Lessons Learned Database



Title:

Age Degradation of High Efficiency Particulate Air Filters

Identifier:

2004-LL-NUCL-0002

Date:

2004-09-30

Lesson Learned Statement:

Implementation of new information, such as the tensile strength testing results published in the recent version of the DOE Nuclear Air Cleaning Handbook, can have a significant impact on the validity of design and safety basis assumptions. Therefore, prompt communication and analysis of new information, such as was performed by DOE and PNNL is critical to promptly maintain the safety basis of a nuclear facility such as the RPL.

Discussion:

In a letter dated February 12, 2004, DOE-RL directed PNNL to review the revised DOE Nuclear Air Cleaning Handbook (HDBK-1169-2003). The direction was to specifically review filter service life and other new information using the PNNL Unreviewed Safety Question (USQ) process. Appendix C of the Handbook discussed the reduction in filter media losing its tensile strength with age that was indicated by testing of HEPA filters. The handbook indicated that HEPA filters operated under dry conditions can be expected to no longer meet the specifications for filter media tensile strength after 13 years.

Analysis:

The Documented Safety Analysis (DSA) for the Radiochemical Processing Laboratory (RPL) specifically assumes the final stage of HEPA filters have sufficient tensile strength to remain intact for any pressure pulse from an explosion that is not large enough to breach the building. The final stage of RPL HEPA filters are approximately 26 years old, twice the age identified in the Handbook where filter media tensile strength may no longer meet requirements. Although the filters are periodically tested for filtration efficiency, and are visually inspected, these tests do not provide indication of tensile strength. The new information provided in the Handbook regarding loss of tensile strength with age calls into question the specific RPL DSA assumption that the final stage of HEPA filters continue to meet design specifications and have sufficient strength to remain intact in an explosion scenario.

This condition was reviewed under the PNNL USQ process and determined to constitute a potential inadequacy in the documented safety analysis (PISA). The USQ Determination performed in response to the PISA determined that an Unreviewed Safety Question (USQ) exists in the RPL safety analysis did exist. Specifically, the new information provided in the DOE Air Cleaning Handbook (HDBK-1169-2003) indicates that the basis for stating in the RPL DSA that the final stage of HEPA filters will remain intact during postulated accidents is no longer valid.

The new information added to Appendix C of the of the DOE Nuclear Air Cleaning Handbook, as well as the findings of the November 2003 DOE (OA-50) Essential Systems Functionality (ESF) review, prompted PNNL to reevaluate the ability of the Radioactive Exhaust Ventilation System (REVS), which includes the final HEPA filters, to accomplish the its credited safety function under certain scenarios and upset conditions. This reevaluation determined that, due to the potential for unfiltered leak paths through the building structure and the reduced tensile strength of the filter media, HEPA filtered exhaust function of the REVS should not be credited. Therefore, PNNL reperformed the accident analysis for those accidents which previously took credit for consequence mitigation by the REVS. The new accident analysis also incorporated the latest International Commission on Radiation Protection (ICRP) dose conversion factors. The revised analysis concluded that the calculated dose consequences for accidents that previously credited the REVS are less than DOE-RL Nuclear Safety Risk Ranking and Control Selection Guidelines and do not warrant engineered safety systems such as the REVS. Therefore PNNL submitted a safety basis revision which removes the REVS as a credited hazard control in the safety analysis. DOE-RL approved issued a Safety Evaluation Report (SER) approving the safety basis revision on August 3, 2004.

Recommended Actions:

Implement revised safety basis as approved by DOE-RL. Implementation and transition to the revised safety basis was completed on September 9, 2004. Specifically, the TSR and several implementing documents have been revised to delete the Limiting Condition for Operations associated with the REVS, to update curie equivalent conversion factors which will be automatically reflected in the Radioactive Material Tracking (RMT) system, and to add the REVS as a defense-in-depth administrative control as part of the Radiation Protection Program TSR Administrative Control. The changes remove REVS as an engineered control credited for dose consequence mitigation in the DSA accident analysis. The REVS remains as a safety-significant system. No changes were made to the way the REVS is operated or maintained.

Originator:

PNNL, William T. Buyers

Validator:

Roger A. Pollari

Contact:

Bill Buyers, (509)376-5746, bill.buyers@pnl.gov

Name Of Authorized Derivative Classifier:

Roger A. Pollari

Name Of Reviewing Official:

Roger A. Pollari

Priority Descriptor:

Blue / Information

Keywords:

Safety Analysis, Limiting Condition of Operation, HEPA Filters

References:

Occurrence Report RL--PNNL-PNNLNUCL-2004-0002
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DOE Function / Work Categories:

Maintenance - Safety Systems

ISM Category:

Analyze Hazards

Hazard:

Personal Injury / Exposure - Radiation / Contamination
Radiological Release

End of Lesson!

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