top of Active Fuel)5)
11/20:30 : Core melting started
6)
11/23:00 : Core collapsed
7)
12/01:00 : Pressure of PVC rose to 0.52 Mpa gauge8)
12/02:45 : The pressures of the Reactor and PCV became the same9)
Reactor penetration failure occurred until this time.
10)
12/04:00 : Molten core fell down on the concrete floor of PCV
11)
12/05:40 : A fire engine supplied cooling water to the reactor vessel
12)
12/14:30 : PCV(S/C) Vent
13)
12/15:36 : Hydrogen explosion

The accident sequence of The 2nd Reactor
1)11/03/11/14:46 : Earth Quake occurred : Reactor scrummed
2)
11/15:35 : Tsunami occurred3)
11/15:39 : RCIC (Reactor Core Isolation Cooling system)operated
4)
14/10:00 ：RCIC lost the cooling capability
5)
14/16:20 ：Water level went down to TAF(Top of Active Fuel)6)
14/18:20 ：Water level went down to BAF(Base of Active Fuel)7)
14/21:00 ：The pressure of Reactor and PCV become same value
8)
14/23:35－15/7:25 : High pressure of 0.65(Mpa g) kept
9)
15/07:25－11:25 : Pressure changed quickly : PCV broken
10)
15/11:25－13:00 ：Much steam was generated in the reactor
11)
15/13:30 ：Reactor vessel broken in the base12)
15/15:25 : Peak value of PCV radiation monitor
13)This show that much X‐133 were released
14)

Simulated Result byTokyo Electric Power
Company (TPCO)

The 1 st Reactor

CoreFig.8 Present status of
the 1 st reactor molten coreby simulations of (TPCO).

Water supply
from fire engines

The core of the 1st Reactor fell
down on the floor of the PCV.
The core concrete reaction wasoccurred after the falling down.

7m

Anyone doesn’t know
the molten core broke the baseplate of the PCV or not.

The 2 nd 3 rdreactors

Fig. 9 TEPCO estimatesthat 60 % of the molten corefell down on the base of
PCV in the maximum.

Cooling water supply
from fire engines