17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007

CONTENT Introduction ATHLET/BIPR-VVER reactor pressure vessel model - mixing at assembly head Exercise 1 of Phase 2 of the CEA-NEA/OECD VVER-1000 Coolant Transient Benchmark Thermocouple correlation Further developments Summary

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 INTRODUCTION The nodalization of the RPV and a correct description of the mixing phenomena in the RPV plays a very big role on the accuracy of the predicted local core parameters which are needed to check the acceptance critera. Recent studies proved that additional modelling of the assembly outlets by the coupled code ATHLET/BIPR-VVER is necessary in order to take into account the fluid mixing phenomena at the thermocouple location Correlation based on measured thermal-couples‘ values at core outlet (for VVER-1000) with the real coolant temperatures at those positions are necessary for correct comparison In order to meet all these additional requirements, new models have been included in the coupled code ATHLET/BIPR-VVER and an international Benchmark problem based on experimental data is recalculated

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 NODALIZATION OF THE REACTOR VESSEL (OPTIMAL nodalization schema) 16 down comers modelled with 16 parallel thermal-hydraulic channels (PTHC) with cross flows (CF). 16x7= 112 bottom plenums (2 levels) modelled with 118 PTHCs with CFs which describe the volume of the reactor bottom part with the perforated elliptical bottom plate up to the fuel assembly support plate = 326 PTHC in the core (2:1) – 2 PTHC per assembly – 163 for the assembly flow – 163 for the control rod guide tube flow 3 different types of guide tube channels – Empty – Burnable absorbers – Control rods

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , (14) 159(14) 160(14) 161(14) 162(14) 163(14) 149(15) 150( 0) 151( 3) 152( 0) 153( 9) 154( 0) 155( 4) 156( 0) 157(13) 139(15) 140( 4) 141( 0) 142( 7) 143( 1) 144( 2) 145( 8) 146( 0) 147( 3) 148(13) 128(15) 129( 0) 130( 8) 131( 0) 132( 0) 133( 5) 134( 0) 135( 0) 136( 7) 137( 0) 138(13) 116(15) 117( 9) 118( 2) 119( 0) 120(10) 121( 0) 122( 0) 123(10) 124( 0) 125( 1) 126( 9) 127(13) 103(15) 104( 0) 105( 1) 106( 6) 107( 0) 108( 0) 109( 6) 110( 0) 111( 0) 112( 6) 113( 2) 114( 0) 115(13) 89(15) 90( 3) 91( 7) 92( 0) 93( 0) 94( 6) 95( 0) 96( 0) 97( 6) 98( 0) 99( 0) 100( 8) 101( 4) 102(13) 76( 0) 77( 0) 78( 0) 79(10) 80( 0) 81( 0) 82( 5) 83( 0) 84( 0) 85(10) 86( 0) 87( 0) 88( 0) 62(16) 63( 4) 64( 8) 65( 0) 66( 0) 67( 6) 68( 0) 69( 0) 70( 6) 71( 0) 72( 0) 73( 7) 74( 3) 75(12) 49(16) 50( 0) 51( 2) 52( 5) 53( 0) 54( 0) 55( 6) 56( 0) 57( 0) 58( 5) 59( 1) 60( 0) 61(12) 37(16) 38( 9) 39( 1) 40( 0) 41(10) 42( 0) 43( 0) 44(10) 45( 0) 46( 2) 47( 9) 48(12) 26(16) 27( 0) 28( 7) 29( 0) 30( 0) 31( 6) 32( 0) 33( 0) 34( 8) 35( 0) 36(12) 16(16) 17( 3) 18( 0) 19( 8) 20( 2) 21( 1) 22( 7) 23( 0) 24( 4) 25(12) 7(16) 8( 0) 9( 4) 10( 0) 11( 9) 12( 0) 13( 3) 14( 0) 15(12) 1(11) 2(11) 3(11) 4(11) 5(11) 6(11) (0) – empty guide tubes; (1-10) – control rod group numbers; (11-16) – burnable absorbers. Location of the different types of guide tube channels in the core

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , bypass THC 24 axial nodes in the active core 2 upper plenums and 1 reactor head = 326 heat structures (HS) in the core – 163 HS for the fuel assemblies – 163 HS for the guide tubes Neutronically the core is modelled 1:1 (1 node per assembly in X-Y plane) All other details concerning nodalization and modelling of the primary and secondary loop can be seen in: S. Nikonov, Lizorkin M., Kotsarev A., Langenbuch S., Velkov K., Optimal Nodalization Schemas of VVER-1000 Reactor Pressure Vessel for the Coupled Code ATHLET-BIPR8KN, 16th Symposium of AER, Bratislava, September 2006.

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 Isolation (closure of SIV-1 and FW valve)of SG-1 of 9.36% Pnom TRANSIENT: Isolation (closure of SIV-1 and FW valve) of SG-1 at reactor power of 9.36% Pnom (Benchmark V1000CT – Phase 2, Exersice 1) Comparison of the cold and hot legs’ temperature agree very well with the measurements. The maximum differences are 1.8 K. These differences are small considering the reported measurements’ error of 2.0 K. The differences in the predicted local coolant temperatures at the begin and at the end of the transient compared with the measured one are small. At t=0 s the maximum assembly coolant temperature deviation is 1.4 K, and at the end of the transient – 5.8 K.

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 COMPARISON WITH MEASUREMENTS – LOOPS‘ COOLANT TEMPERATURES Loop #1 Loop #2

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 Loop #3 Loop #4

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 Comparison of outlet coolant temperature histories for different types of assemblies with different guide tube channel usage

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 Mean ( o C) Max ( o C) Min ( o C) Max - Min ( o C) Maximum Deviation ( o C) (Assembly #) Minimum Deviation ( o C) (Assembly #) SIGMA Experiment Assemblies ( 51) ( 31) Guide tubes ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) % mixing ( 51) ( 31) Comparison of different flow mixing relations on the model accuracy for the end of the experiment

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 THERMOCOUPLE CORRELATION (interpretation of the TC measurements) T TC = (T GT + C M * T ASS ) / ( 1 + C M ) T TC - thermocouple temperature T GT - guide tube coolant flow temperature T ASS - fuel assembly coolant flow temperature C M - mixing coefficient (0.2)

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 FURTHER DEVELOPMENTS Confirmation of the TC correlation for nominal and intermediate reactor power (in preparation ) The TC correlation is derived from data set with a heat up of only 3 o C and reactor power of 9.4 % P nom Dependence of the mixing coefficient at assembly head from the type of the guide tube application (empty, inserted rods, CRs insertion depth, burnable absorbers) Study the influence of different coolant temperature in the guide tubes on the accuracy of the microscopic cross section generation and homogenization procedures

17th Symposium of AER, Yalta, Crimea, Ukraine, Sept , 2007 SUMMARY A method is developed which allows to take into account the correct interpretation of the TC measurements (still subjected to validation) Additional modelling in the coupled code ATHLET/BIPR-VVER is developed to meet the requirements of the correct description of the fluid mixing phenomena at the places where the TCs are located (additional PTHC introduced) The Exercises of Phase 2 of the CEA-NEA/OECD VVER-1000 Coolant Transient are recalculated introducing the new TC correlation and the data are compared with the old ones The coupled system code ATHLET/BIPR-VVER is able to predict the coolant temperature at the assembly outlet within a rather high accuracy even though ATHLET system code is based on 1-D thermal-hydraulic pipe models