Discharge of Fire Water Into a Critical Mass Lab

Content

• Introduction
• Notice Summary
• Applicability
• Background
• Event Summary
• Event Significance
• Corrective Actions
• Hazard Reduction
• 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 areas of nuclear criticality safety and fire safety. It addresses the potential hazards of moderation of fissile material and is based on an incident in which fire suppression water was inadvertently introduced into a Critical Assembly Room of a Critical Mass Laboratory. It is important to note that in this case the potential for criticality did not exist because of the standby status of the laboratory at the time of the event. This notice also presents generic information on analyzing, designing and maintaining water fire suppression systems. An earlier discussion of nuclear criticality safety hazards related to moderation control was presented in Safety Notice 91-1.¹

Applicability

The information in this notice applies to all DOE nuclear facilities installed with water fire suppression systems. No specific action or responses are required solely as a result of this notice.

Background

From 1960 to 1987, Battelle Pacific Northwest Laboratories (PNL) operated the Hanford Critical Mass Laboratory (CML). This facility was used to determine critical mass for various configurations of fissile and other materials. The Hazard Summary Report² for the facility addressed the fire protection features and systems installed in the laboratory. A deluge automatic sprinkler system was originally installed in the critical assembly room.

Deluge automatic sprinkler systems employ open heads with water withheld from the piping by a pneumatically controlled valve. Upon valve actuation, water flows out all heads in the system. The Hazards Summary Report addressed the potential for criticality, flooding and contamination control resulting from the introduction of water into the laboratory. A key factor in the analysis was a design feature that limited the total discharge of water to approximately 500 gallons. Also, an underlying assumption of the original analysis was that the doorways to the "reactor hoods" containing the experimental apparatus would remain tightly closed for contamination control, except when access was required.

In 1984, PNL converted the deluge automatic sprinkler system to a closed head system (i.e., the open sprinkler heads were replaced with closed fusible-link sprinkler heads) and the system was designated as a "preaction" automatic sprinkler system. Preaction automatic sprinkler systems are generally used to protect areas where there is the need to ensure that inadvertent introduction of water from damaged sprinkler heads or broken piping is minimized or avoided. Water is withheld from sprinkler piping by a pneumatic valve until heat activation devices (HADS) respond to heat generated by fire. Typical preaction systems use deluge valves as the controlling device, have closed heads, rely on detection in the area to open the valve and use air to supervise the integrity of the piping. An automatic shut-off valve can be installed to limit the discharge to a specified quantity of water. In 1984 a PNL contractor authorized the removal of the automatic shut-off valve that limited the total water discharge to approximately 500 gallons at CML.

In 1988 PNL prepared an Operational Readiness Plan (ORP)³ for placing the CML on unmanned standby status. Subsequently, on February 11, 1989, the CML was placed in an unmanned standby condition. The ORP addressed fire protection, among other topics. some of the fire protection issues addressed included; 1) The adequacy of provisions for sprinkler coverage in winter; 2) the adequacy of air supply available to operate dry sprinkler systems; and 3) the adequacy of provisions for periodic fire inspections and system tests. The unmanned condition of the critical assembly room in the CML left it unheated and susceptible to freezing temperatures. As part of the actions taken when the CML was placed in an unmanned status the contractor authorized the air supply monitoring system to be disconnected. An inspection by PNL building management in September 1991 indicated that some water had entered the system and frozen. An inspectors test drain valve for the system had cracked and fallen off (no water flowed when the valve broke off). These two conditions indicated that the fused sprinkler heads had been jeopardized by the frozen water in the system (freeze damaged sprinkler heads). Work requests were written to replace the valve and any damaged sprinkler heads. Subsequent inspections conclude that none of the damaged sprinkler were replaced.

Event Summary

On December 22, 1991 the compressed air system failed. It was concluded that the failure of the redundant system of two air compressors allowed the deluge valve to open and water to flow into the system piping. Because the sprinkler heads in the system were freeze damaged, they were unable to contain the pressurized water in the system. The eater discharged into the critical assembly room filling its containment area, and flowed into the hallway as well as the mix room and storage room. A small amount of water flowed out of the storage room door to outside ground level. In addition, the fire alarm system did not perform as designed when the system activated. The sprinkler system discharged approximately 5,720 gallons of water. No criticality event or personnel injury occurred because less than a critical mass of fissile material was present and because the facility was in an unmanned standby status.

In January 1992 the contractor conducted an investigation to determine the cause and contributing factors of this occurrence. The investigation concluded that the following factors contributed to the probable cause of the event.⁴

• The loss of compressed air to the pneumatic release on the deluge valve allowed water to flow into the dry portions of the preaction sprinkler system. Because the sprinkler heads were freeze damaged, water discharged into the critical assembly room.

