Saturday, March 9, 2013

Post # 79: To Vent, Or Not To Vent: That Is The Question!

On the eve of the two-year anniversary of the accident at Fukushima Daiichi, the U.S. Nuclear Regulatory Commission (NRC), and the U.S. nuclear power industry are making progress dispatching the twelve recommendations of the NRC's Near-Term Task Force (NTTF) Review of Insights From the Fukushima Dai-ichi Accident.

The NRC is currently working to resolve a major question about the continued safe operation of the nation's commercial boiling water reactor (BWR) fleet.  The question at hand, driven by NTTF Recommendation 5.1, is "Should hardened, filtered, primary containment venting (FCV) systems be required as backfits to the nation's thirty one commercial boiling water reactors (BWRs) that are similar to the units at Fukushima Daiichi?"  Or, as Shakespeare's Hamlet might put it, "To vent, or not to vent, that is the question..."

(Public Domain image from

Regardless of one's view on the matter, one has to have some sympathy for our colleagues at the NRC.

The NRC staff issued its analysis of the question in SECY-12-0157 (November 2012).  Their conclusion was that engineered filtration systems should be required.

On January 15, 2013, the U.S. House of Representatives Committee on Energy and Commerce  (dominated by Republicans) wrote a 10-page letter to the NRC saying, in essence, "Slow down.  You're moving too fast!"  The letter questioned whether the hardened filtered vents are needed, and reminded the NRC that it took some actions in the wake of the accident at TMI that were later judged to be unnecessary or otherwise ill advised.

Not to be outdone by their House colleagues, on February 20, 2013, the Senate Committee on Environment and Public Works (dominated by Democrats) sent the NRC a letter, saying, in so many words, "You should require the vents.  Get on with it, now!"

In the mean time, the Industry's position on the hardened vents can be summarized as, "Wait a minute!  Not so fast!  We're implementing the FLEX strategy.  We need to understand all the implications of the FLEX strategy before we require the plants to spend buckets of money installing hardened filtered vents.  And oh, by the way, unless one can prevent the containments from failing (thus at least partially bypassing an engineered vent system), money spent on a hardened, filtered vent is money wasted.  Far better to put the money into systems that can prevent the containment from failing."

It is important to remember that none of these questions, and non of these positions are new (see NRC Generic Letter 89-16).  It's "deja vu all over again" as Yogi Berra said.  These same questions and issues arose back in the early-mid 1980s when we were taking the first serious look at BWR severe accidents.  I recall two basic viewpoints about filtered containment vents that arose then and are implicit in the dialog today:  On the one hand, "Vents are a nod to what you don't know you don't know – a last best safety net (or barrier in the multi-barrier containment concept), and thus filtered vents should be required."  The counter argument was, "Yes, we don't know what we don't know, and the unintended consequences of installing a filtered venting system may overwhelm the benefits – so FCVs should NOT be required."

It's fascinating to realize this dilemma has not been resolved despite thirty years of risk-informed regulatory debate.

I recently sat-in via telecon on an NRC / Industry public meeting on the matter of whether hardened filtered vents are to be required and how the NRC plans to make that decision.  Buried in the dialog is a fascinating techno-philosophical issue that juxtaposes the desire for passivity in our safety systems, and the need for reliability and safety system effectiveness.  The attraction of a passive system is that it requires no human action and no outside power sources to act.  Most passive containment venting concepts I'm aware of employ a vent flow path that "once open, is always open."  This also means once the vent is open (functioning), the filter system becomes vulnerable to any dynamics loads or forces that might be placed on it from explosions or energetic events in the containment.  The effectiveness of a passive filtration system after such an event is obviously in question.  In a perfect world, one would prefer the ability to remotely or manually open and close ("modulate") a hardened vent line as the accident progresses, thus protecting the vent filtration system from damage and ensuring its effectiveness when it is needed.

But, as anyone who has analyzed the response of BWR reactor buildings during severe accidents knows (and the accident at Fukushima illustrates), the ability of personnel to maneuver through a reactor building to reach equipment requiring manual operation can be severely limited by the environmental conditions created by the accident.  And the idea of a completely passive filtered containment venting system that can open and close as needed is a bit like the idea of a "one-ended stick"... hard to conceptualize.

Completely aside from the technical issues involved, the NRC faces the very practical, but still highly philosophical issue of HOW to make the filtered vent decision.  There may be a good Ph.D. thesis in decision theory buried in there somewhere!

As I said, regardless of one's position on the matter of filtered containment vents, one has to have some sympathy for our NRC colleagues who are faced with untying this "Gordian knot".  Hamlet could relate to their dilemma.

Just thinking...


  1. and then you have the water/zeolite filtering pond concept, which is hardly likely to lose effectiveness just because of a little thing like a hydrogen blast

  2. "Most passive containment venting concepts I'm aware of employ a vent flow path that "once open, is always open"

    In Sweden, there are valves after the rupture disc that normally are open, that could be closed. There also are closed valves that can be opened in order to bypass the rupture disc, and start venting before the disc bursts.

    If you do not have the power or personnel to open or close these valves, I suggest that that venting through a filter probably is the best idea.

    Regarding filter resiliency, there is the "gravel bed in a concrete vessel"-filter, installed in 1985 at the now decommissioned Bareseback reactor. Its an expensive solution due to its large volume (10 000 m3 is IIRC), but its a very predictable thing due to its simplicity.

  3. Cosmoskitten,

    Thanks for the comment. I basically agree with everything you said. The issues with backfitting a filter are (as always with any backfit): performance, reliability, economics, and "unintended consequences". This dialog was started almost 30 years ago, suspended for a time, and has now come back to life. It's going to be an interesting next year or two as the debate evolves to some resolution.

    Thanks for reading my blog!

  4. Considering that filtered venting at Fukushima would drastically reduce off-site contamination (make it ~100 times less), and considering that most other countries already had filtered vents installed even BEFORE Fukushima, spending such a small sum is a complete no-brainer.