Answer to Question #10009 Submitted to "Ask the Experts"

I have always wondered why radioactive waste from nuclear plants has to stay radioactive for so long. Why can't the leftover "stuff" be melted down and mixed with molten lead, then case hardened with hydrogen gas—kind of like a nitriding process? Would the lead not slow down the radiation inside and the hydrogen skin stop the neutrons? Why could one not just drop this stuff into a volcano where it would become encased in rock? Just wondering.

Each radioactive nuclide has a unique half-life that dictates the rate of decay of the radioactive species and, therefore, the time it will remain radioactive. Certain materials are chemically hazardous and are regulated by the US Environmental Protection Agency; lead is one of them. Mixing radioactive materials and chemically hazardous materials, such as lead, creates mixed waste. Disposing of mixed waste is more complex than disposing of the individual hazards alone. In addition, only certain facilities are authorized to process and dispose of mixed waste at specialized facilities.

In the same manner, hydrogen gas is an extremely unstable and flammable gas. There are known cases where hydrogen gas can form explosive mixtures with air. Hydrogen gas is also an oxygen displacement agent and may cause asphyxia due to the lack of oxygen. Hydrogen-rich materials (e.g., water, polyethylene) are used to shield neutrons. However, their shielding properties depend on their thickness; the thicker the shielding layer, the better they will shield for neutrons. The interactions between the neutrons and the shielding create secondary radiation that also needs to be shielded. Thus, high-density materials (e.g., steel) are used to shield the secondary radiation.

Vitrification is a process used to stabilize and encapsulate high-level radioactive waste. In the vitrification process, radioactive waste is mixed with a substance that will crystallize when heated (e.g., sugar, sand) and then calcined. Calcination removes water from the waste to enhance the stability of the glass product. The calcinated materials are continuously transferred into a heated furnace and mixed with fragmented glass. Upon mixing, the radioactive waste is bonded to the glass material. The melted product is subsequently poured into an encapsulation container (e.g., stainless steel liner). Once the contents cool down, the melt solidifies into the glass matrix. The encapsulation container is ultimately sealed and the waste stored in disposal repositories. Vitrification allows the immobilization of the waste for thousands of years.

The Nuclear Regulatory Commission (NRC) and its Agreement States have strict requirements for the disposal of radioactive waste. The applicable disposal limits are based on the characteristics of the radioactive waste to ensure the protection of members of the public, workers, and environs. Proper disposal of radioactive waste must ensure that the probability of its release to environs upon disposal is minimized to the extent practicable, and that the disposal container retains its integrity and functionality over changes of pressure, temperature, humidity, etc., for the "active life" of the disposal facility. Therefore, approved radioactive waste-disposal facilities are constantly monitored and attended to ensure the public and environs doses remain as low as reasonably achievable (ALARA.)
 

Additional specific information regarding the production and disposal of radioactive waste may be found in publications issued by the US NRC: NUREG-1853, History and Framework of Commercial Low-Level Radioactive Waste Management in the United States; NUREG-6216, Revision 2, Radioactive Waste: Production, Storage, Disposal; NUREG/CR-6567, Low-Level Waste Classification, Characterization, and Assessment: Waste Streams and Neutron-Activated Metals; 10 CFR 20, Standards for Protection Against Radiation; and 10 CFR 61, Licensing Requirements for Land Disposal of Radioactive Waste.

Manuel Diaz
Health Physicist