This work describes spectral distributions of neutrons obtained as function of energy and direction at four workplace fields at the Krümmel reactor in Germany. Values of personal dose equivalent Hp(10) and effective dose E are determined for different directions of a person's orientation in these fields and readings of personal neutron dosemeters—especially electronic dosemeters—are discussed with respect to Hp(10) and 

Bartlett, D.

Boschung, M.

Coeck, M.

Curzio, G.

d'Errico, F.

Derdau, D.

Fiechtner, A.

Itié, C.

Kyllönen, J.-E

Lacoste, V.

Lahaye, T.

Lindborg, L.

Luszik-Bhadra, M.

Molinos, C.

Reginatto, M.

Schuhmacher, H.

Tanner, R.

Vanhavere, F.

RERO DOC Digital Library

ELECTRONIC NEUTRON PERSONAL DOSEMETERS:THEIR PERFORMANCE IN MIXED RADIATIONFIELDS IN NUCLEAR POWER PLANTSM. Luszik-Bhadra1,, D. Bartlett7, M. Boschung3, M. Coeck4, G. Curzio5, D. Derdau8, F. d’Errico5,A. Fiechtner3, C. Itie´2, J.-E. Kyllo¨nen6, V. Lacoste2, T. Lahaye2, L. Lindborg6, C. Molinos7,M. Reginatto1, H. Schuhmacher1, R. Tanner7 and F. Vanhavere41Physikalisch-Technische Bundesanstalt, D-38116 Braunschweig, Germany2Institut de Radioprotection et de Suˆrete´ Nucle´aire, F-92265 Fontenay-aux-Roses, France3Paul Scherrer Institut, CH-5232 Villigen, Switzerland4Studiecentrum voor Kernenergie–Centre d’e´tude nucle´aire, B-2400 Mol, Belgium5Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, I-56126 Pisa, Italy6Swedish Radiation Protection Authority, SE-171-16 Stockholm, Sweden7National Radiological Protection Board, Chilton, Didcot OX11 0RQ, UK8Kernkraftwerk Kru¨mmel GmbH, Elbuferstrasse 82, 21496 Geesthacht, GermanyThis work describes spectral distributions of neutrons obtained as function of energy and direction at four workplace fields atthe Kru¨mmel reactor in Germany. Values of personal dose equivalent Hp(10) and effective dose E are determined for differentdirections of a person’s orientation in these fields and readings of personal neutron dosemeters—especially electronicdosemeters—are discussed with respect to Hp(10) and E.INTRODUCTIONWithin the EC project EVIDOS [‘Evaluation of Indi-vidual Dosimetry in Mixed Neutron and PhotonRadiation Fields’, see recent overviews(1–3)], a seriesof new electronic neutron personal dosemeters andsome passive ones have been irradiated at the nuc-lear power plant at Kru¨mmel in Germany. Placesthat are routinely visited for control measurementswere chosen: a position in the control rod roomunderneath the reactor, another position near thetop of the reactor and two places near a cask con-taining used fuel.Within the project a new spectrometer consistingof six detector capsules—each containing four sil-icon detectors—mounted onto the surface of a poly-ethylene sphere was developed(4,5). This allows thedetermination of the double differential distributionof neutron fluence with respect to energy and direc-tion. Values of ambient dose equivalent and personaldose equivalent Hp(10) for the main direction ofincidence have been derived(2) by multiplying thefluence distributions by the corresponding fluence-to-dose conversion coefficients.This work describes in more detail the directionaldose distributions obtained in the different work-places at Kru¨mmel as well as values for Hp(10)and for the effective dose E for different directionsof a person’s orientation in the fields. Finally,personal dosemeter readings obtained in theseworkplace fields are presented with respect toHp(10) and E.THE DIRECTIONAL SPECTROMETERThe directional spectrometer with silicon detectorsis described in detail in Refs (4) and (5). It consistsof six detector capsules—each containing a stack offour silicon detectors—mounted onto the surface ofa 30 cm diameter polyethylene sphere and electronicsto amplify and record the pulse height spectra of alldetectors. The response function of this device hasbeen determined for a series of directions usingmeasurements in quasi-mono-energetic neutronfields and MCNP calculations for neutrons in theenergy range from thermal up to 15 MeV, and usingmeasurements for photons in the energy region from80 keV to 6 MeV. The pulse height spectra measuredin workplace fields are analysed using unfoldingcodes with respect to energy and direction both forneutrons and photons.One of the codes (MIEKE) does not take intoaccount pre-information on energy and directionaldistribution. One unfolding was performed takingthe full pulse height information and a combinedunfolding for neutrons and photons, a secondunfolding