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Overheated rods & rhetoric

Steve Skutnik's picture
, University of Tennessee
  • Member since 2018
  • 45 items added with 26,922 views
  • May 8, 2012

A little knowledge is sometimes a dangerous thing – particularly when fundamentally incomplete technical knowledge is used to make sweeping engineering recommendations. The latest example of this is the concern over the spent fuel storage pools at Fukushima Daiichi Unit 4, which has been getting attention from several corners. First, there was U.S. Senator Ron Wyden (D-OR), a ranking member of the Senate Energy and Natural Resources committee, who recently toured the stricken Fukushima site and released a very widely reported statement that, “things were worse than reported.” In particular, Wyden has singled out the spent fuel pools at Unit 4 for unique concern, calling on both the Japanese and U.S. governments to see to it that the rods are safely relocated elsewhere, citing their storage in unsound structures close to the ocean. Wyden has pushed the NRC and others to relocate these spent fuel rods to dry cask storage elsewhere.

As for Wyden’s technical credentials for making this assessment? A J.D. in law and his self-assurance in a Senator’s unerring technical omniscience.

I suppose it probably doesn’t occur to the Senator that relocating spent fuel rods out of the damaged building is no mean feat, given that the rods which will be relocated need to maintained underwater while they are transferred into concrete casks (in this case, mostly for radiation shielding purposes) using heavy cranes. Meanwhile, TEPCO has already reinforced the damaged building, addressing the concern he has over future tsunamis further damaging the weakened building and leading to a release into the environment. Its current plans call to begin removing spent fuel for relocation within the next two years. To emphasize – this is not a problem that relevant technical experts were ignorant of until one brave Senator stepped in and decided to lead.

Of course, to be fair to Wyden, as Dan Yurman points out Wyden is clearly not the only politician suffering from an acute hubris on technical matters.

Overheated rods & rhetoric


spent fuel pool
A spent fuel pool (Image: IEEE spectrum)

At least the good Senator can be forgiven for his enthusiasm however, as it’s not nearly as obnoxiously hyperbolic as certain other accounts going around the internet. Anti-nuclear activist and self-described nuclear “expert” (to use the term rather loosely) Robert Alvarez has been shopping around the dangers of spent fuel pools for some time, specifically focusing his ire upon the rods contained in the spent fuel pool at Unit 4. Alvarez has been flogging the dangers of spent fuel pools for sometime, going so far as to argue that such pools are “a ticking time bomb” and that the U.S. needs to move toward dry cask storage of all spent fuel as soon as possible. (More on why this is silly at best and potentially a dangerous misplacement of priorities in a moment.)

unit 4 spent fuel pool
Spent fuel rods at Unit 4 (Image: IAEA)

Alvarez’s latest work, “Why Fukushima Is a Greater Disaster than Chernobyl and a Warning Sign for the U.S.”, hits a new low in terms of outrageous hyperbole. Let’s start with the headline premise – Alvarez asserts that the potential danger – a release of radioactivity from the spent fuel rods at Unit 4 – is already worse than something which actually happened – i.e., the Chernobyl disaster. (Perhaps aware of this seeming logical contradiction, Alvarez walks this back to “may be worse” in the first sentence.)

The basis of his reasoning? 1) Spent fuel contains very large amounts of radioactivity, 2) The spent fuel pools have been exposed to air (due to the hydrogen explosion at Unit 4), 3) A collapse of building containing the spent fuel pool would lead to an overheating of the rods contained at Unit 4, 4) Somehow, this would lead to a zirconium fire and release all of the radioactivity present in the rods.

Alvarez’ blog post is a perfect example of the trouble one can get into when one extrapolates from a small bit of knowledge to a larger technical issue.

