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Initial Look at Lessons Learned From Fukushima

A review of what went wrong, why, and what should be done in the future

Guest Blog Post by: Akira T. Tokuhiro Ph.D *

Fukushima after 1Following a magnitude 9.0 earthquake and as high as ~14 meter tsunami, the Fukushima  Dai-ichi (D1) and Dai-ni (D2) Nuclear Power Plants (NPPs, Units 1-4[U1-4] at D1, U5-6-2 at D2i) experienced a series of multiple incidents caused by inadequate cool down of decay heat in both the reactor and in the co-located spent fuel pool (SFP).

The reactors at D1, U1-6 were constructed as part of a GE/Hitachi/Toshiba collaboration and began commercial operation, during 1971-1979; U1-5 are GE-BWR, Mark-I, U6 is a Mark-II. Two GE ABWRs are due to start construction in April 2012.

(Photo right shows damage from hydrogen explosions (4 bottom, 3 second from bottom, and 1 top)

Impact of loss of power

Although the Units at D1 and D2 automatically shutdown at the onset of the quake and with near immediate loss of off-site power, the back-up diesel generator operated (~30minutes) until the tsunami inflicted considerable (unknown) damage to auxiliary and back-up systems (most prominently the back-up diesel general and batteries).

This initiated the onset of lack of decay heat cooling. Additional aftershocks continued for about one-week. During initial week, March 11-18, there were up to three larger (likely H2 explosion) explosions, vapor/steam jets and fires that further stressed the RPV, the containment and (weather) confinement buildings.

Damage to primary containment?

One of the later explosions conceivably damaged the primary (coolant) containment and thus, water found in the adjacent basement of the turbine building pointed to high-levels of radiation including fission products. Additional large volumes of contaminated water were found in the U-shaped electrical conduit ‘trenches’ off of U1-3 and spreading into other areas such as beneath the reactor site.

Outline of lessons learned

nuclear_power_plant_control_roomThis paper outlines the initial list of lessons learned from the multiple sequence of events, some interpretations of the news releases and the aspects of safety culture that contrast Japan and the U.S. during crisis management.

It is based largely on events of the first three weeks and professional interpretation of publically accessible information. It is being released without peer review and in this summary form. Only the provisionally conclusive lessons learned are noted below.

1) Nuclear R&D institutions must consider alternatives to zirconium-based and zircaloy cladding so that chemical reactions that generate hydrogen is prevented. We (as an industry) need to accelerate development and deployment of non-hydrogren producing cladding materials; that is, assuming that the coolant/ moderator/ reflector remains (light) water.

2) Having multiple (reactor) units at one site, having more than two units on site needs critical review in terms of post-accident response and management. We must consider the energetic events at one unit exacerbating the situation (safe shutdown) at the other.

3) Further, there is a definite need for a backup (shielded) reactor plant control center that is offsite (remote) so that the accidents can be managed with partial to full extent of reactor plant status (P, T, flowrates, valve status, tank fluid levels, radiation levels).

4) There is a need for standby back-up power, via diesel generator and battery power, at a minimal elevation (100feet/31m) above and some distance from the plant (thus remotely located). This is needed to offset loss of off-site power for plants subject to environmental water ingress (foremost tsunami). Spare battery power should also be kept off-site and in a confirmed ‘charged’ state.

5) It is clear that the spent fuel pool (SFP) cannot be in proximity of the reactor core, reactor pressure vessel or containment itself. The SFP, in current form, is essentially an open volume subcritical assembly that is not subject to design requirements generally defining a reactor core.

Yet, unless thermohydraulic cooling is maintained, it is subject to the similar consequences as a reactor core without adequate cooling. Therefore, we need new passive designs of the SFP, away from the actual plant’s reactor core.

6) Thus needs to be a re-definition of the spent fuel pool. A new standard and design requirement is needed for the spent fuel pool. It should be ‘reclassified’ as a subcritical assembly with a potential to go critical with no active or passive control (rod or soluble ‘poison’) mechanism. Further it needs to be some distance from the reactor plant.

7) We need to identify key valves for emergency core cooling and require them to be non-electrically activated. Otherwise these valves need a secondary means of open and closed status that is remotely located.

8) If an ‘in-containment’ SFP is maintained, then the fuel transfer crane system must be designed so that it is available to remove the fuel during a post-accident phase. OR a second means such as a robotic arm needs to be available.

