ML24008A038
| ML24008A038 | |
| Person / Time | |
|---|---|
| Issue date: | 01/04/2024 |
| From: | Steven Garry NRC/NRR/DRA/APOB |
| To: | |
| References | |
| Download: ML24008A038 (60) | |
Text
NRC Update on Radiation Protection Activities ISOE ALARA Symposium January 4, 2024 ML24008A038 Steven M. Garry, CHP Sr. Health Physicist Radiation Protection and Consequence Branch Division of Risk Assessment Office of Nuclear Reactor Regulation Slide 3
Presentation Topics
- Analyzing Occupational Dosimetry Data
- Radiation Monitor Calibration and Emergency Dose Assessment Slide 2
Occupational Dose Assessment EPD vs. TLD/OSL Goal - Evaluate licensee occupational dose assessments Objective - Ensure accurate assignment of occupational dose Methods - Evaluate Electronic Personnel Dosimeter (EPD) vs Thermoluminescent Dosimeter (TLD)/Optical Stimulated Luminescent (OSL) dose using trend charts Slide 3
Nuclear Power Station Radiological Environments
- Power plant dose is mostly from gamma radiation in the
~400 keV to 1 MeV energy range
- 400 keV - 1 MeV photons are an easy to measure gamma spectrum
- Low energy Compton scatter photons do not cause much dose
- Dose by EPDs vs. TLD/OSL dose should be similar Slide 4
EPD vs. TLD/OSL
- American Nuclear Insurers require use of two dosimeters
- EPDs Modern electronic personnel dosimeter measurements are quite accurate Can measure very small doses to 0.1 mrem, but can be subject to error
- Licensee analysis of EPD and TLD/OSL data should be performed to identify outliers for investigation
- NRC oversees licensee performance to ensure occupational dose assignment is adequate Slide 5
Basic differences between EPD vs. TLD/OSL
- EPDs can be intentionally biased high due to geometry factors that can vary from 1.05 to 1.10 for angular and energy dependence
- EPDs can be intentionally biased even higher to add extra conservatism
- TLD/OSL measurements are reliable but have a ~10 mrem threshold
- TLD/OSL measurements can be influenced by processing and background subtraction methods Slide 6
Goal and Methods of Data Review
- Goal:
Understand the performance of EPD vs. TLD/OSL measurements Ensure best performance of both EPD and TLD/OSL Users should have high confidence in dosimetry results
- One Method:
Create a data table and trend chart showing individual dose measurements Display each workers dose by EPDs on y-axis and TLD/OSL on x-axis Establish tolerance bands and identify outliers Identify and investigate outliers
- Accuracy: Dont assume TLD/OSL dose is more accurate than EPD dose!
Slide 7
Evaluating EPD & TLD/OSL data
- Create data spread sheet (template at ML23312A346)
- List each workers EPD and TLD/OSL dose in separate columns
- Sort data set by maximum TLD/OSL dose
- Calculate ratios of TLD/OSL to EPD dose
- Graph data with tolerance bands o INPO band - > 100 mrem and more than +/- 25% difference o Tighter band = 2.5 x sq. root of TLD/OSL value
- Identify outliers for investigation Slide 8
Example: Data table and trend chart Slide 9
Charting of Data
- Charted data by EPD vs. TLD/OSL
- Set tolerance band criteria for investigation INPO criteria - If either EPD or TLD/OSL dose is > 100 mrem, and difference exceeds +/- 25% of TLD/OSL values A tighter band can be set at +/- 2.5 times square root of TLD/OSL values
- Identify individual outlier data points
- Evaluate outliers and observe data trends
- A good reason can be found for most outliers Slide 10
Example Data - Trend Chart for good dosimetry program 0
100 200 300 400 500 600 700 800 900 0
100 200 300 400 500 600 700 800 900 EPD (mrem)
TLD/OSL (mrem)
Example TLD/OSL dose point: Same worker with 497 mrem by TLD/OSL Example EPD dose point:
worker with 508 mrem by EPD Red band is INPO band Green band is 2.5 times square root of TLD/OSL value Slide 11
Example Data -
First generation (c. 1990s) EPDs Data is dispersed, scattered both above and below the trend band Red band is INPO band Green band is 2.5 times square root of TLD/OSL value Slide 12
Low Dose Sensitivity Evaluation 0
50 100 150 0
50 100 150 EPD (mrem)
TLD Results (mrem)
~ 0 - 20 mrem by EPD and
< 10 mrem (recorded as zero) by TLD/OSL Good data correlation between EPD and TLD/OSL Slide 13
EPDs are biased are high, or TLD/OSL is biased low 0
50 100 150 200 250 0
50 100 150 200 250 EPD (mrem)
TLD/OSL Results (mrem)
Slide 14
Too much TLD/OSL background subtraction, or EPDs with too much positive bias 0
100 200 300 400 500 600 700 800 900 0
100 200 300 400 500 600 700 800 900 TLD/OSL (mrem)
Background dose is based on control badges Background dose is difficult to determine precisely, especially for take-home dosimetry Too much TLD/OSL background subtraction results in lower collective dose!!
