ML18207A178
| ML18207A178 | |
| Person / Time | |
|---|---|
| Issue date: | 07/30/2018 |
| From: | Steven Garry NRC/NRR/DRA/ARCB |
| To: | |
| References | |
| Download: ML18207A178 (30) | |
Text
Accident-Range Gaseous Effluent Monitoring Calibration and Time-Dependent Instrument Response Factors Steve Garry, CHP Sr. Health Physicist Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission NEI HP Forum July 30, 2018 Naples, FL 1
- This presentation:
- shares basic information on calibration of accident-range gaseous effluent monitors
- reviews the regulatory basis and guidance for calibration of instruments
- is based in part on proposed guidance for calibration of accident-range effluent monitors given in HPPOS-001 and HPPOS-040
- Other calibration methods may be acceptable 2
TMI Accident Wednesday, March 28, 1979 3
Basic Issue (most detectors)
- Detectors were designed to measure dose, not activity, and most detectors are highly energy dependent
- Effluent monitors were initially calibrated to low-energy gammas from Xe-133
- During emergencies, a high-energy mix of noble gases could be released
- Calibrations based only on low energy gammas from Xe-133 are not representative of a mix of noble gases
- Effluent monitor response factors should be based on a mix of noble gases 4
5
- Footnote 9 Monitors should be capable of detecting and measuring radioactive gaseous effluent concentrations with compositions ranging from fresh equilibrium noble gas fission product mixtures to 10-day-old mixtures, with overall system accuracies within a factor of 2.
6
NUREG-0737 (November, 1980) 7 (NUREG-0660)
procedures or calculational methods to be used for converting instrument readings to release rates per unit time, based on exhaust air flow and considering radionuclide spectrum distribution as a function of time after shutdown (pg 3-96, section II.F.1-3) 8
HPPOS-001 NRR Guidance
- August 16, 1982, memo from NRR to Regional Administrators
- NRR proposed calibration guidance
- Now known as HPPOS-001 9
NEI 99-01 [Revision 6]
10
- Some EALs are based on pre-calculated effluent monitor values corresponding to EPA PAG doses for a 1 hr exposure 11
NEI 99-01, Rev 6 (endorsed by NRC)
- Unusual Event = 2x ODCM release rate limit
- 10 mrem TEDE, or
- 50 mrem CDE (thyroid)
- 100 mrem TEDE
- 500 mrem CDE (thyroid)
Dose Assessment Computer Codes
- Dose code input is in units of µCi/cc or Ci/sec of a mix of noble gases
- The dose assessment computer codes calculates the adjusted radionuclide mix based on decay of the T = 0 source term
- So the input needs to be Ci/sec or µCi/cc of the total MIX of noble gas radionuclides (not Xe-133) 13
cpm & mR/hr (HPPOS-001, pdf 17)
- cpm, or mR/hr, is not a good measure of activity or concentration because of detector energy dependence; i.e.,
different gamma energies and different gamma yields
- Example:
- 1 µCi of Xe-135 (250 keV) produces 7.6 times the dose as Xe-133
- 1 µCi of Kr-88 (~2 MeV) produces 48 times the dose as Xe-133 14
Instrument Response Factors
- Detectors do not measure the concentration of the mix,
- Detectors measure ionizations in cpm, or mR/hr
- An Instrument Response Factor is needed to convert from cpm or mR/hr into concentration uCi/cc or Ci/sec of a mix 15
Time Dependent Instrument Response Factors
- Vendors primary calibration is typically based on Xe-133
- Instrument Response Factors should be based on the calculated isotopic mix as function of time 16
Isotopic Mix
- The isotopic mix has a big effect on the instrument response factors (conventional detectors)
- Instruments calibrated only to Xe-133 will generally over-estimate the total µCi/cc of a mix
- Information on plant conditions and effluent monitor readings (exceeding EALs thresholds) are used to classify emergency conditions and determine if protective actions are needed
- Therefore, effluent monitor calibrations should be performed properly 17
GM Detectors
- GMs are energy compensated (e.g., lead shield)
- So GM detectors intentionally have a strong energy dependency so dose is measured +/- 20%
