Saturday, August 6, 2011

Lessons Learned from Fukushima Daiichi: What the MIT Report Said (and Didn't Say)

The questions that remain from Fukushima Daiichi, as they apply to today's and tomorrow's reactors, could be a very good gauge for the winds of change in the nuclear industry. A lessons learned report from MIT CANES came out recently (hat tip to Steve Darden), and the material is just too important to pass up. It is a great report for anyone wanting to get a grasp on the important questions that stem from the disaster. We have covered the recent IAEA perspective here on The Neutron Economy, which is a great read about the questions regarding what happened. I want to take a moment here to recap and address the enumerated points from the MIT CANES report about "lessons learned".

For some previous attempts at lessons learned, see guests posts from experts at bravenewclimate or Idaho Samazat for just two examples. With that said, here are the points from the MIT CANES report. I'll try to offer a dumbed down version as much as possible for all of these, and of course, plenty of my own spin on things.

Emergency Power following Beyond-Design-Basis External Events
Simplified: How to assure the plant has power to keep the pumps running after a major natural disaster or terrorist attack

This concern is about the Station Blackout (SBO) event. Of course it was the earthquake-tsunami that caused the Daiichi meltdown, but it did it so by causing a SBO, so the relevance to nuclear safety is obvious. Even if we can't prevent tsunamis, we can prevent prolonged SBOs. MIT gives the "place the emergency diesel units on a hill" kind of recommendation, and this is not the first time I've read this. Water-proofing is another possibility, and this was cited in Japanese media as being one reason that the Tokai nuclear power plant did not worsen, but this also applies for the seawater pumps. There are also recommendations to have transportable generators that could be brought. The question on my mind is how much this recommendation does or doesn't overlap with what we already have in place. For the US, there is a good chance, in fact, that the public simply doesn't know about what we have ready to respond to a nuclear emergency with, since many such measures were developed for post-9/11 security, and an anti-terrorism safety feature works best when the public doesn't know the details. The MIT report also mentions passive safety features and asks the question of whether or not new plants should be required to have a mix of passive and active safety features. This question would be most relevant to designs like the EPR, and the performance of a design like the EPR (meaning fewer passive features) during SBO might be indirectly coming under fire here.

Emergency Response to Beyond-Design-Basis External Events
Simplified: Emergency response sums it up pretty well

Not only are things that did happen relevant to the post-Fukushima questions, but what could have happened but didn't is also fair game, this report mentions the possibility of staffing problems due to direct fatalities from an earthquake or tsunami. Additionally, problems in determining the evacuation zone and communicating with the public and with other governments are mentioned. It is suggested that organizations like INPO (an industry group in the US) or an international group could form rapid-response teams for nuclear emergencies. It also notes that there is a tradeoff between evacuation area and stress imposed on the local population, an difficult call that I've written about myself. There is also mention of the need to communicate radiation risk to the public. I would just add, be prepared for outrage. Lately I've been following the Uncanny Terrain documentary effort, which is fantastic on one hand to tell the human story of these tradeoffs between radiation risk and livelihood, but it also shows that people are upset based on social justice issues. This is one of those things you just can't "fix".

Hydrogen Management
Simplified: How to keep Hydrogen from exploding in an accident

The Hydrogen explosions at Fukushima Daiichi are obviously something to address. The MIT report mentions venting via "strong pipes", connecting the pool areas more directly to the plant stack, more hydrogen recombiners and igniters (specifically in the upper building), catalytic recombiners in the ventilation system inside containment (not done as of yet), research into hydrogen flares as a solution for large hydrogen buildup, and use of something other that Zircaloy for cladding. I have previous read about the strong pipes (or hard pipes) topic and in the Idaho Samazat post, using non-Zircaloy cladding materials was mentioned. In my opinion, Changing the cladding material could be very expensive and have a number of unintended consequences, for instance, reduced neutron economy. There is no technical reason the switch could not be made, but it would probably still require significant research and development lead time, if any nation decides to go that route in the first place.

Note: The focus of this point is on the need to vent the egg-shape primary containment

The report suggests directly that the (primary, or inner) containment should be vented to stack instead of to the secondary containment pools, which led to the hydrogen explosions. That would come with the other solutions mentioned in the hydrogen management point, including new catalytic recombining systems. Use of passive containment cooling is mentioned, but no word on if this has any relevance to existing reactor designs.

