Analysis Simulator for Kozloduy NPP Units 5 and 6 N.Rijova (ENPRO Consult), J.Steinborn (GRS mbH)

1. Analysis Simulator for Kozloduy NPP Units 5 and 6N.Rijova (ENPRO Consult), J.Steinborn (GRS mbH) International Nuclear Forum BULGARIAN NUCLEAR ENERGY – NATIONAL, REGIONAL AND WORLD SAFETY BULATOM, May 28-30, 2008, Varna, Bulgaria

2. Analysis Simulators on the basis of ATHLET and ATLAS By means of the best-estimate thermal-hydraulic code ATHLET and plant analyzer system ATLAS, analysis simulators both for German NPPs and for NPPs with WWER type reactors have been developed at GRS during the recent years. Analysis simulators for WWER have been developed, verified and validated in the framework of the technical cooperation between GRS and different Russian organisations: • Balakovo NPP Units 1 ÷ 4 (WWER-1000/320) • Volgodonsk NPP Unit 1 (WWER-1000/320) • Kalinin NPP Units 1 and 2 (WWER-1000/338) • Kola NPP Unit 1 and 2 (WWER-440/230) • Kola NPP Unit 3 and 4 (WWER-440/213) BULATOM

3. Analysis Simulator for Kozloduy NPP Units 5 and 6 • In the framework of the technical cooperation between GRS and ENPRO Consult an analysis simulator for Kozloduy NPP has been developed, based on the AS for a generic WWER-1000/320. • The differences between the generic WWER-1000/320 plant and Kozloduy NPP 5&6 have been specified and the input deck has been modified in order to represent the real characteristics of Kozloduy Units 5 and 6. • Additionally, the initial nodalization of the reactor has been improved to take into account the unsymmetrical location of the circulation loops, which is important for transients with non-symmetrical behaviour of the loops. • The graphical user interface of the simulator is created by means of the GRS developed plant analyser system ATLAS and is also based on the graphical interface of the AS for a generic WWER-1000/320 plant. BULATOM

4. ATHLET code • The thermal–hydraulic system code ATHLET (Analysis of THermal–hydraulics of LEaks and Transients) is being developed by GRS for the analysis of the whole spectrum of leaks and transients in light water reactors. The code is composed of several basic modules for the simulation of the different phenomena involved in the operation of light water reactors: • thermo-fluid dynamics; • heat transfer and heat conduction; • neutron kinetics; • non-condensable gases behaviour; • dissolved nitrogen; • boron transport. BULATOM

5. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (1) The ATHLET model of Kozloduy NPP Units 5 and 6 represents the following components: • reactor; • four loops of the primary circuit; • pressurizer; • steam generators with steam lines; • emergency core cooling systems; • emergency gas evacuation system. BULATOM

6. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (2) For an effective simulation of the main plant controllers a number of FORTRAN subroutines are implemented. They model in detail the following controllers: • reactor power controllers (ARM, ROM); • pressurizer level controller (make-up system); • primary circuit pressure controller (spray system and heaters); • electro-hydraulic turbine control system; • secondary side pressure controllers (BRU-A and BRU-K); • steam generators level controllers and emergency feed water controllers. BULATOM

7. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (3) BULATOM

8. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (4) BULATOM

9. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (5) BULATOM

10. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (6) The basic input data deck models the downcomer with four parallel channels, but without taking into account the neighbouring nozzles asymmetry. Further the four parallel flows of the downcomer are fully mixed in the lower plenum, which is modelled as one channel. The core region is modelled by two parallel thermal-hydraulic channels, representing the central and peripheral part of the core, which are azimuthally equivalent. In each of them a “hot” fuel element is determined with the maximal possible power. The upper plenum region follows the nodalization of the core. Such nodalization of the reactor is suitable for regimes with symmetrical behaviour of the loops. BULATOM

11. ATHLET input data deck for Kozloduy NPP Units 5 and 6 6-channel nodalization of the reactor (1) The symmetrical nodalization of the reactor is suitable for regimes with symmetrical behaviour of the loops. But even the regime with trip of one MCP, which is an anticipated transient for WWER-type reactors and quite an often operational event, could not be simulated adequately if a complete coolant mixing is assumed. As it is known by the plant measurements, the influence of the loop with the tripped MCP is stronger for the loop, located closer, than to the loop, located farther. That is why a new input data deck has been developed, providing a finer nodalization of the reactor: 6 channels in the downcomer, 6 – in the lower plenum, 18 channels in the core (6 central, 6 peripheral and 6 bypasses), 12 – in the upper plenum (6 inner and 6 outer). BULATOM

12. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (2) 6-channel nodalization of the reactor (2) BULATOM

13. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (1) 6-channel nodalization of the reactor (3) BULATOM