was performed taking pulse height signalsabove a higher threshold and doing an unfoldingwith respect to neutrons only.The other code (MAXED) takes pre-informationwith respect to the neutron energy into account. Oneunfolding was performed using the spectral shapemeasured at the same places by Bonner spheresCorresponding author: marlies.luszik-bhadra@ptb.deRadiation Protection Dosimetry (2006), Vol. 120, No. 1–4, pp. 378–382doi:10.1093/rpd/nci556 Advance Access published on April 27, 2006 The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org(BSs) as pre-information, a second unfolding wasperformed using other measurements in similar fieldsas pre-information.The performance of the directional spectrometerhas been checked(4) by measurements in the simu-lated workplace field CANEL at Cadarache. H(10)contributions per angular interval, derived fromMCNP calculations, decrease sharply above 30,whereas the results of the directional spectrometershow significant contributions up to 60. This showsthe coarser angular resolution of the experimentaldevice. The fluence f and the ambient dose equival-ent H(10) obtained by integrating the contributionsper angular interval obtained by the directional spec-trometer have been compared with reference valuesobtained by BS spectrometry. In the case of theMAXED unfolding with BS pre-information, devi-ations were <20%.For the calculation of personal dose equivalentHp(10), fluences obtained in 14 directions (MIEKE)and in 20 directions (MAXED) were multipliedby fluence-to-dose conversion factors taken fromICRP74(6) and new calculations for backward incid-ence(7). Appropriate values for the required energiesand angles were obtained by interpolation.For the calculations of effective dose E, the flu-ences were multiplied by the conversion factors forAP, PA, LLAT and RLAT as given in ICRP74(6).Values for the directions ‘Up’ and ‘Down’ are estim-ated from the difference between the conversion fac-tors given for ISO and ROT by (3·ISO 2·ROT). Inaddition, the influence of revised wR values that arelikely to be included in the 2005 recommendations ofthe ICRP as recently proposed(8) on values of effect-ive dose are estimated. The fluence-to-dose conver-sion factors given in ICRP 74 for AP, PA, LLAT,RLAT, ISO and ROT have been divided by the wRvalues as given in ICRP 74 and multiplied by thenewly recommended values.THE WORKPLACE FIELDSAT KRU¨MMELThe measurements were performed at two positionsnear an NTL11 cask with spent fuel and at twopositions inside the boiling water reactor (Figure 1).In Table 1 details on the positions are given,together with reference values for H˙(10) obtainedby BS spectrometry. The last column in Table 1contains the ratio of neutron and photon contribu-tions of ambient dose equivalent (the latter measuredby a FHT 191N ionisation chamber). At the cask,neutrons contributed with>90% to the ambient doseequivalent. At the position ‘KKK TOP’ the neutrondose was also dominant, while at the position ‘KKKSAR’ the photon dose dominated because of theheavy biological shielding.The fluence spectra obtained by BS spectrometryare shown in Figure 2. The spectra at the cask werequite hard, with a main fluence contribution at a fewhundred keV, whereas the spectra at the reactorcontained a considerable amount of thermal andintermediate energy neutrons. A comparison withfluences obtained by the directional spectrometer(unfolding MAXED) by integrating the contri-butions per angular interval gave deviations ofup to 30%.RESULTS OF DIRECTIONALDISTRIBUTIONS OF DOSERelative contributions to H(10)Figure 3 shows the relative ambient dose equivalentper angular interval for 14 directions, obtainedby the unfolding codes MIEKE and MAXED. The‘Front’ direction was towards the cask or thereactor, in case of ‘KKK SAR’ towards the lock.Six of the directions refer to Front (F), Back (B),Left (L), Right (R), Up (U) and Down (D). The eightdirections in between are named by a combinationof the abbreviations.Figure 1. Measuring positions at an NTL11 cask (a) andinside the reactor (b) at Kru¨mmel.ELECTRONIC NEUTRON DOSEMETERS IN POWER PLANTS379In the case of the position ‘cask midline’ thedistribution decreases by about two orders of mag-nitude from ‘Front’ to ‘Back’. In the case of theposition ‘cask side’ the decrease from ‘Front’ to‘Back’ is smaller, caused by additional backscatterfrom the floor. In addition, higher contributionsfrom ‘Right’ are observed, which is obviouslycaused by the positioning (Figure 1). In the