Initial Startup Procedure Investigation of a BWR-Type Small Modular Reactor Shanbin Shi, Xiaodong Sun Department of Nuclear Engineering and Radiological Sciences University of Michigan, Ann Arbor, Michigan, USA Mamoru Ishii School of Nuclear Engineering Purdue University, West Lafayette, Indiana, USA International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants June 6-9, 2017, Vienna, Austria

Outlines Brief Introduction of SMR New SMR Design at Purdue – NMR Major Issues of SMR Using Natural Circulation Test Design Startup Procedures Slow startup transients Very slow startup transients Pressurized startup transients Summary

Brief Introduction of SMR Necessity of SMR Domestic areas and developing countries without major advanced infrastructure Non-electrical (process heat/desalination) customers Benefits of SMR Non-proliferation Enhanced safety and security Reduced capital cost Shorter construction schedules due to modular design Improved quality due to replication in factory-setting Meets electric demand growth

Novel SMR Design at Purdue - NMR Purdue Novel Modular Reactor Safety Fully passive safety system Lower operational pressure Lower core power density Lower core temperature No SG tube failure No loss-of-flow accident Fewer penetrations on RPV Design basis accident management without AC power Economics Smaller and simpler RPV No need for secondary loop and SGs No recirculation and safety pumps Direct steam cycle for better efficiency Mature BWR technology utilization

Novel SMR Design at Purdue – NMR (Cont’d) NMR-50 Design Parameters

Major Issues of SMR Using Natural Circulation What is the challenge of NC-SMR design? Startup instability Natural circulation induced by density difference between hot and cold legs Oscillations especially prevalent at lower pressures (i.e. startup or accident conditions) due to lower driving head In BWR designs, instabilities could be affected by void reactivity coupling CAORSO BWR in Italy (1982) Neutron flux oscillations in the out-of-phase mode LaSalle Unit 2 in USA (1988) Excessive neutron flux oscillation while it was on the natural circulation after the pump trip

Major Issues of SMR Using Natural Circulation (Cont’d) Startup Instability Causes mechanical vibrations and system control problems, affect normal operations, restrict operating parameters and influence system safety Flashing induced flow instability dominated Furuya, M., F. Inada, and T. H. J. J. Van der Hagen. "Flashing-induced density wave oscillations in a natural circulation BWR-mechanism of instability and stability map." Nuclear engineering and design 235.15 (2005): 1557-1569.

Overview of Instability Project (DOE NEUP) Prototypic design Scaling criteria Scaled model Instability w/o nuclear coupling Instability w/ nuclear coupling Start-up transient test Quasi-steady state test Start-up transient test Quasi-steady state test p, T, v, α p, T, v, α Start-up procedure Stability map Start-up procedure Stability map

Test Design Scaling Analysis

Test Design (Cont’d) Test Facility Schematic of the Test Facility

Test Design (Cont’d) Impedance Void Meter Core section Chimney section

Test Design (Cont’d) Instrumentation Item Uncertainty Manufacturer T-type thermocouple Greater of 1°C or 0.75% Omega Engineering Differential pressure transducer ±0.0375% of the span Honeywell Absolute pressure transducer Magnetic flow meter ±0.5% of the reading SCR power controller ±0.5% of the power output WATLOW Impedance void meter 0.5% in absolute value Homemade

Initial Startup Procedures Slow startup Very slow startup Pressurized startup with different power ramp rates

Slow Startup Transients Startup Simulation Conditions Startup Testing Procedures Set up all instruments (DP, AP, flow meter, thermal couple, impedance void meter) Degassing procedure Start the tests by applying the prescribed power curve Power = 4×10-4 t (kW) (Slow power ramp rate) Pressure (kPa) Coolant Temp. (℃) Coolant Level (m) Core Inlet Subcooling (℃) 50 85 5.85 22

Slow Startup Transients (Cont’d)

Slow Startup Transients (Cont’d) Intermittent Sinusoidal Void fraction (Imp 03) at the core exit for the slow startup transients

Slow Startup Transients (Cont’d) 5.5min Flashing 45sec, 3cm/s DWO Detailed void fraction at the core exit (IMP03) for the slow startup transients

Slow Startup Transients (Cont’d) Phase change △Tcore=5 ℃

Very Slow Startup Transients Similar Startup Procedures Using Very Slow Variable Heat-up Rate t=13 hrs.

Very Slow Startup Transients (Cont’d) Very slow power ramp rate can stabilize the flashing instability In real reactor, there are several power channels with different heat flux. The amplitudes of instability can be further reduced

Pressurized Startup Transients Procedures Set the initial pressure at 300 kPa by filling the steam dome with the nitrogen gas Start the startup transients. Remove the nitrogen when the pressure reaches 500 kPa Power = 4×10-4 t (kW) (Slow power ramp rate) Power = 12×10-4 t (kW) (Fast power ramp rate) Pressure (kPa) Coolant Temp. (℃) Coolant Level (m) Core Inlet Subcooling (℃) 300 85 5.85 53

Pressurized Startup Transients (Cont’d) Venting Venting Two-phase N.C. Pressurized slow startup transients Pressurized fast startup transients

Pressurized Startup Transients (Cont’d) Void fraction (Imp 07) at the chimney exit for the slow startup transients

Pressurized Startup Transients (Cont’d) Modeling of Flashing To determine flashing instability boundary in the stability plane

Summary To Avoid Potential Flashing Induced Flow Instability Normal startup procedure with very small power ramp rate To make the temperature distribution much more uniform Heat the reactor with very small power ramp rate up to 24 hours before the two-phase natural circulation is generated Pressurized startup procedure with venting process Pressurized the reactor to 0.3 MPa with nitrogen at the beginning To vent the non-condensable gas when the pressure reaches 0.5 MPa to generate the two-phase natural circulation

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