Coupling of thermal-mechanics and thermalhydraulics codes for the hot channel analysis of RIA events First steps in AEKI toward multiphysics

Transcription

1 Coupling of thermal-mechanics and thermalhydraulics codes for the hot channel analysis of RIA events First steps in AEKI toward multiphysics A. Keresztúri, I. Panka, A. Molnár KFKI Atomic Energy Research Institute, H-1525 Budapest 114, POB 49, Hungary 121th Symposium of AER on VVER Reactor Physics and Reactor Safety September 19-24, 2011, Dresden, Germany AER-2011 Dresden

2 Contents Introduction: the relation of the presented activity to the final targets of the safety analysis Why the multi-physics treatment can be useful or even necessary? Arguments for using thermal-mechanical codes and coupling them to thermal hydraulic sub-channel codes for the hot channel analysis. The method of coupling the FRAPTRAN fuel behavior and the TRABCO hot channel thermal hydraulic codes Demonstration: results for a Control Rod Ejection event Conclusions AER-2011 Dresden

3 Introduction The hot channel calculation is an important final stage of the safety analysis for checking the acceptance criteria fulfillment or/and for counting up the failed fuel rods, necessary for the activity release evaluation. Quantifying the uncertainties of the final results originating from the hot channel analysis is an essential condition for the applicability of the methods and codes for the safety analyses. For quantifying the uncertainties a best estimate calculation method is necessary, while the present hot channel calculation methods are usually comprise essential assumptions, expected conservative, concerning the gap conductance and the coolant mixing. AER-2011 Dresden

4 Importance of the correct fuel state modeling in a CRE event Time dependence of the heat transferred to the coolant QLE (W/cm) Korrekt alapeset "0" kiégéshez tartozó tabletta hıvezetés és radiális eloszlás "0" kiégéshez tartozó réshıvezetés Egyenletes teljesítményeloszlás Black: Base case (without approximation) Violet: Pellet heat conductance and power distribution according zero burnup Blue: Flat power distribution 100 Green: Gap conductance according to zero burnup Idı ( s) AER-2011 Dresden

5 Power density radial distribution in the pellet at various pellet average burnup values Plutonium build up at the pellet edge, due to the U-238 strong resonance absorption cross section BU= E+00MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU Power density (W/cm 3 ) Relative radius AER-2011 Dresden

6 Local burnup radial distribution in the pellet at various pellet average burnup values. 1.0E E+04 BU= E+05MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU BU= E+05MWd/tU 8.0E+04 Radial burnup (MWd/tU) 7.0E E E E E E E Relative radius AER-2011 Dresden

7 Pellet heat conductivity coefficients depending on the temperature at various local burnup values 6.0 BU=00000MWd/tU BU=10000MWd/tU 5.0 BU=20000MWd/tU BU=40000MWd/tU Pellet heat conductance (W/K/m) T (Kelvin) AER-2011 Dresden

8 Gap conductance depending on the pellet average burnup at various linear heat rate values. Obtained from long term fuel behavior calculations Gap conductance [W/cm 2 K] Ql, kw/m AER-2011 Dresden

9 Hot channel calculations of the safety analysis considering the pin-wise power distributions inside the assembly Phenomena influencing the coolant state: 1. Intensive cooling from coolant MIXING from the not heated wall region and from the less heated adjacent channels 2. Increased resistance or blockage of the hot channel due to the temperature increase or even the void, REDISTRIBUTION of the flow map, the flow can have a tendency avoid the hottest point AER-2011 Dresden

10 DNBR minimum of the CRW ATWS event for different relative pin power distribution in the assembly. The sub-channel outlet temperature and the linear heat rate is limited in the Normal Operation 1,1 2 1,1 1,0 8 1,0 6 DNBR min 1,0 4 1, ,9 8 O u te rm o s t ro w S e c o n d ro w C o rn e r p in 0,9 6 0,9 4 0, ,1 1,2 1,3 1,4 1,5 1,6 K k AER-2011 Dresden

