Nuclear Decommissioning Authority: Case Study Analysis
Hoover Dam
The objective of this study is to conduct research and provide a case study of a human-made system and to report on that system. This work will cover technical and operational details and relate these case study specifics to the course content.
It was reported in April 2006 that the United Kingdom had established the Nuclear Decommissioning Authority (NDA) and that this agency had set out its strategy on how it would address historic nuclear installations in terms of the cleanup and decommissioning of those sites, which includes 20 civil nuclear sites. (Nuclear Engineering, 2006) The strategy was reported to state key principles that included the “accelerated decommissioning wherever feasible and a schedule to create a strong competitive market that aims to achieve value-for-money for the taxpayer.” (Nuclear Engineering, 2006) The publication makes identification of increases that are significant in nature of the “estimated total costs for decommissioning the UK’s nuclear legacy, now conservatively estimated at close to £70 billion ($120 billion).” (Nuclear Engineering, 2006)
In addition to the costs of decommissioning along with cleanup and commercial operations stated at £62.7 billion ($107 billion) there are reported to be “other costs, which will be included in the 2006 LCBL, including R&D directly funded by the NDA, the cost of new low-level waste (LLW) disposal facilities and potential costs for the long-term management of contaminated land.” (Nuclear Engineering, 2006) It is reported that exclusions include such as “costs associated with the long-term management arrangements for intermediate-level waste (ILW) and the treatment and disposition of plutonium and uranic materials, should they be reclassified as waste.” (Nuclear Engineering, 2006)
Four broad objectives have been applied in decommissioning strategies including:
(1) Ensuring the ongoing safety of the public, the workforce and the environment;
(2) Minimization of the impact on the environment of the installation as much as reasonably practicable;
(3) Release of the land for appropriate use; and (4) Minimization of the cost of national resources on decommissioning. (Nuclear Engineering, 2006)
The first step required in decommissioning is reduction of the radiation risk through discharge of the irradiated fuel, which is reported to remove in excess of 99% of the radiation present on the site. Fuel that is removed during decommissioning of a site is handled in the same manner as spent fuel is handled during the life of the nuclear site. There are three stages in the decommissioning process including those as follows:
(1) Stage One — Defueling and other Preparatory Work — complete removal of all fuel from the reactors and its dispatch to the reprocessing site. This will involve two to five years time. Some of the non-radioactive plant and buildings can be removed at this time. Due to the need for covering any inadvertent reactor power faults, the essential and emergency plan will remain in readiness until it is established by calculation and agreed with the Regulator that the possibility of accidentally going critical is vanishingly small.
(2) Stage Two — Safe Store and Care/Maintenance Preparations — this involves the removal of buildings in which there is no radiation present. Included in the important activities of this stage are: (a) retrieval of low level waste and either disposing of it to the Low Level Waste Repository or developing temporary storage solutions on site; (b) retrieval of intermediate level waste (ILW) and construction of adequate passively safe storage arrangement son site pending the development of a national ILW repository in the longer term. Included in stage two is the “commencement of work to secure reactor buildings in which there is no radioactivity. This may involve the replacement of external cladding with high integrity materials and the infilling of unnecessary openings. The construction of the building termed ‘SafeStore’ must provide a robust shell capable of resisting accidental or intentional damage or unauthorized access. The preparatory period is reported to last approximately 20 years and the entire project of decommissioning to last approximately 100 years.
It is reported that Dam construction impacts provide a useful analog for decommissioning of dam although it is stated that “removal is not the opposite of dam construction: some processes are reversible, others are not.” (U.S. Army Corp of Engineers, 2006)
Decommissioning alternatives for dams include:
(1) Partial breaching — this involves using incremental analysis to evaluate 71 alternates: (a) various increments of breaching (100-, 150-, 175-, 200-, 300-, and 400- feet);
(b) Complete removal of dams; and (c) Various combinations of rock ramps and/or backwater refuges. (U.S. Army Corp of Engineers, 2006)
Dam removal takes place for the following reasons:
Environmental 43%
Safety 30%
Economics 18%
Unauthorized structure 4%
Recreation 2% (U.S. Army Corps of Engineers, 2006)
Potential drawbacks:
(1) Mitigation costs may be higher than removal costs;
