Explosion Hazards of Uranium-Zirconium Alloys

Content

• Introduction
• Notice Summary
• Applicability
• Event Summary
• Event Significance
• Causes of the Event
• Site Specific Corrective Actions
• Other 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 presents lessons learned in nuclear and chemical safety. It describes an explosion while processing a bath of scrap nuclear fuel and miscellaneous components. This event occurred last year at the Idaho Chemical Processing Plant (ICPP) a the Idaho National Engineering Laboratory (INEL).^1,2 Several laboratories and DOE facilities have reported the occurrence of explosions during the pickling, etching, and dissolution of uranium-zirconium alloys in nitric acid.³ Adverse reactions with nitric acid was the subject of a recent review at the Savannah River Site (SRS). SRS has experienced a number of adverse situations in handling nitric acid ranging from minor personnel injury to significant explosions.⁴

Applicability

This notice applies to all DOE facilities that handle or process nuclear fuel and items containing uranium-zirconium alloys. Laboratories that have received scrap materials from ANL-East since the 1970s should pay particular attention to this notice.

Event Summary

On February 9, 1991, a small explosion ruptured a 6-inch diameter borosilicate glass vessel containing 1.9 kg of highly enriched uranium fuel and 4 liters of heated nitric acid solution. The explosion sprayed uranium-bearing nitric acid solution out of the confinement hood onto three operators. These individuals did not sustain acid burns or significant contamination. There was no appreciable damage to other nearby equipment.

The preponderance of evidence suggests that a small explosion occurred involving epsilon phase zirconium - uranium (UZr3). Other potential causes include a hydrogen and/or nitric oxide gas explosion or organic contaminants in or on the fuel canister reacting with nitric acid. The post-accident investigation revealed that two pieces of the fuel contained an uranium- zirconium alloy. Depending on composition, homogeneity, and thermal annealing history, uranium-zirconium alloys may react explosively in the presence of nitric acid.³ The magnitude of the explosion depends on the quantity and types of reactants. Also, the use of other chemical media for the dissolution of low zirconium alloys, particularly non-oxidizing acids, may be more hazardous than the use of nitric acid. This could result in an explosion even more violent than that encountered using nitric acid.

The explosion occurred 31 minutes after nitric acid breached the fuel can. This indicates that other fuel present dissolved more rapidly than the UZr3, resulting in a porous, high specific area solid in excess nitric acid. ICPP concluded that "a highly energetic reaction between UZr3, and HNO3 is the most likely cause of the dissolver failure" because "the timing of the occurrence coincides with the expected behavior of HNO3 - UZr3 reactions." It should be noted, however, that the mixture of scrap materials dissolved was complex and included thorium and other potential reactants.

Event Significance

This event was a significant precursor because of the potential for more serious consequences. The uranium was contaminated with plutonium and other radionuclides. Hood doors were open contrary to postings and procedures did not provide adequate worker protection. Flying glass and acid could have inflicted plutonium contaminated wounds.

By itself, this event did not increase the potential for accidental criticality. It would have been necessary to collect more than a minimum critical mass of uranium (about half of the 1.9 kg present) into an unfavorable geometry container (e.g., mop bucket) and add several liters of water before criticality would be possible. The double contingency principle for criticality safety was no violated either during this event or during the recovery phase.

Causes of the Event

The draft DOE-Idaho (DOE-ID) report concluded that the root cause of the accident was failure of Westinghouse Idaho Nuclear Company (WINCO) management to identify, evaluate, and mitigate the hazards involved. Neither the WINCO acceptance criteria nor any internal WINCO documents identified any limit on zirconium; however, there was an unwritten limit of one percent zirconium.

WINCO misinterpreted statements in the shipping papers and concluded that the zirconium content was less than one percent. WINCO also misconstrued the acronym "ND" to mean "not detectable" zirconium content instead of "not determined" as defined on the shipping form. WINCO did not analyze the scrap prior to dissolving it to confirm that zirconium or other potential reactants were within the informal limit. Recent analysis of the residues indicated that two samples were uranium with high zirconium content (estimated to be 15 percent).⁵ The draft report stated that "there was a failure of WINCO management to adequately identify, and preclude from processing, scrap nuclear material determined to be unsuitable." Even if the contractor had sampled batches of the material, the high zirconium content components might have been missed because of imprecise information in the shipping papers.

Site Specific Corrective Actions

WINCO performed immediate corrective actions to decontaminate personnel. After the contractor determined that the situation was stable, shock and reactivity tests were performed on the residue. A check of radiation instruments indicated that no criticality occurred. Contamination surveys found no contamination beyond the facility, and followup bioassays revealed no significant internal doses to individuals involved.

WINCO and DOE-ID then initiated an investigation as to the cause of the event. Contractor management shut down the Custom Process area pending the outcome of its investigation. WINCO collected the remaining undissolved material and solution, decontaminated the area, and flushed the dissolver and process lines. Cognizant personnel reviewed and revised procedures, including procedures for recovering from unwanted events.

