You have three sources of risk:

• radiological. This is probably negligible, because after 500 years anything with a half-life shorter than 50 years is gone. The shielding on the other hand is pretty stable.


• explosive. Nuclear warheads have an explosive primer, containing a sizeable quantity of explosive compounds. Some of these might have become inert, some others might have become dangerously unstable. This might have transformed the warhead in a "dirty bomb".


• chemical. In addition to toxic waste from the primer, plutonium is highly toxic (as well as carcinogenic). Depending on the device's nature (fission or fusion), it may contain other substances that are poisonous, flammable, or both (e.g. lithium deuteride for a thermonuclear design).

²³⁹Pu decay chain

The plutonium in the warhead will slowly decay along the following chain:

Since the half-life of ²³⁵U is way longer than ²³⁹Pu's, we will have mostly alpha emissions and a negligible beta-minus decay from protoactinium.

There is also a more speculative risk (the design of the weapon ought to prevent it, but you never know). Plutonium in nuclear weapons isn't pure plutonium, but rather gallium-stabilized delta-phase plutonium, which has much better characteristics from an engineering point of view. The priming explosion squeezes it in the critical alpha-phase. But it is possible that the same effect can be achieved by aging ("δ phase Pu–Ga is still thermodynamically unstable, so there are concerns about its aging behavior" says Wikipedia), or by "cooking" it at temperatures above 475 °C.

In other words, there might be significant chances for an ill-advised attempt to melt and maybe re-cast the mystery metal to, at minimum, cause poisonous fumes to be released; or, at worst, to trigger a "fizzle melt", which would probably be more than enough to kill everyone in a radius of several meters or more, and possibly contaminate the whole area.

An ancient (and, incidentally, sentient) nuclear bomb appears in Arsen Darnay's The Karma Affair (1978). I seem to remember it being intentionally detonated by letting it fall from a very high tower.

Oh my...

This has happened before. It wasn't a bomb, though.

It was 1987. A hospital in a Brazillian town used caesium-137, which is radioactive, in a radiotherapy machine. The hospital building was abandoned with the equipment in it.

Some thieves scavenged the caesium containing equipment. They pried their salvage open and found an eerily beautiful blue-glowing dust inside...

The thieves took the dust home, and showed it off to their friends and family. People were amazed by the dust and exposed themselves to it in various different ways. One of the thieves used the dust to paint a cross on his abdomen. The other gave some to his six-years-old daughter, who used it as glitter and even swallowed some.

The poor girl died a month later from a very slow and painful death, horribly disfigured and bleeding internally, and alone in a hospital because the nurses were too afraid to come near her (they knew about radiation and had no equipment to deal with it). The kid had to be buried in a lead casket. The populace was mad at her death, but they didn't know who to blame... They were poor, uneducated people. They were also afraid her burial would pollute the cemetery with radiation.

Besides the girl, three other people died in hospital. Other 250 people had enough caesium in them to be picked up by a geiger counter, but only 20 showed any signs of radioactive poisoning, and they all survived.

You can read more about it here or listen to the BBC Witness podcast about it.

I suspect that if people open up a nuke 500 years from now, specially if they don't know what they are doing, a similar incident would happen.

If it's a thermonuclear bomb, the lithium deuteride will likely catch fire as soon as someone breaks its sealing. They won't be able to extinguish the fire (using water, of course, instead of sand), the thing will burn out, including the chemical explosive (no, it won't explode), spreading radioactive char all over the place. The villagers die, nobody else will dare come close for ages.

(Right, solid pieces of LiH don't spontaneously ignite in dry air. So? One moist helping hand, and they have their own little Mini-Chernobyl.)

It depends on how deep you bury it. Radiation can be blocked by soil:
Material Thickness (inches)
Lead 4
Steel 10
Concrete 24
Packed Dirt 36
Water 72
Wood 110

and detonation can also be muted by soil to the point where the Tar Bomba feels like a tremor.

 18 June 1985 

A 2.5 kiloton nuclear device was detonated at the bottom of a shaft 2,850 m (9,350 ft) deep at a location 60 km (37 miles) south of Nefte-yugamsk, Siberia, Russia, on 18 June 1985. The detonation was carried out in an attempt to stimulate oil production. For a comparison, the Hiroshima bomb had a yield of around 15 kilotons.

The Russians carried out 116 nuclear explosions between 1965 and 1988 in a program known as No. 7 - Nuclear Explosions for the National Economy. Known internationally as peaceful nuclear explosions (PNEs), their uses included reservoir and dam-building, mineral prospecting, increasing oil and gas production by liberating material from rocks, creating underground gas stores and putting out subterranean oil and gas fires. The United States had its own PNE program known as Project Plowshare.

http://www.guinnessworldrecords.com/world-records/deepest-nuclear-explosion-underground

Russian testing grounds. The Official CTBTO CC BY 2.0

Radioactive isotopes eventually decay, or disintegrate, to harmless materials. Some isotopes decay in hours or even minutes, but others decay very slowly. Strontium-90 and cesium-137 have half-lives of about 30 years (half the radioactivity will decay in 30 years). Plutonium-239 has a half-life of 24,000 years
Most nuclear weapons these days have a reservoir of Tritium gas, which is a radioactive isotope of hydrogen. The half life of tritium is about 12 years, so the reservoir needs to be periodically replenished to maintain reliability.

Plutonium is very active, decaying at a high enough rate to actually heat itself up significantly. Little is known about the effects of long term radiation damage on plutonium metal, and when coupled with the sustained high temperature, it's conceivable that adverse changes in the crystal structure could take place.

Nuclear weapons are full of unstable material. in most modern weapons, insensitive high explosives are used, but they are exposed to decay heat and a constant stream of gamma radiation from Pu-240 impurities in the bomb's core- this can lead to expansion and cracking of the explosives, damaging them and preventing the bomb from yielding the proper amount. Also, the explosives decay over time; both of these effects necessitate replacement of the explosives.

The bomb's pit (the core) is typically plutonium; plutonium is radioactive and is constantly decaying, and as such microscopic helium bubbles form in the pit, and can affect the symmetry of the core's implosion. Also, the aforementioned decay heat can warp the metal, and radiation from decay can damage the crystalline structure of the pit- plutonium has six common allotropes, and radiation/heat can cause the metal to change allotropes. All of these effects require the pit to be reformed every once in a while.

Modern bombs use tritium boosting gas. Tritium has a half life of about 12 and a quarter years, and as such must be regularly replaced to maintain the ability to yield properly. While nuclear weapon maintenance schedules are obviously classified, those two items probably give you a ballpark estimate of a few years or so. While an unmaintained warhead might still fire, the yield could be significantly reduced. A weapon sitting at the bottom of the sea would be rendered non-functional pretty quickly, but the materials would be salvageable.

I suspect as long as the pit is left alone there is not a problem. A lot of effort is expended in isolating the fissile material from the handlers and the thick metal walls will take eons to corrode unless accelerated by some chemical reaction. However there are the conventional explosives that when detonated compress the pit to produce the nuclear yield. Over time I imagine the conventional explosives will be come unstable and be a monumental risk if someone sneezes at the device or a mouse crawling over it farts.