Validation of the neutron and gamma fields in the JSI TRIGA reactor using in-core fission and ionization chambers

Applied Radiation and Isotopes 96 (2015) 27–35

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Validation of the neutron and gamma fields in the JSI TRIGA reactorusing in-core fission and ionization chambers

Gašper Žerovnik a,b,n, Tanja Kaiba a, Vladimir Radulović a,c, Anže Jazbec a, Sebastjan Rupnik a,Loïc Barbot c, Damien Fourmentel c, Luka Snoj a

a Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Sloveniab Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, SI-1000 Ljubljana, Sloveniac CEA, DEN, DER, Instrumentation, Sensors and Dosimetry Laboratory, Cadarache, F-13108 St-Paul-Lez-Durance, France

H I G H L I G H T S

� Neutron and gamma measurements with fission and ionization chambers were performed.

a r t i c l e i n f o

Article history:Received 11 July 2014Received in revised form24 October 2014Accepted 30 October 2014Available online 11 November 2014

Keywords:Research reactorTRIGAFission chamberIonization chamberMCNPNeutron flux and spectrumThermal power calibration

x.doi.org/10.1016/j.apradiso.2014.10.02643/& 2014 Elsevier Ltd. All rights reserved.

esponding author at: Jožef Stefan Institutea, Slovenia. Tel.: þ38615885362; fax: þ38615ail addresses: [email protected] (G. Ž[email protected] (T. Kaiba), vladimir.radulovic@[email protected] (A. Jazbec), [email protected] ([email protected] (L. Barbot), damien.fourmentel@[email protected] (L. Snoj).

a b s t r a c t

CEA developed fission chambers and ionization chambers were utilized at the JSI TRIGA reactor tomeasure neutron and gamma fields. The measured axial fission rate distributions in the reactor core aregenerally in good agreement with the calculated values using the Monte Carlo model of the reactor thusverifying both the computational model and the fission chambers. In future, multiple absolutely cali-brated fission chambers could be used for more accurate online reactor thermal power monitoring.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The TRIGA Mark II research reactor at the Jožef Stefan Institute(JSI) is a typical 250 kW TRIGA reactor. It is a light water pool typereactor, cooled by natural convection. Like many other researchreactors (Aghara and Charlton, 2006; Jonah et al., 2006; Lin et al.,2006; Meftah et al., 2006; Merz et al., 2011; Nagels et al., 2009;Stamatelatos et al., 2007) it is extensively used for various

, Jamova cesta 39, SI-1000885454.ik),ijs.si (V. Radulović),S. Rupnik),.fr (D. Fourmentel),

applications, such as training and education (Snoj et al., 2011),verification and validation of nuclear data, computational methods and computer codes (Snoj andRavnik, 2008; Snoj et al., 2010, 2011; Trkov et al., 2009), testingand development of experimental equipment used for core physicstests at the Krško Nuclear Power Plant (Trkov et al., 1992) and ir-radiation of various samples. The latter activity occupies almost80% of the reactor operation time and is related mainly to usingthe reactor as source of neutrons for use in nuclear analyticaltechniques, e.g. neutron activation analysis (NAA) (Jaćimović et al.,2010, 2012; Radulović et al., 2013), and as source of neutrons andgamma rays for irradiation of silicon detectors (Cindro et al., 2009;Kramberger et al., 2007, 2007, 2009, 2010, 2013; Mandić et al.,2013) and related radiation damage studies of detector materialand of reading electronics for the ATLAS detector in CERN.

