How radioactive waste is disposed depends on the material's half-life and its level of radioactivity.
Nuclear waste is categorized by its origin, not by its level of radioactivity. Low-level waste includes that which was made radioactive by neutron exposure or became contaminated by high-level waste. Its level of radioactivity can therefore vary widely.
Low-level waste is usually disposed of as ordinary trash after decaying to safe levels. It may also be transported to one of the three facilities in the U.S. that accept low-level waste for burial.
Fission waste is generally left more accessible than low level waste, not due to lower level of radiation but instead to monitor for leaking because of its long-term danger, and to keep options open when more permanent disposal solutions arise.
Things You'll Need
- Pool of water
- Storage area safe from theft
- Metallic, corrosion-resistant canisters
- License to store nuclear waste
Long-term storage of weakly radioactive waste
As a normal effect of fission, nuclear reactors produce transuranic waste, that is, atoms having atomic number greater than uranium's.
Most transuranic waste does not emit high levels of penetrating radiation, but is harmful if ingested and does have a long half-life. For example, the half-life of plutonium-239 is 24,000 years.
Most spent nuclear reactor fuel is stored at the same site as the reactor, in water pools. So the first step of storage is to place the waste in a water pool, to serve as a radiation shield to protect workers.
The fuel is later dissolved in chemicals to recover unfissioned uranium, leaving the waste suspended in liquid.
The next step, in the case of waste from U.S. defense programs, is often to reprocess the waste for use in nuclear weapons or for reuse in new fuel.
Even then, the rest of the steps need to be followed, because reprocessing can be accomplished only to a certain degree due to the difficulty of separating isotopes and finding uses for each of them.
To reduce storage volume, the waste is then concentrated by removing water, through a process called calcination.
The waste is then bonded into glass (through vitrification with fragmented glass) or concrete.
Waste in the United States is then transported for storage at surface sites in stainless steel silos on federal reservations in South Carolina, Idaho and Washington, and at the Nuclear Fuel Services Plant in West Valley, N.Y. The strategy of surface storage is akin to the difference between storing a residential oil tank above ground instead of burying it. In the former case, leaks can be seen as soon as they start. The aim is to keep the waste from leaking and entering the biosphere, especially entering ground water used for drinking.
These facilities concentrate and structurally stabilize the waste in preparation for later consolidation at an as-yet unopened, geologically stable national repository.
Whether such a geological site is ever opened, a remaining step still exists of using future technology, hopefully to be created in the next few decades, to improve recycling and transmutation--the acceleration of decay rates to produce benign end-products.
Storage and disposal of low-level waste
Low-level waste is usually stored on the site of its creation, by licensees who meet regulatory standards in handling the material.
During the time of on-site storage, the radioactivity level decreases, in some cases to levels at which the waste can be thrown out as regular trash.
The more radioactive a material, the faster it decays into atoms that are not radioactive. So the care needed to store highly radioactive low-level waste tends to be a temporary situation.
Low-level waste can range from weakly radioactive items such as contaminated protective shoe covers to more highly radioactive material, such as retired reactor walls made radioactive by neutron exposure. Therefore, storage methods and length of time for decay varies.
Once enough waste has accumulated on-site, it may be transported to one of the three low-level disposal facilities in the United States, where the waste is buried in vaults, boreholes or geologic repositories.
Tips & Warnings
- The difference between storage of highly radioactive waste with a short half-life and of longer-lived, weakly radioactive waste is that the latter is stored in a way that would be accessible for relocation. Short-lived waste is more likely to be buried.
- This may at first seem unwise from a safety perspective, for example to protect against theft or terrorism. However, accessibility by authorities to long-lived waste is desirable to exploit any decision to open a central geological repository, and anticipated progress in recycling and transmutation.
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