The F-82 fighter (a Twin Mustang, like a 2-seater version of Mantell's plane) did not "return to base" then come back again. The F-82 went to Lenoir City, about 10 miles S of Oak Ridge labs, as a staging area for its exercise then came back to look for the unknown target again. Lenoir City was not the home base (which was unclear in this poorly documented report but was the 5th All-Weather Fighter Squadron, Knoxville).
Notice no total duration or any other timing is given for this series of radar contacts other than the initial 4:20 PM time. Given the F-82's cruise speed of 280 mph, it is likely that it flew about 10 miles (rounding off) on first contact at 18,000 yards (the report states clearly "NOTE: The letter "m" ... denotes "thousand." ") or for about 2 minutes, breaking off intercept when it got too close to the Oak Ridge Restricted Area, then flew about 2 minutes south to Lenoir City, then an unknown amount of time in the exercise with the other F-82, then about 2 minutes to fly back to Oak Ridge area, then another unknown period on the mission, then a third run at the unknown target from 20 miles, picking it up at about 17,500 yards, which would be another 2 minutes or so, and breaking off again when they got too close to Oak Ridge, after perhaps still another 2 minutes. Total unknown radar target duration probably 2 + 2 + 2 = 6 minutes but stretched out over an unknown time from 4:20 to at least 4:30 or even 5 PM.
Also note that the F-82's radar observer, Lt Louis B. Cole, noticed the UFO target at 10 to 25 degrees above horizontal. This would seem to be impossible for a ground target reflected through a temperature inversion layer, as radar mirage layers must be within about 1 degree of horizontal. Also, the AMC claim of two inversion layers is from Knoxville at 9 PM -- NOT at "about the time" of the 4:20 PM incident, contrary to AMC's anti-UFO debunking from the infamous Roy James and James J. Rodgers.
This is a huge difference in meteorological conditions. At night, turbulence from solar heating in the daytime diminishes, allowing inversion layers to form. During the day, turbulence and winds usually break up inversion layers. It is important to get winds-temps-humidity aloft from the proper daytime conditions as well as close locations. I find it hard to believe AMC had no data from say 3 PM. I think they did and it showed no temperature inversion so they fudged the data by looking around for 7-8 hours later to try to find an inversion.
Note the X-10 "plant" was the world's 2nd nuclear reactor, the graphite reactor for producing plutonium.
We are losing out on our UFO history by not picking up on the fact that this case was monitored by AF Intelligence (AFOIN) Technical Capabilities Branch (TCB) in the Pentagon. Gen Cabell ordered TCB to conduct its own UFO investigations when Cabell learned in July 1950 that Project Grudge had not been taken underground and run quietly but was being shelved, contrary to his wishes (he had no direct command authority over AMC/Grudge but when he wrested control of AMC Intell away from AMC in May 1951 he effectively fired saucer-hater Col Watson, transferring him to a limbo assignment at USAFE in Europe). Lt Col Milton D. Willis headed up TCB's UFO investigations and even went to Oak Ridge in Dec 1950 to investigate the UFO flap. Willis' UFO job was later taken over by Major DEWEY FOURNET, still within TCB, in early 1952.