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Optimal performance of the studies is often sensitive to theintensity and the shape of the neutron spectra at the points ofirradiation. Some degree of tailoring these parameters is achiev-able by choice of irradiation position, by adjusting the fuel loadingpattern or by the use of various filtering materials. Nowadays suchtailoring is commonly done with advanced computer codes, suchas MCNP (X-5 Monte Carlo Team, 2004). However, the computa-tional model in such calculations should be thoroughly verifiedand validated with experiments. Hence it is important to have averified, validated model to computationally characterize the re-actor irradiation facilities. Accurate and reliable information onneutron flux and spectra in irradiation facilities is one of the keyfactors for performing good research work. The first validation ofthe TRIGA Mark II computational model in MCNP was done bycomparing the calculated keff with the criticality benchmark ex-periment performed in 1991 (Ravnik and Jeraj, 2003), after a majorreconstruction. The evaluated criticality benchmark experimentwas later published in the ICSBEP handbook (ICSBEP, 2009). Untilrecently this was the only publicly available TRIGA criticalitybenchmark featuring homogenous mixture of fuel, moderator andzirconium. Due to UZrH fuel, it is very sensitive to Zr absorptionand scattering cross sections (Snoj et al., 2012). In the last yearscriticality benchmark experiments from Idaho National Laboratory(INL) were also evaluated and published in the ICSBEP Handbook.

In order to expand the utilization of the JSI TRIGA computa-tional model, an extensive experimental campaign was launchedin 2007, with the purpose of experimental verification and vali-dation of the computational model in MCNP for reaction ratedistribution (Snoj et al., 2011). Within this campaign Al–Au(99.9 wt.% Al – 0.1 wt% Au) were irradiated at various irradiationpositions/channels in the reactor. Then measured and calculated27Al(n,α) and 197Au(n,γ) reaction rates were compared. Within thiscampaign the criticality benchmark model of the TRIGA reactorwas upgraded by modelling more reactor components, such asreflector, irradiation channels in the reflector and irradiationchannels in the core. That model was later further expanded bymodelling ex-core irradiation facilities and used for computationalcharacterization of irradiation facilities (Snoj et al., 2012). As thereactor model has been thoroughly verified against experimentaldata, it has been used for power peaking factor calculations (Snojand Ravnik, 2008) and kinetic parameters studies (Snoj et al.,2010). All of the above mentioned activities were performed withthe Monte Carlo neutron transport code MCNP (X-5 Monte CarloTeam, 2004) and various cross section libraries.

Since 2010 many experiments (neutron spectrum character-ization, in-core flux mapping with 235U and 238U fission chambers,βeff measurements) have been performed in collaboration withCEA Cadarache (Snoj et al., 2011; Štancar et al., 2012; Žerovniket al., 2013; Barbot et al., 2013). The purpose of these experimentswas on one hand to test neutron measurement methods andequipment and on the other hand to use the measured data forverification and validation of the computational model. Some ofthese measurements were already analyzed, the analysis of other(such as neutron spectra characterization, βeff) is still in progress.

The use of CEA developed miniature fission chambers (FC)(Geslot et al., 2009) and JSI developed moving mechanism (Štancaret al., 2012) allowed measuring axial in radial neutron flux profilesor to be more exact fission rate profiles with a spatial resolution of1 mm. In contrast to the abovementioned reaction rate measure-ments in irradiation channels, that give information on flux profileon a plane only. The advantage of such system is that enables full3D neutron profile characterization.

In addition to neutron flux measurements, we measuredgamma field as well. The information on the intensity of thegamma field in the reactor is especially important for applicationssuch as radiation hardness studies (Cindro et al., 2009;

Kramberger et al., 2007, 2007, 2009, 2010, 2013; Mandić et al.,2013). The fission chambers operated in pulse mode as well as incurrent mode. Special attention was devoted to background de-termination, linearity tests and dead time correction.

The paper is structured as follows. In Section 2 the TRIGA re-actor and the experimental equipment are described together withthe computational model of the reactor. In Section 3 the linearityof the fission chamber response in pulse and current mode is firstverified, and then the main results, including fission rate dis-tributions in the reactor core, are presented and discussed.

2. JSI TRIGA reactor

2.1. Description

TRIGA research reactor at JSI is a 250 kW TRIGA Mark II reactor.It is a light water pool type reactor, cooled by natural convection.The side and top views are shown in Figs. 1 and 2.

The reactor core is of annular configuration with diameter of44.2 cm and active fuel height of 38.1 cm. As shown in Fig. 3, thereare 90 positions in the reactor core available for fuel elements,control rods, irradiation channels, etc., including two largeropenings in the D-E rings. In the metal grid above the reactor core,there are 26 additional small holes, i.e. measuring positions 8 mmand 10 mm in diameter, enabling access from above the core. Forexample, fission and ionization chambers can be inserted in any ofthese positions to measure the neutron or gamma flux at any axialposition in (and above/below) the reactor core.

