Correction methods for pulsed neutron source reactivity measurement in accelerator driven systems

Transcription

1 NUKLEONIKA 2013;58(2): ORIGINAL PAPER Correction methods for pulsed neutron source rectivity mesurement in ccelertor driven systems Pweł Gjd, Jerzy Jnczyszyn, Włdysłw Pohorecki Astrct. Importnt issue in the perspective of nucler energy development in the ner future is the prtitioning nd trnsmuttion (P&T) of spent nucler fuel. For yers the Europen Commission (EC) hs sponsored this scientific ctivity through the Frmework Progrmmes (FP). One of the milestones for P&T is the development of ccelertor driven systems (ADS). Extensive reserch in this field ws crried out within the EUROTRANS project of 6th FP of EURATOM. Prt of this reserch ws devoted to testing nd development of rectivity monitoring techniques in ADS. This pper concerns the methods of the rectivity mesurement using the pulsed neutron source (PNS). Due to the fct tht sic methods devoted to determine the core rectivity re derived from point kinetics, while rel sucriticl core kinetics differs from this model, there is need to improve these methods in order to del with the oserved sptil effects. There re severl wys to mke these methods work properly nd finlly it should e possile to chieve this. However, they still need vlidtion which is supposed to e done within next FP project FREYA. Key words: ccelertor driven systems (ADS) EUROTRANS mesurement rectivity YALINA Introduction P. Gjd, J. Jnczyszyn, W. Pohorecki AGH University of Science nd Technology, Fculty of Energy nd Fuels, l. A. Mickiewicz 30, Krkow, Polnd, Tel.: , Fx: , E-mil: [email protected] Received: 21 June 2011 Accepted: 2 April 2013 The interest in prtitioning nd trnsmuttion (P&T) of nucler wste hs grown in mny countries over lst two decdes with expecttion tht P&T technologies will llow to reduce the need of geologicl disposl of spent fuel nd high-level wste. The three min resons for pplying P&T technologies re reduced mount of wste, reduced time of storge (since the plutonium nd minor ctinides re minly responsile for the long-term rdiotoxicity) nd etter use of nucler fuel. Although, the first stge of trnsmuttion is possile nd eing done in light wter rectors y reusing plutonium from the spent fuel in the MOX (mixed oxides) fuel, different pproch is required for urning lso the minor ctinides in order to close the fuel cycle. Severl rector technology concepts re, therefore, proposed for the minor ctinide urning. First of ll, fst rectors re considered s relevnt tool due to the fct tht urning-to-production rtio of minor ctinides is more dvntgeous in fst neutron spectr. However, nother importnt issue limiting minor ctinides content in the fuel is their smller delyed neutron frction thn in clssic urnium nd MOX fuel. The solution is coupling of the sucriticl rector with proton ccelertor nd splltion neutron source in the so-clled ccelertor driven systems (ADS). This dditionl neutron source llows operting the rector in sucriticl stte nd mkes its distnce to super-prompt-criticlity

2 288 P. Gjd, J. Jnczyszyn, W. Pohorecki independent of the fuel isotopic composition. As the result, we re not only mking the closed fuel cycle. For ADS, we lso reduce to out 10% the needed shre of ctinide urners from out 35% of instlled power in nucler power plnts when using fst rectors for minor ctinide urning [5]. It is n importnt dvntge due to the fct tht dedicted ctinide urners re expected to e more expensive thn clssic wter rectors. The EUROTRANS project Different spects of ADS development were studied within the extensive reserch clled EUROpen Reserch Progrmme for the TRANSmuttion of High Level Nucler Wste in n Accelertor Driven System (EUROTRANS). It strted in 2005 within the 6th