Answer:
A flawed reactor.
Step-by-step explanation:
The management of the facility was interested in determining if, in the case of a total loss of power, the electrical generator's turbines would continue to spin long enough to keep the cooling pumps running until the diesel backup generators began to operate.
The emergency core cooling system of the reactor was turned off. Due to operator mistake, the reactor's power was reduced to a very low output level. Control rods were removed from the reactor to boost its output because it started operating at a low efficiency. As a result, the reactor became unstable.
Then, the power-producing turbines were turned off. The cooling pumps in the reactor slowed down as a result. Boiling of the coolant began. Power output increased sharply when the reactor began to overheat. The reactor partially burst, preventing the control rods from being put back in. At that moment, the reactor was completely toast. Adios muchachos, the Second Coming couldn't have stopped the reactor's total devastation as a result of the numerous steam and hydrogen gas explosions that occurred.