Criticality Safety Moderator Hazards

Content

• Introduction
• Notice Summary
• Applicability
• Properties of Moderation Hazards
• Event Summary
• Event Significance
• Corrective Actions and Followup
• Building Hazards
• Equipment Hazards
• Nonstructural Hazards
• What To Do if Moderator Control is Violated
• 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 is a lessons learned document in the area of nuclear criticality safety, specifically relating to hazards of moderation (e.g., water) due to an event where air conditioning water was inadvertently introduced into a portion of a dry process area containing highly enriched uranium (HEU), causing a potential criticality safety problem.^1,2 This notice also contains generic information about implementing and maintaining moderation control for criticality safety.

Applicability

The information in this notice applies to all DOE nuclear facilities which utilize moderation control for nuclear criticality safety. The Office of Nuclear and Facility Safety (NFS) advises building managers of applicable DOE facilities to understand the nuclear criticality safety hazards related to moderation control. Knowing the factors that make moderation-controlled equipment containing fissile material vulnerable to accidental criticality is a step in this direction.

Properties of Moderation Hazards

Many DOE facilities that handle, store or process fissile material the quantity of water and other hydrogenous substances (e.g., oil, plastic) in certain areas, equipment, and/or fissile material containers. These materials are called "moderators" because the hydrogen slows down or moderates neutrons, increasing nuclear. Moderation control, sometimes in concert with other criticality safety controls, is relied upon to render large "unfavorable geometry" process equipment, containers, piping, etc. critically safe. That is, the equipment is critically safe for the purpose intended within established limits, but it would not be safe for excessive quantities of moderator and/or fissile material. Moderator poses an additional hazard for fissile material in unwanted locations such as in facility ductwork; e.g., water flowing through ductwork could mix with and relocate a critical mass of fissile materials into an unfavorable geometry configuration. The event described below potentially involved all these hazards.

Event Summary

Last winter workers at the Y-12 plant near Oak Ridge, TN, opened blow down valves for a chilled water system on the roof of Building 9212 as part of a freeze prevention draindown. Workers failed to close these valves prior to refilling the system on June 6, 1991. These blow down valves were located in an enclosure with an air handling unit. As a result, water accumulated in the air handling unit, overflowed the flashing, leaked into a plenum and into a process area. Upon discovery of water in the process area, workers shut off the chilled water system. A total of about 350 liters of water leaked into the dry process area; a small portion of this volume entered the dry vacuum system for the magnesium oxide (MgO) sand recovery system through a High Efficiency Particulate Air (HEPA) filter via a misaligned gasket. The MgO sand contained low concentrations of HEU fines. Water admission to the filer and trap of the dry vacuum system violated criticality safety controls.

Event Significance

This event is very significant from a lessons learned standpoint. It illustrates the importance of communicating, coordinating, and following procedures affecting nuclear criticality safety, including peripheral utility operations such as lockout/tagout of the chilled water system. It also illustrates the need to design a roof enclosure to preserve the integrity of a dry process system. A larger volume of water entering through larger holes might have filled up the bag filters or other unfavorable geometry equipment, including the product breakout glovebox. This glovebox is used to process more than a minimum critical mass of fissile material. However, the product is normally a solid metal "button" which water cannot penetrate; moderation has relatively little effect on solid pieces of HEU such as this product button. Minimum critical mass of HEU occurs when the materials is finely dispersed or dissolved in moderator in an optimum ratio. Therefore, other procedural violations, equipment malfunctions, or fragmentation of the product button would have to occur in addition to loss of moderator control before a criticality would be possible in this glovebox. Nevertheless, since there might be no advance warning of nuclear criticality and its accompanying lethal radiation, criticality safety relies on defense in depth. Even partial loss of one defense is important for both the product button in the glovebox and for residual HEU in other unfavorable geometry equipment.