Frmework Progrmme of EURATOM nd lsted until The project consortium ws constituted y 28 prtners from 14 countries including universities, ntionl reserch orgniztions nd industril compnies with the Krlsruhe Institute of Technology s the project coordintor. It lso included wide coopertion with institutes from non-eu countries [4]. The min long term gol ws to crry out first dvnced design of experimentl trnsmuttion fcility to demonstrte the technicl fesiility of trnsmuttion in ADS. In order to do this wide spectre of topics were nlysed. Among the other spects of the project its domin ECATS (experiments on the coupling of n ccelertor, splltion trget nd sucriticl lnket) ws devoted to exmine some unique fetures of sucriticl core kinetics including development nd testing of rectivity monitoring techniques. An experimentl progrmme ws crried out in the Joint Institute of Power nd Nucler Reserch (JIPNR) in Minsk using the Ylin sucriticl ssemly [4]. Vrious uthors took prt in the preprtion nd performing experiments nd nlysis of the experimentl dt. Tht includes neutronic clcultions to simulte the experiments, nlysis of oserved sptil effects nd development of methods for correction of the rectivity vlues from experiment. Similr reserch ctivities re plnned within consecutive reserch in the 7th Frmework Progrmme, i.e. in FREYA project (fst rectors experiments for hyrid pplictions). These experiments, in which AGH University of Science nd Technology (AGH-UST, Krków) lso tkes prt, include similr rectivity mesurements using GUINEVERE rector. Design of this rector ws lso prt of EUROTRANS project. 1. During power opertion: Current-to-power: sed on the proportionlity etween rectivity nd the current-to-power (or current-to-flux) rtio; Bem trips technique: sed on the dependency of the neutron flux evolution fter short interruption of the em on the rectivity; Noise technique; sed on sttisticl properties of the fission chins. 2. During loding nd strt-up opertion: PNS technique: sed on the kinetic response off the neutron flux fter series of periodicl neutron pulses; Noise technique. All of the methods mentioned ove were tested during the experiments in the Ylin fcility. Some of them (em trips, current-to-power) for the first time. Aprt from the uthors involved, the experimentl tem included lso specilists from Helmholtz-Zentrum Dresden-Rossendorf, Krlsruhe Institute of Technology, Joint Institute of Power nd Nucler Reserch in Minsk, Reserch Centre for Energy, Environment nd Technology (CIEMAT, Mdrid) nd Royl Institute of Technology (KTH, Stockholm). Ylin fcility The Ylin-ooster sucriticl fcility is plced in the Joint Institute of Power nd Nucler Reserch in Minsk, Belrus. It s sucriticl zero-power rector divided into fst nd therml zone surrounded y grphite reflector. The fst zone consist of highly enriched metllic urnium fuel pins in led mtrix, while in the therml one less enriched fuel in polyethylene moderting mtrix is used (see Tle 1). Both zones re seprted with B 4 F vlve zone to prevent therml neutrons to reenter the fst zone. The cross-section of Ylin ooster showing the positions of the experimentl chnnels for detector plcement is shown in Fig. 1. The core is coupled with n NG-12-1 neutron genertor which uses D-T rection neutron source. It Rectivity monitoring techniques in ADS Due to the fct tht ccelertor driven systems will e operted in sucriticl mode nd for sfety resons the current core rectivity must e known. The development of dequte rectivity monitoring techniques is one of the key points for ADS development. Becuse they re supposed to never pproch criticlity ny techniques sed on control rod re indequte. The following techniques re, therefore, proposed for tht purpose [7]: Fig. 1. Ylin-ooster core cross-section.

