Cryogens and ODH at the SNS The Early Years

1. Cryogens and ODH at the SNSThe Early Years SNS was originally designed to use a room-temperature Linac Superconducting RF was recognized as a mature technology with tremendous advantages Thomas Jefferson Laboratory was included as a Partner Lab, bringing their expertise to the collaboration ODH issues were identified as major new hazard, and SNS launched a major effort to identify, quantify, and address the hazards

2. Hazards Identified Areas identified in the Accelerator Safety Order and the Draft Guide were evaluated to provide a framework of the analysis A review of facility documentation and evaluation of issues across the DOE Accelerator Complex was conducted to identify any additional areas Projected releases were modeled in the SNS tunnel geometry; results indicated that ODH issues represented an unacceptable risk, and required mitigation

3. Quantification of the Hazard Potential releases were estimated, based upon JLab analyses, experience at other facilities across the world, and flow calculations Tunnel geometry, incorporating the lentils recommended by the JLab partners, was included in the analyses Using the experience available at the time, personnel safety was acceptable with the "See and Flee" approach To ensure safe escape paths, the tunnel was designed to provide personnel passage even with a cryomodule pulled from the operational position and sitting in the tunnel

4. Process of Selecting Controls Based on a review of the potential releases, and the impacts of the releases, a system to detect and warn of the presence of ODH levels was required The system could be stand-alone or part of another Credited Control, but it must meet selected standards for performance and quality The decision was made to incorporate the ODH system into the PPS that was being developed to control access to prevent exposure to prompt radiation

5. Recommendations The ODH system was required to reliably detect the presence of reduced oxygen, regardless of cause Because of the potential impacts, the system is designated as Credited to protect staff The system would provide audible and visual warning of the hazard Access to areas with ODH potential would be limited to properly trained individuals; access would be linked in real-time to training records At every stage, external experts were convened to review the results of the process; their recommended changes or adjustments were captured, tracked, and addressed at semi-annual Project reviews

6. Implementation The ODH system became part of the Accelerator Readiness Review when the initial portions of the superconducting RF linac were ready to commission The ARR team included experts in ODH and PPS who were not part of the Project - a true external, independent review ODH issues were evaluated at every subsequent ARR, including initial operations and full-power operations ODH issues were present at many of the individual Instruments; ODH issues were addressed in each applicable Instrument Readiness Review

7. Recent Experience at Other Facilities The Upgrade to the Linac Coherent Light Source (LCLS-II) at SLAC will convert the first kilometer of the decades-old tunnel to Superconducting RF technology The cryogens presented a significant new hazard for SLAC, and they have begun to develop programs to address the issue Like many facilities, SLAC is adapting proven technology to their particular situation and needs The potential for significant earthquake activity affects every phase of the new Project

8. SLAC Activities Identified a full range of potential releases from the cryo plants, distribution system, and cryomodules Analyzed the impact of these releases, using best-available information and the SLAC tunnel geometry Convened review panels with outside experts, including staff from ESS Most releases have acceptable impacts, based upon a functioning ODH system and proper training

9. Unique Issues at SLAC Releases from the Worst-Case Earthquake could result in a significant hazard situation: However, the probability of the event occurring with staff present is beyond the level of concern The tunnel does have existing alcoves, and these are being equipped as "safe rooms" for both fire and ODH hazards Based upon input from expert reviewers, NSLS-II will have two cryo plants, and they can be configured to operate independently and together, so the valve configuration will be a challenge Hiring experienced staff is a challenge, simply due to the cost of living in the area

10. Unique Issues at FRIB Upward slope in the tunnel as the beam heads to the Target "Paper Clip" shape of a "folded" linac Extremely close proximity to nearby facilities and student population

11. Lessons To Be Learned Identify recognized standards to define system requirements, including function, design, review, procurement, installation, testing, maintenance, and routine certification Convene a Review committee of external experts, and establish appropriate "hold points" for review and concurrence at every critical stage of the system evolution Ensure appropriate independence of this Credited Safety system within the organizational structure Make the system a routine point of discussion with the Regulators

12. Back-up Information The options that are generally in play at Accelerators * Electrochemical * Zirconium oxide * Laser diode

13. Some Experience notes Electrochemical- (based on SNS experience with MSA equipment): Early on,SNS selected MSA sensors based on testing Kelly Mahoney did at JLab when they discovered that the existing sensors didn't work correctly with helium. The MSA sensors have many problems that were discovered over the years: High failure (safe failure) rate- SNS had problems with infant failure as well as end of life failures (producing false alarms) before the one year service life was up (supposedly a 2 year sensor) Sensor measured partial pressure of O2 instead of concentration which meant that the reading varied based on atmospheric pressure and humidity. Calibration had to be checked every week to avoid the dreaded 2:30 AM Sunday phone call of a false alarm. Most if not all modern sensors include a local pre-amp which means they cannot be installed in the tunnel MSA does have a "SIL-2" rating on their newest units. Certainly a good thing to have as you don't have to estimate equipment failure rates. *

14. Additional Considerations Zirconium oxide- Used at JLab successfully. When SNS considered them years ago they did not come with an extraction system. Had an unusual response on pure He (reading went negative). Sensor is relativity slow to response. Laser diode (Oxigraf)- BNL found an unusual response to pure He also. (one could argue that you would never have a instant transition from air to pure He with sampling system for a large room). As you can see there is no perfect answer- FRIB is using a different brand of eletrochemical sensor for their cryoplant, We may find in time that they don't have the same problems we had at SNS.