ihatelies said:
In my opinion . . . .
First, we acknowledge that in both the reactor and the spent fuel has plutonium in it. The plutonium comes from two sources: First it comes as a by product of the fission reaction in the reactor. I don't think that plutonium is a great risk, because the molecules are interspersed in the rod fuel. In a complete catastrophic explosion, it would not travel far from the reactor.
Pu is produced by n-capture and successive beta decay according to U238 + n => U-239 (ß-decay) => Np-239 (ß-decay) => Pu239. Higher isotopes of Pu are formed similarly by n-capture in U or Np and subsequent beta, or n-capture in Pu 239, Pu 240, Pu 241. Pu-239 and Pu-241 are more likely to fission. Pu is chemically dispersed in the ceramic matrix since it simply is a U atom transformed into Pu in a UO
2 matrix, but there can be complex oxide compounds formed with other fission products, such as Cs
2(U,Pu)O
4.
This is useful - http://nobelprize.org/educational/physics/energy/fission_2.html
The second source of plutonium is the mixing of finely ground (nanometer) plutonium powder with the uranium in the new fuel rods that were placed into the #3 reactor in August. Alternatively known as MOX fuel, they mix between 6% and 15% plutonium powder in. I believe the Fukushima rods were somewhere in the lower half of this range.
Not quite. The Pu and U are in the form of a stoichiometric oxide, PuO
2 and UO
2, which is usually a mechanical blend, or could have been formed from a co-precipitation process. If the Fukushima fuel is nominally 4% enriched in U-235, then the Pu would be about 5-6% Pu - give or take - to match the nuclear characteristics of the UO
2 fuel.
During manufacture, the powder is "sintered" into pellets. What is unclear in everything I have read is whether the sintering melts the powder into solid metal pellets, or whether it simply binds the material into a pellet, but the powder still remains on the inside. Given my knowledge of powder metallurgy it takes a lot of heat and pressure to render powder into solid metal, and I suspect they would not subject the plutonium to enough to completely bind it, for fear of a reaction during manufacturing.
U and Pu are sintered ceramics, not metals. The cold-pressed green ceramic is about 50-55% TD, and is sintered at about 1700-1800C in a reducing environment. PM processes such as HIP do not apply here.
Once the rods are brought to operating temperature in the reactor core, my guess is they reach a high enough temperature to bind the powder completely. I haven't found anything specific on this topic, . . .
The ceramic is a manufactured in solid cylindrical pellet form.
However there exists the possibility that new plutonium enriched rods were waiting in the spent fuel pools to be loaded. If my analysis above is correct, these rods would not have their plutonium bound yet, and in the case of an explosion, the nanometer powder could be released.
According to available records, the 32 MOX assemblies were loaded into the core and were operating. Otherwise fresh fuel was UO
2. Spent fuel contains Pu mixed in the pellets. If the spent fuel pool 'exploded', there would be a significant release of radioactive material. The status of the fuel in the pool is not clear given the large amount of debris that has fallen into the pool. It does not appear to have 'exploded'.
I guess this is more of a set of questions for discussion rather than a statement. My question would be this: 1. Does anyone know if the plutonium powder is bound into solid metal during the sintering process? 2. Did any of the spent fuel pool contain plutonium enriched rods ready to be loaded? and 3. If so, is this a danger if the #3 spent fuel blew up rather than the reactor?
Pu in the fuel is in a form of (U,Pu)O
2 ceramic. The fresh fuel appears to be UO
2. The SFP of unit 3 appears to be intact, although there may have been some damage, and some of the spent fuel could be damaged. That has yet to be determined. Fresh fuel has no fission products, so no decay heat from fission products.
The spent fuel pool of Unit #4 would have been more at risk for loss of cooling since the full core had been offloaded. The SFPs of units 1,2,3 had some fresh fuel and several batches of discharged fuel. One batch would have been discharged last year, one batch the year before, and so on. The older the fuel, the less the decay heat.
The explosions in Units 1 and 3 were attributed to hydrogen from the reactor. That hydrogen is expected to be from oxidation of the zirconium alloy cladding and channels in the core, as has been explained very early in this thread. In unit 4, it was thought that hydrogen was produced in the SFP for oxidation of the cladding/channels. The video of the fuel in SFP#4 seems to show that the fuel is largely intact, but the cladding/channels could have oxidized and produced hydrogen. Some quantity (presently unknown) of fuel rods could have been breached, in which case they would have released Xe, Kr, I and possibly Cs if the fuel temperature was high enough. TEPCO will have to retrieve/lift some assemblies and inspect them for integrity.
The fuel in the cores of Units 1,2,3 were at greater risk of overheating since they have been operating at time of the earthquake, and were generating significant decay heat when cooling was lost.
