NNSA tackles safety concerns at PF-4

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Plutonium: Upgrades underway to reduce earthquake risks

By The Staff

Last week, the National Nuclear Security Administration sent a letter to the Defense Nuclear Safety Board concerning the safety of the Plutonium Facility (PF-4) at the Los Alamos National Laboratory.

The DNFSB asked NNSA and lab officials numerous questions about the safety of the facility at a hearing in Santa Fe late last year.

In the letter, Donald Cook, the deputy administrator for weapons programs wrote, “Our evaluation of the existing PF-4 facility in 2008 established a Documented Safety Analysis (DSA) for the facility that was compliant with the Department’s Nuclear Safety Rule.

“The DSA concluded that safety measures at PF-4 were reasonable and adequately protected the public under current standards for existing facilities. However, the evaluation highlighted that the facility was not as resilient and its safety systems were not as effective as we would require of a new facility constructed today.

“Therefore, taking the most conservative approach, we concluded that additional analysis and modifications to PF-4 were warranted to minimize the potential for a radiological release following an accident triggered by a rare earthquake and subsequent main-floor fire. Upon analysis, the actual risk to the public was so small, given the rarity of the posited earthquake and fire event, that NNSA concluded that it was prudent to allow continued operations while we took measures to modernize the facility.”

Cook also addressed the seismic issues. He said the NNSA conducted a more detailed response of the PF-4 facility to earthquakes.

He said “when we evaluated new seismic information that had not earlier been available, we learned that the PF-4 facility could undergo a partial collapse in a severe earthquake. We concluded that the possible structural damage increased the potential for a radiological release from the facility.”

The memos sent to the safety board document the various seismic upgrades to PF-4.
Key actions completed include:
• Strengthened the roof, thereby addressing the most significant known weakness – a building collapse failure mode.
• Braced ventilation room columns, addressing the next most significant known weakness.
• Braced ventilation fan pads, addressing another major weakness.
• Imposed restrictive material-at-risk limits to reduce the amount of plutonium that could be released in an accident.
• Removed plutonium from vulnerable glove-boxes underneath five mezzanines that could fail.
• Repaired two of the five mezzanines, including the weakest one.
• Installed a seismic shut-off switch to isolate non-vital electrical loads in a seismic event.
• Upgraded the necessary equipment and established a process to completely isolate ventilation, if required.
• Packaged 1.2 kg of heat-source plutonium in robust containers to prevent release, equivalent to 170 kg of weapons-grade plutonium.
• Braced four shield walls.
• Remediated captured ventilation room columns.

By April, the following actions will be complete:
• Repair the three remaining mezzanines.
• Reinforce lab room ceiling supporting steel, to prevent buckling.
• Complete confirmatory analysis for roof cold joints.
• Establish evaluation criteria and complete initial non-linear analysis for corridor columns.

By May, the following actions will be complete:
• Assumptions in the 2011 DSA will be protected and the JCO will no longer be necessary.
• Mitigated radiation dose will be less than 25 REMs.
• The 2011 DSA estimates the probability of the post-seismic fire event at 3 x 10-6 Iyr.

By June 2012, non-linear analysis for corridor columns will be complete.

The memo states that the PF-4 was recently re-evaluated in accordance with national consensus standards using the ground motion that would occur on the order of once in 2,500 years.

The report stated, “The results showed that the structure met industry codes and standards with a few exceptions. The elements that were the exceptions, if not strengthened, limit the structure’s ability to confine hazardous materials and could lead to partial collapse of the facility.

Lab officials created seven upgrade projects and five have been completed and the remaining two will be complete early this year.

The NNSA also unveiled a long-term safety plan for PF-4, which includes complete seismic upgrades to the fire suppression system, complete glove box stand upgrades and installing glove box fire suppression, improving fire barriers and completing seismic upgrades related to active confinement ventilation.

“While the risks should be further reduced, they are also very low,” the memo said. “An earthquake large enough to significantly damage PF-4 and cause these increases in long-term cancer risk will also cause significant damage and acute injuries and fatalities in the surrounding communities. The latter would likely be the dominant public health concern if the postulated major earthquake occurred.”