Superman had once explained to Batman that, for him, exposure to Kryptonite was seemingly a "Lose/Lose" situation. The first thing he explained was that it didn't matter how much or how little he was exposed to; it could be sliver the size of a grain of rice, or a mound the size of Mt. Everest; the effects were the same. The only difference was the effective range. The smaller the source, the closer he'd have to be to feel the effects, the larger, and the further away he could be and still be affected.
The other side of that was it didn't matter how much of it his body was exposed to it. He could be in a lead suit holding a one pound rock of Kryptonite. Cut a hole the size of a penny into the suit and he'd feel it, just as much as he would if he was standing naked in a kryptonite shower.
The effects were always the same: his powers just slowly drained away. His strength, his speed, heat vision, x-ray vision, flight, etc.; they all just seemed to slowly fade until they were gone. Within the first few minutes, he still had access to nearly all his abilities, but they were considerably reduced. After three minutes; he could only hover, his heat vision could barely start a fire, lifting a small car took tremendous effort, his speed was all but gone, and his other vision's were lost. Five minutes and he could barely jump let alone fly, his heat vision was little more than a burning sensation in his eyes, he could barely manage to lift his own arms, let alone a ton, and his super-breathe wouldn't have been enough to blow out a match.
His invulnerability was similar, but with a few exceptions. First off, Kryptonian physiology varied from human physiology; Kryptonian molecules were naturally denser. His invulnerability wasn't so much due to his exposure to yellow sun radiation, as much as a result of his natural structure. And while the exposure to Earths yellow sun significantly boosted his body's dexterity and resistance to harm, the elements, and nearly every other form of external influence; he was still extremely tough in comparison to a normal person. He could absorb more damage, survive longer in extreme conditions, and his bones were harder to break.
The second exception was that his invulnerability was always the last thing to fade. Even after five minutes of constant exposure to kryptonite, he could still withstand a blast from a high powered shotgun at close range with little more than a bruise.
Recovering from exposure was a little tricky to explain. The moment he was either out of range of the kryptonite, or completely shielded from its radiation, his powers came back on line, but at a significantly reduced rate. Flight, heat vision, strength; everything worked, but at a reduction that was directly relative to the amount of exposure he sustained.
Basically, the longer he was exposed, the weaker he was when his powers came back online. After five minutes of exposure, his powers returned at twenty-five percent their effectiveness. They would increase gradually, but it would take time.
Conversely, exposure to Yellow sun radiation or yellow sun UV rays significantly increased the healing process. Bruises faded in seconds, bones mended in minutes, cuts healed and vanished without so much as a scar in record time.
The upside to it all? When Metallo lived up to his word and tore the suit from his limp body, it didn't make things much worse. The downside? When Superman depicted how he was affected by kryptonite, he never took into consideration a kryptonite fuel rod in a nuclear core.
He'd never felt anything like it, the remains of the Stangin Institute the possible exception.
It was like his body was on fire. Sweat was dripping from his skin. He felt like he was both freezing on the inside and roasting on the outside. His vision was blurry, he wanted to vomit with every breath, and his arms and legs felt like they each weighed a ton.
And that was without Metallo punching him repeatedly.
Every blow landed with the force of a freight train at high speed. Superman's face was a swollen mass of flesh. His eyes refused to focus, his jaw was fractured, if not broken, and blood trickled from his nose.
And he was completely helpless to fight back.
Metallo punched him again. "I have to admit," the metal skull spoke. "I'm a little disappointed in you, Superman. I thought you'd put up more of a fight."
"L-let's… take… it… outside… T-t-then… we'll… see…" Superman whispered.
Metallo actually laughed. "Nice try, but I like it in here." He punched him again, the blow shaking the reactor. "It's roomy." He punched him again. "It's private." And again. "And the acoustics are amazing!" He punched him four times in rapid succession. "You hear that?" he joked. "It's like an opera house! Plus, we have front row seats to the end of Metropolis!"
"Not… just… Metropolis…" Superman struggled to speak. "H-h-half… the… country… T-the… kryptonite..."
"Even better!" Metallo beamed. He rose to his full height, standing over his enemy. "The last time we faced off," he began. "I was lucky to escape intact. You really did a number on me. I've been dreaming about returning the favor."
He reared back and kicked Superman like a ball, and like a ball, Superman sailed through the room and crashed into the wall.
Superman grabbed his stomach and rolled to his back, the wind completely knocked out of him. He tried to breathe and coughed blood.
"But don't worry," he spoke as he approached. "I'm not going to kill you, at least, not right away. I'm going to wait until after the fireworks! When half the nation is a charred ruin, a post apocalyptic wasteland, when every ones else is dead and dust, when you realized how completely and utterly you have failed, and when I have heard the sweat sound of your breaking heart. That's when I will end you life, Superman. That's when I will have my revenge."
Metallo reached down and grabbed Superman up by his hair, pulled him into the air, and held him a foot off the ground. Blood was dripping from his mouth, his eyes fluttered open and closed, and his breathing was raspy and labored.
"I just hope you survive long enough, but if not… no biggie!"
And then the shooting started.
The first two bullets hit the back of Metallo's metal skull, the third, forth and fifth bounced off his back and spine.
Without moving his body, Metallo's head swiveled one-hundred and eighty degrees, his emerald eyes falling on the lone figure standing in the door way of the reactor core.
There, in a pale green protective suit, a large black Desert Eagle high powered hand gun firmly in his gloved hands and aimed directly at Metallo's head, was Special Agent Davison.