was used, so it contained of uranium and of graphite. No cooling system was provided as it only ran at a few kilowatts. CP-2 became operational in March 1943, with a k of 1.055. During the war Walter Zinn allowed CP-2 to be run around the clock, and its design was suitable for conducting experiments. CP-2 was joined by Chicago Pile-3, the first heavy water reactor, which went critical on 15 May 1944. The reactors were used to undertake research related to weapons, such as investigations of the properties of tritium. Wartime experiments included measuring the neutron absorption cross-section of elements and compounds. Albert Wattenberg recalled that about 10 elements were studied each month, and 75 over the course of a year. An accident involving radium and beryllium powder caused a dangerous drop in his white blood cell count that lasted for three years. As the dangers of things such as inhaling uranium oxide became more apparent, experiments were conducted on the effects of radioactive substances on laboratory test animals. Though the design was held secret for a decade, Szilard and Fermi jointly patented it, with an initial filing date of 19 December 1944 as the neutronic reactor no. 2,708,656. The Red Gate Woods later became the original site of Argonne National Laboratory, which replaced the Metallurgical Laboratory on 1 July 1946, with Zinn as its first director. CP-2 and CP-3 operated for ten years before they outlived their usefulness, and Zinn ordered them shut down on 15 May 1954. Their remaining usable fuel was transferred to Chicago Pile-5 at the Argonne National Laboratory's new site in DuPage County, and the CP-2 and CP-3 reactors were dismantled in 1955 and 1956. Some of the graphite blocks from CP-1/CP-2 were reused in the reflector of the TREAT reactor. High-level nuclear waste such as fuel and heavy water were shipped to Oak Ridge, Tennessee, for disposal. The rest was encased in concrete and buried in a trench in what is now known as the Site A/Plot M Disposal Site. It is marked by a commemorative boulder. By the 1970s there was increased public concern about the levels of radioactivity at the site, which was used for recreation by local residents. Surveys conducted in the 1980s found strontium-90 in the soil at Plot M, trace amounts of tritium in nearby wells, and plutonium, technetium, caesium, and uranium in the area. In 1994, the