Its not a nuke: A nuke works by getting a nucleus to split up, unleashing a small number of electrons from the atomic shell, the main source of energy from the blast.
What we're faced with isn't the energy release of energy from cracking an atom: Its the total death of the entire atom - a perfect conversion of matter as we know it into energy, carried by radiant photons as radiation.
All matter has a nucleonic half-life, not just an electronic half-life. The simpler it is, the longer it lasts.
There's a special kind of magnet here, called a monopole that causes an "error" in matter, causing it to mass-cascade fail: collapsing again and again and again until nothing is left.
Unlike antimatter, there isn't some equal reaction: it isn't like burning wood, going out when the wood finally runs out of fuel.
Its more like a black hole. It swells and gets worse. And worse. And worse. Growing exponentially in size, sinking into the planet and then only stopping when the monopoles finally equalize electrically with the planet's magntosphere.
There isn't a reaction here. You need to understand that.
In fact, what's happening isn't some triggered reaction either but a controlled orchestrated containment failure.
Shooting at it has a high risk of making things even worse. Or slowing the process, giving you a few more minutes.