Radioactive heating of the Earth

              Watts/kg     half life  mantle      Watts/kg
              of isotope   (years)    abundance   of mantle
Uranium-238    9.46e-5       4.47e9    30.8 e-9   2.91 e-12
Uranium-235   56.9 e-5        .70e9      .22e-9    .125e-12
Thorium-232    2.64e-5      14.0 e9   124   e-9   3.27 e-12
Potassium-40   2.92e-5       1.25e9    36.9 e-9   1.08 e-12

The Earth loses heat at a rate of .087 Watts/m^2.
Global heat loss = 4.42e13 Watts.

80% of the Earth's heat is from radioactivity and 20% is from accretion.

The radioactive heating rate 3 billion years ago is twice that of today.

The Earth's core temperature is ~ 7000 K.

Given the above data, estimate how long will it take for the Earth's radioactive power
to decrease by 1/2.