Establishing the Viability of Drying Wet and Corroded Magnox Spent Fuel for Interim Dry Storage

This project aims to develop and test a process which will convert wet and corroded Magnox fuel to a stable physical form which allows interim extended storage in dry conditions, prior to disposal and consider how it may behave in dry storage.

Magnox reactors were the first commercial nuclear power reactors to be operated in the UK, and the fuel elements consisted of a uranium metal fuel bar clad in a Magnox outer can. The spent fuel from these reactors is typically stored underwater for shielding and cooling purposes then reprocessed at Sellafield, but a quantity of Magnox fuel remains unprocessed after the reprocessing plant closure in July 2022. Additionally there are large quantities of spent Magnox fuel which have been held in underwater storage ponds for several decades, and the material in these ponds has corroded significantly producing secondary wastes in the form of sludges, and hazardous sites for expensive remediation. Therefore there is a risk that any outstanding inventory of wet stored Magnox fuel could develop with no available disposal route as reprocessing will be incompatible or unavailable. Therefore an alternative solution is required for storage of Magnox fuel between reactor discharge and final geological disposal. One possible solution is to dry the wet fuel and hold in a dry store.

To initially investigate the feasibility of drying corroded Magnox, the corrosion products of inactive and unirradiated Magnox simulant have been characterised by X-ray diffraction, thermogravimetric analysis and computed tomography. Published data from these analyses are not widely available for both real and simulant materials, and in regard to computed tomography are novel in this context. From this analysis mechanisms for potential physical and chemical means for water hold up have been identified. Further to this, water removal has been performed by heating corroded Magnox under vacuum. The process is monitored by observing pressure, dew point, temperature and gas flow changes supported by measuring the sample mass loss as water is removed. Temperature during drying has been tested from 40-120 °C to observe the optimum temperature with respect to drying rate and level of dryness achieved by measuring the mass loss from water removal over time. From these tests it was observed that several hours vacuum/heat exposure is required before no further water is removed, and increasing temperature was seen to enable greater extent of dryness to be achieved over all temperatures investigated. It is beneficial to minimise water carryover into dry storage to reduce fuel/cladding corrosion but also to reduce the amount of hydrogen generated via water radiolysis. Despite the industrial interest in implementing vacuum drying, there are no externally published technical reports or papers that detail the Magnox fuel drying process or conditions. This work provides some important and relevant technical basis for realistic vacuum drying conditions and material behaviour observations.

Given the requirements for elevated temperature and extended drying times, some residual water is almost guaranteed to be present during storage. To investigate the impacts of small quantities of water carryover, gamma and alpha radiolysis experiments were undertaken at the Dalton Cumbrian Facility to observe the hydrogen generation yields of low water content/vacuum dried magnesium hydroxide and hydromagnesite. These tests showed that at water content <25% the hydrogen formation rate is higher than that for bulk water, which is an important consideration if estimating radiolytic hydrogen generation for low water content corroded Magnox fuel. Additionally, irradiation of vacuum dried powders still yielded measurable hydrogen, with decreasing yields at increasing drying temperature up to 120 °C. Radiolysis of adsorbed water on metal oxides/hydroxides is important to various areas of the nuclear fuel cycle, but surprisingly little is published surrounding Magnox spent fuel/corrosion products when compared to the scale of the challenges faced by the industry. This work builds on the existing knowledge and provides further data to understand this interesting physical effect, and also states evidence for occurrence in a new and industrially important context.