explaining the unexpected: early analysis of the fukushima dai- ichi fuel pools
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Explaining the Unexpected: Early Analysis of the Fukushima Dai-ichi Fuel PoolsNorth Carolina Health Physics Society
Chapter MeetingRaleigh, NC
6 October 2011
Andrew Sowder, Ph.D., CHPSenior Project Manager
Used Fuel & HLW Management
2© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-Ichi: Applying Industry and Government Resources
• Substantial early engagement among DOE/EPRI/INPO/NEI/NRC
• Each organization focused on its core capabilities and role
• Direct and indirect support to TEPCO/Japan
• Integrated response to policy, regulatory and technical lessons learned underway
US Department of Energy
US Nuclear Regulatory Commission
Nuclear Energy Institute
Institute of Nuclear Power Operations
+ Utilities, Vendors, and International Organizations
3© 2011 Electric Power Research Institute, Inc. All rights reserved.
U.S. Based Institutions - Event Response Role
Nuclear Energy Institute
Institute of Nuclear Power Operations
Leads industry communications to media, public, and government
stakeholders and leads interface with federal government
Coordinates industry response to operational aspects of an event
Provides technical support to industry
4© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
5© 2011 Electric Power Research Institute, Inc. All rights reserved.
Industry Roles under “The Way Forward”
6© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-ichi Before 11 March 2011
Units 1 - 4
Units 5 - 6
Dry Storage
Common Fuel Pool
7© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-ichi After Tsunami
Source: TEPCO
8© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-ichi After Tsunami
Source: TEPCO
9© 2011 Electric Power Research Institute, Inc. All rights reserved.
11 March Fukushima Dai-ichi Tsunami Strike
10© 2011 Electric Power Research Institute, Inc. All rights reserved.
11 March Tsunami Strike at Fukushima Dai-ichi
Source: JNES
11© 2011 Electric Power Research Institute, Inc. All rights reserved.
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?
12© 2011 Electric Power Research Institute, Inc. All rights reserved.
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.
13© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-ichi Design
Source: NEI, 2011. http://www.nei.org/filefolder/BWR_illustration_3.jpg
14© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-ichi Fuel Pool
Source: TEPCO
15© 2011 Electric Power Research Institute, Inc. All rights reserved.
Spent Fuel Pool Details
Pool Elevations Fuel Racks
16© 2011 Electric Power Research Institute, Inc. All rights reserved.
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 408Common pool 6,291 6,840
Total 10,149 15,558
Source: TEPCO
17© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fukushima Dai-ichi Fuel Pool Inventories
Reactor Power Level (MWt/MWe)
Core Fuel Assemblies
Most Recent Addition of
Irradiated Fuel to Pool
Irradiated fuel
Assemblies in Pool
Unirradiated fuel
Assemblies in Pool
Total Number of
Assemblies in the Pool
Pool AssemblyCapacity
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
18© 2011 Electric Power Research Institute, Inc. All rights reserved.
1F4 Fuel Pool Map Indexed by Assembly Thermal Power
16 0.19 kW
24 0.16 kW
14 0.20 kW
10 0.22 kW
12 0.21 kW
9 0.23 kW
5 0.30 kW
8 0.24 kW
2 0.55 kW
4 0.40 kW
1 1.12 kW
IF 3.60 kW
0 1 2 3 4 5 6 7 8 9 0 1 2 4 5 6 7 8 9
2 1 1 1 1 1 1 1 1 1 2
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 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 2 5 5 5 16 16 5 5 5 5 5 2 1 1 1 1 SBG 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
1 IF IF IF IF IF IF IF IF IF IF 1 IF IF IF IF 5 5 5 5 5 5 1 IF IF IF IF DF DF DF DF DF DF 2 4 2 IF IF IF IF IF IF IF IF 2 9 9 9 9 9 9 9 9 9 9
0 IF IF IF IF IF IF IF IF IF 5 0 5 5 5 5 IF IF 5 5 5 5 0 FF FF FF FF DF DF DF DF DF DF 1 IF IF IF IF IF IF IF IF IF IF 1 9 9 9 9 9 9 2 2 2 2
