Overheated rods & rhetoric

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

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.)

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. 

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.

 

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.

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.