"They're inside the protected fence, so only badged people -- and employees -- can get in there," said Fran Poda of the site's office of external affairs.
In the past, markers were placed along the nearest state highway even if the site was some distance away, but more recently, markers have been erected at the site itself even if they are away from state highways and public roads, according to the South Carolina Department of Archives and History Web site, which said the markers cost $1,725 apiece.
The markers are the first such signs to be placed inside SRS' fence, and were created to recognize the prominent role the P and R reactors played in winning the Cold War.
The reactors were among five production reactors at SRS and both produced plutonium and tritium for national defense uses.
"They were true pioneers, and what they built is still significant today," said Elizabeth Johnson of the South Carolina Historic Preservation Office. "We take care to preserve our history for future generations, because what's been done here is important."
R Reactor began operations on Dec. 28, 1953, and ran until 1964 and was the prototype for the four subsequent reactors at the site. P Reactor ran from 1954 until its final shutdown in 1988.