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 9 0 IF IF IF IF IF IF IF IF IF IF 0 1 9 1 9 9 9 9 9 9 9
2 5 5 5 5 5 5 5 5 5 8 2 5 5 5 10 10 10 10 10 10 10 2 FF FF FF FF IF IF IF IF DF DF 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
1 8 4 4 4 4 8 8 8 8 4 1 4 4 4 4 5 5 5 5 IF 10 1 FF FF FF FF IF IF IF IF 2 IF IF IF IF IF IF IF IF IF IF 2 1 14 9 9 8 8 9 8 9 9
0 4 8 8 8 8 IF 10 10 10 10 0 5 5 5 5 5 5 10 5 5 5 0 FF FF FF FF IF IF IF IF 1 IF IF IF IF IF IF IF IF IF IF 1 14 9 9 9 9 9 9 14 14 14
0 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
2 1 1 1 1 1 1 1 1 1 1 2 9 9 9 9 9 9 9 9 9 9
1 1 1 1 1 1 1 1 1 1 1 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
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 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
2 8 8 8 5 IF IF IF IF IF IF 2 5 IF 10 10 10 10 5 5 5 10 2 FF FF FF FF FF FF 2 1 1 1 1 1 1 1 1 1 1 2 8 8 8 4 8 8 8 8 8 8
1 5 8 8 5 5 5 5 5 IF 8 1 10 IF IF IF 10 IF IF IF 10 10 1 FF FF FF FF FF FF 1 1 1 1 1 1 1 1 1 2 2 1 8 8 8 8 9 9 9 9 9 1
0 8 8 10 IF 10 10 10 10 10 10 0 IF IF IF IF IF IF IF IF 10 IF 0 FF FF FF FF FF FF SBG SBG 0 2 2 2 2 2 2 2 2 2 2 0 9 9 9 9 9 10 10 10 10 9
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 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
2 5 5 5 IF IF IF IF IF 5 12 2 IF IF IF 5 IF IF 5 9 IF IF 2 FF FF FF FF FF FF FF FF FF FF 2 2 2 2 2 2 2 2 2 2 2 2 9 1 9 9 9 9 1 9 9 1
1 5 IF IF IF 8 IF 10 10 10 8 1 9 9 IF IF IF IF IF IF IF IF 1 FF FF FF FF FF FF FF FF FF FF 1 2 2 2 2 2 2 2 2 2 2 1 10 10 10 10 9 9 9 8 8 8
0 8 10 8 8 IF 8 8 10 10 10 0 9 9 10 10 12 12 12 12 IF 9 0 FF FF FF FF FF FF FF FF FF FF 0 2 2 2 2 4 4 4 4 2 2 0 8 8 8 8 8 8 2 2 2 2
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 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
2 IF 8 10 10 10 IF 10 10 10 IF 2 IF IF IF IF IF IF IF IF IF 10 2 FF FF FF FF FF FF FF FF FF FF 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
1 10 10 10 10 10 IF 10 IF 10 10 1 IF IF IF IF IF IF 10 10 10 10 1 FF FF FF FF FF FF FF FF FF FF 1 2 2 2 2 4 4 4 4 4 4 1 4 2 2 2 2 2 4 4 2 2
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 4 4 4 2 4 2 4 2 4 8 0 4 4 2 2 2 2 2 2 2 2
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 9 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 SBG 2 4 4 4 2 4 8 2 2 4 4 2 2 2 2 2 2 2 2 2 2 2
1 10 10 10 10 10 IF IF 10 10 10 1 IF 10 10 10 24 24 IF IF IF IF 1 FF FF FF FF FF FF FF FF FF FF 1 4 4 4 4 4 4 4 4 4 4 1 2 2 2 2 2 2 2 2 2 2
0 IF IF IF IF IF IF IF IF IF IF 0 24 24 10 10 10 10 10 IF IF IF 0 FF FF FF FF FF FF FF FF FF FF 0 IF 4 4 4 4 4 4 4 4 4 0 2 2 2 2 2 2 2 2 2 2
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 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 IF IF IF IF IF IF IF IF IF IF 2 FF FF FF FF FF FF FF FF FF FF 2 IF 2 2 2 2 4 4 4 4 4 2 2 2 2 2 2 2 2 2 2 2
1 IF IF IF IF IF IF IF IF IF IF 1 IF IF IF IF IF IF IF IF IF IF 1 FF FF FF FF FF FF FF FF FF FF 1 4 4 4 4 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0 IF IF IF IF IF IF IF IF IF IF 0 IF IF IF IF IF IF IF IF IF IF 0 FF FF FF FF FF FF FF FF FF FF 0 4 4 4 4 4 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 9
2 IF IF IF IF IF IF 4 IF IF IF 2 IF IF IF IF IF IF IF IF IF IF 2 8 8 8 8 8 8 8 IF IF IF
1 IF IF IF IF IF IF IF IF IF IF 1 IF IF IF 5 5 IF IF IF IF IF 1 8 8 8 8 8 IF 4 IF IF IF
0 IF IF IF IF IF IF IF IF IF IF 0 IF IF IF IF IF IF IF IF IF IF 0 IF IF IF IF IF IF IF IF 9 9
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 9
2 IF IF IF IF IF IF IF IF IF IF 2 IF 5 IF 12 IF 5 IF IF IF IF 2 9 9 9 9 9 9 9 9 9 9
1 IF IF 4 4 4 IF IF IF IF 4 1 5 IF 12 IF IF IF IF IF IF IF 1 9 9 9 9 9 9 9 9 9 9
0 IF 4 IF IF IF IF IF IF 4 IF 0 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 0 1 2 3 4 5 6 7 8 9 yrs
2 IF 4 IF IF IF IF 4 IF IF 4 2 12 12 12 IF IF IF IF IF IF IF 2 9 9 9 9 9 IF 8 IF 8 IF 365-729 = 1
1 IF IF IF IF 4 IF IF 4 4 IF 1 IF IF IF IF IF IF IF IF IF IF 1 IF 8 8 8 IF 8 8 8 8 IF 730-1094 = 2
0 4 4 IF IF 4 IF IF 4 IF IF 0 IF IF IF IF IF 4 IF IF IF IF 0 8 8 8 IF 8 8 IF IF 8 IF 1460-1824 = 4
