Over the past few days there’s been buzz around whether the melted-down reactors at Fukushima Daiichi are near “cold shutdown”. Since the nuclear crisis began, achieving cold shutdown has been the major goal of the Japanese government and the Tokyo Electric Power Company (TEPCO), which runs the plant. Loosely speaking, it would mean that the stricken reactors at the plant no longer require active cooling and that the immediate nuclear crisis is more or less over.
The US Nuclear Regulatory Commission (NRC) defines cold shutdown as “a reactor coolant system at atmospheric pressure and at a temperature below 200 degrees Fahrenheit.” (That’s 93 degrees Celsius for the rest of the world.) By that standard, Fukushima’s reactors do seem very near a cold-shutdown state. According to the latest data from the Japan Atomic Industrial Forum, Unit 1 is at 36.5 degrees C and falling, and Units 2 and 3 are gently declining at 68.1 degrees C and 68.8 degrees C respectively. The pressure in the drywell of Unit 1 is at 1.18 atmospheres (120 kpa), and Units 2 and 3 are at 1.08 atm (109 kpa) and 1.01 atm (102 kpa).
But cold shutdown is about more than numbers. As World Nuclear News points out, it’s difficult to be sure of the actual temperature of the uranium fuel inside the reactors because most of it is thought to have melted. The meltdowns also make these reactors unlike any others on the planet. It’s difficult to say whether the same parameters for cold shutdown can be applied to Fukushima’s ruined fuel.
Finally, there is a political element to cold shutdown. If the government and TEPCO agree that cold shutdown has been reached, they are in some sense declaring the immediate crisis at Fukushima over. That might be a controversial announcement at a time when thousands are still out of their homes, and farms in several prefectures are contaminated by radiation. For all of these reasons, politicians, not engineers, will decide when to declare a cold shutdown at the plant.