3 Correction methods for pulsed neutron source rectivity mesurement in ccelertor driven systems 289 Tle 1. Ylin-ooster core configurtions pplied in mesurements Zone Inner ooster Outer ooster Therml zone k eff (MCNP) Enrichment 90% 36% 36% 10% SC SC SC SC cn e operted in oth pulsed nd continuous mode. When using the continuous mode short interruptions of the em (em trips) re lso possile. Different core loding ptterns re possile to otin different core rectivities. Core configurtions used in the experiments re shown in Tle 1 where the expected rectivity vlues clculted with MCNPX code re lso given. It is worth to notice tht SC3 nd SC3 hve the sme rectivity otined y using different loding ptterns. Slight rectivity chnges cn e lso done y moving the control rods plced in therml region. During the pulsed neutron source (PNS) experiments, the neutron genertor ws operted in the pulsed mode to generte 5 μs long pulses with repetition rte etween Hz depending on the core configurtion (respectively 50 for SC0 nd SC3, 56 for SC3 nd 166 for SC6). He-3 detectors nd fission chmers (1 mg nd 500 mg U-235) were used to mesure the time evolution of the neutron flux in the rector core during the consecutive pulses. A liquid scintilltion counter ws pplied for monitoring of the neutron source intensity. Mesurements for ll core configurtions were done oth with control rods inserted nd extrcted. Mesurement methods Two sic methods pplied to otin the vlue of core rectivity were: the Sjöstrnd method (lso clled re method) nd the prompt decy constnt fitting method. Theoreticl core response to the neutron pulse injection is shown in Fig. 2. Its shpe depends on severl core prmeters, mong them on the rectivity. At the eginning, we cn oserve the rise of the neutron flux fter the pulse, then the exponentil decy nd t the end the ckground in which core is eing fed y delyed neutrons. The prompt decy constnt method relies on fitting the decy constnt of prompt neutron popultion (α) to the prt of the core response where exponentil decy is oserved. Rectivity (ρ) is then given y [3] ρ α Λ (1) = + 1 β β The vlues of delyed neutron frction (β) nd neutron genertion time (Λ) must e known vlues nd for experiments they cn e clculted using MCNP code for ech core configurtion. The Sjöstrnd method is n old method proposed y N. G. Sjöstrnd in 1956 to mesure negtive rectivities. It is sed on the fct tht the rtio of prompt (F p )-to-delyed neutron re (F d ) is dependent on the rectivity nd the delyed neutron frction. The vlue of the rectivity normlized to β is, therefore, given y the simple reltion [6] ρ Fp (2) = β Fd This method does not require ny dditionl dt to otin such vlue from the experiment. Preliminry results Exmple of kinetic response mesured in the core fter the neutron pulse injection is shown in Fig. 3. It is showing signl from detectors plced in different core regions (fst zone EC1B, therml zone EC6T nd reflector MC2). We cn oserve some differences in the neutron flux ehviour depending on the prt of the core where the detector ws plced. Significnt differences occur in the very eginning of the pulse. Afterwrds, prompt decy slopes cn e oserved nd then the delyed neutron ckground. As expected, slow decy of the delyed neutron level ws oserved ut its influence on the results ws negligile thus it Fig. 2. Neutron flux fter pulse [3]. Fig. 3. Exmple results plot SC3, control rods inserted (CRI).

4 290 P. Gjd, J. Jnczyszyn, W. Pohorecki Tle 2. Results for prompt decy constnt method SC3 CRE CRI α (s 1 ) ρ ($) α (s 1 ) ρ ($) EC1B 1014 ± ± ± ± 0.29 Booster zone EC2B 1108 ± ± 0.33 EC3B 1091 ± ± 0.23 Therml zone EC5T 1029 ± ± ± ± 0.46 EC6T 1060 ± ± 0.21 Reflector MC2 985 ± ± ± ± 0.26 MC3 901 ± ± ± ± 0.37 CRE control rods extrcted. Tle 3. Results for prompt decy constnt method SC3 Booster zone Therml zone CRE CRI α (s 1 ) ρ ($) α (s 1 ) ρ ($) EC1B 1036 ± ± 0.13 EC2B 1029 ± ± ± ± 0.15 EC3B 1039 ± ± 0.15 EC5T 1032 ± ± 0.13 EC6T 1030 ± ± ± ± 0.17 Reflector MC2 983 ± ± ± ± 0.14 CRE control rods extrcted. Tle 4. Results for Sjöstrnd (re) method SC3 SC3 CRI ρ ($) CRE ρ ($) CRI ρ ($) CRE ρ ($) EC1B ± ± ± 0.31 Booster zone EC2B ± ± ± 0.43 EC3B ± ± 0.18 Therml zone EC5T ± ± ± 0.93 EC6T 7.63 ± ± ± 1.21 Reflector MC ± ± ± ± 0.16 MC ± ± 1.79 CRE control rods extrcted. cn e considered constnt in ny further clcultions. It should e noticed tht despite eing fst-therml coupled ssemly, the core kinetics is clerly dominted y the therml prt. For exmple, such prmeter like neutron genertion lifetime (~60 μs) [1] resemles rther therml rectors. The results for core configurtions SC3 nd SC3 re shown in Tles 2 4. In those preliminry clcultions only the detector ded time correction ws pplied. The vlues of the neutron lifetime nd delyed neutron frctions for different core configurtions necessry to otin rectivity vlues from prompt decy constnt were clculted using MCNPX 2.7 code nd JENDL 2.2 nucler dt lirries. The rectivity vlues used for comprison with experimentl results were clculted in the sme wy. It is clerly visile from the results in Tle 4 tht the mesured vlue of rectivity strongly depends on the respective detector position. This sptil distriution of mesured vlues with comprison to MCNPX simulted rectivity vlue is shown in Fig. 4. It is cused y the fct tht ll methods of clculting the rectivity re derived from the point kinetic equtions while ehviour of the Rectivity [$] EC1B EC2B EC3B EC5T EC6T Prompt decy const. Are method MCNP Fig. 4. Sptil distriution of mesured rectivity vlues SC3, control rods extrcted (CRE). MC2

