Radionuclide Effects and Remediation Tactics: Chernobyl and Fukushima Case Studies

Nuclear accidents are menacing manmade disasters that pose great dangers to people across very broad regions. Reactors are an area where these incidents tend to occur, with two very famous accidents being Chernobyl and Fukushima. These nuclear accidents involve the release of large quantities of radioactive materials outside the reactor containment zone. Wind and water currents further disperse these radioactive materials into the atmosphere and surrounding environment. A large portion of radioactive elements also settle into soil, where they will negatively impact life for years to come. 

When a fallout occurs, particles disperse over a large radius with concentration levels diminishing as the distance from the source increases. The largest concentration of radioactivity is in the immediate vicinity of the accident and the highest threat to human health. Fallout over larger distances tends to be less intense, relatively insignificant to human health, and unfeasible to clean up for many logistical reasons. Therefore, remediation efforts are often concentrated in the immediate area of the accident.

An exclusion zone is one of the first steps in the remediation process to minimize exposure to radiation. The zone also allows workers access to the site to assess damages and stabilize the reactor. This involves cooling the reactor core until the cold shutdown temperature is reached, which will prevent a nuclear meltdown, or the melting of fuel inside the reactor. Any excess melting will likely cause a breach of the containment system that will cause further radioactive particle emission. Because of this, cooling the reactor is of paramount importance, and the exclusion zone assists with that.

Additional remediation is often necessary to reduce the amount of radioactive contaminants released.  This can come in many forms; for example, in Chernobyl, a physical barrier was set up around the site to contain the radioactive emissions. In Fukushima, about 5 cm of topsoil, to which radioactive particles tend to stick to, was removed and disposed of (Gavett, 2012) as part of remediation efforts.

Because these two accidents also happened in different circumstances, examining each site separately can provide important insights on specific aspects of nuclear accidents. For Chernobyl, looking at the dispersion of radionuclides into the environment and their effects on humans is important for preparedness in the future if another accident were to occur. For Fukushima, the many remediation tactics used can provide important insight in how to clean up a site effectively and efficiently, as well as the challenges operators may face when doing so.

The 1986 Chernobyl incident was the largest uncontrolled radioactive release in history. An explosion at the no. 4 reactor occurred after a failed safety experiment, propelling massive amounts of radioactive materials into the atmosphere. Shortly after, several fires ensued that were put out by dumping water and a sand mixture into the reactor(World nuclear, 2020).

Intense levels of radiation still permeated the site after the fires were put out due to the left-over debris. Initially, robots were used to clean up the debris, but began to fail as the radiation destroyed their circuitry. Therefore, men known as liquidators were employed to clean up the debris and once cleared, a concrete sarcophagus was built around the reactor to contain the radioactive emissions (National Geographic, 2016).

An extensive number of radionuclides were dispersed throughout the region due to the explosion. Short lived radionuclides were mostly present in soil deposits; ¹³¹I was the most radiologically significant of these due to their adverse effects on human health. After some time passed, ¹³⁴C and ¹³⁷C were the most radiologically significant due to their abundance and longevity, especially since they tend to stick to shallow depths. In fact, about 95% of the soil contaminants remained in the top 8 cm two decades after the accident (International Atomic Energy Agency, 2001).

Due to the nature of radionuclide travel in the soil, it is best to categorize soil contamination into two different groups: Forest and Agricultural.

Forest Soil Contamination

The rate of radioactive migration in the soil plant system is influenced by many natural factors, with the most important ones being the soil’s acidity, cation exchange capacity, mineral composition, physical composition, and organic composition (International Atomic energy Agency, 2001). However, the ecosystem will also have a significant effect on the duration and severity of contamination. In the forest surrounding Chernobyl for example, there exists elements such as mushrooms, shrubs, and decaying matter that continuously recycle radioactive materials (UN Chernobyl Forum). This process keeps radionuclides in the upper soil levels where they are available for plant uptake.

Forest contamination initially occurred mostly as surface contamination on trees. Over time, surface contamination became second to contamination via uptake through the roots, which keeps radionuclides at ground level. The most bioavailable radionuclide for plant uptake is ⁹⁰Sr, with a study conducted in 2020 showing that about 90% of tree samples about 40 km south from Chernobyl had ⁹⁰Sr levels above permissible limits (ScienceDaily, 2020). This study provides some insight into the natural processes that circulate radioactive particles.

