Toxic Waste packet questions
EE 105: Crises of Planet Earth Lab #9 Laboratory #9 Waste Disposal Name: PART I. INTRODUCTION – RADIATION FACTS Radiation is a form of energy, and all living beings require some kind of radiation just to live. Light and heat, for example, are two basic forms of radiation necessary for all life on Earth. Radioactivity is the spontaneous emission of energy from certain elements, and from other elements under special conditions, in the form of particles or electromagnetic waves. Radiation and radioactivity occur naturally in the physical world. Nature is the source of 80% of exposure More than 80 percent of the radiation we are exposed to comes from such natural sources as sunlight, soil, and certain types of rocks. Cosmic rays filtering down through the atmosphere, and radon gas filtering up through the soil, are sources of natural radiation. This radiation is called background radiation. It is present everywhere, all the time, and varies greatly depending on our geographical location. In addition, people are exposed to radiation from man-made sources such as color televisions, smoke detectors, computer monitors, and X-rays. These sources account for less than one-fifth of our total radiation exposure. But, there is no difference between natural radiation and its effects and man-made radiation and its effects. Radioactivity Scientists have learned how to split atoms in a controlled process to capture the energy stored in them. When atoms are split, heat and radioactivity are produced. The intense heat produced when an atom is split can be used to turn water into steam to run turbines that produce electricity. This is the basis for nuclear power production. The radiation produced from radioactive atoms is emitted in several forms; most commonly, alpha and beta particles, and gamma rays (see Figure 1). 1 EE 105: Crises of Planet Earth Lab #9 Measuring radiation exposure and average exposures A person’s exposure to radiation is measured in units called millirem. A millirem measures the effects of radiation on the human body as much as degrees measure temperature. In the United States, a person’s average exposure to radiation is about 360 millirem per year. 2 EE 105: Crises of Planet Earth Lab #9 Where people live, as well as their lifestyles, can play a part in how much radiation they receive. The natural or background radiation exposure a person receives can vary depending on how high above sea level he or she lives and on the radioactive content of the soil and rocks in the vicinity. People who live at higher altitudes receive more exposure to radiation that comes from space. Some examples of exposure • A person taking a cross-country flight would receive about two to five additional millirem of radiation per roundtrip, depending on flight altitude and shielding on the airplane. Due to the thinner atmosphere at the altitudes involved in cross-country flights, a traveler is exposed to more cosmic radiation. • A person undergoing a full set of dental X-rays would receive about 10-39 additional millirem per set. • A person working in a nuclear power plant would receive approximately 300 additional millirem per year (the Nuclear Regulatory Commission’s limit is 5,000 millirem per year for occupational exposures). • A person living directly outside a nuclear power facility would receive approximately one additional millirem per year. EPA sets exposure limits The Environmental Protection Agency is responsible for establishing exposure limits to protect public health and safety and the environment for a repository. Federal law has directed the agency to consider recommendations by the National Academy of Sciences, and, through a rulemaking process, to establish environmental standards for a repository. The rulemaking process allows for public input into how these environmental standards are developed and instituted. The Nuclear Regulatory Commission will then incorporate the results into its criteria for licensing a repository. Reference: U.S. Department of Energy, Office of Civilian Radioactive Waste Management, 2002. 3 EE 105: Crises of Planet Earth Lab #9 PART II. INTRODUCTION TO NUCLEAR WASTE CASE STUDY OF YUCCA MOUNTAIN There are four types of nuclear waste: ∙ high-level waste ∙ low-level waste ∙ transuranic waste (e.g. clothing, rags, equipment, tools, etc.) ∙ mill tailings (leftover crushed rock). High-level waste is the most radioactive category of nuclear waste. It includes spent fuel (used fuel) from power plants and defense activities. Pellets of uranium oxide are the fuel for most nuclear reactors. These solid pellets are sealed in metal tubes approx. twice the diameter of a pencil and about 12-13 feet long. The tubes are bundled together into fuel assemblies, each containing between 50-270 tubes. The assemblies are placed in reactors and kept there for approx. 3 years, depending on the type of reactor. Then the assemblies become “spent fuel.” (aka spent fuel rods) (reference: www.citizenalert.org) This waste is now in temporary storage awaiting disposal — the question is…..where can it be stored forever? Many scientists agree that geologic disposal is the most desirable, safest, and most acceptable of permanently disposing of high-level radioactive waste. The U.S. Department of Energy was studying Yucca Mountain, Nevada as the most likely site to build a repository. 1. Radioactivity can be measured in units called Curies (1 Curie = 3.7×1010 disintegrations/second). One key isotope to dispose of is Pu-239. This isotope of plutonium has a half-life of 24,000 years. It is estimated that there will be about 10 million curies of this isotope in existence in the year 2000, most generated by the production of nuclear weapons. On the graph, illustrate the amount of Pu-239 remaining vs. time (assume no more is produced after the year 2000). About how many years will be required to reduce this inventory to 1 million Curies? 4 EE 105: Crises of Planet Earth Lab #9 YUCCA MOUNTAIN GEOLOGY Yucca Mountain consists of a thick sequence of ash-fall tuffs deposited from the Timber Mountain caldera complex to the north between 13.25 and 11.45 m.y. before the present. Block-bounding normal faults form the topography of Yucca Mountain. A brief timeline: 1957 – The National Academy of Sciences recommends geologic disposal for nuclear waste. 