7/27/2019 Evaluation of the Response of Tritium-in-air Instrumentation to HT in Dry and Humid Conditions and to HTO Vapour

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Evaluation of the Response ofTritium-in-air Instrumentation to HT in Dryand Humid Conditions and to HTO Vapour

Hilary Phillips1, Edwin Privas2, and Julian Dean1

1National Physical Laboratory, Teddington, United Kingdom2Grenoble INP-Phelma

email: hilary.phillips@npl.co.uk 

 The responses of ion chamber basedtritium-in-air monitors to tritium gas inboth dry and humid (45% RH) atmospheresand exposed to HTO vapour were found tobe within the uncertainty of measurementfor each of the trials performed. Theresponse of the proportional countingsystem was found to vary with humidity,

however calibration with the gas mixtureencountered in service would significantlyreduce this variation.

Abstract:Nuclear plant (power generation, decommissioning and reprocessing operations) have a legal requirement to monitor releases of tritium speciesfor regulatory compliance and radiation protection purposes during normal operating conditions. Tritium monitoring is routinely performedusing tritium-in-air gas monitoring instrumentation based either on flow though ion chambers or proportional counting systems. Tritium-in-airmonitors are typically calibrated in dry conditions but in service may operate at elevated levels of relative humidity. The NPL radioactive gas-in-aircalibration system has been used to study the effect of humidity on the response to tritium of two tritium-in-air ion chamber based monitors andone proportional counting system which uses a P10/air gas mixture. The response of these instruments to HTO vapour has also been evaluated. Ineach case instrument responses were initially obtained for HT in dry conditions (RH ~2%), HT in 45% RH, and finally HTO at 45% RH. Instrumentationresponse to HT in humid conditions has been found to slightly exceed that in dry conditions but response uncertainties may overlap.

Acknowledgments The authors wish to thank Berthold, Canberra and the UK agents for Overhoff, LabImpex Systems for the loan of instrumentation for testing. This work was funded bythe UK National Measurement System, an Executive Agency of the UK Department ofBusiness, Innovation and Skills.

 The NPL radioactive gas-in-air monitor calibration system used to containthe radioactive and humid atmospheres used during this study wasdesigned to enable the exposure of monitoring instrumentation to wellcharacterized activity concentrations of radioactive gas.

 This system consists of a pipe work loop with connection ports for vessels

or instrumentation under test. Each port has an isolation valve to enablethe inclusion or isolation of attached vessels and monitors (for examplethe trapping of radioactive gas in characterized volumes whilst the testinstrument remains at its background response).

A vacuum line is used to remove radioactive gases from the systemmanifold and gas lines to enable the addition of inactive or active gases tospecified volumes attached to the system. All system volumes are accuratelyknown enabling a well quantified dilution of standardized radioactive gasfrom a vessel of certified volume into trapped inactive gas contained withinthe rest of the system. A pump circulates active and inactive gases withinthe system to form the homogeneous gas mixture used during calibration.

A Perspex enclosure is routinely attached to the system to enable thecalibration of several monitors simultaneously and was used to contain theion-chamber based monitors during all exposures.

 The proportional counting system was attached directly to the calibrationsystem and the necessary operational gas mixture of P10 and air preparedinside the system prior to instrument exposure to radioactivity.

Instrument exposure at NPL is generally performed using dry air (relativehumidity, RH 2%). To determine the effect of ambient humidity (RH 45%)on instrument response the system was either filled with laboratory air,or for the testing of the proportional counting system a vessel was filled

and isolated containing laboratory air. A dreschel flask containing eitherwater or HTO was used to enable the addition of water/HTO vapour to thesystem. Vapour was condensed from the system using a flask immersedin an acetone and cardice solution to return monitors to backgroundresponse after exposure to HTO vapour. Liquid scintillation countingwas used to confirm the activity concentration of the supplied atmosphereduring the trial.

Response Ion chamber Monitor (%) Proportionalcounting system(%)1 2

HT Dry 86 ± 4 97 ± 4 100a ± 2.3

HT at40% RHb

99 ± 5 102 ± 4 165

HT at

40% RHc

110 ± 3 -

HTO at45% RH

80 ± 15 125

 

Exposure Trial 1 Trail 2

Dr y HT Dr y responserange

Humid responserange

HTOvaporadded

50% anticipatedresponse to HTO,reduced responseto HT

Response range todry HT and to HTOvapor

Dr y HT Dr y responserange

Humid responserange

 a response normalised to HT dry b initial response to humidity c response after previous exposure at ambient humidity