CONVERSION OF THE THERMAL HYDRAULICS COMPONENTS OF ALMARAZ NPP MODEL FROM RELAP5 INTO TRAC-M

Transcription

1 International Conference Nuclear Energy for New Europe 2002 Kranjska Gora, Slovenia, September 9-12, CONVERSION OF THE THERMAL HYDRAULICS COMPONENTS OF ALMARAZ NPP MODEL FROM RELAP5 INTO TRAC-M César Queral, Javier Mulas, Ignacio Collazo, Alberto Concejal, Nicolás Burbano Departamento de Sistemas Energéticos (DSE). ETSI Minas. Universidad Politécnica de Madrid (UPM). Ríos Rosas, Madrid, Spain. 1 INTRODUCTION Inés Gallego Iberdrola Ingeniería Alfredo López Lechas CC.NN. Almaraz-Trillo AIE In the scope of a joint project between the Spanish Nuclear Regulatory Commission (CSN) and the electric energy industry of Spain (UNESA) on the USNRC state-of-the-art thermal hydraulic code, TRAC-M, there is a task relating to the translation of the Spanish NPP models from other TH codes to the new one. As part of this project, our team is working on the translation of Almaraz NPP model from RELAP5/MOD3.2 to TRAC-M. At present, several portions of the input deck have been converted to TRAC-M, and the output data have also been compared with RELAP5 data. This paper refers to the translation of the following thermal hydraulic models: pressurizer, hot and cold legs (including the pumps and the injection systems), and steam generators. The comparison of the results obtained with both codes shows a good agreement. However, some difficulties have been found in the translation of some code components, like pipes, heat structures, pumps, branchs, valves and boundary conditions. In this paper, these translation problems and their solutions are described. 2 BOUNDARY CONDITIONS The relation between the boundary conditions models of RELAP5 and TRAC-M is: Velocity or mass flow boundary condition: RELAP5: Time-Dependent Volume (TDV) + Time-Dependent Junction (TDJ) TRAC-M: FILL Temperature (or quality) and pressure boundary conditions: RELAP5: Time-Dependent Volume (TDV)+ single-junction

2 TRAC-M: BREAK + single junction Velocity and temperature boundary conditions: RELAP5: Time-Dependent Volume (TDV) + Time-Dependent Junction (TDJ) TRAC-M: FILL (IFTY=10, 11) + control blocks Only one problem has been observed in the transformation from TDV to BREAK. In TDV it is usual to take the volume and the area quite large; however for the BREAK components the length (DXIN), the volume (VOLIN) and the area (VOLIN/DXIN) are usually the same than those at the last cell of the adjacent component. We have checked if these rules are applied different results between RELAP5 and TRAC-M are obtained. The solution is, 1. To take a very large area (VOLIN/DXIN) in the BREAK component. With this condition the velocity inside the BREAK is null, like in the TDV. Also, It must be taken into account that it is necessary to use the option NFF=1 to avoid the impact of area change on the pressure drop between the BREAK and the first cell of the PIPE. 2. To put a very small length (DXIN). This option avoids the existence of a gravity term inside the BREAK. With these rules we have obtained equivalent results with TDV and BREAK. The FILL component has no translation problems. 3 PIPES The translation rules that we have applied are, The areas in TRAC-M (FA) are defined at the junctions, but in RELAP5 they are defined at the cells and junctions. A possible way to convert from RELAP5 to TRAC-M is to take the junction area as the smaller area of the two volumes adjacent to the junction. Loss coefficients. If IKFAC = 1 is taken then FRIC(NCELLS+1) is the same than the K factors of RELAP5. If Reverse flow energy loss coefficient have to be included it is necessary to put NFRC1=2. In this case the dimension of FRIC is 2 (NCELLS + 1). First the forward coefficients are introduced and later the reverse coefficients. In TRAC-M there are several options for the friction model (NFF= 0, 1, -1, -100). We have checked that the option that better reproduce the RELAP5 model is NFF=-1, excluding the junctions with BREAK (NFF=1) as commented above. Gravity array. The vertical angle (ccc06nn) and the elevation change (ccc07nn) of the cells in RELAP5 can be translated into two ways:

