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%

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)…