On his first day back aboard the Enterprise, he forces himself to go to Engineering.
He's some time in working up to it, stalling with a tour of the bridge to check out the sensor enhancements, a look at medbay to see if Bones' new equipment is installed (no), and a quiet minute in his quarters going through some of his things. By then he has to admit that he's avoiding the very thing he came up to the ship to do, and so steels himself and makes the walk.
With the Enterprise functioning entirely on Starfleet 6's power, only the most essential personnel are present, and large portions of the ship sit dark and empty. This leaves her quiet compared to what he's used to as captain, and it's more than a little unnerving. He thinks of the painful silence that haunted the Academy's halls after the Nerada, the crater in London where the Kelvin Memorial Library once stood, and the charred corpse of the Vengeance lying in the ashen grave it carved out of San Francisco, and worries that loss is now a staple of Starfleet's diet.
Though he's intentionally neglected his rank shirt, numerous crew members pause to give him a nod or formal greeting. He keeps his acknowledgements low-key, since he's not here to scrutinize anyone except himself. Sometimes he can feel them staring after him with wariness and wonder in equal measure, and he resolves to try and put them at ease. He's still Jim Kirk and still their captain. Mostly.
Engineering is the one place with some life to it. The old reactor has been gutted and the new one is being tested and prepped for its first firing. While he doesn't know the intimate differences between them, he thinks he can spot one now and again as he meanders through the labyrinth of tanks and conduits and focusing bays. (Scotty's report comparing the two is somewhere around the bottom third of the slowly diminishing queue of reports on his tablet; he makes a mental note to bump it to the top.)
The core's exterior hasn't changed much. The beam path aggregators are narrower and the walls aren't so croweded, but the overall configuration is how he's always known it: a tear-drop shaped chamber with the pointed, lower half dropping beneath the floor, its blue shell coated in housings and wiring, and ringed by a long, white chamber with a solid glass door at one end.
Despite the core sitting quiet and inert, the door is shut and locked to anyone lacking authorization. He wonders if that's his fault, and his mouth twitches at a flash of black humor laced with irony; the captain's override code allows access to anywhere on the ship. He doesn't open it, though; he just looks down into the hallway that he lately crawled through, dying.
Sometimes he thinks he can remember things between when he lost consciousness and woke up in the hospital bed, but they're not the kind of memories that surface under scrutiny. They wait for moments of distraction and lurk in the corners of his mind's eye until he's least ready for them, and any time he recognizes one for what it is, he becomes less certain he wants to know the rest. (The roar of an icy, alien river closing over his head while something screams; a vision of himself on the other side of the glass door and Spock being the one to die; the gray dust of Tarsus IV getting under his fingernails as he and Kevin pry open another array control tower in a desparate bid to survive; the Ambassador's crushing grief at the loss of Vulcan; the sound of his mother crying in another room.) The process of coming back has smashed him apart and rearranged and compacted the resulting pieces, like spacedust gathering up to form something new.
He stares at the glass door for a long time, trying to figure out what Pike would want him to learn from all of it. He's so focused inward that he doesn't know someone's there until he hears a familiar voice say, "Captain?"
Jim turns to find Sulu standing behind him, holding a large bin of parts. "Mr. Sulu."
"It's good to see you back on board, sir." Sulu's eyes flick to the chamber door, and Jim murmurs an apology and steps clear, reaching over to punch in his authorization code into the panel out of habit. By the time he has the last number in he can feel his heart pounding. The readout switches from red to green, followed by the loud, metallic clank of the lock bolts sliding back and then the smooth shhh of the door opening.
Jim realizes he should say something. "Good to be back on board." He ignores the cold sweat that's broken out along his back. As Sulu moves into the chamber, Jim finds he can't follow; his breath catches in his throat and he feels rooted to the spot. The hallway is a bottomless cave yawning before him, holding only pain and death and regret. Then he hears Spock's voice in his mind asserting that any comparison involving caves is flawed in numerous ways, and exasperation at his First Officer's imagined critique propels him across the threshhold.
Small victory aside, it takes a good deal of willpower to keep his voice from shaking. "Volunteering with the engineering crew?"
If Sulu notices any of this, he doesn't show it. (This is something Jim has always appreciated about Sulu; he knows when not to push.) "Yes sir. It's not often you get an opporunity to help install a new warp core." At the back of the corridor a panel sits open into a mass of pipes and cables. Sulu sets his bin down beneath it and stats taking things out.
"No it's not." Jim stops some distance short of Sulu, in front of the core access hatch. Being in the corridor is enough, he knows that it is, except for all the ways it's not, and so he enters his code and opens that door too.
Behind him he hears Sulu stop what he's doing and stand up. The helmsman doesn't say anything, and Jim steps up to the ledge.
It's a very different view without the blue glare of the functional core; the running lights cast harsh, angular shadows and leave large portions in darkness behind the bulk of the projectors and their bundles of cabling. The interior has only subtle changes: the projector braces are heavier, and there's proper stairs and ladders bolted into the smooth, concave sides. Otherwise it's the same.
Sulu moves to stand next to him, looking into the dim space. "How did you do it, sir?" His voice is hushed, like their talking might awaken the core and compell it to finish its handiwork.
"I braced myself against the top housing," he points at the railing, "and kicked the bottom back into place."
"You kicked it, sir?"
"Kind of jumped on it, really. It's heavier than it looks."
Sulu's laugh is small and quiet, and out of the corner of his eye Jim sees a look of 'you are completely insane' cross the his features. "Isn't this the sort of thing we could use robotic arms for, sir?"
Jim considers the idea. There's been discussion of some contingency measures, but most of those measures have focused on preventing the deadly fix rather than actually making a safe way to do it. "I like that," he says, and gives Sulu an appraising look. "One of your secondaries was robotics, wasn't it?"
Jim nods. "If you've got the time for it, I'll tell Mr. Scott and Mr. Chekov to find you."
Sulu looks nervous and excited. "Absolutely, sir. Thank you."
"I'm the one who should be thanking you for keeping her from falling," Jim says. "A functioning warp core's not worth much to a ship without a good pilot. So, thank you."
Sulu's reply is less automatic this time. "Yes sir."
Jim turns away from the dark room, and is glad it doesn't feel like turning his back on a treacherous enemy. "Anything I can help you out with here? The shuttle doesn't head back for another couple of hours."
Sulu hesitates, then says, "I could use help testing the actuators on the beam aligners, sir. There's an awful lot of them between here and the generators, and every one of them needs to be checked."
Jim kneels down and pulls a tri-corder and tablet out of the bin of supplies. "At your service, Mr. Sulu."