Information Notice 2000-03:High-Efficiency Particulate Air Filter Exceeds Mass Limit Before Reaching Expected Differential Pressure


February 22, 2000



All Nuclear Regulatory Commission (NRC) licensed fuel-cycle conversion, enrichment, and fabrication facilities.


The NRC is issuing this information notice to alert addressees to a potentially significant nuclear criticality risk for high efficiency particulate air (HEPA) filters which could in some instances accumulate special nuclear material (SNM) beyond a safe mass. Recently, the SNM mass limit was exceeded in a HEPA filter housing because the established differential pressure control was based on SNM with significantly different flow restriction characteristics than the actual process SNM. Review of a change in application of the HEPA filter failed to consider the effect of the different SNM on the filter housing differential pressure control. Recipients are expected to review this information for applicability to their facilities and consider actions, as appropriate, to control the risk of a nuclear criticality. Suggestions contained in this information notice are not NRC requirements. Therefore, no specific actions or written response is required.

Description of Circumstances:

On June 3, 1999, a fuel cycle facility licensee reported that a nuclear criticality safety mass limit within a primary SNM bearing HEPA filter had been exceeded. Subsequent licensee investigation revealed that the SNM bearing filter was being used to remove dry triuranium octoxide (U3O8) with a mass limit based on differential pressure that would occur if dry uranium dioxide (UO2) were the sole process material in the air stream. The inappropriate differential pressure limit degraded the margin of safety to the extent that a criticality safety limit was violated. The failure to consider the effect of the different material on the established differential pressure control is considered to have high potential safety significance.


On June 2, 1999, a fuel cycle licensee discovered that approximately 50 kgs (110.23 lbs) of U3O8 were held up in a primary HEPA filter in excess of nuclear criticality safety limits. The HEPA filter had a mass limit of 25 kgs (55.12 lbs) UO2 equivalent, which was one of the two primary criticality controls for the equipment. The licensee investigation determined that the HEPA filter was designed so that a differential pressure of 10.16 cm (4 inches) would be reached before 25 kgs (55.12 lbs) of fissile material had accumulated. This differential pressure limit was established for filtration of dry UO2 by HEPA filters widely used in the facility. The HEPA filter in which the mass accumulated was connected to equipment that produced dry U3O8 which exhibited different characteristics for flow at pressure drop across the filter media.

One cause of this event was the failure of the licensee to determine that a change did not comply with the basis for an original operating limit. When evaluating changes in the use of equipment for which risk significant limits and controls are established, licensees should consider all aspects of the equipment or operation upon which the limits and controls were originally based.

An additional cause of this event appeared to be low airflow through the filter. The vacuum for the filter was set just above the 10.16 cm (4 inch) differential pressure limit. This airflow was sufficient for UO2 but due to the different particle characteristics of U3O8, the 10.16 cm (4 inch) differential pressure limit may never have been reached regardless of the amount of material trapped. Licensees should consider reviewing recent changes to equipment or operations to see if operating parameters have been changed in a way that undermines risk significant safety limits and controls.

This event highlights the need to understand clearly the basis for established safety limits when reviewing new installations or configuration changes. A new or changed configuration may undermine the basis for proposed limits which were valid in other applications.

It is expected that addressees will evaluate the above information for applicability to licensed activities. This information notice requires no specific actions or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate regional office.


Michael F. Weber, Director
Division of Fuel Cycle Safety and Safeguards
Office of Nuclear Material Safety and Safeguards

Technical Contact: Dennis C. Morey, NMSS
Attachments: 1. List of Recently Issued NMSS Information Notices
2. List of Recently Issued NRC Information Notices

(ADAMS Accession Number ML003685310)
Template: NRR-052

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