Radioactive Waste Engineering and Management

All kinds of activities associated with radioactive wastes generated as a result of nuclear utilization are often collectively referred to as “radioactive waste management.” This term is used in a broader sense than a mere series of technical processes to convert generated wastes into waste forms for storage and disposal.

Mining refers to the act of collecting ore containing a target metal from mines. This process generates gangue—rock that is commercially valueless and therefore subject to disposal. The process of extracting metal from its ore is called melting. Uraninite, pitchblend and brannerite are uranium containing ores, and they contain uranium in the form of oxide. Their grade generally ranges from 0.1 to 0.3 % triuranium octoxide (U₃O₈) equivalent. Although extracted ore of ordinary metals undergoes “dressing,” the process of separating valueless rocks based on physical or chemical characteristics by such methods as fire refining and aqueous refining, the dressing process is not effective for low-grade uranium ore. This kind of ore is crushed into pieces and dissolved in acid or alkali solution. Then uranium is refined and concentrated, followed by precipitation using strong alkali. An intermediate from this milling process is uranium concentrate, which is U₃O₈ powder, called yellow cake for its color. Its uranium content (U₃O₈ content) is about 70–80 %. This material is further refined (or purified) to increase purity and is converted to forms such as UF₆, UO₂ or metallic uranium, suitable for use as reactor fuel in the next process at fuel fabrication facilities. In this connection, refining means increasing the purity of metal resulting from melting by electrolysis or other processes; in the uranium melting process, purification corresponds to refining.

Decommissioning is a series of measures taken after the main activities associated with a licensed activity or reactor have been terminated and before the regulations set forth in the Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors [1] (hereinafter referred to as “Reactor Regulation Act”) are fulfilled, including the transfer of nuclear fuel material, elimination of contamination caused by nuclear fuel material, and disposal of nuclear fuel material or other materials contaminated with nuclear fuel material. Therefore, the dismantling of facilities, which is undertaken after the main activities associated with a licensed activity or reactor have been terminated, is also included in decommissioning. Decommissioning is thus a process to reduce the residual radioactivity of such facilities to the levels necessary for fulfilling the regulations set forth in the Reactor Regulation Act. Because these measures produce various types of radioactive wastes in large amounts in a short period of time, the concept of radioactive waste management needs to be actively incorporated into the planning and implementation of decommissioning. If a decommissioning plan is not adequately formulated, there is a possibility that material that does not need to be handled as radioactive wastes may be improperly classified and handled as such. Furthermore, depending on the dismantling method selected, the amount of secondary wastes generated may increase or decrease and the disposal method for the wastes may also vary. It is therefore important to develop a decommissioning plan based on analytical evaluation, operating history surveys, measurement evaluation and other advance surveys as well as the latest dismantling technology studies. As explained above, there is a close relationship between decommissioning and radioactive waste management.

The term “clearance” refers to the idea that if an exposure dose due to a very low-level artificial radioactive material is sufficiently smaller than natural background radiation and human health risk is negligibly small, that artificial radioactive material does not need to be treated as a radioactive material, and therefore the material may be released from regulatory control even if the category to which the material belongs is under regulatory control for radiation protection.

As discussed in Sect. 1.​3.​1 “Radioactive waste treatment and disposal processes,” the treatment of radioactive wastes includes three steps: pretreatment, treatment and conditioning; these three processes are not performed independently of each other. Instead, appropriate pretreatment, treatment and conditioning methods need to be selected based on the characteristics of the wastes to be treated (e.g., the amount, physical and chemical properties, contained nuclides, and radioactivity level) and also by taking into account both the interconnectedness of the individual steps of the treatment processes and the disposal process subsequent to the treatment processes. These matters are discussed as Principle 8 “Radioactive Waste Generation and Management Interdependencies” of the IAEA’s Principles of Radioactive Waste Management Safety Fundamentals [1].

The first concept of geological disposal proposed in history is probably the direct disposal of high-level radioactive liquid wastes in salt formations indicated in a 1957 report [1] prepared by the U.S. National Academy of Sciences (NAS). The concept, however, differs considerably from today’s concept of geological disposal in that, for example, the plan in those days was to directly inject liquid waste and the time span considered was only 600 years. The basic ideas of today’s disposal systems are from the concept indicated in the so-called Polvani Report [2] in the 1970s and the KBS concept [3] developed in Sweden in the early 1980s. The basis, therefore, had been established by the end of the 1980s. Today, R&D on geological disposal systems is underway in more than 30 countries [4].

When geological disposal is planned, the first question to be answered is: How safe is it? This is the most fundamental question to be answered, and it comes not only from the regulatory authorities but also from the general public, waste generators and waste disposers. Performance assessment, therefore, has been carried out over the years to answer this question.

Like the derivation of exhaust air and waste water radioactivity concentration limits based on the reference dose values for radiation protection, clearance levels are derived by following these steps:

Chapter 7 explained that the transport of radionuclides affecting the performance assessment of radioactive waste disposal can be described with diffusion equations or advection–dispersion equations. This chapter shows some basic mass transfer problems incorporating diffusion, advection/dispersion, radioactive decay, and sorption retardation along with methods of solving them.

In thermodynamics, the total energy that the system under consideration has is called internal energy (U). Let w represent the amount of work done to a system and let q represent the amount of energy added to the system in the form of energy. Then, the amount of resultant change in internal energy, ΔU, can be calculated as Eq. (10.1).