1. 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

2. 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

3. 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

4. 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

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

6. 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

7. 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  (2005):

8. 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

9. Test Design
Scaling Analysis

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

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

12. 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

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

14. 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

15. Slow Startup Transients (Cont’d)

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

17. 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

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

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

20. 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

21. 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

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

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

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

25. 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

26. Thank you for listening!
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