Analysis of a Natural Circulation Transient at the VISTA Facility

1. Analysis of a Natural Circulation Transient at the VISTA Facility Young-Jong Chung Korea Atomic Energy Research Institute IAEA’s 4rd Research Coordination Meeting on the CRP on Natural Circulation Phenomena, Modeling, and Reliability of Passive Systems that Utilize Natural Circulation IAEA/Vienna, Austria, 10-14 Sept. 2007

2. CONTENTS • INTRODUCTION • SMART Plant • VISTA Experimental Facility • ANALYSIS CODE • MARS code • TASS/SMR code • INITIAL and BOUNDARY CONDITIONS • RESULTS and DISCUSSIONS • SUMMARY

3. CEDM MCP Annular Cover Pressurizer Steam Generator Reactor Vessel Core Side Screen Displacer INTRODUCTION • SMART (System-integrated Modular Advanced ReacTor) : • SMART plant is designed to supply an electricity and portable water • Maximum thermal power : 330 MWt • Portable water production : 40,000 tons/day • Electricity generation : ~90 MWe • Major components • Pump Type : Canned motor MCPs • Eliminates pump seal leak SBLOCA • SG Type : Helically coiled • Reduce possibility of SGTR acc. • PZR : A passive type with a large volume • The system pressure is self-controlled by N2 gas • Remove active component (spray and heater)

4. INTRODUCTION • SAFETY SYSTEM • Passive residual heat removal system • Safety injection system • Over pressure protection system (POSRV) • Shutdown cooling system

5. INTRODUCTION PRHRS • Consist of Hx, compensating tank, RWT, check and isolation valve • For normal operating condition; This system is isolated from the secondary sys. • For accident condition; Close MFIV and MSIV Open PRHRS isolation valves Established a natural circulation flow by gravity • Characteristics Remove decay heat for 3days without operator action • In order to verify the performance and the safety of the SMART plant, the VISTA experimental facility is constructed

6. INTRODUCTION – VISTA Facility • VISTA Facility • Design to test system behavior of the integral reactor, SMART • Design for full pressure and full temperature operation • Primary components are simplified to be a loop type • Secondary and PRHR systems have a single train • Major components • Vessel • 1 SG cassette, • 1 MCP • 1 train of PRHRS

7. Pressurizer Gas Cylinder CEDM MCP PZR Steam Generator SG Reactor Vessel Reactor Vessel Core Shielding INTRODUCTION : SMART vs VISTA MCP

8. VISTA Test • Overall Thermal-hydraulic Tests • Main components characteristics (pump, valve, heat exchanger) • Heat transfer at heater rod, PRHR Hx, Helical SG • System behavior during power change • Performance Tests • Reactor shutdown operation • Start-up, power change and MCP transient operations • PRHRS natural circulation operations • Transient and Accident Tests • Increase and decrease of feedwater flow • Increase steam flow • Power increase transient

9. NATURAL CIRCULATION ANALYSIS CODE • MARS 3.0 code • 1-D and 3-D system analysis code for thermal hydraulic analysis of the light water reactor transients • Developed at the KAERI by consolidating and restructuring the RELAP5/MOD3.2 and COBRA-TF codes • SMART specific models • Helically coiled SG • Pressurizer with non-condensable gas • Performed verification and validation using • Comparison of RELAP5/MOD3 results • OECD collaborative works such as SETH, PKL, BEMUSE, TMI-2 • Data of VISTA experiment

10. NATURAL CIRCULATION ANALYSIS CODE • Heat transfer correlation for natural circulation (MARS code) • For vertical region (Churchill-Chu) • For horizontal region (McAdam) • For energy down-flow • For energy up-flow

11. NATURAL CIRCULATION ANALYSIS CODE • Nodalization of MARS 3.0

12. NATURAL CIRCULATION ANALYSIS CODE • TASS/SMR code • 1-D system analysis code for thermal hydraulic analysis of an integral reactor, SMART • Developed by KAERI • Basic conservation equation : mixture mass, liquid mass, non-condensable gas mass, mixture energy, steam energy, mixture momentum • The code incorporates slip effects between the phases by using empirical correlations • SMART specific models are included Vertical Region Horizontal Region

13. NATURAL CIRCULATION ANALYSIS CODE • Nodalization of TASS/SMR

14. NATURAL CIRCULATION ANALYSIS CODE • Summary of input model • Volume and Junctions • RCS : 0.13 m3 • secondary system : 0.035 m3 • ECT tank : 9 m3 • 1-dimensional 233 nodes, 252 junctions – MARS code 117 nodes, 125 junctions – TASS/SMR code • Heat structure of the primary system • Heater rods • SG helical tubes • Wall heat structure and heat loss – MARS code No wall heat structure excluding heat loss – TASS/SMR code • Pipe heat structure of the secondary system • PRHR heat exchanger tubes • Not model other structure

