Thursday, March 24, 2011

Recent presentations on the events at Fukushima

Each of these presentations was designed to engage and inform the general public about the events which have occurred at Fukushima, including a specific explanation of what we understand to have happened so far from the perspective of experts and ultimately what is being done now to bring the situation under control. The advantage of these types of seminars is that the offer the ability to convey more detailed explanations of the specific sequences of events that lead to the current situation which are typically not available in traditional media accounts.


  1. My understanding of the explosions is that they were caused by a build up of hydrogen gas. The hydrogen gas is a by product of a reaction between the zirconium cladding and cooling water. Is there a cladding material that wouldn't evolve explosive gas when exposed to high levels of heat and water?

  2. I'll readily admit that I am not a materials science expert, so with that caveat, my understanding is that cladding material is inherently a compromise of necessary conditions. Cladding needs to be 1) Corrosion resistant (given high temperatures and exposure to water), 2) Have reasonable mechanical strength, 3) Be "transparent" to neutrons (i.e., have a low absorption cross-section).

    The oxidation reaction which produces free hydrogen generally only occurs when the clad gets very hot - i.e., when it's uncovered. Normally, the plant is designed to be able to vent this hydrogen safely - one thing that will likely be studied in light of this is how to safely vent hydrogen if circumstances like this present again.

    As far as replacement materials for clad though, I am uncertain that it would be easy to find a suitable replacement that meets all of these conditions. (Further, I'm not sure that we wouldn't see a similar oxidation reaction in other materials under similar conditions.)

  3. Note that the zirconium alloy is not the source of hydrogen, the water is. With that in mind, read on:

    You also have to worry about pellet clad interaction (PCI) which is very important. You also are concerned with the buildup of CRUD (it's an industry jargon term) and different materials react differently. You also have to worry about thermal expansion properties in the sense of which direction how much and at what temperatures. You have to worry about the machinability and whether or not the cladding can be pressed or extruded into these particular geometries. You have to know the change in properties and whether or not the embrittlement in a high radiation field is acceptable. You have to have a material that can withstand extreme differences in pressure in the event of depressurization in the core. You have to have a material with high enough melting point while at the same time having good thermal conductivity and specific heat properties. You need a material that can endure mechanical creep and fatiguing over a period of years.

    I'm definitely not a materials expert either but this is the list I came up with in 5 minutes. Hydriding needs activation energy. That comes in the form of the zirconium alloy reaching 2200F. Chances are you never have nor ever will (with the exception of staring into the heart of Kilauea on your vacation) encounter something that hot. Even for the reactor, we are talking about a design basis event (actually two with both earthquake and tsunami).

    Typically the cladding is a special alloy (see which have taken years by some of the best to design; hence, they are proprietary. I would assume that the Fukushima plants were using Zircaloy 2.

    There is also talk of they should have just vented to atmosphere. Well the whole point of venting from pressure vessel to suppression pool to primary containment to secondary containment then through special filters is to minimize the amount of gaseous fission products that will leave containment in the event of core damage. Now typically there are devices called glow plugs that constantly burn off the hydrogen (just like at a refinery they always have a vent stack on fire). These prevent hydrogen from building up and causing a large release of energy all at once. I don't know if Fukushima plants have these or not.

    There is no doubt that standards will be quadruple checked to make sure that some happenings (hydrogen explosions) at Fukushima don't happen again.

    The one other place I could steer this would be to take the water out of the equation. There are plenty of designs floating around that use a sodium based coolant. Each has its pros...and cons but I'll leave that for another day.