Nuclear Waste Hazard Reduction

Abstract

This chapter reviews the history of nuclear fuel reprocessing. The implementation of President Carter’s International Nuclear Fuel Cycle Evaluation (INFCE) Program hinders the sound fostering of nuclear fuel cycle technologies in the USA and scattered their nuclear engineers to other fields of industries. They once wanted to contribute to developing the “Atoms for Peace Policy.” The statement by President Carter changed the nuclear fuel policy as if direct disposal of spent nuclear fuels was quite normal and nuclear fuel reprocessing exceptional. Although the purpose of INFCE is to stop the proliferation of atomic bombs, we experienced and witnessed the proliferation of atomic bombs, despite the banning of nuclear fuel recycling policy for civil purposes. This chapter focuses on “Atoms for Peace” and gives the future perspectives of the nuclear power system at a glance at global ecology. The unnecessary fear of radiation and radioactive substances through the mushroom cloud over Nagasaki and Hiroshima will be discussed as societally important aspects for our future.

1. Introduction

This chapter reviews the history of nuclear power development for civil purposes in the context of nuclear fuel and radioactive substances. Soon after the Atoms for Peace speech addressed by Mr. Dwight D. Eisenhower, President of the United States of America, to the 470th Plenary Meeting of the United Nations General Assembly [1], confidential documents were declassified and delivered to the nonnuclear weapon states to have world share the technology of nuclear power for peaceful uses. The United States of America had led the development of nuclear power to develop and construct light water reactor (LWR), witnessing a sharp rise of a nuclear share in the power grid. At the same time, following the nuclear power growth, nuclear fuel supply was becoming one of the key interests. Recycling uranium and plutonium had been envisaged to be the most feasible at this stage. The nuclear fuel reprocessing plant for civil purposes was planned and constructed, first in West atomic Valley in New York State in 1966 [2] and second in Barnwell in South Carolina State in 1970 [3]. The diagram of process flow in those plants was similar to that of plants for military purposes. However, their plutonium product will be recycled and mixed with uranium as mixed oxide fuel that uranium is planned to be transferred to the UF₆ conversion plant and fed to the uranium enrichment plant [4].

The typical concept of nuclear fuel cycle is shown in Figure 1 [5].

Figure 1.

Concept of nuclear fuel cycle [5].

As is described in Figure 1, spent fuel reprocessing plants were once constructed and operated in the U.S. but are now omitted. This chapter discusses the history and thinks about where the rationale is.

Reprocessing plant is a key component of the nuclear fuel cycle to recover uranium and plutonium, and nuclear fissile materials are to be used for nuclear fission, so that we can recycle nuclear fuels as much as possible. It must be an environmentally friendly means of saving nuclear fuel consumption to lessen its impact on the earth through human activities. It contributes to extracting less amount of uranium through mining for a sustainable society.

Nevertheless, the nuclear fuel recycling policy had to be ended in the USA. Nuclear Non-Proliferation Act of 1978 Statement is signed into law [6]. It refers to the Atomic Energy Act of 1954 and the adoption of the Non-Proliferation Treaty by the United Nations in 1968. The statement shows disagreement with the unnecessary commitment to the commercialized use of plutonium, which may not be able to prevent the proliferation of nuclear weapons. Thus, nuclear fuel reprocessing was omitted and the so-called once-through fuel cycle, deleting the fuel reprocessing, and recycling illustrated in Figure 1, was adopted as the U.S.A. policy of nuclear energy utilization.

Current nuclear fuel cycle in the U.S. is shown below [7].

Spent fuel reprocessing is not shown in Figure 2 but remarked in used fuel as “pending possible reprocessing or permanent storage.” This note is the reflection of the statement of the Nuclear Non-Proliferation Act of 1978, which states that “I continue to oppose making premature and unnecessary commitments to commercialization of the fast breeder reactor and reprocessing, as exemplified in the United States by the Clinch river and Barnwell projects” [6]. The Clinch River project is the development of the fast breeder reactor, which consumes and produces plutonium simultaneously. The Barnwell projects are the development of the commercial spent fuel reprocessing plant and its related facilities. The facilities at the planning stage are a uranium conversion plant for recovered uranium and a mixed oxide (MOX, a mixture of uranium oxides and plutonium oxides) fuel plant for the recovered plutonium planned to be built in the adjacent area.

