Nuclear Reactor Core
noun
The furnace heart is the heart of the furnace. The heart of this particular furnace, the one we're talking about here, is the nuclear reactor core.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
どうして奴らは原子炉の炉心が溶解してしまったことを隠そうとしたんだ？
Why did they try to hide that the core of the nuclear reactor had melted?