Presumably this is Phantasmaplasmagoria by Herbert Jacob Bernstein.
It was published in the April 1968 edition of Analog Science Fiction.
The man had obviously been a fool or a liar; but Zave decided to humor him. "Go on."
"I shall come to the point promptly. I did not quite comprehend the meaning of the contour plots. Perhaps you would be good enough to enlighten me?"
I think I know what you mean, the terrain maps of the moon. Right?"
"Quite," the man had said with obvious glee. "What do the valleys represent?"
"No, no. I want to know what they are valleys in. The energy output of some system?"
"Only in gravitational potential energy. Those are simply terrain maps of the moon, an academic joke. You're only the third person to learn..." Zave had stopped when the man turned a shade of purple, and ran out of the office.
This is a critical point, and the questioner panicked because the people running the experiment in London (that was about to go horribly wrong) had assumed that those valley represented stable lower-energy points in the fusion reaction, and were tuning their reactor to them. And we know how well that worked out.
(In fact it did more than just destroy London.)
A fireball 3.5278 kilometers in diameter flared into brief existence. Oxford, Gloucester, Wilts, Berks, and Buckingham were gone. A fault formed, and half of northern England slid on molten limestone into the North Sea. Kent shifted with great majesty until the White Cliffs of Dover were blocking the port of Calais. The tidal waves finished damage the war had started in the coastal regions of Europe.