Garlas

I don't really remember but I'll go through the mssion again to find the way. I'm pretty sure you just follow the sub's passages untill you reach an area that glows red. This is one of the few areas that are unsubmerged (not underwater) and there are a few gaurds so just be ready for them. Then you need to attach your gappling hook to a point and you'll either go down it or up in then you'll reach a chamber with renard and the reactor. Renard is ensearting the reactor into the core and is incased in glass. You'll need a few shot you break through it but after about 4-5 shots it will break and you'll need to shoot Renard in the head.

Hope this helps you finish the game