NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
February 22, 2000
|NRC INFORMATION NOTICE 2000-02:||FAILURE OF CRITICALITY SAFETY CONTROL TO PREVENT URANIUM DIOXIDE (UO2) POWDER ACCUMULATION|
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 problem recently noted with safety-significant level probes that are not self-checking. A level probe in a uranium dioxide (UO2) powder hopper failed without indicating a failed condition due to a broken connector in the level-probe circuit. This allowed UO2 powder to accumulate in a hopper and approach the criticality safety mass limit before discovery. Recipients are expected to review this information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. Suggestions contained in this information notice are not NRC requirements. Therefore, no specific action nor written response is required.
On August 5, 1999, a fuel cycle facility operator noticed that dry UO2 powder was not coming out of a granulator while the granulator was operating. Powder was being automatically fed to the equipment at the time and a level probe was monitoring the powder level in the feed hopper as a primary nuclear criticality safety control. The licensee determined that powder had blocked the compaction section of the granulator and an unusual amount of dry UO2 powder had accumulated in the powder hopper. The powder accumulation was not detected by the level-probe on the feed hopper because the probe had failed due to a broken connector in the level-probe circuit.
A fuel cycle licensee achieves uniform fuel particle size with equipment that compacts UO2 powder into a ribbon and then grinds up the compacted powder. Powder is added to the equipment from safe-geometry containers called polypacks. A known difficulty with this equipment is that the ribbon of compacted UO2 can jam and block the material flow. The safety of the operation was assured by limiting mass and moderator in the equipment. Primary criticality safety controls on mass included operator monitoring of the equipment, an interlock system limiting the number of polypacks that can be added, and a level-probe in the powder hopper that feeds the compactor.
An operator assigned to the compactor/granulator equipment observed that powder was not coming out of the granulator while the equipment was in operation and powder was being automatically fed to the compactor. The operator subsequently discovered that an unusual amount of dry UO2 powder had accumulated in the hopper that feeds the compactor. Licensee investigation revealed that the powder hopper level-probe, which was intended to detect this type of failure, did not work due to a broken connector in the level-probe circuit. Additionally, the licensee determined that the limit on the number of polypacks that could be added to the powder hopper was set too high to prevent the mass limit in the hopper from being reached. Also the licensee determined that required visual checks for powder accumulation were set at twice per shift without specific instructions on the required interval. Operators were allowed to perform the checks at the beginning and end of a shift which was too long an interval to ensure that the mass of UO2 powder did not exceed the mass limit in the hopper.
The primary contributing factor to the event was that the level-probe on the compactor/ granulator powder hopper did not self-indicate failure and alert the licensee to a lost control. A properly functioning level probe would have indicated the powder accumulation in the hopper before the powder approached safety limits. A self-indicating circuit would have informed the licensee of probe failure so that corrective actions could be taken to restore the control.
Additional contributing factors were that the limit on the number of polypacks automatically added to the hopper was set too high to protect against exceeding the mass limit and that the interval between required visual checks was allowed to be too long to assure that accumulations would be seen before the mass limit was exceeded.
This event highlights the need to establish the availability and reliability of safety-significant controls involving nuclear criticality safety under all credible upsets. Licensees should evaluate the need for safety-significant electronic equipment to be self-checking. Licensees should also establish that supporting criticality safety controls will actually perform their intended function.
It is expected that addressees will evaluate the above information for applicability to licensed activities. This information notice requires no specific actions nor 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
|Technical Contact:||Sheryl A. Burrows, NMSS |
|Attachments:||1. List of Recently Issued NMSS Information Notices |
2. List of Recently Issued NRC Information Notices
(ADAMS Accession Number ML003685215)