1 mole (0.226 kg) of radium has 6.0 * 10^23 atoms (Avogrado's number N)
[tex]\frac {N \ln{2}} { T }[/tex] of those will be decaying each second.
where T is the half life of radium in seconds, and ln(2) the natural logarithm of 2 (about 0.693)
The half life of radium is about 1600 years.
This is 8.2*10^12 decays/second.
The decay energy is 4.9 MeV, producing radon, which is a gas, which might escape.
If you contain the radon and all other short-lived reaction products, about 28 MeV/decay is produced.
1eV is 1.6 * 10^-19 J.
Energy production is 8.2*10^12 * 4.9 *10^6 * 1.6 * 10^-19 = 6.4 W/mole or 28W/kg
from just the radium decay, or 160 W/kg including the reaction products.
It can get as hot as you like if you stack up enough radium, and pack it in with heat
isolation. Radium is extremely expensive however, and not safe, unless youmake sure that none of the decay products can ever get out.