9) There needs to be a volumetric guidance analysis for ultimate (decay heat) cooling contingency plans so that not only limitations on volume are understood but also transfer of liquids from one volume to another.

Spare tanks and water-filled tanks need to be kept on site as uptake tanks for ‘runoff’ in case of addition of cooling during accident management phases. Spare means to produce boric acid needs to be available off-site. Earthquake-proof diesel generator housing also need to be water-proof. Remote diesel generators are also needed with access to equally remote diesel fuel tanks (also see 4).

10) For nuclear power plants located in or near earthquake zones, we cannot expect structural volumes and ‘channels’ to maintain structural integrity. We should also expect the immediate ground underneath these structures to be porous (earth). Thus design of these volumes and channels should be such that they minimize connections to other (adjacent) volumes from which contaminated (liquid) effluents can flow.

11) Color-code major components so that in case of an accident such as the Fukushima NPP accident, we will be able to quickly identify the major components from digital images.

12) An international alliance of nuclear reactor accident first responders and thereafter, a crisis management team is needed. This does not seem to be available at any significant level at this time. We (the global nuclear industry) cannot wait 3 weeks for international participation.

13) We should consider and work toward international agreement on standards for regulated levels of radiation (activity) and radiation exposure to the general public and separately, those under emergency and extended ‘recovery’ phases.

We should also be consistent in definition and practice of evacuation zoning. We should also strongly encourage acceptance and use of SI unit for activity and exposure and not use culturally-based numbering customs (in Japan, one counts in orders of (‘man’)104, (‘oku’)108, 1012 etc.)

14) Under emergency and crisis management, wider access roads are needed to and from NPPs. The access roads need to be clear of debris and of such width to accommodate large-scale trucks needed as first response and thereafter. A means to access the plant via water, such as ocean, calls for infrastructure (boats, water-containing barge, jet-skis etc) is needed as part of a contingency plan for those plants located near bodies of water.

______________

* Akira T. TokuhiroAuthor ID: Akira T. Tokuhiro (right) (email: tokuhiro@uidaho.edu) (web site) Department of  Mechanical Engineering, University of Idaho, 1776 Science Center Drive, Idaho Falls, Idaho 83402 USA

Keywords: nuclear power plant, accident, meltdown, spent fuel pool, loss of off-site power, earthquake, tsunami

Submitted as short communication to: Nuclear Exchange, First published at www.nuclear-exchange.com April 2011. Reprinted in electronic form at Idaho Samizdat with permission of the author and publisher.

 

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Andy Dawson's picture
Andy Dawson on April 26, 2011

A quite excellent piece – good to see that we’re starting to think beyond the immediate.  for what it’s worth, my comments below:

1) Nuclear R&D institutions must consider alternatives to zirconium-based and zircaloy cladding so that chemical reactions that generate hydrogen is prevented.

I couldn’t agree more.  We’ve now seen two accidents (Fukushima and TMI) where hydrogen generation from overheated Zircalloy fuel played a significant role.  There are alternatives out there.

The UK moved from a zircalloy-analogue (MAGNOX) when it implemented the AGR programme.  While no-one in their right mind would protray that programme as a success, it showed a potential way forward re fuel cladding.  MAGNOX had been selected for similar reasons to zircalloy in the early PWRs – good thermal properties, adequate but not startling mechanical strength, and excellent neutron economy.  We recognised, however, it had downsides re potential reactiveness even in a CO2 atmosphere, and lack of strength to resist accident conditions.  The AGR used stainless steel (austenitic, 20Cr, 25Ni/Nb).  Compared to zircalloy, it has better mechanical strength, similar thermal properties, but worse neutron absorbtion.  The latter (along with other design aspects) required going to 2 to 2.5% enrichment, contrasting with the natural uranium fuelling of the MAGNOX plants.

I don’t have access to the sorts of tools necessary to evaluate the enrichment requirements of stainless steel clad fuel in an LWR, but I’d be surprised if it meant adding more than 1-1.5 percentage points to current eernichment levels.  Obviously, that increases costs, but the cladding itself is cheaper, and more usefully (to the best of my knowledge) non-reactive with water even at high temperatures. 

