In Part I the theoretical and experimental conditions which must be satisfied in order to determine lattice constants with a precision of a few parts per hundred thousand are discussed with particular application to the symmetrical focusing type of camera. Cohen's method of calculation of lattice constants for the elimination of ``drift'' and experimental errors is applied. Methods are developed for the evaluation of standard errors and fiduciary limits of results from a single film and from a set of films. The usefulness of x-ray targets made of alloys rather than pure elements for the purpose of securing a larger number and better distribution of lines is indicated. The influence of the number and Miller indices of diffraction lines on the values of lattice constants in noncubic systems is shown. The importance of the methods of sample preparation for precision work is emphasized. In Part II, precision measurements on Al, Ni, Ag, Au, Si, Fe, Mo, W, Mg, Zn, Cd, Sb, Bi and Sn are reported for materials of a high degree of purity. The fiduciary limits of these lattice constants vary between 2 and 7 parts per hundred thousand and are so chosen that the probability of the correct value lying between the stated limits is 19 out of 20.

A series of iron oxides with compositions between 76.08 and 76.72 percent Fe, all of them lying within the single phase solid solution area known as Wüstite has been prepared and the lattice constants and densities of the individual members determined. The NaCl structure found by earlier investigators and considered by them to be the structure of FeO has been found throughout this series. Contrary to the earlier results the size of the unit cell decreases as the iron content decreases. The solid solution is of a complex type; an oxygen ion replaces an FeO group with an accompanying increase in valence of 2 Fe++ to 2 Fe+++. The results are discussed from the chemical and structural points of view.

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The second core for EBWR is planned to consist of thoria-urania pellets enclosed in tube sheets, similar to the Borax-IV loading which is now-under test. Higher temperature and strength requirements for the EBWR, however, make necessary the substitution of tube sheet material different than the aluminum alloy used for Borax-IV. Zircaloy-2 is a possible choice for tube sheet material, but extrusion of this material, unlike the aluminum, would be expected to be very difficult. Alternate methods, including rolling and expansion, are being tried. These take the form of bonding two Zircaloy-2 plates together by rolling, but leaving lengthwise strips of unbonded areas which later can be expanded into tubes pneuinatically. Efforts to date have been unsuccessful in establishing proper conditions for the intermittent bonding.

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