This memorandum is the first in a series reporting progress in the program of critical experiments in the ORNLArmy Package Power Reactor Project. The critical assembly, designated as CA-25, is analogous to the APPR design core and consists of 45 fuel boxes, in a 7 x 7 array with the corners removed, contaIned in a large water tank. Two sides of each box are slotted for the insertion and positioning of any desired loading of eighteen plates of fuel, structural material, and poison. The array is submerged in water to provide a moderated and reflector. Enriched uranium metal, in two-mil-thick foils 2.5 x 22 in, is encased in type 304 stainless steel sheets, 2.7 x 23 x 0.0105 in., to form fuel plates. Stainless steel plates, 2.7 x 23 x 0.025 in. are used to simulate additional steel in the APPR core. It is, therefore, possible to maintain an essentially constant metal to water ration in the assembly when the fuel content is varied. The fuel is distributed as uniformly as possible in all boxes and a symmetrical distribution of materials is maintained in the core at all times. Fuel plates containing half-width (1.25 in) uranium foils are provided for …