Site A / Plot M Disposal Site The Site A / Plot M Disposal Site is located within Red Gate Woods and situated on the former grounds of Argonne National Laboratory and its predecessor , the University of Chicago Metallurgical Laboratory in Cook County , Illinois and is now part of the Palos Forest Preserve . The site contains buried radioactive waste from contaminated building debris , and the Chicago Pile - 1 ( CP - 1 / CP - 2 ) , and Chicago Pile - 3 ( CP - 3 ) nuclear reactors . Site history . Shortly after the December 1942 demonstration of the first self - sustaining chain reaction at the University of Chicago , the research group led by Enrico Fermi needed to move to a larger , more remote laboratory campus . The U . S . Army Corps of Engineers leased from the Forest Preserve District of Cook County , and placed the wartime reactor laboratory on within the forest . The first reactor , CP - 1 , was disassembled and moved to Site A in March 1943 , renamed as Chicago Pile 2 . In May 1944 the laboratory first operated a second , heavy water - moderated reactor , CP - 3 on the site . Argonne National Laboratory obtained an even larger , permanent site in Du Page County in 1947 and began moving its operations out of Site A to the new site . The two reactors operated until 1954 , conducting reactor research and production of tritium . Decontamination and demolition of the buildings began in 1955 . The reactors were defueled and the concrete shell for CP - 3 was imploded and buried . In 1956 the property was returned to the forest preserve . Two granite monuments mark Site A and Plot M . The Site A marker reads : THE WORLD ' S FIRST NUCLEAR REACTOR WAS REBUILT AT THIS SITE IN 1943 AFTER INITIAL OPERATION AT THE UNIVERSITY OF CHICAGO THIS REACTOR ( CP - 2 ) AND THE FIRST HEAVY WATER MODERATED REACTOR ( CP - 3 ) WERE MAJOR FACILITIES AROUND WHICH DEVELOPED THE ARGONNE NATIONAL LABORATORY THIS SITE WAS RELEASED BY THE LABORATORY IN 1956 AND THE U . S . ATOMIC ENERGY COMMISSION THEN BURIED THE REACTORS HERE . The Plot M marker reads : CAUTION - DO NOT DIG Buried in this area is radioactive material from nuclear research conducted here 1945 - 1949 . Burial area is marked by six corner markers 100ft from this center point . There is no danger to visitors . U . S . Department of Energy 1978 The markers have since been vandalized , with vandals chiseling off the word " NO . " Plot M was a dump for low - level radioactive waste generated at the site between 1943 and 1949 . Initially buried in trenches , later in steel bins , the waste included tritium , uranium , and fission products in various forms including contaminated equipment , animal carcases , and solids . In 1949 the burial of waste at the site was halted , and the dump was covered with grass until 1956 , when a concrete cover was installed to protect the landfill from rainwater . Surveillance of the site since the demolition in the 1950s has found small amounts of soil contamination with uranium and fission products , and some wells in Red Gate Woods had tritium concentrations as high as 13 nCi / l in the late 1970s . In April 1998 the fence separating Site A from the rest of Red Gate Woods was taken down after a DOE determination that the risk to the public while enjoying the forest preserve is minimal . Location . Site A is located near . Plot M is located near . During the Glenwood stage of Lake Chicago , this was part of Mount Forest Island , a triangular island long and wide , rising 80 to above the surrounding waters .