Yashar Rahmani Department of Physics, Faculty of Engineering, Sari Branch, Islamic Azad University, Sari, Iran

Transcription

1 Study of Thermohydraulc Parameters of the Bushehr s VVER-1000 Reactor durng the Intal Startup and the Frst Cycle Usng the Couplng of WIMSD5-B, CITATION-LDI2 and WERL Codes Yashar Rahman Department of Physcs, Faculty of Engneerng, Sar Branch, Islamc Azad Unversty, Sar, Iran Abstract In ths paper, by desgnng a thermo-neutronc code, the three-dmensonal changes of the thermohydraulc parameters of the Bushehr s VVER-1000 reactor as well as the temperature dstrbuton of the fuel elements and coolant n each assembly were studed durng the ntal startup and the frst cycle. In order to perform the tme-dependent cell calculatons and obtan the concentraton of fuel elements, the WIMSD5-B code was used. Besdes, by utlzng the CITATION-LDI2 code, the effectve multplcaton factor and the thermal power dstrbuton of the reactor were calculated. For consderng the real geometry of VVER-1000 fuel rods and also the effects of the gaseous fsson products n calculatng of the temperature dstrbuton n the reactor core, a thermo-hydraulc software (WERL code) was desgned usng the Enveloped Pn method. The Dttus-Boelter, Ross-Stoute and Lee-Kesler models were used n the calculatons of the heat transfer coeffcent of coolant, gap conductance coeffcent and gap pressure, respectvely. In addton, to estmate the concentraton of the released gaseous fsson products nto the gap space, the Wesman model was used. After calculatng the temperature of fuel, clad and coolant n each axal sub volume of the fuel assembles (n each tme step), the temperature of these elements was nserted nto the nput fles of the WIMSD5-B code (n each assembly). Thus a sequence of neutronc and thermohydraulc calculatons was formed based on the couplng of WIMSD5-B, CITATION-LDI2 and WERL codes. Study of the results demonstrated that the BUSEHR VVER-1000 reactor enjoyed the desrable thermohydraulcal safety thresholds durng the ntal startup and frst cycle. Fnally, t s worth mentonng that the comparson between the results of ths modelng and the fnal safety analyss report of ths reactor made clear that the results presented n ths paper are satsfactorly accurate. Keywords VVER-1000, Thermohydraulc analyss, Frst cycle, WERL code, Ross-stoute,Wesman Introducton Durng the startup process (from the cold condton), the thermal power of the reactor ncreases gradually untl t reaches the nomnal value (3000 MW). In ths regard, the negatve reactvty caused by the control rods and borc acd should be dmnshed to overcome the temperature negatve feedbacks. Afterwards, to stablze the reactor s thermal power n ts nomnal power (3000 MW), the concentraton of borc acd should be gradually decreased durng the cycle. In ths research, the smplfcatons common to former researches are avoded n modelng of the geometry of the fuel assembles and the reactor core (n the neutronc and thermohydraulc sectons), and the real geometry s taken nto account. Moreover, due to the mportant effects of the coolant and the fuel temperature feedbacks, the couplng of neutronc and thermo-hydraulc calculatons was utlzed. It s worth notng that a computatonal program was desgned based on the Enveloped Pn method [13, 18] n thermo-hydraulc calculatons. Furthermore, n order to accurately model the effects of the gaseous fsson products on the process of heat transfer from the fuel to the clad, the Ross- Stoute [14, 18] and Lee-Kesler methods [17] were used n calculatng the gap conductance

2 coeffcent and gap pressure, respectvely. In addton, the Wesman model [19] was employed to calculate the amount of the gaseous fsson products released nto the gap space. Several researches have been performed n order to tme-dependent modelng of nuclear reactor cores [1,5,6,7,8,9,11,16], whch some of them have already been conducted n the feld of ntal startup modelng [7,16] and burnup calculatons of Bushehr s VVER-1000 reactor [6]. However, consderng the lmted tme nterval durng whch these researches were conducted and the fact that temperature feedbacks, fsson gas release and the modelng of real geometry of the reactor core have not been nvestgated n these researches, t seems that the calculatons performed n ths study are nnovatve and more accurate comparng to those of the former researches conducted. The operatonal condtons of the Bushehr s VVER-1000 reactor durng the ntal startup and the frst cycle The thermo-neutronc parameters of the VVER-1000 reactor were changed durng the frst cycle. In ths regard, the changes of the reactor s thermal power and the nlet coolant temperature are shown n Fgures 1 and 2, respectvely [3]. Fgure 1. The reactor s thermal power versus tme durng the ntal startup and the frst cycle. Fgure 2. The reactor s nlet coolant temperature versus tme durng the ntal startup and the frst cycle. Furthermore, the crtcal concentraton of borc acd and the entrance heght of the control rods n the group No. 10 were changed durng the frst cycle. Fgures 3 and 4 llustrate the tme-dependent changes of these parameters respectvely [3].

