Controlling exposure to nuclear waste is challenging
types of exposure | the acceptable level of exposure
This reading is part of a series: Nuclear Waste Disposal
Nuclear energy produces waste that is highly radioactive for a very long time. Given that radiation is dangerous, something must be done to control our exposure to it. Before understanding what can be done to prevent exposure, it may be instructive to understand the different types of exposure and why each has to be targeted differently.
Relevant from the archive:
Why is nuclear waste from a reactor so radioactive? (link)
How can radioactivity from spent fuel be harmful? (link)
External irradiation:
Exposure can happen externally when a person is sufficiently near a radiation-emitting object but not necessarily touching it. This exposure is called external irradiation.
For how long is external irradiation a concern?
One year after being discharged from the reactor spent fuel’s radioactive dose rate is 100,000 rem/hr. A lethal dose is about 500 rem received at once [1].
During this early phase, the spent fuel is temporarily stored in a 40-foot-deep cooling pool of water with thick steel-reinforced concrete walls, usually underground. After around five years, it is transferred to dry cask storage (steel and concrete casks), sealed in an inert atmosphere and kept above ground. Protection from external irradiation is achieved by keeping the spent fuel behind lead, water or concrete barriers.
After 40 years, when it’s time to transfer the spent fuel to a repository for long-term storage, the dose rate is still hazardously high at 6,500 rem/h. However, given the protective canister that holds the waste and at least 500 meters of land that might separate a geological storage repository (ideal for long-term storage) from us, external irradiation is not a top concern for repository designers. A deep repository provides vastly oversized protection against external irradiation.
The main aim of a geological repository is to isolate and contain the radionuclides (another term for unstable atomic nuclides that emit radiation and decay into more stable forms) that might escape, reach the above-ground environment and be ingested by us, causing internal irradiation [1].
Internal irradiation:
Radioactive material can enter the body by ingesting contaminated food and water or inhalation. This exposure is called internal irradiation.
Radiotoxicity measures the effect of a radioactive substance when it enters the body. It considers that different radionuclides have different dose factors and migrate and accumulate in different ways inside the body [1].
Inhalation:
Most radionuclides are more toxic if they are inhaled than if they are ingested. However, such a concern is primarily theoretical. Given the way nuclear waste is managed, it is not likely for this to be a reasonable path for radionuclides into the body. However, if it was somehow inhaled, actinides (e.g. plutonium) would pose the most hazard. The hazard from fission products is less and becomes nonexistent after a few hundred years. Actinides continue to pose a significant threat from inhalation for 100,000 years.
Ingestion:
Ingestion is the primary and the most significant path through which nuclear waste could threaten us over its lifetime. It is of singular concern when designing a long-term storage repository.
The scientific community generally agrees that storage of nuclear waste deep in a geological repository will safely isolate it from the biosphere as long as it poses significant risk [2]. The primary pathway of radionuclides from such a repository is groundwater and using that water for drinking or growing food [3].
Given the long life of radioactive waste, different radionuclides contribute to this hazard at different points in time in the life of radioactive waste.
In the first few days of the life of nuclear waste, isotopes of iodine pose a significant threat. Iodine is easily soluble in water and highly mobile in biological systems. Humans accumulate iodine in the thyroid gland. Iodine was the leading cause of cancer in the Chornobyl region. Its path inside the body was from the ingestion of milk soon after the Chornobyl disaster [1].
In about 40-60 years, when it is time to entomb nuclear waste, americium-241 starts to dominate in the radiotoxicity of nuclear waste and continues to do so for more than 1000 years.
After about 10,000 years, plutonium isotopes are the primary concern.
Geological repositories are designed to hold in place nuclear waste that might escape from its canister. The canister itself has been designed to withstand harsh corrosive conditions for more than a million years. But if the canister should fail, the geology of the repository is carefully chosen to slow the escape of radionuclides sufficiently that by the time they reach the surface, they are no longer a threat. Design specifically considers the chemical composition of the fuel itself, e.g., most actinides are very poorly soluble in water and tend to cling to the rock much more strongly, which slows their escape. In some designs, the canister is surrounded by bentonite clay, creating an almost impervious barrier to water and making the escape of long-lived actinides even more difficult. A good design uses multiple barriers to add layer upon layer of protection so that if each by itself can sufficiently isolate the waste, together, they add surety to its long-term confinement.
Acceptable level of exposure:
The EPA defines safety requirements for a geological repository in the US. The maximum exposure must not exceed 15 mrem/yr during the first 10,000 years and 100 mrem/yr from 10,000 to 1 million years for the most exposed person living near the repository site boundary, using the local groundwater for drinking and growing crops. For reference, the average annual radiation dose in the US is around 360 mrem/yr [3].
Since radioactivity can be seriously harmful to us and nuclear waste is radioactive for over 100,000 years, there is a clear need to isolate it for a very long time. The method used for isolating waste must be scientifically and technically sound, and many credible plans offer such a solution. No matter what plan is selected, it must offer confidence to the public that isolation will be lasting and that there will be several barriers between them and the waste.
Safety must not be contingent on plausible future conditions, and the viability of the method selected should be independent of future scenarios. And even in the worst possible scenario, sufficient protection must be there to limit the harmful effects. In the extreme, failure should be allowed, but it should be “graceful and controlled” with a predetermined outcome. In the end, any plan must seek to give rise to the realization that the risk emanating from nuclear waste and the nuclear fuel cycle is far lower than other risks that we take to generate electricity. If this can be established and believed, the concept of nuclear energy will be looked at in a very different light.
References:
[1] Hedin, A. (1997). Spent nuclear fuel - how dangerous is it? A report from the project ‘Description of risk.’ (SKB-TR--97-13). Sweden.
[2] MIT (2003). The Future of Nuclear Power: An Interdisciplinary MIT Study. Massachusetts Institute of Technology.
[3] MIT (2011). The Future of Nuclear Fuel Cycle: An Interdisciplinary MIT Study. Massachusetts Institute of Technology.
