I have noticed that my RBMK test reactor (Just one fuel rod) exploded even though I had gamerule dialDisableMeltdowns set to true, and I managed to irradiate the desert with it after the core went off, how do I fix this, or did I type it incorrectly?

Where did you find this gamerule? I don't see it in the code nor in the game?

I used the gamerule in 1.7.10 to run highly dangerous RBMK types for power production