Figure 2.

Current nuclear fuel cycle in the U.S.A. [7].

The statement of President Jimmy Carter states that “we are premature to commercialize the utilization of plutonium as an energy source, so it reflects in Figure 2 as noted if we become mature enough to get rid of nuclear weapon proliferation risks, it may be turned to be “possible reprocessing,” otherwise, “permanent storage” is the only one option [6].

It states further that “the U.S.A. withdrawn all the program of commercial utilization of plutonium, however, it also asks the international community to abandon their promotion of plutonium and weapon-grade enriched uranium for commercial purposes.” It extends the setup of INFCE, International Nuclear Fuel Cycle Evaluation with more than 40 nations is now going. One of the agreements reached by INFCE is that uranium enrichment is limited to up to 20% maximum for the civilian reactors. This mainly targets test reactors that use almost weapon-grade enriched uranium to replace the 93% enriched uranium with 20% enriched uranium. Not limited to research reactors, INFCE ruined Germany’s high-temperature reactor development. The German development policy for high-temperature reactor development and how the INFCE affects them will be discussed below.

In 1961, the German prototype reactor of AVR started its construction [8]. Its nuclear fuel is the so-called pebble bed design fuel. Still, this fuel type is one of the candidate fuel designs expected to be adopted in our next-generation reactors. AVR is a helium-cooled high-temperature prototype reactor and foresees the future utilization of nuclear energy even for iron manufacturing. Its nuclear fuel cycle shall be focused on INFCE and its influence on fuel cycle development. Originally, the drive fuel of AVR was highly enriched, around 93% enriched uranium. The core is surrounded by thorium fuel, and the neutron irradiation breeds ²³³U, which can be used as fissile material to replace highly enriched uranium used in its initial core.

Figure 3 shows the ²³³U recovery process from the irradiated thorium fuel [9]. The ²³³U produced in the irradiated thorium and the thorium as matrix materials are dissolved in highly concentrated nitric acid and extracted together with tri-butyl phosphate (TBP). Most of the fission products, including protactinium, are separated, leaving in the aqueous phase through the liquid–liquid two phases (aqueous and solvent, TBP with diluent) extraction. Then, the extracted thorium and ²³³U are stripped to the aqueous phase separately, using the difference in their distribution ratios between the aqueous to solvent phases. Since protactinium decays to ²³³U, the recovery of protactinium is also proposed in the effluent treatment.

Figure 3.

Schematic diagram of ²³³U recovery process [9] (courtesy of International Atomic Energy Agency, copyright IAEA).

The recovered ²³³U will be recycled for fuel fabrication, as in the plutonium case in Figure 1. It is widely known that ²³⁹Pu and ²³⁵U are fissile materials used for nuclear weapons, although ²³³U is not as famous as them. Since it was only produced through irradiated thorium fuel reprocessing, Germany is the only state that tried to recover ²³³U through reprocessing, using a new type of reactor on an industrial scale. INFCE set the criteria of 20% enriched ²³⁵U as the upper limit of enrichment for civil purposes. Thus, the research reactors were forced to redesign their core configuration to lower the enrichment down to less than 20%. For ²³³U, INFCE put the threshold of 12% diluted by ²³⁸U [10]. In the original perspective of the thorium fuel cycle, the incentive is to obtain ²³³U, which is not contaminated with other uranium isotopes. However, INFCE had ordered to dilute the recovered ²³³U with ²³⁸U, which ruined the feasibility of the thorium fuel cycle in Germany.

As discussed before, the statement of President Carter on nuclear non-proliferation has not only abandoned nuclear fissile material recycling in the U.S.A. but also limits the fissile materials reuse internationally. The statement named France and Japan as those states to choose the policy of adopting the nuclear fuel cycle and are still keeping the policy of recycling plutonium for civil purposes [6].