3 Fgure 3. The crtcal concentraton of borc acd versus tme durng the ntal startup and the frst cycle. Fgure 4. The entrance heght of the control rods (group No. 10) versus tme durng the frst cycle. Fgure 5, also shows the arrangement of the Bushehr s VVER-1000 reactor core n the frst cycle [2]. Fgure 5. The arrangement of the fuel assembles n the core of Bushehr s VVER-1000 reactor n the frst operatonal cycle.

4 Methods In order to estmate the tme-dependent changes of the thermo-neutronc parameters of Bushehr s VVER-1000 reactor durng the ntal startup and the frst cycle, the couplng of neutronc and thermo-hydraulc calculatons was used. To do ths, the physcal group constants of the fuel assembles and reflectors were calculated usng the WIMSD5-B code [15]. Furthermore, to obtan the tme-dependent changes n the fuel composton and calculate the rate of burnup n each fuel assembly, the computatonal capabltes of the WIMSD5-B code were utlzed [15]. By nsertng the physcal group constants obtaned from the WIMSD5-B code nto the nput fle of the CITATION-LDI2 code [4] and defnng the geometry of the reactor core, the effectve multplcaton factor and the three-dmensonal dstrbuton of the reactor s thermal power were calculated. In ths study, a thermo-hydraulc computatonal program (WERL code) was used for calculatng the temperature dstrbuton of the Bushehr s VVER-1000 reactor core based on the thermal power dstrbuton obtaned from the CITATION-LDI2 code. By usng the results of the thermo-hydraulc calculatons, the temperature and densty of the fuel, clad and coolant elements (n each fuel assembly) were appled to the neutronc calculatons and thus, a contnuous sequence of neutronc and thermo-hydraulc calculatons was created. In the followng paragraphs, the methods of calculatons n each of the neutronc and thermohydraulc sectons wll be explaned n detal. Neutronc calculatons Use of the WIMSD5-B code n the cell calculatons of the VVER-1000 reactor core Calculaton of the physcal group constants of the fuel assembles and reflectors was performed usng the WIMSD-B code (wth ENDF-BVII lbrary). In ths code, the neutron transport equaton was solved n the real geometry of each fuel assembly usng the Dscrete S N method [15]. To nsert the real geometry of each fuel assembly nto the WIMSD5-B code, 36 radal arrays were employed to poston the fuel rods. Moreover, to defne the complement space of the fuel rods n each assembly, ther nteror space was dvded nto 54 annuluses. In the cellular calculatons of radal reflectors such as downcomer, core barrel, core baffle, water holes and pressure vessel, the entre geometry of Bushehr s VVER-1000 reactor core was modeled usng the WIMSD5-B code. In ths regard, the materals of each fuel assembly were homogenzed at frst. Then the hexagonal geometres of the assembles were transformed nto crcular form so that the fuel assembles could be consdered as a cylndrcal rod n modelng the entre reactor core (by WIMSD5-B code). In order to determne the poston of the 163 fuel assembles and the 138 water holes, 28 and 23 arrays were used, respectvely. Besdes, 8 and 4 radal annuluses were employed to defne the complement spaces of the fuel assembles and the radal reflectors n the reactor core respectvely. To obtan the tme-dependent changes of the fuel composton and calculate the rate of burnup n each fuel assembly, the computatonal capabltes of WIMSD5-B code were utlzed. For ths purpose, the thermal power of each fuel assembly and duraton of each tme step were defned n the nput fle of the WIMSD5-B code. Use of CITATION-LDI2 code n calculatng the neutronc parameters of the VVER-1000 reactor core After enterng the physcal group constants of each fuel assembly (obtaned from the WIMSD5-B code) nto the nput fle of the CITATION-LDI2 code and defnng a three-dmensonal geometry for the reactor core n the latter code, the neutron-dffuson equaton was solved three-dmensonally by usng fnte-dfference method [4]. Thus, the effectve multplcaton factor and the reactor s thermal-power dstrbuton were calculated three-dmensonally at each tme step. Snce the reactor core has a symmetrc arrangement n the frst cycle, the meshng carred out n the CITATION-