11 Necessity of the multi physics approach including thermal mechanical and thermal hydraulic calculations The gap conductance gap size and pressure - play essential role in case of fast transients. Both the initial and time dependent values, the latter ones changing during the transient, is important. The mixing effects play also essential role, because the heat transfer regime, cladding temperature depend on the coolant state. Some acceptance criteria are of mechanical character: strains and stresses are to be determined. There is a recent tendency to define acceptance criteria, which are closer to the real reasons of the fuel failure and can be less conservative. AER-2011 Dresden

12 PHENOMENA LEADING TO FUEL FAILURE AND ACTIVITY RELEASE Hoop strain Stress corrosion cracking of the cladding Reaching ultimate strain or stress of cladding Fuel rod collapse Cladding fatigue Large deformations Cladding diameter reduction Cladding elongation Fuel rod internal pressure Corrosion (oxidation) AER-2011 Dresden

13 The two codes used in the calculations TRABCO: Thermal hydraulic hot channel code, single channel, 1D, 4-equation model Without thermal mechanics, geometrical changes can be followed only in an implicit way: parameterized conservative - gap heat conductance is to be applied. FRAPTRAN: Transient fuel behavior code, single channel thermal hydraulics, frequently not used (cladding outer surface temperature is prescribed), no mixing with the adjacent channels AER-2011 Dresden

14 Computational method of the on-line coupling INTEL FORTRAN service function USE DFWIN for sharing selected memory parts between separately parallel running processes Possibility to develop our own FORTRAN subroutines for assuring synchronization Minimizing the necessary modifications in the standalone codes Only few control points for the data transfer and synchronization had to be built in for using its own separated memory of each separate parallel task, and at the same time to define the shared part of the memory for the communication between them. AER-2011 Dresden

15 Reactor Physics Region-wise power distribution, neutron flux, isotopic composition Region-wise temperature distribution Coolant temperature Fuel behavior: Thermalmechanics, heat conduction Heat transfer coefficient, bulk coolant temperature Thermal hydraulics Cladding wall temperature, heat flux AER-2011 Dresden

16 The three investigated ways of the hot channel calculation Standalone TRABCO calculation with parameterized conservative enveloping gap heat conductance according to the preceding long term stationary fuel behavior calculations results. Beside the pellet burnup, which is constant in these investigations, the average pellet temperature was selected for the parameterization. Two subcases: Method 1.a no DNB, Method 1.b for methodical reasons, DNB is supposed conservatively. Similarly to Method 1, standalone TRABCO calculation with parameterized conservative enveloping gap heat conductance, but the gap conductance to be parameterized is taken from the FRAPTRAN results of the coupled calculation. On-line coupling of the TRABCO and FRAPTRAN codes. At the end of each time step data exchange. AER-2011 Dresden

17 Gap heat transfer coefficients from the preceding stat. calc. or from the FRAPTRAN Gap conductance [W/m 2 /K] Axial level 1 Axial level 2 Axial level 3 Axial level 4 Axial level 5 Axial level 6 Axial level 7 Axial level 8 Axial level 9 Axial level 10 Axial level 11 Enveloping curve Avaraged fuel temperature [K] From stationer calculation AER-2011 Dresden

18 Fission power for the CRE event starting from HZP state 4500 Axial Power [kw/m] ,29 0,3 0,31 0,32 0,33 0,34 0,35 0,36 0,37 0,38 Time [s] AER-2011 Dresden

19 Surface heat flux for the three cases, no DNB 400,00 350,00 300,00 Heat flux [W/cm 2 ] 250,00 200,00 150,00 100,00 50,00 Coupled FRAPTRAN/TRABCO TRABCO stand alone (FRAPTRAN gap conductance) TRABCO stand alone (gap conductance from stationer calc.) 0,00 3,0E-01 3,2E-01 3,4E-01 3,6E-01 3,8E-01 4,0E-01 4,2E-01 4,4E-01 4,6E-01 4,8E-01 5,0E-01 Time [s] AER-2011 Dresden

20 Heat transfer coefficients to coolant, no DNB Surface Heat Transfer Coefficient [W/(m 2 K)] ,5 1 1,5 2 2,5 3 3,5 Time [s] Switching between the different heat transfer regimes: convective heat transfer ( Dittus-Boelter ), sub-cooled boiling ( Thom ) AER-2011 Dresden

