Calculations from a few previous posts (with updates):
Given that Uranium, Thorium, and Potassium are the most common radioactive elements, we can get an estimate of the energy released by looking at them. Both Uranium and Thorium require about 1 half-life to go by in six months, and Potassium-40 about 4 to produce the measured ages. Given their abundance in the earth and earth’s mass, that is roughly 6x10^44 becquerels. Alpha decays emit about 10 MeV each. Hence, 4.5x10^32 joules get poured into the earth in that time. For reference, that is slightly more than Earth’s binding energy, or about the amount of sunlight that hits the earth every billion years. That is also enough to raise temperatures at 1500K/s, re-melting the crust in about a second and vaporizing the earth in 5. By the end of six months, the ball of plasma once known as earth would be (assuming that it stayed the same size) about 55 MK, producing a Planck curve which peaks in keV x-rays and outshining the sun by 9 orders of magnitude, thus contributing 15% of the energy outputted by the galaxy.