Nuclear Waste
Nuclear Waste

Waste produced with the nuclear energy is one of the greatest problems. The waste is generally radioactive, and thus toxic. There are also a few different kinds of waste, depending on how it was produced. Nuclear waste is produced in many different ways. There are wastes produced in the reactor core, wastes created as a result of radioactive contamination, and wastes produced as a bi-product of uranium mining, enrichment, and refining.

In nuclear waste the vast majority (99%) of radiation is given off from spent fuel rods. However, fuel rods make up a relatively small percentage of the volume of waste. The largest volume of nuclear waste is composed of the leftovers from the mining process. This waste, however, doesn't give off much radiation. Some of the nuclear waste is extremely long-lived; meaning that it lasts a long time without its toxicity decreasing all that much, and some of it is very short-lived. Some types of nuclear waste are considered high-level and some are considered low level. The difference is in the amount of radioactive nuclei in relation to the mass of the waste. If there are a large amount of radioactive nuclei relative to the amount of waste, it is considered high-level nuclear waste.

When a 235U atom splits, it can produce a number of different products. Many of these are radioactive elements. For example, the following reaction produces 90Sr, which has a half-life of about 29 years.

1 neutron + 235U 2 neutrons + 90Sr + 144Xe

Although its half-life is 29 years, a quantity of 90Sr is not considered safe for 290 years. After 290 years, 10 half-lives would have passed. So, if we started out with half a ton (1000 lbs.) of 90Sr, after 290 years there would be 1000 x (1/2) 10 left. This is about a pound. The rest of the 90Sr would have undergone ß- decay, producing 90Y. 90Y is also radioactive, but is has a very short half-life of about 2.67 days. The 90Y undergoes ß- decay, forming 90Zr, which is a stable, non-radioactive isotope. 90Sr is particularly dangerous because it shares many of the same chemical properties as calcium (Ca), and, if ingested, can take calcium's place in your bones. Then, when 90Sr decays, the radiation released in your body can cause cancer.

The fission products, or fragments, usually remain within the fuel rods of the reactor. When most of the 235U in a fuel rod is spent, the rod must be removed. The radioactive fragments are what make the spent rods toxic. The fission products can be long-lived or short-lived.

Transuranics

The 238U in a fuel rod is not fissile and is a neutron absorber. Because it absorbs neutrons, it stops the chain reaction in a nuclear power plant from running away (and producing a nuclear bomb effect). This is a good thing. However, think about what it means when we say "238U is a neutron absorber". The following reaction expresses that statement:

1 neutron + 238U 239U

When 238U "captures" a neutron, it is added to the original uranium nucleus, producing the radioactive isotope of uranium, 239U. This isotope has a half-life of 23.45 months. It decays, through ß-, into 239Np. 239Np is also radioactive and decays into 239Pu. 239Np has a short half-life of about 2 days. This sequence of decays can be expressed like this:

1 neutron + 238U 239U

239U (ß- decay) 239Np

239Np (ß- decay) 239Pu

239Pu is also radioactive, and has a half-life of approximately 24,000 years. That's a long time!! A lot of 238U is turned into 239Pu through this sequence of decays. 239Pu is called a transuranic element. Any element with a higher atomic number (and thus more protons) than uranium is considered to be transuranic. This applies to all of the elements to the right of uranium in the Periodic Table. In the equations above we showed how a neutron can be captured by a nucleus and, through a series of ß- decays, can produce an isotope with a higher atomic number than the original atom. More than one neutron can be captured. So, for instance, a neutron can be captured again by 239U. This produces 240U, which decays into 240Np. If 240Np captures another neutron, it becomes 241Np, which then decays into 241Pu and then into 241Am, which has a half-life of about 400 years. This sequence of decays and neutron additions can be expressed in the following reactions:

1 neutron + 238U 239U

1 neutron + 239U 240U

240U (ß- decay) 240Np

1 neutron + 240Np 241Np

241Np (ß- decay) 241Pu

241Pu (ß- decay) 241Am

The transuranic neutron addition products usually remain in the fuel rods, where the original 238U from which they were produced was located. This adds to the rods' toxicity, and makes it harder for them to be disposed. In general, transuranic wastes are long-lived. However, this depends on the isotope produced. The biggest transuranic waste produced is 239Pu. This is an extremely toxic and extremely long-lived compound. 239Pu is fissile. In fact, when a nuclear reactor's fuel rods are almost spent, as much as 30% of the reactor output can come from the fissioning of 239Pu. Thus, the plutonium transuranic "waste" produced in a nuclear reactor can actually be used as fuel.

