The paper presents the main conclusions and recommendations derived from the EVIDOS project, which is supported by the European Commission within the 5th Framework Programme. EVIDOS aims at evaluating state of the art neutron dosimetry techniques in representative workplaces of the nuclear industry with complex mixed neutron-photon radiation fields. This analysis complements a series of individual papers which present detailed results and it summarises the main findings from a practical point of view. Conclusions and recommendations are given concerning characterisation of radiation fields, methods to derive radiation protection quantities and dosemeter result

Bartlett, D.

Bolognese-Milsztajn, T.

Boschung, M.

Coeck, M.

Curzio, G.

d'Errico, F.

Fiechtner, A.

Kyllönen, J.-E

Lacoste, V.

Lindborg, L.

Luszik-Bhadra, M.

Reginatto, M.

Schuhmacher, H.

Tanner, R.

Vanhavere, F.

RERO DOC Digital Library

EVALUATION OF INDIVIDUAL DOSIMETRY IN MIXEDNEUTRON AND PHOTON RADIATION FIELDS (EVIDOS).PART II: CONCLUSIONS AND RECOMMENDATIONSH. Schuhmacher1,, D. Bartlett2, T. Bolognese-Milsztajn3, M. Boschung4, M. Coeck5, G. Curzio6,F. d’Errico6, A. Fiechtner4, J.-E. Kyllo¨nen7, V. Lacoste3, L. Lindborg7, M. Luszik-Bhadra1,M. Reginatto1, R. Tanner2 and F. Vanhavere51Physikalisch-Technische Bundesanstalt, D-38116 Braunschweig, Germany2Health Protection Agency, Chilton, Didcot OX11 0RQ, UK3Institut de Radioprotection et de Suˆrete´ Nucle´aire, F-92265 Fontenay-aux-Roses, France4Paul Scherrer Institut, CH-5232 Villigen, Switzerland5Studiecentrum voor Kernenergie- Centre d’e´tude de l’e´nergie nucle´aire, B-2400 Mol, Belgium6Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, I-56126 Pisa, Italy7Swedish Radiation Protection Authority, SE-171-16 Stockholm, SwedenThe paper presents the main conclusions and recommendations derived from the EVIDOS project, which is supported by theEuropean Commission within the 5th Framework Programme. EVIDOS aims at evaluating state of the art neutron dosimetrytechniques in representative workplaces of the nuclear industry with complex mixed neutron–photon radiation fields. Thisanalysis complements a series of individual papers which present detailed results and it summarises the main findings from apractical point of view. Conclusions and recommendations are given concerning characterisation of radiation fields, methodsto derive radiation protection quantities and dosemeter results.INTRODUCTIONThe EVIDOS project, supported by the EuropeanCommission within the 5th Framework Programme,aims at evaluating state of the art dosimetry tech-niques in representative workplaces of the nuclearindustry with mixed neutron–photon radiation(1).Seven European institutes with recognised expertisein radiation protection instruments and methodsjoined efforts with end users at nuclear power plants,at fuel processing and reprocessing plants, and attransport and storage facilities. A major task of theproject was to develop methods to characterise theneutron component of mixed radiation fields atworkplaces and to derive reference values of radia-tion protection quantities from energy and directiondistributions of the neutron fluence.The results from the project available to dateinclude spectra and dosimetric data for 14 differentworkplace fields (boiling water reactor, pressurisedwater reactor, research reactor, fuel processing, stor-age of spent fuel), instruments and procedures toderive reference values for personal dose equivalentand other radiation protection quantities, and resultson the dosimetric and technical performance ofpersonal dosemeters for mixed radiation.While other papers in this volume describe inmore detail the methods developed and the instru-ments used, this presentation summarises the mainresults, draws conclusions and discusses generalrecommendations. A number of questions, of rele-vance for the routine monitoring of such complexmixed radiation fields, are addressed in the followingsections, e.g., which methods permit the determi-nation of H(10) and Hp(10); what is the influenceof the energy and direction distributions of neutronson the ratios