Crackpot incoming...
What if it wasn't a nuke at all that Kellhus found, but the nominal False Sun, buried when the Nogaral was destroyed, and what if KELLHUS made it overload and explode, destroying the horde and killing Saubon, while simultaneously telling his Ordeal to flee to save as much as possible. Surely Kellhus had heard of the betrayal of Titurga and the False Sun he carried.
Probably not the case, but an interesting thought. It wasn't Tekne at all.
So, when I first read this in the draft, I too thought it was the Diurnal. Then it blew up and that thought got pushed aside. The whole scrolling graphics thing made me then think it had to be something other than that. Kind of weird to bury an ancient Nonman artifact, though, then have Kellhus dredge up another device, from the very same spot. It's just strange to me, but that's just my opinion.
However, this thought just occurred to me, based on what you said, gtownwr. The Day Lantern is described as showing what appears to be the sun in full daylight, regardless of the fact it was night when Titirga confronted Shaeonanra. Like a sorcerous Lens, but always trained on the sun. Shae even feels it, the promise of a sunburn. So, it not only shows the sun, but
transmits the heat/uv/energy of the sun. With this in mind, the device could have been the Diurnal, but 'Lensing' a device located somewhere else. A detonation of that device would have funneled through the Diurnal to Dagliash. Maybe causing it to destruct, as well. A Tekne/sorcerous explosion. Shae obviously knew of the Day Lantern, and maybe figured out how to divert the subject of the Lens to a nuke instead of the sun.
And here I go debunking my own theory... The description of the device didn't really sound like what Titirga was holding, but the physical description of the Day Lantern was sparse at best. Meh.