Heat Exchanger Temperature Response for Duty-Cycle Transients in the NGNP/HTE R. B. Vilim Argonne National Laboratory Fourth Information Exchange Meeting – Nuclear Production of Hydrogen Chicago, Illinois April 16, 2009

Background Need to Meet Operational and Safety Goals while Achieving a 40 Year Plant Lifetime Intermediate Heat Exchanger is a Limiting Component Elevated service conditions : 850 C inlet temperature and 2-5 MPa pressure differential Temperature transients induce thermal stresses Important to Limit Thermally Induced Damage to IHX Need a Control System that minimizes temperature changes in the IHX for Duty Cycle transients

Outline Objectives Plant Description Duty-Cycle Event List Plant Control System Load Schedule Step Load Change and Loss of Load Conclusions

Objectives Develop Requirements for Control and Protection Systems to Operate the NGNP as a Producer of Hydrogen Design Plant Control System to Minimize Temperature Transients Perform Transient Simulations to Obtain IHX Temperature Response for Duty Cycle Events Characterize IHX Temperature Response

Plant Description Primary System Power Conversion Unit

Plant Description High Temperature Electrolysis (HTE) Unit

Duty-Cycle Event List Defines Types and Frequency of Transients that Form the Plant Design Basis Temperature Time History Calculated to Generate the Corresponding Thermally-Induced Stresses Damage Plant Components will Sustain over a 40 Life is Estimated from this Data Simplifying Observation: Over 85 percent of the Useful Reactor Power ends up being Communicated to the Chemical Plant as Electric Power through the NGNP/HTE Interface Thus transients in the chemical plant act on the reactor through changed electric power load and therefore from the standpoint of the nuclear plant are classic electric-load transients

Plant Control System Primary System Reactor Outlet Temperature Setpoint via Reactor Power IHX Flow Differential Setpoint via Primary System Inventory Primary System Comp Speed Power Conversion Unit PCU Shaft Speed via PCU Inventory Precooler Powers via Cold Side Water Flowrate High Temperature Electrolysis Unit Constant current density setpoint via Cell Area Mass flowrates proportional to hydrogen production rate Supervisory Control System Coordinates above setpoints specific to particular transient

Load Schedule He Hot-Side Temperatures HTE Temperatures He Cold-Side Temperatures

Step Change in Hydrogen-Production Load Able to Maintain Essentially Constant IHX Hot-End Temperatures PCU Shaft Speed IHX Temperatures

Loss of Hydrogen-Production Load: Protected Short-Term Transient is Does not Present Significant Temperature Changes PCU Shaft Speed IHX Temperatures

Loss of Hydrogen-Production Load: UnProtected IHX Hot-End Temperature Change Limited to ~1 C/s PCU Shaft Speed IHX Temperatures

Conclusions Near-Constant Temperatures can be Achieved over the Load Schedule IHX Differential Flow Controller is Effective in Maintaining Constant Hot-End Temperature Gradients in the IHX during Transients Partial Load Plant Efficiency is Maintained via Inventory Control In Rapid Transients Turbine Bypass Control Limits Shaft Over-speed In General Hot-End Temperature Changes in the IHX were Limited to ~1 C/s Stability of Closed Loop Brayton Cycle was Found to be Sensitive to Operating Point on Turbo-Machine Curves More stable operating point has reduced efficiency