There's a moment, however brief, where Tony almost thinks that he's going to get away with saying what he said without protest. And, granted, he can't say he's surprised when it doesn't work. If the roles were reversed, and Lydia was saying she was going to power some weapons with a device that is keeping her alive, he'd probably (definitely) have a few things to say about that.

The main difference is that Tony doesn't exactly have the best self preservation skills - especially now. It's dangerous since powering two reactors isn't an easy task, but he can't bring himself to care. He doesn't have a problem with being an energy source for something that can help the greater good. Well, the greater good being him and Lydia, but still. It's weapons that they need and could use and, considering this environment, it's worth the risk. Absolutely and completely. Is it risky? Yeah, sure. Does Tony care? No. Not really.

That is why, even if he's trying to not just ignore her concern, he still shrugs as if it's not a big deal. "I mean, I'll probably have to take it easy after I'm done just while my arc reactor recovers, but nah. It should be fine."

Honestly, the only reason he's bringing it up is because he knows he'll have to use this lab instead of the one in HQ to accomplish all this. Mainly because he's avoiding a certain someone, and the last thing he wants to do is freak Lydia out if she were to walk in on it.

"I've used the reactor as an energy source before," he assures her. "It's fine, Lydia."