characterization of neutron and gamma fields and dosimetry...

Radiation Safety & Control Services, Inc.

Characterization of Neutron and GammaFields and Dosimetry Response from SpentFuel in Transit and Stored at ISFSI Facilities

Eric L. Darois M.S., CHPNasser B. Rashidifard M.S., CHP

2014 RETS REMP Conference

June 24 - 26, 2014Savannah, GA

Two ISFSIsNo Operating Plant Nearby

ConnecticutYankee

MaineYankee

Goal

• Establish the Unrestricted Area Boundary– 40CFR190 Dose Limit– 25 mrem/year– 100% Occupancy– 2.85 urem/hr

Gamma Exposure Measurements

• High Pressure Ionization Chamber– Reuter Stokes RS-111– 8 Liter Chamber– 25 ATM Argon

• Omni-Directional• Flat Energy Response• Excellent Sensitivity• Ten 30 sec Integrations at Each

Location• 21 Pre-Op Locations for BKG (10.89

uR/hr +/- 9%)

Neutron Measurements

• Tissue Equivalent ProportionalCounter, TEPC– Far West Technologies (FWT) “Hawk”

TEPC– Initially Conceived for Air-flight

Dosimetry– Measures Gamma and Neutron Dose

and Dose Equivalent– Based on Micro-dosimetry Principles

Hawk (TEPC) Operation• 5 inch Hollow Sphere of A150 Plastic

in SS Cylindrical Housing– 7 Torr (0.0135 psig) TE Proportional Gas– Simulates 2 um Tissue Sphere– Bragg-Gray Principle– Measures LET Distribution– High and Low Gain MCAs

• 15 Minute to 3 Hour Integrations (1minute Measurements)

• NIST Traceable Calibration (Cf-252& Cs-137)

• QFs from ICRP-21 vs ICRP-60

Maine Yankee ResultsGamma Rate (uR/hr)

Maine Yankee ResultsGamma Rate Detail, uR/hr

Maine Yankee Results, urem/hr(“Short Axis”)

Dose Equiv. Rate25 mrem/yr Limit100 mrem/yr Limit

Distance from Center, feet

Maine Yankee Results, urem/hr(“Short Axis”)

Distance from Center, feet

Maine Yankee Results, urem/hr(“Long Axis”)

Distance from Center, feet

Maine Yankee Summary

• 169 Gamma Locations• 55 Neutron Locations

Boundary AxisDistance to 25-mrem/yr

from Center-of-ISFSIArray, ft

South Long -379.8North Long 357.6West Short -441.4East Short 321.5

Connecticut Yankee Results, urem/hr(“Long Axis”)

Distance from Center, feet

Connecticut Yankee Results, urem/hr(“Short Axis”)

Distance from Center, feet

Connecticut Yankee Summary

• 142 Gamma Locations• 40 Neutron Locations

Boundary AxisDistance to 25-mrem/yr

from Center-of-ISFSIArray

South Short -534.5North Short 713.3West Long -553.3East Long 555.1

Operating Sites…Why is it Important to Characterize?

• New Dosimetry Types• New Dosimetry Vendors• Modification of shielding• Dry Fuel Storage Campaigns• Independent Spent Fuel Storage Installations

Why is it Important to Characterize?• Dosimeter neutron energy response curve variations.• Should be accurate or reasonably conservative (dose to

personnel or public).• Most sites can have a variety of fields all having very

different spectrums.• Sites will typically use a vendor recommended neutron

correction factor.• These values have a sound technical basis but site specific

characteristics may make them not representative.• ANI Bulletin 11-02 Neutron Monitoring

Where to Start

• What activities contribute the most to the site’sneutron dose?

• What new activities are going on?• What activities are no longer being performed?• What activities may be expected in the future?• Can measurements occur at the location?

Collecting Data

• Neutron measurements done using a Tissue EquivalentProportional Counter (TEPC).

• Dosimeters irradiated on ANSI N13.11(2009) 30 cm x 30 cm x15 cm Phantoms.

• Active area of 10x10 cm to prevent edge effects.• Performed in a uniform field in parallel.• Performed in series for smaller unchanging fields.• Ensure enough dose on the dosimeters.

Independent Spent Fuel StorageInstallation (ISFSI)

• Low dose rates• Typically highly

thermalized neutrons• Require long integration

times on dosimetry• Additional shielding

added after fuel loadingcan make access tofields difficult.

Independent Spent Fuel StorageInstallation (ISFSI) at Operating Sites

SitesDosimeterDose Rate(mRem/hr)

TEPCDose Rate(mRem/hr)

TEPC/Dosimeter

1 45.2 0.8 0.02

2 68.2 2.3 0.03

3 30.9 0.7 0.02

4 0.3 0.2 0.67

5 4.9 0.2 0.04

6 1.7 0.6 0.35

7 7.5 0.5 0.07

• Highly thermal neutronscause certaindosimeters can overrespond by a significantamount.