1970 – Lyons, Kansas selected as first repository site; rejected in 1973. 1983 – Nine sites under study in six states. 1985 – Three sites selected for further study: Deaf Smith, Texas; Hanford, Wash., Yucca Mt., Nevada. 1987 – Yucca Mountain was designated to be the only site studied. Here are some reasons: ∙ Remote location ∙ very dry climate – less than 15 cm. (or 6 in.) per yr. ∙ extremely deep water table – approx. 300- 240 m (or 720 ft.) below the level of the potential repository ∙ repository would be located in layers of volcanic tuff – a very hard, consolidated rock 2010 – Yucca Mt. no longer being considered as repository. However, geological factors will be an important consideration regardless of the next site to be studied. 5 EE 105: Crises of Planet Earth Lab #9 GROUNDWATER Groundwater is the most efficient transport mechanism to move radioactivity into the environment, both below and above ground. Groundwater was the most studied part of Yucca Mt. One of Yucca Mountain’s significant features is its deep water table (see Figures 2a and 2b). The repository would be located at a depth of 300 m (approx. 984 ft.) and still be 300-240 m above the level of the groundwater table within the zone of aeration. 6 EE 105: Crises of Planet Earth Lab #9 2. Let’s think more about groundwater. If Pu-239 was leached from the repository by groundwater, about how far could it travel in 100,000 years? Assume that groundwater flow in this area is 300 cm/yr (probably an upper limit for the Nevada desert) and that the retardation factor for Pu is 1000 (a lower limit). Hint: Pu Travel Distance = (water velocity) x (time)/(retardation factor) Recent discoveries, however, indicate that at least four zones of perched water occur naturally in the type of volcanic rock that formed Yucca Mt. Perched water is a pocket or zone of water trapped within layers of rock or rock fractures and is isolated from the main water table (see Figures 3). 3. What does the presence of a perched water table indicate about the ability of water to travel through the subsurface, and why might this be of concern? Consider this — In 1995, a government geologist discovered traces of Chlorine 35, a byproduct of the 1950s-era atmospheric nuclear weapons testing, in small amounts of rainwater that had seeped down through cracks in Yucca Mt, to about 800 ft. below the surface. If it took only half a century for rainwater to penetrate into the ground, then is it possible that the groundwater could be contaminated with this chemical, and could it reach nearby dairy farms? This raises the possibility that geologic isolation alone might not be enough to protect the groundwater. 7 EE 105: Crises of Planet Earth Lab #9 CLIMATE CHANGE ⇒ CHANGE IN WATER TABLE Today, the region near Yucca Mt. has a very dry climate – less than 15 cm. (or 6 in.) of rain per year. Climate, however, changes throughout geologic time. There was higher moisture for 70-80% of the last 2 million years. 4. How would the water table be affected by an increase in precipitation – and what are the potential effects with respect to the repository? PART VI. VOLCANISM AT YUCCA MT. Yucca Mt. was formed between 13.25 and 11.45 million years ago by a series of volcanic eruptions. These eruptions deposited ash and material which compressed together to create layers of rock called tuff. Seven small cinder cone volcanoes are located near the proposed repository site. Cinder cones generally form during a single eruptive event. Two cones are located about 19 to 43 km (12-27 mi.) away and may have been active within the last 10,000 years. The other five, located 13 to 43 km (8-27 mi.) away, had their last eruptions from 300,000 years to 1.2 million years ago. 5. If a cinder cone eruption were to occur at Yucca Mountain in the next 10,000 years, could such a volcanic eruption affect the repository? Explain. 8 EE 105: Crises of Planet Earth Lab #9 EARTHQUAKE ACTIVITY The mountain crest achieved its present form largely from faulting activity, particularly along its western base. Major trenching programs will study fault movement during the past two million years. Modern Global Positioning System (GPS) studies indicate that tectonic deformation is still occurring in Nevada, which means that there must be active faults somewhere in the region. 6. Examine the cross-section of this area on page 10. a) How would you describe the orientation of the rock layers? (Think back – we know that rock layers start off as horizontal and laterally continuous. Is that the case for these rock units?) b) Note that the offset (or the distance along a fault that a rock unit has been displaced) is quite variable for the rock layers in this region. Make an estimate of one of the larger amounts of offset that you can see in this figure. (Remember that such large offsets do not occur in a single earthquake event, but happen due to recurrent events) PART VIII. WHAT DO YOU THINK? 7. Considering all of the above discussion/information, in your opinion is Yucca Mountain a safe place to store nuclear waste? Why or why not? Include in your answer what you consider the most important environmental or geologic factors. 8. Can you think of possible alternatives to nuclear storage at Yucca Mountain? 9 EE 105: Crises of Planet Earth Lab #9 10 EE 105: Crises of Planet Earth Lab #9 PART III. OTHER PROBLEMS WITH WASTE DISPOSAL 1. Talkin’ Trash. On average, every man, woman, and child generates 2 kg of trash per day!!! a. Assuming a population of 4.7 million people, how much trash does the Boston area produce in 1 day? b. If the trash has a density of ~500 kg/m3, estimate the volume of this trash. c. The volume of TD Garden is about 640×103 m3. How many days would it take to fill TD Garden with the trash we generate? 2. Landfill Sites. Two sites (A and B, sketched below) are considered as potential landfill sites. We want to prevent the potential pollution leaching from a landfill from reaching groundwater (and thereby reaching wells). Which would you recommend as the better site? Briefly explain your reasoning. 11
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