In total there are four control rods (Fig. 3), however duringnormal operation only regulating (R) and compensating (C) rodsare used while the pulse (P) and safety (S) rods are always com-pletely withdrawn. As of 2014, the current excess reactivity issmaller than the integral worth of any control rod thus one controlrod is sufficient to shut the reactor down.

2.2. Measurements

2.2.1. Fission and ionization chamber designThe CEA manufactured fission chambers (FC) were designed

considering the operational constraints and the expected neutronflux in research reactors. CEA manufactured water-tight fissionchambers with integrated mineral cable (Fig. 4) (Geslot et al.,2009). Some technical characteristics of the FC are given in Table 1.Mechanical parts are purchased from the PHOTONIS France SASCompany. Fissile deposits, detectors assembly and preliminarytests performed in the CEA Cadarache facilities. The IC is me-chanically identical to the FC but with no fissile coating (Filliatreet al., 2011; Fourmentel et al., 2013).

Fission chambers are designed to be operated in the threepossible modes: pulse, Campbell and current regarding the en-countered neutron flux level and the associated signal acquisitionsystem.

CEA manufactured fission chambers are all tested and absolutelycalibrated in the MINERVE zero-power reactor at CEACadarache site. The calibration technique relies on a primarycalibration at ·SCK CEN Mol site (Belgium) and a secondarycalibration at the MINERVE reactor facility at the CEA Cadarachesite. The used 235U reference FC is calibrated with an overalluncertainty of 1.4%.

2.2.2. Experimental setupA FC containing approximately μ10 g of 98.49% enriched 235U

was used to perform axial measurements (23 axial positions) ofthe fission rate along the complete core height at radial mea-surement positions MP5 and MP8 (Fig. 3). These measurements

Fig. 1. TRIGA reactor at JSI, side view (Jeraj and Ravnik, 1999).

Fig. 2. TRIGA reactor at JSI, top view (Jeraj and Ravnik, 1999).

G. Žerovnik et al. / Applied Radiation and Isotopes 96 (2015) 27–35 29

Fig. 3. Positions (A–F) in the reactor core including the small upper grid openings –measuring positions (MP) for in-core measurements.

Table 1Mechanical and physical characteristics of the fission chamber.

MaterialsStructure Stainless SteelInsulator Al2O3

Fissile deposit (mass) 98.49% enriched 235U (∼ μ10 g)Filling gas (pressure) Argon 96% Nitrogen 4% (500 kPa)

Detector dimensionsNominal diameter 3 mmDetector length ∼55 mmSensitive length ∼4 mm

Integrated cableNomenclature Coaxial mineral insulated cable

(Copper core and a double copper/SS sheath)Outer diameter 2.2 mmLength 20 mImpedance Ω50Connector HNReference THERMOCOAX 1CCAcSi22 Ω50

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are used for verification and validation of our computationalmodel as well.

The FCs were deployed into the reactor core by using a speciallydesigned FC positioning system, composed of Al guide tubes, drivemechanism and data acquisition system. Fig. 5 shows a schematicview of the system where FC integrated cable is also used for in-serting and withdrawing the FCs into and out of the reactor core.The FC position was regulated by a commercially available pneu-matic drive consisting of a series of valves and pistons, all con-trolled by a micro controller. The axial positioning was ensured byan incremental system which measures the FC position relative tothe reference position at the end of the guide tube. The accuracy ofthe FC positioning system was ∼0.1 mm and the repeatability ofthe FC position was within 0.3 mm.

2.2.3. Measurements in the JSI TRIGA reactor coreWhen FCs are used in current mode, additionally to the (mostly

thermal) neutrons the signal includes contributions of bothprompt and delayed gamma flux. Most of the delayed gamma fluxcan be subtracted by zero-power measurement. Namely, the

Fig. 4. Schematic view of a PHOTONIS CFPR fiss

contributions of delayed gammas originating from relativelylong-lived precursors do not change significantly with reactorpower while the prompt gamma flux is proportional to reactorpower.

