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Nuclear Waste
A nuclear reactor "burns" uranium in a nuclear fission chain reaction, under controlled conditions so that the heat output can be harnessed to generate steam used to power turbines and thus produce electricity. All commercial reactors are currently thermal reactors of this type.
A nuclear power plant produces some 3 m³ (25 to 30 tonnes) of highly radioactive spent nuclear fuel per year. Nuclear waste consists of spent fuel and all other contaminated items (buildings, equipment, clothing, tools, etc.)

Nuclear Waste Categories

CategoryVolumeRadioactive ContentDescription
High-level (HLW)3%95%Used nuclear fuel. Requires very careful handling, storage and disposal. After 50 years cooling, it will be disposed of deep underground in geological repositories.
Intermediate-level (ILW)7%4%Used filters, steel components from within the reactor, effluents. etc. Disposed of in established ground repositories.
Low-level (LLW)90%1%Contaminated items like tools and work clothing usually disposed of by incineration and burial.
After 300 years waste remains radioactive even if reprocessed, that is after separation of transuranic elements for further usage in a power plant, and is considered as hazardous waste.
Transuranic elements, like plutonium and curium, have very long-term radioactivity. Only after more than 10,000 years will waste cease to be a threat to public health or safety.
In 2018, no definitive solution exists to dispose of waste effectively, although the consensus points in the direction of geological entombment. For this reason, waste is kept stored, pending further research and advancement of long-term waste disposal projects currently under way. The World Nuclear Association, a gathering of nuclear industrialists whose mission is to promote a wider understanding of nuclear energy among key international influencers explains that:
High-level wastes can remain highly radioactive for thousands of years. They need to be disposed of deep underground in engineered facilities built in stable geological formations. While no such facilities for high-level wastes currently operate, their feasibility has been demonstrated and there are several countries now in the process of designing and constructing them.
One may beg to disagree. Moving from a laboratory mock-model to the terrain opens mother nature's box of daemons: tsunamis, earthquakes, volcanoes, wildfires, cyclones, floods, landslides. The 2011 Fukushima nuclear disaster caused by an earthquake is still fresh in our minds. There is very little engineering can effectively do to mitigate such big natural cataclysms.
Setting aside mother nature, the highest hazard is still the social one. Society is crossed by conflicting interests that cause priorities to shift, and drive both private and public executives to take reckless approaches to items that do not appear to demand immediate attention, while they seem to cost an arm and a leg.
On 14 August 2018 the Morandi viaduct in Genoa, Italy, collapsed leaving 43 people dead as dozens of cars fell some 30 meters to the riverbed, railway tracks and dusty streets below. In early November 2018, two buildings entirely collapsed, and a third partially crumbled in downtown Marseille, France. Six bodies have been found in the rubble, while the search continues to find others still believed to be missing. In both cases, the city hall had been repeatedly warned for years, and was totally aware of the risk. Notwithstanding, municipal executives considered uncritical to allocate resources to the maintenance, replacement or disposal of those rotten structures. Then it was too late.
We could easily find scores of similar situations of catastrophes caused by neglect, or by scarcity of resources to keep things adequately maintained and fully functional during their planned lifetime. The list extends from buildings to highways, bridges, dams, railroads, power stations, telecommunications networks, software systems, etc. Closer to our subject, the Associated Press reported from Albuquerque, U.S.A, on 16 November 2018:
Operations at the federal government's nuclear waste repository in southern New Mexico resumed Friday as managers acknowledged there was radioactive waste in the area where a portion of the underground facility's ceiling collapsed earlier this week.(...) The repository was carved out of an ancient salt formation about a half-mile below the desert, with the idea that the shifting salt would eventually entomb the radioactive tools, clothing, gloves and other debris that make up the waste. (…) Access in the underground disposal area has been limited in the wake of the 2014 radiation release, which was caused by an inappropriately packed drum of waste that had come from Los Alamos National Laboratory. That release contaminated part of the area, forcing the closure of the repository for nearly three years and resulting in a costly recovery.
The question now is: why should it be any different with the deep underground facilities built in stable geological formations to bury high-level wastes?

Source: World Nuclear Association: What are nuclear wastes and how are they managed?