4 septembre 2014
Un ingénieur a découvert que, suite à la quantité de matière fissile transportée sur les chariots, lors d’un basculement de ces derniers, il y avait un risque de départ de réaction en chaîne incontrôlée (risque de criticité) qui n’avait pas été envisagé. Cette hypothèse avait simplement été écartée au motif qu’elle n’était pas crédible, mais elle est aujourd’hui considérée comme possible. Les chariots ont été retirés de la circulation et des mesures correctives seront appliquées dès qu’il y aura les retours de l’enquête.
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Event Number : 50425
Facility : B&W NUCLEAR OPERATING GROUP, INC. - RX Type : URANIUM FUEL FABRICATION
City : LYNCHBURG State : VA Event Date : 09/04/2014 - Event Time : 11:00 Emergency Class : NON EMERGENCY 10 CFR Section : PART 70 APP A (b)(1) - UNANALYZED CONDITION
Event Text
UNANALYZED CONDITION RELATED TO POTENTIAL MATERIAL TRANSFER CART TIPPING
"I. EVENT DESCRIPTION : On September 4, 2014 at approximately 1100 [EDT], a Nuclear Criticality Safety (NCS) engineer identified a safety concern. While working to consolidate information in the safety basis for the safe geometry storage and transport carts, it was determined that an unanalyzed condition existed that did not meet the performance requirements of 10 CFR 70.61. Tipping or impact of a cart during transport had not been considered as a credible upset condition.
"II. EVALUATION OF THE EVENT : At B&W’s NOG-L [Nuclear Operations Group] facilities, safe geometry storage and transport carts are used to transfer uranium bearing materials between radiologically controlled areas. The carts are typically used to transfer scrap and waste materials in favorable volumes less than or equal to 2.5 liter containers to the Drum Count Area for 235U assay. Because the 235U content of such containers is not known until they have been assayed, they are referred to as ’unknowns’ and are subject to a net weight limit. These containers are limited to a maximum of 7,000 grams net weight (approximately 15 pounds) of uranium bearing material in any form.
"There are forty storage locations on a cart, twenty per side. The locations on each side are arranged in an array of 4 columns, each column contains 5 storage locations. During transport each column of storage locations is protected by closure of a door. The four column doors on each side of the cart are secured by a common locked bar.
"The NCS evaluation of the safe geometry storage and transport carts did not address possible tipping during transit. Although unlikely, it is believed at this point the event is credible. If a cart were to tip, no controls were identified to retain the containers on the cart. Although the doors on the cart are secured by a robust locking bar, this action is taken for security purposes and is not credited as an IROFS [Item Relied On For Safety]. Assuming the containers on the cart were fully loaded (7 kg net weight) with a U-metal water mixture at optimum H/X and the cart tipped over, and more than three containers fell out of the cart, a configuration could result that would exceed the keff safety limit of 0.95 in NRC License SNM-42 for a single contingency.
"The requirement of 10 CFR 70.61 (d) states in part : ’... the risk of nuclear criticality accidents must be limited by assuring that under normal and credible abnormal conditions, all nuclear processes are subcritical, including use of an approved margin of subcriticality for safety.’
"Therefore the performance requirement of 10 CFR 70.61 (d) would not be maintained during this credible abnormal condition.
"The as-found condition had no actual safety significance. There was no immediate risk or threat to the safety of the workers, the public, or the environment as a result of this condition. The safe geometry storage and transport carts did not contain any uranium bearing materials. There was no actual tipping event. The carts were immediately removed from service.
"Ill. NOTIFICATION REQUIREMENTS : B&W is making this 24 hour report in accordance with 10 CFR 70, Appendix A, (b)(1)—Any event or condition that results in the facility being in a state that was not analyzed, was improperly analyzed, or is different from that analyzed in the Integrated Safety Analysis, and which results in failure to meet the performance requirements of 70.61.
"IV. STATUS OF CORRECTIVE ACTIONS : The carts have been removed from service. An investigation of the root causes of this condition is ongoing. Corrective actions will be determined as a result of the investigation."
https://www.nrc.gov/reading-rm/doc-collections/event-status/event/2014/20140915en.html#en50425
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