14. ATHLET input data deck for Kozloduy NPP Units 5 and 6 (1) 6-channel nodalization of the reactor (4) BULATOM

15. CONDRU code The CONDRU code was developed at GRS to model in a simple way the thermal-hydraulic parameters in the containment during LOCA. It has a two-phase, three-component model. The containment is represented by two nodes. Another node represents the environment. The code is coupled with ATHLET. In case of LOCA the data for the mass and energy rates of the leak are transferred from ATHLET to CONDRU. Also the spray system mass flow rate and the heat, added to the containment from the structures of the reactor facility, can be transferred to CONDRU. The main output of CONDRU is the calculated containment pressure. It is used in ATHLET code as a back pressure for the leaks and also for determination of the time of the signal for containment isolation, which is important for the correct modelling of the status of many systems. BULATOM

16. Plant analyzer system ATLAS (1) The plant analyser tool ATLAS (ATHLET Analysis Simulator) was developed by GRS with the aim to create a multi purpose tool for analyses in the field of nuclear and industrial plant safety. It is based on computer codes, modelling the dynamic processes in the plant, and offers a simulation environment in which the presentation and evaluation of the numerous results is supported by an interactive visual display system. In this way, it provides possibilities to intervene directly into the simulation as the calculation proceeds. The visual display system is supplemented by the graphics editor APG (ATLAS Picture Generator) which creates the images interactively. BULATOM

17. Plant analyzer system ATLAS (2) Special features of the visualisation system are: • Graphics based on OpenGL (WINDOWS version) and GKS (UNIX version) for portability; • All geometrical and graphical attributes can be dynamically changed by using data from the simulation; • All simulation data are available as trends; • Trend group images with several axes and several functions per axis can be created; • Handling of the process by interactive mouse-clicks on symbols; • Automatic or manual scaling of the parameters and the time section. • The plant analyser is controlled by the mouse. A menu bar with the most important functions always appears on each frame. Image related functions can be activated by mouse-clicks on buttons in each image. Clicking on the symbols either activates interventions in the simulation program or calls up the trend of the associated process variable. BULATOM

18. BULATOM

19. Graphical user interface (1)Synopsis picture BULATOM

20. Graphical user interface (2)Main interactive picture BULATOM

21. Graphical user interface (3)Reactor power control BULATOM

22. Graphical user interface (4)Reactor protection and interlocks BULATOM

23. Graphical user interface (5)Main parameters of the unit BULATOM

24. Graphical user interface (6)Containment and ECCS BULATOM

25. Graphical user interface (7)Secondary side BULATOM

26. Graphical user interface (8)Reactor BULATOM

27. Graphical user interface (9)Steam generators BULATOM

28. Cross-verification with Relap 5 (1) The first application of the newly developed analysis simulator for Kozloduy NPP Unit 5 and 6 was performed on the transient with inadvertent opening of one pressurizer safety valve with failure to close. This scenario had been previously analyzed in ENPRO Consult using RELAP 5 code. The comparison demonstrates a very good agreement between the codes. BULATOM

29. Cross-verification with Relap 5 (2)Scenario of the transient BULATOM

30. Cross-verification with Relap 5 (3)Scenario of the transient BULATOM

31. Cross-verification with Relap 5 (4)Inadvertent opening of one pressurizer safety valve Mass flow rate through the opened valve BULATOM

32. Cross-verification with Relap 5 (5)Inadvertent opening of one pressurizer safety valve Integral mass flow through the opened valve BULATOM

33. Cross-verification with Relap 5 (6)Inadvertent opening of one pressurizer safety valve Power, released through the valve BULATOM

34. Cross-verification with Relap 5 (7)Inadvertent opening of one pressurizer safety valve Power, transferred through one steam generator BULATOM

35. Cross-verification with Relap 5 (8)Inadvertent opening of one pressurizer safety valve Pressure above the core BULATOM

36. Cross-verification with Relap 5 (9)Inadvertent opening of one pressurizer safety valve Mass flow rate from ECCS BULATOM

37. Validation with operational event № 747 (1)Trip of MCP 3 Temperature in the cold leg of loop 3 BULATOM

38. Validation with operational event № 747 (2)Trip of MCP 3 Temperature in the hot leg of loop 3 BULATOM

39. Validation with operational event № 747 (3)Trip of MCP 3 Temperatures in the cold legs of non-affected loops (1, 2 and 4) BULATOM

40. Validation with operational event № 747 (4)Trip of MCP 3 Temperatures in the cold legs of non-affected loops (1, 2 and 4) BULATOM

41. Conclusions • The analysis simulator for Kozloduy NPP is based on the well known best-estimate code ATHLET and plant analyser tool ATLAS. • The model represents all important reactor systems and controllers. It is based on the generic input data deck for WWER-1000/320, which has been constantly developed, verified and validated during approximately 15 years. It has been used for analysing wide spectrum of accidents for different NPP units, including operational events. • The first applications of the Analysis simulator for Kozloduy NPP Units 5 and 6 demonstrated its ability for adequate modelling of the thermal hydraulic processes at the plant. • Further improvement, verification and validation of the AS are planed. For instance, coupling with the GRS code COCOSYS for a better simulation of the processes in the containment is possible in the near future. • The possibilities of the graphical user interface for on-line visualisation and interactive initiation of equipment failures and operators actions make it a very powerful instrument for verification of emergency operating procedures. BULATOM