Taking it point-by-point – first we have this:

Several pools are now completely open to the atmosphere because the reactor buildings were demolished by explosions;

First of all, it should be noted that spent fuel pools are generally kept at room temperature and atmospheric pressure to begin with. A spent fuel pool, at its core, is essentially a very deep, very large swimming pool (which is also very radioactive as you reach the bottom). At the top, radiation levels are low enough to safely work without problems – you can even look down inside and see the eerily beautiful blue Cerenkov glow if the lights are dark enough. As for containment? The explosion at Unit 4 was in the secondary containment, which is essentially a thin metal shell – again, namely because spent fuel rods are un-pressurized and not at the kinds of temperatures found in the reactor. (In other words, the same kinds of phenomena involved in a core melt aren’t relevant here.) The primary containment in any spent fuel pool is the water itself, which isn’t hot enough to be going anywhere.

Moving on:

As more information is made available, we now know that the Fukushima Dai-Ichi site is storing 10,833 spent fuel assemblies (SNF) containing roughly 327 million curies of long-lived radioactivity About 132 million curies is cesium-137 or nearly  85 times the amount estimated to have been released at Chernobyl. 

So what does this mean? Without context – absolutely nothing. What Alvarez is trying to imply is that in the circumstance that these materials were released into the environment, the consequences would be far worse than Chernobyl. The problem? Alvarez presents no credible physical mechanism for this to happen.
Then there’s this:

Also, it is not safe to keep 1,882 spent fuel assemblies containing ~57 million curies of long-lived radioactivity, including nearly 15 times more cs-137 than released at Chernobyl in the elevated pools at reactors 5, 6, and 7, which did not experience melt-downs and explosions.

Why is it not safe? Well, other than the fact that spent fuel is radioactive, Mr. Alvarez doesn’t say. An industrial blast furnace is also not a safe place to be, but that certainly doesn’t prevent their use. Instead, we actually take precautions to use them safely – the same way spent fuel pools use deep levels of water to both cool the fuel and shield the high levels of radioactivity.

To wit: certainly no one would want to be next to a spent fuel assembly without the shielding provided by the deep pool of water. (With this shielding, the levels of radiation are low enough where it is quite safe to stand above the pool and look down inside – something I have had the opportunity to do before). But for this radioactivity to be truly disastrous (rather than simply being a dangerous but extremely localized nuisance), something has to cause the radioactive materials in the fuel to change state – i.e., to either melt or be carried away (“lofted”) by a fire.

In the beginning of his article, Alvarez eludes to the possibility of a zirconium fire, which he asserts could happen if the rods grew too hot. (Alvarez provides no further explanation or reference to credible technical resources beyond this.) Yet there are several significant problems with this theory. First, this would require the rods growing hot enough to ignite (if this is even possible – zirconium in solid form will not ignite, and its melting point is 1852° C). It second assumes that all of the radioactivity is uniformly lofted into the atmosphere; one of the main reasons for the magnitude of the Chernobyl disaster had to do with the fires in the reactor building which lofted radionuclides high into the atmosphere, where they spread across Europe. (Incidentally, this fire was also not from zirconium – it was a graphite fire from the reactor and control rod design being used.)

(Alvarez also rides his hobby-horse in inveighing against spent fuel reprocessing – a topic beyond the scope of this post but one which we’ve covered previously.)


A background on spent fuel

Spent fuel heat (click for larger)

Meanwhile, let’s back up for a moment such that everyone understands what’s going on. As we’ve covered on this blog before, spent fuel does still produce heat after the fission reaction shuts off. The remaining radioactive materials in the fuel, created both by fission and absorbing neutrons – are decaying. The quickest-decaying materials produce very high levels of radioactivity, and much of this energy is trapped in the fuel itself, heating it. Thus why spent fuel needs to be cooled following the reactor shutdown (which was the resulting source of problems at Units 1, 2, and 3).

Both this radioactivity and decay heat fall off dramatically with time, as the shortest-lived fission products decay away. Within 100 days, the heating rate and the radioactivity in spent fuel have dropped by a factor of 10; within 10 years, this drops to 1/100th of the original values.