2. Long-lived radioactive nuclides produced by nuclear power generation

Nuclear energy is delivered when the fissile material is fragmented into fission products. They are radioactive. Sometimes, the utilization of nuclear energy is criticized because they produce human-made radioactivity. However, fossil fuels have also been producing carbon dioxides and the author believes that to what extent we can afford to accept them must be important. Most of the half-lives of fission products are relatively short, and their radioactivity decreases quickly with time. Figure 4 shows the typical decrease of radioactivity in spent fuel [11].

Figure 4.

Radioactivity decreases with time (in the case of PWR fuel with burn up of 50 MWd/kg(Copy of source captions) and initial enrichment of 4.5%) [11].

Figure 4 uses the unit of Ci/MTHM, and curies are easier for us to grasp the magnitude of radioactivity. The unit Curie is named following her honorable achievement of purifying radium from thousands of tons of ores from the mines of St Joachimsthal, Germany (today, the mines of Jáchymov in Check) [12]. The unit of a Curie is defined as radioactivity in 1 g radium. In Figure 5, radioactivity starts with 3 x 10⁶ curies, equivalent to 3 tons of radium in radioactivity from each 1 ton of nuclear fuel. The radioactivity falls to be equivalent to 3 g radium when one million years have passed. If we believe Mrs. Curie collected 1 g of radium, we may accept 3 g of radium equivalent in our world. However, if 3 tons of radium exist in our vicinities, the residents may become awful to abandon the generation of radioactive waste immediately. It is a typical sentiment but acceptable since we witnessed the atomic bomb cloud over the peaceful city of Hiroshima and Nagasaki in 1945.

Figure 5.

Radiotoxicity and time after discharge, radiotoxicity shown as dilution water volume to be accepted as not hazardous [11].

However, the author believes scientists must deliver the rationale for assessing radioactivity hazards. As discussed before, the world of nuclear energy is divided into two major groups in terms of nuclear fuel cycles. One is the so-called once-through cycle, which does not process the spent fuel; thus, the radioactivity it generates is left as it is. In this case, the radioactivity stored or disposed of is the same as in Figure 4.

On the other hand, if we reprocess the spent fuel, the activity of plutonium and uranium would be -thousandth of the values shown in Figure 5. As shown in this figure, fission products generated through nuclear power are relatively quickly down to 3 g of radium in around a few hundred to one thousand years. After the decay of most fission products, the major contributors to the residual radioactivity are actinide elements. Thus, if we wish to reduce the radioactivity to a few grams of radium within one thousand years, the actinide elements, such as americium and curium, must be recovered in addition to plutonium and uranium. Those elements are recovered through the existing reprocessing plant in the PUREX process. The details of actinide recovery (among nuclear technology experts, the term minor actinide is widely used to identify actinide elements except for plutonium and uranium, focusing on the elements not recovered by PUREX reprocessing plant) are discussed later. However, further discussion shall be made here if the once-through fuel cycle is viable or not in its nature in ecology.

The radioactivity of each nuclide does not directly indicate the radiotoxicity to human health. The higher the energy irradiated from the radioactive substances is, the more severe their consequences are to our body showing radiation detriment. Radiotoxicity is normally expressed as the amount of water needed to dilute the radionuclide concentration to tolerably acceptable levels. As indicators of acceptable levels, annual levels of intake (ALIs) are used [13, 14]. ALIs are levels of specific radioactivity concentration acceptable to our daily lives. ALIs are evaluated to be equivalent to our exposure received by the radionuclides to be 1 mSv/year for the general public. The 1 mSv/year is expressed as the dose limit in radiation protection practices agreed upon with International Committee on Radiation Protection (ICRP) [14].

We receive 2.4 mSv/year averaged over the population in our globe [15]. Thus, it would be strange if we could keep the code of dose limit of 1 mSv/year, while we exceed the limit in living every day. To manage the discrepancy, ICRP prepared a note of implementation in 1 mSv/year, when they first introduced the dose limit concept, saying that the level of 1 mSv/year was applied to the cases in controlling radiation exposure and not to be applied to those cases in an uncontrollable situation. In other words, 1 mSv/year is not a bounding limit for judging if the environment is safe or not [16].