3 x 2 2 φ R2 = 55 o x 1 1 φ T M φ R1 = 35 o Figure 1: Relation between the vertical angles of RELAP5 and the GRAV term of TRAC-M 1. In TRAC-M the cell elevation is specified by the gravity term of the GRAV array. The GRAV gravity term is defined as the ratio of the change in elevation to the length of the flow path between cell centers. The relation with the vertical angles of RELAP5 φ 1,R5 and φ 2,R5, is (Figure 1), GRAV = sin φ T M = x 1 sin φ 1,R5 + x 2 sin φ 2,R5 x 1 + x 2 (1) 2. The user select a reference elevation and all other elevations are relative to that reference elevation. The relation with the change in elevation of the cells in RELAP5 is straightforward, ELEV T M,j = 1 j 1 2 ELEV R5,0 + ELEV R5,i ELEV R5,j (2) TRAC-P takes this cell-center ELEV elevation data and internally converts it to GRAV gravity-term data for use in the calculation. The wall roughness is uniform along the pipe in TRAC-M. If the control volumes in the RELAP5 pipe have different wall roughness, several TRAC-M pipes must be defined in the input file, one for each roughness. Hydraulic diameters: RELAP5 hydraulic diameters are cell-centered, while TRAC-M ones are junction-centered. In this case it is possible to follow the strategy similar to that for the junctions areas. 4 BIFURCATIONS There are several problems in the translation of the BRANCH (RELAP5) into TEE (TRAC- M), A BRANCH (RELAP5) could have N entrances but a TEE (TRAC-M) only could have two. This problem is well known and has been commented in several papers. The solution is to transform one BRANCH with N entrances in N-1 TEE. In TRAC-M it is required that several areas of a TEE have the same value: i=1

4 The average areas (VOL/DX) of cells JCELL and JCELL±1 The junction areas of the edges of the cell JCELL. This condition was not included neither in TRAC-PF1/MOD1 nor in the first versions of TRAC-PF1/MOD2. It was included in last versions of TRAC-PF1/MOD2 and has been maintained in TRAC-M. There are different ways to solve the limitations generated by this restriction. An example is shown in Figure 2. x 1 1 BRANCH x 1 D/2 1 x 1 D/2 TEE A j x 3 3 D A j D/2 x 3 3 D D x 3 D x 2 2 x 2 D/2 2 D/2 x 2 D/2 RELAP5 TRAC M Figure 2: Transformation of a BRANCH in a TEE Gravity term evaluation in TEEs The parameter GRAV of a TEE is calculated in a similar way to a PIPE. There is only one exception in the cell-edge interface between main-tube cell JCELL and side tube cell #1, Where x JCELL is defined as, GRAV = ELEV 1,Side-tube ELEV JCELL x JCELL + x 1,Side-tube ( ) x HDJCELL 1/2 + HD JCELL+1/2 x JCELL JCELL = min, 2 sin θ T M cos θ T M (3) (4) where θ T M is the angle between the main-tube and side tube cell #1. Its relation with the vertical angles of RELAP5 is, 5 HEAT STRUCTURES θ T M = φ R5,1ST φ R5,JCELL (5) Among the different problems that we have found in the translation of heat structures there are several ones interesting to mention, The principal difference between both models is the relation between the hydraulic mesh and the heat structure mesh,

5 In RELAP5 the position of the heat structures nodes coincides with the hydraulic nodes. In a PIPE with N cells there are N heat structure nodes and therefore N wall temperatures. 2. In TRAC-M the position of the heat structure nodes coincides with the edges of the hydraulic cells. In a PIPE with N cells there are N+1 heat structures nodes and therefore N+1 wall temperatures. In RELAP5 the wall heat transfer coefficient is calculated with properties in the hydraulic nodes. On the other hand, in TRAC-M the wall heat transfer coefficient is calculated with properties in the edges of the hydraulic cells (junction properties). Therefore in RELAP5 the wall heat transfer coefficient depends on the average velocity in the hydraulic cell while in TRAC-M depends on the junction velocity. For example, in a RELAP5 model with contractions/expansions in the junctions, Figure 3(a), it will be necessary to perform two changes in the model to make a correct translation, 1. to take the junction area as the smaller area of the two volumes adjacent to the junction (in RELAP5 the junction area could be smaller than the areas of the two volumes adjacent to the junction). With this change the average cell velocity and the junction velocity are the same and therefore the wall heat transfer coefficients will be quite similar, Figure 3(b). 2. On the other hand, if we change the junction area it will be necessary to adjust the loss coefficient in the junction to obtain the same pressure drop with both codes. RELAP5/MOD3 Primary side Secundary side Heat structure TRAC M Primary side Secundary side Heat structure RELAP5/MOD3 TRAC M CONTRACTIONS/EXPANSIONS ARE ELIMINATED Primary side Secundary side Primary side Secundary side Heat structure Heat structure (a) Comparison between RELAP5 and TRAC-M (b) Transformation from RELAP5 to TRAC-M Figure 3: Heat structures In TRAC-M the option of spherical geometry is not included. This limitation forces to use a cylindrical geometry (ROD), and afterwards adjust the area relation with the parameter RDX.