Spent fuel radioactivity (click for larger)

Doing my own calculations using ORIGEN-S (a tool for nuclear licensing evaluation which is used to simulate spent fuel inventories), a typical assembly of the type found in the spent fuel pool would produce about 3-4 kW of heat after being stored around 1.5 years (and even less as it grows older) – or about 17-20 watts per pin (which themselves are over a meter long). In other words, while fuel which has just been ejected from a reactor poses a challenge in terms of cooling, it is difficult to conceive of how one gets the type of scenario Mr. Alvarez describes, in which something producing so little heat manages to cause these assemblies to melt or spontaneously catch fire.


A solution in search of a problem

Dry storage casks
Dry storage casks for spent fuel

Getting back to the main thread now – let’s assume for a moment that this scenario, one already demonstrated to be of extremely questionable plausibility, is true – i.e., that there remains a real threat spent fuel pools, in which the cooling water is lost and the rods subsequently overheat and either catch fire or otherwise change state. So Alvarez’s solution, to prevent these rods from overheating? Put them into thick concrete casks cooled by circulating air. Apparently, the same rods at risk of spontaneous combustion when exposed to air are fine if put into thick concrete casks. The logical inconsistency beggars belief.

Note that I am most explicitly not criticizing dry storage – in fact, dry storage casks have been demonstrated to be an effective, medium-term solution for isolating spent fuel from the environment. But to simultaneously assert a danger of spent fuel rods melting when exposed to air while simultaneously advocating to put them in thick concrete casks exposes a basic failure of physics reasoning, one which both Mr. Alvarez’s employer and the ever-reliable science reporting of the Huffington Post are happy to embrace.
Alvarez and his sponsors at the liberal think tank Institute for Policy Studies are of course using this reasoning to go a step further, arguing that all spent fuel pools at U.S. reactors are at risk and thus need to be moved to dry storage. Let’s just watch the errors compound…
First, let’s go back to the decay heat issue. Generally speaking, spent fuel isn’t suitable for moving into dry storage until it has cooled for a few years in a spent fuel pool – a general rule of thumb for dry storage is 5-10 years cooling time, although less is possible. The heat generated by 10-year old spent fuel assemblies are a hundredth of that generated by recently-ejected assemblies – in other words it would take one hundred assemblies stored for ten years to equal the contribution of one “fresh” ejected assembly.
If the reasoning here is to give greater safety margins for spent fuel pools in the event of a loss of cooling, dry storage is an extremely inefficient mechanism for doing so – namely because of the fact that the assemblies which are eligible to be moved into dry storage casks make at best a marginal contribution to the spent fuel pool heating. In other words, a large expense for very marginal gains in safety.
So here’s how it breaks down: “newer” spent fuel rods are too hot to go into dry storage casks, and thus must be kept in the spent fuel pool to cool. Therefore, the integrity of the spent fuel pool must be maintained. Yet if the integrity of the spent fuel pool is maintained, there is no real safety reason (at least in terms of heat or radioactivity) to move older rods, which can be moved into dry storage. (Note: there are other reasons one may choose to do so – spent fuel pools are limited in terms of total capacity, based on a number of safety-related factors, including total heat as well as how closely the assemblies can be placed together in order to prevent assemblies from going “critical” and restarting the fission chain reaction. However, these are far less limiting circumstances.)

What we have is thus a classic case of a solution in search of a problem. Alvarez (and others, for that matter) have found a solution they like – dry storage – and have (by process of scientifically incomplete reasoning) connected this with a problem they see – the vulnerability of spent fuel in wet storage pools – and naturally put the two together. Regardless, that is, of whether that square peg will actually fit in said roundish hole – the solution is, apparently, to just keep pounding.

When well-meaning ignorance actually becomes dangerous

This is where I think Alvarez’s (possibly well-meaning) concern actually becomes dangerous. Maintaining the integrity of spent fuel pools for “younger” fuel is vitally important – which is why some of the most recent changes recommended by the NRC as well as industry call for improvements such as better monitoring and instrumentation at spent fuel pools, along with other kinds of contingency plans to ensure water can be delivered to the pool in the case of a loss of coolant. Likewise, ensuring the integrity in the design of spent fuel pools indeed should be a priority.

