Early Analysis of the Fukushima Dai-ichi Fuel Pools

Report
Explaining the Unexpected:
Early Analysis of the
Fukushima Dai-ichi Fuel
Pools
North Carolina Health Physics Society
Chapter Meeting
Raleigh, NC
6 October 2011
Andrew Sowder, Ph.D., CHP
Senior Project Manager
Used Fuel & HLW Management
Fukushima Dai-Ichi: Applying Industry
and Government Resources
• Substantial early engagement
among DOE/EPRI/INPO/NEI/NRC
Institute of Nuclear Power Operations
• Each organization focused on its
core capabilities and role
Nuclear Energy Institute
• Direct and indirect support to
TEPCO/Japan
• Integrated response to policy,
regulatory and technical lessons
learned underway
+ Utilities, Vendors, and International Organizations
© 2011 Electric Power Research Institute, Inc. All rights reserved.
2
US Nuclear Regulatory
Commission
US Department of Energy
U.S. Based Institutions - Event Response Role
Institute of Nuclear Power Operations
Nuclear Energy Institute
Coordinates industry response to
operational aspects of an event
Leads industry communications to
media, public, and government
stakeholders and leads interface
with federal government
Provides technical support to industry
© 2011 Electric Power Research Institute, Inc. All rights reserved.
3
EPRI Role / Industry Path Forward
• Participation on “Industry
Support Team”
• Direct support for TEPCO by
various EPRI groups
– Plant Technology
– Chemistry, LLW, RM
– Used Fuel & HLW, FRP
• The Way Forward Initiative
www.nei.org/filefolder/TheWayForward_060611_FinalA2.pdf
© 2011 Electric Power Research Institute, Inc. All rights reserved.
4
Industry Roles under “The Way Forward”
© 2011 Electric Power Research Institute, Inc. All rights reserved.
5
Fukushima Dai-ichi Before 11 March 2011
Units 5 - 6
Common Fuel Pool
Dry Storage
Units 1 - 4
© 2011 Electric Power Research Institute, Inc. All rights reserved.
6
Fukushima Dai-ichi After Tsunami
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
7
Fukushima Dai-ichi After Tsunami
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
8
11 March Fukushima Dai-ichi Tsunami Strike
© 2011 Electric Power Research Institute, Inc. All rights reserved.
9
11 March Tsunami Strike at Fukushima Dai-ichi
Source: JNES
© 2011 Electric Power Research Institute, Inc. All rights reserved.
10
Early Event Analysis
• Accurate understanding essential
for applying lessons learned
• Early analysis drives focus on
credible, significant issues
• Understanding still evolving
(incomplete data, many theories)
Focus for this Presentation:
What role, if any, did the fuel pools play in the events
at Fukushima Dai-ichi?
© 2011 Electric Power Research Institute, Inc. All rights reserved.
11
What is a Spent Fuel Pool?
• Water-filled, stainless-steel-lined,
concrete basin for storing irradiated fuel
• Provides
– cooling
– radiation shielding
– sub-critical condition
• Refueling every 18-24 months in US,
12-15 months in Japan
– reactor shutdown, vessel opened, and
fraction of core replaced with fresh fuel
– used or spent fuel moved to pools
Pool provides large thermal inertial provided, but “young”
used fuel requires substantial heat removal.
© 2011 Electric Power Research Institute, Inc. All rights reserved.
12
Fukushima Dai-ichi Design
Source: NEI, 2011. http://www.nei.org/filefolder/BWR_illustration_3.jpg
© 2011 Electric Power Research Institute, Inc. All rights reserved.
