Radiation Streaming at Hot Cells

Content

• Introduction
• Notice Summary
• Applicability
• Event Summaries
• Event Significance
• Corrective Actions
• Hazard Mitigation
• References

Introduction

This notice is one in a series of publications issued by the Office of Nuclear and Facility Safety to share nuclear safety information throughout the Department of Energy complex. For more information, contact Dick Trevillian, Office of Operating Experience Analysis and Feedback, Office of Nuclear and Facility Safety, U.S. Department of Energy, Washington, DC 20585, telephone (301) 903-3074. No specific action or responses are required solely as a result of this notice.

Safety Notices are distributed to U.S. Department of Energy Program Offices, Field Offices, and contractors who have responsibility for the operation and maintenance of nuclear and related facilities, and to other organizations involved in nuclear safety. Written requests to be added to or deleted from the distribution of Safety Notices should be sent to: BR Richard L. Trevillian, EH-33, Room E-460 GTN, U.S. Department of Energy, Washington, DC 20585.

The ESH Office of Information Management maintains a file of Safety Notices and supporting information. Copies can be obtained by contacting the Office of Information Management at (301) 903-0449 or by writing to the Office of Information Management, U.S. Department of Energy, EH-72/Suite 100, CXXI/3, Washington, DC 20585.

Notice Summary

This notice presents lessons learned in radiation safety, specifically with respect to the potential for unmonitored radiation exposures during the operation of hot cell facilities. Radiation streaming was discovered at several different Department of Energy (DOE) hot cell facilities. This notice describes these incidents, provides generic information on the conditions that can lead to radiation streaming, and discusses recommended corrective actions.

Applicability

This notice applies to hot cell facilities of all hazard classifications which handle sources, irradiated material, and fission products.

Event Summaries

The first event was discovered on March 18, 1992, during routine radiation surveys at the Test Reactor Area (TRA) Hot Cells, Idaho National Engineering Laboratory (INEL).¹ Radiation protection personnel discovered a radiation stream measuring 2 R/hr outside of the hot cell building adjacent to a pedestrian walkway. This radiation stream, which originated at a collimator opening in the four foot thick shielding wall of Hot Cell No. 1, measured 18 R/hr three feet from the front face of the shielding wall. The collimator opening, located approximately 3.5 feet above the floor, was taped over with black tape and had been unused for over a decade. The radiation readings returned to normal when the Cobalt 60 source was moved to the far corner of the hot cell. Previous radiation surveys had identified radiation streaming problems, however, adequate corrective actions were not taken.

On April 1, 1992, at TRA Hot Cell No. 3, an EGG operatorís digital alarming dosimeter (DAD) alarmed. The operator was standing at the Shift Supervisor's office, approximately 21 feet from the hot cell wall. The DAD, which was worn at the waist, alarmed during a radiation survey of Hot Cell No. 3. Health Physics technicians subsequently determined that, at contact, a 780 mR/hr radiation stream was emanating from an unused piping penetration. The dose rate was 25 mR/hr at the Shift Supervisor's doorway.²

On June 16, 1992 EGG personnel moved a processing tray thought to be empty in front of the TRA Hot Cell No. 1 collimator. This evolution was part of an engineering evaluation of the potential historical radiation exposures prior to the March 18, 1992 discovery of radiation streaming. However, Health Physics technicians did not conduct a radiation survey for this change until June 17, 1992. The survey discovered a radiation beam measuring 700 mR/hr on contact with the lead blanket shielding over the collimator. The dose rate was 300 mR/hr at 3 feet and 25 mR/hr at the outside of the Hot Cell Building.³

In addition to these specific events, NS observed that the INEL TRA Hot Cell safety basis documentation was based on incorrect assumptions. The Safety Analysis Report (SAR) for the Hot Cells specifies that in areas with full time occupancy, radiation levels should not exceed 0.1 mR/hr. The SAR stated that the worst case analyzed would result in 0.23 mR/hr on contact at the surface of Cell No. 1 during Cobalt 60 processing with a 160,000 curie source. However, routine radiation surveys showed several other locations at or near TRA Hot Cells with doses rates ranging from 0.5 to 10 mR/hr while operating under an informal limit of 20,000 curies.

On June 29, 1992, as a result of hot cell evaluations initiated by the Office of Nuclear and Facility Safety's (NS) Idaho Site Representative, Oak Ridge National Laboratory (ORNL) discovered another case of radiation streaming. Radiation streaming was discovered during a radiation survey of Hot Cell No. 2 port shield plugs at the Irradiated Materials Examination and Test Facility (IMET), Building 3025-E.⁴ Health Physics technicians recorded radiation levels of approximately 60 mR/hr at the outside surface of a shield plug located approximately 3.5 feet above the floor. This survey involved placing a source directly against the inside surface of a shield plug. When the source was returned to its normal work location, the radiation levels returned to approximately zero. Subsequent surveys discovered additional radiation streaming including a 170 mR/hr stream at the Cell No. 6 periscope penetration located approximately seven feet above the floor. The Limiting Condition for Operations (LCO) for this facility specify a 2.5 mR/hr maximum radiation level at the distance of one foot.

