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Transitioning of Spent Advanced Gas Reactor Fuel from Wet to Dry Storage

Goode, James Bruce (2017) Transitioning of Spent Advanced Gas Reactor Fuel from Wet to Dry Storage. PhD thesis, University of Leeds.

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Abstract

Upon removal from a reactor spent nuclear fuel is placed into water storage for a period of time since water provides both a decay heat sink and radiation shielding. In the UK this fuel has traditionally been reprocessed however the decision has been made that this will cease in 2018 in favour of an open fuel cycle and direct disposal to a degological disposal facility, although this is not expected to be available until 2075. Current experience with pond storage has found that fuel can be safely stored in caustic dosed ponds for at least 25 years and therefore the current plan is to extend water storage until around 2040, however how the fuel will be stored for the remaining 35 years is yet to be decided. One option that is being considered is the use of dry storage which is being used successfully in the USA for the storage of fuel from Light Water Reactors. One of the most important factors when utilising dry stores is an effective drying step and this PhD is aiming to look at this area in particular. Commercial drying methods have been developed for use with Zircaloy and aluminium clad fuels however this PhD intends to develop a method that can be used to dry Stainless Steel clad fuels such as those used in the UK's Advanced Gas Reactors (AGR). Since intact AGR fuel has yet to be examined following storage the first part if this thesis looked at preparing samples for later macro scale testing. Samples were treated to induce defects known to be of concern and analysed with the conclusion being that bound water is of no concern for drying operations and can be disregarded when preparing samples. Since AGR is known to have failed by stress corrosion cracking water trapped within microcracked pins is a likely issue. A section of tube was prepared by compression and boiling in magnesium chloride which micro CT imaging found to have images of a similar size to known failures. This tube was prepared into a test piece for later testing. The second part of the thesis looks at macro scale testing using the test piece described above and a similar test piece with a 300 um pinhole. A multipurpose drying rig capable of vacuum drying and flowed gas drying was constructed. In the first phase of drying tests vacuum drying and flowed gas drying were compared using a test piece with a 300 um pinhole and the impact of different cover gas was assessed. vacuum drying was found to be significantly more effective than flowed gas drying with the type of cover gas having little impact. Most importantly this work guided further improvements to the rig. The second phase of this work carried out more detailed drying tests on vacuum drying and flowed gas drying and utilised the pinholed and cracked test piece. The greatest influencing factor on the drying rate (the rate at which water could be removed from inside the test piece) was found to be the water level inside the test piece with broadly similar rates being found in all conditions and with both test piece when the test piece was full. As the water level within the test piece dropped, vacuum drying quickly became more effective. This was not helped by considering energy usage. There was however a feeling that flowed gas drying was limited due to the pipework size. In the final phase of testing the ability to detect the point which all water was removed (the end point) from the test piece using online instrumentation was assessed. Currently a vacuum rebound test is used which is both time consuming and if flowed gas drying is being used requires additional plant equipment. It was found that dew point measurements appeared to give a relatively clear indication of the end point when for both vacuum drying and flowed gas drying. For vacuum drying only the measurement of mass flow and pressure also gave a reasonable indication of end point.

Item Type: Thesis (PhD)
Keywords: Spent Nuclear Fuel, Interim storage, Drying,
Academic Units: The University of Leeds > Faculty of Engineering (Leeds) > School of Chemical and Process Engineering (Leeds)
Identification Number/EthosID: uk.bl.ethos.745516
Depositing User: Dr James Bruce Goode
Date Deposited: 29 Jun 2018 12:49
Last Modified: 25 Jul 2018 09:57
URI: http://etheses.whiterose.ac.uk/id/eprint/20585

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