Background dose is also different for supplemental (outage) dosimeters, shipped at different times, with different anneal dates, different transit dose, and different control badge storage dates 50 mrem by EPD and 90 mrem by TLD/OSL, maybe security officers at BWR are getting N-16 shine patrolling outside the RCA while not wearing EPDs 120 mrem by EPD and 20 mrem by TLD/OSL Investigate discrepancies Slide 15
TLD/OSL higher than EPDs 0
50 100 150 0
50 100 150 EPD (mrem)
TLD/OSL Results (mrem)
Possible BWR workers outside RCA, wearing TLD/OSL badges but not wearing EPDs Slide 16
Background Subtraction vs. EPD Geometry Factor Slide 17
Reasons EPDs readings are too high Bias set too EPD geometry factor (bias) is set too high EPD data processing differences o
Errors associated with low doses; e.g., workers wearing EPDs all day getting 0.1 mrem background dose o
Tenths of a mrem - some access control software (e.g., Sentinel) will record tenths; however, other software may not record tenths (recorded as zero, sometimes even 0.9 is recorded as zero) o Rounding errors (0.4 is recorded as zero) (e.g., 0.4 and out the door)
(0.5 is recorded as 1 mrem)
Slide 18
Reasons TLD/OSL readings are low Control badges are used to estimate background dose to occupational badges (not EPD dose)
Control badge dose is intended to be representative of the dose to occupational badges when are not in use o Control badges must be stored properly o Improper storage of control badges can result in non-representative background dose being measured o Results in too much background dose subtracted o Reported doses are too low Slide 19
TLD/OSL Background Subtraction Issues
- Badges stored onsite o
When personnel badges are stored onsite adjacent to control badges, there is a more accurate background subtraction than for take-home badges o
Background dose is dependent on storage location (BWRs need to find a place that minimizes facility-related dose rates from shine and ISFSI pads)
Take-home badges o
Home background dose rates vary by 20 - 30 mrem per six months o
River/lakeside/ocean background is shield by water, doses ~ 12 mrem/qtr o
Hillside background doses are higher, ~ 20 - 30 mrem/qtr Slide 20
Take-home Dosimeters
- 6-month issue period
- Take-home dosimeters are subject to different background doses o One size fits all background dose does not apply to all take-home dosimeters o Background dose varies by location, stored in cars or on granite countertops o Background at low elevations near lakes, rivers, oceans is lower than average, e.g., 10 - 15 mrem lower per six months) o Background dose at higher elevations on hills/rocks is higher than average background (10 - 15 mrem higher per six months)
Slide 21
Background Subtraction - Transit Dose
- Transit dose varies between dosimeter shipments o Not all dosimeters are shipped at the same time o Supplemental outage dosimeters are needed for contractors o A second incoming shipment of dosimeters for outages could easily have different transit doses o Outgoing early-return shipments may be made having different transit doses o One-size fits all background dose may be incorrectly assumed based on control badges from the first shipment Slide 22
Background Subtraction - Transit Dose (continued)
- Some shipments may be X-rayed
- Tech-99m generators are commonly shipped weekly to medical facilities on Thursday or Friday
- During transit, some TLD/OSL badges may be exposed to Technicium-99m generators, etc. (for medical purposes) reading 200 mR/hr Slide 23
Recording thresholds - TLD/OSL collective dose is too low TLD/OSLs LLD recording threshold is ~10.0 mrem Example: TLD/OSL are processed on a 6-month basis A person received a true 23 mrem occupational dose Control badges measure background; i.e., are not stored in a lead shield A worker badge has a 6-month background dose of 45 mrem (vs. 59 mrem control badge)
A gross TLD/OSL reading is 68 mrem. The TLD/OSL net dose would be calculated as 68 mrem - 59 mrem background = 9 mrem, but is recorded as zero Comparison: True dose is 23 mrem, but is recorded as zero Other workers doses (below 23 mrem) are also recorded as zero Result: Collective doses are too low Workers question the dosimetry results, i.e., why did I get 23 mrem but my dose reports shows zero dose?
