Yes, the landscape is scattered with nuclear debris.
Most nuclear power plant designs will fail spectacularly with long lasting effect on the surrounding area.
The notion that nuclear reactors can run for extended periods of time without human intervention is simply not founded in reality.
Keeping a nuclear reaction from becoming critical is a feat within itself. It's balancing act of maintaining a state that is less than critical.
It will fail within a few months to a few years working under the assumption that it is not getting it's cooling water replenished which is entirely plausible.
Most nuclear power plants in the United States require are light water reactors or pressurized water reactors and require the circulation of water in order to stay within a 'steady state' of operational limits. The two failure scenarios that come into play under an unattended state both involve overheating and a change of state to critical. Other incidents occur on the timeline to critical such as hydrogen explosions but these events are essentially ancillary and aren't central to root cause.
Background
There are two types of nuclear reactors that can suffer from water related failure.
* BWR - Boiling Water Reactor
* PWR - Pressurized Water Reactor
Failure Scenario - Lack of Water
Both water cooled reactor types suffer cataclysmic failure due to a lack of water. Some reactors can be more robust than others depending on the design. Many reactors derive their cooling water directly from their environment using ocean, lake or river water. These reactors are prone to having their water intake ducts clogged with debris thereby restricting the flow of cooling water to their segregated cooling systems. A lack of human intervention in these reactor types can lead to failure.
Cooling towers are used with reactors that are not in close proximity to ocean, river and lake water. Palo Verde Nuclear Power Plant in Arizona is one such example as was Three Mile Island.
In the event of an overheating reactor in the United States the Federal Government only requires a 30 day supply of cooling water. This cooling water, called a UHS (Ultimate Heat Sink) is a finite source and dissipates over time due to a number of reasons including evaporation, steam release and lack of re-circulation of primary cooling circuits due to radiation (the water for cooling is used one time only, in some cases, due to the amount of radiation contamination of the water)
As the water supply for cooling dwindles and water pressure decreases enough for flow rates to diminish beyond preset thresholds, the reactor undergoes an automatic shutdown called a SCRAM. 435
A SCRAM event does not require electricity. Neutron absorbing control rods are held in place by electromagnets above the fissile pile and upon loss of electricity the electromagnets lose their magnetism and the rods are dropped into place bringing fission to a near halt in the core. These systems are automated and do not require human intervention, however, the continued decay heat of the fissile material continues to create issues with cooling and with a finite water supply, eventually, the reaction pile becomes exposed to air which cannot cool the fissile rods due to simple lack of density.
At this point water changes it's state to a gas and becomes steam. The heat increases further and more steam and pressure buildup in the chamber. Water becomes superheated and takes on properties and attributes more akin to an organic solvent. The pressure is so high within the chamber that it eventually prevents the water from boiling. This superheated water's hydrogen bonds are eventually broken and the chamber becomes filled with superheated highly pressurized hydrogen which eventually explodes due to combustion or the failure of the reactor pressure vessel to contain the extremely high pressures. This failure has been made famous by the hydrogen explosion at the Fukushima Daiichi reactor containment vessels in Japan due to the tsunami which did not damage the reactors, it damaged generators that prevented the circulation of water for the cooling systems.
The loss of integrity to the reaction chamber is the penultimate step to the catastrophe, all systems are essentially destroyed and now the fissile material can become molten and eventually melt through any concrete casements that require cooling features in order to prevent the molten nuclear pile from actually melting through it. Once this molten pile of nuclear material hit's moisture an explosion can occur sending nuclear debris into the atmosphere and contaminating the surround landscape with fallout. Mind you, this isn't a nuclear explosion it's just an explosion....but you see what the problem is here, it's called a meltdown.
Conclusion
Water inlets that use water from the environment (rivers, lakes, oceans) for secondary cooling systems require regular maintenance to prevent debris from clogging their inlets.
US located closed loop cooling reactors require only 30 days of backup cooling water.
Manual events such as steam or pressure release to prevent containment vessel explosions won't be occurring. There are disaster prevention events that require human intervention. 435