PWR Core Response to Boron Dilution Transient

Transcription

1 Interntionl Conference Nucler Energy for New Europe 2003 Portorož, Sloveni, September 8-11, ABSTACT PW Core esponse to Boron Dilution Trnsient Angelo Lo Nigro, Frncesco D Auri Dept. of Mechnicl, Nucler nd Production Engineering University of Pis, vi Diotislvi 2, Pis, Itly ngelo.lonigro@rin.org, f.duri@ing.unipi.it This pper illustrtes the steps followed in order to set up tool (composed of plnt model nd of procedure) tht llows ccounting for boron rectivity feedbcks during plnt trnsients. The procedure tht hs been developed llows to find out the vlues of the boron feedbck coefficients, given the differentil boron worth, nd to properly initilize the Therml Hydrulic nd the Neutronic (TH/NEU) system. Once the tool hs been developed, it hs been used to nlyze different scenrios, resulting from deborted wter injection from the rector mke-up system. The most importnt prmeter, during this ectivity Insertion Accidents (IAs), is the Energy elesed to the Fuel (EF) nd it hs been monitored, in order to identify the situtions when the fuel might be dmged [EF > 250 kj/kg, for high burnup fuel]. The nlyses hve been performed using the ELAP5-3D computer code. The conclusion of the study is tht the limited cpbility of modeling mixing phenomen provided by most common plnt codes (such s ELAP5-3D ) is not suitble to perform BE nlyses of IAs, since those ccidents re so sensitive to boron concentrtion chnges tht the effect of uncertinties cnnot be neglected. The use of Computtionl Fluid Dynmics (CFD) codes could reduce uncertinties enough to perform BE nlyses nd thus it should be recommended. 1 INTODUCTION During the opertion of rector, core prmeters such s temperture, power level nd void frction chnge. These chnges ffect the rectivity of the core requiring compensting chnges. These compenstion chnges cn be mde by the opertor through the use of control rods or neutron bsorbers dissolved in the coolnt. One of the systems used to such extent, the rector mkeup system, supplies rector grde wter nd boric cid, in vrious combintions, to the chemicl nd volume control system nd other primry systems. It opertes in different modes nd utomticlly controls the boric cid trnsfer pumps, the rector mkeup wter pumps nd vrious vlves. Due to the mlfunctioning of some equipment, or during specific plnt conditions (such s the plnt restrt fter smll LOCA), the boron concentrtion in the rector coolnt system (CS) cn experience lrge fluctutions, which cn be very dngerous. The objective of the present reserch ctivity is to ssess the core response to rpid boron density vritions nd to chrcterise the situtions tht might cuse dmge to the fuel. 2 METHOD OF ANALYSIS All nlyses hve been performed using the digitl computer code ELAP5-3D [1]

2 ELAP5-3D Computer Code Description The ELAP5-3D, [1], is multipurpose therml hydrulic code tht, in ddition of clculting the behvior of the CS during trnsient, cn be used for wide vriety of hydrulic nd therml trnsients, in systems involving stem-wter non condensble solute fluid mixtures. It is the ltest in the series of elp5 code nd it is widely used by the interntionl scientific community. Six prtil derivtives blnce equtions for seprted stem nd liquid phses re solved; energy/conduction-het-trnsfer equtions re solved within solids. Specil models necessry for the simultion of components like pump nd seprtor or specific physicl models like criticl flow or convection het trnsfer, re embedded into the code. The most prominent ttribute distinguishing ELAP5-3D code from previous versions is the internlly fully integrted, multi-dimensionl therml-hydrulic nd kinetic modeling cpbility. The multi-dimensionl neutron kinetic model is bsed on the NESTLE code, developed t North Crolin Stte University, which solves the two or four group neutron diffusion equtions in either Crtesin or hexgonl geometry using the NEM. We ssume tht the qulifiction level of the dopted ELAP5-3D version is comprble with the elp5/mod eference Plnt The reference plnt is the TMI-1, 2772 MWe, 2 loops PW. A sketch of the rector is given in fig. 1, whilst fig. 2 provides the plnt lyout. The two Cold Legs (CL) in ech loop nd the OTSG lyout cn be observed. The nominl working condition for the plnt cn be derived from Tb. 1. The plnt sttus reltes to hot full power t the end of the cycle. Fig. 1 - Sketch lyout of the TMI-1 Plnt Fig. 1 Sketch of the TMI-1 Plnt

