WHAT HAPPENED,
WHAT IS GOING ON
IN FUKUSHIMA NO.1
NUCLEAR POWER STATION?

Yoichiro Shimazu
Research Institute of Nuclear Engineering
University of Fukui

WHERE IS FUKUSHIMA NUCLEAR POWER STATION?

SapporoDistance; about 600km
South of Sapporo

Fukushima Nuclear Power StationWHAT HAPPENED?
The largest earthquake in history hit North-East Japan at 14:46 on
11, March, 2011; Magnitude of 9.0
 The largest Tsunami caused by the earthquake hit the coastline ofPacific Ocean; the height of the wave thought to reach over 15
meters at the maximum at the site, although the design value wasabout 5.7 meters. Almost of the plants systems were covered withsea water of 4m in depth.
 In Fukushima-1 site, operating units (Unit No. 1 to 3) succeeded toshutdown as designed. ( Unit No.4 to 6 were during inspection.) The electrical power from external grid was lost due to the earthquake.
 Back-up diesel driven generators started but, they all failed in one
hour or so due to the loss of cooling.
 The final heat sink was completely lost because all of the sea waterpumps were damaged by the tsunami.
SYSTEM CONFIGURATION of BWR (Mark-1)
Spent Fuel Pool
Reactor Pressure VesselContainment Vessel

Suppression Chamber

Sea Water Pump

WHY IS THE HEAT SINK REQUIRED?
 Nuclear fission reactions produce fission

products (FP) in nuclear fuel.
 Some of the nuclides of FP are unstable, thus
they emit energy in the form of radiation inorder to be stabilized, which result in heatcalled as “Decay Heat.” This distinguishes the nuclear fuel from thefossil fuel. (The latter generates heat only whenit burns.)

HOW LARGE THE DECAY HEAT?

MIT NSE Unclear Information Hub
http://web.mit.edu/nse/

DECAY HEAT (NUMERICAL DATA)
Date/Time (Fukushima Time)
Fukushima Daiichi-1
Decay Heat (MW)
3/11/11 2:46 PM
3/11/11 2:47 PM
3/11/11 2:48 PM
3/11/11 2:50 PM
3/11/11 3:00 PM
3/11/11 3:30 PM
3/11/11 8:00 PM
3/12/11 8:00 AM
3/12/11 8:00 PM
3/13/11
3/14/11
3/16/11
3/20/11
4/1/11
7/1/11
10/1/11
3/11/12

92.0
44.7
36.9
31.4
24.1
19.1
12.8
10.1
9.1
8.5
7.8
6.9
6.1
5.2
3.7
3.3
2.9

Fukushima Daiichi-2 & 3
Decay Heat (MW)
156.8
76.2
62.8
53.5
41.0
32.5
21.9
17.3
15.5
14.5
13.2
11.8
10.5
8.8
6.3
5.6
5.0

Percent of Full Reactor Power
6.60%
3.21%
2.64%
2.25%
1.73%
1.37%
0.92%
0.73%
0.65%
0.61%
0.56%
0.50%
0.44%
0.37%
0.26%
0.23%
0.21%

Decay heat needs to be cooled for a long time

WHAT HAPPENED WITH NUCLEAR FUELS?
 When cooling capability was lost, the watertemperature rising in the core and also in spent fuelpool, some of the water began to vaporize andeventually uncovered some of the fuel rods.
 The fuel rods have a layer of cladding material madeof a zirconium alloy. If zirconium is hot enough and isin the presence of oxygen (The steam provides theoxygen) then it can undergo a reaction that produceshydrogen gas. (Zr+2H2OZrO2+2H2)
 Hydrogen at concentrations above 4% is highly
flammable when mixed with oxygen; however, notwhen it is also in the presence of excessive steam
This hydrogen caused the explosions in Unit 1through 4.

RELEASE OF RADIOACTIVITY
 Increasing temperature results in vaporizationof coolant, thus increasing pressure of reactor
vessel In order to protect reactor vessel integrity,pressure must be lowered by venting the steam Radioactive fission products came out withsteam to environment through containment
vessel and broken reactor building

RELEVANT RADIOACTIVE NUCLIDES
I-131
Melting Temp. 113.6C
Boiling Temp. 182.8C
Half life
8 daysRadiation
Energy(Mev)
Radio Toxicity

Cs-13728.4C
671 C
30 years(Effectively 70days)
Beta
Beta & gamma
0.606
0.514 & 0.66
Thyroid cancer Leukemia

RADIATION AND SHIELDING
A paper

Alfa rayBeta ray
Gamma ray
NeutronAluminumsheet

Ion plate

Water or Concrete

RADIATION UNITS Becquerel: Bq

Number of decays per second
Ex. Average human has about 7000 Bq
 Sievert: Sv ; 1mSv=1000μSv
Measure to evaluate effect on human bodyEx. 100mSv is assumed to causes no healthproblems
Do not confuse numbers and units reported by media.

RADIATION DOSE IN DAILY LIFE

EXTERNAL AND INTERNAL DOSES FROM
NATURAL SOURCES (WORLD AVERAGE)
External dose
Cosmic ray
0.39mSv/y

From the ground0.48mSv/y

Food
0.29mSv/yInternal doseTotal
2.4mSv/yInhalingRadon
1.26mSv/yUnited Nations Scientific Committee on
the Effect of Atomic Radiation(1993)

CURRENT SITUATION Cooling by poring water into the reactors and spent

fuel pool is going on with temporary pumps, thus theplants condition holds a stability.
 The recovery of the cooling capability is still underway.
 High level of radiation from the water inside thereactor and turbine buildings prevents the progress ofthe operation.
 Cooling still needs to be provided from externalsources, using fresh water and pump trucks, while
work continues to establish a stable heat removal
path to external heat sinks.

RADIATION DOSE DATA

http://d.hatana.ne.jp/oxon/20110318/1300381733

ANNUAL DOSE IN SAPPORO External radiation

0.03*24*365=263 µSv/yLess than the world average of 870 µSv/y
µSv/h

0.05
0.04
0.03
0.02
0.01
00

5

1015
2025
Days after 17, MarchMaximum radiation level (µSv/h) in Sapporo
http://atmc.jp/

PROTECTION FROM RADIATION
 Studies on Hiroshima and Nagasaki atomic bomb,accidents in Three Mile Island, Chernobyl andJapan Nuclear Fuel Conversion Company , doselimits for health have become well understood.
 Based on the facts learned from the above,procedures for health protection have been wellcontrolled The regulations are set based on the ICRP
international guide lines with large margin andopenly measured data
 Thus, nothings to worry about right now in Sapporo