ML18207A178

2018 NEI RP Forum - NRC Accident-Range Rms Calibration Process Presentation (Final)

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)

(NUREG-0660) 7

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

• Alert (1% EPA PAG)

- 10 mrem TEDE, or

- 50 mrem CDE (thyroid)

• Site Area Emergency (10% EPA PAG)

- 100 mrem TEDE

- 500 mrem CDE (thyroid)

• General Emergency (100% EPA PAG)

- 1 rem TEDE

- 5 rem CDE 12

Dose Assessment Computer Codes

• URI, MIDAS, RADDOSE-V, RASCAL, others

• 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 Regular size paper clip 19

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