But herein lies the problem with “experts” like Mr. Alvarez, who has no actually technical background to speak of – starting with the faulty premise that “wet storage” (i.e., spent fuel pools) can be eliminated entirely (they can’t), we are then assaulted with faulty recommendations to move fuel out of these spent fuel pools at large expense and very marginal contributions to safety. Yet arguably these are resources that could be better spent on improvements to the safety of spent fuel pools – things like better instrumentation to know what is going on in said pools and improved emergency response capabilities (such as designing easier means of supplying auxiliary water to the pools). The focus on dry storage as a safety measure thus makes for a dangerous distraction which commits attention and resources away from more productive ends, thus potentially compromising safety as a whole.

Alvarez isn’t the only one guilty of a single-minded focus on dry storage as a “solution” to spent fuel storage pools – all kinds of individuals (such as Senator Wyden above, and even some people I know of in real life who should know better…) have jumped all over this. The problem comes down to a simple failure to think things through – again, if spent fuel is too hot to be exposed to air, it’s too hot to go inside a thick (thermally insulating) concrete cask. If it isn’t too hot for dry storage (i.e., older fuel), then it isn’t what is driving the safety issue at the spent fuel pool. Thus, in either case, it’s a solution in search of a problem – given the fact that hotter fuel cannot be removed from the pool itself, it is more useful to focus upon the problem at hand.

The underlying pathology here – in other words, why seemingly simple-sounding solutions like this are so seductive – is because it gives the illusion of “doing something” about the (perceived) problem. In this case, this is done through a somewhat primitive technical analogy – we  have a thick concrete containment for the reactor as a safety mechanism, therefore spent fuel should similarly always be in a thick concrete containment. It simultaneously ignores where the solution is technically inappropriate (“younger,” hotter fuel) and how it fails to address the root problem (i.e., keeping the spent fuel both cooled and well-shielded – which is done by ensuring the integrity of the water levels in the spent fuel pool). Fundamentally, it is an example of how not to do engineering – engaging in a top-down method of choosing a solution first and making it work to fit the problem.

Under ordinary circumstances, this leads to bad outcomes – wasted money and sub-optimal solutions (or even solutions that are simply inappropriate). In the worst-case scenario, this kind of thinking actually makes things worse, namely by committing time and resources away from evaluating actual safety improvements – and thus where well-meaning concern of outsiders who are fixed upon a particular solution without understanding the actual problem can actually do more harm than good.

Steve Skutnik's picture
Steve Skutnik on May 8, 2012

I can’t help but feel as if you’ve missed the broader point here. Spent fuel that is relatively “young” cannot be relocated to dry storage. This means that it has to stay in the pool until it’s cooler – which means maintaining the integrity of the pool itself is the top priority. “Older” fuel generally isn’t going to be susceptible to the kinds of problems which would lead to widespread release, like melting or cladding rupture. In that sense then, worrying about pulling it out of the pool is a misplaced priority. Again, the priority here is to maintain the integrity of the pool such that the “younger” fuel remains cool. Full stop.


Further, there’s the issue of just “what” is going to happen if the spent fuel pool fails. Again, you seem to blithely ignore this issue. Yes, it will be a problem, but the so-called “experts” here have failed time and time again to demonstrate any plausible mechanism by which this would produce the catastrophe they predict. To emphasize – a failure of the spent fuel pool would be very bad indeed and all reasonable precautions should be taken to avoid this contingency, but it is not nearly the kind of catastrophe being predicted.


Finally, I must wonder if you sneer at the idea of other credentialed experts. Do you seek out someone who is not a physician and has no medical background to diagnose a medical problem? Why in the world then are we supposed to be trusting the demonstrably incorrect assesments then of someone with no relevant technical credentials? Again, it’s not simply a matter of credentials here – it’s the fact that these arguments can very easily be demonstrated to be factually incorrect, while being given by someone who has no relevant background in the topic.