5 LDI2 code was ntended for one -sxth of the reactor core. In lne wth ths, the reactor core s dvded nto 10 axal sub volumes, and the radal sector s composed of 7938 trangular meshes. Thermo-hydraulc calculatons Durng the frst cycle, a fracton of the gaseous fsson products (ncludng xenon and krypton) s released nto the gaseous space of the gap between the fuel and the clad. Consderng the fact that the gap s gaseous space s flled wth helum n the begnnng, therefore, due to the lower thermal conductvty of krypton and xenon gases compared to helum, the release of these gaseous fsson products nto the gap leads to a decrease n the rate of heat transfer from the fuel to the clad. Therefore, for consderng the real geometry of fuel rods and also the effects of the gaseous fsson products n calculatng of the temperature dstrbuton n the reactor core, a thermo-hydraulc software (WERL code) was desgned usng the Enveloped Pn method [13,18]. The Dttus- Boelter[18], Ross-Stoute [14,18] and Lee-Kesler [17] models were used n the calculatons of the heat transfer coeffcent of coolant, gap conductance coeffcent and gap pressure, respectvely. Calculaton of fuel elements and coolant temperatures: Now, n ths part and n order to complete the computatonal cycle, the temperature dstrbuton n fuel elements should be calculated, whch the clad's outer surface temperature, could be calculated as follows: q (1) Tclad ( t) = + T ( t) out coolave 2 π R h ( t) Co cool The nternal surface temperature of the clad wll be obtaned through the followng correlaton [18]: RCo ln( ) 1 R (2) c Tclad n ( t) = q ( + ) + Tcool ( t) 2πR h 2πK Co cool clad For the calculaton of the outer surface temperature of fuel, the followng correlaton wll be appled [18]: RCo ln( ) 1 1 R (3) c T fuel out ( t) = q ( + + ) + Tcool ( t) 2πR h 2πR h 4πK gap gap Co cool clad The central temperature of fuel wll be calculated as follows [18]: R fo 2 ln( ) q R f T fuel ( t) = [1 ] + T ( t) n fuelout 4 π K R f fo 2 ( ) 1 R f (4) Where R, R, R c co f R fo are the nner and outer radus of clad and fuel respectvely. f k and clad k are the thermal conductvty coeffcent of fuel and clad, and cool h s the heat transfer coeffcent of coolant. By usng control volumes for each fuel, clad and coolant elements, the average temperatures of these tems wll be calculated at dfferent tmes as follows:

6 m fuel T ( ( n+ 1) fuel T t fuel ) = P h gap A fo ( T fuel _ out T clad ) (5) m clad h T ( cool ( n+ 1) clad A co T t ( ( T clad n) clad _ out ) = h T gap cool ) A fo ( T fuel _ out T clad ) (6) m cool h ( ( n+ 1) cool = 1,...,50 h t cool ) = m& cool ( h np h cool ) + h cool A co ( T clad _ out T cool ) (7) Where, m fuel, mclad and m cool are the mass of fuel, clad and coolant respectvely. Furthermore, A fo and A co are the area s of the outer surface of fuel and clad. Therefore by usng the fnte dfference method n solvng equatons No. 5, 6 and 7 and also solvng correlatons No. 1 up to 4 n each tme steps, we wll be able to calculate the temperatures at dfferent surfaces of fuel, clad and the coolant, whch are needed n the chaned calculatons. For completon of ths computatonal cycle and performng precse calculatons, the estmaton of probable two phase condton n calculatons of the coolant's heat transfer coeffcent has been consdered [18].In the stage of temperature calculatons of coolant, after calculatng enthalpy of each axal sub-volume, the mass qualty of flud s calculated by usng thermodynamc tables consdered n the structure of the program and through followng formula: h (8) h ff X = h h gg ff It's clear that other thermodynamc parameters of coolant can also be calculated va ths method. Also along the accomplshment of abovementoned temperature calculatons, n order to calculate gap conductance coeffcent (n gaseous space between fuel and clad), the ROSS-STOUTE gap model [18] has been used, whch the manner of applyng t, s expressed n a computatonal cycle exstent n the papers wrtten by authors n reference [13,14]. Wth regard to the mportant pont that at the begnnng of the calculatons, gap pressure s 2 MPa, therefore, we could not use the complete gas model n the pressure calculatons, and n order to solve such a defcency, we used the Lee-Kesler model [17]. One of the outstandng ponts that should be mentoned here s the mpact of pressure parameter changes on radus changes n fuel and clad. Pressure ncreasng causes formaton of stress on the fuel and clad surfaces whch, wth regard to the elastcty characterstc present n fuel and clad, such a stress wll develop stran n fuel and clad surfaces, thus causng change n ther radus. Of course, n addton to the radus changes caused due to the elastcty phenomenon, reference should be also made to the radus changes caused as a result of thermal expanson n fuel and clad, whch play a key role n the gap thckness changes n these calculatons. In ths study the effects of the both phenomena have been consdered n calculatng the gap thckness.

