“The codes got better and better” after the accident at Three Mile Island revealed the poor state of reactor assessment, said Michael W. Golay, a professor of nuclear science and engineering at the Massachusetts Institute of Technology.

These portraits of the Japanese disaster tend to be proprietary and confidential, and in some cases secret. One reason the assessments are enormously sensitive for industry and government is the relative lack of precedent: The atomic age has seen the construction of nearly 600 civilian power plants, but according to the World Nuclear Association, only three have undergone serious accidents in which their fuel cores melted down.

Now, as a result of the crisis in Japan, the atomic simulations suggest that the number of serious accidents has suddenly doubled, with three of the reactors at the Fukushima Daiichi complex in some stage of meltdown. Even so, the public authorities have sought to avoid grim technical details that might trigger alarm or even panic.

“They don’t want to go there,” said Robert Alvarez, a nuclear expert who, from 1993 to 1999, was a policy adviser to the secretary of energy. “The spin is all about reassurance.”

If events in Japan unfold as they did at Three Mile Island in Pennsylvania, the forensic modeling could go on for some time. It took more than three years before engineers lowered a camera to visually inspect the damaged core of the Pennsylvania reactor, and another year to map the extent of the destruction. The core turned out to be about half melted.

By definition, a meltdown is the severe overheating of the core of a nuclear reactor that results in either the partial or full liquefaction of its uranium fuel and supporting metal lattice, at times with the atmospheric release of deadly radiation. Partial meltdowns usually strike a core’s middle regions instead of the edge, where temperatures are typically lower.

The main meltdowns of the past at civilian plants were Three Mile Island in 1979, the St.-Laurent reactor in France in 1980, and Chernobyl in Ukraine in 1986.