In pulse mode, the signals resulting from gammas are dis-criminated thus measuring almost exclusively neutrons. However,due to dead-time limitations, the FC may not operate in pulsemode at higher powers. For the measurements in the centre of thecore, the upper limit for using the pulse mode the reactor power isorder of 1 kW.

Ionization chamber (IC) with exactly the same geometry andmaterial composition as the fission chamber has been used todetermine also the prompt gamma response. The difference of theFC and IC signals should be sensitive only to neutrons. The pro-blem of this combination is that it is required to perform the FCand IC measurements sequentially. Possible positions for onlinemeasurements would be in irradiation channels, however they areused for different purposes. For higher reactor powers the non-linear gamma contribution at a fixed position becomes negligible.

Because of (almost) symmetry, the variations due to control rodmovement in normal operation mode, when compensating(C) and regulating (R) rod are partially inserted, are the smallest inthe central plane perpendicular to the R and C rods (Fig. 3).Measuring positions MP5 and MP8 have been chosen due tohigher flux levels in the centre of the core.

All measurements were performed relative to the reactor on-line power monitoring signal P, which is performed with a com-pensating ionization chamber on the so-called linear channel. For

ion chamber assembled at CEA Cadarache.

Fig. 5. Schematic figure of the FC positioning system (left). Side view of the fuel element, FC positioning system (right).

G. Žerovnik et al. / Applied Radiation and Isotopes 96 (2015) 27–35 31

fixed positioning of the control rods, this signal is assumed to bereliable and is therefore taken as a reference. On the other hand,when comparing FC signals for different control rod positions, thepower signal P is corrected using MCNP calculated neutron fluxredistribution factors (Podvratnik et al., 2011).

2.3. Computational model

A detailed neutron spectrum and spatial flux distribution cal-culation has been performed using Monte Carlo neutron transportcode MCNP5 (X-5 Monte Carlo Team, 2004) and ENDF/B-VII.0

(Chadwick and et al., 2006) cross section library. A detailed 3Dmodel of the JSI TRIGA reactor, including core, graphite reflector,and all irradiation channels, has been developed in order to ac-curately calculate physical parameters of the TRIGA reactor. Ourmodel is based on the criticality benchmark model, which isthoroughly described by Jeraj and Ravnik (1999) and published inthe International Handbook of Evaluated Criticality Safety Ex-periments (ICSBEP, 2009). The main advantage of our computa-tional model is that it has been thoroughly verified and validatedwith experiments for calculations of the effective multiplicationfactor keff (Ravnik and Jeraj, 2003), power peaking factors (Snoj

Fig. 7. Measured fission chamber count rate C as a function of reactor power P(points) and fitted dead-time corrected response function (line). FC located inmeasuring position MP8 slightly above the centre of the active fuel height

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and Ravnik, 2008), reactor kinetic parameters (Snoj et al., 2010),flux and reaction rate distributions (Snoj et al., 2011), or spectraand other reactor parameter calculations.

For the calculations presented in this paper, track length neu-tron flux estimators were used for neutron flux and reaction ratescalculations. The actual calculations of neutron fluxes and reactionrates at certain position were made by using the “meshtally” op-tion in MCNP, where a predefined region is divided into a mesh ofcell. Then the neutron flux and reaction rates is calculated in eachof the cells in a mesh. The fission chambers have not been mod-elled explicitly since it has been assumed that the relative dis-turbance of the signal due to the neutron interactions with thefission chamber and its guide tube is independent of the FC po-sition. Previously, the perturbation of the thermal neutron flux dueto FC and guide tube has been estimated to be of the order of 2%(Podvratnik et al., 2011). Self-shielding in the active part of thefission chamber is negligible. 7.5�109 active neutron histories andcorresponding number of secondary gamma particle histories havebeen simulated for each calculation.

(301.8 mm above the lowest axial point).

3. Results

3.1. Power range determination of the fission chamber

One of the objectives of this investigation is also the determi-nation of the power ranges, where the signal from the fissionchamber is accurate and reliable. For the future, reactor powermeasurements with multiple fission chambers positioned at dif-ferent locations in the reactor core, are considered (Žerovnik et al.,2013). Using an optimized system with multiple detectors, theaccuracy of the online power monitoring system can be sig-nificantly improved compared to the existing system using a singleex-core large compensating ionization chamber. Secondly, themultiple-detector system could be used for multiple-point-reactorkinetics studies. With these far-lying objectives in view, the de-tector responses in both current mode (also applicable to ioniza-tion chamber) and pulse mode were studied.

