RADIOACTIVE ELEMENTS produce heat as they decay. Nuclear plants draw power from this process, and typically stabilize the temperature with water. But during a power outage, H2O-which needs pumps to flow-can't always prevent meltdowns. Molten salt reactors, which instead control heat with melted lithium and potassium fluorides, have a fail-safe: If the electricity dies, a plug will melt, causing the salts to seep down a safety drain and solidify around the uranium, preventing overheating. After a decades-long lull in development, countries from China to Denmark are building new molten salt reactors. Here's how they work.
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