6 VALVES In TRAC-M this kind of component is compound by cells (like the PIPE component). However, in RELAP5 the VALVE component has no volume: therefore it is necessary to give a volume to the VALVE of RELAP5. For example, if we have a valve in the middle of a pipe, it is possible to take a portion of the length and volume of the PIPE (RELAP5) and assign it to the VALVE component of TRAC-M. There are other more complex cases, like in the regulation valves of spray lines, which transformation is shown in Figure 4. RELAP5 VALVE BRANCH BRANCH 1 2 x1 x2 VALVE x1 x2 TRAC M VALVE TEE TEE x1 x2 Figure 4: Transformation of a VALVE in a bifurcation Apart from this, we have found no difficulty to perform the transformation of the different kinds of valves from RELAP5 to TRAC-M. 7 PUMPS We have found two difficulties in the translation of this kind of component, In the second cell edge of the PUMP-TM component the elevation change across the interface and the frictional losses (both wall friction and additive losses) are considered to be identically zero regardless of the input values for GRAV(2) or ELEV(2), FRIC(2) or KFAC(2), and NFF(2). Due to this model, it is necessary to transform the PUMP-R5 model as is shown in Figure 5. Also the whole transformation of the loop is shown in the same figure. TRAC-M has a different coastdown model than RELAP5. In TRAC-M and RELAP5 the model of the friction torque is, Ω Ω Ω Ω 3 T f = C 0 + C 1 + C 2 + C Ω R Ω 2 3 R Ω 3 R (6) where Ω angular velocity; Ω R reference angular velocity; C 0, C 1, C 2 and C 3 input constants.

7 TDV TDJ PUMP SJ TDV PUMP 1m 325 FILL BREAK 1m Figure 5: Transformation of the loop 3 from RELAP5 to TRAC-M In TRAC-M if the pump-impeller angular velocity (pump speed) drops below the input specified value (TFRB), then a second set of constants are used to determine T f, T f = C 0 + C 1 Ω + C Ω Ω 2 Ω R Ω 2 R + C 3 Ω 3 Ω 3 R (7) where C 0, C 1, C 2 and C 3 are input constants. When the pump speed drops to 1/10 of the rated speed (Ω 10% ), C 0 and C 0 decrease in a linear way, Ω C 0 C 0 C 0 C Ω 0 (8) Ω 10% Ω 10% In order to obtain the same coastdown with both codes we have take TFRB = Ω 10%. Therefore, if Ω > Ω 10% the expression of T f is the same in both codes, but when Ω < Ω 10% the expressions for each code are different, Ω Ω Ω Ω 3 T f,r5 = C 0 + C 1 + C 2 + C Ω R Ω 2 3 R Ω 3 R T f,t M = C 0 Ω + C Ω 1 + C Ω Ω 2 + C Ω 10% Ω R Ω 2 3 R Ω 3 Ω 3 R (9) We have imposed that T f,r5 = T f,t M for three values Ω = Ω 10%, Ω 5%, Ω 1%. With this adjustment we have obtained a quite similar coastdown with both codes.

8 PRESSURIZER SPRAY In TRAC-M, the flow area of the entrance of the spray inside the pressurizer must be sized so that the liquid velocity at the PRIZER-component top cell edge exceeds 4.0 m ft ( ). s s This will ensure that the condensation model in the PRIZER component is activated to provide a more accurate pressure response during spraying. With this in mind it is necessary to change the area of the TRAC-M model and to adjust the pressure drop in the spray line line model to maintain the same mass flow than in RELAP5 model, Figure PZR HEAD 1m 1m FIRST: ADJUST THE AREA SECOND: ADJUST THE LOSS COEFFICIENTS 153 Figure 6: Transformation of the spray line 9 CONCLUSIONS The difficulties that we have found in the translation of some code components, like pipes, heat structures, pumps, branchs, valves and boundary conditions have been solved. The rules to make the translations are very easy in the majority of the cases, only a few of them have a little difficulty. In the next months the translation of Almaraz NPP model from RELAP5/MOD3.2 to TRAC-M will be finished and the validation with plant transients will be performed. It has been necessary to use the original data base document to perform in the steam generator model. 10 REFERENCES 1. C. Queral et al., TRAC-M Almaraz NPP Model Translation from RELAP5 to TRAC-M, Final Report, CTC/CSN-DSE Project, December TRAC-P User s Guide, Vol. II, Los Alamos National Laboratory, November 1996.