Criticality Safety Hazards Associated With Large Vessels

Content

• Introduction
• Notice Summary
• Applicability
• Event Summary
• Event Description
• Event Significance
• Causes of the Event
• Site Specific Corrective Actions
• Criticality Safety of Large Vessels at DOE Facilities
• Equipment Hazards
• Other Hazards
• 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 nuclear criticality safety, specifically with respect to large vessels. It describes an events in which 150 kilograms of low-enriched uranium (LEU) in solution were inadvertently introduced into a large waste treatment vessel. Waste treatment chemicals in this vessel precipitated in LEU, potentially creating conditions favorable for a criticality accident.¹ This notice also contains generic information about implementing and maintaining criticality safety control in large vessels, and it provides useful lessons for DOE and contractor personnel responsible for nuclear criticality safety and management of DOEís nuclear facilities.

Applicability

This notice applies to all DOE facilities that use nuclear criticality safety controls over fissile material in solution.

Event Summary

The event in question involved an inadvertent transfer of LEU from a solvent exchange process to a waste treatment process at the General Electric (GE) Nuclear Fuel and Component Manufacturing Facility, located near Wilmington, North Carolina. (Figure 1 is a simplified schematic of portions of the wastewater treatment process.) This incident was evaluated by a Nuclear Regulatory Commission Incident Investigation Team (IIT). The IIT report noted that when this particular waste recovery and treatment process was licensed, criticality safety controls were adequate. However, over time most of the criticality safety controls had deteriorated or were altered through design modifications, operational changes, or neglect. Changes to the solvent extraction process included:

• Operating in a manual override mode during upset conditions, thus defeating automatic control system safeguards.

• Disabling audible alarms and an interlock and raising alarm setpoints for uranium solution density monitors without considering the impact on the waste treatment process.

• Using new uranium sampling instruments without developing the necessary procedures or sufficient testing.

Changes to the waste treatment process included:

• Sampling the quarantine tanks manually. (There was automatic sampling equipment, but it did not work properly.)

• Removing a density probe from service.

After these and other changes had been made, the uranium mass limit contingency barrier was no longer adequate. For example, the new sampling equipment could not produce a representative sample during certain upset conditions. This led to inadvertent transfers of highly concentrated LEU. Sparging was the sole remaining workable barrier to criticality in the large waste treatment process tanks, and even sparging would not have protected against all scenarios.

Event Description

On the evening of May 28, 1991, a flow control valve failed to open. This valve was manufactured by Masoneilan (model number 2630A "Splitbody") and was located between the solvent exchange process and the aqueous waste treatment process. Concentrated LEU was transferred from the solvent extraction process over a period of several hours as operators tried to control the symptoms without shutting the process down. Approximately 150 kilograms of LEU (3.2 weight percent U-235) were inadvertently transferred from safe geometry process tanks through other tanks in a series of transfers to a nominal 20,000 gallon tank at the on-site waste treatment facility. During the first hours of the event, operators did not fully realize what was happening, took inappropriate actions (including blocking the failed valve in the open position, and failed to sample some of the transfers for uranium concentration). At 5:20 AM on May 29th, after receiving the first sample indicating a high uranium concentration, operators shut down the solvent exchange process. The 150 kilograms transferred exceeded the safe mass limit for this waste tank (35 kilograms uranium at a maximum enrichment of 5 weight percent I-235). Water treatment chemicals already in this tank caused the uranium to precipitate out of solution, thus creating the potential for a criticality accident. GE evacuated non-essential personnel from the area and removed the uranium precipitate from this tank via centrifuging operations over the next few days. In a parallel effort, some of the uranium in this tank was transferred to other available tanks to reduce the mass in any one tank below the minimum critical value (approximately 100 kilograms uranium for 3.2 weight percent U-235). (Safe mass is nominally 45 percent of minimum critical mass.)

FIGURE 1 SOLVENT-EXTRACTION PROCESS WASTE FLOW is not available in this document. If you would like to obtain a copy of it, you may contact the Nuclear Safety Information Center at (301) 903-0449 or by writing to NSIC, U.S. Department of Energy, EH-15/Suite 100, CXXI/3, Washington, DC 20585.

Event Significance

This event was significant because under different circumstances ceasing sparging could have resulted in a criticality accident. The IIT report noted that if the entire 275 kilograms of LEU in the solvent extraction process had been transferred to this tank, "sparging would likely be necessary to prevent the criticality accident." The potential for accidental criticality would have been even greater had there been a greater volume of higher enriched uranium and a lesser volume of non-uranium solids in the tank.