Slide 24
Accident-Range Radiation Monitoring Effluent monitors Containment/Drywell monitors Slide 25
Plant staff responsibilities Plant staff should:
o know which department is responsible and who is the SME for radiation monitoring systems (RMS) o ensure the SME understands NUREG-0737, Item II.F.1 o know what type of equipment is installed and how equipment works (vendor manuals and calibrations) o know how calibration checks are performed and the basis for efficiency factors o know what the monitor output means (is it uCi/cc of Xe-133 or uCi/cc of a mix of isotopes?)
o whether the monitor output interfaces correctly with dose assessment codes Slide 26
RG 1.21 - Discusses Post-TMI Requirements NUREG-0578 (July 1979) ~ NRR Short Term Recommendations (ML090060030)
NUREG-0660 (May 1980) ~ NRC staff Action Plan (ML072470526)
NUREG-0737, Clarification of TMI Action Plan Requirements (ML051400209) (approved by NRC Commission)
NRR Letter to Regional Administrators, (August 16, 1982) Proposed Guidance for Calibration and Surveillance Requirements to meet Item II.F.1 (ML103420044)
(Additional Accident Monitoring Instrumentation)
HPPOS-001, Guidance on Calibration and Surveillance to meet Item II.F.1, Additional Accident Monitoring Instrumentation (ML093220108)
Slide 27
~ Instrumentation For Accident Response
- Rev. 2 - 1980 and Rev. 3 - 1983
- RG 1.97 establishes performance criteria for:
o Effluent monitors o Containment high-range monitors Slide 28
NUREG-0737, Post-TMI Action Plan
- NUREG-0737, Post-TMI Action Plan Requirements, Item II.F.1 (ML051400209)
- Item II.F.1 is Additional Accident-Monitoring Instrumentation, requiring:
o Noble gas effluent monitoring (Item II.F.1-1) o Iodine and particulate sampling and analysis (Item II.F.1-2) o Containment high range radiation monitoring (II.F.1-3)
- Specifications for radiation monitoring equipment are in Tables II.F.1-1, II.F.1-2, and II.F.1-3 Slide 29
NUREG-0737, Item II.F.1-1 Noble Gas Effluent Monitoring Slide 30
NUREG-0737 Table II.F.1-1 Noble Gas Effluent Monitoring Stated Objective:
o Provide calculational methods to convert instrument readings to release rates, considering radionuclide distribution as a function of time after shutdown Specifications:
o Develop calculational methods to convert instrument readings to release rates o Consider radionuclide distribution as a function of time after shutdown o Calibrate to Xe-133 and display as equivalent Xe-133 concentrations Slide 31
Accident-Range Gaseous Effluent Monitoring Calibration and Time-Dependent Instrument Response Factors ADAMS ML18171A035 Steve Garry, CHP Sr. Health Physicist Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Radiological Effluents and Environmental Workshop June 27, 2018 New Orleans, LA 2018 REEW Presentation Calibration of Effluent Monitors Slide 32
2018 NEI HP Forum Effluent Monitor Calibration NEI HP Forum July 30, 2018 Naples, FL Accident-Range Gaseous Effluent Monitoring Calibration and Time-Dependent Instrument Response Factors (ML18207A178)
Steve Garry, CHP Sr. Health Physicist Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Slide 33
Effluent Monitors
- GM detectors are ionization detectors o detector efficiency is strongly energy dependent o detectors are energy compensated to dose (do not directly measure activity concentrations in uCi/cc)
- Ionization chambers are dose detectors o do not directly measure activity concentrations in uCi/cc o ion chambers measure electrical current (amps) o ion chambers are used to measure exposure (aka dose in R/hr) o amps are converted to R/hr using efficiency factors (amps/R/hr) o typically, 1E-11 amps / R/hr o R/hr is NOT a good surrogate for activity concentration (uCi/cc) o there is a corollary to friskers that measure cpm, then convert to dpm with a 10% efficiency factor Slide 34