- GM detectors under-respond at low gamma energy
- GM readouts are in cpm // µCi/cc of Xe-133
- Should be adjusted to readout in terms of a mix of nuclides 18
Mid-Range (V)
Energy-Compensated GM Detector 19 Regular size paper clip
Ion Chambers
- Measures electrical current (amps) caused by radiation exposure
- Detector output is electrical current (e.g. picoAmps);
- current is directly proportional to exposure rate
- Programmable microprocessor converts picoAmps to mR/hr or µCi/cc or Ci/sec 20
Instrument Response Factors
- At T = 0 (worst case), detectors calibrated to Xe-133 (81 keV gamma),
- GM detectors could over-respond by a factor ~30
- Ion Chambers could over-respond by a factor of ~ 10
- Cd/Te detectors could over-respond by a factor of ~ 5
- Flow-through ion chambers may be within factor of ~ 2 21
Time-Dependent Response
- GM response factors that are based only on Xe-133 could over-estimate the release: e.g.,
- 0 to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />
- Gas Gap
- T = 0 high estimate by a factor of ~ 5
- Core Melt
- T = 0 high estimate by a factor of ~ 30 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />
- Gas Gap
- T= 8 hrs, much better estimate
- Core Melt
- T = 8 hrs, much better estimate
- > 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />
- T > 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, under estimate 22
Example Calibration Process Step 1. Vendor Calibrations
- Step 1.1 Gas calibration
- Step 1.2 Linearity check
- Step 1.3 Transfer calibration
- Step 1.4 Detectors energy response characterization Step 2. Secondary Calibration check at Plant Step 3. Energy Response Factors Step 4. Instrument Response Factors 23
IP 71124.06 - Radioactive Gaseous and Liquid Effluent Treatment Inspection Objectives
- To ensure that the gaseous and liquid effluent processing systems are maintained so that radiological discharges are properly mitigated, monitored and evaluated with regards to public exposure.
02.04 Instrumentation and Equipment (1 Sample)
- (c) High-Range Effluent Monitoring Instrumentation
- Verify instruments are calibrated and available
- Review methodology for calculating EAL thresholds (e.g.
instrument response factor determination) associated with system modifications since the last inspection
- Verify licensee can collect and analyze high-range, post-accident iodine and particulate samples 24
IP 71124.06 - Inspector Guidance Guidance 03.04(c) :
- Refer to FSAR, technical specification requirements and NUREG-0737
- Since monitors are used for PARs, ensure regional EP staff is aware of monitoring issues that could impact the monitors function
- Interview personnel who perform calculations
- Detailed review of EAL threshold calculations not intended
- Focus on changes since last inspection Refer significant questions or issues to the program office for further guidance 25
Items to Consider
- Which plant departments are responsible for the effluent monitors?
- Radiation Protection, Chemistry, I&C, or EP?
- Is there good communication between departments?
Calibration QA
- Typically required by T.S. commitment to RG 1.33 26
Plant Commitments
- Letters to NRC describing implementation of NUREG-0737 items
- Letters to NRC describing implementation of RG 1.97
- Documents that contain compliance-related commitments 27
Vendor Documents to Review during 71124.06 Inspections
- Equipment Design Documentation
- Technical manuals
- User manuals
- Vendor Calibration Documentation
- Initial gas calibration data
- Transfer calibration data
- Detector energy-dependence studies 28
Plant Calibration Procedures and Detector Energy Response Studies
- Plant Calibration Procedure(s)
- Plant Engineering Studies
- Programmed instrument calibration constants for converting detector output into source term
- cpm into Ci/sec or micro-Ci/cc
- mR/hr into Ci/sec or micro-Ci/cc
- Evaluation of instrument response to changing radionuclide mix
- Instrument response factors 29
Questions 30