Nuclear waste management at Yucca Mountain

Nuclear waste management at Yucca Mountain May 2002

Shani Wallis, TunnellTalk

Following 20 years of intense study, Yucca Mountain in Nevada has been recommended to the US Congress as the site for long term management and underground storage of the country's growing stockpiles of high-level radioactive nuclear waste. In February 2002, the Secretary of Energy recommended to the President that Yucca Mountain be developed as the first US repository for spent nuclear fuel and high-level radioactive waste. Endorsement the day after by President Bush launched the project on what will remain a long road to gaining Congressional approval and obtaining the many necessary permits before construction of the facility can continue and storage of the waste can begin.

In 1987 Yucca Mountain was selected from an initial nine possible sites and a short list of three, as the one location to be taken forward for study in detail. Over the two short listed alternatives - a granite rock site in Washington State and a bedded salt formation in Texas - Yucca Mountain had the decisive advantages of an extremely arid climate, and a geological formation within which the hydro geologic conditions have changed little, if at all, for millions of years. In addition, the site, about 145km northwest of Las Vegas, is remote from centers of concentrated population and is on federally protected land adjacent to the government's nuclear test site, established more than 50 years ago.

Since 1987 the 1,000-acre site has been subjected to intense investigation. More than 75,000ft of core has been retrieved, 451 bore holes of up to 5-6,000ft (1,500-2,000m) have been drilled, 214 pits and trenches have been excavated, and in the early and mid-1990s contractor Peter Kiewit excavated two TBM-driven exploratory tunnels. The larger 7.6m diameter main tunnel on its U-shaped alignment and with a portal at both ends will eventually become the main railway access to the planned repository. The smaller 5m diameter exploratory adit branches off the main tunnel and runs uphill to access an alignment some 15m above and across the width of the planned repository.

If approved, the scope of future excavation for phased development of the repository is tremendous. The footprint of the facility is 1km wide x 6.5km long and comprises some 50 storage tunnels or 'emplacement drifts' for a total of some 56 miles or 90km of 5.5m to 7.6m diameter tunneling. In addition there is an elaborate ventilation system comprising several deep shafts and a shaft connection from each emplacement adit to a central service tunnel that runs the length of the footprint beneath the repository.

"The methods by which this volume of excavation will be procured, is yet to be determined," said Patrick Rowe, Senior Engineer/Scientist with the Management & Technical Support division of the project's Site Characterization Office. "But if approved, the facility will be the largest single construction project undertaken by mankind. Designed to hold 70,000 metric tonne of waste, the current recommended proposal for the facility will cost an estimated $58 billion in fiscal-2000 dollar values. It is planned to be built in phases over a 22-year period and will take 24 years to fill."

As with the earlier access and exploratory tunnels, excavation of the principal underground structures will be by TBM with roadheaders excavating TBM assembly chambers and cutting the turnouts into the emplacement adits. The various shafts are planned as raisebores. To avoid damaging the host material, drill+blast is considered for only limited excavation.

Suitable environment
The arid environment and geological character of Yucca Mountain is its most significant benefit as a repository. The greatest potential for contamination from an underground repository is via contact with water. "There can be no transportation of radio active nuclei, under the expected conditions, without water," said Rowe. "At Yucca Mountain, rain fall at the top of the ridge is about 7.54 inches/year and the hydrological basin of the valley is completely contained. There is no link at all with any adjacent watershed and no connection of the deep ground water table to any river, lake or sea. There is a: natural moisture content of about 8-10% in the layers of predominantly welded volcanic tuff geology, but the repository is on average 300m below the top of the mountain and about another 300m on average above the ground water table. Porosity in the welded volcanic tuff is low at 10-12% and even if saturated at 80% water content, there is little water movement through the material. Moreover, in this isolated hydrological basin, across the ridge from Death Valley in the Amargosa Desert, some 95% of the low rainfall is given up to evaporation or transpiration by desert vegetation, with only about 5%, about 0.1in of precipitation under the current climate state, percolating very slowly towards the deep water table."

