Calcium atoms can't withstand extreme heat according to this link. http://abyss.uoregon.edu/~js/ast122/lectures/lec09.html "Why do some stars have strong lines of hydrogen, others strong lines of calcium? The answer was not composition (all stars are 95% hydrogen) but rather surface temperature.
As temperature increases, electrons are kicked up to higher levels (remember the Bohr model) by collisions with other atoms. Large atoms have more kinetic energy, and their electrons are excited first, followed by lower mass atoms.
If the collision is strong enough (high temperatures) then the electron is knocked off the atom and we say the atom is ionized. So as we go from low temperatures in stars (couple 1,000K) we see heavy atoms, like calcium and magnesium, in the stars spectrum. As the temperature increases, we see lighter atoms, such as hydrogen (the heavier atoms are all ionized by this point and have no electrons to produce absorption lines)."
So, everything including heavier elements such as calcium will be reduced to helium and hydrogen in the ground zero of nuclear explosions because the temperature at ground zero reach millions of degree Celsius. Correct?