ANALYSIS OF THERMAL STRATIFICATION IN THE PRIMARY CIRCUIT WITH THE CFX CODE

Transcription

1 ANALYSIS OF THERMAL STRATIFICATION IN THE PRIMARY CIRCUIT WITH THE CFX CODE Ildikó Boros, Dr. Attil Aszódi Budpest University of Technology nd Economics, Institute of Nucler Techniques Abstrct The therml strtifiction cn led n importnt role in the ging of the NPP piping becuse of the stresses cused by the temperture differences nd the cyclic temperture chnges. These stresses cn limit the lifetime of the piping, or led to penetrting crcks. For the strength nlyses, the determintion of the therml hydrulic prmeters of the strtified flow is necessry, which cn be simulted by Computtionl Fluid Dynmics (CFD) codes. The results of the simultion show the time development nd the breking up of the strtifiction nd the temperture distribution of the strtified flow. The min difficulty of these CFD simultions is the uncertinty of the boundry conditions becuse of the unknown flow circumstnces. In this pper some results of CFX simultions re presented concerning the pressurizer surge line, nd the injection pipe of the HPIS for VVER-440 type rectors. 174

2 Introduction If two medium with different densities (i.e. with different tempertures) flows inside pipe, therml strtifiction cn occur. The development nd the stbility of the strtified flow depend on the temperture difference nd on the reltive velocity between the fluids. The strtified flows induce therml stresses in the piping through different mnners. There is globl stress cused by the temperture differences of the upper nd lower lyer of the coolnt, which cn induce new crck in the pipe wll, or the growing of n existing crck. The wving of the middle mixed coolnt lyer (the so-clled therml striping) nd the so-clled turbulent penetrtion cuse stresses fluctuting with high frequency. The mximum vlue of these fluctuting stresses is on the inner pipe wll, nd it decreses going outside of the pipe, becuse of the inerti of the pipe wll. Turbulent penetrtion occurs when coolnt of higher mss flow penetrtes into pipe with stgnting or nerly stgnting coolnt. A typicl occurrence of turbulent penetrtion is t the connection of the pressurizer surge line nd the min loop. According to the interntionl operting experiences, the mteril ftigue of the NPP piping cused by therml strtified flows my limit the lifetime of the pipes, therefore the considertion of therml strtifiction is crucil in the ging mngement nd for the lifetime-extension of nucler power plnts. In PWRs the most ffected pipes re usully the feed wter lines of the stem genertor, the pressurizer surge line, nd the injection pipes of the emergency core cooling systems. The strtifiction is cused minly by leking vlves. In the US, more thn dozen penetrting crcks hve been identified since 1979, s result of therml strtifiction. Crcks due to the strtifiction were found in Frnce, Belgium nd Jpn, s well. The Loviis NPP in Finlnd reported penetrting crck t the pressurizer surge line in 1994 s common result of therml strtifiction nd mteril defects. The settled temperture monitoring system confirmed the existence of strtified flow with temperture differences of o C. The prticulr importnce of the problem of the therml strtifiction in the Pks NPP is cused by the fct, tht the significnce of the phenomenon ws not known t the construction of the plnt (s in other plnts of similr ge), therefore it is not included in the design bse of the plnt. For the determintion of mechnicl effects of the strtified flows, strength nlyses hve to be done, for which the therml-hydrulic prmeters of the flow (e.g. the mximl temperture differences long the given pipe section nd the frequency of the temperture chnges) re required s n input prmeter. The therml-hydrulic behvior of the flow cn be simulted by Computtionl Fluid Dynmics (CFD) codes. Becuse of the quite simple geometry, the modeling my be performed with resonble low computtionl efforts. The min difficulties of the simultions re the uncertinties of the boundry conditions, becuse the necessry prmeters re usully not mesured t the boundries of the flow region. In this pper, some CFD clcultions re presented concerning the development of therml strtifiction in VVER-440 type rectors. With the CFX code, strtified flow in the pressurizer surge line, nd in the injection pipe of the HPIS of VVER-440 hs been investigted. 175

