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“Don't you see how dangerous this all is!” I shouted. “If something goes wrong it could destroy all Ponyville.”

Pinkie looked me in the eye. She paused before saying solemnly, “Twilight. It's safe. I promise.”

“You promise?”

“Cross my heart and hope to fly, stick a cupcake in my eye.”

She looked at me with a deadly serious stare.

“How can you be so sure?” I asked.

She smiled. “Because there's loads and loads of security features, and an auto-shutdown safety system, and a backup system to back up the auto-shutdown system, and a backup backup system to back up the backup system, and a backup backup backup system... And it's designed to be super stable so it won't overheat. And if anything goes wrong we drop the control rods right down. Or failing that we can squirt a neutron absorber into the water. Then there are four independent cooling systems... It's built to be totally idiot-proof!”

“But how can you be sure it will all work as designed?”

“Earth ponies have been running these plants for many years,” she said. “Without any accidents...” She paused a moment and stopped grinning. “Well, with very few accidents... There was that occasion when some streamers got stuck in the fan on the secondary cooling circuit, but nopony has hurt and there was hardly any release of radioactive gas... And there was that case in Fillydelphia when that colt blocked the safety valve with bubble gum... And that one unfortunate incident in Vanhoover – that was a stupid reactor design – rock candy does not make a good neutron moderator. But relatively speaking, it's totally-ultra-super-mega-safe.”

http://www.fimfiction.net/story/153810/2/rock-farms-and-nuclear-reactors/chapter-2-which-describes-the-refuelling-of-a-nuclear-reactor