13
Fukushima Dai-ichi Fuel Pool
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
14
Spent Fuel Pool Details
Pool Elevations
© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fuel Racks
15
Used Fuel Management at Fukushima Dai-ichi
Storage method
Inventory
as of
March
2010
Total
Capacity
# Assemblies
Spent fuel pool at
each reactor unit
3,450
8,310
Dry cask
408
408
Common pool
6,291
6,840
Total
10,149
15,558
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
16
Fukushima Dai-ichi Fuel Pool Inventories
Reactor
Core Fuel
Power Level
Assemblies
(MWt/MWe)
Most Recent
Irradiated Unirradiated
Total
Pool
Addition of
fuel
fuel
Number of
Assembly
Irradiated Fuel Assemblies Assemblies Assemblies
Capacity
to Pool
in Pool
in Pool
in the Pool
Pool
Decay
Power
(MW)
Unit 1
1380/460
400
March 2010
292
100
392
900
0.07
Unit 2
2381/784
548
Sept 2010
587
28
615
1,240
0.5
Unit 3
2381/784
548
June 2010
514
52
566
1,220
0.2
Unit 4
2381/784
0
Nov 30, 2010
1,331
204
1,535
1,590
2.3
Unit 5
2381/784
548
Jan 2010
946
48
994
1,590
0.8
Unit 6
3293/1100
764
Aug 2010
876
64
940
1,770
0.7
Common
Pool
-
-
-
6,375
-
6,375
6,840
1.2
© 2011 Electric Power Research Institute, Inc. All rights reserved.
17
1F4 Fuel Pool Map Indexed by Assembly
Thermal Power
A
C S
EFPM
0
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
2
13 1
IF
IF
IF
IF
IF
IF
IF
IF
1
1
0
IF
IF
IF
IF
IF
IF
IF
IF
1
0
1
2
3
4
5
6
7
8
2
IF
IF
IF
IF
IF
IF
IF
IF
12 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
5
0
1
2
3
4
5
6
7
8
9
2
5
5
5
5
5
5
5
5
5
11 1
8
4
4
4
4
8
8
8
0
4
8
8
8
8
IF
10
10
2
9
0
1
2
3
4
5
6
7
8
4
B
9
C
0
1
2
3
4
5
6
7
8
9
5
5
5
16
16
5
5
5
5
5
2
IF
IF
IF
IF
5
5
5
5
5
5
37 1
0
5
5
5
5
IF
IF
5
5
5
5
0
1
2
3
4
5
6
7
8
9
8
2
5
5
5
10
10
10
10
10
10
8
4
24 1
4
4
4
4
5
5
5
5
10
10
0
5
5
5
5
5
5
10
5
2
25 1
WFP M
9
1
2
3
1
1
1
IF
IF
IF
IF DF DF DF DF DF DF
2
0
FF FF FF FF DF DF DF DF DF DF
4
5
6
10
2
FF FF FF FF
IF
IF
IF
IF DF DF
IF
10
36 1
FF FF FF FF
IF
IF
IF
IF
5
5
0
FF FF FF FF
IF
IF
IF
IF
0
1
2
5
6
7
8
D
0
1
9
SBG
3
7
8
9
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
2
IF
IF
IF
IF
IF
IF
IF
IF
2
9
9
9
9
9
9
9
9
9
9
45 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
53 1
9
9
9
9
9
9
2
2
2
2
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
1
9
1
9
9
9
9
9
9
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
2
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
2
1
14
9
9
8
8
9
8
9
9
44 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
52 1
14
9
9
9
9
9
9
14
14
14
1
1
1
1
1
1
1
1
1
1
0
14
9
9
1
1
9
9
1
1
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C
R
A
CR
1
0
3
5
7
30
9
CR
2
4
C
R
B
1
6
3
0
1
2
3
4
5
6
7
8
9
2
8
8
8
5
IF
IF
IF
IF
IF
IF
5
8
8
5
5
5
5
5
IF
8
0
8
8
10
IF
10
10
10
10
10
10
0
1
2
3
4
5
6
7
8
9
2
5
5
5
IF
IF
IF
IF
IF
5
12
09 1
5
IF
IF
IF
8
IF
10
10
10
8
0
8
10
8
8
IF
8
8
10
10
10
7
6
8
9
2
10 1
5
0
8
4
7
1
0
3
CR
1
2
4
3
CR
5
7
6
8
4
5