Atomik Rabbit's picture
Atomik Rabbit on May 8, 2012

In the public interest, I am blowing the whistle on the fact that in the CVS pharmacy less than one mile from my home, there are 20 BOXES of sewing implements EACH containing 1000 pins. That is equivalent to 2000 TIMES the amount of stabbing devices that were released by the Charles Manson gang in their psychopathic rampages of 1969.

Sharpened sewing implements, also called “needles,” are extraordinarily pointy.  In a matter of seconds, an unprotected human eye contacting a SINGLE freshly removed sewing needle would receive a BLINDING dose of pointyness. As one of the most dangerous materials in the world, sewing needles pose significant long-term risks, requiring isolation in a double time-locked vault that can protect the human environment for TENS OF THOUSANDS OF YEARS!

To reiterate, if an earthquake or other event were to cause this pharmacy to explode, as experts predict, this could result in catastrophic blindings involving nearly 2000 TIMES the amount of carnage released by the Manson incident. 

Nearly ALL of the 20,000 sewing needles at the CVS pharmacy sit on shelves vulnerable to future earthquakes, with roughly 2000 TIMES more dangerous pointyness than released at BenedictCanyon! Look for my expose of this pending cataclysm in the next Bulletin of the Airhead Seamstresses – if they don’t revoke my literary license completely before then.

Paul O's picture
Paul O on May 9, 2012

The reason it is bad is that if the integrity of the pools are good, then this it is better to allow the rods to continue coolling, and if there is no science and safety driven need to touch the rods, the senator is merely playing politics with a serious matter better handled rationally and dispassionately.

Steve Skutnik's picture
Steve Skutnik on May 9, 2012

I have to wonder why you seem to be intentionally misrepresenting the argument here. 

There’s nothing wrong with moving older fuel to dry storage casks – and certainly, if you want to say that we should get the older fuel out of the pool due to concerns over stability, fine. But the “younger” fuel generally is going to have to stay parked for awhile – which means that keeping the structural stability of the pool should be the absolute highest priority. Which, it would seem, is what TEPCO is in fact doing. 

The point here is that by the marginal gains from prioritizing dry storage for older fuel are a lot smaller than the issues surrounding the “younger” fuel. Again, not a difficult argument here – the issue here is in priorities. 

Steve Skutnik's picture
Steve Skutnik on May 9, 2012

Here’s the thing – the argument you express here is a sound argument, and one I have no quarrel with. But it’s not the argument the Senator or Alvarez is making.

If the argument is, “We need to clear space such that we can safely relocate fresh rods from SP4 to a more structurally sound common pool,” you’ll get no argument from me. This is actually a sound and reasonable argument. The problem with the argument is that older rods themselves present the risk – which clearly they do not. The problem is further compounded by folks like Alvarez pushing a broader agenda against wet storage writ large.

Again, I’m not arguing that losing the integrity of SP4 is insignificant – but 1) it’s not the doomsday scenario being described, and 2) The solutions being proposed are inappropriate. I actually think TEPCO is handling the situation correctly, here, in the sense that they are applying an appropriate triage – stabilize the SFP first, move “younger” rods to a more stable location if possible, then worry about “older” rods. It’s Alvarez and Senator Wyden who appear to disagree.

Steve Skutnik's picture
Steve Skutnik on May 9, 2012

I think you’ve misinterpreted my position, here. I actually do think a failure in the structure of SP4 would be a very bad thing indeed, precisely for the reasons you indicate. I maintain my doubts about the zirc fire contingency, namely because outside of the freshest fuel, none of the fuel in the pool has sufficient heat to get anywhere close to this. Meanwhile, the “freshest” fuel is now a over year old – meaning the source term on the decay heat has decreased substantially. Again, I have provided my data for this, and you are free to check my assumptions. 