between H(10), Hp(10) and E; howmuch do the readings of area monitors and personaldosemeters deviate from these radiation protectionquantities and do they give conservative estimatesof E; do new active (electronic) personal dosemetersgive better estimates of Hp(10) than passive ones;and can the overall response of area monitors andpersonal dosimeters in workplaces of nuclear indus-try be improved by a suitable calibration?SUMMARY OF RESULTSNew methods were developed for the characterisa-tion of workplaces using direction spectrometry(2–4).These new instruments were used together with well-established spectrometers, which determine the angle-integrated energy distribution(5). The result is a fullcharacterisation of the radiation fields in terms ofthe neutron fluence (or fluence rate) as a function ofenergy and direction(6). Conversion coefficients(7,8)were then used to derive the relevant radiation pro-tection quantities, ambient dose equivalent, H(10),personal dose equivalent in a slab phantom asused for calibration purposes, Hp(10), and effectivedose, E(5,6).Corresponding author: helmut.schuhmacher@ptb.deRadiation Protection Dosimetry (2007), Vol. 125, No. 1–4, pp. 281–284 doi:10.1093/rpd/ncm167Advance Access publication 20 February 2007INVITED PAPER The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.orgResults were obtained from a large number ofarea monitors(9) and personal dosemeters(10). Theseinclude commercial devices and instruments devel-oped by the partners outside or within the project.The first investigations were performed in thesimulated workplace fields SIGMA and CANEL atCadarache (FR) in order to test the various instru-ments used(11). Then three measurement campaignswere performed in nuclear facilities at Kru¨mmel (DE),Mol (BE) and Ringhals (SE). The result is a compre-hensive set of data from 14 workplaces in the nuclearindustry, namely at reactors (BWR, PWR, researchreactor), at transport casks and for fuel processing.A final campaign is planned in a fuel processingfacility.CONCLUSIONS AND RECOMMENDATIONSCharacterisation of radiation fieldsThe measurements with reference spectrometryrevealed significant differences in the energy distri-butions at the workplaces(5). While all distributionsexhibit similar structures—a thermal peak, a ratherflat intermediate-energy region and a high-energy peakwith a maximum between 100 keV and 1 MeV—thecontributions of these neutrons to the fluence variessignificantly(5).The importance of the different neutron energiesfor dosimetry depends on the fluence-to-dose equi-valent conversion coefficient. Because of its strongenergy dependence, the contributions of high-energyneutrons to dose equivalent is dominant in all spec-tra. To give an example, the spectrum with the largestcontribution from the thermal and intermediate-energy region was found in a workplace at a researchreactor: about 90% of the neutron fluence is obser-ved in the energy region below 10 keV but these neu-trons contribute only about 25% to ambient doseequivalent.For the analysis of dosemeter results in the nextsections, the average fluence-to-ambient dose equi-valent conversion coefficient, h, was used as a para-meter to indicate the ‘hardness’ of a spectrum.Although, in principle, different spectra can lead tothe same h, the use of a single parameter allows tocategorise spectra for analysing the instruments’behaviour in the different radiation fields. Figure 1shows h for the different radiation fields. All spectraat the reactors are soft because they are influenced bymassive shielding between the reactor cores and themeasurement positions. At the transport casks twotypes of spectra were observed: Hard spectra at onecask and much softer spectra at another one, whichinclude an additional neutron shield. For fuel pro-cessing, the spectra vary significantly dependingon the specific shielding conditions. The values of hindicate that the simulated workplace fields arerepresentative for fields with significant shielding(e.g., at reactors) but not for those with littleshielding.The ratio Hp(10)/H(10), shown in Figure 2, canbe used as a crude indicator of the