case of‘KKK TOP’ a slightly higher contribution from‘Down’ is observed. In the case of the position‘KKK SAR’ an almost isotropic distribution isobtained.Hp(10) and E for different directions of a person’sorientationValues of Hp(10) and E normalised to H(10) areshown in Table 2 for six different directions of aperson’s orientation.For fields with chiefly directed neutrons Hp(10)values decrease strongly from values close toH(10) for the ‘Front’ direction to values about oneorder of magnitude smaller at the ‘Back’ direction(see ‘cask midline’). They stay almost constant at avalue of about one-third of H(10) in fields with analmost isotropic distribution (see KKK SAR).Table 1. Measuring positions at Kru¨mmel.Position Distance 1 Distance 2 Height abovefloor (m)H˙(10) mSv h1 H˙n(10)/H˙g(10)Cask midline Centre of cask To cask: 1.0 m 2.65 152 23Cask side 10th ring, left side To cask: 1.35 m 1.50 54.6 8.2 40KKK TOP, 40 m level To door: 0.8 m To wall: 0.68 m 1.50 39.0 5.8 4.20KKK SAR Centre of room Centre of room 1.20 46.8 7.0 0.24Figure 2. Spectral neutron fluence per logarithmic bin-width as a function of energy at workplaces investigatedat Kru¨mmel using BSs (dotted line)(1,2) and the directionalspectrometer with silicon detectors (full line, MAXEDunfolding with pre-information taken from BSs).Figure 3. Relative contributions to H(10) from variousdirections (see text) using the MIEKE unfolding (dottedlines, neutron/photon unfolding) and the MAXEDunfolding (full lines, unfolding with pre-informationtaken from BSs).M. LUSZIK-BHADRA ET AL.380E values show smaller changes with respect todirection. In fields with chiefly directed neutrons,Hp(10) values overestimate E for ‘Front’ directionand underestimate E for ‘Back’ direction (see ‘caskmidline’, factors 1.8 and 6.4). In almost isotropicfields Hp(10) values and E values almost coincide(see KKK SAR).RESULTS OF PERSONAL DOSEMETERSPersonal dosemeters (active and passive devices)were irradiated on ISO phantoms in the positions‘cask side’, ‘cask midline’ and ‘KKK TOP’ for 2 hand inside the SAR under nitrogen atmosphere for38 h. In the latter case, the etched track dosemeters(CR-39 /PADC from PSI and NRPB) were sealed inair in order to avoid track-fading due to oxygendeficiency. The readings of the personal dosemetersattached on the front side are shown in Figure 4. Inall cases, dosemeters were also attached on thebackside of the phantom. The readings were mostlyzero or close to the detection limit of the dosemetersand are not shown in Figure 4 besides those meas-ured at the SAR, where the radiation field was muchmore isotropic.The dosemeter readings are shown with respect tovalues of Hp(10) and E calculated using the H(10)values given in Table 1 and ratios of Hp(10)/H(10)and E/H(10) given in Table 2. The uncertaintiesgiven in Tables 1 and 2 are estimated on standarddeviation of results of different unfoldings. Theabsolute uncertainties of Hp(10) and E contain addi-tional uncertainties of the raw data and the instru-ment calibration itself. A full uncertainty analysis isTable 2. Ratio of personal dose equivalentHp(10) to ambient dose equivalentH(10) and effective dose E and Enew to ambientdose equivalent H(10) for a person facing different directions (see text).Position Front Up Down Left Right BackCask midlineHp(10)/H(10) 0.78(8) 0.31(4) 0.23(4) 0.23(1) 0.25(2) 0.05(3)E/H(10) 0.44(5) 0.24(1) 0.24(1) 0.28(1) 0.27(2) 0.31(3)Enew/H(10) 0.36(5)Cask sideHp(10)/H(10) 0.61(14) 0.27(5) 0.31(8) 0.15(5) 0.33(6) 0.18(6)E/H(10) 0.40(9) 0.27(5) 0.27(4) 0.28(4) 0.28(2) 0.32(6)Enew/H(10) 0.34(9)KKK TOPHp(10)/H(10) 0.58(5) 0.29(1) 0.29(3) 0.25(2) 0.25(2) 0.10(5)E/H(10) 0.44(3) 0.32(2) 0.32(2) 0.31(2) 0.31(3) 0.34(3)Enew/H(10) 0.30(3)KKK SARHp(10)/H(10) 0.30(2) 0.31(5) 0.34(5) 0.28(2) 0.30(3) 0.28(6)E/H(10) 0.32(3) 0.35(1) 0.35(1) 0.31(2) 0.31(3) 0.31(2)Enew/H(10) 0.23(1)The uncertainties are standard deviations using four different unfoldings (see text).Figure 4. Personal dose equivalent rates H˙p(10) ofdosemeters at Kru¨mmel. The dashed lines indicateestimated values of H˙p(10), the dashed/dotted linesestimated values of E˙. The local device was a TLDalbedo dosemeter. The full/open symbols indicate valuesmeasured at the front side/back side of the phantom[H˙p(10) value dotted for back side].ELECTRONIC NEUTRON DOSEMETERS IN POWER PLANTS381not yet available. A conservative estimate of theuncertainty of Hp(10) and E values is 30%.The dosemeters based on superheat drop detectors(HpSLAB, BTI-PND and PND-BDT) show