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 9 1825-2189 = 5
2 IF IF 4 IF IF 4 4 IF 4 IF 2 IF IF IF IF IF IF IF IF IF IF 2 IF 8 4 4 4 IF IF IF IF IF 2920-3284 = 8
1 4 4 4 4 IF IF IF IF IF IF 1 IF IF IF IF IF IF IF IF IF IF 1 FF FF FF FF FF FF FF FF FF FF 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
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 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
1 12 IF IF 5 IF 12 12 IF IF IF 1 IF IF IF IF IF IF IF IF IF IF 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 DF Dummy Fuel SBG Single blade guide
01 14 26
C R D02 15 27
03 16 28
days
04 17 29
Cooling t ime years
05 18 30
06 19 31 38 46
07 20 32 39 47
08 21 33 40 48
09 22 34 41 49
10 23 35 42 50
44 52
CRA
CRB
CRC
43 51
45 53
11 24 36
D E
12 25 37
13
B C
A C S
3
0 9
8642
1 3 5 7
0 9
8642
1 3 5 7
0 9
8642
1 3 5 7CR
CR
CR CR CR
CR CRCRCR
CR30
0
2
4
6
8
9
1
3
5
7
CR
CRCR
CR
CR
CR
CR
CR
C
E F P M W F P M
B G A
01 03 05
19
17
15
13
11
09
07
05
03
01
19© 2011 Electric Power Research Institute, Inc. All rights reserved.
Understood: Hydrogen Explosions in Units 1 and 3
Zr + 2H2O → ZrO2 + 2H2 + energy
20© 2011 Electric Power Research Institute, Inc. All rights reserved.
Unexpected: 15 March Unit 4 Damage
Source: Air Photo Service Co. Ltd., Japan
21© 2011 Electric Power Research Institute, Inc. All rights reserved.
Early Focus on 1F Unit 4 Pool
• 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
BEFORE
AFTER
22© 2011 Electric Power Research Institute, Inc. All rights reserved.
March 16: Grave Statements from US Officials
http://abcnews.go.com/Business/wireStory?id=13150227
23© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
• 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.
Zr + 2H2O → ZrO2 + 2H2 + energy
24© 2011 Electric Power Research Institute, Inc. All rights reserved.
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• 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
• Unconfirmed reports of water in pool• Initial water spray from ground level on 20 March• 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*
3/17 dose rates:
• 87.7 mSv/hr at 100 m el.
• 400 mSv/hr near Unit 3 west wall
• 100 mSv/hr near Unit 4
*Reported to be “several dozens” of mSv/hr above refueling floor – consistent with water above fuel.
25© 2011 Electric Power Research Institute, Inc. All rights reserved.
Lots of Unfiltered Information
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Milestone: 13 April 2011 Water Analysis Data for Unit 4 Pool
Unit 4 fuel pool water analysis
Unit 2 fuel pool water analysis Data Source: TEPCO
27© 2011 Electric Power Research Institute, Inc. All rights reserved.
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.
28© 2011 Electric Power Research Institute, Inc. All rights reserved.
Milestone: Underwater Images of Unit 4 Pool Conditions
• No damage to storage structure
• No major fuel damage
Source: TEPCO
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Closing in on Cause for Unit 4 Damage
• 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
Hydrogen from Unit 3 currently most credible theory.
• Visual evidence of fuel, racks, and pool integrity does not support catastrophic pool drainage and Zr oxidation event
Source: Air Photo Service Co. Ltd., Japan
30© 2011 Electric Power Research Institute, Inc. All rights reserved.
TEPCO 16 May 2011 Theory for Source of Hydrogen in Unit 4
Source: TEPCO
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TEPCO 16 May 2011 Theory for Source of Hydrogen in Unit 4 (cont’d)
Source: TEPCO
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1F4 Emergency Gas Treatment System – Post Accident Configuration
Source: TEPCO27 August 2011
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June 15: The Correction
http://abcnews.go.com/US/wireStory?id=13845733
34© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
35© 2011 Electric Power Research Institute, Inc. All rights reserved.