On July 16, 1992, in response to the discovery of radiation streaming at IMET, ORNL performed a radiation survey at the High Radiation Level Examination Laboratory (HRLEL), Building 3525. Preliminary results of this survey have identified a 35 mR/hr contact radiation stream from a viewing window while the source was located in its normal work area within the hot cell.⁵ This radiation stream was located approximately three feet above the floor. No LCO is specified for this facility. This location was not posted as a radiation area in accordance with the ORNL Health Physics Manual or DOE 5480.11.⁶

Subsequent review of these incidents and examination of the hot cell facilities at the Idaho Chemical Processing Plant and the Hot Fuel Examination Facility at Argonne National Laboratory - West identified similar equipment deficiencies. These deficiencies included covering of hot cell penetrations with duct tape, missing shield block bolts, and missing or incorrectly designed penetration plugs. These reviews also identified the following unsatisfactory conditions, including:

1. Lack of facility drawings depicting the current configuration of the hot cells.

2. Failure to conduct detailed radiation surveys when the facility hazard changed, i.e., after introduction of a source of a different type or magnitude into the hot cell.

3. Failure to properly document special radiation surveys conducted by the facility staff or subcontractors.

Deficiencies were also identified with hot cells SARs including:

1. SARs which do not reflect the current facility configuration.

2. SARs which do not identify safety systems.

3. SARs which contain invalid shield calculations and technical assumptions for determining personnel exposure and nominal occupancy times.

4. SARs which lack, or contain inadequate, safety limits and surveillance requirements.

Event Significance

These events are significant because the existence of undetected radiation streams could result in personnel overexposures, and these radiation streams could produce exposures that are not reflected in dosimetry readings. Furthermore, routine radiation surveys of hot cell exteriors may be insufficient if they fail to address the repositioning or movement of source material within the cell, because repositioning of the source may result in vastly different radiation fields. Finally, although the deficiencies were initially reported in the DOE Occurrence Reporting and Processing System (ORPS) database on March 20, 1992, radiation streaming problems were not discovered at ORNL until NS initiated evaluations to ascertain whether the problems similar to those at the TRA hot cells might exist at other DOE facilities. These discoveries underscore the need for DOE Program Offices, Filed Offices, and DOE MO Contractors to evaluate occurrence reports for applicability to their facilities.

Corrective Actions

DOE and its contractors performed immediate corrective actions and followed up. Actions were taken to suspend hot cell activities, establish radiation zones, and where possible, sources were moved into locations to minimize radiation streaming. Ongoing corrective actions at ORNL and INEL include performing detailed radiation surveys and dose reconstruction, locking and tagging hot cell manipulators, and verifying hot cell configuration. Longer term corrective actions at INEL include preparing a recovery plan for a Type 2 restart of the TRA Hot Cell facilities. TRA management committed to providing additional supervision as a compensatory measure.³ Oak Ridge Field Office has submitted a preliminary schedule for performing hot cell shielding verification to DOE headquarters.⁷

Hazard Mitigation

Actions which could be taken to minimize the potential for personnel over-exposures at hot cell facilities include:

1. Perform detailed radiation streaming surveys.

   • Conduct systematic surveys using well-formulated plans, including provisions for contingency actions, to minimize personnel exposures.

   • Consider the potential for streaming to cause unmonitored radiation exposure to body parts not typically monitored by dosimetry.

   • Use a source approximating the maximum permissible source permitted by the SAR.

   • Mimic source movement among all possible work locations within the hot cell.

   • Include all accessible surfaces and adjacent walls, floors, and ceilings.

   • Revise routine radiation surveying techniques to specifically address the existence of radiation streams.

   • Factor the results of these surveys into the program for extremity dosimeters.

2. Field verification of the hot cell configuration.

   • Verify that drawings reflect the as-built conditions.

   • Verify proper installation of all shielding components including bolting hardware.

   • Verify penetrations are designed properly; for example:

     • Incorrectly designed penetrations will utilize a path of least resistance (in-line/non-offset cell port plugs) to penetrate the shielding wall.

     • Cell port plugs which incorporate a helical utility line configuration to eliminate streaming are a typically accepted industry practice.

   • Examine the shielding of floor drains and drain lines to ensure that hot particles cannot be inadvertently swept, flushed, or vacuumed into locations that could result in an uncontrolled radiation source.

   • Review hot cell designs and modifications to ensure that they specifically address ALARA concepts during installation and hot cell operations.

3. Verification of shielding calculations.

   • Ensure the current as-built hot cell configurations are bounded by the original shielding calculations.

   • Confirm that the calculations reflect the current source loading limits imposed on the hot cell.

4. Perform radiation streaming surveys after configuration changes.

   • Conduct additional radiation streaming surveys when the configuration of the hot cell is changed.

     • I.e., a hot cell modification or after introduction of a source of a different type or magnitude into the hot cell.

   • Review the potential for a new source to change radiation hazards of the hot cell but remain undetected until the performance of routine surveys

References

1. Occurrence Report, "Radiation Streaming From TRA Hot Cell No. 1," ID--EGG-TRAHC-1992-0003, dated April 29, 1992.

2. EGG Idaho, Inc. "Radiological Evaluation and Dose Analysis of the Test Reactor Area Hot Cell Operation for January 1 - July 1, 1992" Report Number RP-A-92-001, SSRE- A-92-001, dated July 1992.

3. Occurrence Report, "Radiation Beam from Collimator Port in Hot Cell No. 1," ID--EGG-TRAHC-1992-0009, dated June 30, 1992.

4. Occurrence Report, "Cell Port Shielding Survey," ORO--MMES-X10METCER-1992-0016, dated July 13, 1992.

5. Occurrence Report, "Hot Cell Shield Inadequacy," ORO--MMES-X10METCER-1992-0018, dated July 15, 1992.

6. DOE 5480.11, "Radiation Protection for Occupational Workers," Change 1, dated July 20, 1989.

7. Memorandum, R.L. Egli DOE/OR to E.C. Brolin, "Performance of the Oak Ridge National Laboratory (ORNL) Hot Cell Shielding Verification," dated July 22, 1992.