The Dense-packing Fiasco
Corrected cost deferral associated with dense-packing. It’s not 8 years. It’s more like 25 years.
Figure 1. Open-racking left; dense-packing right.
In a solid fuel reactor, the used fuel elements are transferred from the core to a spent fuel pool where they are allowed to cool under water for at least four years. The water provides both shielding and cooling. The pool water is circulated through a heat exchanger to keep the pool water at around 30C, The original plan was that after cooling for four or so years the fuel elements would be sent to a reprocessing facility or a federally provided deep geologic repository But in the US, both reprocessing and a repository got hung up in political wrangling and neither materialized. The obvious fallback was on-site dry cask storage. But dry cask storage adds about 0.1 cents per kWh to the cost of the electricity.
Figure 2. Fukushima Unit 4, Spent Fuel Pool
Most spent fuel pools are outside containment; and some such as the GE designs at Fukushima, Figure 2, are elevated.1 They could be damaged and drained either by a screw up, a natural event such as an earthquake, or terrorist attack. If the fuel elements overheat to about 600C, the gas pressure inside the elements will burst the cladding and cause a release. To make matters worse, we could easily reach temperatures at which the zirconium cladding will react exothermically with oxygen and nitrogen starting a fire. Therefore, the original plan called for open-racking. The fuel elements were spaced far enough apart so that, even if the pool drained, air cooling by natural circulation would keep the elements below the temperature at which the cladding would rupture. It was a good plan.
But when the spent fuel pools started filling up, the NRC approved dense-packing, Figure 1, which quadrupled the capacity of the pools by encasing each bundle of fuel elements in a neutron absorbing shield to avoid criticality. All US nuclear plants now use dense-packing.
The problem is NRC's own study indicated that air cooling would no longer keep the elements intact if the pool were drained.\cite{sandia-1979} The NRC justified dense-packing by doing a Probabilistic Risk Assessment which came up with a probability of pool draining of less than one in one million per pool year. I have no idea how they arrived at this probability. The NRC itself admitted than the probability does not take into account terrorist attacks.
But simply losing active cooling is a potential release for a dense-packed pool. Fukushima Dai'ichi used dense packing. When the tsunami hit, Unit 4 was partway through an outage. The entire core had been moved to the spent fuel pool. The plant lost active cooling on March 11 with the pool water temperature at 27C. The water temperature was up to 84C by March 14th. At that point, the 87 tons per day of evaporation matched the decay heat and the temperature stabilized.\cite{wang-2012} But the tank water level was falling at 0.7 meters per day. Fortunately on March 20, they were able to get new water to the pool, first by very high pressure sprayingand then by a concrete boom pump. Otherwise fuel would have been uncovered by March 23.
Finally, there are all sorts of questions about what happens to the neutron absorbing shields if the fuel become uncovered. It is possible to come up with scenarios in which the shield and fuel damage is such that the mess goes critical.
So in the US we now have some 35,000 tons of used fuel sitting in close-packed spent fuel pools, waiting for something bad to happen and cause a major release. Why?
The only thing that dense packing does for you is to defer the cost of dry cask storage
while the spent fuel pool fills up. So you get a one time saving of pushing back dry cask storage cost (roughly 0.1 cents/kWh) for something like 25 years. Absolutely nuts.
This fiasco raises at least two obvious questions:
1) Why does an NRC, which is prepared to impose all sorts of horribly expensive paperwork and procedures, which have little or no impact on the probability of a release, allow dense packing?
At least part of the answer is bureaucratic infallibility. Dense-packing crept up on the NRC. It was just a short-term response to a temporary problem. When it morphed into a universal, standard operating procedure, NRC was faced with
a) admitting they had screwed up and allowed an unnecessarily risky situation to develop, or
b) claiming that it's all part of the plan.
Deskpots are never wrong. It had to be (b).
2) How would Underwriter Certification (UCert) handle this issue?
Operators tend to discount low probability events, especially events that have never happened, even if the consequences are dire. Part of this is our hardwired it-will-be-the-other-guy response.