15. INITIAL/BOUNDARY CONDITIONS

16. Experiment Results – VISTA NC • Power changeswith constant FW flow • System behaviors are nearly the same • From these results, • effect of initial power is small for NC

17. Experiment Results – VISTA NC • Repetition test performs to enhance a reliability of the experiment • System behaviors show nearly the same results • From these results, • obtains a reliability of the NC test • at the VISTA facility

18. 그림 3-1). VISTA 증기발생기 설계 Primary inlet Steam outlet FW inlet Primary outlet RESULTS and DISCUSSIONS Boundary Condition + For the 100% operation, HT at active = 688.3, HT at inactive=10.4, heat loss at primary loop=16.8 Total power = 715.5 + For the NC test, the heater power is zero + Initial power of MARS has the same value with the exp. + Initial power of TASS/SMR is smaller value => doesn’t model heat loss to the atm. heat transfer at SG inactive

19. ECT Cooling water inlet RESULTS and DISCUSSIONS Boundary Condition • Cooling water temperature at ECT inlet • + Hx is cooled by chilled water in the CWS • + Cooling of chilled water is used by an air cooler

20. RESULTS and DISCUSSIONS • Secondary Natural Circulation • +NC flow decrease 10% of initial flow early stage • +NC loop is formed within a few minutes • +Cal. results are predicted well the exp. result

21. RESULTS and DISCUSSIONS Forced circulation flow Natural circulation flow • Primary Natural Circulation Flow • + Pri. NC flow fails to measure at the exp. • + NC flow of two codes predict nearly a same value • + Flow rate decrease 9% of initial flow early stage • + the flow decreases gradually to 2%

22. Primary inlet Steam outlet FW inlet Primary outlet RESULTS and DISCUSSIONS • Primary Pressure • +Primary pr. becomes to decrease with transient • +Primary pr. depends on coolant temperature under NC • +Calculated depressurization rate is higher than the exp. pressure at the early stage • => final pr. is lower than the exp.

23. RESULTS and DISCUSSIONS SG Steam Pressure + From experiment, Sec. pr. increases after it decreases +Overall pressure is predicted well by codes +Two codes doesn’t predict decrease at beginning of transient +Calculated min. pr. is initial pressure max. pr. is higher than exp. pressure two codes over-predicts peak pr. => to find discrepancy

24. Primary inlet Steam outlet FW inlet Primary outlet RESULTS and DISCUSSIONS • Primary temperature • +Two codes predict well overall trend • => NC HT model of helical tube • and bundle

25. Primary inlet Steam outlet FW inlet Primary outlet RESULTS and DISCUSSIONS • SG Inlet and Outlet Temperature • + From exp., Sec. temp. is higher than Pri. Temp. • => Steam temp. affects on heat capacity of steam pipe wall • + SG inlet temp. is predicted reasonably • + Code doesn’t predict steam temp. • => the codes should be modeled the heat structure of steam pipe

26. RESULTS and DISCUSSIONS Hx Inlet and Outlet Fluid Temperature (Tube side) + Steam temp. is a saturated steam and liquid temp. is a subcooled liquid + Steam temp. is predicted well by two codes + Liquid temp. is slightly higher than the exp. => the codes seem to under predict heat transfer at the heat exchanger

27. RESULTS and DISCUSSIONS Surface temperature in the Hx tube +Surface temperature in the upper part is always above 100 ℃ (≈106 ℃) => local boiling occurs at the upper part +The lower part maintains a sub-cooled liquid condition +Calculated temp. at the lower part is over-predicted since the cal. heat transfer in the Hx is smaller than the exp.

28. RESULTS and DISCUSSIONS Liquid temperature in the ECT + The overall fluid temp. in the ECT is lower than the sat. temp + Fluid temp. in the upper part maintains a constant value and the lower part increase with time + Cal. Liquid temp. in the upper partis nearly same with exp. and the lower part is slightly higher

29. The natural circulation test at the VISTA facility is performed to find thermal hydraulic characteristics in the PRHRS of SMART The PRHRS removes well the primary heat source as long as the Hx is submerged the water in the ECT. Natural circulation flow rate is around 10% of the initial flow for the integral reactor The local boiling is occurred at the top of the heat exchanger bundle Natural circulation of the VISTA facility depend on Heat loss to the atmosphere at the reactor vessel and connected pipes Latent heat in the reactor vessel Friction and form loss of the geometry Heat transfer at the SG and the heat exchanger SUMMARY From Experiment

30. The realistic calculations is performed to find analysis capability of the TASS/SMR and MARS codes The model of the heat loss and heat capacity for the system are important to predict well the natural circulation From the code calculation results Two codes calculate reasonably the natural circulation at the integral reactor It is important to model properly the boundary condition such as heat loss, heat capacity of the structure Tow codes over-predicts depressurization rate of the primary pressure Under-predicts heat transfer slightly at the SG and the heat exchanger SUMMARY From Calculation