in mostcases an overresponse (roughly a factor 2) withrespect to H˙p (10) besides the dosemeter devotedto thermal fields (BTI-BDT), which gave lowervalues.The electronic dosemeter EPD-N devoted tomeasuring thermal and intermediate energy neutronsshowed very low or zero values in all cases.The electronic dosemeters (EPD-N2, DOS-2002and Saphydose-n, all based on one or more silicondetectors) showed reasonable agreement with H˙p(10)at the cask besides the EPD-N2 that showed too lowvalues at ‘cask midline’ and values a factor 3–5 timestoo high inside the reactor.The electronic dosemeter ALOKA PDM-313(also based on a silicon detector) showed in all fieldswith substantial contributions of scattered neutronsmuch too high values (factor 5–10) compared withH˙p(10). Only in the case of the position ‘cask mid-line’ was the value indicated close to the referencevalue.The DISN devices, which are under investigationat PSI, using different wall materials and absorbers,showed in general underreadings compared withH˙p(10) and high uncertainties owing to a photonsubtraction procedure and could not be used in fieldswith high photon dose (KKK SAR).The dosemeters based on CR-39/ PADC showedin general values lower than H˙p(10) or even zerovalues. In the case of the SAR, the low values mayhave been caused by track-fading owing to oxygendeficiency.The ‘Local device’ used in routine at the Kru¨mmelnuclear power plant is an TLD albedo dosemeterand was used with calibration factors as foreseenfor the different application areas (N1 inside thepower plant and N2 at the cask). At the cask, thevalues agreed within the uncertainties with H˙p(10),while inside the power plant, H˙p(10) is overestimatedby a factor of 3–4.With respect to _E, the overresponse observed withelectronic dosemeters inside the power plant stays atalmost the same level, since H˙p(10) and E˙ do notdeviate strongly. At the cask, the dosemeter readingsbecome more conservative with respect to E˙.This conservative behaviour of personal dose-meter readings with respect to _E will still increaseif new radiation weighing factors wR as recentlyproposed by members of the ICRP(8) are used. Theresulting values of Enew are given in Table 2 in thelast column. They are 20% (cask) to 40% (reactor)lower than the old values.CONCLUSIONSDepending on the radiation field, Hp(10) is onlybetween 30 and 80% of H(10). Most of the elec-tronic dosemeters show conservative (too high) read-ings in the fields investigated at the Kru¨mmelreactor.ACKNOWLEDGEMENTThis 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.REFERENCES1. Bolognese-Milsztajn, T. et al. Individual neutron monit-oring in workplaces with mixed neutron/photon radiation.Radiat. Prot. Dosim. 110, 753–758 (2004).2. Luszik-Bhadra, M. et al. EVIDOS: optimisation ofindividual monitoring in mixed neutron/photon fieldsat workplaces of the nuclear fuel cycle. In: 4th ISOEEuropean workshop on occupational exposure manage-ment at NPPs, Lyon, France, 24–26 March 2004.Available on http://isoe.cepn.asso.fr/. Copyright CEPN1998–2005.3. Vanhavere, F. et al. Evaluation of individual monitoringin mixed neutron/photon fields: mid-term results from theEVIDOS project. Radiat. Prot. Dosim. 120(1–4),263–267 (2006).4. 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).5. Reginatto, M., Luszik-Bhadra, M. and d’Errico, F. Anunfolding method for directional spectrometers. Radiat.Prot. Dosim. 110, 539–543 (2004).6. International Commission on RadiologicalProtection. Conversion coefficients for use in radiologicalprotection against external radiation. ICRP Publication74. Ann ICRP 26 (3–4) (Oxford: Pergamon Press)(1996).7. d’Errico, F., Giusti, V. and Siebert, B. R. L. A newneutron monitor and extended conversion coefficients forHp(10). Presented at the IM2005 European Workshopon individual monitoring of ionising radiation, Vienna,April 11–15 (2005). Radiat. Prot. Dosim. (to bepublished).8. Dietze, G. and Harder, D. Proposal for a modifiedradiation weighting factor for neutrons. In: Proceedingsof the 11th International Congress of the InternationalRadiation Protection Association, 23–28 May 2004,Madrid, Spain. Published on CD. Produced in Spainby SENDA. ISBN: 84-87078-05-2.M. LUSZIK-BHADRA ET AL.382

Electronic neutron personal dosemeters: their performance in mixed radiation fields in nuclear power plants

Electronic neutron personal dosemeters: their performance in mixed radiation fields in nuclear power plants

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