5 Correction methods for pulsed neutron source rectivity mesurement in ccelertor driven systems 291 Tle 5. Deep-sucriticl configurtion SC6 results (with control rods extrcted) Prompt decy constnt method Are method MCNP α (s 1 ) ρ ($) ρ ($) ρ ($) EC2B 2598 ± ± ± 2.67 EC5T 2603 ± ± ± ± 0.37 MC ± ± ± 0.78 rector core coupled with neutron source plced in its centre is sptilly dependent. When the prompt decy constnt method is used, the vlues from detectors plced in the core re differing just sutly. They lso fit the simultion vlue within the rnge of uncertinties. Both these fcts suggest tht they cn e considered correct. However, the vlues otined from detectors plced in the reflector re noticely lower. It is pprent especilly in the results from deep-sucriticl core configurtion SC6 shown in Tle 5. One of the resons cn e the longer neutron lifetime expected in the reflector which cuses tht the oserved decy is slower. However, it ffects strongly only the detectors in the reflector thus mesurements mde in the core cn e recognized s relile. The results otined y the Sjöstrnd method re strongly ffected y sptil effects nd none of the single detector vlues should e considered relile. The rectivity vlues otined from detectors plced in different core regions for the sme core configurtions differ more thn y fctor of two. Appliction of this method requires, therefore, correcting of the errors introduced y sptil effects. Sptil effects correction Development of possile corrections or lterntive wys of the core rectivity clcultion ecme the next gol of the reserch. The first ttempt ws mde y clculting the correction fctors for ech detector position using MCNPX code to simulte entire PNS experiment. Such fctor is ssumed s the reltion etween the expected vlue of mesured rectivity in the position where the detector is plced (respective to the kinetic response of the core in tht region) nd the simulted glol rectivity of the core Ap, MCNP ρlocl, MCNP Ad, MCNP (3) c.f. = = ρ ρ glol, MCNP glol, MCNP It ws expected tht the sme reltion should occur etween experimentl vlues thus the corrected vlue of rectivity is given y ρmesured (4) ρ= c.f. Correction fctors were clculted using MCNPX 2.7 code nd JENDL 2.2 nucler dt lirries. Wht is importnt, the use of different dt lirries (such s JENDL 3.1, JEFF 3.1 nd ENDFB/VIII.0) did not hve significnt influence on the correction fctors vlue [7]. However, importnt disdvntge of this method lys in the need of mking computer simultions for the support of mesurement. Correction fctors depend not only on the detector position ut lso on core configurtion nd possily on other core prmeters (i.e. temperture). The use of this method for rel-time rectivity monitoring is, therefore, limited. Another pproch to correct the results ws sed only on experimentl dt nd moreover did not require ny dditionl simultions. In Fig. 5 the neutron source pulse nd the response of the detectors plced in ooster zone t the very eginning of the pulse is shown. One cn oserve there huge pek of the signl, so-clled prompt hrmonic effect. For these detectors, even hlf of the detected neutrons re locted in this erly prt of the pulse nd it strongly ffects the finl results. The shpes of the very eginning of the detector response nd the signl from the source neutrons monitor look very similr. It indictes tht the source neutrons ffect results in the ooster re. The proposed method of the correction relies on sutrcting the source signl from the detector signl fter normliztion. This normliztion cn e done y compring the source monitor nd the detector signl t the sme time point where source signl reches its mximum or t oth signls mxim. Simplified method ssumes skipping the detector dt from the eginning up to the point where the source influence is considered significnt. However, for ll methods used the results fter correction were similr for ll detectors positions (see Tle 6). The lst method used for evlution of the experimentl results ws using the Gozni method insted of the Sjöstrnd one. It is lso sed on comprison of prompt nd delyed neutron fields ut it uses extrpoltion sed on the prompt decy constnt to determine the prompt neutron re nd is not ffected y the prompt hrmonic effect. Rectivity is there given y (Fig. 2) [4] Fig. 5. Source monitor nd ooster zone detectors signls.

