Defense in Depth through Filtered Venting
Posted July 2, 2012
Defense in Depth. When I hear this phrase, a very common one at the Nuclear Regulatory Commission (NRC), I imagine a design philosophy that takes multiple safety systems and procedures, and layers them like multiple ring walls protecting a city at their center. This philosophy takes these layers and links them together, helps them flow from one to the next. The goal: make reactors as close to disaster proof as reasonably possible. If one safety system fails, another is in place right behind it. I imagine that the ideal guiding principle of defense in depth is protecting the public and environment from the effects of a severe accident irrespective of preserving revenue for plant owners. Defense in depth means providing safety nets under the most critical reactor elements, such as backup diesel generators and emergency core cooling systems. Engineering safety nets for the safety nets. But how does this ideal concept of Defense in Depth compare with how the NRC regulates its reactors?
The NRC is expected to meet next month to discuss, among other items, the implementation of their March 12, 2012 order that requires operators of Boiling Water Reactor (BWR) Mark I and II containments to install reliable hardened vents. In the order, the NRC finally owned up to a vulnerability that has been known for decades, a vulnerability that significantly contributed to the disaster in Japan last year: that BWR Mark I and II containments have a significantly smaller volume than Pressurized Water Reactors (PWRs) and could become dangerously over-pressurized during a severe accident. The addition of a hardened venting system helps to relieve excess pressure and heat produced in a severe accident that could damage the containment barrier, increasing the quantity of radioactivity released in a severe accident.
The requirements set forth in the March 12 order cover a number of features that NRDC agrees are vitally important to maintaining adequate mitigation measures during a severe accident and protecting workers from unnecessary risk. However, during this month’s upcoming meeting the NRC staff will focus on one item that was unfortunately not mandated in the previous order: the inclusion of high-capacity filters in the design of the venting systems. During a severe accident, it may become necessary to vent the primary containment, such was attempted by operators in the Japanese accident. In certain scenarios, the vented steam and other gases, such as hydrogen, will need to be further routed to the outside environment in what would, hopefully, be seen as a “controlled release.” Controlled does not necessarily mean harmless, because within this flow are various radioactive fission products that would exit the plant along with the gas and heat. To catch these before they leave the plant, you need a filter.
Modern-day filtration systems can achieve upwards of 99% efficiency in particulate removal and vastly reduce the potential for offsite contamination during an accident. They are available from multiple vendors and can be retrofitted to or incorporated into almost any design. Numerous reactor operators in Europe have taken the necessary precaution of installing these filter systems to “scrub” the vent flow of the majority of radioactive particulates, should they need to vent to the outside environment. Nearly 20 years ago, France upgraded all of their pressurized water reactors (PWR) to include high-capacity filters and all German BWRs currently have venting systems outfitted with filters.
The U.S. nuclear industry is arguing that filtered vents are unnecessary and that there are other less costly safety improvements that could preempt the need for a filtration system. But nuclear operators of U.S. BWRs could plausibly still need to use the vents in a severe accident; therefore industry’s position is not logical. I believe the American public deserves the right to be just as safe from the radiation released in a nuclear accident as citizens in France and Germany, and I insist that U.S. plant operators treat safety contingencies like their European counterparts: minimize the potential impact of an accident to the public and the environment, don’t try to convince regulators that an accident with venting won’t happen, even thought it just occurred in Japan. Will the NRC hold to the principle of Defense in Depth, or leave gaps in the walls protecting the public from the consequences of a severe nuclear accident?
For more information about the background and details on vent filtration systems and their safety benefits, please see Post-Fukushima Hardened Vents with High-Capacity Filters for BWR Mark Is and Mark IIs written by NRDC consultant Mark Leyse.
 Licensees are required to submit for review, by February 28, 2013, an overall integrated plan for the installation of the vents, for which full implementation should be completed no later than two refueling outages following submittal of their plans, or December 31, 2016, whichever is sooner.