2
1
1
1
1
1
1
1
1
1
1
2
9
9
9
9
9
9
9
9
9
9
43 1
1
1
1
1
1
1
1
1
1
1
51 1
9
9
9
9
4
8
9
9
9
9
0
1
1
1
1
1
1
1
1
1
1
0
9
9
9
9
9
9
9
9
8
8
9
CR
CR
2
0
CR
CR
CR
CR
0
1
2
3
4
5
8
9
5
IF
10
10
10
10
5
5
5
10
2
FF FF FF FF FF FF
23 1
10
IF
IF
IF
10
IF
IF
IF
10
10
35 1
FF FF FF FF FF FF
0
IF
IF
IF
IF
IF
IF
IF
IF
10
IF
0
FF FF FF FF FF FF
0
1
2
3
4
5
6
7
8
9
2
IF
IF
IF
5
IF
IF
5
9
IF
IF
2
FF FF FF FF FF FF FF FF FF FF
22 1
9
9
IF
IF
IF
IF
IF
IF
IF
IF
34 1
FF FF FF FF FF FF FF FF FF FF
0
9
9
10
10
12
12
12
12
IF
9
0
FF FF FF FF FF FF FF FF FF FF
2
6
C
R
C
E
0
4
6
7
8
9
01
03
05
SBG
SBG
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
2
1
1
1
1
1
1
1
1
1
1
2
8
8
8
4
8
8
8
8
8
8
42 1
1
1
1
1
1
1
1
1
2
2
50 1
8
8
8
8
9
9
9
9
9
1
0
2
2
2
2
2
2
2
2
2
2
0
9
9
9
9
9
10
10
10
10
9
IF
3.60 kW
1
1.12 kW
2
0.55 kW
4
0.40 kW
5
0.30 kW
8
0.24 kW
9
0.23 kW
10
0.22 kW
12
0.21 kW
14
0.20 kW
16
0.19 kW
24
0.16 kW
19
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
2
2
2
41 1
2
2
2
2
2
2
2
2
2
2
49 1
3
4
5
6
7
8
9
9
1
9
9
9
9
1
9
9
1
10
0
10
0
2
2
2
2
4
4
4
4
2
2
0
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
2
40 1
2
2
2
2
4
4
4
4
0
4
4
4
2
4
2
4
2
0
1
2
3
4
5
6
7
8
9
2
4
4
4
2
4
8
2
2
4
39 1
4
4
4
4
4
4
4
4
4
IF
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
1
10
2
10
9
9
9
8
8
8
0
8
8
8
8
8
8
2
2
2
2
0
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
2
2
2
4
4
48 1
4
2
2
2
2
2
4
4
2
2
4
8
0
4
4
2
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
4
2
2
2
2
2
2
2
2
2
2
2
4
47 1
2
2
2
2
2
2
2
2
2
2
4
4
0
2
2
8
9
17
15
13
11
0
1
2
3
4
5
6
7
8
9
IF
8
10
10
10
IF
10
10
10
IF
0
1
2
3
4
5
6
7
8
9
IF
IF
IF
IF
IF
IF
IF
IF
IF
10
0
1
2
3
4
5
6
7
8
9
09
2
2
2
FF FF FF FF FF FF FF FF FF FF
07
08 1
10
10
10
10
10
IF
10
IF
10
10
21 1
IF
IF
IF
IF
IF
IF
10
10
10
10
33 1
FF FF FF FF FF FF FF FF FF FF
0
IF
IF
10
10
IF
10
10
10
10
IF
0
IF
IF
10
10
10
10
10
10
IF
IF
0
FF FF FF FF FF FF FF FF FF FF
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
2
IF
10
10
10
10
10
IF
10
10
IF
2
9
9
IF
IF
IF
IF
IF
IF
IF
IF
2
FF FF FF FF FF FF FF FF FF FF
SBG
07 1
10
10
10
10
10
IF
IF
10
10
10
20 1
IF
10
10
10
24
24
IF
IF
IF
IF
32 1
FF FF FF FF FF FF FF FF FF FF
B
0
IF
0
24
0
FF FF FF FF FF FF FF FF FF FF
05
03
0
1
2
3
4
5
6
7
8
9
01
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
1
2
3
4
5
6
7
8
9
2
24
10
10
10
10
10
IF
IF
IF
0
1
2
3
4
5
6
7
8
9
IF
2
IF
IF
IF
IF
IF
IF
IF
IF
IF
06 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
19 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
1
2
3
4
5
6
7
8
9
2
IF
IF
IF
IF
IF
IF
4
IF
IF
IF
IF
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
1
2
3
4
5
6
7
8
9
2
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
2
05 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
18 1
IF
IF
IF
5
5
IF
IF
IF
IF
IF
30 1
IF
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
SBG
G
A
0
9
IF
2
FF FF FF FF FF FF FF FF FF FF