7 In addton, to estmate the concentraton of the released gaseous fsson products nto the gap space, the Wesman model was used. After calculatng the temperature of the fuel, clad and coolant n each axal sub volume of the fuel assembles (n each tme step), the temperature of these elements was nserted nto the nput fles of the WIMSD-B code (n each assembly). Thus a sequence of thermo-neutronc calculatons was formed based on the couplng of WIMSD5-B, CITATION-LDI2 and WERL codes. Fgure 6 provdes a schematc descrpton of the appled computatonal flowchart. Fgure 6. Schematc descrpton of the appled computatonal flowchart. Calculaton of the concentraton of gaseous fsson products released nto the gap space A fracton of the gaseous fsson products such as xenon sotopes and krypton, whch are produced n the fuel pellet, s released nto the gap space through the knockout and recol processes[12]. Snce they leave mportant effects on the heat transfer process and also on the neutronc calculatons of the reactor, here ther releasng process was modeled usng the Wesman method[19]. To ths end, the concentraton of the gaseous fsson products of each fuel assembly was calculated usng the WIMSD5-B code (n each tme step). Then the concentraton of the fsson products released nto the gap space was calculated usng the correlatons 1-5 [19]. C rel + C = C ret pro 1 K ( t ( K (1 exp( K2 t)) )[1 exp( K 2 t)]) + (9) C ret = C tot C rel (10)

8 C pro = Ctot C ret K1 = exp( ) T K 2 = exp( ) T (13) C tot C Where, pro C, rel C ret and are the concentratons of the total, produced, released and trapped gases (n moles) n the th tme step, respectvely. In addton, t s the length of the tme step (n seconds), and T denotes the fuel average temperature (n Kelvn). Snce the gaseous spaces of the gap and the upper capsule are nterconnected, the released fsson gases are dstrbuted n the entre space of ths gaseous space. From neutronc pont of vew, a fracton of the gaseous space, ncludng the upper capsule and central hole, s almost enumerated as nactve space. Therefore, when the modelng of the fsson gases release s taken nto account, the negatve reactvty caused by these gases wll be lower than the case where ths process s not taken nto account. Moreover, because of the hgh absorpton of neutron by these gases, once the gaseous fsson products are entered nto the central hole of a pellet, the fsson rate n the central secton s decreased and as a result, the central temperature of the fuel s reduced. To take nto account the effects of the released fsson gases on the calculaton of the temperature dstrbuton n the reactor core, frst the pressure changes caused by these gases were calculated usng the Lee-Kesler model. Next, after calculatng the mole fracton of each of the released gaseous fsson products, ths parameter was appled to the calculaton of the gap conductance coeffcent (based on the Ross-Stoute model). Calculaton of the crtcal borc acd concentraton durng the cycle In order to ncrease the precson of the calculatons, the cycle length was dvded nto small tme steps. Therefore, the concentraton of borc acd had to be calculated n each tme step. Employng the conventonal teratve methods requres a great deal of tme to be spent on the related calculatons; therefore, the crtcal borc acd concentraton was drectly calculated usng the followng correlaton [10]: 3 ρ B = C B (1 f0) (14) In ths correlaton, whch s employed to estmate the negatve radoactvty caused by borc acd, ρ B s the amount of negatve radoactvty caused by borc acd wth a concentraton of C B (ppm) f s the thermal utlzaton coeffcent n the absence of boron. The effectve multplcaton and 0 factor of the reactor n each tme step was calculated usng the CITATION-LDI2 code and subsequently the amount of the reactvty n each tme step was also calculated. Therefore, by replacng the amount of the excess reactvty wth the ρ B parameter, the correspondng crtcal borc acd concentraton was calculated. (12) (11) Results: In ths paper, the tme dependent or axal changes of the thermal power, mass qualty, gap conductance coeffcent of fuel, heat transfer coeffcent of coolant, gap pressure & thckness and the temperature dstrbuton of fuel elements and coolant of Bushehr s VVER-1000 reactor core has been studed durng the ntal startup and frst cycle. In fgures 7 to 11 the tme dependent changes of fuel, clad and coolant temperatures of fuel assembles n the Bushehr s VVER-1000 reactor has been shown.

