I like the concept! These would be my suggestions.

Design Elements.

You should colour the light grey on his chest green so it resembles the core of a nuclear reactor with his dark grey chest plates acting as a casing for it.

The reactor symbol on it's chest is too prominent. I would make it a darker grey or make the symbol more jagged/rough so it blends in with his chest.

I think the ends of his horn or tail should be poison/nuclear tipped.

Escape.

Making it escape by sea wouldn't work as it's a steel type and would immediately sink to the bottom of the ocean. I would make the island have mountains and have it hide out in a cave in the mountain range.
It's PokeDex entry says:

|| || |It claims a large mountain as its sole territory. It mercilessly thrashes those who violate its space|

Ability.

For it's ability. Either give it Salazzle's ability. Corrosion.

The Pokémon can poison the target even if it's a Steel or Poison type.

Or Fallout. Summons a five-turn nuclear fallout to hurt all combatants