direction distri-bution of the neutrons. If neutrons impinge mainlyfrom the normal direction to the front surface ofthe phantom, like in simulated workplace fields,this ration is close to 1. The larger the contributionfrom neutrons from other directions of incidence,the smaller this ratio gets, since neutrons comingfrom these other directions of incidence are essen-tially shielded by the phantom. The lowest value of0.22 was observed for the control rod room under-neath the core at the Kru¨mmel power plant where alarge isotropic component was present with the maindirection being from the top. In general, the direc-tion distributions in the investigated workplacesare more isotropic at reactors and more directedat transport casks and fuel processing. The specificFigure 2. Ratio of personal dose equivalent for theassigned front direction, Hp(10), to ambient doseequivalent, H(10), for the different radiation fieldsinvestigated. For position VENUS C, Hp(10)-data are notavailable because measurements with the directionspectrometer could not be performed.Figure 1. Average fluence-to-ambient dose equivalentconversion coefficient, h, for the different radiation fieldsinvestigated.H. SCHUHMACHER ET AL.282results for a particular workplace are influenced bythe details of its geometry, in particular with respectto shielding and scattering material.The photon dose equivalent rate was measuredwith a FHT 191 N ionisation chamber and low-pressure proportional counters(10,12). The fractionsof photon to total ambient dose equivalent rangedfrom 17 to 98% in the workplace fields investigated.Radiation protection quantitiesThe reference method to determine H(10) ofneutrons in unknown radiation fields is that of theBonner sphere (BS) spectrometry. BS spectrometryis well-established and has been benchmarked againstother methods, e.g., calculations and allows H(10)to be determined with small uncertainties in the orderof 5% (one relative standard uncertainty) if the res-ponse matrix of the spectrometer is precisely knownand if the data analysis is performed carefully(5).At present, a reference method to determine Hp(10)does not yet exist. Energy and direction spectrome-ters are still research tools for which further valida-tion is needed. The results from these instrumentscan be improved if the angle-integrated spectra fromBS are used as pre-information. In this case the uncer-tainties are in the order of 30%(6,10). An alternativemethod, which may result in similar uncertainties, isbased on a superheated drop detector located at10 mm depth inside a slab phantom(8). However,further research is needed for this device, e.g., a fullcharacterisation with calibration measurements andtransport calculations.Dosemeter resultsIn this paper, the observed responses (readingsdivided by the reference value or best estimatederived from spectrometry) are analysed in a generalway as a function of h. For specific results concern-ing particular instruments or workplaces the readeris referred to the papers which give the full resultsfrom area monitors(9) and personal dosemeters(10).The area monitors slightly underrespond in hardspectra while the results are between 0.5 and 1.5(Figure 3). Despite the small underestimates interms of H(10), the instruments do provide gene-rally conservative estimates of Hp(10) or E. In highlyscattered fields the overestimates can be substantial.For the personal dosemeters, a significant spreadof the results is observed (Figure 4). While the bestresults are obtained in hard spectra, many dose-meters overrespond significantly in soft spectra. Thenew active (electronic) personal dosemeters (APDs)do not generally give better results than passive onesin terms of the spread of responses—at least for theworkplace fields investigated. APDs show, however, amuch lower detection limit. In reactor fields, 20 mSvcan be measured with a statistical uncertainty of 10%with most APDs.If the application is restricted to highly scatteredfields at reactors, the field-specific calibration (e.g.,using a simulated workplace field) improves theresults for some area monitors and personal doseme-ters. Further