6 292 P. Gjd, J. Jnczyszyn, W. Pohorecki Tle 6. Corrected results of mesured rectivity (in $) SC3 CRI Correction method EC1B EC2B EC3B EC5T EC6T MC2 MC3 MCNP c.f ± ± ± ± ± ± ± 1.73 Source 8.48 ± ± ± ± ± ± ± 1.64 Source (norm. mx. to mx.) 8.40 ± ± ± ± ± ± ± 1.58 Source (skipping dt 8.45 ± 0.26 from eginning) 8.44 ± ± ± ± ± ± 1.66 Gozni 8.26 ± ± ± ± ± ± ± 2.71 Tle 7. Corrected results summry SC3 SC3 Method CRI CRE CRI CRE Prompt decy constnt 8.51 ± ± ± ± 0.15 Sjöstrnd CRE control rods extrcted. ρ N0 R (5) = β α Nd where R is the frequency of neutron pulses. However, the vlue of the extrpolted strting point (N 0 ) is strongly dependent on the dely of the core response in the position of the detector compring to the neutron pulse. This effect ecomes the stronger the frther detector is plced nd is limiting the use of this method to detectors plced in ooster re. The dely is there reltively smll nd the dt from those detectors need this correction most. Results fter correction Uncorrected ± ± ± ± 0.34 MCNP c.f ± ± ± ± 0.36 Source 8.71 ± ± ± ± 0.27 Source (mx. to mx.) 8.65 ± ± ± ± 0.27 Source (simpl.) 8.62 ± ± ± ± 0.27 Gozni ± ± ± ± 0.41 Gozni/Sjöstrnd 8.33 ± ± ± ± 0.33 Simultion vlue 7.86 ± ± 0.11 The results of the mesurement fter pplying different methods of corrections re listed in Tle 6 (for core configurtion SC3 with control rods inserted). It is clerly visile tht compring to the preliminry results (Tle 2) vlues re not ffected y sptil effects nymore. Slight differences etween detectors re still oserved, however they re cused rther y sttisticl dispersion thn y ny systemtic effect. Only exception is the Gozni method when pplied to the therml zone nd reflector detectors due to core response dely. The differences etween methods of sutrcting the source monitor signl re negligile. However, detectors with smll overll numer of counts nd therefore poor sttistics, s for this core configurtion EC5T nd MC3, re still giving doutful results, significntly different from the others. In this cse the difference cn e considered not s the effect of wrong method used, especilly if other detectors in this region re giving results similr to the verge vlue. It should e lso noted tht the correction method sed on MCNP clculted correction fctors gives us systemticlly higher vlues thn other methods. Comprison of the results from core configurtions SC3 nd SC3 otined y different methods fter clculting weighted verge over the detectors is shown in Tle 7 nd Figs. 6 nd 7. The MCNP simultion vlues re lso shown for comprison, ut they should not e tken s the reference vlues, due to the possile smll differences etween core specifictions used for MCNP input file nd the rel core prmeters. For exmple, even smll difference in polyethylene density used s mtrix for therml zone cn hve slight, ut not negligile, influence on the simultion result. It would ffect not only the rectivity vlue ut lso the correction fctors clculted y MCNP nd it cn explin why the results otined using the correction fctors Rect. [$] Slope Gozni Sjostrnd MCNP c.f. Source Fig. 6. Methods comprison SC3. Source mx CRE CRI MCNP CRE Source simpl. Goz/Sjo

