Ensuring Spent Fuel Pool Safety Michael WeberDeputy Executive Director for S Nuclear Regulatory Commission American Nuclear Society Meeting June 28, 2011

7/31/2019 Ensuring Spent Fuel Pool Safety Michael WeberDeputy Executive Director for S Nuclear Regulatory Commission American Nuclear Society Meeting June 28, 2011

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Ensuring Spent Fuel Pool Safety

Michael WeberDeputy Executive Director for Operations

U.S. Nuclear Regulatory Commission

American Nuclear Society MeetingJune 28, 2011

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Nuclear emergency at Fukushima-Daiichi

3 nuclear reactors

4 reactor spent fuel pools

1 common spent fuel pool

Insights from Fukushima


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Spent fuel rods stored inspent fuel pools (SFPs)under at least 20 feet ofwater

Typically ~1/4 to 1/3 offuel in reactor replacedwith fresh fuel every 18 to24 months

Spent fuel stored in poolsminimum of 5 years

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U.S. Spent Fuel Pools


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Spent Fuel Pools (SFP) originallydesigned for limited storage of spent fuel

until removed off-site

Safety of spent fuel in pools achievedprimarily by maintaining water inventory,

geometry, and soluble boron (PWRs)

Drain down can lead to uncovered fuel,heat-up, and the release of radionuclides

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U.S. SFP Safety


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SFP risk is low, due to the low frequency ofevents that could damage the thick reinforcedpool walls Frequency of fuel uncovery; 6E-7 to 2E-6/yr.

NUREG-1738 Consequences have been assessed to be large

due to the potential for heatup of all the fuel in thepool

Heatup of the fuel in the pool can lead tozirconium fire initiation and propagation

Large inventory of Cs-137

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Risk of Large Release


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NRC extensively reexamined pool safetyand security after 9-11 attacks Vulnerability to attack Significantly improved analysis of fuel

coolability / heatup Assessment of mitigation measures to

improve coolability of fuel Improved fuel configuration within the pool

achieves substantially greater passive coolingcapability by natural convection

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SFP Safety and Sec

urity


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Additional analyses of a spray system for spentfuel pool cooling

NRC required spray capability for each site toimprove active cooling capability

Licensees performed site-specific assessments;NRC inspected

Coolability of fuel within pools has been enhancedby measures identified and assessed as part of

post-9/11 research Conducting research to confirm understanding and

validate analytical modeling

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SFP Safety and Security


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Prototypic full length 99 BWRhardware Single pool rack cell Upper & lower tie plates with seven spacers Water tubes and channel box

74 electric heater rods with Zr-2 cladding(eight partial length)

5000 W simulating a 100 day old assembly

Measurements Temp profiles: Axial and radial

Induced flow: Effect of ignition on flow O2 concentration: Determine depletion Nature of fire: Initiation location & axial burn

rate

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Cross sectionabove partial

length rods

0.61

1.22

1.83

2.44

3.02

3.66 m

O2 Sensor

Light Pipe

Thermocouple

Cross section offull bundle

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Zirconium Fire Investigations DuringSFP Loss of Coolant Accident (LOCA)


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Full LengthSingle

AssemblyIgnitionMovie

Zirc Fire Investigations DuringSFP LOCA Postmortem
http://bwr_full_zr_ign_test.wmv/http://bwr_full_zr_ign_test.wmv/http://bwr_full_zr_ign_test.wmv/http://bwr_full_zr_ign_test.wmv/http://bwr_full_zr_ign_test.wmv/http://bwr_full_zr_ign_test.wmv/


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NRC has considered benefits of removal of fuel fromthe pool and returning to a low density racking typeconfiguration

There are competing factors in such a consideration Storage in dry casks must be consistent with certificate

Discharging of fuel increases the risk of cask drops andworker doses

Removal of fuel will decrease the inventory of Cesium-137 Removal of fuel does not appreciably reduce decay heat

(most of the decay heat is from recently discharged fuel)

Reduction in potential land contamination and economicimpacts, if a large release occurred

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Removing Fuel from Pools


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0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

0 500 1000 1500 2000 2500 3000

Decay

heat(rela

tive

to

fullpool)

Number of Assemblies Removed from Pool

5+ year old fuel 3+ year old fuel

5+ year old fuel

3+ year old fuel

Impact of Removing AssembliesReduction of pool thermal heat load


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NRC is initiating an updated SFP study

Estimate the change in accidentconsequences associated with removingolder fuel from the SFP and placing it indry storage

Limited scope analysis (e.g., singleSFP/operating cycle for low/high densityracking)

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Comparative Consequence Study


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Technical approach relies on realistic analysisusing expedient but technically-defensibledeterministic methods and assumptions.

Elements of study include Information gathering

Seismic and structural assessment

Accessibility, decay heat, and radionuclide inventoryassessment

Accident progression (MELCOR) and offsiteconsequence analysis (MACCS2)

Emergency planning assessment

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Comparative Consequence

Study for SFP


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Conclusions

No immediate safety concerns based onFukushima nuclear emergency

Confirmed the existing safety measuresfor SFPs

Examining both the near-term and long-term reviews

Spent fuel needs to be managed safelyand securely

Documents

Ensuring Spent Fuel Pool Safety Michael WeberDeputy Executive Director for S Nuclear Regulatory Commission American Nuclear Society Meeting June 28, 2011