Waste from Uranium Mining and Enrichment

While mining, Uranium has to be separated from rock. This produces pure uranium ore and "tailings", essentially leftover rock that has had the uranium stripped from it. This rock often still contains radioactive nuclides and is somewhat dangerous. The tailings are generally long-lived, but are considered to be low-level waste. That is, the concentration of radioactive nuclei in them is small, and thus they are not extremely radioactive.

Uranium ore is only about .7% 235U. It must be enriched to bring the percentage of 235U up to about 4%. The enrichment process produces a lot of waste. This is because for every gram of enriched uranium fuel produced, there are about 4 grams of 238U waste. 238U is radioactive and has a half-life of 4,468,000,000 years. This means that it is long-lived, but not extremely dangerous. However, some of its "daughter products" are radioactive. Thus, wastes produced as a result of enrichment must be kept in storage. By the way, a "daughter product" is an isotope that results from a decay of another, "parent", isotope. For example, when 238U decays, it produces 234Th, which is very radioactive and has a half-life of about 24 days. The decay can be expressed in the following equation:

238U (decay) 234Th + particle

Contaminated Materials

Nuclear waste’s major portion is comprised of spent fuel rods. These contain the fission products and transuranic wastes we mentioned above. However, a lot of other waste is produced in the reactor besides the fuel rods. This results radioactive contamination.

A nuclear reactor is extremely hot. This means that the particles inside the reactor are very energetic and are flying around at incredible speeds. Occasionally, an atom that is in a fuel rod can get knocked out. These atoms that get knocked out can be many different types, ranging from fission products to uranium to transuranic elements. Most are radioactive. Atoms that escape the fuel rod careen all over the inside of the reactor core. Eventually these atoms can strike something solid. This is a lot like a bullet hitting a wall. If the wall is small, it might pass through. However, if the wall is big enough, the bullet will smash into the wall and "stick" there. So it is with a nuclear reactor. Occasionally an atom can smash into a structural component of the reactor, implanting itself into it. Because many of the nuclides (fancy term for atomic nucleus) careening about the core of a fission reactor are radioactive, when they smash into a structure and "stick", they make that structure appear to be radioactive. This is because there are many radioactive nuclides embedded in it, which give off radiation. Thus, many of the structural components of a reactor become radioactive over time, as they absorb radioactive nuclei into themselves. Also, many of the pipes and other components of a reactor become radioactive. These must be replaced eventually because over time the extreme radiation inside the reactor weakens them. The biggest problem, however, arises when a nuclear reactor is turned off for good, or "decommissioned". Due to extremely radioactive the disposal of the reactor core is a huge problem.

Primary Radiation Waste and Nuclear Waste Products

In the United States, it is the process from which the waste was produced, and not the characteristics or radioactivity of the waste itself, which determines the waste's classification. This is the fundamental problem with radioactive waste classification in the United States. Radioactive waste is divided into 5 legal categories for classification purposes:

Types of Nuclear Waste:

  • High-level Waste (HLW)

High Level Waste (HLW) is produced by nuclear reactors. It contains fission products and transuranic elements generated in the reactor core. It is highly radioactive and often thermally hot. HLW accounts for over 95% of the total radioactivity produced in the process of nuclear electricity generation. The amount of HLW worldwide is currently increasing by about 12,000 metric tons every year, which is the equivalent to about 100 double-decker busses or a two-story structure built on top of a basketball court.

  • Transuranic Waste (TRU)

Department of Energy (US) mostly uses TRU classification. Waste generated outside of DOE facilities which meets the definition of TRU is generally classified as LLW in either the "Class C" or "Greater than Class-C" categories. The reason for this is that the categories of LLW had not yet been implemented or developed when the DOE required an intermediate level of waste, which were not high-level waste, or what they had defined as low-level waste at the time. Thus, the "low-level" waste category may consist of waste, which qualifies as TRU waste under DOE rules, but TRU waste must always meet the exact definition to be called transuranic waste.