2
IF
2
2
2
2
4
4
4
4
IF
31 1
FF FF FF FF FF FF FF FF FF FF
38 1
4
4
4
4
4
1
1
1
1
IF
0
FF FF FF FF FF FF FF FF FF FF
0
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
8
8
8
8
8
8
8
IF
IF
IF
8
8
8
8
8
IF
4
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
9
9
0
1
2
3
4
5
6
7
8
9
IF
IF
IF
IF
IF
IF
IF
IF
IF
04 1
IF
IF
4
0
IF
4
IF
0
1
2
3
IF
4
4
IF
IF
IF
IF
4
IF
IF
IF
IF
IF
4
IF
4
5
6
7
8
9
2
17 1
IF
5
IF
12
IF
5
IF
IF
IF
IF
2
9
9
9
9
9
9
9
9
9
9
5
IF
12
IF
IF
IF
IF
IF
IF
IF
29 1
9
9
9
9
9
9
9
9
9
9
IF
IF
IF
IF
IF
12
12
12
12
IF
0
9
9
9
9
9
9
9
9
9
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
2
IF
4
IF
IF
IF
IF
4
IF
IF
4
2
12
12
12
IF
IF
IF
IF
IF
IF
IF
2
IF
IF
IF
IF
4
IF
IF
4
4
IF
16 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
28 1
4
4
IF
IF
4
IF
IF
4
IF
IF
0
IF
IF
IF
IF
IF
4
IF
IF
IF
IF
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
9
9
9
9
9
IF
IF
8
IF
IF
8
8
8
IF
8
8
8
8
IF
8
8
8
IF
8
8
IF
8
IF
8
IF
0
1
2
3
4
5
6
7
8
9
IF
8
4
4
4
IF
IF
IF
IF
2
2
2
2
2
3
4
5
6
7
8
9
4
2
2
2
2
2
2
2
2
2
2
1
46 1
1
1
1
1
1
1
1
1
1
1
9
CR
7
8
CR
6
5
Cooling time years
days
yrs
CR
CR
03 1
2
2
CR
CR
0
2
1
0
CR
2
2
0
2
3
4
365-729
=
1
730-1094
=
2
1
2
1460-1824
=
4
1825-2189
=
5
CR
C
IF
IF
4
IF
IF
4
4
IF
4
IF
2
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
2
IF
0
2920-3284
=
02 1
4
4
4
4
IF
IF
IF
IF
IF
IF
15 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
27 1
FF FF FF FF FF FF FF FF FF FF
CRD
3285-3649
=
9
0
4
8
8
10
10
IF
8
10
8
4
0
IF
IF
IF
IF
IF
IF
IF
IF
9
IF
0
FF FF FF FF FF FF FF FF FF FF
3650-4014
=
10
2
8
0
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
4380-4744
=
12
2
12
IF
IF
IF
IF
IF
12
IF
IF
IF
2
IF
5
12
IF
IF
IF
IF
5
IF
IF
2
FF FF FF FF FF FF FF FF FF FF
5110-5474
=
14
01 1
12
IF
IF
5
IF
12
12
IF
IF
IF
14 1
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
26 1
FF FF FF FF FF FF FF FF FF FF
5840-6204
=
16
0
12
5
12
12
12
5
5
12
12
5
0
IF
IF
IF
IF
IF
IF
IF
IF
IF
IF
0
FF FF FF FF FF FF FF FF FF FF
8760-9124
=
24
FF Fresh Fuel
IF Irradiated Fuel
0
DF Dummy Fuel
0
1
2
3
4
5
6
7
8
9
SBG Single blade guide
© 2011 Electric Power Research Institute, Inc. All rights reserved.
18
Understood: Hydrogen Explosions in
Units 1 and 3
Zr + 2H2O → ZrO2 + 2H2 + energy
© 2011 Electric Power Research Institute, Inc. All rights reserved.
19
Unexpected: 15 March Unit 4 Damage
Source: Air Photo Service Co. Ltd., Japan
© 2011 Electric Power Research Institute, Inc. All rights reserved.
20
Early Focus on 1F Unit 4 Pool
BEFORE
• Damage to Unit 4 reactor
building unexpected
– Unit 4 reactor was offline for
maintenance
– Defueled ~100 days before
earthquake/tsunami
• Full core offload in fuel pool
– youngest, hottest fuel (2.3 MW
decay heat load in 1F4 pool vs. <1
MW in other pools)
– most reactive fuel (first cycle for
BWR)
– other pools less challenged
© 2011 Electric Power Research Institute, Inc. All rights reserved.