For excavation, the welded tuff at the level of the repository provides for ideal TBM tunneling conditions. Rock strengths are medium to strong at up to 8.9 GPa and while there are fractures, there are no major faults in the repository block. The site is within the influence of the Pacific Rim earthquake zone but the layered strata of tuff deposits are relatively undisturbed. For long-term durability, the facility's surface and underground structures are designed to withstand a 7.5 Richter scale earthquake at an epicenter some 45km distance away and a 6.3 scale event with an epicenter at 6km.

As with earlier access and exploratory tunnels, the greater concern during excavation is to avoid introducing water. Only limited water spraying can be used for dust suppression and other construction needs.

To perform as geological exploratory drifts, lining in the existing two tunnels is limited comprising steel sets, wire mesh, rock bolts, and shotcrete at some critical intersections. The detailed design of the primary support and final lining systems of the emplacement adits and other underground structures of the proposed repository, is yet to be confirmed, said Rowe, although precast or in-situ concrete linings are not envisaged and shotcrete application will be limited. "This will prevent any possible leaching of a1kali in the cement should the hydrology of the area change during the facility's extraordinary design life of many hundreds of years."

Isometric of the repository within the geological strata of Yucca Mountain

Dissipation of heat is a major design concern. Nuclear waste is not only 'hot' radioactively but also thermally, emitting temperatures of 200-250ºC. According to Rowe, it is impossible for the specially graded and packed waste to go critical in a repository, but controlling the build up of heat is required. Through selective packaging, heat from the envisaged storage canisters or 'packages' could heat the walls of the emplacement drifts to 150ºC. Because the rock at Yucca Mountain is a good thermal insulator, and because of the existence of long-term actinides the rock surrounding the waste could be above the boiling temperature of water for between 300 and 1,000 years. Positive long-term ventilation is required to cool air temperature in the repository to about 85°C.

Heat energy, like the radioactivity of the waste, decays with time and starting with an initial half-life of about 55 years for cesium and about 29 years for strontium, most of the radioactive nuclides in the waste will have decayed within seven half lives or a total of 300-500 years. Temperatures will also have reduced significantly.

Other radionuclides remain radioactive indefinitely or eternally and since these will be held in high concentration in the repository, the proposal is to entomb the waste packages after 300 years using concrete backfill and bentonite plugs.

Until then, phased filling of the repository will be a non-man-entry operation. All transportation of storage packages in shielded rail cars from the surface handling and packaging facilities into the repository and placement in the drifts will be undertaken using remotely controlled systems.

In storage, the waste is contained within several engineered barriers designed to prevent escape of nuclides or contact with water. The package canisters have walls about 20 times thicker than a propane tank and are built of the highly corrosion-resistant Alloy 22 and stainless steel. There is a titanium drip shield over the waste packages and a special ballast material in the invert of the emplacement adits to limit transport of radionuclides. Additional natural barriers for containment include the presence of ziolites in the volcanic tuff that behave much like charcoal to absorb radionuclides and the fact that the very slow velocity of minute volumes of percolating ground water will flow naturally around, rather than into the tunnels of the repository. Also the immense heat given off by the waste will heat the natural moisture content of the host rock, driving it away from the heat source to where it can condense once again and flow downward with gravity.

Suggested water movement around a waste package emplacement drift

In addition to forced ventilation, other factors will help control the build up of heat. The packages will be stored at a minimum distance from each other and the parallel emplacement adits will be spaced at about 90m centers. Within the emplacement adits, the free space between the 5.5m diameter of the tunnel and the proposed 1.3-2m diameter of the packages, required for efficient remotely-controlled placement and possible retrieval of the packages, will assist efficient ventilation circulation.

Following approval of the development recommendation by the Bush administration in February and transfer of the Bill to Congress for debate, the Governor of Nevada has given official notice of his disapproval of the proposal as was expected and as was his right within 60 days of the President's announcement. Congress now has 90 working days to overturn the Governor's veto if the project is to move forward. A straight majority in both houses of Congress is sufficient to pass the Bill and depending on the schedule of the

summer recess and other interruptions, a vote and decision from Congress is expected by early August. If Congress fails to act or to act in time, the Governor's veto stands and - the project is abandoned.

However, as Rowe explained that, “doing nothing is not an option”. Some 20% of electricity generated in the US is nuclear and nuclear power generation remains significantly less expensive at 1.83c/kWh than coal at 2.2c/kWh or natural gas or oil at more than 4c/kWh. This includes the 0.1c/kWh deposited by all the nuclear generating companies into the 'superfund' set up by Congress to fund waste management.