TRU is not simply made up of materials that appear after uranium on the periodic table, as the name implies. TRU waste contains more than 100 nanocuries (3700 becquerels) of alpha-emitting transuranic isotopes, with half-lives greater than twenty years, per gram of waste, except for (1) high-level radioactive waste; (2) wastes that the Secretary of Energy has determined, with the concurrence of the EPA, do not need the degree of isolation required by EPA's high level waste rule; or (3) has approved for disposal on a case-by-case basis in accordance with National Research Council’s (NRC) radioactive land disposal regulation. TRU is mainly produced from the reprocessing of spent nuclear fuel, reactor fuel assembly, and the use of plutonium to fabricate nuclear weapons. TRU waste is categorized as either Contact Handled (CH) or Remote Handled (RH). RH-TRU is any transuranic waste with a surface dose rate of 200 millirem per hour or greater, and CH-TRU is any TRU waste with levels which are lower. DOE is currently proceeding with plans for TRU waste disposal at the geologic repository called the Waste Isolation Pilot Plant (WIPP).

  • "Low-Level" Waste (LLW or LLRW)

Low-level waste (LLW) is generated from hospitals and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters, etc., which contain small amounts of mostly short-lived radioactivity. Commonly, LLW is designated as such as a precautionary measure if it originated from any region of an 'Active Area', which frequently includes offices with only a remote possibility of being contaminated with radioactive materials. Such LLW typically exhibits no higher radioactivity than one would expect from the same material disposed of in a non-active area, such as a normal office block. Some high activity LLW requires shielding during handling and transport but most LLW is suitable for shallow land burial. It is often compacted or incinerated before disposal to reduce its volume.

"Low-level" waste is defined by what it is not and consequently is the broadest category of waste. In fact, it was been said that LLW as a bulk category contains more radioactivity than high-level waste; because the amount of material by it's sheer volume has a greater amount of radioactivity in curies. Industries, hospitals and medical, educational, or research institutions; private or government laboratories; and nuclear fuel cycle facilities (e.g., nuclear power reactors and fuel fabrication plants) using radioactive materials generate "low-level" wastes as part of their normal operations. These wastes are generated in many physical and chemical forms and levels of contamination.

LLW has four subcategories: Classes A, B, C, and Greater-Than Class-C (GTCC), described below.

On average, Class A is the least radioactive while GTCC is the most radioactive.

Class A

- Primarily contaminated with shorter-lived radionuclides.

- Contains the least radioactive materials in the LLW classes.

- Average concentration: 0.1-curies/cubic foot.

Class B

- More short-lived radionuclides than class A.

- Average concentration: 2-curies/cubic foot.

Class C

- More long- and short-lived radionuclides than classes A or B.

- Average concentration: 7-curies/cubic foot.

Greater-than-Class C (GTCC)

- The most radioactive of all of the LLW classes.

- Average concentration: 300-to 2,500 curies/cubic foot.

  • "Low-Level" Radioactive Waste:

  • Irradiated Components and Piping -- reactor hardware and pipes that are in continual contact with highly radioactive water for the lifetime of the plant. The metal becomes activated, or radioactive, by the bombardment of neutrons in the reactor area. Also referred to as "irradiated primary system components."

  • Control Rods -- from the core of nuclear power plants, these rods regulate and/or stop fission chain-reactions in the reactor by absorbing neutrons.

  • Poison Curtains -- also absorb neutrons, but from the water in the reactor core and the irradiated fuel pool

  • Resins, Sludges, Filters, and Evaporator Bottoms -- residues and cleaning wastes from the water that circulates around the irradiated fuel in the reactor vessel and in the fuel pool, which holds the irradiated fuel when it is removed from the core.

  • Entire Nuclear Power Plants -- When decommissioned, everything from the entire reactor vessel (minus the spent fuel rods) to the concrete floor is considered "low-level" waste. A typical 1000-megawatt reactor-building floor contains 13,000 cubic feet of contaminated concrete, and 1,400 cubic feet of contaminated reinforcing steel bar.

