The D Pile was down five times for scheduled shutdowns. The fifth shutdown was of short duration for the purpose of discharging temporary ``P`` columns. Details of the shutdowns are given. Three new bismuth columns were established, Special Requests were charged into seven tubes and three Special Requests were discharged. Three tubes were charged with four-inch slugs. The power level of the F Pile has been corrected for the discrepancy noted last month. Material for irradiation under the Special Request program was charged into six tubes and the B experimental hole during the month. The effect of poison columns and rods on the temperature distribution of the pile was checked for the overall content of the pile. The F Pile gained two in hours in cold, clean reactivity during the month if allowance is made for xenon poison values corresponding to 190 MW for the beginning of the month and 200 MW at the end of the month. The graphite stringer removed from the ``D`` test hole of the D Pile on 3-12-46 was studied physical radiation effects.

The magnitude and distribution of the fission rate per unit area produced by three energy groups of moderated neutrons reflected from a water tamper into one side of an infinite slab of active material is calculated approximately in section II. This rate is directly proportional to the current density of fast neutrons from the active material incident on the water tamper. The critical slab thickness is obtained in section III by solving an inhomogeneous transport integral equation for the fast-neutron current density into the tamper. Extensive use is made of the formulae derived in The Mathematical Development of the End-Point Method by Frankel and Goldberg. In section IV slight alterations in the theory outlined in sections II and III were made so that one could approximately compute the critical radius of a water-tamper sphere of active material. The derived formulae were applied to calculate the critical dimensions of water-tamped slabs and spheres of solid UF{sub 6} leaving various (25) isotope enrichment fractions.