Agricultural Soil Contamination

Agricultural lands tend to have distinct characteristics in regard to radioactive particle distributions over time due to tillage. Additionally, the regular harvest of agricultural products can remove some of the contamination from the biological cycle (UN Chernobyl Forum).

¹³⁷Cs and ⁹⁰Sr are the most significant radioisotopes because of their long half-life. ⁹⁰Sr was strongest in areas closer to the reactor while ¹³⁷Cs had a larger spread across the site. In the first few years after the accident, the presence of radionuclides in agricultural products significantly diminished due to physical decay, vertical migration, and bioavailability reduction. High permeability soils around agricultural areas and tillage in particular facilitated vertical migration of radionuclides. 

In contrast to the forest regions, the agricultural regions seemed to exhibit lower concentrations of radionuclides due to these factors. In the same study mentioned earlier about ⁹⁰Sr concentrations in the forest, it was shown that only 50% of grain samples contained ⁹⁰Sr above permissible limits, as opposed to 90% (ScienceDaily, 2020).

Health Effects

Radiation is a topic of great concern for human health. Chernobyl is the nuclear accident that has had the most impact on human lives.

Short Term Consequences

In the short term the initial reactor explosion took the lives of 3 plant operators.  An additional 27 people died shortly after the accident due to acute radiation sickness from the extremely large radiation doses they received trying to extinguish the fires. With acute radiation sickness the symptoms included nausea, vomiting and loss of appetite, with infections, dehydration, bleeding and confusion following in the later stages (World Nuclear, 2020).

Long Term Consequences

In the long term radiation exposure is known to cause a host of illnesses. A minimum of 5000 cases of cancer are linked with exposure to radiation at Chernobyl although some claims place the number much higher. Thyroid cancer is one of the more prominent ones resulting from radiation, with an estimated 50% of exposed people who developed thyroid cancer attributing their cancer to their exposure. 

Other than cancers, exposed groups also experienced other types of disease from radiation. Vision was affected and autoimmune diseases were not uncommon. Birth defects went up 200% and congenital birth deformities went up 250% in children born in the Chernobyl fallout area since 1986. Although some connections are clear, other links between radiation and specific illnesses are still being studied.

Going further than physical consequences, a plethora of psychological effects occurred as a result of Chernobyl. One noteworthy impact was a large number of abortions following the accident as many physicians advocated for it to prevent child abnomaliites. However, there were no clear scientific indications that the level of exposure experienced by many of the aborters would lead to birth defects. 

Another significant impact was the mental health issues of a large amount of the employed liquidators. While they lacked diagnosable physical ailments, many of them went on to suffer from PTSD as a result of their exposure (World Nuclear, 2020).

Remediation Tactics

Many remediation techniques were implemented after Chernobyl. A concrete sarcophagus was built around the reactor after the explosion, which was replaced by a new safe confinement facility in late 2017 (World Nuclear, 2020).

The cleanup process for Chernobyl was an extensive one. Some forests in the area were extensively contaminated and had to be cleared. The most notable one was the red forest which had turned red as it died from radiation contamination. Upon clearing, the soil was sprayed with Bourda Decon Gel which entrained radioactive particles. The gel would then dry up and could be rolled up and disposed of. This gel was also used to clean up buildings and roads in the area.

In rural areas mechanical measures were taken to bury radioactive particles under the soil. In a few cases removal of the top layer of soil was used to get rid of radioactive particles. Fertilizers from clean areas were also imported to help improve the soil health. Despite their best efforts, huge amounts of land remained highly radioactive and remain part of the exclusion zone today (UN Chernobyl Forum).

Further remediation techniques have been experimented within the 30 km zone around Chernobyl. Among the remediation techniques piloted, mulching agricultural lands has proven to be effective because it inhibits radionuclide propagation due to resuspension. This has been further confirmed by a scientific study conducted in 1994 and 1995 that found mulching decreased the amount of radioisotopes on crops by up to 70 % (Sauras-Yera, 2003). This is a significant improvement, and an example of one among a few remediation techniques being piloted in the area.

