How hot was Nebuchadnezzar's fiery furnace?
Certainly to the point of nuclear fusion of hydrogen into helium through a C–N–O cycle and associated nuclear reactions
[Daniel 3:19] — Then was Nebuchadnezzar full of fury, and the form of his visage was changed against Shadrach, Meshach, and Abed-nego: therefore he spake, and commanded that they should heat the furnace one seven times more than it was wont to be heated.
It is not clear if that "one" is part of a figure that would have given the "seven" a place value of sixty in right-to-left script, as the ancient Babylonians would have written the number four hundred and twenty-one, in a base-60 system that we still use to this day for hours or degrees, minutes and seconds of angular arc or passage of time.
And the king's servants that put them in, ceased not to make the oven hot with rosin, pitch, tow, and small wood. So that the flame streamed forth above the furnace, forty and nine cubits: And it passed through, and burnt those Chaldeans it found about the furnace.
Rosin (or "resin") is dried tree pitch or sap, like hard plastic in consistency, "pitch" probably refers to petroleum tar or crude oil (as in the common expression "pitch black,") and "tow" in this sense is flammable hay, straw and dry brush, (as in the common expression "tow-headed" for blonde hair the color of hay.)
When flames "stream forth" in such a manner, they are burning at hundreds of millions of degrees like the streaming crown of the Sun's rays of charged particles which cause the Aurora Borealis when they hit the Earth's magnetic field. Those Chaldeans would have received a lethal dose of ionizing radiation from that streaming flame of charged particles which passed through brick walls, solid objects, thick heavy garments and even their bodies.
A fire that is so hot becomes positively charged because the electrons are driven off by the intense heat, leaving the atomic nuclei to interact directly with one another, and emit ionizing radiation.