As I noted in a post four days after the Japan earthquake, salt accumulation due to emergency use of seawater to cool the damaged Dai-ichi reactors might seriously interfere with the cooling of fuel-assemblies in the primary containment vessels. The reason is that as the seawater boils off, it leaves mineral deposits at the metal-water interface. This crust accumulates with time and reduces the heat transfer efficiency. It's possible that uranium metal inside that crust will melt with the trapped heat of decay regardless of how much seawater is pumped in or dumped in.
Now worries about salt's side effects are on the rise. This item notes concerns by the French authorities.
Here's an excerpt from the NYTimes published March 23, "Optimism on Hold at Japan Plant as New Problems Arise:"
Richard T. Lahey Jr., who was General Electric’s chief of safety research for boiling-water reactors when the company installed them at the Fukushima Daiichi plant, said that as seawater was pumped into the reactors and boiled away, it left more and more salt behind.
He estimates that 57,000 pounds of salt have accumulated in Reactor No. 1 and 99,000 pounds apiece in Reactors No. 2 and 3, which are larger.
The big question is how much of that salt is still mixed with water and how much now forms a crust on the reactors’ uranium fuel rods. Chemical crusts on uranium fuel rods have been a problem for years at nuclear plants.
The NYTimes article continues:
A Japanese nuclear safety regulator said on Wednesday that plans were under way to fix a piece of equipment that would allow freshwater instead of seawater to be pumped into at least one of the reactors.
He said that an informal international group of experts on boiling-water reactors was increasingly worried about salt accumulation and was inclined to recommend that the Japanese try to flood each reactor vessel’s containment building with cold water in an effort to prevent the uranium from melting down. That approach might make it a harder to release steam from the reactors as part of the “feed-and-bleed” process that was being used to cool them down, but that was a risk worth taking, he said.
Why is that particular risk worth taking? I read the regulator's comment as suggesting there are concerns that the fuel may overheat, then slump to the bottom of the primary containment. If the fuel rearranges itself into a glob that's not under the influence of control rods and boron neutron absorbers, and has some water remaining to act as a neutron moderator, it could lead to a criticality accident.
That means an unwanted, uncontrolled fission reaction. In this case fission would restart in fuel that had been undergoing fission before the March 11 earthquake, but that had stopped reacting soon after the earthquake, as each operating reactor's fission-damping control rods were inserted automatically. (Unfortunately the fission byproducts in the fuel rods didn't stop decaying, which produces the unwanted heat the reactor operators have been struggling with.)
Uncontrolled fission would add to the inventory of radioactive cesium and iodine. It would heat up the vicinity even more than decay heat. It would be detectable by a sharp, sudden rise in hard radiation, as in the 1999 criticality accident at the Tokai fuel facility. If it's of any reassurance, there have been at least two dozen accidental criticalities in Atomic-Age history and none led to an atomic blast. Some stayed critical only for an instant, but in other cases criticality lasted for minutes, irradiating people in the line of sight.
Out in the open, a critical mass that produces a lot of energy tends to be self-extinguishing; it gets so hot that it melts or otherwise takes itself apart, which eliminates the conditions necessary for a critical mass. So it stops reacting. But if a mass of melted uranium or uranium/plutonium is trapped inside a containment vessel and can't blow itself apart in a pressure wave, the stopping point for the uncontrolled criticality is harder to predict.
Right now there are just two principal barriers against this happening: (1) that the overheated fuel assemblies stay intact and don't crumble or melt into a fissionable shape, and (2) the boron added to the water being injected by emergency pumps.
That's not many barriers left, given the deleterious effect of salt crust on cooling. It suggests to me that experts should be thinking about the physics of a possible criticality, and gathering remotely-operated equipment to have on hand given the high radiation levels that would be encountered if it happens. It may already be a subject of quiet discussion in Japan nuclear circles.
Maybe it won't be a problem: maybe a criticality will just boil away the coolant water that moderates neutrons and somehow settle down without something really bad happening, as with the ancient African natural reactor I mentioned in an earlier post. For now, all we amateurs can do is speculate.
Perhaps coincidentally, Ed Lyman the Union of Concerned Scientists posted a statement on March 23 that is rather critical of the Japanese government for not extending the evacuation zone around the reactor now, rather than waiting to see how things turn out. Excerpt:
Despite the US advisory [that American citizens keep a 50-mile distance from the reactors], the Japanese government is still maintaining its current order, which is evacuation only to a distance of 12 miles, and “shelter in place” for those between 12 and about 18 miles from the reactor site. “Shelter in place” means that people are directed to stay indoors and seal their windows and doors. Our assessment is that the Japanese government is squandering the opportunity to initiate an orderly evacuation from larger areas around the site–especially of sensitive populations, like children and pregnant women. It is potentially wasting valuable time by not undertaking a larger scale evacuation at this time.
(Note: There is a news report in the Japan Times that a low-intensity neutron flux was detected 13 times in the general vicinity of the damaged reactors. That's not necessarily a sign that criticality has happened, because fissile materials -- particularly plutonium -- release neutrons spontaneously.)