As @AlexP calculated, the atmosphere needs to be replenished with Radon at 1.25e16 kg/day = 1.45e11 kg/s. Let's have a look at all the numbers.
The planet's surface area is around 5.11e14 m^2; for most quantities, it makes sense to consider them per square meter of surface area.
At 1.3 atm pressure, Radon gas has a volume of ~ 7.9e-2 m^3/kg, giving a replenishment rate of 1.15e10 m^3/s; each square meter needs to vent 22 milliliters of Radon gas per second.
The decay chain of Radon-222 involves many tasty heavy metals and eventually ends in Lead-206, which will accumulate at 1.34e11 kg/s, or 2.63e-4 kg/s per m^2; lead dust will sediment at 2.31e-8 m/s = 2 mm/day or 0.73 m/year.
The decay chain starts at U-238, which has a half-life time of 4.5 billion years. To produce 1.45e11 kg/s of Radon-222 would require 3.16e28 kg of U-238 -- 3.1e4 times the mass of your planet, so... U-238 cannot be the origin of the Radon.
The next-longest-lived element of the decay chain is U-234 with a half-life time of 245k years, you'd need 1.70e24 kg of that -- still heavier than the planet.
The next option is Thorium-230, with a half-life time of 75k years. We'd need just 5.14e23 kg of it, or 51 % of the mass of the planet. As a bonus, its density is around 1.17e4 kg/m^3, very close to the density of the planet. Of course, this means that the planet must be very young - at most 75k years old, and its radon production will greatly reduce in the next tens of thousands of years.
Finally, decay heat. Over the entire decay chain from Th-230 to Pb-206, a fraction of 1.87e-4 of the mass is released as energy. Some of that will be carried away by neutrinos, but must will heat the planet (inside or in the atmosphere).
This is a mass-energy conversion rate of 2.81e7 kg/s or 2.52e24 W, or 0.66 % the power output of the sun. 921
Per area, it's 4.93e9 W/m^2. Earth provides around 8.2e-2 W/m^2.
Assuming a perfect black body, this means the surface temperature of your planet is 1.72e4 K (T^4 ~ P/A). Of course, this means that the planet will be made entirely of plasma, and evaporate quickly, so unfortunately your idea looks impossible.
However, Radon is a very heavy gas, heavy enough not to mix with the atmosphere too much. On Earth, it likes to accumulate in basements. Conceivably, if your planet has no wind for some reason, Radon might accumulate in the very lowest layers of the atmosphere, maybe just in the lowest 2 meters. If you limited yourself to that, you'd need much less radon, which would mean less heat, less accumulation of lead dust and fewer geothermal vents. Just scale all of the numbers down accordingly - except for temperature. Due to <PERSON>'s law, if you reduce the amount of radon 10000x, you can only reduce temperature 10x.
If you're happy with a still-extremely-deadly* 1 ppm of Radon instead of 8.3%, and just confined to the lowest 1/1000th (by mass) of the atmosphere, your planet will be somewhere around room temperature. You could even go for the longer-lived U-238 as source material. You should move the planet well away from the parent star to avoid any extra heating.
*) 1 ppm of Radon at 1.3 atm has an activity of 1.94 Ci/L, a comfortable 4.9e11 times the EPA action limit.
If you really want a radioactive gas that makes up a single-digit percentage of the atmosphere, may I interest you in some other gasses, like H-3? Kr-81? Ar-39?
I also hear that Unobtainium, a recently-discovered element in the Island of stability, has a density and half-life similar to that of U-238 while decaying directly into a gas that has properties that happen to be exactly right for your story. 921