Chestnut Ridge

Accelerators

Oak Ridge National Laboratory

Oak Ridge National Laboratory

Name: Fulvia Pilat

Telephone No.: (865) 576-9315

Name: CHANCE, TRACY D

Telephone No.: (865) 574-8430

Name:

Date:

  1. Occurrence Report Number: SC-OSO--ORNL-X10CHRIDGE-2019-0001

      Unanalyzed Pressure Transient Event in the Spallation Neutron Source (SNS) Mercury Loop

  2. Report Type and Date: FINAL

Report Level: I

  5. Secretarial Office: SC - Science

  6. System, Bldg., or Equipment: Building 8700

  7. UCNI?: No

  Reviewed for Public Release:

  8. Plant Area: Building 8700

9. Date and Time Discovered:     04/18/2019    15:00  (ETZ)

10. Date and Time Categorized:     04/18/2019    15:36  (ETZ)

11. DOE HQ OC Notification:

12. Other Notifications:

13. Subject or Title of Occurrence:

      Unanalyzed Pressure Transient Event in the Spallation Neutron Source (SNS) Mercury Loop

10(1) - An event, condition, or series of events that does not meet any of the other reporting criteria, but is determined by the Facility Manager or line management to be of safety significance or of concern for that facility or other facilities or activities in the DOE complex.

At 7:26 am on March 21, 2019, the Spallation Neutron Source (SNS) circulating mercury system experienced an unanticipated pressure transient during a normal top-off fill of the mercury loop using approved procedures. The pressure transient forced radioactive material/mercury into helium lines contained within unshielded conduit in the experiment hall area. The unshielded radioactive material in the helium lines caused elevated radiation levels in a localized portion of the North Experiment Hall, impacting some experimental areas. Elevated radiation levels persisted for over 9 hours before being identified by a Radiological Control Technician. Upon discovery, affected areas were cleared of personnel and appropriate radiological area boundaries and postings were put in place. Line management was promptly notified. Dosimetry was collected from personnel in the affected area and promptly analyzed. A maximum whole-body dose of 24 millirem (mrem) was recorded for an experimenter working in the affected area. Two other experimenters received a whole-body dose of approximately 20 mrem and another experimenter received a shallow dose equivalent of 19 mrem. Dosimetry was collected the next day from 12 other individuals who had been in the affected area of the building that day and no other whole-body dose was recorded.

Engineering and Safety staff analyzed system indications and determined that an overfill of the mercury loop, compounded by intermittent fill valve operation and higher than usual fill pressure, caused a pressure transient that burst the pump tank rupture disk and transported highly radioactive material into helium service lines into a location outside of engineered shielding.

On March 23rd, a revised mercury fill procedure was approved that included multiple hold points to allow for data collection and to ensure the system was stable before adding additional mercury. With radiological support, the revised procedure was performed by Operations and Engineering staff. A similar pressure transient was experienced. The recurrence of the pressure transient indicated that an overfill of the mercury loop was not the cause of the pressure transient. The mercury system was put into a safe state and RCTs surveyed the conduit area of the experiment hall. Radiation levels in the area were not significantly affected. The ability to fill the mercury loop was disabled by application of a Radiation Safety Lock.

On April 18, 2019 a conservative analysis identified a previously unanalyzed event potentially leading to significant dose rated in occupiable areas with a potential for radiological exposure to personnel. The ORNL Laboratory Shift Superintendent (LSS) was notified, and the event was categorized as a 10(1) Management Concern.

- The affected areas were cleared of personnel and appropriate radiological boundaries and postings were put in place.

- Dosimetry was collected from personnel in affected areas and readings were promptly analyzed.

- A team of technical and operations personnel was appointed to investigate the event, determine cause, and propose corrective actions.

- A critique was held regarding the unplanned exposures.

- The mercury loop was drained to the mercury storage tank and the ability to pressurize the Storage Tank was locked out.

- The SNS Operations Manager decided to start the planned outage early in lieu of continuing efforts to restart.

- Building access records were examined, personnel contacted, and additional dosimetry was collected for expedited read.

      3) Develop and Implement Hazard Controls

A3B2C04 - Previous success in use of rule reinforces continued use of rule -->couplet - A2B6C01 - Defective or failed part

The cause of both pressure transients was injection of pressurized helium into the mercury loop. This outcome was possible due to an unrecognized low mercury level in the Storage Tank. This was due to a leak at a flange in the mercury loop. In addition, the installed Storage Tank mercury level device had been inoperable since before initial SNS operation. An ancillary sensor, the Collection Basin leak detector, is designed to identify the presence of mercury or water in the Collection Basin. The leak sensor did not perform as expected and only indicated the presence of water in the Collection Basin.

Until this event, a significant leak out of the mercury loop had not occurred in the operating history of the SNS. Note that mercury leaking out of the system is contained in a Collection Basin per the system design.

01B--Inadequate Conduct of Operations - Loss of Configuration Management/Control 01H--Inadequate Conduct of Operations - Inadequate Safety Analysis/USQs/TSRs 01Q--Inadequate Conduct of Operations - Personnel error 01R--Inadequate Conduct of Operations - Management issues 05D--Mechanical/Structural - Mechanical Equipment Failure/Damage 06G--Radiological - Intake 12B--EH Categories - Conduct of Operations 14D--Quality Assurance - Documents and Records Deficiency 14E--Quality Assurance - Work Process Deficiency

On March 21, 2019, the Spallation Neutron Source circulating mercury system experienced an unanticipated pressure transient during a normal top-off fill of the mercury loop. The pressure transient forced radioactive material/mercury into helium lines contained within an unshielded conduit in the experiment hall area, causing elevated radiation levels in a localized portion of the North Experiment Hall impacting some experimental areas. Elevated radiation levels persisted for over nine hours, before being identified by a Radiological Control Technician. Upon discovery, the affected areas were cleared of personnel and appropriate radiological area boundaries and postings were put in place. Dosimetry was collected from personnel in the affected area and analyzed. A maximum whole-body dose of 24 millirem (mrem) was recorded for an experimenter. Two other experimenters received a whole-body dose of approximately 20 mrem, and another experimenter received a shallow dose equivalent of 19 mrem. Dosimetry was collected the next day from 12 other individuals, no other whole-body dose was recorded. On March 23, a revised mercury fill procedure was approved that included multiple hold points to ensure the system was stable before adding additional mercury. The revised procedure was performed and a similar pressure transient was experienced. The mercury system was put into a safe state. The ability to fill the mercury loop was disabled by application of a Radiation Safety Lock.