Figure 5 shows an example of radiotoxicity interpreted from the radioactivity as shown in Figure 4. The volume of water needed to dilute the radioactivity into the exposure levels of 50 mSv/year is shown as radiotoxicity. The exposure limit adopted for waste disposal is one of the key societal issues since the regulators support the principle of the hypothesis that there are no threshold levels of radiation exposure in terms of their effects on human health. In most cases, the general public believes that radiation is as low as possible. However, the concept of no threshold can also be applied to an idea of acceptable radiation to be decided by the society or community, namely, to which extent the additional man-made radionuclides may be accepted in the community even though they generate a slight rise of environmental radiation levels near the disposal site.

Figure 5 indicates that most of the fission products represented by Cs and Sr. are to be decayed in several hundreds of years, and most residual activities come from actinides. Suppose we shorten the time frame of the isolation period of radioactivity from one million years to a few hundred years. In that case, the sentiment of fear of radioactive waste might be eliminated. It will be discussed in later sections.

3. Partitioning and transmutation of minor actinides

Spent fuel reprocessing plant using PUREX process partitions uranium and plutonium to recycle them to nuclear power plants. The solvent of TBP was found to be suitable for those two elements to separate them from other fission products. As shown in Figure 5, to reduce the radioactivity over the extended period of over 1000 years, residual actinides, the so-called minor actinides must be separated from fission products. If the reprocessing plant could be regarded as an ecologically friendly plant, it must be the due course of our destined R&D step further to recover minor actinides.

Figure 6 shows the contribution of separating uranium and plutonium by spent fuel reprocessing and separating minor actinide, respectively [17].

Figure 6.

The contribution of reprocessing and minor actinide partitioning on radiotoxicity [17].

In Figure 5, radiotoxicity is indicated by m³ water to dilute the radioactivity to give the exposure levels of 50 mSv/year, while Figure 6 shows the radiotoxicity by Sv/tsm. They are indicating the radiotoxicity, the same but in a different index. Figure 5 shows the amount of water diluted to reach a certain level of exposure to the radionuclides. If not diluted, the exposures to be taken are to be multiplied by the volume of water for dilution. Even though each researcher defines the radiotoxicity scales differently, choosing the radiotoxicity levels to be targeted must be mutually agreed upon among researchers. The target levels are not the dose limit given by ICRP [14], exemption or clearance levels given by IAEA [18], but the levels observed in our surrounding environment. In Figure 6, the natural uranium levels are shown as target of radiotoxicity in time of isolation from our biosphere. Figure 6 indicates that the isolation period for radioactive waste resulting in nuclear power generation can be dramatically shortened from 170,000 years to 330 years. Thus, researchers’ challenge to remove minor actinides has never been given up in the past, now, and in the future. The major elements of minor actinides are americium and curium. In the U.S.A, “octyl(phenyl)-N,N-diisobutyl Carbamoyl Methyl Phosphine Oxide” (CMPO) is selected to investigate the recovery of americium from the raffinate of the extraction cycle in PUREX process in 1980s [19]. The process is named as TRUEX process. Since then, many other solvents have been proposed, but the investigation has continued to industrialize the partitioning of minor actinides to the end. In addition to the recovery process of minor actinides, it must be noted that transmuting the minor actinides is also facing obstacles. If we recycle those minor actinides into light water reactor (LWR, widely adopted as a standard nuclear power plant globally), they absorb thermal neutrons again to be transformed into higher atomic number actinides. They may not be able to transform them into shorter half-life nuclides.

To transform the minor actinides into nuclear reactors, we need a reactor using fast neutrons, the so-called fast reactors (FRs). FR used to be called fast breeder reactor (FBR) when the concept of breeding plutonium was accepted. Irradiated by fast neutrons, ²³⁸U can be transformed into ²³⁹Pu, which is fissionable. Since INFCE, the ²³⁹Pu breeding feature of fast reactors is becoming gradually unpopular, and the word “breeding” is omitted now [19].