3 Therml strtifiction in the pressurizer surge line The surge line of the VVER-440 connects the lower nozzle of the pressurizer to the hot leg of the loop 6 (in Units 1 nd 3) or to the loop 1 (in Units 2 nd 4). Under the pressurizer, fter short verticl section, the surge line is divided into two legs signed s YP10 nd YP20 legs tht follow shifted pth to the min loop (see Fig. 1). The inner dimeter of the surge line is 207 mm, the length of one leg is bout 14 m. The surge line is thermlly insulted. During the opertion of the pressurizer heters, the high temperture medium of the pressurizer diltes, nd flows into the surge line. This wrmer coolnt cn be strtified on the lower temperture coolnt of the surge line nd flows strtified bove the cooler lyer towrd the min loop. Figure 1: Arrngement of the pressurizer surge line In norml opertion, the pressurizer heters operte periodiclly. The pressurizer temperture is then bout 325 C, while the coolnt of the hot leg of the min loop hs temperture of bout 297 C. In the surge line, there is n lternting flow with very low coolnt velocities in the surge line in both directions. The heting of the primry circuit during the strt-up of the rector is mnged in prt with the pressurizer heters tht men they operte lmost continuously. Therefore, the temperture of the pressurizer is lwys higher thn the min loop temperture, so there is slow permnent flow downwrd from the pressurizer during the het-up period. The mximl temperture differences cn rech even C. T2 T3 T4 T5 T6 T7 Fig. 2: Plnned mesuring positions on the surge line (relized only on the YP20 leg) [2] Fig. 3: Thermocouples t one mesuring position [2] 176

4 Temperture monitoring system in the pressurizer surge line in the Pks NPP In the Pks NPP, n extensive temperture mesurement progrm ws implemented in 2000 in order to find out the possible therml strtifiction in the pressurizer surge line. Altogether 70 Pt resistnce thermometer ws settled on the YP20 leg of the surge line of the Unit 1. The temperture of the pipe wll ws mesured t ten positions between the pressurizer nd the min loop (see Fig. 2). In one mesuring position there were 7 thermocouples operting, rrnged verticlly equidistnt (see Fig. 3). The monitoring system collected temperture dt minutely for bout three months. The operting period included the strt-up of the unit, nd norml opertion s well. The mesured tempertures bout dt per thermometer were recorded by the unit computer. The mesurements show tht in norml opertion there is periodic strtifiction in the surge line with cycle period of bout 45 minutes which corresponds to the opertion of the pressurizer heters. However, the mximum of the temperture differences is bout 30 C, therefore no criticl ftigue occurs in norml stte. On the other hnd, the monitoring system confirmed the existence of stble strtifiction during the het-up period (see Fig. 4). The mximum of the temperture differences reches C. The strtifiction is very stble, prticulrly in the first horizontl section of the pipe. (The monitoring system ws settled only on the YP20 leg of the surge line, so the possibly symmetric flow fields could not be proven.) 280 o C 315 o C 200 o C 190 o C Fig. 4: Mesured tempertures t the cross-sections 0 (ner the verticl inlet nozzle) nd 4 (lower horizontl section) for the sme 24-hours period, on the top nd the lowest thermometers, during the het-up of the Unit 1 CFX simultion of therml strtifiction in the surge line The development of the therml strtifiction during the strt-up of the rector hs been investigted with the CFX code. For the simultion of the strtifiction, the CFX model of the pressurizer surge line hs been built, which contined the T-junction t the pressurizer, the two legs of the surge line, nd short (1 m) section of the min loop (see Fig. 5). For the simultion, volume mesh of hexhedrl elements ws creted. The mximl edge length ws set to 4 cm, the mximl height to 1 cm. At the pipe wlls, inflted lyer hs been defined with mximl height of 4 mm (Fig. 6). 177