"Back in the 1950s, Congress, under the Eisenhower administration, took upon itself the two gravest concerns about nuclear energy as expressed by the power generating industry," said Rowe. "One was the public liability statutes should there be an accident of any kind and the second was the unknown cost of managing the industry's waste.”

Under the Price Anderson Act Congress assumed all responsibility to indemnify Americans against any possible nuclear incident or accident, and the 0.1c/wKh superfund started in the 1980s currently stands at some $16 billion.

Temporary storage
Today, waste in the US is stored temporarily at some 131 locations in 39 states including some 72 commercial nuclear power stations. Most of these are in the eastern states, and many are close to centers of high population and near major waterways or the sea. Between them, all 131 locations are temporarily storing some 45,000 tonne of spent nuclear fuel and high-level radioactive waste. In addition there is a further 14,000 tonne of high-level nuclear waste being generated and stored temporarily at US defense facilities. Law limits the first phase of possible extensions to the Yucca Mountain facility to a storage capacity of 70,000 tonne until a second repository in a different location is in operation. By 2040 the stockpile of high-level nuclear waste in the US is expected to be about 119,000 tonne.

Management of nuclear waste is an international issue but the urgency for a solution in the US came in 1998 when a group of nuclear power companies, concerned for the security of high-level waste stored temporarily at their facilities, sued the federal government for reneging on its legal responsibility to use the superfund to manage the waste.

Submission of the scientific recommendation for development of Yucca Mountain was a function of timing rather than who was or was not in the White House at the time T&TNA was told. "Given a positive vote by Congress in August, operation of the facility remains about 10 years off with construction unlikely to start until 2006-7. " said Rowe.

With a congressional approval in hand, the Secretary for Energy will first apply to the Nuclear Regulatory Agency (NRA) for authorization to start Construction and once constructed another approval is required from the Nuclear Regulatory Commission (NRC) for authorization to receive and possess waste. These permits will also involve thorough investigation of the methods used for transporting waste by road and rail across the country to Nevada and to the repository. Should the project stall at any of these political milestones, the problem of how to manage the nation's nuclear waste goes back to members of Congress to provide the Department of Energy with new direction.

Schematic of the the repository's forced ventilation system

As client for this immense and politically emotive project, the federal Department of Energy (DOE) has spent more than $7 billion on investigating different possible sites for a repository. Of that, more than $4 billion has been invested in Yucca Mountain. To handle the project's many different aspects, the DOE employs a league of specialist consultants, engineers, contractors, and science laboratories, all under the direction of a project management contractor. In 2001, the previous management contract expired and was rebid. As a result the current five-year management contract with a five-year renewal clause was awarded to the Bechtel SAlC Company, a limited liability joint venture between Bechtel and Science Applications International Corporation. As an employee of Stone & Webster, Patrick Rowe is seconded to the Booz-Allen & Hamilton/Golder Associates/Stone & Webster/SRS Technologies JV engaged as the site characterization management and technical support division of the project. "Some 70% of the project's current funding is financed from the superfund with the remainder allocated from various agencies including the US defense budget," explained Karen Threlkeld, of the Bechtel SAlC Communications Department.

If phased development of the facility is to go ahead several different construction procurement options are being considered according to Rowe. Management is also considering procurement of the excavation equipment for the full project and leasing it to the contractors engaged for each construction phase.

The TBMs used on the two existing tunnels were procured for the project by the client and remain on site. The new 20ft (705m) diameter shielded TBM designed and manufactured by CTS for the larger diameter access tunnel will not be used on future development phases and the stored machine is now on the market for resale. The smaller 5.5m diameter refurbished Robbins gripper TBM procured from Peter Kiewit for the exploratory tunnel remains in place at the tunnel heading and may be restarted to extend the exploratory heading into other areas.

Of all the different options considered for managing or disposing of high-level nuclear waste, the international scientific community agrees almost unanimously that storage underground in favorable geological formations remains the most appropriate and responsible long-term solution. Yucca Mountain and the US is the national program most advanced in providing the world's first long term high-level underground nuclear waste repository but the international tunneling community can expect that excavation for similar repositories around the world will become a major source of activity in the decades to come.

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