The Dry Cask Storage Process

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Dry casks in rows at the proposed interim storage facility

The current South Australian nuclear discussion revolves around the lucrative potential of importing cooled and contained used reactor fuel from partner nations and temporarily storing it at a secure above-ground facility prior to eventual underground disposal.

The long-term profitability and desireability of this nuclear material stewardship proposal has faced sustained criticism from established opponents, yet the inadequacies of their most comprehensive offical protestations have been exposed in a submission to the Joint Committee examining the royal commission’s findings.

The sizeable potential net benefits to South Australia warrant further investigation and public consultation. As one prominent local family business leader stated,

The establishment of a nuclear spent fuel repository will require the construction of ships to transport the fuel rods, cited as seven ships to be replaced at 20-year intervals.

This would enhance our ship-building industry, along with a requirement for the ships to be manned by Australian mariners.

The fuel rods need to be transported and stored in canisters and steel casks which could also be manufactured in South Australia.

There is also substantial indication that notions of broad public opposition are over-stated (perjorative terms notwithstanding).

To facilitate the accessibility of knowledge relevant to this proposal, this blog has reprinted a  segment of a discontinued wiki page which clearly describes and illustrates the dry cask used fuel management process. It is based on the US experience, which has witnessed not a single worker health impact from decades of operations. Units have been converted to metric, with minor changes to improve readability.


The Dry Cask Storage Process

featured at the “Gunn-Lee-Thorson Nuclear Power Plant

Spent nuclear fuel must be isolated from any interaction with the ecosystem until its radioactivity has diminished to a safe level. Initially, nuclear waste is usually stored in a spent fuel pool of water which serves to cool the spent fuel as it continues to generate decay heat after its use, and also shields the surrounding environment from radiation from the fuel. The water is circulated through a number of heat exchangers to remove heat from it. Reactors typically have a spent fuel pool that can hold several years of spent fuel discharge.

Due to increasing quantities of spent nuclear fuel and nuclear waste accumulating at the nation’s nuclear power plants and the uncertain future of a federal nuclear waste depository, a new method of storing this radioactive waste was necessary. A common solution is dry cask storage, which is being utilized by many nuclear sites currently with more examining this option. This section will provide a detail explanation of what this process entails.

The method that will be specifically detailed is the Holtec dry fuel storage assembly. The first component of this storage assembly is the Multi-Purpose Container (MPC).

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Holtec Multi-Purpose Container (MPC)

This enormous stainless steel (SS) structure is approximately 4.8 m in height and 1.7 m in diameter. The MPC has a 63.5 mm thick SS base and 241 mm thick SS lid equipped with 2 vents or fill ports. Each MPC holds 68 fuel bundles and weighs 40.8 metric tonnes (mt) when fully loaded.

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Holtec MPC lid

The next element of the assembly is the HI-STORM Overpack.

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HI-STORM Overpack container

This container is approximately 6.08 m tall and 3.37 m in diameter. It is composed of a carbon steel shell filled with 0.8 m 2’ of concrete internally. The container alone weighs at least 122 mt when empty, 163 mt when fully loaded.

The HI-TRAC Transfer Cask:

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HI-TRAC Transfer Cask

This container is approximately 5.1 m tall and 2.9 m in diameter. The walls of this container are 114 mm thick lead and the empty container weighs 65 mt. The image below illustrates how these containers work together.

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Compatibility of Holtec dry cask storage containers

Once the MPC has been placed within the HI-TRAC transfer cask, and the apparatus is on the refueling floor of the nuclear power plant, a rubber ring must be secured in the gap between the two concentric cylinders. This is necessary to prevent the MPC from rising out of the HI-TRAC from the buoyant force of the water when the empty assembly is lowered into the water of the fuel pool. The empty MPC is also filled with water before being submerged for the same reason. Next, a stainless steel sheet that serves as a “diaper” for the assembly is setup.

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Pad for holding the loaded HI-TRAC assembly

This pad is necessary to prevent radioactive contamination by the loaded HI-TRAC after it is removed from the spent fuel pool. Next, the entire assembly is lowered via a large crane into the spent fuel pool.

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Submerging of Hi-TRAC assembly into spent fuel pool

Another crane is then used to load the spent fuel rods into the MPC.

Loading of spent fuel rods into submerged MPC

Loading of spent fuel rods into submerged MPC

The MPC lid is then put into place prior to lifting the loaded HI-TRAC transfer cask assembly from the spent fuel pool and placing it on the stainless steel “diaper” sheet. A machine is then used to weld the lid to the MPC.

Machine used to weld the lid on the MPC

Machine used to weld the lid on the MPC

At this point, the vents in the lid of the MPC are used to further cool the spent fuel rods, utilizing water and nitrogen gas in different steps.

Cooling of the spent fuel in the MPC

Cooling of the spent fuel in the MPC

The HI-TRAC transfer assembly is then lowered down from the assembly floor and placed on top of the HI-STORM. The bottom of the HI-TRAC can then be slid out to the side and a crane used to lower the full MPC into the HI-STORM.

chematic of MPC being lowered from HI-TRAC into HI-STORM

chematic of MPC being lowered from HI-TRAC into HI-STORM

At this point, the entire assembly is resting on the Low Profile Transporter (LPT) illustrated below as a view from the bottom.

Low Profile Transporter for moving the loaded HI-STORM assembly

Low Profile Transporter for moving the loaded HI-STORM assembly

The LPT is approximately 3.66 m by 2.59 m and weighs approximately 2.6 mt unloaded. The LPT is equipped with 6 Hilman Roller Assemblies (3 on each side). Each Hilman Roller Assembly is approximately 1.1 m by 0.4 m, 0.23 mt, and made up of 26 rollers, each with a 136 mt rating. The LPT is used to get the loaded HI-STORM outside of the plant so that it can be loaded to the transporter.

Transporter used to move loaded HI-STORM to storage pad

Transporter used to move loaded HI-STORM to storage pad

This transporter then moves at about 3.2 km/h to take the assembly to the dry-fuel storage facility.

Dry-fuel storage facility

Dry-fuel storage facility


 

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