Transient modeling of sulfur iodine cycle thermo-chemical hydrogen generation coupled to pebble bed modular reactor Nicholas Brown, Volkan Seker, Seungmin Oh, Shripad Revankar, Thomas Downar, Cheikhou Kane Purdue University West Lafayette, IN Apr. 16, 2009 Research supported by: DOE NERI Project U.S. Department of Homeland Security Fellowship Program

Outline Hydrogen production Thermo-chemical hydrogen production cycles Sulfur-Iodine cycle (SI) Pebble Bed Modular Reactor - 268 (PBMR 268) Purdue hydrogen cycle models Point kinetics model Model coupling scheme Transient results

Energy demand in 21st century Global energy demand – met by fossil fuels It is almost universally accepted that peak oil production will occur before 20201 Sustainable, CO2 free energy sources are required: Nuclear Renewables Biomass Hydrogen is widely recognized as a promising energy carrier for the 21st century 1 – “Strategic Significance of America’s Oil Shale Resource. Volume I” U.S. Department of Energy, March 2004

Sulfur-Iodine (SI) Cycle Section 1: SO2+I2+2H2O H2SO4 + 2HI (Bunsen Reaction, 120 oC, exothermic) Section 2: H2SO4 ½ O2 + SO2 + H2O (Sulfuric Acid Decomposition, 900 oC, endothermic) Section 3: 2HI I2 + H2 (Hydrogen Iodide Decomposition, 450 oC, endothermic)

Transient cycle modeling 1st priority in nuclear system: Safety Important to understand coupled nuclear reactor - chemical plant behavior Methodology Treat each chemical reactor as a closed system and a control volume, Perform a chemical kinetics analysis of each chemical reactor, Heat/mass balance of each chemical reactor, Couple model to THERMIX-DIREKT nuclear reactor thermal code and point kinetics model

SI cycle simplified schematic

SI cycle kinetic behavior HI decomposition (Section III) dominates chemical plant behavior, since it has very slow response time 7

Reaction chamber model Mass balance: Energy balance: Intermediate heat exchanger:

Helium (or helium-xenon) cooled with graphite moderator PBMR-268 Pebble bed type high temperature gas cooled reactor with a power of 268 MWth Helium (or helium-xenon) cooled with graphite moderator Inner reflector is dynamic and composed of graphite spheres Figure reproduced from: Matzner, Dieter. “PBMR Project Status and the Way Ahead.” Proc. Of 2nd International Topical Meeting on HIGH TEMPERATURE REACTOR TECHNOLOGY Beijing, China, September22-24,2004.

PBMR-268 model simplifications THERMIX PBMR-268 model: Flattening of the pebble bed’s upper surface Flat bottom reflector Flow channels are parallel, equal speed Constant central column and mixing zone width Control rods in the side reflector are assumed as a cylindrical skirt with a given B-10 concentration Point kinetics model Flux profile does not change, is treated as a single point and scaled up or down based on feedback Six delayed neutron groups are used Doppler feedback coefficient is 0.0095

Model integration and coupling Three codes integrated: THERMIX PBMR-268 model Point kinetic model Purdue SI-cycle models Relevant calculations are performed each timestep (0.1 sec) Transfer between codes occurs via thermal conditions at Intermediate Heat eXchanger (IHX) GA flowsheet scaled to a thermal power input of 268MW Steady state hydrogen generation rate of ~ 1.4 kmol/sec Power distribution: 26% in Section 2, 74% in Section 3

Model integration flowsheet

Reactor initiated transients/accidents Examples of PBMR-268 reactor transients or accidents: Control rod insertion or withdrawal Depressurized Loss of Forced Cooling with or without SCRAM Pressurized Loss of Forced Cooling with or without SCRAM Load following accidents Control rod ejection Results shown here: insertion and removal of $0.25 of reactivity via control rod

Reactivity insertion +$0.25

Reactor power +$0.25

IHX Temperatures +$0.25

Reactivity insertion -$0.25

Reactor power -$0.25

IHX Temperatures -$0.25

Conclusions HI decomposition section is rate limiting step of the SI cycle Preliminary scaling of GA flowsheet yields an energy distribution of 25% in Section 2, 75% in Section 3 Hydrogen generation rate of SI plant coupled to PBMR is 1.4 kmol/sec