3.1.1. Current modeIn current mode, the operation power range of the FC is limited

by the (non-linear) contribution of delayed gammas. In order todetermine the lower limit, linearity of the FC response in currentmode has been studied. The FC response has been measured at

Fig. 6. Measured fission chamber current I as a function of reactor power P (points)and fitted background corrected response function (line). FC located in measuringposition MP26 approximately in the centre of the active fuel height.

measuring position MP26 approximately in the centre of the activefuel height. Fig. 6 reveals almost perfect linearity of the signaldown to around 10 W under the condition of well-known zero-power background contribution.

Consequently the current mode may be used from about 10 Win combination of background determination before start-up un-der condition that the reactor did not operate at high power(above ∼10 kW) in the last couple of 10 s of hours. In case reactorrecently operated at higher powers, the current mode is com-pletely reliable only above ∼1 kW.

3.1.2. Pulse modeIn pulse mode, the operation power range of the FC is limited

on the lower side by count rate and statistics requirement, and onthe upper side by the electronics setup performances (dead-time,etc.).

The approximate count rate of the FC in the centre of themeasuring position MP8 is 260 s�1W�1. For example, if the targetaccuracy is 1% in 1 s, the lower operation limit for FC approxi-mately 40 W. For lower powers, more sensitive fission chamberswith higher fissile material mass would have to be used.

Fig. 7 shows the response of the FC (in count rate C) in pulsemode as a function of reactor power. In the centre of the mea-suring position MP8 (similarly for MP5) the deviation from line-arity becomes significant at around 20 W. The dead-time correctedcount rate Ccorr can be expressed as

=−

CCt C1

,(1)

corrDT

where tDT is the dead-time of the electronics and C is the measuredcount rate.

Assuming proportionality of the corrected count rate Ccorr tothe reactor power P, the dead-time corrected function has beenfitted (Fig. 7) to the data, and the fitted dead-time of the electronicsetup was = μ ± μt 4.69 s 0.04 sDT . The uncertainty of the dead-timehas been obtained by random sampling of the measured countrate within their uncertainties assuming Poisson statistics andperforming non-linear fit of tDT for each sample.

However, at higher powers the signal starts to significantlydeviate even from the standard dead-time corrected function(Fig. 7). We assume that at higher count rates the pulses start tosuperimpose, i.e. they are piling up which results in the unavail-ability of the system to take into account new coming pulses

Fig. 8. Measured fission chamber count rate C (black) and dead-time correctedcount rate Ccorr (red) as a function of reactor power P (points) and correspondingfitted dead-time corrected and linear response functions (dashed and solid line,respectively). FC located in measuring position MP8 slightly above the centre of theactive fuel height (301.8 mm above the lowest axial point). (For interpretation ofthe references to color in this figure caption, the reader is referred to the webversion of this paper.)

Fig. 10. Comparison of the calculated (solid curve) axial prompt gamma flux dis-tribution with the measured total gamma flux distributions at different reactorpowers (dashed and dotted curves). Measuring position MP5. All distributions arenormalized to the variance-weighted average over the total axial range.

G. Žerovnik et al. / Applied Radiation and Isotopes 96 (2015) 27–35 33

during this extended dead-time. This can also be interpreted as an“effective dead-time” increase at higher power (above ∼1 kW).

By applying dead-time correction, the count rate linearity isextended to significantly higher powers (Fig. 8). The linearity ofthe dead-time corrected count rate is within ∼1% up to 300 W.

3.2. Gamma background at zero power

When the reactor was in shutdown condition, the axial dis-tribution of the gamma background has been measured in positionMP5 using both the fission and ionization chambers (Fig. 9). Insuch state, the response of the FC and IC was expected to be equalin first approximation due to the negligible contribution of theneutrons and the same geometry of both detectors. Indeed theshape of the axial distributions is practically equal, howeverslightly shifted. The difference in “zero current” of approximately0.1 nA is presumably due to a different current offset (i.e. differentleaking currents) between the two detectors due to different in-sulating resistances ( Ω1.6 T for FC and Ω28 T for IC).