21 DNBR for the three cases DNBR [-] 1,50E+00 1,45E+00 1,40E+00 1,35E+00 1,30E+00 1,25E+00 1,20E+00 DNBR COUPLED FRAPTRAN-TRABCO STANDALONE TRABCO, GAP HEAT CONDCTUNCE FROM THE FRAPTRAN CALC. STANDALONE TRABCO, CONSERVATIVE GAP HEAT CONDCTUNCE FROM THE STAT: CALC. 1,15E+00 1,10E+00 1,05E+00 1,00E+00 0,36 0,37 0,38 0,39 0,4 0,41 0,42 0,43 Time [s] Gap conductance [W/m 2 /K] Axial level 1 Axial level 2 Axial level Axial level 4 Axial level Axial level 6 Axial level Axial level 8 Axial level Axial level 10 Axial level Enveloping curve From stationer calculation Avaraged fuel temperature [K] AER-2011 Dresden

22 Surface heat flux for the three cases, DNB in the worst case 350,00 300,00 250,00 Heat flux [W/cm 2 ] 200,00 150,00 100,00 50,00 Coupled FRAPTRAN/TRABCO TRABCO stand alone (FRAPTRAN gap conductance) TRABCO stand alone (gap conductance from stationer calc.) 0,00 3,0E-01 3,2E-01 3,4E-01 3,6E-01 3,8E-01 4,0E-01 4,2E-01 4,4E-01 4,6E-01 4,8E-01 5,0E-01 Time [s] AER-2011 Dresden

23 Maximum cladding temperatures for the three cases, DNB in the worst case 8,00E+02 7,00E+02 Maximum cladding temperature [C] 6,00E+02 5,00E+02 4,00E+02 3,00E+02 TRABCO (Base case) FRAPTRAN/TRABCO (coupled) TRABCO (using FRAPTRAN based gap conductance) 2,00E+02 1,00E+02 0,00E ,5 1 1,5 2 2,5 3 3,5 Time [s] AER-2011 Dresden

24 Heat transfer coefficients to coolant, DNB in the worst case Surface Heat Transfer Coefficient [W/(m 2 K)] ,34 0,36 0,38 0,4 0,42 0,44 Time [s] Switching between the different heat transfer regimes: convective heat transfer ( Dittus-Boelter ), sub-cooled boiling ( Thom ), transition boiling AER-2011 Dresden

25 Gap gas pressure [MPa] 2,5 2,4 2,3 2,2 2,1 2 1,9 1,8 1,7 1,6 1,5 Gap gas pressure DNB No DNB 0,1 0,3 0,5 0,7 0,9 1,1 1,3 1,5 Time [s] AER-2011 Dresden

26 Thermal mechanical processes, gap size, no DNB 0,035 0,03 0,025 Radial Gap [mm] 0,02 0,015 0,01 0, ,5 1 1,5 2 2,5 3 3,5 Time [s] AER-2011 Dresden

27 Thermal mechanical processes, gap size, DNB in the worst case 0,035 0,03 0,025 Radial Gap [mm] 0,02 0,015 0,01 0, ,5 1 1,5 2 2,5 3 3,5 Time [s] AER-2011 Dresden

28 Gap heat transfer coefficient Gap Heat Transfer Coefficient (W/(m 2 K) DNB No DNB Gap Heat Transfer Coefficient (W/(m 2 K) ,3 0,32 0,34 0,36 0,38 0,4 0,42 0,44 0,46 Time (s) AER-2011 Dresden

29 Cladding hoop strain at different axial levels in case of DNB No fuel failure can be expected! 0,005 0,0045 0,004 Cladding Hoop Strain 0,0035 0,003 0,0025 0,002 0,0015 0,001 0, ,5 1 1,5 2 2,5 3 3,5 Time [s] AER-2011 Dresden

30 Summary The gap conductance, gap size and pressure play essential role in case of fast reactivity transients. Both the initial and the time dependent values, the latter ones changing during the transient, are important. In case of fast RIA transients, a thermal mechanical code has to be connected on-line to the thermal hydraulics for the best estimate calculation the latter approach necessary for uncertainty analysis -, or even only for justifying a method, expected conservative. A not negligible reserve, concerning our earlier conservative method, could be explored by the on-line coupling the TRABCO and FRAPTRAN codes. AER-2011 Dresden