• The primary building air compressor failed when the electric motor burned out and the secondary air compressor failed due to overheating caused by rapid cycling.

• Numerous leaks in the compressed air pipe system resulted in the air bleeding off and not maintaining pressure.

• Previous freeze damage to three sprinkler heads and a pipe tee had not been corrected.

Event Significance

This event is significant from a lessons learned standpoint. It illustrates the importance of analyzing the possibility of water intrusion into moderator control areas from fire suppression systems^5,6, including the possibility of introducing larger amounts of water, due to removal or failure of valve initiating devices. This event also illustrates the need for proper design and maintenance of fire suppression systems, particularly for unmanned facilities in a standby status.

Corrective Actions

As a result of the investigation, the contractor performed the following short-term corrective actions 1) repaired and tested the sprinkler piping, 2) replaced the deluge valve, 3) repaired the air compressors, and 4) restored the alarm and building monitoring systems to an operable status. The contractor also identified follow-up actions that included 1) ensuring proper functioning of the building alarm systems and evaluating the adequacy of the alarm system preventive maintenance program, 2) A recommendation that the deluge valve clapper diaphragm be replaced at 5-year intervals to prevent valve failure, and 3) Review of material and transportation needs to response to after hour off-normal events.

Hazard Reduction

To ensure that the introduction of water from fire suppression systems in moderator and contaminated control areas is adequately analyzed and controlled, and to maintain the historically high reliability of sprinkler systems, NS suggests that DOE facilities consider the following guidelines:

• Fire protection systems should be considered "Safety Related Systems" pursuant to DOE 5480.5⁷ and ANSI N46.1⁸ and maintained accordingly.

• Changes to systems affecting the potential introduction of water into moderator control or contamination control areas should be carefully analyzed to ensure that adequate measures have been established for criticality, flooding, water containment, and drainage capability.

• Readiness reviews should verify that system requirements, design features, and assumptions important to safety are valid. Changes to operational status should be analyzed and treated in the same manner as design changes.

• Although properly designed, installed and maintained sprinkler system has a level of inherent reliability, changes to the original design basis should be properly evaluated and validated.

Fire Protection Design Changes

• Fire protection systems should be designed and maintained in accordance with applicable codes and standards.⁹ For example, in accordance with design standard NFPA 13¹⁰, preaction systems require pipe supervision (either air or nitrogen under slight pressure) to indicate leakage.

• When changing an open-head deluge system to a preaction system consideration should also be given to:

  • reducing or eliminating the potential for condensation and subsequent freeze damage,
  • providing dependable air supplies that include dryers or dehydrators on the air supply line to prevent condensation in the air lines.

Building Protection

• Consideration should be given to maintaining adequate environmental conditions, such as building heating, ventilation, etc.

• Requirements should be spelled out in detail, e.g., water-tight doors and equipment features that must be secured when not in use.

• In unmanned buildings proper maintenance and inspection of fire protection systems¹¹, including the air supply system on which air operated systems depend, should be conducted to ensure that the historically high reliability of sprinkler systems is not jeopardized.

• Identified deficiencies should be acted upon in a timely manner to ensure that risks are minimized. In the event discussed above, deferral of corrective actions was compounded by the impending transfer of the facility to another contractor.

References

1. Criticality Safety Moderator Hazards, Safety Notice 91-1, dated September 1991, USDOE, Office of Nuclear and Facility Safety.

2. Hazards Summary Report for the Hanford Plutonium Critical Mass Laboratory, HW 66266, date August 1, 1960; Supplement 1 dated October 1963; and Supplement 2 (PNL 3749) dated April 1981.

3. Operational Readiness Plan for Placing the Critical Mass Laboratory on Unmanned Standby Status, PNL, dated October 4, 1988.

4. Investigation of Equipment Failure and Flooding at Building 209-E (Critical Mass Laboratory) on December 22, 1991, PNL, dated January 1992.

5. ANSI N304-86 "Nuclear Fuel Facilities - Facilities for Reprocessing Fuel - Fire Protection", 1986.

6. NFPA 801, "Facilities Handling Radioactive Materials," 1990.

7. DOE 5480.5 "Safety of Nuclear Facilities'" dated September 23, 1986.

8. ANSI 46.1 "Guidance for Defining Safety Related Features of Nuclear Fuel Cycle Facilities," dated July 1981.

9. NFPA 72 "Installation, Maintenance and Use of Proprietary Protective Signaling Systems," 1990.

10. NFPA 13 "Installation of Sprinkler Systems," 1991.

11. NFPA 13A "Inspection, Testing and Maintenance of Sprinkler Systems," 1987.