Radioactive waste is generally referred by Industry and government in terms of volume, even though there can be tremendous concentrations of radioactivity in a small volume, and small concentrations in a larger package. All of the medical waste from diagnosis and treatment shipped in a year from most states usually gives off a fraction of one curie in radiation. The average pressurized water reactor produces 1,900 curies per year, and the average boiling water reactor produces 4,000 curies in "low level" waste every year.

  • Byproduct Wastes

Byproduct Wastes is generated during uranium milling, where wastes known as tailings are left behind, and in the extraction or concentration of uranium or thorium from ore. Much of the byproduct wastes also originates from Cold War bomb development projects, which were conducted at sites across the country.

Uranium mill tailings are the earthen residues that remain after the extraction of uranium from ores. Tailings are generated in very large volumes and contain low concentrations of naturally occurring radioactive materials. These materials comprise a potential health hazard; the isotopes of major concern are Radium-226 and its daughter, Radon-222.

In 1978, the Uranium Mill Tailings Remedial Act (UMTRA) was passed, which created a legal definition of the Atomic Energy Act of waste generated from extracting uranium or thorium from ore. It did not cover waste from uranium mining. The reason for this is that uranium mining is not formally considered part of the uranium fuel chain. Interestingly, however, it does cover some waste from uranium conversion (pre-enrichment). Natural ores that are processed for rare earth or other metals have significant concentrations of radioactive elements. The tailings produced (which consist of the crushed depleted ore and the depleted solution after recovery of metals and rare earths are not material. This is because the ore was not processed primarily for its source material content but for the rare earth or other metal.

DOE created the Formerly Utilized Site Remedial Action Program (FUSRAP) to address radiological contamination at sites used by DOE's predecessor agencies, the Manhattan Engineer District (MED) and the Atomic Energy Commission (AEC) from the 1940s through the 1960s. The FUSRAP program was transferred from the DOE to the Army Corps of Engineers by the Energy and Water Development Appropriations Act for FY98.

When UMTRA passed, DOE sites, which qualified, were placed into two categories, either UMTRA Title I or Title II. Title I were sites operated prior to 1978, and Title II were sites operated afterward. At milling sites, contaminated materials such as cattle fencing are classified as Byproduct wastes due to the activities designated at the property. As in other waste categorizations, it is the process from which the waste was produced, and not the characteristics of the waste itself, which determines the classification.

Naturally Occurring or Accelerator Produced Radioactive Materials (NARM/NORM/TENORM)

These are radioactive materials not covered under the AEA that are naturally occurring or produced by an accelerator. These materials have been traditionally regulated by States. Natural occurring NORM contains radio nuclides (e.g., Ra-226, Rn-222, Th-232, U-238) existing throughout the earth's crust. NORM waste with more than 2 nCi/g of 226Ra or equivalent is commonly referred to as discrete NORM waste; below this threshold, the waste is referred to as diffuse NORM waste. NORM waste is not covered under the AEA, not a form of LLW And is not regulated by NRC. Accelerator wastes include accelerator targets, wastes from accelerator maintenance or D&D, and wastes from radiopharmaceutical manufacture.

Discrete NORM wastes have a relatively small volume but large radioactivity and include industrial gauges, old radium watch and industrial dials, radium needles in medical equipment, resins (filters) that remove radioactive radium from public drinking water, and some radio pharmaceutical waste. Diffuse NORM wastes are characterized by a relatively large volume with small radioactivity. These materials result from industrial processes and include: coal ash and slag from utility electrical generation; solid wastes from geothermal energy production; slag, leachate, and tailings from the mining and processing of metals other than uranium or thorium (e.g., copper); sludge from drinking water treatment; scale, sludge, produced water And equipment from oil and natural-gas production containing NORM; and wastes (phosphogypsum and slag) from mining phosphate ores for fertilizer (ammonium phosphate) production.

Naturally Occurring Radioactive Materials (NORM) is a subset of NARM and refers to materials whose radioactivity has been enhanced (radionuclide concentrations are either increased or redistributed where they are more likely to cause exposure to humans) usually by mineral extraction or processing activities.

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