Fukushima Daiichi Updates From IRID Part 4; Fuel Debris Study

JAEA and IRID have been working on various project for removal and storage of the corium (melted fuel) in the reactors at Fukushima Daiichi. The projects are tied together since understanding the composition of the melted fuel will impact how it can be safely removed and eventually stored long term. Efforts to understand the debris look at: -Mechanical properties: hardness, elasticity, fracture toughness, etc. -Thermal properties: melting point and thermal conductivity, specific heat, etc. -Other: particle size, shape, porosity, density, chemical forms, etc. Part of this effort is to also understand how the various substances in the melted debris are formed. If they are a uniformly mixed material or layers of different materials. Sample materials are then tested vs. cutting techniques. Computer modeling of the melt scenarios based on the melting point of the various components of the reactor core and how they react under intense heat has been studied but they admit the models have limits. The core model below estimates the hottest portion at 2400k, was in the top center of the reactor core.  The section of the fuel rods (before melting and collapse in the diagram) is considered to have a different mix of substances than the bottom head that is mostly metals. The fuel rods section is considered to be mostly oxides (red) with a smaller portion of metals, mostly zirconium from the fuel cladding. The bottom head section is considered to be mostly metals containing zirconium and steels or iron due to the melted metal of the reactor vessel bottom head. Reactor internal structures could also contribute to the metal content of this section. This may have formed separate layers as the reactor melted down or may have become more combined depending on temperatures and the progress of the melt down. Some rough estimates of the types of fuel debris created by region of the reactor core have been given. They largely base these assumptions off of findings from Three Mile Island.  The top section is assumed to be chunky broken debris. The middle section solids and powders and the portion that has melted into the concrete base mat as a solid. Since they based these off of TMI there was a strong caution that these assumptions could change as they learn more about the situation inside the reactors at Fukushima Daiichi. The two meltdowns are drastically different so these assumptions of course could change as more is learned. Not clearly mentioned in the handout, they also identify inside the torus tube as a location of potential fuel debris.    Two melted fuel simulated materials. Left pellets done by sintering and right side by arc melting. These various samples can be compared to samples they hope to obtain from the reactors to test various cutting methods. Of these samples tested those with boric oxides were considered the hardest mixed substances. JAEA also ran an experiment to try to estimate the role that the sea water injections played. One of the simulated melted fuel pellents was heated to 1200c for 12 hours immersed in salt. It did influence the fuel melt sample, leaving something that looked like this.   Another melt test looked at the total substances when melting a fuel rod sample that consisted of the substances found in the fuel rods and the control rods. It created some interesting compounds. Substance key: B4C = Boron Carbide Zr = Zirconium Fe = Iron Ni = Nickel UO2 = Uranium Dioxide Cr = Chromium ZrB2 = Zirconium Diboride ZrC = Zirconium Carbide The test took the combined sample and arc melted it. What they found when they examined it with a scanning electron microscope were some new compounds. The before and after of the sample. The melted sample contained Zirconium Diboride and Zirconium Carbide. Zirconium diboride (ZrB₂) is a highly covalent refractory ceramic material.  ZrB₂ is an ultra high temperature ceramic with a melting point of 3246 °C. This along with its relatively low density of and good high temperature strength makes it a candidate for high temperature aerospace applications such as hypersonic flight or rocket propulsion systems. (wiki). Zirconium carbide (ZrC) is an extremely hard refractory ceramic material,^[7] commercially used in tool bits for cutting tools. ZrC seems suitable for use in re-entry vehicles, rocket/SCRAM jet engines or supersonic vehicles in which low densities and high temperatures load-bearing capabilities are crucial requirements. ZrC reacts with water and acids and is pyrophoric. (wiki)

JAEA obtained samples of corium from Three Mile Island. Tests were performed on the samples to understand the physical, mechanical and chemical properties of the sample. This may help in some ways to understand what they may be dealing with at Fukushima. The meltdown at TMI did not leave the reactor vessel (remained “liquid full”) and did not involve the addition of salt water. It may be considerably different from the corium that will be found at Fukushima Daiichi. So these samples are of limited value. What could significantly improve the estimates and understanding gained so far would be to obtain samples of actual corium to analyze. Right now there is no announced plan to do this. The first inspection by robot into the containment structures is planned for late 2014 – early 2015. This could potentially provide photos of corium along with locations inside containment but doesn’t have the capability to obtain samples.

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