9 Of course due to one-twelfth symmetry of the Bushehr s reactor core n the frst cycle, the results were presented only for a symmetrc secton of the core. Internal temperature of fuel pellet (oc) Tme (Day) Fgure7. Tme dependent changes of the nternal surface temperature of fuel pellet n each assembly durng the frst cycle (n the mddle of the core) External surface temperature of fuel pellet(oc) Tme (Day) Fgure8. Tme dependent changes of the external surface temperature of fuel pellet n each assembly durng the frst cycle (n the mddle of the core) Internal surface temperature of clad (oc) Tme (Day) Fgure 9. Tme dependent changes of the nternal surface temperature of clad n each assembly durng the frst cycle (n the mddle of the core)

10 External surface temperature of clad(oc) Coolant outlet temperature (oc) Tme (Day) Fgure10. Tme dependent changes of the external surface temperature of clad n each assembly durng the frst cycle (n the mddle of the core) Tme (Day) Fgure 11. Tme dependent changes of the outlet coolant temperature n each assembly durng the frst cycle. Fgures 12 to 16 descrbe the tme dependent changes of produced thermal power, mass qualty, gap conductance coeffcent, gap pressure and thckness n each assembly of Bushehr s VVER-1000 reactor. Fuel assembles power (MW) Tme (Day) Fgure 12. Tme dependent changes of the reactor s thermal power n each assembly durng the frst cycle.

11 Mass qualty Tme (Day) Gap conductance coeffcent (W/m^2k) Fgure 13. Tme dependent changes of coolant s mass qualty n each assembly durng the frst cycle Tme (Day) FA-82 FA-83 FA-84 FA-85 FA-86 FA-87 FA-88 FA-97 FA-98 FA-99 FA-100 FA-101 FA-102 FA-112 FA-113 FA-114 FA-115 FA-126 FA-127 Fgure 14. Tme dependent changes of the gap conductance coeffcent n each assembly durng the frst cycle (n the mddle of the core) Gap pressure (MPa) Tme (Day) FA-82 FA-112 FA-83 FA-84 FA-97 FA-85 FA-98 FA-86 FA-99 FA-87 FA-100 FA-113 FA-88 FA-101 FA-126 FA-102 FA-115 FA-127 FA-114 Fgure 15. Tme dependent changes of the gap pressure n each assembly durng the frst cycle

12 Gap thckness (m) Tme (Day) FA-82 FA-83 FA-84 FA-85 FA-86 FA-87 FA-88 FA-97 FA-98 FA-99 FA-100 FA-101 FA-102 FA-112 FA-113 FA-114 FA-115 FA-126 Fgure 16. Tme dependent changes of the gap thckness n each assembly durng the frst cycle (n the mddle of the core) FA-127 In fgures 17 to 24, the axal varatons of mass qualty, heat transfer coeffcent and temperature of coolant as well as the central temperature of fuel has been shown n the end of both startup process(day=100) and frst cycle(day=289.71). Mass qualty Hcool(W/m^2oC) Day= Core's axal length (m) Fgure17. The changes of mass qualty of coolant n the axal drecton of the core (n the end of ntal startup) Day= Core's axal length (m) Fgure18. The changes of heat transfer coeffcent of coolant n the axal drecton of the core (n the end of ntal startup)

13 Coolant temperature(oc) Fuel nsde temperature (oc) Mass qualty Day= Core's axal length (m) Fgure19. The changes of coolant temperature n the axal drecton of the core (n the end of ntal startup) Day= Core's axal length (m) Fgure 20. The changes of central temperature of fuel n the axal drecton of the core (n the end of ntal startup) Day= core's axal length (m) Fgure 21. The changes of mass qualty of coolant n the axal drecton of the core ( n end of frst cycle )

14 Hcool(W/m^2oC) Coolant temperature (oc) Fuel nsde temperature (oc) Day= Tme (Day) Fgure 22. The changes of heat transfer coeffcent of coolant n the axal drecton of the core (n end of frst cycle) Day= Tme (Day) Fgure 23. The changes of coolant s temperature n the axal drecton of the core (n end of frst cycle) Day= Tme (Day) Fgure 24. The changes of central temperature of fuel n the axal drecton of the core (n end of frst cycle) Fgures 25 and 26 compare the calculated results of maxmum power peakng factor and the reactor s radal power peakng factor dstrbuton wth the data presented n the safety analyss report of Bushehr s VVER-1000 reactor (Atomenergoproekt, 2003b).

