A neutron burst from an untamped cylindrical Oy assembly occurred at the Pajarito Remote Control Laboratory at 1800 hours on 18 April 1952. The circumstances causing the burst and a description of the assembly involved are given There was no personnel hazard. Normal operations could have been resumed on other assemblies withing two or three hours after the burst. No evidence of damage to the oy was observed. It was determined that the burst resulted in 1.5 x 10(16) fissions in the oy. Also reported are some results of the computations and post-burst tests and a discussion of probable time behavior of power level during the burst.

Part I of this report covers critical-mass determinations for pseudospheres of oralloy hydride composition (approximating UH3) in 8"-thick Tu and Ni tampers and in the Tu tamper with Ni liner. The critical mass of a hydride cube in the thick Tu also is given. Data on weight and dimensional changes of hydride pieces during the period of use are included. In Part II are presented the results of Rossi time-scale measurements on the hydride assemblies. Values of alpha at delayed critical and its variation with mass in the neighborhood of delayed critical are given. Measurements on the activation of various detectors within the hydride assemblies are described in Part III. Results as a function of radial position are given for Au, for Au shielded by Au and by Cd, for S and for fission catchers with U235 and U238. Reactivity changes resulting from the introduction of foreign materials into the hydride assemblies are discussed in Part IV. Apparent regularities with respect to Z and qualitative interpretations of variations with radius are pointed out. From data for various radial positions, changes in critical mass corresponding to small changes in composition and density are computed.