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ike the terms cause and effect, the terms energy and environment are a natural fit. One of the most dramatic manifestations of this relationship is the problem of what to do with the estimated 24,000 metric tons of high-level radioactive waste now on hand in the United States as a result of decades of nuclear reactor operations. Even without the addition of any new nuclear power plants, this waste continues to accumulate and is expected to exceed 85,000 metric tons early in the next century. The waste must be sealed in containers that will last at least 1,000 years. These containers must, in turn, be buried within geologic barriers that will isolate the waste for at least 10,000 years-a time span ranging from before construction of the Egyptian pyramids until the year 5,000 AD. The Department of Energy has proposed constructing the nation's first permanent underground repository at Yucca Mountain in Nevada. However, before the repository can be constructed, the Yucca Mountain site must be thoroughly characterized to ensure that once the nuclear waste is buried, none of it can ever escape into the environment. Berkeley Lab earth scientists, working in conjunction with the U.S. Geological Survey, have developed a three-dimensional site-scale computer model of Yucca Mountain. The model simulates the flow of moisture, gas, and heat through the unsaturated zone (the soil between the ground surface and the water table) of the site. For the safe containment of the nuclear waste, it is essential to know how much water percolates through the unsaturated zone and to what degree it picks up radionuclides. Waste managers must also know how rapidly air that could contain gaseous radionuclides flows to the ground surface. The Berkeley Lab computer model allows users to predict the results of any abrupt changes in climatic conditions on moisture and gas flow through the unsaturated zone and its major faults and fracture zones.

Another dramatic manifestation of the cause and effect relationship between energy and environment can be seen-literally-in the Los Angeles basin, where the enormous reliance on automobiles has taken a severe toll on air quality. A recent plan to reduce ozone in the Los Angeles air basin calls for a 75-percent reduction in the emission of nitrogen oxides and reactive organic gases using unspecified technologies. Many have questioned the cost and technical feasibility of this plan. To provide a better understanding of ozone-formation in air basins, Berkeley Lab scientists have begun to develop an ambitious computer model. This model will enable users to investigate-through computer simulations-alternative ozone reduction strategies that could prove more practical and cost-effective than the current plan for Los Angeles. Early in its development, the model has already been used to demonstrate that reducing emissions to different levels in different areas of the basin would achieve the same air quality as the current strategy with only modest controls required. For example, emissions reductions of 30 to 50-percent in select basin areas yield air quality results comparable to the 75-percent basin-wide reductions now proposed.

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