3 219.3 Fig. 2 Lyout of the TMI-1 Plnt QUANTITY UNITS DESIGN VALUE MODEL Core power MWt CL temperture & subcooling K / K (51.30) 564 (53) HL temperture & subcooling K / K (23.63) 592 (22) Lower plenum pressure MP Outlet plenum pressure MP CS pressure MP Totl CS flow rte kg/s Core flow rte kg/s Bypss flow rte (totl) kg/s Pressuriser level m FeedWter flow per OTSG kg/s OTSG outlet pressure MP OTSG outlet temperture K OTSG superhet K FeedWter temperture & subcooling K / K (42.12) (43.1) MCP velocity d/s Initil SG mss inventory Kg Tb. 1 elevnt initil conditions for nominl opertion of the TMI-1: design nd model vlue

4 ELAP5-3D Plnt Model Description The dopted reference plnt ELAP5-3D model hs been set up by slightly modifying previous model developed nd qulified by the University of Pis, representing ll the mjor primry, secondry nd sfety system components 1. The model is composed by the following generl zones, ech in turn composed by single components: rector pressure vessel, pressurizer nd surge line, piping nd pumps, stem genertors nd stem line, LPIS nd HPIS. The two loops of the plnt re seprtely modeled, ech including the hot leg (HL), the stem genertor, the cold legs (CLs) nd the ssocited min coolnt pumps (MCPs). Tble 2 contins the min model fetures. Model feture Number of volumes 1508 Number of hydrulic stcks in the core region 18 Number of junctions 1543 Number of prllel core bypss regions 2 Number of het structures 1879 Number of elements in ech core stck 2 26 Number of mesh points Tb. 2 Min TMI-1 model fetures 2.4 Derivtion of Boron ectivity Coefficients The tsk of this phse hs been to find out the poison concentrtion coefficients in order to rech differentil boron worth of 7 pcm/ppm 3. ELAP5-3D requires poison concentrtion coefficients for ech neutron group in ech composition of the neutronic model. 4 In order to evlute the rectivity feedbck due to boron concentrtion vrition, typicl vlues of microscopic cross section in PW hve been used 5. The mcroscopic dsorption cross section chnge due to vrition of 1 ppm in the boron concentrtion cn be evluted: Σ = N σ, where N represents the tom concentrtion (toms / cm 3 ). Given the fluid density of g/ cm 3, it is possible to obtin the vlues of Σ /. 6 ρboron The vlues obtined hve been divided by Σ (different in ech composition) nd the results represent the boron rectivity feedbck coefficients used in the ELAP5-3D input file. The differentil boron worth hs been evluted performing run with the stedy-stte option on. With this option on, the code clcultes coefficient, nmed eigenvlue, tht represent the K eff of the rector nd the totl power is defined in the input file nd it is ssumed to be constnt. Indeed, this option is usully dopted in order to initilise the TH system neglecting neutronics feedbcks. During this run, the boron concentrtion chnges (due to injection of deborted wter from the rector mkeup system), whilst ll the other quntities do not chnge (coolnt temperture, fuel temperture, voiding ). Hence the eigenvlue chnges even if power level is constnt, since the code evlutes the rectivity chnge due to the boron 1 the model developed nd qulified during the OECD/NEA/NSC Benchmrk on TMI-1 Min Stem Line Brek. The min modifictions performed re the following: - 4 ccumultors modeled (p = 26 br; liq. vol. = 33.2 m 3 ; N 2 vol. = 11 m 3 ; initil mss flow rte = 300 kg/s/ccum.) - LPIS modelled. - MCPs ssumed to trip following scrm. - convective boundry condition type for verticl bundle without cross-flow used for the fuel rods het structures (w(3) = 110, insted of 111) - stem genertor wter inventory control dded during the stedy stte 2 hydrulics, het conduction, neutronics 3 this vlue hs been considered representtive of the technology modeled (TMI-1 PW) 4 the quntity required by ELAP5-3D is ( Σ ) / ( Σ * ρboron ). 5 typicl Westinghouse PW dt used: brn for the fst neutrons group nd brn for the therml group. 6 the coefficients hve been evluted for boric cid (H 3 BO 3 ), where boron occurs with the nturl isotopic distribution (19% 10 B 81% 11 B)

