SC-OSO--ORNL-X10CHRIDGE-2019-0001 FINAL
Occurrence Report
After 2017 Redesign

Chestnut Ridge


(Name of Facility)

Accelerators


(Facility Function)

Oak Ridge National Laboratory Oak Ridge National Laboratory


(Laboratory, Site, or Organization)

Name: Fulvia Pilat
Telephone No.: (865) 576-9315


(Facility Manager/Designee)

Name: CHANCE, TRACY D
Telephone No.: (865) 574-8430


(Originator/Transmitter)

Name: Date:


(Authorized Classifier (AC))

  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

Date Time
Notification: 05/03/2019 16:09  (ETZ)
Initial Update: 05/03/2019 16:09  (ETZ)
Latest Update: 05/03/2019 16:09  (ETZ)
Final: 05/03/2019 16:09  (ETZ)

Report Level: I


  4. Division or Project: Research Accelerator Division

  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:

Date Time Person Notified Organization
NA NA NA NA

12. Other Notifications:

Date Time Person Notified Organization
04/18/2019 15:20  (ETZ) Martha Kass DOE ORNL
04/18/2019 15:20  (ETZ) Michael Herr DOE ORNL
04/18/2019 15:28  (ETZ) Johnny Moore DOE ORNL
04/18/2019 15:00  (ETZ) Paul Langan ORNL NSD

13. Subject or Title of Occurrence:

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


14. Reporting Criteria:

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.


15. Description of Occurrence:

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.


16. Is Subcontractor Involved? No


19. Immediate Actions Taken and Results:

- 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.


20. ISM:

      3) Develop and Implement Hazard Controls


21. Cause Code(s):

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


22. Description of Cause:

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.


25. Corrective Actions

            (* = Date added/revised since final report was approved.)
        1.
Procure and install additional radiation monitoring equipment related to the Radiation Safety Officer analysis (ACTS 18573.41.3) that will detect changes in radiation levels and audibly alarm. (ACTS 0.38184.1)
Target Completion Date: 05/10/2019 Completion Date: 05/09/2019
        2.
Complete the Unreviewed Safety Issue Evaluation of the existing system configuration to identify any USIs associated with the discovered system vulnerabilities. (ACTS 0.38184.2)
Target Completion Date: *05/17/2019 Completion Date: 05/14/2019
        3.
Identify and evaluate effective controls and diagnostics for the mercury system to ensure safe operations as supported by safety evaluation. (ACTS 0.38184.3)
Target Completion Date: 06/30/2019 Completion Date: 06/17/2019
        4.
Implement the effective controls specified in ACTS 0.38184.3. (ACTS 0.38184.4)
Target Completion Date: 07/07/2019 Completion Date: 06/18/2019
        5.
Complete assessment of readiness to resume operations, with identified controls in place, conducted by a team including internal and external peers. (ACTS 0.38184.5)
Target Completion Date: 07/15/2019 Completion Date: 06/21/2019
        6.
Obtain authorization to resume safe operations. (ACTS 0.38184.6)
Target Completion Date: 07/31/2019 Completion Date: 06/24/2019


26. Lessons Learned:


27. Similar Occurrence Report Numbers:


30. HQ Keyword(s):

01B--Inadequate Conduct of Operations - Loss of Configuration Management/Control
01H--Inadequate Conduct of Operations - Inadequate Safety Analysis/USQ/TSR
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


31. HQ Summary:

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.