I'm still pretty sure it wasn't Freedom . If it actually was Freedom, Shinn wouldn't have that grin on his face
1) The explosion followed too fast after the final stab
2) The initial explosion after the TH-blast was indeed too small so I assume those were the first seconds of the explosion.
3) Shinn destroyed Freedom's power source. That could explain the fact that Freedom completely washed away by the explosion while Shinn still floates in the sky. The penetration of the power source doesn't automatically result in a nuclear exlosion.
4) The following text(I got it from another forum) gives a good explanation for an explosion caused by the Tannhäuser:
There's two important things, as far as this show is concerned, that one needs to know about positron weapons:
1. Antimatter, like normal matter (ignoring 'strange matter'), consists of two basic elements - Positrons and Electrons (differing only by charge and spin). Positron weapons fire the former which is the antiparticle of the normal matter electron.
2. Normal matter and anitmatter cannot occupy the same space at the same time. Matter and antimatter violently annihilate each other when they come in contact.
If the two particles, the antiparticle and the particle, collide while at relative rest (little to no difference between the energies of the particles), the two annihilate each other resulting in two gamma ray photon travelling in opposite directions (universal background radiation). If however, there is a large difference between the energies of the particles, the resulting reaction puts nuclear fission reactions to shame. Unlike in a nuclear reaction where the material breaks down or is vaporized and no longer reactive, a particle-anitparticle annihilation, if there is a large difference between the energies, will generate more antiparticles furthering the duration the reaction until the energy difference balances out. (the ships in GS and GSD fire highly charged positron weapons).
Put bluntly, the energy released by a matter-antimatter explosion is between 10X to 100X greater than that of a nuclear explosion of the same mass. The radiation emmited is between 25x and 250x greater.
The previous two times it was used in the show (excluding Junius 7), the weapon was countered by a shield which cancelled out the antimatter (positively charged weapon to negatively charged shield, zero charge - zero reaction). This time was different, the highly charged positron beam impacted a mass of low energy normal matter.