5 219.5 concentrtion chnge. The differentil boron worth obtined using the coefficients evlute s.m. is 6.5 pcm / ppm. In order to mtch the vlue of 7 pcm / ppm (Fig. 3), the coefficients hs been slightly modified nd the following vlues used: Σ / ρboron for fst group: 1.95e-07 Σ / ρboron for therml group: 8.49e pcm 500 pcm / 70 ppm = 7.1 pcm / ppm 70 ppm Fig. 3 ectivity vs. Boron Concentrtion curve during the run crried out with stedy stte option on Setting the poison density reference vlue equl to 950 ppm, Keff of hs been obtined. A boron serch hs been crried out in order to hve Keff = 1. The boron concentrtion required for criticlity is ppm. This mens tht the uncertinty of boron concentrtion is lrger thn ppm Anlyzed Scenrios In order to test the model, 9 runs hve been performed, simulting 9 preliminry scenrios 8. The objective of these runs ws to check the core sensibility to plugs of deborted wter. During the two runs to which Fig. 4 refers, the sme mss of deborted wter hs been injected (nd indeed, the symptotic condition is the sme in the two cses), but the limiting condition (mximum power excursion) depends on the trnsient evolution, which is different in the two cses. These runs hve highlighted the importnce of the mixing in the PV downcomer. Indeed, the observed power excursion is lrgely dependent on the mixing in DC nd LP: given mss of deborted wter tht enters in the PV t certin time, the less is the mixing, the lrger is the power pek. The preliminry scenrios re clerly cdemic 9, but there re situtions in which deborted wter might ccumulte in limited zones of the primry system 10. In order to chrcterize the situtions tht might cuse power excursions, cpble to dmge the fuel, twenty-four runs hve been performed, simulting scenrios with different plugs dimensions nd mixing level. 7 ginst mximum cceptble vlue of 50 ppm for uncertinty in boron concentrtion, given in ref. 3 8 runs from 11 to 19 9 i.e., they do not represent rel trnsients for NPPs 10 following smll LOCA, the rector is in shut down condition nd the core hs been flooded by the ECCS; the core is refrigerted by nturl circultion: wter boils long fuel ssemblies nd the stem generted flows through the hot legs nd condenses in the stem genertors. In typicl Westinghouse PW, with U-tubes SG, portion of the stem tht condenses t the top of the U-tubes will pss the 180 U curve nd will collect in the loop sel. This wter is completely deborted. When the rector is restrted, this plug of deborted wter flows to the PV. The mixing phenomen which occur before the plug reches the core determine the level of de-bortion nd, thus, the severity of the trnsient. Potentilly, more thn one plug of deborted wter could rech the core t the sme time, one per cold leg.

6 Totl rector power (W) These trnsients cn be very dngerous nd cn seriously dmge the fuel ssemblies. The most significnt prmeter during these trnsients is the EF. 3.20E E E E E E E E Fig. 4 Exmple of boron concentrtion chnges feedbck (preliminry runs 16 & 17) 2.6 Energy elesed to the Fuel As lredy mentioned in the previous prgrphs, the Energy elesed to the Fuel is very significnt prmeter. Indeed, for high burnups, eqution 1 represents the threshold between core excursions tht do not dmge the core nd excursion tht dmge it 11 : The EF cn be evluted with the following simple reltionship: EF = 250 kj/kg. (1) Energy relesed to the fuel [kj/kg] = c p [kj/kg/k] x T [K] (2) where c p is UO2 specific het cpcity 12, to evlute with the following eqution (MATPO): c c p P 60 run run = 2 T e = 503 time 2 ( ) T 110 e T Averge core boron concentrtion (ppm) time T + 60 run run 17 7 γ T 5 e T [ 300K < T < 3113K] [ T > 3113K ] where T is the temperture (K), γis the oxygen-to-metl rtio nd is the universl gs constnt = (J/mol/K). Fig. 5 grphiclly represents eqution time Deborted wter injection flow (kg/s)) run 17 run time (3) cp (J/kg/K) cp (J/kg/K) [MATPO, [MATPO, p. 35 IAEA-TECDOC-949] p. 35 IAEA-TECDOC-949] The vlue of 320 J/kg/K hs been considered significnt for the rnge of temperture in which the core of PW opertes nd it hs been ssumed constnt. Putting c p = 320 J/kg/K into eqution 2, eqution 1 becomes T [K] T = 780 K. [4] Eqution 4 provides the threshold in terms of the therml excursion. Fig. 5 UO2 Specific Het Cpcity (MATPO) 11 In the cse of IA, the limits for energy deposition in the fuel re 1170 kj/kg to gurntee shut down coolbility nd 710 kj/kg to preclude fuel dmge. However, recent IA experiments crried out in Frnce n Jpn hve shown tht the limits for fuel dmge need to be reduced for fuel operted to higher burnup levels. At 60 MWd/kgU, the limit drops to pproximtely 250 kj/kg. (see ref. 4) 12 Dt of vrious uthors exist for the specific het cpcity of UO 2 (see ref. 5)