2) Having multiple (reactor) units at one site, having more than two units on site needs critical review in terms of post-accident response and management. We must consider the energetic events at one unit exacerbating the situation (safe shutdown) at the other.

I can certainly forsee a requirement for spacing between units being increased on new build, and detailed consideration of blast effects between buildings.  One other thought would be for protected access routes to each individual unit, such that where radiation leakage occurs from one unit on a site, there is no restriction on access to others.

3) Further, there is a definite need for a backup (shielded) reactor plant control center that is offsite (remote) so that the accidents can be managed with partial to full extent of reactor plant status (P, T, flowrates, valve status, tank fluid levels, radiation levels).

Again, agreed – I’ve been amazed by the limited redundancy in terms of instrumentation and control access at Fukushima.

4) There is a need for standby back-up power, via diesel generator and battery power,

In my (UK) experience, no consideration was given in safety cases for the usage of equipment located off-site, and accessed during fault conditions.  Very definitiely, this can provide valuable additional redundancy.

5) It is clear that the spent fuel pool (SFP) cannot be in proximity of the reactor core, reactor pressure vessel or containment itself. The SFP, in current form, is essentially an open volume subcritical assembly that is not subject to design requirements generally defining a reactor core.

You are right re the nature of the pond itself (although I’m sceptical about the real potential for criticality-producing incidents).  I’m not sure, however, that separation is the best answer, dependent on overall design.  There’s an argument that the pool should be enclosed within the (a?) containment volume – and operational considerations suggest some intermediate spent fuel storage is needed in close proximity to the reactor itself, for use during refuelling and other outage activities.

What’s much more questionable is the practice of holding fuel in such a pond for extended periods. 

Therefore, we need new passive designs of the SFP, away from the actual plant’s reactor core.

Absolutely correct.  There’s a very good case for making these “dry” where possible – The UK’s experience of the Wylfa dry storage facility has been extremely good, albeit I’m not sure it qualifies as passive.

8) If an ‘in-containment’ SFP is maintained, then the fuel transfer crane system must be designed so that it is available to remove the fuel during a post-accident phase. OR a second means such as a robotic arm needs to be available.

I’m not sure I entirely follow the argument here?

9) There needs to be a volumetric guidance analysis for ultimate (decay heat) cooling contingency plans so that not only limitations on volume are understood but also transfer of liquids from one volume to another.

Spare tanks and water-filled tanks need to be kept on site as uptake tanks for ‘runoff’ in case of addition of cooling during accident management phases. Spare means to produce boric acid needs to be available off-site. Earthquake-proof diesel generator housing also need to be water-proof. Remote diesel generators are also needed with access to equally remote diesel fuel tanks (also see 4).

A very good suggestion, and not expensive to implement.  I’d also suggest some standby facilities for the basic treatment of water – iodine and caesium removal, particulate filtration, and possibly filtration/evaporation.

10) For nuclear power plants located in or near earthquake zones, we cannot expect structural volumes and ‘channels’ to maintain structural integrity. We should also expect the immediate ground underneath these structures to be porous (earth). Thus design of these volumes and channels should be such that they minimize connections to other (adjacent) volumes from which contaminated (liquid) effluents can flow.

Yes, and no – I was of the belief that the siesmically qualified parts of the Fukushima plant had performed well, even beyond DBA?  However, I agree re the interconnections – the degree to which water from R2 has shown up in unexpected places is worrying.

12) An international alliance of nuclear reactor accident first responders and thereafter, a crisis management team is needed. This does not seem to be available at any significant level at this time. We (the global nuclear industry) cannot wait 3 weeks for international participation.

This is one for when the “dust settles”.  My impression is that considerable assistance was available from very early, but the political structures (in particular) were slow to accept outside assistance.  Perhaps this should take the form of an obligation to accept such assistance?

13) We should consider and work toward international agreement on standards for regulated levels of radiation (activity) and radiation exposure to the general public and separately, those under emergency and extended ‘recovery’ phases.

We should also be consistent in definition and practice of evacuation zoning. We should also strongly encourage acceptance and use of SI unit for activity and exposure and not use culturally-based numbering customs (in Japan, one counts in orders of (‘man’)104, (‘oku’)108, 1012 etc.)

The evacuation zoning suggestion is particularly good – this is an issue too open to manipulation by politicians seeking to play to the gallery.

 

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