Detector Efficiency Factors
- Noble gas monitoring instruments are GM detectors, ion chambers, plastic scintillators, and CdTe(Cl) solid-state detectors
- Each detector has a counting efficiency for each gamma energy
- GM and ion chambers are typically calibrated to Xe-133; i.e., to low energy, 81 keV photons with low yield (~36%)
Slide 35
Manufacturer Primary Calibrations
- Vendor calibrations do not convert from Xe-133 to a mix of noble gas nuclides
- Plant staff need to do this conversion
- Transfer calibrations are Cs-137 calibration checks to verify that the detector is working properly Slide 36
Radionuclide Mix Gaseous effluent is not just Xe-133 Gaseous effluent is a mix of noble gases, and the fractional mix is very time dependent The average energy of the mix declines quickly Generally, short-lived noble gas nuclides have higher energy gammas than long-lived nuclides GM detectors and ion chamber efficiency factors are 10 - 30 times higher for high energy gammas Therefore, a time-dependent detector efficiency factor is needed to convert detector output into dose assessment input Slide 37
NUREG-0737 Item II.F.1-1 Noble Gas Effluent Monitoring
- Manufacturers provide transfer calibration using Cs-137
- Some/most manufacturers do not provide energy response characterization (~81 keV to 3 MeV)
- Plants may need to do their own energy response characterizations (or hire an engineering vendor)
Slide 38
Noble Gas Efficiency Factors
- Detector efficiency factors are a function of gamma energy
- Each noble gas has its own efficiency factor (cpm/uCi/cc)
- Most dose assessment codes need a source term input based on a gross mix of noble gases (not based on Xe-133)
- Detector output needs to be corrected to a gross mix of gases
- The correction factor is time-dependent because high energy gamma emissions decay quickly Slide 39
Effluent Monitor Over Response 1.0 6.0 11.0 16.0 21.0 26.0 31.0 36.0 0.1 1
2 4
8 12 24 48 168 720 Instrument Over-Response Factors Core Melt Accident Gas Gap Relative Response to Xe-133 Time after Rx trip (hours)
Slide 40
How to Develop Isotope-Specific Efficiency Factors
- 1. Use Monte Carlo methods, compare calculations for each radionuclide to Xe-133 gas calibration
- 2. Use solid source efficiencies and correlate to the gas calibration (Xe-133, 81 keV, 36% yield)
Develop a graph of solid source detector efficiency vs gamma energy Calculate isotopic-specific efficiency factors based on energy and yields of each noble gas Calculate fractions of noble gas as function of time Apply isotopic-specific efficiency factors to a fractional mix of decayed radionuclides Slide 41
RMS gross correction factors Gross efficiency factor = Eff (Xe-133) x fraction Xe-133 + Eff (Xe-135) x fraction Xe-135 + Eff (Xe-135m) x fraction Xe-135m + Eff (Kr-87) x fraction (Kr-87) +
For each dose code time step, multiply RMS output by the gross efficiency factor Slide 42
Dose assessment, emergency classification and protective action recommendations
- Plants are responsible for:
o Performing a transfer calibration check to verify the detector is working properly o Determining the detector response to a mix of noble gases as a function of time after Rx shutdown o Plants are responsible for entering the correct source term into the dose assessment code o Plants are responsible for dose assessment, emergency classification and protective action recommendations Slide 43
