SNS Modulator Fires; Causes, Mitigation, and Long-Term Plans David E. Anderson

2Managed by UT-Battelle for the U.S. Department of Energy HVCM Simplified Block Diagram

3Managed by UT-Battelle for the U.S. Department of Energy HVCM Major Subsystems

4Managed by UT-Battelle for the U.S. Department of Energy Cavity/Klystron/Modulator Layout Multiple HVCM/Klystron Configurations Peak Power 11 MW, Average Power 1 MW design 115 kV 125 kV ≤135 kV ≤75 kV 75 kV

5Managed by UT-Battelle for the U.S. Department of Energy HVCM “Smoke Generating” Events 70 Total Events 1 during last production run Most do not result in fires but response is consistent until incident evaluated

6Managed by UT-Battelle for the U.S. Department of Energy HVCM Fire Causes – Bus Arcing 1 st fire, none since, in RF Test Facility Workmanship or residual dirt believed responsible Repeated arcing acted as ignition source for combustibles Corrected with improved training of assemblers, no faults w/ same root cause since (Jan 07)

7Managed by UT-Battelle for the U.S. Department of Energy HVCM Fire Causes – Insulation Degradation Cause of 2 fires and likely many of the IGBT failures Original design relies on single layer of DMD to insulate cooling tubes from different polarity bus Interference fit between tube and bus compresses DMD and can cut material if sharp edges present Corona degrades insulation over time, resulting in arc event Insulation double, short-term sol’n., cutout long-term

8Managed by UT-Battelle for the U.S. Department of Energy HVCM Fire Causes – Capacitors Mfgr.’s cap lifetime ratings 100, V, Expanded the lifetime on the spec. to 1 million hours Experience indicates ≤ 2300 V Replaced all warm linac caps w/ higher lifetime spec caps Replacing all other caps with new caps, starting to see failures of RSO units

9Managed by UT-Battelle for the U.S. Department of Energy HVCM Capacitor Comparison Capacitor Type Oper. Temp. Range IPB °F BTDP small °F BTDP large °F RSO °F Solid Potted<100°F IPB no longer available, original batch of capacitors as delivered Others tried –Reconstituted mica, failed in tens of minutes –Another manufacturer’s RSO, failed in 18 hours IPB Isopropyl Biphenyl BTDP Benzyltoluene Diphenylethane RSO Rapeseed Oil

10Managed by UT-Battelle for the U.S. Department of Energy HVCM Fire Causes – IGBTs Usually less severe, lower collateral damage At transition to 60 Hz operation, incidents increased significantly Improved thermal bonding procedures implemented Overvoltage problem solved, minimal problems since

11Managed by UT-Battelle for the U.S. Department of Energy HVCM Fire Mitigation – CO 2 Suppression Dedicated CO2 system installed Smoke detector installed EPICS screens updated Manual discharge from CCR if smoke detector trips Prevent or minimize system damage

12Managed by UT-Battelle for the U.S. Department of Energy HVCM Fire Mitigation – Additional Over Current detection on energy storage capacitor bank to limit fault energy Retraining workers / rewriting procedures for operators Implement Alarm Handler for smoke detector events Installing shrapnel shields around capacitors to minimize collateral damage Emergency Off modified to remove 2100 V primary energy source automatically Replace combustible materials inside Enclosure with UL-94 V0 rated plastics (G9 Phenolic) Replace cables with plenum-rated cables Design new IGBT drive circuitry to shut IGBTs off if over current detected

13Managed by UT-Battelle for the U.S. Department of Energy Future Capacitor Replacement ICIC V CE VCVC RSO Plastic Case Self-Clearing Metallized Polypropylene Ran capacitors until July shutdown Assess self-clearing capacitor degradation Simulate 10 years of faults on preferred capacitor Choose and order capacitor selected Install when delivered during 1 st FY10 Extended Shutdown Period

14Managed by UT-Battelle for the U.S. Department of Energy Future “Series Switch” ICIC V CE VCVC Decouples primary energy storage when fault detected Minimizes energy delivered to fault, prevents collateral damage Can be used to add a future redundant H-bridge

15Managed by UT-Battelle for the U.S. Department of Energy Capacitor / IGBT Preventative Maintenance ICIC V CE VCVC Liquid dielectric – Monitor case temperature – Inspect periodically for leakage – Inspect for case dimensions out of tolerance – Return suspect units to manufacturer for analysis and design improvements Solid potted self-clearing – All of above – Periodically measure capacitance value and replace when 5% degradation occurs IGBTs – Monitor for changes in Turn on time Turn on delay Turn off time Turn off delay Gate characteristics Monitor substrate temperature with thermal interlocks All monitored via transformer flux monitoring system Periodically monitored during shutdowns

16Managed by UT-Battelle for the U.S. Department of Energy ORPS Reporting ORPS reporting threshold is the activation of an automatic fire suppression system but NOT a manually-actuated or detection system – No automatic suppression employed – Building central detection system (VESDA) rarely detects HVCM events – ORPS considers burn times > 5 minutes significant – No HVCM event has passed the reporting threshold – All events have been contained inside the aluminum / stainless steel Safety Enclosure UT/Battelle and DOE ORO periodically review the HVCM smoke event history through self-assessment and oversight activities

17Managed by UT-Battelle for the U.S. Department of Energy Conclusion Many “smoke generating” events to date, none severe Engineering and procedural controls and protection systems put in place Nature of component failures believed to be well understood, PM and component replacement underway to address Additional protection systems in design to further enhance system availability Hopeful that these event can be significantly reduced in the future QUESTIONS?