• Even though the doseto personnel is low,correction factorselection is important tomeet dose estimates.

Additional Dosimetry Considerations

• Personnel NCFs cannot be applied to environmentaldosimeters, a separate set of measurements willneed to be done.

• These corrections factors are easy to implementwhen the plant is no longer operating.

• For sites conducting operations or active fueltransfers to the ISFSI pad other correction factorsmay need to be applied or considered.

Pressurized Water Reactor

• Have the Highest neutrondose of the industry.

• Surveys in containmentwhile at 100% power.

• Online Maintenance.• Routine Operations walk-

downs.• Variety of neutron fields.

Pressurized Water Reactor

LocationDosimeterDose Rate(mRem/hr)

TEPCDose Rate(mRem/hr)

TEPC/Dosimeter

1 409.0 24.3 0.06

2 113.5 8.3 0.07

3 116.0 7.5 0.06

4 29.2 3.5 0.12

5 41.9 13.2 0.32

6 142.0 5.5 0.04

7 398.1 13.8 0.03

8 196.0 14.9 0.08

• Values can vary greatlyeven within onecontainment.

• Measurements wereperformed at a variety oflocations:

• Outside Bioshield• Instrument Penetrations• Refueling floor• Upper elevations

• Up to a factor of 10difference.

Boiling Water Reactor

• Can be measuredoutside the DrywellHatch or in pipingpenetration rooms

• No measurementsinside.

• Lower neutron doserates that inside PWRs

• Positioning is criticaldue to high potential ofstreaming

Boiling Water Reactor

SitesDosimeterDose Rate(mRem/hr)

TEPCDose Rate(mRem/hr)

TEPC/Dosimeter

1 1.1 0.7 0.65

2 22.2 2.8 0.13

3 6.2 9.2 1.48

• Can very greatly sinceevery measurement canrepresent a different areaoutside the drywell.

• Some measurements arethrough penetrationsproviding lessthermalized fields.

Spent Fuel Movements

• Measurements collected afterdraining of the inner canisterduring the vacuum dryingphase.

• Typical irradiation is 16-36hours.

• Setup is generally on middlelevels of scaffolding due touniformity of fields andaccessibility.

Spent Fuel Movements

SitesDosimeterDose Rate(mRem/hr)

TEPCDose Rate(mRem/hr)

TEPC/Dosimeter

1 23.3 10.1 0.43

2 28.7 2.9 0.10

• Each operation has its ownshielding setup that caneffect the spectrum.

• Water jackets andPolyethylene wraps canchange the neutronspectrum

Example 1• An operating BWR that is actively performing spent fuel transfers for dry

storage.• Using CR-39 dosimetry with a neutron correction factor of 0.68 based on

BWR energy spectrum from their NVLAP accredited vendor for the wholesite.

Area Original NCFsUsed

Actual NCFscalculated

BWR 0.68 0.65

SFC 0.68 0.90

• The result of this is that the site was under estimating the dose of thespent fuels group by approximately 20%.

• This is due to the higher average neutron energy of the spent fuel.• The site issued two sets of dosimetry, one for the routine site work and

another for the spent fuels work.

Example 2• ISFSI site was conducting site repairs over the course of a week using a local

contractor.• The workers were considered escorted non-rad workers since the pre-job

dose estimate was well below 100 mrem and they would not be handlingany radioactive materials. General area surveys were less than 2 mrem/hr.

• There was no need to issue dosimetry but spare badges were rush orderedfor the workers anyway as good idea……

Example 2 (cont.)• An investigation was conducted to find the cause by gathering and reviewing the

following information:– Area surveys– Reconstructing the activities of every worker– Interviewing all escorts and site staff– Dosimetry storage– Neutron correction factors

Don’t Forget Survey Meters!

• Accurate surveys of areas with large or high dose rate neutron fields.• Particularly applicable to PWR containments, spent fuel operations, ISFSI

installations, or Cyclotrons where neutrons are almost as large as thephoton components.

• Instrument response of neutron survey meters can vary up to a factor of 4or more.

• Important for establishing boundaries, ALARA planning, occupational orpublic dose estimates.

Remball and Time Keeping• Some sites still use Remballs

and time keeping to assigndose.

• During a neutron study at anoperating PWR surveyinstruments were evaluated totheir response in a variety ofareas.

• Since the neutron dose rates arefairly high this factor representsa significant portion of theirdose.

• Effect work planning and doseestimates when using timekeeping methods.

LocationNormalizedResponse tomeasured DE

1 1.35

2 3.57

3 4.17

4 3.45

5 1.61

6 0.91

7 2.56

8 1.32

Questions?