Spent Fuel Pools
Note: BWR spent fuel pools are unique and several recommendations are specific to those

It is believed that the spent fuel pools accounted for a lot, if not the majority, of the radioactive releases, so obviously this is one of he most major lessons learned items. The MIT report gives prompt movement of spent fuel to dry storage as an option... with many provisos. There is a safety benefit to doing this, although there are fundamental limitations associated with it, which is a debate the authors of this blog have been through several times. The MIT report notes the following shortcomings of dry storage
  • The casks must be secured so they do not tip during an earthquake
  • A break in a cask will result in direct and unmitigated release to atmosphere
  • The decay heat in spent fuel pools is mostly unaffected by using more dry cask storage because only the old fuel can be moved which don't contribute much to the heat
Very good points. Put together, this means that dry storage may or may not be preferable to other options, which the MIT report identifies as being on-site spent fuel pools (I'm guessing this means not close to the reactor) and centralized interim storage. The interim storage option is being talked about today as a result of the Blue Ribbon Commission on spent fuel management, by the way, although, for reasons that have limited overlap with these post-Fukushima concerns.

Some of the other recommendations are also very worthy of mention. Passive cooling of the spent fuel pools, and the policy of moving fuel out during outage are mentioned as things that need to be looked at for current plants. For the future, it is suggested that spent fuel could have its own containment, regional spent fuel storage facilities are relevant (with a hat-tip to Rokkasho, which is unfinished business), and a national repository could be created.

Plant Siting and Site Layout
Simplified: How do you keep an accident at one reactor from affecting other units?

This report notes the Fukushima Daiichi plant's "compact layout", and of course, cross-unit complications. The tsunami disabled a staggering 13 emergency diesel generators. Also, the Daini and Onagawa plants were at risk, reflecting concerns about regional clustering of nuclear plants. The phrase to remember here is common cause failure, and the report also coins the term unit-to-unit contagion.

A major conundrum that still remains is how to deal with earthquake risk. Saying that we should avoid major fault lines is obvious, but sites in places like Japan and Taiwan are always vulnerable to seismic risk. How do we regulate for this? This is a hard question. The report suggests to develop criteria with which a permissible number of units at a site could be evaluated.

Things not mentioned in the report

As someone who's followed this event and some of the technical aspects of it, I already had my own list of "lessons learned", but almost everything was covered by this report. Of course, there are a few things I had down which were not covered, as well as items other people have mentioned that weren't in there.
  • Beyond Design Basis - I had been wondering if the way we look at nuclear safety will be changed due to the sheer magnitude of the event beyond the tsunami magnitude that was planned for. Risk informed analysis and regulation seeks to not place an upper limit on events that can be tolerated (such as a x.x earthquake), and instead seeks to set acceptable accident frequency. I think Fukushima Daiichi strengthens the impetus for this perspective, since if nature is going to exceed the limits you set anyway, then you might as well quantify the risks in a more appropriate manner. Then again, if the problem with Fukushima Daiichi is that the tsunami risk was underestimated, then changing the way we look at risks will not fix that. That is a different type of error.
  • A comprehensive approach to flood events - A great read for past presidents that could have helped at Fukushima Daiichi, if it had been applied, is the 1999 flood of a reactor in France. Not to mention, there are plants in the US like Browns Ferry that have both a similar design to Fukushima and a flood risk. Don't forget about Fort Calhoun either, which the media was all over, predicting a catastrophic flood event. Since flooding keeps coming up as a concern, I had been wondering if this could be a sort of Achilles heel that needs more attention. The Japanese plants have certainly drafted plenty of flood mitigation plans in the wake of the Daiichi disaster, could a worldwide revamp of flood safety be in order? Looking at the lessons learned from the 1999 French flood, it would appear that safety measures can be expensive, easily in the $100 million range. Maybe there really isn't a easy fix to this problem, although I admit to have a sense of irony, since the problem that leads to meltdown is the insufficiency of water. Maybe we will ultimately switch to floating nuclear plants and be done with both the concern of flood and long term cooling at the same time.
  • Reevaluation of liquid effluent danger - The battle for stability of the stricken Daiichi plants was wrought with problems of radioactive water. Groundwater contamination will be topic for many years to come, and the ocean in the vicinity did prevent fishing for some time. I wonder if any specific solutions in this area will be called for.
  • Global regulation and crisis management - This was really addressed by the MIT report in emergency planning, but it could have effects more far reaching than what was mentioned. Accusations that the Japanese government made things worse by failing to promptly accept US assistance were rife. Ultimately, however, the safety of the public in a nation is the responsibility of the national regulator. I wonder to what extent people will continue to be satisfied with this structure. It is personally frustrating to have so many good things to say about the US industry safety record, only to be stonewalled by questions about the safety of the Japanese industry because it's simply not in our regulatory purview. I wondering if something more fundamental to the nature of safety and responsibility could be in the cards.