Fig. 9. Measured fission chamber (black) ionization chamber (green) currents I as afunction of axial distance z from the reference point in measuring position MP5.(For interpretation of the references to color in this figure caption, the reader isreferred to the web version of this paper.)

Due to equal sensitivity to photons, the pure sensitivity of theFC to neutrons can be estimated by subtracting the response of theIC from the measured FC response and taking into account the“zero current” correction.

3.3. Axial gamma and neutron flux distributions

MCNP is capable of simulating the transport of prompt gammarays, only. The axial distribution of the delayed gamma ray flux(originating from the decays of fission and activation products) isdifferent, however it roughly follows the same (cosine) shape.Therefore, at higher powers (above 10 kW) the total gamma dis-tribution (measured by IC) becomes similar to the prompt gammadistribution (MCNP calculated) even though the delayed gammacontribution remains in the order of a few %. This effect is shownin Fig. 10.

Axial distributions of the fission reaction rates were measuredusing CEA manufactured fission chambers at the measuring posi-tions MP5 and MP8 in the JSI TRIGA reactor core in current andpulse mode.

Fig. 11 shows the axial fission rate distribution in measuringpositions MP5 and MP8 for fully withdrawn compensating rod andregulating rod at the critical position (around step 580). The axialdistribution in current mode systematically deviates from thecalculated axial distribution due to contribution of the gamma rayswhich are not taken into account in the simulation. The agreementbetween the calculated and measured distributions in pulse modeis generally relatively good, especially taking into account the ageof the JSI TRIGA reactor and relatively scarce information onreactor geometry and material composition. This result furthervalidates the computational model and will serve as a basis forfurther applications such as the characterization of the neutronand gamma fields thermal column and dry chamber which can beused for neutron irradiation of larger objects and will improve theoverall utilization of the reactor.

In the next step, the sensitivity of the fission chamber responseto control rod movement will be determined (both experimentallyand by simulation) and the feasibility of installation of a moreaccurate online power monitoring system will be studied.

Fig. 11. Relative 235U fission rates as a function of axial position z of the fission chamber in measuring positions MP5 (left) and MP8 (right). In MP5, the measurements wereperformed both in current and pulse mode of the fission chamber. The discrepancy of the current mode is due to a significant contribution of photons to the measuredcurrent. All distributions are normalized to the variance-weighted average over the total axial range.

G. Žerovnik et al. / Applied Radiation and Isotopes 96 (2015) 27–3534

4. Conclusions

Linearity of the CEA manufactured miniature fission chamberresponse has been confirmed in the current and pulse mode, andsuitable power intervals in the chosen measurement positions ofthe JSI TRIGA reactor core have been determined. Neutron andgamma flux distributions have been determined in the JSI TRIGAreactor core. The measured axial gamma field and fission ratedistributions have been compared with MCNP calculated dis-tributions. In general, the agreement is very good providing ad-ditional validation of the existing Monte Carlo model of the TRIGAreactor.

An online power monitoring system using multiple in-corefission chambers, which have been studied in this work, is con-sidered at the JSI TRIGA reactor. In order to achieve this goal, theresponse of the detectors as a function of control rod movementwill be studied in immediate future in the frame a new bilateralcollaboration project with CEA Cadarache.. Simultaneous mea-surements with multiple identical fission chambers will be per-formed using a CEA recently developed FC acquisition systemoperating simultaneously in pulse and Campbell mode over a widereactor power range (Barbot et al., 2013; Thévenin et al., 2014).Finally, the system will be adapted and constructed for routineoperation in the JSI TRIGA reactor.

Acknowledgments

The research is funded by the bilateral project “Experimentalverification of neutron flux form factors and Qualification of a newwide range multichannel neutron instrumentation” between theMinistry of education, science and sport of the Republic of Sloveniaand Commissariat à l’énergie atomique et aux énergies alter-natives (CEA) under contract number Q2-0012 1000-13-0106.

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