15 Fgure 25.Comparson of the results of tme dependent changes of the maxmum power peakng factor of Bushehr s VVER-1000 reactor wth the data of FSAR. Fgure26.Comparson of the results of the power peakng factor dstrbuton of the Bushehr s VVER-1000 reactor wth the data of safety FSAR n the end of the frst cycle.

16 The mole fractons of the released fsson gases at the end of the frst cycle are shown n Fgure 27. Fgure 27. The mole fractons of the released fsson gases at the end of the frst operatonal cycle. Gven that for the frst tme these calculatons have been done n Bushehr reactor, therefore, expermental data was not avalable for benchmark. However, wth regard to the fact that there was a graph for gap conductance coeffcent changes for the hot fuel pn (versus Burnup changes) durng frst cycle n the fnal safety analyss report of VVER-1000 Reactor of Bushehr NPP, n order to ensure from the authentcty of the calculatons made n ths research, we were forced to make the smlar calculatons n ths regard, and the comparson results were ndcatve of mnor error n ths case, whch are outlned n fgure28. Fgure 28. A comparson between the gap conductance coeffcent resulted by FSAR data [2] wth results obtaned through Ross-Stoute calculatons (for the hot fuel pn)

17 Dscusson and concluson Through study of fgures 7 to 16, t s notced that by ncreasng the thermal power of Reactor (durng startup process) and subsequently ncrease n temperature(fgures7 to 11), the gap effectve thckness(fgure.16) wll reduce as a result of thermal expanson, whch as a result of ths the gap pressure(fgure. 15) wll go up. By reference to the equatons presented n the Ross-Stoute model [14, 18] and also study of fgure.14, t wll be notced that such changes n these parameters wll ncrease the gap conductance coeffcent. Through observaton of such phenomena, t could be concluded that the VVER-1000 Reactor typcally operates under a self-control and nherent safety status aganst ncrease n thermal power and temperature of the Reactor. Furthermore, wth the addton of gaseous fsson products nto the gap area, the gap pressure wll ncrease, and as t was notced n the Ross-Stoute model, wll ncrease the gap conductance coeffcent, and also n ts second role as a control feedback, by exertng stress n the fuel and clad surfaces, wll create stran n them (due to the presence of elastc characterstc n fuel and clad). It should be noted that ths phenomenon wll reduce the radus ncreasng caused by thermal expanson n fuel whch, as a control feedback, wll prevent extra decrement of the gap thckness. Because no study has been conducted to calculate the tme dependent changes of the thermohydraulc parameters of Bushehr s VVER-1000 reactor durng ntal startup and frst cycle, and no report s gven n the fnal safety analyss report of ths reactor (FSAR), there was no opportunty to compare our results wth other studes. However, wth regard to the calculatons and modelng whch were publshed by author n references no. [13]&[14] and also the comparson drawn between the results of the power peakng factor dstrbuton n the end of cycle(fgure.26) and the tme dependent changes of maxmum power peakng factor(fgure. 25) durng the ntal startup and frst cycle of Bushehr s VVER-1000 reactor wth FSAR data, t can be observed that the calculatons performed n ths paper are satsfactorly accurate. Bearng n mnd the partcular structure of the fuel rods of Bushehr s VVER-1000 reactor, makng use of conventonal codes (such as COBRA-EN) was not feasble for the thermo-hydraulc modelng of ths reactor. Furthermore, the prevous codes had defcences wth regard to modelng and estmatng the heat transfer process n the gaseous space of the gap. To ths purpose, a thermo-hydraulc computatonal program was desgned to correct these flaws, so that t would have the capablty of estmatng the concentraton of the released gaseous fsson products nto the gap and applyng t to the heat transfer process n ths area. Fnally by observng the tme dependent changes of fuel elements and coolant temperatures and also the value of mass qualty (fgure.13), gap pressure and thckness durng the ntal startup process and frst cycle, t can be concluded that the Bushehr s VVER-1000 reactor s completely safe durng ths perod.

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