This specific event could not have led to a criticality accident in any of the other unfavorable geometry equipment in this process area because there was a negligible mass of fissile material in the MgO sand. (For example, each bag filter was estimated to contain less than five grams HEU.) To understand how the HEU in the MgO sand could become concentrated to cause a criticality safety concern, a brief description of the process is given: MgO sand is used to line a crucible containing HEU as uranium tetrafluoride (UF4) and other materials, primarily calcium metal. This crucible is heated and the chemical reaction yields HEU metal, calcium fluoride (CaF2), and slag. When a HEU metal button s knocked out of a crucible, the slag adds a small quantity of fissile material to the sand. The MgO sand is limited to one percent HEU by weight, a critically safe mixture for the sand recycle system equipment even if fully water moderated and reflected. The MgO sand is vacuumed into the sand recycling equipment, routinely sampled for uranium and other contaminants, and reused if acceptable. Because the HEU was well within the limit, criticality was not possible in this unfavorable geometry equipment regardless of the amount of water present.

Corrective Actions and Followup

The DOE contractor, Martin Marietta Energy Systems (MMES), made short term corrective actions and is working on long term actions to further reduce or quantify the risk of accidental criticality. The HEPA filter system involved in this event was relocated from the plenum to the operating area. Drains will be added to the air handling unit mounted on the roof. Procedures will be reviewed will all personnel involved. MMES has initiated a systematic evaluation of the other dry vacuum systems on site; facility safety personnel have not discovered any other obvious design flaws. Nevertheless, a mechanism will be developed to advise facility management of maintenance and utilities activities. The contractor is also conducting a review of all enriched uranium process areas to determine of there are unidentified credible scenarios for major liquid inundation of fissile operations. NS suggests that applicable DOE contractors consider the MMES response to this event, using the following guidelines to provide a framework for judging criticality safety of moderator hazards at their facilities.

Building Hazards

• The facility is located in a geographical area with moderate to heavy precipitation, especially facilities with buildings with flat roofs and the potential for roof drains to become plugged.

• Buildings or moderation controlled areas within buildings have overhead water lines.

• The building roof or other barriers to moderator entry are not in the original design condition due to facility age or modification.

Equipment Hazards

• Air handling or process offgas equipment and overhead sources of moderator are located inside enclosures without adequate drains.

• Process equipment, facility ductwork, and containers for which moderation control is necessary are approaching or exceeding design life.

• Facility ductwork or duct components are known or suspected to contain more than a minimum critical mass of fissile material.

• Drains in unfavorable geometry equipment that are intended to prevent buildup of moderator are nonexistent, obstructed, or could easily become obstructed during the same credible event that causes violation of moderator limits.

Nonstructural Hazards

• In addition to dangers posed by buildings and equipment, personnel at DOE facilities should be aware of moderator hazards and the need to prevent violations of moderation control. Efforts to minimize hazards posed by moderators should include the following.

• Coordinate work on water lines and equipment over of moderation control areas with workers in those areas.

• Have criticality safety management concur with lockout and tagout controls activities with moderation controlled materials to minimize risk during severe storms or severe storm warnings, when snow or ice loading is high, or if leaks are observed.

• Properly cover or close moderation controlled equipment and containers when not in use. During extended equipment shutdown, disconnect items such as vacuum wands that could provide a credible pathway for moderator.

• Preplan fire prevention and firefighting strategies for moderation controlled areas, minimizing combustibles. For example, restrict use of high pressure, high volume water sprays within moderation controlled areas and near entrances to or on roofs of such areas.

• Develop site-specific guidelines and planned responses to loss of moderation control and train workers to nuclear safety requirements including understanding of what moderators are and specific equipment hazards.

What To Do if Moderator Control is Violated

• Because of the diversity of DOE applications of moderator control for criticality safety no single answer exists. However in the absence of other site specific guidance, immediately do the following: Evacuate the area, notify management, and request assistance from criticality safety personnel.

References

1. Occurrence Report, "Level 2 Criticality Incident," ORO-MMES-Y12MAINTU- 1991-1007, date June 7, 1991.

2. T.S. Tison, DOE/ORO, memo to M. Williams, DOE/NS-10, "Off-Normal Occurrence - Level II Criticality Incident; Summary of Incident and Lessons Learned," dated July 25, 1991.