5 With the model of the surge line, trnsient CFX clcultion ws performed concerning the development of the strtified flow. The simultion run 1250 s, with timestep of 1 s, ssuming lminr flow. The boundry conditions were estimted bsed on the unit opertionl indictors nd were set s follows: the coolnt with higher temperture (240 C) enters from the pressurizer into the surge line with velocity of 0.03 m/s. In the min loop, coolnt with lower temperture (140 C) flows with mss flow of 1500 kg/s. As outlet boundry condition, 0 P reltive pressure ws set t the min loop. Fig. 5: CFX model of the surge line Fig. 6: Hexhedrl mesh of the model The simultion confirms the development of strtified flow in the surge line. The strtifiction is very stble in the first horizontl section of the surge line, the mximl temperture difference between the upper nd lower lyer is bout 100 o C during the whole trnsient. However, the results show very symmetric flow in the surge line tht eventutes in n symmetric strtifiction in the lower sections of the surge line (see Fig. 7). Fig. 7: Coolnt temperture in the surge line t t=1200 s The cuse of this specil flow field is tht - ccording to the clcultion - there is permnent recircultion of colder coolnt in the lower lyer cused by the symmetric rrngement of the surge 178

6 line legs nd the symmetric connection of the two legs into the min loop. The recirculting coolnt blocks the flow of the wrmer coolnt coming from the pressurizer in the lower section of one of the legs, therefore it cn flow only in the other leg. This result is not supported by the mesurements in the Pks NPP, where only one of the legs hs been monitored, nd no definite sign of flow blocking ws noticed. The differences between the mesured nd clculted development of the strtifiction cn be originted possibly from the uncertinties of the boundry conditions (minly the inlet velocity, which is clculted from the pressurizer level). To bypss this problem n extensive prmeter study should be mde, which is quite time-demnding. However, some mesurements in the Bohunice NPP confirm the existence of this type of recircultion. M1 M2 M3 Fig. 8: Position of the monitor-lines M0 Height (m) 2,30 2,25 2,20 2,15 2,10 2, M1 monitor-line 100 s 200 s 400 s 600 s 800 s 1000 s 1200 s Temperture (K) Fig. 9: Temperture t M1 during the trnsient 0,10 0,10 0,05 0,05 Height (m) 0,00-0,05-0,10-0,15 M2 monitor-line 100 s 200 s 400 s 600 s 800 s 1000 s 1200 s Height (m) 0,00-0,05-0,10-0,15 M3 monitor-line 100 s 200 s 400 s 600 s 800 s 1000 s 1200 s Temperture (K) Fig. 10: Temperture t M2 during the trnsient Temperture (K) Fig. 11: Temperture t M3 during the trnsient Figures 9-11 show the coolnt temperture during the trnsient t three verticl monitor-lines (M1, M2 nd M3) shown in Fig. 8. In Fig. 9 it cn be seen tht the strtifiction with bout 100 C temperture difference remins during the whole trnsient, but the position of the mixed lyer sinks lower in time. At the M2 monitorline (see Fig. 10), there re much lower temperture differences: the mximum is bout C. The wrmer coolnt from the pressurizer reches the monitor-line t 200 s nd then the temperture of ll points of the monitor-line increses during the trnsient. The temperture of the lower point of the M3 line increses much slower which mens tht strtified flow develops in the pipe section (see Fig. 11). The mximl temperture differences re bout C. 179

7 Therml strtifiction in the surge line of the high pressure injection system (HPIS) According to the interntionl operting experiences, the surge lines of the emergency cooling systems cn be ffected concerning the therml strtifiction. The surge line of the High Pressure Injection System is connected to the cold leg of the loops 2., 3. nd 5. The surge line runs from the HPIS pump to the box wll on one pth nd then it is divided into two legs tht re closed with pneumtic quick-stop vlves. Inside the box wll, there re inner quick-stop vlves nd fter it, the pipe legs join together gin (see section C in Fig. 12) nd connects fter check vlve (see B in Fig. 12) to the min loop (see section A). The surge line is thermlly insulted long the section A, but fter the check vlve, it is not insulted. A B C Figure 12: Pth of the HPIS surge line between the min loop nd the box wll In the first ( A ) section of the surge line, the coolnt temperture corresponds to the temperture of the primry circuit nd therml strtifiction my occur due het losses through the (insulted) pipe wll. Between the check vlve nd the quick-stop vlves, the temperture corresponds to the so-clled box (hermetic comprtment) temperture, i.e. bout 50 o C. In this section, the lekge of the check vlve tht could be cused e.g. by therml expnsion my led to therml strtifiction. Some demonstrtive clcultions were performed with the CFX code for the sections A nd C of the surge line to simulte the development of the therml strtifiction. In section A, the effect of het losses through the pipe wll ws investigted with ssuming tht here ws no lekge through the check vlve. Het flux through the pipe wll ws determined by nlyticl clcultion nd through the check vlve by dditionl CFX simultion. The model of the section A contined the surge line up to the check vlve (without exct modeling of the vlve itself) nd smll prt of the min loop. The volume mesh of the model contined tetrhedrl volume elements (see Fig. 13). 180

