Windscale Piles: Cockcroft's Follies avoided nuclear disaster

   Construction was well under way when Sir John, the director of the Atomic 
   Energy Research Establishment, insisted the filters be installed.
   
   "He saw that if there was a fire, which was probable, there would be no way 
   of stopping radioactive dust escaping into the atmosphere," his son said.
   
   Because they were last-minute additions, the filters were placed atop the 
   two 360ft (110m) tall chimneys rather than at the base.
   [...]
   
   They were roundly criticised by the engineers building the nuclear facility.
   
   Engineers, who had been told by the government to make the UK a nuclear 
   power by 1952, nicknamed the filters Cockcroft's Follies, mocking them as an 
   expensive piece of pointless delay.
   
   However, as one of Sir John's physicists Terence Price said after the fire, 
   "the word folly did not seem appropriate after the accident".
   http://www.bbc.com/news/uk-england-cumbria-29803990

http://en.wikipedia.org/wiki/Windscale_fire#Fire

"There was no doubt that the reactor was now on fire, and had been for almost 48 hours. Reactor Manager Tom Tuohy[10] donned full protective equipment and breathing apparatus and scaled the 80-foot ladder to the top of the reactor building, where he stood atop the reactor lid to examine the rear of the reactor, the discharge face. Here he reported a dull red luminescence visible, lighting up the void between the back of the reactor and the rear containment. Red hot fuel cartridges were glowing in the fuel channels on the discharge face. He returned to the reactor upper containment several times throughout the incident, at the height of which a fierce conflagration was raging from the discharge face and playing on the back of the reinforced concrete containment — concrete whose specifications required that it be kept below a certain temperature to prevent its disintegration and collapse.

Initial fire fighting attempts

Operators were unsure what to do about the fire. First they tried to blow the flames out by running the fans at maximum speed, but this fed the flames. Tom Hughes and his colleague had already created a fire break by ejecting some undamaged fuel cartridges from around the blaze, and Tom Tuohy suggested trying to eject some from the heart of the fire by bludgeoning the melted cartridges through the reactor and into the cooling pond behind it with scaffolding poles. This proved impossible and the fuel rods refused to budge, no matter how much force was applied. The poles were withdrawn with their ends red hot; one returned dripping molten metal. Hughes knew this had to be molten irradiated uranium, causing serious radiation problems on the charge hoist itself."

By association and contrast that brings Chernobyl where catastrophe resulted from experimentations/tests outside of the established [existing] SOPs.