Several other reactors, such as molten salt reactors, had been discussed for minor actinides transmutation [19]. However, in this chapter, FR is adopted as the representing type of reactor to transmute minor actinides. Partitioning and transmutation of minor actinides are not yet industrialized. However, it is worth continuing to work for it since it might be a key to accepting the radioactive waste generated through nuclear power through our highest ethical value to give the lowest damage to the globe.

4. How safe is the safe for radiation protection

As concluded by the Ethical Committee in Germany, most people believe that we have not yet been wise enough to give a solution to radioactive waste [20]. The committee’s report says that if we cannot get the expected probability value, we must judge if it is acceptable, assessing the worst-case consequences. What would be the worst case for radioactive waste? Since we cannot imagine our world 170,000 years later, far in the future, let someone believes that the worst case might be a virtual scenario of the waste coming up to the surface of our biosphere in 10,000 years.

From Figure 6, direct disposal is 10 to 10² times higher than natural uranium in once-through spent fuel as radioactive waste. In contrast, reprocessing is a few times higher, and reprocessing plus minor actinides removal is 10 to 10² times lower. Those scientists supporting the R&D for minor actinides removal take it for granted to use the radiotoxicity of natural uranium as reference levels to judge whether the waste’s radioactivity is safe to leave the waste as it is in our biosphere environment. A similar approach to our interpretation in uranium mining, extraction, and enrichment is shown in Figure 7 [21].

Figure 7.

Development of activities in different fractions of uranium in uranium supply to nuclear power plants (assuming that the 8-ton natural uranium equilibrated with daughters are processed into 7 tons of depleted uranium, 1 ton of enriched uranium) [21] (through the courtesy of Svensk Kärnbränslehantering AB).

Figure 7 shows the radioactivity of natural uranium had never been changed through our activities up to the loading of nuclear fuel to nuclear power plant but relocating the extracted uranium and uranium progenies (daughters in Figure 8), and the extracted uranium is divided into the enriched uranium and depleted uranium. It is clear in the scientific background that chemical or physical processing does not cause any nuclear reaction. Thus, the nuclides supplied to the chemical/physical process cannot be more radioactive than before.

Figure 8.

Relative activity of spent nuclear fuel of type SVEA 64 with a burnup of 38 MWd/kgU(Copy of source captions) [21] (through the courtesy of Svensk Kärnbränslehantering AB).

Most scientists might not be interested in Figure 7 because it is clear. However, the author is keen on the scale of elapsed time to be 10 million years. In scientific meaning, 10 million years is ten times one million years, but in societal meaning, it implies our extension of an unbelievable period. No one can imagine how our society would be. Nevertheless, the radioactivity survived as it is now.

Figure 7 indicates two facts; one is that the radioactivity of natural uranium is not to be decreased even after ten million years, and the other is that our activity of uranium mining, extraction, and enrichment does not influence our radioactivity for a total of natural background. The former gives the idea that radiation could not be gotten rid of in this globe and the latter gives the idea that uranium extraction and enrichment do not affect radioactivity on earth. The author believes that Figure 8 enhances the understanding of radiation in the natural environment.

Once we understand the illustration in Figure 7, acceptance of radioactive waste disposal becomes easier. Figure 8 is another example of Figure 6 but uses radioactivity left behind compared to natural uranium [21].

Some may criticize that radiotoxicity must be calculated, and a simple comparison of radioactivity has no meaning. It is true to scale the effect of radiation shall be measured with the dose of exposure. However, Figure 8 suggests the ethical values of our common world that the radioactivity generated by energy generation can be remediated to the same levels in the initial stages. Radiation protection controls radiation dose exposure to men/women in its narrower sense. However, once natural radioactivity is included, the concept of exemptions or existing exposures has been introduced to balance the flexibility of radiation protection philosophically. Thus, it is inevitable to introduce some indicators other than exposure doses.

This kind of idea in choosing indicators other than doses always invites rigorous discussions among radiation protection experts. Some radiation protection experts always emphasize that the risk must be measured via doses caused by the radiation sources to assess the detriment to our health. They are turning a deaf ear to the assessment of radiotoxicity by cumulative radioactivity or specific radioactivity alone or related parameters, implacably hating the attitude of not mentioning the doses caused by the radioactivity.