Effluent Monitor Digital Display Slide 44
Iodine and Particulate Monitoring NUREG-0737 Item II.F.1-2 Slide 45
NUREG-0737 Item II.F.1-2 Iodine and Particulate (I&P) Monitoring Real-time I&P monitoring is not practical Licensees must develop procedures for collection and analysis of samples NUREG-0737 says assume 30 minutes sampling time and average energy 0.5 MeV using shielding and distance factors Iodine release rates can also be estimated based on scaling factors to noble gas Scaling factors to noble gas are assumed but not well known That makes iodine and particulate sampling and analyses even more important Slide 46
Containment / Dry Well High Range Monitors (CHRMs)
NUREG-0737 Item II.F.1-3 Slide 47
RG 1.97 design criteria for CHRMs
- CHRMs design criteria
- +/- 200% from 60 keV to 100 keV
- +/- 20% from 100 keV to 3 MeV
- CHRMs calibration check accuracy:
- ANSI N323-1978 and ANSI N323D-2002 design criteria
- Manufacturer establishes calibration check criteria
- Accuracy requirement, generally, +/- 20%
Slide 48
NUREG-0737 Item II.F.1-3 Containment High Range Monitor (CHRMs)
- High range dose rates up to 10 million R/hr
- Output used is estimating containment conditions and assessing Core Damage
- Manufacturer provides the instrument response factor, e.g., ion chamber is ~ 1E-11
/
- Licensee perform a periodic solid source calibration check in the R/hr range
- Perform electronic calibration above 10 R/hr Slide 49
CHRM vendor transfer calibration
- Manufacturer does the primary calibration:
o Determines broad beam Cs-137 dose rate as function of distance o Exposes the detector to the broad beam and measures amperage o Calculates the detector efficiency (~ 1E-11 amps/R/hr) o Also provides plants a transfer calibration check method
- Builds a Field Calibrator jig on CHRM with Cs-137 source
- Determines the expected dose rate value (R/hr)
- Prepares calibration summary report for both the CHRM and the Field Calibrator jig Slide 50
Calibration Certificate Slide 51
Plant performs calibration checks
- Electrical - Instrument and Control (I&C) technicians perform an electrical calibration check on all scales
- Radiological - I&C does a one-point radiological calibration check (in the 1 - 10 R/hr range)
- Efficiency factors are NOT normally adjusted
- Is there SME review of calibration check results?
Slide 52
Slide 3 Slide 53
Slide 3 Slide 54
CHRM meter Readout Slide 55
Violations - Significance Determinations Violations are evaluated under the Emergency Planning Cornerstone NRC Reactor Oversight Manual Chapter 609, Appendix B (ML15128A462)
Slide 56
GM Detector Over-Response (based on calibration to Xe-133) 1.0 6.0 11.0 16.0 21.0 26.0 31.0 36.0 0.1 1
2 4
8 12 24 48 168 720 Instrument Over-Response Factors Core Melt Gas Gap Relative Response to Xe-133 Hours post-shutdown Slide 57
Potential Violations
- Geometry - licensees use wrong calibration check geometry
- Isotopic-specific efficiency factors; i.e., licensees only use Xe-133 efficiency factor for the radionuclide mix
- Maintenance - Fail to adjust efficiency factors o Replace solid state detectors or components and fail to update efficiency factors (particularly General Atomics CdTe(Cl) solid state detectors which have their own efficiency factors) o Licensees may use wrong efficiency factors o Assume 1 efficiency factor fits all detectors Slide 58
NUREG-1940, RASCAL Radiological Assessment System for Consequence Analysis o NUREG-1940, section 1.2.8 and o Supplement 1, Section 2.6 Slide 59
Questions and Discussion Slide 60