But fusion was essentially a thermal reaction, not inherently different in its kindling from an ordinary fire; it required no critical mass and was therefore potentially unlimited. Once ignited, its extent depended primarily on the volume of fuel—deuterium—its designers supplied. And deuterium, Harold Urey’s discovery, the essential component of heavy water, was much easier and less expensive to separate from hydrogen than U235 was from U238 and much simpler to acquire than plutonium. Each kilogram of heavy hydrogen equaled about 85,000 tons TNT equivalent.1622 Theoretically, 12 kilograms of
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