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.
36© 2011 Electric Power Research Institute, Inc. All rights reserved.
Fuel Pool Evolution Following Loss of Cooling
• 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.
Stage Sequence1 Nominal condition of pool: 30-
45°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
37© 2011 Electric Power Research Institute, Inc. All rights reserved.
Estimated Time Margins for 1F Pools Based on Simple Calculations
Unit Time to Uncovering Top of Fuel (days)
Time to Uncovering 50% Fuel Length (days)
1 250 350 2 35 50 3 88 120 4 7.6 11
NOTE: For Unit 4, additional inventory from refueling cavity and D/S pit could provide up to 14 days of additional margin.
Assumes:
Initial Pool Temp = 35 °C
Initial sloshing removes 1.5 m of water inventory
Refueling Cavityr = 20'
D/S Pit 20' x 49'
Fuel Pool32.5' x 40'
38© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
Water inflow from the gate
SFPD/Sピット
RPV
WellDS Pit
Water inflow from the gate
SFPD/Sピット
RPV
WellDS Pit SFPD/Sピット
RPV
WellDS Pit
Water injection
SFPD/Sピット
RPV
Well
Gate sealed
DS Pit
Water injection
SFPD/Sピット
RPV
Well
Gate sealed
DS Pit SFPD/Sピット
RPV
Well
Gate sealed
DS Pit
39© 2011 Electric Power Research Institute, Inc. All rights reserved.
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 water radiolysis in 1F4 storage pool (fuel offload)
Hydrogen from zircaloy cladding oxidation not likely for pool levels covering at least half the fuel height
Bounding production rate calculations indicate insignificant H2 production relative to building
volume to cause explosion (<<1% in gas mixture)
40© 2011 Electric Power Research Institute, Inc. All rights reserved.
Status of Unit 3 Fuel Pool
14 March: Unit 3 explosion
Source: TEPCO; Video image obtained 8 May 2011
Source: NHK
Data Source: TEPCO13 April – Unit 3 pool reported to be “full”Source: Air Photo Service Co. Ltd., Japan
41© 2011 Electric Power Research Institute, Inc. All rights reserved.
Status of Units 1 & 2 Fuel Pools
• 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
Uni
t 2 fu
el p
ool
wat
er a
naly
sis
Data Source: TEPCO
Source: Air Photo Service Co. Ltd., Japan
42© 2011 Electric Power Research Institute, Inc. All rights reserved.
Units 1 - 4 Summary as of 16 August 2011Unit 1 Unit 2 Unit 3 Unit 4
Physical Status Roof collapsed on operating floor and
SFP
Roof intact; operating floor
condition unknown
Roof and structure destroyed; debris in
pool, exposed concrete
Roof structure damaged; debris in
pool. Support beams installed July.
Loading of SFPs,# assemblies
Used 292 New 100
Used 587 New 28
Used 514 New 52 (32 MOX)
Used 1,331 New 204
Heat Loads in March, estimated
180 kW 620 kW 540 kW 2,300 kW
SFP Volume in m3 1020 1425 1425 1425
SFP Rack Material Aluminum & Boral Aluminum & Boral Aluminum & Boral SS 304
Fuel Burnup 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.Estimate of Fuel Damage
likely none, believed always covered
likely none, believed always covered
Some mechanical damage from fallen
debris
<1% based on water sample on 4/12/11
Temperature of SFP Current 36 CEst. peak <80 C.
Current 38 C Peak of 80 oC.
Current 33 C Peak of 62 oC
Current 45 C. Peak >90 oC.
Date closed loop cooling in service
8/10/11 Using original HX
with new secondary system
5/31/11 Alternate SFP
Cooling system
6/30/11 Alternate SFP
Cooling system
7/31/11 Alternate SFP
Cooling system
Volume of seawater injected early on
none 90 tons 4,560 tons 700 tons
Evaporation rate before cooling set up
Unknown; but well <10 tons/day
21 tons/day 17 tons/day 72 tons/day
Source: TEPCO
43© 2011 Electric Power Research Institute, Inc. All rights reserved.
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• ND - Not Detectable
Source: TEPCO
44© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
45© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
Post 9/11 or B.5.B
Mitigation Strategies
TMI Modifications
IPE / IPEEE and PRA
Modifications
Design Basis Accidents
FukushimaStrategies
All H
azar
d Risk
s
Mitigation
46© 2011 Electric Power Research Institute, Inc. All rights reserved.
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
47© 2011 Electric Power Research Institute, Inc. All rights reserved.
Together…Shaping the Future of Electricity
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