Franklin Thompson Matthias (13 March 1908 – 3 December 1993) was an American civil engineer who directed the construction of the Hanford nuclear site, a key facility of the Manhattan Project during World War II.

A graduate of the University of Wisconsin–Madison, Matthias joined the Tennessee Valley Authority (TVA) as a Junior Hydraulic Engineer in 1935 and worked on hydroelectric projects.

As such, he supervised the enormous construction effort, which included three chemical separation plants so large that they were known as "Queen Marys", and the world's first three production-scale nuclear reactors.

[9] Initially, the intention had been to locate the plutonium plant with the Manhattan Project's other production facilities at the Clinton Engineer Works in Oak Ridge, Tennessee, but concerns had arisen about the dangers of an atomic explosion, and Knoxville lay only 20 miles (32 km) away.

[9] When he returned from the meeting, Matthias was met by Groves, who instructed him to take a survey team and find a suitable site.

Matthias later recalled that:As far as my working relationship with Du Pont was concerned, of course, we had differences from day one.

I remember one time Granville Read (Du Pont's assistant chief engineer) called up Groves and said Matthias and Church were having a big argument about something and what should we do?

[11] The size of the construction work force eventually reached 44,900 in June 1944;[12] over 150,000 workers were employed at the site between 1943 and 1946.

Because the slugs were dangerously radioactive, the Queen Marys had concrete walls 7 feet (2.1 m) thick, and operations were by remote control.

[14] In 1942 Matthias allowed about 30 Wanapum Native Americans to use their winter camp on the Columbia River with access to their customary fishing ground in the middle of the federal reservation, although white farmer families were being moved off it.

But after WWII, the tribe were totally excluded from the federal reservation (although it contained graves and cultural sites).

[15] The first production batch of uranium slugs were dissolved at the separation plant on 26 December 1944, and in January 1945 Matthias personally couriered the first batch of plutonium nitrate to Los Angeles, where he handed it over to another courier, who took it to the Los Alamos Laboratory.

[1][5] Matthias was a Registered Professional Engineer in Wisconsin, California, Nebraska, Pennsylvania, and British Columbia.