Meanwhile, your point about the integrity of SP4 is the reason why I’ve argued that maintaining the integrity of the pool is the top priority – focusing on dry storage is an unhelpful distraction when the “problem” rods are those least likely to be suitable for dry storage. Again, the issue here is priorities – if the integrity of SP4 is in doubt, this should be your first priority no matter what. Moving out older rods does little in terms of marginal gains if the pool fails.

EDIT: To emphasize a point here – assuming that SP4 is essentially a total loss (i.e., no way to triage the structure itself), one still must deal with the issue of what to do with the “young” rods. This would obviously mean that again, the focus on dry storage is largely a distraction; it’s just fine for older rods (and TEPCO obviously doesn’t need a non-expert like Alvarez telling them this), it’s inappropriate for “young” rods, which means the driving focus still should be on what to do about these rods (such as, as has been pointed out by others, transfering them to the common storage pool). END EDIT


But going further, the main point here is that folks like Alvarez are jumping over this as a means to condemn all storage pools, something which is clearly silly at best and diverts resources from more productive safety improvements at worst. 


Nathan Wilson's picture
Nathan Wilson on May 11, 2012

Steve, it is disappointing that people are so willing to believe theses ludicrous claims of extreme danger from spent fuel ponds.  However, I think you are underestimating the public relations value of dry cask storage.  After all, Japan doesn’t have a nuclear safety problem, it has nuclear confidence problem, and placebos are often the best treatment for psychosomatic disorders.

Dry cask storage can be a win not just for politicians and publicity seekers who embrace it, but also for the utilities who must bring their reactors back on-line in order to save the Japanese economy (the public doesn’t care that we’re only talking about the oldest spent fuel).  To quote “Dogbert’s Top Secret Management Handbook” (Scott Adams, 1996):  “…you don’t live in an ideal world, … you are left with only two logical choices: 1) Do nothing and get fired for doing nothing. 2) Do irrational and unproductive things, and get rewarded for being a can-do manager who makes things happen.”

Atomik Rabbit's picture
Atomik Rabbit on May 12, 2012

The last sentence of the reference you cited is “Because of the large inherent safety margins in the design and construction of spent fuel pools, this [genereic] issue was RESOLVED and no new requirements were established.”

That’s why they are required to be constructed to seismic category 1. The SFP at North Anna experienced a beyond design basis quake last year and was completely undamaged.

Steve Skutnik's picture
Steve Skutnik on May 17, 2012

The events to produce a serious radiological release at a spent fuel pool would require a combination of several very unlikely circumstances. First, for any spent fuel pool at ground level, generally it is difficult to conceive of a scenario where the integrity of the pool itself is compromised; that is, as long as it is capable of holding water, the remediation is simply to refill the pool. Part of the security measures post-9/11, and those following from Fukushima, have included preparing for better ways to ensure adequate auxillary water supplies to the pools. That being said, again – there is the further issue of the fact that to a limited degree, spent fuel pools are an inescapable fact of life – fuel which has not been cooled for several years (generally at least 4-5 years) is not suitable for relocation; it must be kept under water as inventories of shorter-lived fission products decay away.

Given that, part of what has been done is to look at the accident planning and response scenarios, which both the industry and the NRC have been busy doing since the 9/11 attacks.

That being said, reprocessing – in my opinion – is a good longer-term strategy for managing spent fuel, particularly for the reasons you list. Heat is not as much of an issue for the fission products here, as the remaining fission products (which are not recycled as new fuel) are generally vitrified into glass logs (borosilicate glass – pyrex, basically) to immobilize the fission products. This of course has a much higher melt temperature (and is not at risk of spontaneous combustion), so thermal management is not much of an issue here.

Steve Skutnik's picture
Steve Skutnik on May 17, 2012

Wayne, I am not an expert on steam generators, but I would refer you to Meredith Angwin’s excellent piece on the matter, “San Onofre and Steam Generator Design.” Meredith covers the topic in far greater detail than I could hope to.

Steve Skutnik's picture
Thank Steve for the Post!
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