Defense in Depth Equipment was Out of Service due to a Normalized Deviation

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Type:  Lessons Learned

Publisher:  CH2MHill Plateau Remediation Company (Richland, WA - Hanford Site), Richland, WA - Hanford Site (CHPRC)

Published As:  Public


Topics:  Nuclear/Nuclear Safety, Maintenance, Radiation Protection


Changes in operating practices which have no immediate consequence may become accepted over time, resulting in normalized deviations. Training on Defense in Depth (DID) Important To Safety (ITS) equipment may be limited; consequently, personnel may not adequately recognize the significance of DID equipment relative to the facility safety basis.


The Integrated Water Treatment System (IWTS) was operated without operable radiation monitoring equipment, resulting in an out of service DID feature described in the safety basis. The IWTS Garnet Filters are a set of three filters, each with an associated radiation detector installed. The Safety Basis identifies these detectors as DID equipment and a method to determine when the Garnet Filters require regeneration. The radiation monitors are not associated with any Technical Safety Requirements or Surveillance Requirements. Shortly after installation of the filters in 1999, Operations learned that differential pressure (dP) was the governing measure in practice and began using it as the method for determining when to regenerate. Due to the conservative measures for backwashing, the radiation detectors would not reach a point where they would alarm.

In September 2010, Detector 2 experienced a series of spurious alarms and ultimately failed during the summer of 2011. In February 2012, the calibration cycle expired for Detectors 1 and 3. Detector 2 remained out of service. Operations asked if the calibration cycle could be extended or cancelled. In March 2012, the alternate Design Authority (DA) for the Garnet Filter system changed the recall status from Periodic (Recall 1) to As Needed (Recall 3). There was no evidence that any of these changes received a technical analysis through the applicable change management.


The radiation detectors were credited in the safety basis as a method to determine when the Garnet Filters require regeneration. The language in the FSAR did not result in a clear understanding of the significance of the radiation detectors. Since the radiation detectors never reached an alarm point (with the exception of a single spurious alarm), neither Operations nor Engineering recognized the safety basis role of this equipment. A repercussion of this normalized deviation was that limited priority was assigned to repairing or replacing the out of service radiation detector. The condition influenced Operations and the alternate DA to change the recall schedule for the radiation detectors. Given the low priority and low operational consequence of having the equipment out of service, there was no discussion or attempt to modify the safety basis and change the significance of the radiation detectors.

A review of the Safety Basis training determined that DID ITS equipment is mentioned but no detailed information is provided. The content of the training was insufficient to alert Operations to the significance of the radiation detectors or change behaviors related to prioritization of repairs. The Safety Basis training reflects a general emphasis on the Safety Significant and Safety Class Structures, Systems, Components (SSCs), but does not specify the DID ITS equipment.

RECOMMENDATIONS - to prevent recurrence

Verify that DID equipment is maintained in accordance with safety basis requirements.

Review safety basis training content to verify that it contains sufficient information to communicate the significance and requirements associated with DID equipment.


Occurrence Report EM-RL-CPRC-SNF-2012-0013


Beth Poole 373-0522 or or

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