(2) Altered hydrological and hydraulic regime;
Sediment transport issues especially for contaminated sediments. (U.S. Army Corps of Engineers, 2006)
Considerations for dam decommissioning include:
(1) Acceptable risk and uncertainty;
(2) Degree of potential impact
(3) Recovery potential;
(4) Physical and economic constraints;
(5) Public impacts and perceptions
(6) Quality and quantity of available data;
(7) Costs
(8) Benefits
(9) Multi-objective optimization model. (U.S. Army Corps of Engineers, 2006)
Dam decommissioning alternatives include:
(1) Discontinued use of a hydroelectric power plant, partial removal of the dam;
23) complete removal of the dam and all associated structures including spillways, outlets, power plants, switchyards, etc. (U.S. Army Corps of Engineers, 2006)
Other considerations include the type of material used in dam construction and necessary for determination are:
(1) How much of the dam to remove;
(2) The volume of material for disposal; and (3) The removal process itself. (U.S. Army Corps of Engineers, 2006)
Engineering considerations are reported to “influence the amount and rate of sediment erosion, transport and deposition.” (U.S. Army Corps of Engineers, 2006) The rate of dam removal and reservoir drawdown is reported as having a strong influence on the rate that sediments are eroded and then transported to the river channel downstream. The impacts from a large volume of reservoir sediment release into the downstream channel is such that a reduction can be realized “through slowing the rate of reservoir drawdown.” (U.S. Army Corps of Engineers, 2006) Progressive removal of the layers of the dam over several weeks or even months or years can be expected based upon the dam size and the reservoir sediments volume. The rate of the reservoir drawdown is stated to be required to be slow enough “to avoid a flood wave of reservoir water spilling into the downstream river channel.” (U.S. Army Corps of Engineers, 2006) Drawing the reservoir pool down is dependent on how the release of flows can occur “through, over, or around the dam. If the dam has a low-level, high-capacity outlet works or diversion tunnel, the reservoir could be emptied at a prescribed rate and the dam could be removed under dry conditions. However, if the width of the outlet works is narrow relative to the reservoir sediment width, then a substantial portion of the sediments would remain in the reservoir until the dam is removed. A bypass channel could be constructed around the dam, but it would need the ability to at least partially drain the reservoir. For concrete dams, it may be acceptable to release flows over the dam or through notches cut into the dam (U.S. Army Corps of Engineers, 2006).
Dam removal and reservoir drawdown plans have to prepare for the possibility of floodflows occurring during dam removal. The occurrence of a flood may simply mean the temporary halt of dam removal and reservoir drawdown activities. However, an overtopping flood could cause a failure of the remaining structure and a downstream flood wave that would be many times larger than the reservoir inflow. If the remaining structure can withstand overtopping flows, then floods may help to erode and redistribute delta sediments throughout the reservoir. In a wide reservoir, a floodflow may help to leave the reservoir sediment in a more stable condition after dam removal.” (U.S. Army Corps of Engineers, 2006)
There are several numerical models that have been applied in predicting erosion after removal of a dam and these models are such that can be divided into “case-specific models and general application models.” (U.S. Army Corps of Engineers, 2006) Case specific models are reported as being empirical in nature and such that tend to be supported by field data. The primary components of the model based on physical reasoning and data form a drawdown experiment are the following:
(1) Dam notching;
(2) Assumption of stable slope which can be calculated to be equal to the delta slope;
(3) Calculate new delta shape;
(4) Calculate reservoir trap efficiency;
(5) When delta meets the sill of the dam, start to move sediment out of the dam; and (6) Continue until removal is complete. (U.S. Army Corps of Engineers, 2006)
Two basic categories of sediment models are: (1) one-dimensional; and (2) two-dimensional models. One-dimensional models are reported to “generally solve steady-flow or unsteady equation of 1D open channel flow.” (U.S. Army Corps of Engineers, 2006) Two — dimensional and hydraulic and sediment transport models have the ability t o model the variation of hydraulic and sediment properties across the reservoir cross section” as well as modeling the failure of the reservoir banks. (U.S. Army Corps of Engineers, 2006, paraphrased) Processes that are not well represented in sediment transport models include those of:
(1) Headcut migration through cohesive material;
(2) Bank erosion;
(3) Large width changes; and (4) Stratified bed sediment. (U.S. Army Corps of Engineers, 2006 )
Thesis statement
Decommissioning of the Hoover Dam will be a process that requires a span of several years to ensure proper and effective safe decommissioning.