7 ESULTS The core is modelled with 432 het structures, which represent 432 core zones. A preliminry run hs been performed in order to identify the most heted zones nd the verge rod temperture excursion hs been observed in these zones, during the scenrios nlysed. Tble 3 briefly describes the four types of scenrios tht hve been considered: Scenrio 1 Scenrio 2 Scenrio 3 Scenrio 4 A plug or deborted wter, deposited in CL1 flows through the core. LP divided in 4 qudrnts, withouth crossflow. The prmeter vrying mong runs is the mss of deborted wter A plug of wter prtilly deborted, deposited in CL1 flows through the core. LP divided in 4 qudrnts, withouth crossflow. The prmeter vrying mong runs is the boron concentrtion The portion of LP relevnt to FA hs been isolted. No mixing in LP. The plug of deborted wter reches this LP portion nd then reches intct one FA only. The phenomenon is completely loclized (extreme scenrio) A plug of deborted wter, deposited in CL1 flows through the core. Complete mixing in LP: t BAF elevtion, wter hs the sme boron content in ech FA. The phenomenon is completely de-loclized (extreme scenrio) Tb. 3 Types of scenrios considered 13 Tble 4 reports, for ech run, the mss of the plug, the therml excursion, the EF, nd the mximum nucler power reched. UN Deborted Wter Mss [kg] Min. verge boron concentrtion t BAF [ppm] MAX D T [K] EF [kj] Mximum Nucler Power eched / Nominl Power Type of Scenrio (*) (*) (*) (*) runs stopped by code filure. Tb. 4 Summry of runs performed Fig. 6 nd fig. 8 represent the reltionship between the mss of deborted wter nd the EF, for the different scenrios. Ech run is represented by point in these grphs. Such representtion does not ddress the trnsient evolution, which vries from scenrio to scenrio, but records the mximum EF reched during the trnsient. The objective of these representtions is to identify the conditions tht could bring the rector to be dmged. Notes 13 Scenrios 1 nd 2 re similr. In both cses the deborted wter plug reches ¼ of the fuel ssemblies, since the LP is modelled with 4 prllel chnnels without cross flow. The difference is in the plug boron concentrtion: in the first scenrio 1 the plug is completely deborted, whilst in the second it hs reduced boron concentrtion, but it is not completely deborted.

8 219.8 Fig. 7 nd fig. 9 represent the mximum power reched during the trnsient, before the scrm eventully occurs. The whole trnsient is drmticlly rpid nd the power pek is reched bout 2 seconds fter the plug reched the BAF elevtion. The scrm is ssumed to occur with dely of 0.4 seconds on the hi-power signl (nucler power > 3.16 GW, corresponding to 114% of nominl power). The C bnks need 2.2 seconds to completely insert. Hence, when the scrm begin, the plug hs lredy flown through the core nd the criticl condition hve lredy been reched. Anywy, the shut down voids 2 nd trnsient to occur when the plug will re-enter (diluted) in the core (see fig. 10) 14. During rel trnsient, n eventul plug ccumulted in the CLs, due to the mixing, will rech the BAF elevtion diluted nd will ffect portion of the core, which we cn ssume between 25% to %, i.e. somewhere in the dshed zone of fig. 6. The lrge dimension of this zone is due to uncertinties, which cnnot be reduced without using CFD tools, implementing specific models for the representtion of the boron mixing. Without this tools, it is not possible to perform BE nlysis of IA trnsients nd the criticl mss of deborted wter cn be evluted conservtively only. 15 Scenrio 3 is the most criticl for EF, since during this scenrio the plug of deborted wter does not mix nd reches smll portion of of the ctive core 16 nd it is nticipted tht the locl therml excursion will be very rpid. Fig. 7 shows tht scenrio 3 is the most criticl even from the totl power point of view. Scenrio 2 Scenrio 1 Scenrio 3 Scenrio 4 Scenrio 2 Scenrio 1 Scenrio 3 Scenrio EF (kj/kg) kj/kg mx. nucler power reched before scrm (GW) 10 Nominl Power Level (2.772 GW) mss of deborted wter (kg) Fig. 6 Mx. EF vs. deborted wter mss mss of deborted wter (kg) Fig. 7 Mx power vs deborted wter mss Scenrio 2 Scenrio 1 Scenrio 3 Scenrio 4 Scenrio 2 Scenrio 1 Scenrio 3 Scenrio EF (kj/kg) kj/kg mx. nucler power reched before scrm (GW) 10 Nominl Power Level (2.772 GW) Min. vg. boron concentrtion reched t BAF (ppm) Fig. 8 Mx. EF vs. boron concentrtion t BAF Min. vg. boron concentrtion reched t BAF (ppm) Fig. 9 Mx. power vs boron concentrtion t BAF 14 Should EF > 1170 kj/kg, see note 11, we would expect problems for core coolbility nd more detiled nlyses would be needed to understnd whether the rpid fuel rods therml excursion could interfere with the C bnks insertion 15 the criticl mss of deborted wter, ccording fig. 4 is between 400 nd 3500 kg. Without using CFD tools to perform rel BE nlyses, the vlue of 3000 kg shll be conservtively ssumed. 16 one fuel ssembly only

