Estimates of ISFSI Site Boundary Doses

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Estimates of ISFSI Site Boundary Doses
Andrew Hodgdon, Jo Ann Pelczar Radiological
Engineering Group and Mel Gmyrek Environmental
Laboratory FRAMATOME ANP DES 400 Donald Lynch
Boulevard Marlborough, MA 01752 Telephone
[phone] e-mail Melvin.Gmyrek_at_Framatom
e-ANP.com
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ISFSI Independent Spent Fuel Storage
Installation
Wet Pool
Dry Pad
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Feds Regulate to 25 mrem/yearto Real Member of
Public

• There are
• Plants without ISFSIs
• Plants with ISFSIs
• Decommissioned Plants with ISFSIs
• Regulations
• (plant) 40CFR190 provide reasonable assurance
  that annual dose does not exceed 25 millirems
• (ISFSI) 10CFR72.104 annual dose to real
  individual beyond controlled area must not exceed
  25 mrem

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Factors Affecting Compliance
1. State limit is lower than Feds (2
plants) 2. Small Site (lt 3000 feet to
boundary) 3. Public access to the site (roads,
camp) 4. Nearby dwellings (homes and
condominiums) 5. Nearby schools 6. Nearby
commercial properties
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Plant Modifications That Increase Dose

• Add ISFSI
• 20 mrem/h contact (typical)
• Dose is higher at vents
• Hydrogen Water Chemistry
• BWR only
• 100 mR/hr turbine area

• Store Old Steam Generators
• 1 mR/h contact
• Sell-off property
• Up-rate power
• Store waste (old turbine blades)

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Options to Demonstrate Compliance

• 1. Take credit for Occupancy
• 2. Predict with Models
• 3. Make Measurements
• 4. Model and Measure (MM)
• 5. Buy Property
• 6. Build Shield or Berm
• 7. Limit ISFSI Loads
• less fuel
• lower burnup
• longer decay time

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Options to Demonstrate Compliance(contd)

• 1. Take Credit for Occupancy
• Start with conservative calculation(s)
• Residents (100)
• School, roads, visitors center, hikers, clam
  diggers, etc. (lt100)
• Administrative requirements to update basis yearly

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Options to Demonstrate Compliance(contd)

• 2. Predict With Models (Without Measurements)
• Highly uncertain and/or very expensive
• MicroSkyshine
• Simple
• Greater than 100 errors
• MCNP - very detailed, more accurate but expensive

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Options to Demonstrate Compliance(contd)

• 3. Make Measurements (Without Models)
• Background variation (noise) swamps dose rate
  (signal)
• Detection Limit is 70 mrem/year
• Distance curve is required
• ( The detection limit for on/off sources, like
  BWR N-16, is 10 mrem/year)

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Background Variation
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Distance Curves

• Highly problem dependent
• Add significant error
• Are very hard to measure
• Can be measured for rare on/off sources
• Turning the source on and off allows the
  background to be measured at the same location
  and at nearly the same time. Unfortunately, an
  ISFSI is always on.
• Here are the four Distance Curves we know about

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Distance Curves for On/Off Sources
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Options to Demonstrate Compliance(contd)

• 4. Model and Measure (MM)
• Pro
• More accurate
• Model (MCNP) provides distance curve
• Surveys validate the results
• Model suitable for further engineering
• Con
• Need experts in modeling and measurements

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Recipe forM M (Model and Measure)

• 1. Set up model (MCNP)
• contact model
• distance model
• 2. Take contact surveys to calibrate model
• 3. Take near surveys to validate distance curve
• 4. Run model at boundary to get answer

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Contact versus Distance Model

• Contact Model
• Details inside the building or bunker
• Complex
• 10 to 20 hours to run
• Results in contact dose rates
• Distance Model
• Starts where contact model leaves off
• Simple
• Runs in 10 minutes
• Results in site boundary dose rates

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Three Examples
Ex. 1 BWR N-16 Streaming (presented at ANS RPSD
98) Ex. 2 NUHOMS ISFSI (presented at ANS RPSD
02) Ex. 3 NAC ISFSI (in process)
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Ex. 1 - BWR N-16 Streaming
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Assignment

• Shorten outage by a half day by removing some
  N-16 shielding while at power.
• Risk - violate boundary limit.
• Solution - use MM to
• Guide shield removal
• Validate boundary dose at end of year

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Dose Points
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Turbine Building Section
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Plan
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Contact vs Distance Model
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Calibrated Distance Curve
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Zoom at Fenceline
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Ex. 2 - NUHOMS ISFSI(20 bunkers shown)
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Assignment

• Build contact dose rate model to
• Permit further engineering (more fuel in bunker)
• Permit site boundary calculations

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Facility Description

• 900 PWR Assemblies in 36 bunkers
• 10 metric tons fuel per bunker
• 16 kW heat
• 4E16 photons/second/bunker (measured value)
• 4E9 neutrons/second/bunker (measured value)
• Major Nuclides - Cs-137, Co-60, and Cm-244

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Canister For 24 Assemblies
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Canister Being Pushed Into Bunker
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Results

• 1. Built MCNP Model
• 2. Used Contact Surveys to Calibrate Model
• neutrons lt1 mrem/h (not reported here)
• photons 20 to 100 mR/h
• 3. Distance Model Not Yet Completed

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Model / Measured
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Ex. 3 - NAC Type ISFSI
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Assignment

• Demonstrate compliance
• Reduce control area

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Concrete Bunkers
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Contact Model
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Model Predictions

• Contact Model
• Runs take 8 hours
• 20 mrem/h contact
• Neutrons at vents and top
• Distance Model
• Runs take 10 minutes
• 25 mrem/year at about 740 feet
• Neutron dose less than 4
• Model provides spectrums

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Measuring Background(Reuter-Stokes HPIC - ISFSI
Empty)
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Measurements In Progress

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As a Results of Using M M(Modeling and
Measurement)

• Ex. 1 Saved a half day during the refuel
  outage
• Ex. 2 - Built a validated model
• will be used to analyze higher loading
• will be used for site boundary doses
• Ex. 3 - Work in progress to reduce Control Area

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Must Not Exceed 25 mrem per year

• Measurement alone cant demonstrate compliance
• Models alone cant demonstrate compliance
• What works is modeling and measurement combined

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CONTACTS
Andrew Hodgdon (Radiological Engineering
Group) [phone] Andrew.Hodgdon_at_Framatome-AN
P.com Framatome ANP DES 400 Donald Lynch
Boulevard Marlborough, MA 01752
Mel Gmyrek (Environmental Laboratory) (978)
568-2522 Melvin.Gmyrek_at_Framatome-ANP.com Framatom
e ANP DES Environmental Laboratory 29 Research
Drive Westborough, MA 01581-3913