HTTF Analyses Using RELAP5-3D Paul D. Bayless RELAP5 International Users Seminar September 2010

11 Outline HTTF description Initial scoping analyses Steady state and transient simulations Code user observations

22 High Temperature Test Facility (HTTF) Integral experiment being built at Oregon State University Electrically-heated, scaled model of a high temperature gas reactor – Reference is the MHTGR (prismatic blocks) – Large ceramic block representing core and reflectors – ¼ length scale – Prototypic coolant inlet (259°C) and outlet (687°C) temperatures – Less than scaled power – Maximum pressure of ~700 kPa Primary focus is on depressurized conduction cooldown transient

33 Initial Scoping Studies Reference reactor simulations Simulations using a scaled-down MHTGR model Concerns with laminar flow and initial structure temperatures

44 MHTGR RELAP5-3D Scoping Model Features Three systems – Primary coolant – Reactor cavity – Reactor cavity cooling system (RCCS) Coolant gaps between the core blocks modeled Each ring modeled separately 2-D (radial/axial) conduction in all vertical heat structures Conduction between fuel blocks and to adjacent reflector blocks Radiation across gaps between reflector rings Radiation from core barrel to vessel to RCCS Core barrel divided azimuthally

55 MHTGR Reactor Vessel Core Region Cross Section Reactor vessel Core barrel Coolant channels Central reflector Fuel blocks Side reflector Control rod channels

66 MHTGR RELAP5-3D Core Region Radial Nodalization Reactor vessel Core barrel Coolant channels Central reflector Fuel blocks Side reflector Coolant gaps

77 Fuel Block Unit Cell Coolant hole Fuel MHTGRRELAP5-3D

88 130, , 134, ,160, 145,162, , Reactor Vessel Nodalization

99 Reactor Cavity and RCCS Nodalization

10 Base Calculation Set Steady state Low pressure conduction cooldown (LPCC) – 10-s forced depressurization to atmospheric pressure – Both reactor inlet and outlet open to He-filled volumes Conduction cooldown with intact coolant system – 60-s flow coastdown – Reactor inlet closed – Reactor outlet pressure reduced over 4-hr period – Three outlet pressures Normal operation (~6.3 MPa) 3.0 MPa 0.7 MPa

11 Base Calculations – Peak Fuel Temperature

12 Base Calculations – Peak Vessel Temperature

13 Base Calculations – RCCS Heat Removal

14 Base Calculations – Axial Conduction Effect

15 MHTGR/HTTF Sensitivity Calculations 25% power case – Nominal coolant temperatures – Transient response uninteresting, no heatup 10% power case – Nominal coolant temperatures; laminar flow in bypass channels – Full power decay heat – Transient response similar to reference plant but with higher, earlier temperature peaks ¼ scale model – Nominal coolant temperatures – Laminar flow in all flow channels – Core and reflector temperatures much higher than reference plant – Much higher transient fuel temperatures

16 HTTF RELAP5-3D Model Description Same components and approach as for MHTGR No gaps between core and reflectors All coolant holes are open at both ends without flow restrictions – Loss coefficients adjusted to provide 11% core bypass flow Control rod holes in reflectors modeled separately from solid regions Radial heat transfer by conduction in core, central and side reflectors Simplified model of the RCCS

17 HTTF RELAP5-3D core region radial nodalization Reactor vessel Core barrel Coolant channels Central reflector Core region Permanent reflector Coolant gaps Heater rod Coolant hole Side reflector

18 HTTF Model Initial Unit Cells Coolant channel Heater rod Ceramic ReflectorCore Helium gap

19 HTTF RELAP5-3D Model Unit Cells Coolant channel Ceramic ReflectorCore Heater rod Radiation

20 Initial Steady State Calculations Initial HTTF power was ~600 kW, but calculations showed that the power needed to be >1250 kW to get turbulent flow in the core cooling channels Facility power subsequently upgraded to 2.2 MW Sensitivity calculations looked at different reflector cooling hole geometries to investigate effect on initial temperature and bypass flow rate Cooling hole geometry still being determined

21 Transient Boundary Conditions Decay power (compared to MHTGR) – Power factor of 1/32 – Time factor of 1/2 Scram at transient initiation Power held constant until decay power drops below 2.2 MW 10-s depressurization in depressurized conduction cooldown (DCC) 60-s flow coastdown in pressurized conduction cooldown (PCC)

22 DCC Core Average Temperatures

23 DCC Radial Temperature Profile (1)

24 DCC Radial Temperature Profile (2)

25 Peak Fuel Temperature Comparison

26 Reactor Vessel Average Temperatures

27 Heat Removal and Generation

28 Transient Calculation Observations Temperature response seemed reasonable and representative Not a significant difference between DCC and PCC calculations

29 Code User Observations These studies exercised new or seldom-used models in the code – 2-D conduction – Control variable-driven heat flux boundary condition on a heat structure – Decoupled heat structures Code shortcomings – Inability to model both conduction and radiation from a heat structure surface – No 2-D conduction in structures with an imposed boundary condition