7 Correction methods for pulsed neutron source rectivity mesurement in ccelertor driven systems 293 Rect. [$] re slightly, ut systemticlly, different from the other correction methods. Additionlly to Gozni method, the mixed Sjöstrnd/Gozni ws used to clculte the verge using Gozni method results from ooster zone detectors nd uncorrected Sjöstrnd method results from the others. After pplying the corrections, the vlues of rectivity otined using different methods fit ech other nd the simultion vlue within the rnge of uncertinties. However, slight difference etween the results, depending on the method used, is still present. Conclusions Slope Sjostrnd Gozni MCNP c.f. Source mx Source -16 Fig. 7. Methods comprison SC3. Goz/Sjo Source simpl. CRE CRI MCNP CRE Despite the fct tht the sucriticl core kinetics differ from the point kinetic eqution which ws used to derive sic mesurement methods their usge is still possile. However, their reliility for rectivity monitoring depends on the possiility of pplying proper modifictions or corrections to these methods. The experimentl results hve shown tht we re le to get rid of sptil effects nd get vlues tht re consistent independently of the detector position or the clcultion method. Eventully it ws possile to otin the ctul core rectivity from the mesurement so the min gol of the reserch ws chieved. However, we hve to keep in mind tht the Ylin core ehviour is dominted y therml neutrons wht differs it from the future ccelertor driven systems which re designed to work in fst neutron spectrum. Bsing on the results of the erlier MUSE project, it is expected tht this fct will hve vitl influence on rectivity monitoring methods [2] nd this hs to e put in reserch s one of the the min gols. Other spects tht re to e studied re the influence of neutron source position nd reflector position nd mteril composition on neutron spectrum time evolution nd sptil distriution nd, therefore, on the rectivity mesurement. Such work is foreseen to e prt of FREYA project long with development of other spects of rectivity mesurement in ADS. It is supposed to crete complex rectivity monitoring system for further ppliction. Acknowledgment. We pprecite the efforts nd support of ll the scientists nd institutions involved in EUROTRANS nd the present work. Finncil support y the Europen Commission (through the contrct FI6W- -CT ) nd y the Polish Ministry of Science nd Higher Eduction is grtefully cknowledged. References 1. Bécres V, Fernndez-Ordones M, Villmrin D et l. (2010) Rectivity monitoring with imposed em trips nd pulsed mode detectors in the sucriticl experiment YALINA-ooster. In: Proc of the Interntionl Topicl Meeting on Nucler Reserch Appliction nd Utiliztion of Accelertors 4 8 My 2009, Vienn. IAEA Proceedings Series CN-173, ADS/P Crt M (2004) Rectivity ssessment nd sptil time- -effects from the MUSE kinetics experiments. In: Proc of PHYSOR 2004 The Physics of Fuel Cycles nd Advnced Nucler Systems: Glol Developments, April 2004, Chicgo, USA 3. Kiełkiewicz M (1990) Mesurements in nucler rectors. Wydwnictw Nukowo-Techniczne, Wrsw (in Polish) 4. Kneel JU (2010) The EUROTRANS Project: Some thoughts nd perspectives. NEA/OECD Technology nd Components of Accelertor Driven Systems. Krlsruhe, Germny 5. NEA/OECD (2002) Accelertor Driven Systems (ADS) nd Fst Rectors (FR) in dvnced nucler fuel cycles: A comprtive study. Nucler Energy Agency, Pris, Frnce 6. Sjöstrnd NG (1956) Mesurement on sucriticl rector using pulsed neutron source. Arkiv för fysik 13: Villmrin D, Becres V, Berglöf C et l. (2010) Experimentl vlidtion of the industril ADS rectivity monitoring using the YALINA-ooster sucriticl ssemly. NEA/OECD Technology nd Components of Accelertor Driven Systems, Krlsruhe, Germny