A store of value?

Uranium's chemical interaction with hydrogen represents both a threat and an opportunity. At room temperature uranium, in common with other metals, tends to absorb hydrogen to form a stable metal hydride, UH3. In a waste context, uranium hydride is cited as a hazard during storage and handling of uranium metal. According to a report produced for UK body Radioactive Waste Management (RWM) this is because of its potentially pyrophoric behaviour and the risk of fire. The report says bulk accumulations of uranium hydride can exhibit a highly exothermic oxidation reaction on exposure to water or water vapour with atmospheric oxygen. The reaction with atmospheric oxygen is characterised at its most reactive extreme by high temperatures, red-hot glow, and some particulate release.

A number of events may have been connected to ignition of wastes containing uranium as a result of uranium hydride formation during storage and “these observations, and confusion about where and when uranium hydride could form and how it could react, have historically led to a highly conservative approach to the design of treatment, storage and handling processes for metallic uranium fuel and associated wastes”.

The caution appears to be abating: the report also says significant progress has been made in understanding the formation and properties of uranium hydride under fuel and waste storage conditions in recent years and it now seems there may be either little reaction on exposing uranium hydride to atmospheric oxygen, or a delayed reaction.

On the side of opportunity, the fact that depleted uranium can absorb hydrogen atoms and, when heated, then release them means it can potentially be used as a storage medium. It is already used in this way for tritium, one of the hydrogen isotopes that will be required to fuel fusion reactors.

Swiss company Tritec offers depleted uranium ‘beds’ for tritium handling on a small scale as a commercial product. The range of sealed ‘beds’ with one or two valves for tritium input and extraction have from 1g to 30g of depleted uranium in products up to 20cm long, and the total capacity of the largest is 10 kCi.

Depleted uranium has also been chosen as the solution for storing tritium at the next generation fusion test reactor, ITER, in preference to other metals with similar hydrogen absorption properties. ITER says depleted uranium (U-238) has the most suitable physicochemical properties.

This experience of storing tritium has sparked interest in uranium as a storage medium in the context of the growing need for hydrogen as an energy vector to complement electricity in a decarbonised energy system. There is already a significant and direct need for hydrogen in producing steel or chemicals, but it is also expected to be an important energy vector in its own right in fuelling industrial heat or heavy transport, as well as in generating power using relatively familiar gas turbines adapted to be fuelled by hydrogen instead of natural gas.

On-site storage

Meanwhile, the global hunt is on for new forms of long duration storage to absorb renewable (and nuclear) power generated at times of excess supply and retain it – for months or seasons – for use at times when low-carbon power supplies fall short of demand.

Hydrogen is seen as a valuable storage medium, but most discussions of hydrogen storage refer to large storage options such as underground caverns similar to those used for gas storage (possibly even repurposed from natural gas storage). However, as industry has found in using other forms of generation, storage at different scales is required to operate the energy system at least cost and most reliably for its users.

This model is familiar already: sites that are connected to the electricity grid nevertheless have emergency arrangements in case supplies are interrupted, such as diesel generators – along with stocks of diesel fuel.

On-site hydrogen buffer stores can be seen as an extension of this arrangement. And unlike diesel generators (but like batteries, which are increasingly attractive to companies where power supply is mission-critical) they can also give the owner an opportunity to shift power use in both directions – managing the state of charge, or in this case absorption, using more power when it is cheap and reducing import from the grid when it is expensive.

Using depleted uranium for storing hydrogen is the opportunity being investigated in the Hydrogen in Depleted Uranium Storage (HyDUS) project, led by EdF, which has now won support from the UK government.

The HyDUS project

The HyDUS development consortium will explore and develop the chemical storage of hydrogen at ambient conditions through the reversible formation of heavy-metal hydride compounds.

Along with EdF UK R&D the consortium also includes uranium enrichment company Urenco, the University of Bristol and the UK Atomic Energy Authority. In January the consortium was awarded £7.73m (US$9.3m) by the UK government’s Department for Business, Energy and Industrial Strategy (BEIS) to develop a hydrogen storage demonstrator utilising depleted uranium. The project is to be located at UKAEA’s Culham Science Centre in Abingdon, Oxfordshire, and is part of BEIS’s Net Zero Innovation Portfolio (NZIP), which aims to accelerate the commercialisation of low-carbon technologies and systems.

The HyDUS project will demonstrate the chemical storage of hydrogen at ambient conditions by chemically bonding the hydrogen to depleted uranium to form heavy-metal hydride compounds. In the storage demonstrator, hydrogen would be absorbed on a depleted uranium ‘bed’, which could then release the hydrogen when needed for use.

According to Urenco, the HyDUS project will deliver a modular demonstrator system within the next 24 months with an ambition to initially install the technology on nuclear sites, thereby enhancing the profitability of nuclear power plants. Later, however, it is hoped that the technology could be more widespread and used to support transport and heavy industries such as aluminium and steel smelting.

Consortium member, Professor Tom Scott, said: “The hydride compounds that we’re using can chemically store hydrogen at ambient pressure and temperature but remarkably they do this at twice the density of liquid hydrogen. The material can also quickly give up the stored hydrogen simply by heating it, which makes it a wonderfully reversible hydrogen storage technology.”

As a project partner, Urenco will contribute depleted uranium material also known as tails, which is made as a by-product of the uranium enrichment process. Following the successful project demonstration, Urenco will contribute to the commercial implementation of this innovative hydrogen storage technology. David Fletcher, Head of Business Development at Urenco, said in a statement: “We are proud to be a part of this exciting project which brings together proven fusion technology and a potential commercial use for Urenco’s stock of depleted uranium tails to develop a sustainable, low carbon energy storage solution for the emerging hydrogen economy.”

In a previous discussion of its decarbonisation research and development EdF said it was developing relationships with stakeholders for the production, distribution and consumption of hydrogen. Its R&D UK Centre now has a Zero Carbon Hub which aims to couple low-carbon generation with existing and new technology to decarbonise demand. The Zero Carbon Hub’s team completed the feasibility study for HyDUS, which looked at the potential advantages in terms of volume density, purity and long life.

Following the grant announcement, the HyDUS team has also begun seeking engagement with supply chain companies. The process requires a compressor to recover high-purity hydrogen from initial pressures down to ~0.1 barA and compress it for storage at 10-20 barA. UKAEA is seeking to engage with manufacturers and suppliers of hydrogen compressors to understand the operating requirements and abilities of these units and investigating conversion of gaseous hydrogen into electricity via an electrochemical fuel cell.

In January UKAEA sought expressions of interest from manufacturers and suppliers of fuel cells to understand the operating requirements and abilities of these units.

Author: Janet Wood, an expert author on energy issues