Materials and methods
This work in writing addresses system engineering (SE) concepts and uses appropriate analytical SE methods including the details of the process used in research. The procedure is chronological in nature and detailed leading the reader to the results. Data collection required for the decommissioning of the Hoover Dam include data collected on:
(1) Hazardous waste inventory;
(2) Environmental data;
(3) Regulatory considerations; and (4) Regulatory permits and processes. (American Society of Civil Engineers, Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities, 2006)
Required is a six-phase evaluation process including the following six phases:
(1) Initial retirement study;
(2) Consultation;
(3) Data collection and analysis;
(4) Evaluation of alternatives;
(5) Implementation; and (6) Long-term obligations. (American Society of Civil Engineers, Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities, 2006)
It is reported that engineering and constructability issues are key in conducting the evaluation and planning the options for the retirement dams and associated hydroelectric facilities. Primary engineering and constructability issues may involve consideration of hydrology and hydraulics, geotechnical engineering, structural engineering, construction and demolition techniques, hazardous materials, cost estimates, and construction/demolition scheduling.” ( )
Major engineering issues include:
(1) Flood requirement and spillway/river channel capacity;
(2) Diversion and cofferdam arrangements;
(3) Fish passage facilities;
(4) Structural removal of part or all of the dam, hydroelectric facilities and appurtenant structures;
(5) Removal of part or all of the mechanical and electrical equipment;
(6) Treatment of facilities partly or entirely left in place;
(7) Engineering and constructability relating to proposed mitigation for sediment management, environmental requirements, flood effects and effects on wetlands;
(8) Construction and demolition costs estimates and schedules; and (9) Operation and maintenance costs. (American Society of Civil Engineers, Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities, 2006)
The processes utilized in the Hoover Dam Decommissioning initiative include:
(1) Hazard Classification;
(2) Flood Studies;
(3) Structure Assessment and Analysis
(4) Removal or Modification of Dams and Appurtenant Structures;
(5) Removal or Modification of Hydroelectric facilities;
(6) Removal or modification of water conveyance structures;
(7) Construction requirements;
(8) Costs and schedules;
(9) Engineering and constructability summary. (American Society of Civil Engineers, Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities, 2006)
It is reported that Stillwater Sciences has developed DREAM (Dam Removal Express Assessment Models), a model that is applicable to dam removal. The assumption of the model is that the channel through the reservoir sediments “has a simplified trapezoid shape. The user inputs the initial width and the model calculates the evolution of this channel based on transport capacity. The model ignores sediment that would travel as wash load (i.e., silts and clays).” ( ) Greimann et al. (2003) derived the following equation:
Source: American Society of Civil Engineers, Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities (2006)
Results
The processes set out in this proposal for the decommissioning of the Hoover Dam follow required engineering regulatory requirements and suggested guidelines for decommissioning of such as the Hoover Dam. Following these processes will result in the effective decommissioning of Hoover Dam at minimal risk to the environment and people in the area of the Hoover Dam and areas of potential impact from environmental and ecological risks and hazard associated with decommissioning the Hoover Dam.
Discussion
This work in writing has outlined the processes required in the decommissioning of Hoover Dam and has as well examined the engineering solutions for mitigation of sediment changes and shifts during the process which will require several years to complete and will span several decades.
References
Current Dam Decommissioning Activities: The Importance of Making Connections. Dam Decommissioning Workshop, Traverse City, MI. April 2006. U.S. Army Corps of Engineers. Retrieved from: http://www.crrel.usace.army.mil/sid/Dam_decom/pdfs/Overview_Dam_Decommissioning.pdf
Chapter 8 Dam Decommissioning and Sediment Management. Retrieved from: http://www.usbr.gov/pmts/sediment/kb/ErosionAndSedimentation/chapters/Chapter8.pdf
McCully, P. (1996) Getting Old: Dam Ageing and Decommissioning International Rivers. Excerpt from Silenced Rivers: The Ecology and Politics of Large Dams, Zed Books, London, 1996. Retrieved from: http://www.internationalrivers.org/node/1463
Pacca, Sergio (2007) Impacts from Decommissioning of hydroelectric dams: a lifecycle perspective. Climatic Change (2007) 84:281 — 294. Retrieved from: http://www.hydroreform.org/sites/www.hydroreform.org/files/Pacca-%20Impacts%20of%20decommissioning%20of%20hydroelectric%20dams.pdf
Current Dam Decommissioning Activities (2006) U.S. Army Corps of Engineers. Retrieved from: http://www.crrel.usace.army.mil/sid/Dam_decom/pdfs/Overview_Dam_Decommissioning.pdf
Fischenich, C. And Conyngham, J (2006) Socio-Economic Aspects of Dam Decommissioning. Dam Decommissioning Workshop: Options, Opportunities and Challenges. 24-25 April 2006. Retrieved from: http://www.crrel.usace.army.mil/sid/Dam_decom/pdfs/SocioeconomicDamDecommissioning.pdf
Impacts from decommissioning of hydroelectric dams: a life cycle perspective (2007) Hydropower Reform Coalition. Retrieved from: http://www.hydroreform.org/tags/content-bibliographic_reference/decommissioning
Appendices:
Decommissioning Timeframe
Time Frame in Years
0 Cease Electricity Generation
2 to 5 Decommissioning
(1) Progressive dismantling of radioactive and non-radioactive plant and buildings, cooling ponds and ancillary plant;
(2) Cleanup of radioactive materials — disposal of site low level waste to the national LLW repository;
(3) Construction of new intermediate level waste packaging and interim storage facilities;
(4) Hazardous and non-hazardous wastes dispatched off-site for disposal or recycling;
(5) Preparation of safe store reactor building.
20 to 100 Care and Maintenance (only significant building remaining safe store reactor and ILW store
40 to 50 Dispatch ILW packages off site to national repository
100-200 Dismantle reactor buildings and ILW store. Ground remediation and landscaping.
120 Delicensing/final site clearance for potential re-use or to original state.
