The stockpile of cores grew to 13 in 1947, and 53 in 1948.[35] Oppenheimer noted that the bombs were "still largely the haywire contraptions that were slapped together in 1945."[35] 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.[36] 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.[37] 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.