Overall, the low-tech remediation techniques employed at Chernobyl were reflective of their times. An intensive clean up process was undergone to minimize the effects of the fallout as the Ukranian government tried to deemphasize the severity of the problem in order to protect their interests in nuclear energy. This remediation process continuously placed the populace in contact with radioactive materials which was highly detrimental. However, many of the reactors that did remain were improved thanks to better safety regulations that were put in place as a result of the incident. Additionally, an international community of nuclear scientists was formed to share expertise on reactor safety. The devastation and lessons learned from the accident helped usher in an era of improved nuclear safety and technological advancement in the remediation sector.

The 9.0 Mw Tohoku earthquake struck Japan on March 11th, 2011, which caused the reactors inside the Fukushima Dai-ichi Nuclear Power Plant to automatically shut down in response. Back-up generators located underground were initialized to maintain coolant processes within each reactor and kept the power plant safe from earthquake shaking. However, the plant was not adequately protected from the subsequent tsunami, having a seawall significantly lower than what was recommended by Japanese government officials. As a result, the tsunami inundated the plant that caused the generators to fail. Fuel rods inside each reactor melted from extreme heat and eventually caused several reactor explosions that released radioactive chemicals to the surrounding environment.

While there were no deaths directly related to the incident, around 156,000 people were evacuated in a 12-mile radius of the site due to radioactive activity induced by radionuclides. Among these nuclides was the radionuclide ¹³⁷Cs, which has a long half-life of 30 years and is known to adsorb strongly to clays (Okumura et al, 2018).  These two factors restrict Cs-contaminated soils for other land uses, especially since human exposure to ¹³⁷Cs can lead to radiation poisoning, burns, increased cancer risk, and even death (EPA, n.d.).  Thus, early remediation efforts were targeted towards Cs-contaminated soil in residential and agricultural areas to minimize the impact of the site on human health.

Remediation Tactics: Soil and Water

The most effective remediation tactic employed by the Japanese government was perhaps the simplest.  ¹³⁷Cs is not known to penetrate deeply into contaminated soil deposits due to its high sorption rate; therefore, most of the cesium was concentrated in the top 5 centimeters of the soil. Removal of this layer reduced the amount of contaminant present in underlying soil by about 80% and cesium concentrations in flood deposits from rivers by about 90% (Burke, 2021). It has also been proven to reduce the air dose rate of ¹³⁷Cs surrounding the site and the area of the quarantine zone, allowing previous residents to move back into select areas if they so choose (Okumura, 2018). However, this remediation effort has been costly to the Japanese government, with an approximate cost of JPY 3 trillion (~28 billion USD) that produced about 20 million m³ of soil waste. Transport of the soil waste to interim facilities for decontamination and storage purposes remains a major challenge to remediation efforts, and Japan is expected to run out of storage capacity soon (Evrard et al., 2019).

Though soil remediation has gone relatively smoothly at Fukushima, the same cannot be said about water remediation efforts. Radionuclides  such as ¹³⁷Cs, ⁹⁰Sr and Tritium leak into the groundwater over time and eventually flow into the Pacific Ocean, irradiating sea life and food sources over time. While Tritium is not known to be dangerous to humans or sea life even in large doses, ⁹⁰Sr poses high health risks to humans as it can weaken bones by acting like calcium (Hsu, 2013). Thus, it is important to reduce the flow of groundwater through the plant in order to prevent further radionuclide contamination, while also preventing contaminated water from exiting the plant.

Furthermore, reactors must be continuously kept stable on-site by cooling down leftover fuel debris via injecting water, which creates contaminated water as a by-product. If given the channels to do so, this water can easily mix with groundwater underneath the site and flow continuously into the ocean, causing a slow but steady radionuclide stream (CNIC, 2018).

One interesting remediation tactic was the erection of an underground frozen soil barrier, implemented by plant operators of the Tokyo Electric Power Company (TEPCO) due to its relative ease of construction, proven technology, and ability to operate without power sources.

A combination of this wall along with an extensive subdrain system and an additional steel wall on the seaward side has been effective in reducing the amount of contaminated water generated on-site, with TEPCO estimating a water-flow reduction of about 75%. Closer inspection of this data, however, reveals that the ice wall was attributed to reducing water flows by about 95 out of 380 tons in the months following construction, a reduction of around 25% (Nikkei ASIA, 2018). As such, criticism has been levied towards the effectiveness of the ice wall itself, with specific criticism towards the shallow depth of the frozen soil wall relative to groundwater flow channels. Because of this, groundwater can still seep into the plant and become contaminated, which causes the amount of contaminated water to slowly increase over time as more injected water is used as coolant. (Sheldrick & Foster, 2018). A government-commissioned panel in 2018 agreed that the ice wall was only partially effective in remediation efforts, and additional measures should be taken to mitigate water flows. Public perception of the ice wall has not been favorable either, with many pointing at the expensive construction costs ($324 million USD) and operations costs ($9.5 million USD per year) as wasteful and less effective than traditional, cheaper solutions such as drains.  