9 Nucler Power [W] Averge od Temperture [K] 3.50E E Nucler Power 2.50E E E s ector shut down Averge od Temperture 1.00E E E Hi-power signl Time (s) Fig. 10 Exmple of trnsient evolution (run 23) Hence, the locl conditions of the fuel ssembly where the deborted wter flows ffect the whole core behviour. This is due to the locl neutron flux pulse, which in turn cuse power pek tht propgtes throughout the core. Fig. 11 nd fig. 12, which represent the locl het genertion in the 18 core chnnels, show this phenomenon: the ssemblies locted fr from the criticl ssembly, experience smller pek 17 with lrger dely. Het genertion (W/m^3) 1.1E E E E E E E E E E E+08 Serie1 Serie2 Serie3 Serie4 Serie5 Serie6 Serie7 Serie8 Serie9 Serie10 Serie11 Serie12 Serie13 Serie14 Serie15 Serie16 Serie17 Serie18 Chnnel reched by the plug 0.0E time (s) Fig. 11 Het genertion in the core chnnels q>10 9 W/m 3 q>5x10 8 W/m q>4x10 8 W/m q>3x10 8 W/m 3 q>2.5x10 8 W/m q>2x10 8 W/m 3 q>1.5x10 8 W/m q>10 8 W/m 3 Fig. 12 dil distribution of het genertion during power pulse Chnnel reched by the plug 17 neutron flux is proportionl to (1 / r 2 ), where r is the distnce from the criticl ssembly 219.9

10 CONCLUSIONS The procedure developed cn be used to clculte the boron feedbck coefficients nd to properly initilize the TH/NEU system. The nlyses performed hve shown how the IAs resulting from deborted wter injection cn be dngerous. Indeed the core behvior is drmticlly sensitive to boron concentrtion chnges nd the boron dilution trnsient evolve so fstly tht the sfety systems cnnot prevent dmges to occur to the core. Given the core sensitivity to the boron concentrtion, it is fundmentl to use tools which llow to reduce the uncertinties ssocited to this quntity. Hence, in order to perform BE nlysis of those IA it is necessry to use CFD codes coupled with the code modeling the core behvior (e.g. ELAP-3D ). Without the cpbility to properly model the mixing phenomen occurring in the vessel, it is possible to perform conservtive nlyses only, considering the extreme scenrios tht could occur. ACONYMS BAF Bottom of Active Fuel BE Best Estimte CFD Computtionl Fluid Dynmics CL Cold Leg C Control od DC Downcomer ECCS Emergency Core Cooling System EF Energy elesed to the Fuel FA Fuel Assembly HL Hot Leg HPIS High Pressre Injection System LOCA Loss Of Coolnt Accident LP Lower Plenum LPIS Low Pressure Injection System MCP Min Coolnt Pump MSLB Min Stem Line Brek NEA Nucler Energy Agency NEM Nodl Expnsion Method NSC Nucler Science Comitee OECD Org.. for Economic Coopertion & Development OTSG Once Through Stem Genertor PW Pressurized Wter ector CS ector Coolnt System IA ectivity Insertion Accidents PV ector Pressure Vessel SG Stem Genertor TH/NEU Therml Hydrulic nd Neutronic TMI Three Miles Islnd EFEENCES [1] The ELAP5-3D Code Development Tem, ELAP5-3D Code Mnul, INEEL-EXT ev. 1.2, [2] F. D Auri, Nodlistion Development nd Qulifiction, IAEA TCM on Intercomprison nd Vlidtion of Computer Codes for Therml-hydrulic Sfety Anlyses of Hevy Wter ectors, Vienn, 2000, pp [3] J. Svrny, Proc. 3th Symposium of AE, KFKI Atomic Energy eserch Institute, Budpest, 1993, pp [4] "Sfety Aspects of Advnced Fuel Assemblies under Intense Discussion, Service&Fuel, Siemens Power Genertion 2/98, 1998, pp [5] TECDOC-949, Interntionl Atomic Energy Agency, pp