But for insurers evaluating such events is at the core of their existence. They know that, if the event happens, it will not be the other guy. They would assemble the best experts they could find, and ask for their advice. Ideally the advisors would say: you need to charge an additional premium of x for plants that use dense-packing. The plants would then decide whether to pay the premium or eliminate dense-packing. In this case, it's more likely the advisors would say, we don't have enough data to come up with a number; but the cost to the plants of eliminating dense-packing is almost certainly less than the expected cost of a release. If they won't open-rack, don't insure them.
That's the way UCert addresses all such issues. Can we make money insuring this risk? If not, we won't insure it. The way NRC addresses all such issues is: what's best for the NRC? When it comes to regulatory systems, those are your choices. Take your pick.
Only VVer-1000, EPR, and Konvoi pools are inside containment
Adding my two cents:
The NRC Commissioners erred terribly when they rejected the recommendation of the NRC staff's Fukushima Near-Term Taskforce to phase out high-density pool storage of spent fuel by accelerating the deployment of dry casks. As I see it, two major misrepresentations in the staff's analysis supporting that decision led to both the frequency and consequences of pool draining events being grossly understated.
1. Pool Draining Events
First, and as noted by others, was the NRC staff’s seemingly disingenuous selection of an extremely rare beyond design basis earthquake as the so-called "prototype event" for assessing the risks of pool draining leading to zirconium fires. There are obviously other credible events that could lead to pool boil-off or draining. These would include insider sabotage or missile attacks by domestic or foreign terrorists.
Relevant events would further include extended regional grid blackouts resulting from electromagnetic pulses (EMPs) as induced by either Carrington-scale solar storms or high-altitude thermonuclear blasts by an adversary. Note that the frequency of Carrington-scale solar storms has been estimated at 12% per decade. Such intense EMPs could fry large transformers that would take months to fix or replace. The result would be vast regionwide grid blackouts lasting several months or longer.
In the ensuing dystopian chaos, it is far from clear that mitigative actions could reliably be taken. Traffic chaos could make it extremely difficult for trained responders to reach affected reactor sites. And communication infrastructure failures could prevent responders from learning of the affected sites. Moreover, responders might understandably tend to prioritize defending home and family over all else. Such EMPs could thus result in unmitigated crises at all reactor sites in the blackout affected regions.
2. Pool Draining Event Progression and Consequences
As likewise noted in part by others, the NRC staff's analysis failed in two ways to adequately model the progression and consequences of pool draining events. First, the intensity of spent fuel burning was greatly underestimated due to MELCOR code's acknowledged inability to model exothermic zirconium nitriding reactions in air.
Second, no consideration was given to the likelihood and consequences of potential criticality excursions that could occur during pool draining or boil-off or while refilling an extensively drained or boiled-off pool. Almost all high-density pool racks use aluminum based neutron absorber plate materials (i.e., Boral, Metamic, Carborundum, and others) that, when no longer submerged, could readily disintegrate and/or melt from overheating by spent fuel nuclear decay and eventual zirconium burning. The absence of effective absorber plates could then give rise to pool criticality excursions.
Because the pools at pressurized water reactors (PWRs) are heavily borated, criticality-induced local pool boiling would produce strongly positive feedback and, thus, highly destructive energetic excursions akin to the one that destroyed Chernobyl.
Given that high-density spent fuel storage pools generally contain many core inventories of cesium and strontium and are located outside containment, it is clear to me that such events have far larger potential consequences than any conceivable reactor events.
Those were my two cents. Any questions?
Donald E. Carlson, PhD
Retired nuclear engineer and regulator
505-490-9137
Very interesting and informative.
"So in the US we now have some 35,000 tons of used fuel sitting in close-packed spent fuel pools, waiting for something bad to happen and cause a major release. Why? In order to put off spending 0.1 cents per kWh for a few years." - Question - Once the used fuel is put in spent fuel pools, is it too late to transfer them to dry storage casks? Or can it still be done?