Causes of the Event

The IIT report concluded that there were three interrelated root causes of this event:

1. There was a pervasive attitude that a nuclear criticality was not credible.

2. Management did not provide effective guidance for safety.

3. A production-oriented mindset was not tempered with a "safety first" attitude.

The IIT report faulted the facilityís nuclear safety organization, GE management, and the NRC regulatory oversight function for failing to monitor for, detect, and analyze the impact of plant changes. The report noted that facility management initially underestimated the seriousness of the event and delayed declaring an emergency for 12 hours. The report also cited weaknesses in maintenance, training, engineering, and other process support elements, as well as in areas such as emergency planning for criticality accidents. The report noted that the on-site criticality safety organization was adequately staffed, but that it placed insufficient emphasis on observing day-to-day plant operations and analyzing the impact of changes on criticality safety. The report listed weaknesses in NRC regulatory guidance, including insufficient review of the nuclear safety analyses that supported facility changes.

GE concurred with the facts in the IIT report, but felt that the conclusions were overstated.² For example, GE stated that training emphasized that accidental criticality is a potential hazard and is to be avoided by following approved operating procedures. However, the operatorsí actions to keep the solvent extraction process operating in spite of the difficulties encountered indicated that they were highly inclined to maintain production.

Site Specific Corrective Actions

In addition to the immediate corrective actions, GE performed other corrective actions and initiated an in- depth evaluation of root causes. GE removed, disassembled, and inspected the control valve because its failure had initiated the event. The inspection revealed that the Moore positioner for the flow control valve was stuck; thus, the positioner was unable to control the valve actuator. The positioner was manufactured by Moore (model number 750P). It appeared that interference between the spool piece and the bushing of the valve positioner had occurred in a previous event and an erroneous root cause evaluation, resulted in a decision to file down the spool. GE reinstalled the valve with a new Moore positioner, and it functioned properly.

Criticality Safety of Large Vessels at DOE Facilities

Most DOE facilities that process fissile material bearing liquids use large "unfavorable geometry" process or waste stream vessels. Therefore, it is necessary to maintain constant control over mass and/or concentration of the fissile material. DOE contractors maintain criticality safety through a number of measures: e.g., controlling transfer paths via valve alignments, sampling, monitoring tank levels, checking for leaks and spills, and adding solid neutron poisons. Some designs use siphon breakers, pipe spools, and other devices to prevent flow of fissile material in solution into unfavorable geometry vessels or other equipment. However, fissile material accumulations in excess of safety limits have occurred in unfavorable geometry equipment in DOE facilities. Sumps, sinks, trash cans, gloveboxes, and ventilation ductwork are but a few examples where solutions or mixtures of fissile material might not be critically safe due to unfavorable geometry.

The following guidelines provide a framework for judging criticality safety of DOE nuclear facilities that process fissile liquids.

Equipment Hazards

Personnel at DOE facilities should be aware of equipment- related criticality safety hazards posed by large vessels. Some examples of these hazards follow:

• Large numbers of unfavorable geometry vessels and diverse interconnections to and from favorable geometry process vessels. (This includes interconnections to or through tanks containing precipitating agents or ion exchange media.)

• Valves that are leaking, stuck, or deteriorating, or valves that cannot routinely be inspected for leaks or position (e.g., due to high radiation fields).

• Deteriorating process equipment, especially interconnecting components and barriers to inadvertent transfer, and the associated instrumentation and controls.

• Unfavorable geometry tanks located at lower elevations from process tanks or piping; and unpoisoned, unfavorable geometry sumps that do not contain solid nuclear poisons (e.g., Raschig rings).

• The use of a spool piece design that allows pumps to be installed backwards.

• Favorable geometry tanks whose holdup capacity is insufficient to obtain sample results prior to transferring liquids to unfavorable geometry tanks.

• Outdoor equipment that lacks adequate freeze protection.

Other Hazards

In addition to dangers posed by equipment, personnel at DOE facilities should be aware of the need to prevent violations of concentration or mass control for large vessels. Efforts to minimize hazards posed by large vessels should include checking for the following:

• Precipitation which can result in a critical mass near the bottom of the vessel.

• Non-representative sampling due to inadequate mixing, especially if precipitation is credible.

• Use of precipitating reagents or solvent exchange reagents in the process or available via interconnections, including, in some cases, water. (For example, reducing the acidity of plutonium nitrate via dilution can cause the plutonium to precipitate).

• Changes to processes, procedures, and maintenance in order to work around problems instead of taking appropriate steps to resolve them. For example, bypassing automated safety or process control equipment and repetitive use of temporary hose connections.

• Evaporation due to excessive sparging, which could cause concentration of fissile materials to increase above criticality safety limits.

References

1. NUREG-1450, "Potential Criticality Accident at the General Electric Nuclear Fuel and Component Manufacturing Facility, May 29, 1991," dated August 1991. (Figure 1 in this Safety Notice was adapted from a figure in this report.)

2. General Electric Management Briefing to NRC Commissioners, Public Meeting held on October 18, 1991, at Bethesda, MD.