Lesson Learned Statement:Assumptions in Safety Analysis Reports (SAR) for High-Efficiency Particulate Air (HEPA) filtration efficiency during design basis accidents should be consistent with values actually achievable during periodic testing.
Discussion:During a review of requirements for HEPA filters in the 242-A Evaporator SAR, an engineer identified that the filter efficiency cannot actually be tested to the degree assumed in the technical basis for a design basis accident. An Unreviewed Safety Question Screening/Evaluation form was prepared and submitted to the 200 Area Liquid Waste Processing Facility Plant Review Committee (PRC).
Analysis:HEPA filter are relied upon for accident mitigation in the 242-A authorization basis. The filter assembly consists of a pre-filter with two stages of HEPA filtration. The fully mitigated accident analysis took this configuration into account in the calculation of the dose consequences. The receptor dose consequences were calculated based upon multiple HEPA filters in series, resulting in an assumed efficiency of 99.999%. Actual filter aerosol testing was performed over the filter bank housing as a whole (both HEPA filters at the same time), with acceptable efficiency being at least 99.95%. The minimum detection level for this testing method is 0.002% transmission, or 99.998% efficient, which is less than the efficiency assumed in the accident analysis. Therefore, current testing methodology did not sufficiently verify that the HEPA filters are functioning as assumed in the design basis accident analysis.
On March 24, 2000, the PRC voted unanimously that there was discovery of a potential Unreviewed Safety Question (USQ). The results of a USQ evaluation were reviewed by the PRC on March 28, 2000, at which time they determined that a USQ existed due to an inadequacy in the approved authorization basis.
Dose consequences of the lower efficiency of the filters during a design basis accident remain less than one percent of the risk evaluation guidelines. Therefore the SAR was changed to reflect the correct efficiency.
Recommended Actions:Ensure that all assumptions made in the technical bases and evaluation of design basis accidents for nuclear facilities can be validated or physically measured.
Originator:Fluor Hanford, Inc. Submitted by Greg Millward
Contact:Project Hanford Lessons Learned Coordinator; (509) 373-7664; FAX 376-6112;
Name Of Authorized Derivative Classifier:Not required
Name Of Reviewing Official:John Bickford
Priority Descriptor:Blue / Information
Keywords:design basis accident, HEPA Filter, aerosol penetration testing
References:USQ evaluation LW-00-006
Occurrence report RL--PHMC-200LWP-2000-0002
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DOE Function / Work Categories:Authorization Basis
Engineering Design and Construction - Nuclear
ISM Category:Feedback and Improvement
Hazard:Personal Injury / Exposure - Radiation / Contamination
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