21
AFTER
March 16: Grave Statements from US Officials
http://abcnews.go.com/Business/wireStory?id=13150227
© 2011 Electric Power Research Institute, Inc. All rights reserved.
22
Rampant Speculation on Cause of Unit 4
Damage
• Hydrogen gas from spent fuel in pool from high temp
reaction of steam w/ Zr cladding after loss of water
Zr + 2H2O → ZrO2 + 2H2 + energy
• Hydrogen gas from :
– radiolysis (radiation induced breakdown of H2O)
– other sources in Unit 4
– Unit 3 shared piping or other connectivity
• Other combustible gas in Unit 4
• Combustion of soot from lube oil fire
Venting of hydrogen gas from Zr oxidation in Unit 1 and 3
cores implicated in explosions of those units.
© 2011 Electric Power Research Institute, Inc. All rights reserved.
23
Early Information for Unit 4 Assessment
• 15 March - Unit 4 explosion at day 4 (<< nominal time for
boil off)
• Ongoing venting of Unit 1 – 3 containments
3/17 dose
rates:
• Increasing flow of dose rate and environmental data
– early exposure readings in vicinity of Unit 4 building
– contamination of sea water near plant
– low (near ambient) Pu soil concentrations onsite
• 87.7 mSv/hr
at 100 m el.
• 400 mSv/hr
near Unit 3
west wall
• Unconfirmed reports of water in pool
• 100 mSv/hr
• Initial water spray from ground level on 20 March
near Unit 4
• Water additions via concrete pump boom on 22 March
• 12 April – Water sample collected, water temp (90 ºC) &
level (2 m above fuel), dose rate measured prior to filling*
*Reported to be “several dozens” of mSv/hr above refueling floor – consistent with water above fuel.
© 2011 Electric Power Research Institute, Inc. All rights reserved.
24
Lots of Unfiltered Information
© 2011 Electric Power Research Institute, Inc. All rights reserved.
25
Milestone: 13 April 2011 Water Analysis Data
for Unit 4 Pool
Unit 4 fuel pool water analysis
Unit 2 fuel pool water analysis
© 2011 Electric Power Research Institute, Inc. All rights reserved.
Data Source: TEPCO
26
Reported Water Additions to 1F4 Consistent with
Evaporation – Not Leakage
TEPCO concludes on 4/28 fuel pools not leaking based
on daily evaporation rates of 140 to 210 tons (kiloliters)
of water daily that match water additions.
© 2011 Electric Power Research Institute, Inc. All rights reserved.
27
Milestone: Underwater Images of Unit 4 Pool
Conditions
• No damage to storage
structure
• No major fuel damage
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
28
Closing in on Cause for Unit 4 Damage
• Visual evidence of fuel, racks, and pool integrity does not
support catastrophic pool drainage and Zr oxidation event
• Other Unit 4 sources of hydrogen
or other combustible material not
considered significant or credible
• Unit 3 as source of hydrogen
remains most credible suspect
–
–
–
–
–
timing of Unit 3 & 4 events
hydrogen role in Units 1 & 3 damage
shared vent stack and piping
consistent failure of venting
16 May TEPCO analysis
Source: Air Photo Service Co. Ltd., Japan
Hydrogen from Unit 3 currently most credible theory.
© 2011 Electric Power Research Institute, Inc. All rights reserved.
29
TEPCO 16 May 2011 Theory for Source of
Hydrogen in Unit 4
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
30
TEPCO 16 May 2011 Theory for Source of
Hydrogen in Unit 4 (cont’d)
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
31
1F4 Emergency Gas Treatment System – Post
Accident Configuration
Source: TEPCO
27 August 2011
© 2011 Electric Power Research Institute, Inc. All rights reserved.
32
June 15: The Correction
http://abcnews.go.com/US/wireStory?id=13845733
© 2011 Electric Power Research Institute, Inc. All rights reserved.
33
Interim EPRI Assessments
• Fukushima Dai-ichi Fuel Pool Criticality Assessment
• Summary of fuel pool evolution following loss of cooling
– preliminary gap analysis
– calculation of time required to evaporate 1F fuel pool
water inventory for key scenarios
– identifying important scenarios and mitigating factors
• Evaluation of proposed scenarios directly implicating Unit
4 pool in hydrogen generation
– from cladding oxidation in blocked fuel channels
experiencing DNB in a pool with water level above fuel
– from enhanced radiolysis in high radiation fields in a
fuel pool at or near boiling
© 2011 Electric Power Research Institute, Inc. All rights reserved.