8 Fig. 13: CFX model nd meshing of the first section of the HPIS surge line The trnsient simultion performed with the model run 180 s with time step of 1 s, with ssuming lminr flow nd with the following boundry conditions: 1500 kg/s mss flow in the min loop (ccording to the norml opertion) with temperture of 267 C W/m 2 het flux ws specified t the wll of the surge line (ccording to the nlyticl clcultions, ssuming no insultion conservtively), 5000 W/m 2 het flux t the closed end of the surge line (ccording to the CFX simultion of the check vlve). According to the clcultion, no strtified flow evolved in the first section of the HPIS surge line becuse the mximl temperture differences re only 4-5 C (see Fig. 14). The coolnt cools down t the vlve disk but this is not enough for the formtion of strtifiction. The colder coolnt flows in lrge eddies nd mixes with the wrmer wter in the surge line. Fig. 14: Coolnt temperture in the surge line t t=180 s For the simultion of the second section of the HPIS surge line (see section C in Fig. 12), the CFX model of the pipe section hs been built, ssuming the lekge of the check vlve. The model contined the surge line between the check vlve nd the quickstop vlves. The vlves re modeled s simple closed end of the pipe. The volume mesh of the model contined hexhedrl volume elements (see Fig. 15). Fig. 15: CFX model of the second section of the HPIS surge line 181

9 The development of the strtifiction ws investigted with trnsient simultion which rn for 420 s, with time step of 1 s, ssuming lminr flow in the pipe. As inlet boundry condition, 1 t/h mss flow from the primry circuit with temperture of 267 o C, s outlet boundry condition, 0 P reltive pressure t the quick-vlves were ssumed. The het losses through the pipe wll were neglected. The results show the development of therml strtifiction in the surge line but the strtified flow is not stble, t t=300 s the lyers brek up in the first section of the investigted pipe. The mximl temperture difference reches 200 C in the first section of the pipe for short period (see Fig. 16). The therml striping (the wving of the mixed lyer) cn be observed s well (see Fig. 17). Fig. 16: Therml strtifiction in the HPIS surge line Fig. 17: Therml striping in the surge line 182

10 Summry In the pper, some results of CFX simultions concerning the therml strtifiction in primry piping of VVER-440 were presented. The clcultions in ccordnce with the vilble mesured dt confirm the development of strtified flow in the surge line of the pressurizer during the het-up of the rector. However, the CFX simultion shows stble symmetric flow in the surge line, which is not directly confirmed by the mesurements. In the second prt of the pper, simultions of therml strtifiction in the HPIS injection line were presented. The investigtion of the surge line of the HPIS shows similr strtifiction, in cse the check vlve is ssumed to be leking. These simultions cnnot be vlidted by Hungrin mesurements. For the precise description of the boundries (s the lekge mss flow rte on the check vlve) the extensive study of the interntionl operting experience is necessry. The results showed tht the CFD codes provide n effective method for the demonstrtion of the therml strtifiction when sufficient opertionl dt re vilble for the concerned piping. Otherwise, mesurements or dditionl simultions re required, nd dditionl sensitivity studies should be performed for the ccurte nlysis. Qulittive CFD simultions cn lso ply n importnt role in the geing mngement of NPP piping: they cn help deciding, whether the possibility of therml strtifiction exists in given pipe sections. References [1] VEIKI: Ageing mngement of nucler power plnt equipment - Piping, Bckground mteril for regultory guidelines, in Hungrin, 1998 [2] Assessment of the results of the temperture monitoring system, settled on the pressurizer surge line, Pks NPP, 2001, in Hungrin 183