Reply #80: Here's the really bad news. If plutonium rods have melted, they're at 3000 degrees.

[leveymg]

Here's something I wrote Saturday before last, but withdrew after I learned that the material in MOX rods is plutonium oxide and uranium oxide, which have a significantly higher melting and vaporization point than pure Pu and Ur. Now that we've learned that traces of plutonium particles have been detected at a distance from the containment sites, we have to conclude two things:

1) One or more MOX rods have melted or vaporized;

2) Those rods were at least 3000 degrees, hot enough to melt through the containment structure.

With that in mind, here's what I posted. The rest of it is accurate:

leveymg (1000+ posts)
Sat Mar-19-11 08:49 AM
Original message
Plutonium Rods Melting In #3 - Some Bad News, Some Good News, and Some Really Bad News.
For a while, a lot of us have been asking -- and the authorities aren't really telling -- one thing: are the plutonium fuel rods in the #3 reactor melting down? The answer, I'm afraid, is right before us: if the rods have gotten hot enough to melt their cladding, liberating hydrogen, the nuclear material IS hot enough to melt.

The reason numbers 2,3, and 4 containment buildings have blown out is because of a buildup of hydrogen gas. The fuel rods are wrapped in an alloy (zirconium) that when it burns (1200 degrees C, 2200 F) pulls oxygen out of water, releasing explosive hydrogen and steam. The hydrogen gas has exploded, bursting the containment structures. Because the explosion wasn't contained, the plume escaped into the atmosphere.

The fumes and smoke are radioactive steam and burning materials from inside the reactor containments and the melted cladding. Here's the good news: that plume is not as radioactive as Chernobyl because the radioactive fuel is not burning as intensely as it did inside the Russian graphite reactor. The real danger in any nuclear event is melting and vaporization of the rods themselves. For a while, I was not sure how close we were to that point of melting at Fukuyama. They aren't telling us. As I said, the plutonium rods in #3 melt at a lower temp than the others. But, a bit of checking reveals a startling and disturbing fact:

Plutonium melts at a much lower temperature (1200 degrees F) than the cladding material, and the melting point of uranium is just about the same as zirconium insulation, about 2200 degrees F. So, if there has been a hydrogen explosion, that means the rods have heated to a temperature sufficient to melt them. This is particularly true with the MOX rods in #3, which as we can plainly see from photos, is far more melted from extreme heat than the other structures.

But, here's the really bad part - vaporized plutonium is even more dangerous as a dirty bomb than as a fission bomb. The stuff is a high alpha particle emitter - only a few tiny particles, if you breath or ingest it, will cook your tissues and eventually kill you. Not quickly like a bomb blast, but relatively slowly as a Cancer.

First, a bit of technical background: (NOTE: 3/28/10 - The information below applies to pure plutonium and uranium. MOX rods are plutonium and uranium oxides, which have a melting point of about 3000F and 4000F, respectively. Now that they appear to have started to melt down, this is even worse news, as they are that much hotter than when they burned off their cladding.)

Chemical Elements.com - Plutonium (Pu)
Name: Plutonium Symbol: Pu Atomic Number: 94. Atomic Mass: (244.0) amu. Melting Point: 639.5 °C (912.65 K, 1183.1 °F) Boiling Point: 3235.0 °C - http://www.chemicalelements.com/elements/pu.html

It is possible that some of the plutonium in the uncooled MOX rods contained in Reactor building #3 have started to melt.

However, the vaporization (boiling) point - 3235 degrees C is still so high that it's unlikely that we'll see massive airborne release unless criticality is reached. That's the good news.

By comparison, the melting point of Uranium is considerably higher than Plutonium and about the same as the insulation it is clad by:
Chemical Elements.com - Uranium (U)
Name: Uranium Symbol: U Atomic Number: 92. Atomic Mass: 238.0289 amu. Melting Point: 1132.0 °C (1405.15 K, 2069.6 °F) Boiling Point: 3818.0 °C (4091.15 K, ...
www.chemicalelements.com/elements/u.html - Cached - Similar

Now, for More Bad News

Maybe, here's the really bad news about Reactor 3. The critical mass of pure plutonium (at which it will support a chain reaction is only 4.4 kilograms - about as much pure plutonium as is contained in two MOX rods. Each MOX rod contains about ten percent Pu mixed with reactor grade uranium and other radioactive isotopes, which raises the critical mass substantially. Nonetheless, any two MOX fuel rods (each weighs about 60 pounds) contains a sufficient mass of plutonium to achieve criticality, a "chain reaction", if the uranium and other elements were removed. Of course, MOX fuel rods do not explode for two reasons: they aren't normally mixed together, and because they contain blended nuclear materials of various types that do not have a sufficient purity of any type or combination of types to cause a run-away nuclear reaction.

Theoretically, if one were to take many MOX rods and mix them together in a molten mass, it would not explode like an atomic bomb. Nonetheless, these materials, left uncooled and uncontained, in sufficient quantities long enough, would be so hot and radioactive as to burn through and radiate practically any material, including steel, concrete, stone, and dirt. Once they had burned through the floor of a containment structure, they would continue to "melt down." It is unclear as to how far below ground this glowing molten mass would descend, but in its present location, it would reach the water table within a few meters, sending up a large plume of radioactive steam and particles of molten sand, radioactive silicate.

The situation is really bad. We are at rod melting, next comes melt-down and a pooling of large masses of melted plutonium and enriched uranium if they are unable to flood the rods in a few days or weeks.

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BACKGROUND: What is MOX?

"Mixed oxide, or MOX fuel, is a blend of uranium and plutonium which behaves similarly to the enriched fresh uranium fuel. The plutonium in MOX fuel can be derived from either spent fuel discharged from reactors or nuclear weapons material. Reactor grade MOX, derived from commercial reactor spent fuel, contains quantities of fissile (U-235, Pu-239, and Pu-241) and fertile (U-238) material. Uranium and recovered plutonium constitute the basis for reactor grade MOX fuel. Weapons grade MOX is derived from surplus nuclear weapons. The main difference between weapons grade plutonium and reactor grade plutonium is the percentage of the plutonium isotopes present in each fuel type. . . reactor grade MOX has higher concentrations of Pu-238, Pu240, Pu-241, and Pu-242. In addition, Pu-241 decays to Am-241. The concentrations of these radioactive isotopes necessitate additional shielding measures for reactor grade MOX fuel. . . typical reactor grade PWR MOX fuel assembly contains approximately 10% of plutonium per weight of heavy metal," - http://webcache.googleusercontent.com/search?q=cache:ug ...

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An estimate of the MOX fuel assembly weight was derived from the weight of a ... Weight of control rod assemblies for the MOX core design is given as 60 Ibs ...
http://www.osti.gov/bridge/servlets/purl/889278-GsWJAs /
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- Mark


http://www.ornl.gov/~webworks/cpr/v823/rpt/109264.pdf