analysis is needed to understand whyfor several instruments this procedure does not leadto an improvement.SUMMARY AND OUTLOOKThe EVIDOS project will be completed byOctober 2005. Further measurements in fuel proces-sing are scheduled. To date, a huge number of datahave been accumulated in comprehensive measure-ment campaigns and from accompanying calcula-tions. These include spectra (energy and direction)for 16 radiation fields, about 300 dosimetric dataderived from spectrometry, 150 results from areamonitors and 500 results from personal dosemeters.Data analysis is partly still in progress, e.g., to checkthe observed responses with calculations.Figure 4. Personal dose equivalent response of thedifferent personal dosemeters as a function of the averageconversion coefficient, h.Figure 3. Ambient dose equivalent response of thedifferent area monitors as a function of the averageconversion coefficient, h.CONCLUSIONS FROM THE EVIDOS PROJECT283ACKNOWLEDGEMENTSThis research is partly funded by the EuropeanCommission under the auspices of the Euratom5th Framework Programme: Nuclear Energy,1998–2002, Contract No: FIKR-CT-2001-00175.The authors wish to express their gratitude to staffat Cadarache, Kru¨mmel, Mol and Ringhals for theirhelp during the campaigns.REFERENCES1. d’Errico, F., Bartlett, D., Bolognese-Milsztajn, T.,Boschung, M., Coeck, M., Curzio, G., Fiechtner, A.,Kyllo¨nen, J.-E., Lacoste, V., Lindborg, L. et al. Evalu-ation of individual dosimetry in mixed neutron andphoton radiation fields (EVIDOS). Part I: Scope andmethods of the project. In press.2. Luszik-Bhadra, M., Reginatto, M. and Lacoste, V.Measurement of energy and direction distribution ofneutron and photon fluences in workplace fields. Radiat.Prot. Dosim. 110, 237–241 (2004).3. d’Errico, F., Giusti, V., Reginatto, M. and Wiegel, B.A telescope-design directional neutron spectrometer.Radiat. Prot. Dosim. 110, 533–537 (2004).4. Reginatto, M., Luszik-Bhadra, M. and d’Errico, F. Anunfolding method for directional spectrometers. Radiat.Prot. Dosim. 110, 539–543 (2004).5. Lacoste, V., Asse´lineau, B., Muller, H. and Reginatto, M.Bonner sphere neutron spectrometry at nuclear work-places in the framework of the EVIDOS project. Inpress.6. Luszik-Bhadra, M., Bolognese-Milsztajn, T.,Boschung, M., Coeck, M., Curzio, G., d’Errico, F.,Fiechtner, A., Lacoste, V., Lindborg, L.,Reginatto, M. et al. Direction distributions of neutronsand reference values of personal dose equivalent inworkplace fields. In press.7. International Commission on Radiation Units andMeasurements. Conversion coefficients for use in radio-logical protection against external radiation. ICRUReport 57 (Bethesda, MD: ICRU) (1998).8. d’Errico, F., Giusti, V. and Siebert, B. R. L. A newneutron monitor and extended conversion coefficients forHp(10). In press.9. Tanner, R. J., Bolognese-Milsztajn, T., Boschung, M.,Coeck, M., Curzio, G., d’Errico, F., Fiechtner, A.,Hager, L. G., Hussien, M., Kyllo¨nen, J-E. et al. Neu-tron area survey instruments in the EVIDOS project:response in workplaces that have been characterized interms of energy and direction distributions. In press.10. Luszik-Bhadra, M., Bolognese-Milsztajn, T.,Boschung, M., Coeck, M., Curzio, G., Derdau, D.,d’Errico, F., Fiechtner, A., Kyllo¨nen, J.-E.,Lacoste, V. et al. Summary of personal neutron dose-meter results obtained within the EVIDOS project. Inpress.11. Bolognese, T., Boschung, M., Coeck, M., Curzio, G.,d’Errico, F., Lacoste, V., Lindborg, L.,Luszik-Bhadra, M., Pochat, J.-L., Reginatto, M. et al.Individual monitoring in workplaces with mixed neutron/photon radiation. Radiat. Prot. Dosim. 110, 753–758(2004).12. Kyllo¨nen, J. and Lindborg, L. Photon and neutron dosediscrimination using low pressure proportional counterswith graphite and A150 walls. In press.H. SCHUHMACHER ET AL.284

Evaluation of individual dosimetry in mixed neutron and photon radiation fields (EVIDOS). Part II: conclusions and recommendations

Evaluation of individual dosimetry in mixed neutron and photon radiation fields (EVIDOS). Part II: conclusions and recommendations

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