34
Early EPRI Criticality Assessment for
Fukushima Dai-ichi Fuel Pools
• A spent fuel pool criticality event remains highly unlikely in
damaged Fukushima Dai-ichi spent fuel pools.
– includes the extreme case of the introduction of water
to a dry fuel pool
– total fuel inventory present in the form of rubblized fuel
pellet fragments lying at the bottom of the pool
• Primary objective for mitigation of drained fuel pool at
Fukushima should be covering spent fuel pool with
water to reduce the dose to workers.
© 2011 Electric Power Research Institute, Inc. All rights reserved.
35
Fuel Pool Evolution Following Loss of Cooling
Stage
1
Sequence
Nominal condition of pool: 3045°C, normal water level
2
Pool water temperature gradually
approaches water boiling point
3
Boiling of pool water: Boiling
begins in racks where hottest fuel
assemblies are stored; pool water
level gradually decreases towards
top of fuel assemblies’ levels
4
Gradual uncovering of fuel
assemblies
5
Thermal ramping of fuel rods in
the presence of water (water level
above rack base plate).
6
Thermal ramping of fuel rods in
the absence of water (pool water
below rack base plate).
7
Dispersal of fuel materials
© 2011 Electric Power Research Institute, Inc. All rights reserved.
• The level of water at which
temperatures start ramping up will
depend on decay heat and assumed
heat transfer mechanisms, including
boiling heat transfer at the submerged
fuel rod-water interface.
• GAP: More realistic assessment of
the transition point from sufficient
to insufficient axial cooling as a
function of rack and assembly
design and decay heat.
36
Estimated Time Margins for 1F Pools Based on
Simple Calculations
Unit
1
2
3
4
Time to Uncovering
Top of Fuel (days)
250
35
88
7.6
Time to Uncovering 50% Fuel
Length (days)
350
50
120
11
Assumes:
NOTE: For Unit 4, additional inventory from refueling
cavity and D/S pit could provide up to 14 days of
additional margin.
D/S Pit
20' x 49'
Refueling Cavity
r = 20'
© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fuel Pool
32.5' x 40'
37
Initial Pool Temp = 35 °C
Initial sloshing removes
1.5 m of water inventory
TEPCO 20 June 2011 Theory for Evolution of
1F4 Fuel Pool Water Inventory following LOCA
• Refueling well and
D/S pit were flooded
and interconnected.
• Spent fuel pool gate
was in place.
• Following
evaporation of pool
inventory, leakage
from adjoining
refueling cavity
provided an
alternative source of
makeup water
© 2011 Electric Power Research Institute, Inc. All rights reserved.
W a te r in flow fro m th e g ate
D S P it
W e ll
SFP
D /S ピ ッ ト
RPV
W a te r
in je c tio n
G a te s e a le d
D S P it
W e ll
D /S ピ ッ ト
RPV
38
SFP
Early EPRI Evaluation of Scenarios Implicating
Fuel Pool in Unit 4 Explosion
• Hydrogen generation from cladding oxidation in pool with
water covering fuel due to localized departure from
nuclear boiling (DNB) in debris-blocked fuel channels
Hydrogen from zircaloy cladding oxidation not likely
for pool levels covering at least half the fuel height
• Hydrogen from water radiolysis in 1F4 storage pool (fuel
offload)
Bounding production rate calculations indicate
insignificant H2 production relative to building
volume to cause explosion (<<1% in gas mixture)
© 2011 Electric Power Research Institute, Inc. All rights reserved.
39
Status of Unit 3 Fuel Pool
14 March: Unit 3 explosion
Source: NHK
Source: TEPCO; Video image obtained 8 May 2011
Source: Air Photo Service Co. Ltd., Japan
© 2011 Electric Power Research Institute, Inc. All rights reserved.
13 April – Unit 3 pool
reported to be “full”
40
Data Source: TEPCO
Status of Units 1 & 2 Fuel Pools
© 2011 Electric Power Research Institute, Inc. All rights reserved.
