What caused the Fukushima accident to lose cooling capability?

• A. An earthquake and tsunami that led to loss of electrical power to cooling equipment.
• B. A deliberate shutdown without backup power.
• C. A meltdown due to design flaws in the core.
• D. A cyberattack on cooling pumps.

Answer: (A)

This item focuses on how loss of cooling happens when power to cooling systems is interrupted. After the reactors were shut down, decay heat still needed removal, so cooling systems had to run to keep temperatures under control. An offshore earthquake disrupted the plant’s electrical grid, cutting off power from the outside source. The tsunami that followed flooded on-site backup equipment, including diesel generators that would normally provide emergency power for cooling pumps. With both primary and backup power unavailable, the cooling systems could not circulate water, so heat built up in the reactors and spent fuel, leading to overheating and, in the broader sequence, core damage and hydrogen buildup. The essential point is that the cooling capability was lost because the natural events damaged electric power supplies to the cooling equipment, not because cooling was intentionally shut down, due to a design flaw in the core, or due to a cyberattack.