I am trying to think of some materials that an extraterrestrial spacecraft owned by an extraterrestrial species, known to humans as the Visitors, can use to survive a nuclear explosion in Low Earth Orbit.
The spacecraft is capable of hosting at least ten railguns, six nuclear torpedo launchers, each carrying a total of 20 nuclear torpedoes each, and three particle beams as spacecraft to spacecraft weapons, while also being able to store 40,000 large 20 metre long and 2 metre in diameter tungsten rods, and additional 200,000 0.30 metre in diameter and 6.1 metre long tungsten rods ready to bombard a planet (in this case, Earth). This thing is also capable of sustaining a total of up to 4 million scientists, engineers, security personnel, crewmembers, etc. and 10 million robotic troops and drones combined. It also has 200 km/s of delta-v for interplanetary travel using its ICF engines, as well onboard research labs, manufacturing facilities, habitat modules and storage facilities full of enough raw materials and 3D printers to get small operations elsewhere in the Solar System started. It is 10 km long and 6 km in diameter. Each antimatter beam core stage has a total length of 724 km and is basically ~72,000 Mt in dry mass, and capable of pulling 0.35 Earth gees of acceleration, and reaching a cruising velocity of 0.75 c.
This spacecraft has a magnetic shield to protect against radiation, not to mention the spacecraft has four layers' worth of armour, mainly 100 metres' worth of Inconel, 50 metres' worth of carbon nanotubes, 130 metres' worth of Grade 38 titanium alloy, and a 30 metre thick layer of reinforced carbon-carbon.
Do take note: this is a K2 civilisation. If you happen to not be aware of the Kardashev Scale, here is some links to the scale itself.
Briefly explaining, however, it is basically a civilisation that can harness the energy of an entire star, spam multiple star systems at once, build large-scale megastructures like Dyson Swarms and the like to harness the energy of the star itself for whatever purpose it can come up with, as well as manufacture billions of spacecraft at any given time, not to mention use relativistic missiles (aka: RKKVs/RKVs/relativistic bombs), advanced AI, Von Neumann probes and the like.
So my question is how does the Visitor spacecraft (nicknamed the Leviathan by the pesky humans) survive a 475 kt nuclear explosion at an altitude of 1,500 km above the Earth's surface?