Committee directed that the AFSWP should remain a tri-service organization answerable to the three service chiefs. Field operations Groves and the wartime director of the Los Alamos Laboratory, Robert Oppenheimer, had begun the move of ordnance functions to Sandia in late 1945. The laboratory's ordnance-engineering division, known as Z Division, after its first director, Jerrold R. Zacharias, was split between Los Alamos and Sandia. Between March and July 1946, Z Division relocated to Sandia, except for its mechanical engineering (Z-4) section, which followed in February 1947. Z Division worked on improving the mechanical and electrical reliability of the Mark 3 Fat Man bomb, but this work was disrupted by the Crossroads tests. The 1947 nuclear stockpile consisted of nuclear weapons components, not weapons. Meeting with Truman in April 1947, Lilienthal informed him that not only were there no assembled weapons, there were only a few sets of components and no fully trained bomb-assembly teams. By August 1946, Sandia Base held electrical and mechanical assemblies for about 50 Fat Man bombs, but there were only nine fissile cores in storage. The stockpile of cores grew to 13 in 1947, and 53 in 1948. Oppenheimer noted that the bombs were "still largely the haywire contraptions that were slapped together in 1945". With a half-life of only 140 days, the polonium-beryllium modulated neutron initiators had to be periodically removed from the plutonium pits, tested, and, if necessary, replaced. The cores had to be stored separately from the high-explosive blocks that would surround them in the bomb because they generated enough heat to melt the plastic explosive over time. The heat could also affect the cores themselves, provoking a phase transition to a different allotrope of plutonium. They had to be periodically inspected by technicians wearing gloves and respirators. The bomb's electrical power for its radar fuzes and detonators came from a pair of lead-acid batteries similar to those used in cars. These had to be charged 24 hours before use. After a few days the bomb had to be partially disassembled so they could be re-charged (and, only three days after that, replaced). The 38th Engineer Battalion's electrical group studied the batteries, the electrical firing systems and the radar fuzes which detonated the bomb at the required altitude. The mechanical group dealt with the exploding-bridgewire detonators and the explosive lenses. The nuclear group moved to Los Alamos to study the cores and initiators.