Source: Air Photo Service Co. Ltd., Japan
Unit 2 fuel pool
water analysis
• No video or
photographic
images of either
pool
• Unit 2 pool had
2nd highest
thermal load
(0.5 MW)
• Unit 2 water
additions start
3/20
41
Data Source: TEPCO
Units 1 - 4 Summary as of 16 August 2011Source: TEPCO
Unit 1
Unit 2
Roof collapsed on
operating floor and
SFP
Roof intact;
operating floor
condition unknown
Used 292
New 100
180 kW
Used 587
New 28
620 kW
Used 514
New 52 (32 MOX)
540 kW
Used 1,331
New 204
2,300 kW
SFP Volume in m 3
1020
1425
1425
1425
SFP Rack Material
Aluminum & Boral
Aluminum & Boral
Aluminum & Boral
SS 304
Most recent S/D
9/27/10
Most recent S/D
11/18/10
Most recent S/D
9/23/10
Most recent S/D
11/29/10. All
assemblies in SFP.
<1% based on water
sample on 4/12/11
Physical Status
Loading of SFPs,
# assemblies
Heat Loads in March,
estimated
Fuel Burnup
Estimate of Fuel
Damage
Temperature of SFP
Date closed loop
cooling in service
Volume of seawater
injected early on
Evaporation rate
before cooling set up
Unit 3
likely none, believed likely none, believed
always covered
always covered
Current 36 C
Est. peak <80 C.
8/10/11
Using original HX
with new secondary
system
none
Unknown; but well
<10 tons/day
© 2011 Electric Power Research Institute, Inc. All rights reserved.
Current 38 C
o
Unit 4
Roof and structure
Roof structure
destroyed; debris in damaged; debris in
pool, exposed
pool. Support
concrete
beams installed July.
Some mechanical
damage from fallen
debris
Current 33 C
o
Current 45 C.
o
Peak of 80 C.
5/31/11
Alternate SFP
Cooling system
Peak of 62 C
6/30/11
Alternate SFP
Cooling system
Peak >90 C.
7/31/11
Alternate SFP
Cooling system
90 tons
4,560 tons
700 tons
21 tons/day
17 tons/day
72 tons/day
42
Units 1 - 4 Summary as of 16 August 2011
• Structure reinforcement of Unit 4 pool completed
• Hydrazine has been added to Units 2, 3, 4 for corrosion concerns
• Boric acid has been added to Unit 3 for pH purposes
• A truck-mounted desalination unit is being planned for CL- reduction; movable
between units; not in service as of 8/15/11
• Unit 3 water sample of 5/9/11 showed CL- at 2,400 ppm and pH 11.2
Source: TEPCO
• ND - Not Detectable
© 2011 Electric Power Research Institute, Inc. All rights reserved.
43
Status of Fuel in Dry Storage and Common Pool
• 17 March - While also inundated with water, TEPCO
confirms integrity of dry storage casks and building
• 18 March - TEPCO confirms stability of common fuel pool
Source: TEPCO
© 2011 Electric Power Research Institute, Inc. All rights reserved.
44
Broader Picture: Event Analysis ►
Understanding ► Lessons Learned
• U.S. plant features and
operating practice have
evolved in light of operating
experience and knowledge
• Still early in Fukushima
recovery phase
• Event analysis remains
incomplete
• Implications for U.S. plant
design and operations not
fully understood
• Post-Fukushima:
– vulnerabilities identified
and corrected
– mitigation strategies
developed for credible
beyond design basis
hazards
Design Basis
Accidents
TMI
Modifications
Post 9/11 or
B.5.B
Mitigation
Strategies
Fukushima
Strategies
Mitigation
© 2011 Electric Power Research Institute, Inc. All rights reserved.
IPE / IPEEE
and PRA
Modifications
45
Post-Fukushima R&D Path Forward
• Verify current understanding
• Identify and address gaps
– Risks posed by external hazards
– Severe accident progression,
including combustible gas control
– Monitoring and instrument needs
– Radiological releases and paths
– Fuel pool phenomena and relative risk
– Integration and execution of mitigation
actions
• Establish and preserve pedigreed
Fukushima accident knowledge base
© 2011 Electric Power Research Institute, Inc. All rights reserved.
46
Together…Shaping the Future of Electricity
© 2011 Electric Power Research Institute, Inc. All rights reserved.
47

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