That's due to the heat that remains in the nuclear core, both from the recently-disabled but still-hot fuel rods and from the various byproducts of the fission process.
Nuclearfission produces insane amounts of energy — we're talking several million times more energy than you'd get from a similar mass of a more everyday fuel like gasoline.
核裂变产生的能量极其巨大——相比同等质量的普通燃料如汽油等,核裂变材料释放出的能量要多出几百万倍。
3
The uranium fuel generates heat by neutron-induced nuclearfission.