We must unify those arguments among scientists to identify the ideologies behind the struggles. They are due to the history of the inventions of radiation applications. In our immature age, we do have much misuse of radium and X-rays in our medical and aviation industries (radium painting for airplane cockpit displays). We witnessed a series of injured patients, X-ray technicians, and radium painters. In the 1930s, the first trial of protecting human rights was completed. ICRP had proposed radiation protection standards and guidelines for protecting radium ingestions [12]. Nevertheless, the world has aligned the radiation applied to the most powerful weapon in killing people. At the same time, research on radiation protection had been enhanced to protect workers of the alliance who developed and produced the atomic bomb [22].

The orchestration of the promotion of atomic bombs to kill the enemy as much as possible and the arrangement of radiation protection to lower the exposures of the alliance side attributes the conclusions to be that radiation is harmful even a bit of exposure to our body. Our hostile attitudes toward the tiny dose of exposure, avoiding any number of milli- or micro-Sieverts could be regarded as the saga of our history in atomic bomb development.

The linear no-threshold (LNT) hypothesis is widely believed in our community, especially among media people. It can be interpreted as radiation exposure being harmful indifferent of the levels of exposure. Even if the exposure is so small, no safety allowance exists for being irradiated. Major experts in radiation protection believe that LNT hypothesis is only a hypothesis and not practically applicable to our society [23]. However, the media values the sensation of radioactive substances as a source of readers’ fear, hardening the spell of the LNT hypothesis. They explained that the scientists have not yet agreed with concluding the denial of the applicability of the LNT hypothesis to our societies. In their sense, until the last scientist who supports the LNT application to our daily life of radiation exposure and radioactive substances intake becomes silent, radioactive waste is the risk of cancer in our society even if its exposure is so small.

It has taken less than 10 years to develop an atomic bomb since the discovery of fission in uranium. At the same time, a rational understanding of harmful radiation to our body seems to take more than a century. The latter is needed for the implementation of atoms for a peace accord. The scientists who can give rationales to the background of radiation protection in history are the only contributors to disseminating the meaning of nuclear energy in our society.

5. Future perspectives

The author believes nuclear energy plays an important role in saving the globe. Dr. Takashi Nagai, who submitted the first rescue report of Nagasaki atomic bomb victims, mentioned in his “Atomic Bomb Rescue and Relieve Report” that peaceful uses of atomic energy are the only one offering dedicated to the souls lost in the atomic bomb explosion in Japan [24]. The author introduces the countermeasures for reducing radiotoxicity in radioactive waste generated through our nuclear power generation, although they are still in the stage of R&D.

The once-through cycle option is selected in the U.S.A. for political reasons. The Fast Breeder Reactor cannot be in line with the nuclear power plant, since banning plutonium utilization for civil purposes has not yet been lifted. No one can predict when we can freely discuss the utilization of plutonium in our energy production. Without perspectives of fast neutron resources, the separated minor actinides may not be transmuted on an industrial scale.

Our common goal is a sustainable energy supply and nuclear energy is one of the vital candidate energy resources. Radioactive waste is believed to be the last but invulnerable to stop nuclear. We have to recall the history of nuclear energy development, and the scientists with full knowledge of radiation protection in nature will open the gate to receive the acceptance of radioactive waste for society to use nuclear energy in the end.

It must be a long way, tens of or hundreds of years, to accept nuclear energy, but the author believes in the rationales of radiation protection in wider arrangements to our environment, not limited to the narrower arrangement of controlling exposure of doses. The day will come when our society is not too sensitive to radiation as of today, but to accept radiation exposure as it is.

6. Conclusions

It has been discussed how to tame nuclear energy to achieve sustainable development goals (SDGs). Some understand it as one of the vital tools to be chosen, while others refute it. Irrational fear of extra radiation exposure to our health can only be rectified by the power of natural and social science studies for our society to understand the history of nuclear energy both for military and civil purposes. That is the only way to understand why nuclear fission is discovered, plutonium production and separation are invented by human beings.