WINCO issued a corrective action plan as part of its internal investigation report. This plan recommended the following long-term corrective actions:

• Increase the depth and detail of safety evaluation by improved use of techniques and methods described in the WINCO Safety Analysis Manual.

• Evaluate existing equipment and processes for conformance to safety criteria and DOE orders.

• Assure that protective devices are provided to mitigate and contain the consequences of credible accidents.

• Prepare a lessons-learned bulleting for DOE distribution to alert shippers to the need for complete information about contents.⁶

• Review the information for current fuel scrap inventory for completeness, and request additional information as necessary.

• Improve the availability of information about previous experiences, both at ICPP and other DOE sites.

• Expand and document the evaluation of hazards to include materials used in the process.

• Review the response to the accident and improve communications equipment, understanding of responsibilities during accident response, and availability of spare clothing and safety showers.

Other Corrective Actions

The shipper, Argonne National Laboratory - East (ANL-E), performed an independent review of its program for packaging and transportation of unirradiated Special Nuclear Material (SNM).⁷ This review acknowledged that there were areas that needed correction or improvement to provide an acceptable level of performance in packaging. Proposed corrective actions included revising shipping procedures, training, and line management oversight and self assessment of packaging and shipping operations. Specifically, the review recommended two-person confirmation of the contents of each off-site shipment of SNM before the containers are sealed.

Hazard Reduction

Recently, DOE/EH issued a Safety Note concerning problems in packaging and transporting hazardous materials.⁸ The Safety Note emphasized the importance of properly specifying hazardous materials in packages. Personnel at DOE facilities should be aware of the potentially hazardous conditions that may evolve when conducting custom dissolution process operations. In addition to the aforementioned corrective actions, the following precautions should be considered:

• Technical and safety disciplines should evaluate nuclear and chemical hazards of anticipated processing of fuel and component materials in conjunction with scrap repackaging.

• Handling procedures for scrap repackaging should require identifying evaluated hazards and providing detailed descriptive information on shipping papers, avoiding non-specific acronyms.

• Facilities should formally establish and actively enforce material receipt criteria on all shippers. Specific characteristics of proposed shipments should be assessed to avoid mishaps or exceeding the safety envelope.

• Potentially explosive materials should be inspected and certification should be validated to ensure batches meet specifications.

Materials to be processed, particularly those with hazardous impurities such as uranium-zirconium alloys, should be analyzed to accurately determine the contents. Scrap nuclear materials determined to be unsuitable should be excluded from processing.

• A suitable plastic-lined metal dissolve should be used as a primary barrier to allow adding fluoride ion to the dissolution medium. Addition of fluoride has been found to prevent explosion hazards involving dissolution of epsilon uranium-zirconium alloys.³ Additionally, glass and low fracture strength plastic dissolvers are vulnerable to other types of accidents (e.g., gas phase explosions).

• Secondary barriers should be used to protect workers against the forceful expulsion of materials and to minimize worker and facility contamination. (For example, use lucite or other safety glass plus an intermediate air gap to dissipate sufficient energy so that the secondary barrier will hold.)

• In addition, DOE 6430,1A requires double containment for new or significantly modified equipment for processing plutonium.

• Workers should complete regularly scheduled training regarding specific safety precautions to be taken when processing scrap uranium fuel. DOE contractors should provide periodic assessment of procedure compliance.

• Dissolution processes should be implemented in a conservative, formal, and disciplined manner using the principles of Conduct of Operations. Independent verification should be implemented for critical process steps or system line-ups.

References

1. DOE Facility Occurrence Report, "Rupture of Dissolver No. 1 in the Hot Chemistry Lab," ID--WINC-ICPP-1991- 0026, update of February 12, 1992.

2. Final Draft, Uranium/Nitric Acid Processing Explosion at Idaho Chemical Processing Plant: Follow-up Investigation Report, DOE/ID-10368, dated January 1992.

3. "A Study of the Explosive Properties of Uranium/Zirconium Alloys," ANL-5135, R.P. Larsen, et al., July, 1954.

4. "Adverse Experiences With Nitric Acid at the Savannah River Site," WSRC-TR- 91-22, Rev. 1, Durant, et. al., January, 1991.

5. Letter, J. D. Zahrt, LANL, to B. Wheeler, WINCO, dated March 5, 1992.

6. Letter, W.C. Moffitt, INEL, to S. M. Blush, DOE/NS, "Transmittal of Safety Bulletin," dated April 20, 1992.

7. Memorandum, S. Ludwig DOE/CH to D.T. Goldman, "Chicago Field Office Review of the Argonne National Laboratory - East (ANL-E) Program for the Packaging and Transportation of Unirradiated Special Nuclear Material," dated January 23, 1992.

8. Safety Note, "Problems in Packaging and Transportation of Hazardous Materials," DOE/EH-0210, March 1992.