Europium Hexaboride

This material is used in the CCRs and EPRs of the LMRs installed in submarine factory number 601. The Group could find no published data on the corrosion behavior of this material, and assumptions had to be made for modelling purposes. Although pure Eu is a reactive metal, in ceramic Bj form it is expected to be stable and relatively impervious to corrosive attack. The BCR of 0.01 mm-a was selected on the assumption that it was unlikely to be any higher than this. The importance of this parameter in the LMR model relates to the criticality issue and its value is shown in Section 5.2 (Reactor Criticality) to be insensitive in the criticality analysis, and therefore not of significance with regard to accuracy. 

The Pb-Bi eutectic is used as a coolant in the LMRs installed in submarine factory number 601. In the absence of information on the performance of this material in sea water, the model assumes the same corrosion rate as Pb. Heiser and Soo [23] report uniform corrosion rates of 0.008 to 0.013 mm-a, and a BCR for Pb of 0.01 mm-a is therefore not unreasonable. There is clearly some uncertainty over the performance in sea water of the Pb-Bi eutectic compared with pure Pb. However, until further data becomes available on the former, it is considered a representative rate to use in the model at this stage. 

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Europium hexaboride is a semiconductor (Eg — 0.38 eV) but becomes a metallic conductor if 1 % of the europium atoms is replaced by lanthanum. The boron nets can accommodate some of the valence electrons from the lanthanum. The compound is metallically conducting because of the excess of valence electrons that go into interstitials. The hexaborides have a high melting point and a high strength. They are hard and stiff because of the strong covalent bonds in the lattice. 

A note about the bonding and electron requirements is that the Be octahedron has been calculated to be two-electron deficient when bonding in solids, namely, it lacks two electrons to satisfy the bonding orbitals. Therefore, for trivalent rare earth atoms, which supply three electrons, there is one extra electron in the conduction band, and RBg for trivalent rare earths can be well understood to be good metals. When the metal atom is divalent, such as for europium in the divalent state and calcium, the hexaboride becomes semiconducting or similar to a semimetal. 

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