Remediation Tactics: Treated Water

Contaminated water is treated comprehensively by TEPCO in a multi-stage process. Water is filtered in Cesium and Strontium filtering devices that are then transferred to multi-nuclide removal facilities of varying effectiveness (called Advanced Liquid Processing Systems, or ALPs) depending on the concentration of radionuclides. ALPs will then remove 62 of the 63 radioactive elements from contaminated water with the exception of Tritium; once removed, the treated water is sent to water storage tanks, where it sits until a disposal method is found.

 

While the ALPs system seeks to remove as many contaminants as it can, prioritization of different elements of ALPs prevents the system from removing enough radionuclides to meet regulatory standards. According to a report by the International Atomic Energy Agency, only 28% of treated water sitting in tanks met the regulatory standards as of December 2019 (IAEA, 2020).  This may pose problems down the road when a disposal method is implemented, as the low radionuclide removal rate of ALPs may not inspire confidence in the efficiency of its systems.

Storage has been slowly depleting as more irradiated water is treated and placed into tanks. To alleviate some tank capacity, the Japanese Government announced in April 2021 that it would dump over one million tons of treated water into the Pacific Ocean over a span of several decades. The water will contain Tritium that will be diluted with seawater to regulatory levels before release (Sheldrick, 2021).

This has created backlash from foreign countries such as China and South Korea, as they believe that releasing radioactive water into the ocean will decrease public health and negatively impact sea life. Further dishonestly from the Japanese Government about the distribution of radionuclides and the lack of support from local fisherman guilds have caused domestic backlash to manifest as well (McCurry 2021). However, experts and researchers from the nuclear energy industry generally approve of this move, citing that the only radionuclide remaining in the treated water, Tritium, will have little to no impact on human and sea life if released into the ocean. In fact, the same expert panel that criticized the frozen soil wall remediations recommended releasing radioactive water into the ocean for this exact reason (Conca, 2019).

This conflict highlights an important factor in the remediation process: the community. Although the Japanese government has effectively removed all dangerous contaminants from the water, the negative perception of radioactive materials prevents Japan from dumping relatively safe treated water without reputational consequences. As such, Japan must spend more time and money to convince the public of their actions via studies and campaigning, which provides a significant barrier to the full recovery of Fukushima as the treated water takes up valuable tank space that could be used for other purposes.

Overall, treated water dumping into the ocean leaves Japan in a rather sticky situation. If Japan were to appease their citizens and neighbors by not releasing the water, they will be left with a lingering storage problem that will only compound over time. On the other hand, releasing the water will cause permanent damages to their reputation both locally and internationally despite the solution’s scientific benevolence. Either case comes with long-lasting consequences, and how Japan deals with this situation could inform how these remediation situations should (or shouldn’t) be handled in the future.

In conclusion, both Chernobyl and Fukushima show many different remediation tactics that can be used in the case of a nuclear meltdown. Perhaps the most important conclusion that can be made is that remediation is a direct function of the site’s location and the circumstances surrounding an incident. Because Chernobyl happened in a forest-like area, remediation efforts were mostly concentrated on preventing radionuclides from propagating through the soil and cleaning up the area surrounding Chernobyl. On the other hand, Fukushima happened in a coastal region, which meant that most of the effort was focused on curbing the spread of radionuclides through groundwater. Remediation tactics were also tailored to fit the need for the specific incident, with their own challenges; for Chernobyl, the concrete sarcophagus was used to great effect to prevent the spread of intense concentrations of radionuclides immediately following the incident. This intense concentration in Chernobyl still remains a challenge to this day, and it will take decades before Chernobyl returns to some form of normalcy. In contrast, the nuclear meltdown at Fukushima was contained relatively quickly, directly corresponding to lower concentration of radionuclides in the surrounding area. The abundance of contaminated water and soil, though, caused these remediation efforts to be costly, and Japan must now deal with a lingering storage problem. Both these incidents provide important insights on the remediation process, and the challenges that may arise from it.

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