First off, that isn't what the rules say.
The rod can hold up to 8,000 pounds of weight. More weight causes the rod to deactivate and fall. A creature can use an action to make a DC 30 Strength check, moving the fixed rod up to 10 feet on a success.
It says 'weight', not 'force'. You can make some physics argument about how those are two ways to interpret the same thing, but reading the rules strictly, there are only two ways to move the immovable*, and those are overloading its weight capacity or having a creature succeed on a check.
Now, if you want to be a little more flexible with the rules and think about how much force is required to move the Rod, that DC 30 Strength check is probably the right place to start. As the DM, you could give your explosives a fixed strength check bonus, adding the bonus again for each time you double the amount of explosives used, and then roll against that DC 30, and on success, the rod is shoved 10 feet away from the origin of the blast.
But if it were me, I'd just handle it narratively. I'd simply declare that if they use the correct amount of explosives and place it correctly, it'll push the rod ten feet when detonated. The challenge is then in acquiring, moving, and planting the explosive, not in rolling the right number to see whether it works or not when it's detonated.
How much explosive would they need? That just depends on what I want them to struggle with. A massive barrel of gunpowder seems appropriate if I want them trying to shift heavy barrels around and figure out how to get them in place without the guards spotting what they're doing, Guy Fawkes style. But if I want them to deal with making deals with criminals or stealing explosives before they even get to the point of planting them, maybe I'll talk about some fancy dwarven blasting clay that the clans use for mining purposes, which is difficult for outsiders to get. The story takes precedence over the mechanics, and the "right" answers to those questions are the ones that make a story I want to tell.
*row, row, fight the power