A new concept for compact radiation shielding: Reactive sintered tungsten borocarbides

There has been no greater existential threat to humanity to date from anthropogenic climate change as a result of CO2 emissions. The effects are already apparent in terms of more extreme weather, loss of polar ice and rising sea levels. Power generation contributes to much of the CO2 emissions from fossil fuel burning as the worldwide demand for power continues to outstrip supply. Renewable energy (wind, solar, hydro) is limited by weather dependency, with associated issues such as energy storage and land use. Nuclear fusion has a significant role in decarbonizing global power generation but radiation shielding is a limiting factor. Creating miniature Suns is one part, but materials must exist that can withstand fusion conditions for practical fusion reactors. W-based alloys and other refractory metals are current solutions in fusion reactors, but the engineering requirements for power-generating fusion reactors exceed those in current materials.

The goal of this fellowship will demonstrate the feasibility of radiation shielding based on the Cemented Tungsten Carbides and Reactive Sintered Borides (cWC-RSB) concept in Compact Spherical Tokamaks (cSTs). cWC-RSBs can bridge the gap between current materials and the engineering requirements for a power-generating cST. cWCs-RSBs have excellent radiation absorption properties by combining heavy (W) and light elements (C, B) with the strength and toughness by combining WC with a ductile metal binder. However, cWCs have never been used in nuclear reactors to date since the use of Co (and Ni) metal as a binder alloy prevented the use of cWCs as radiation due to Co and Ni being activation hazards.

In 2014, I discovered that non-activating FeCr alloys are suitable as cWC binder alloys, with RSB development following on investigating boron additions in cWCs. Combined cWC-RSB shields have greater radiation attenuation overall, compared to cWCs alone. The first objective evaluates the thermo-mechanical properties and the safety case for shielding candidates, including high-temperature oxidization and thermal shock to in terms of worst-case scenarios, such as exposure of hot shielding to air. Experimental data on Si-coated cWCs showed that Si-coating retarded oxidization rate by 4 orders of magnitude relative to tungsten in the temperature range 900C-1200C. I will evaluate the properties of cWC-RSBs over cryogenic to failure (> 1200C) temperatures predicted for power-generating fusion reactors. While considerable data on the thermo-mechanical properties exist for cWCs since the 1930s, there is little on RSBs, given their novelty and it is crucial that thermo-mechanical properties of RSBs are well-known prior to industrialization.

The novelty of RSBs means that very little is known about their chemistry and routes to fabrication. Current processing methods are not fully optimized for dense, crack-free RSBs. The second objective aims to fill these gaps using the calculation of phase diagram method (CALPHAD) for predicting the most suitable compositions and design of experiment (DoE) methodology for the most efficient processing trials. This research demonstrates how new solutions can be derived from existing materials and techniques when a critical gap in current solutions is apparent. Recent simulations of the neutron and gamma attenuation of WC- and RSB-based shielding concepts show considerable promise. However, there is little data on the radiation response of cWCs and none on RSBs to date. For this third objective, I intend to build on current research using simulated cSTs to inform radiation experiments and experimental work simulating the range of conditions inside a cST, including ion bombardment, charged particles, and secondary radiation. Data from cWC-RSB shields in a simulated fusion reactor alongside demonstrated oxidization resistance indicates that cWC-RSB materials exceed current radiation shielding candidates in terms of radiation attenuation and safety.

The potential of fusion power for the UK and its key to decarbonizing power generation and fighting anthropogenic climate change cannot be understated. With industrial manufacture of high-temperature superconductors (HTS) reaching maturity and the development of the International Experimental Reactor (ITER) paving the way for industrialization, fusion power is a practical candidate for zero-carbon power generation by 2030. One of the limiting factors for practical fusion power is that generic engineering challenges must be overcome and candidate materials must satisfy radiological, regulatory and safety concerns. W-based alloys and other refractory materials and metals are currently used in experimental fusion reactors, but the engineering requirements for power-generating fusion reactors exceed the properties of current materials. Bridging the materials gap for power-generating fusion reactors will enable fusion power to fulfil its potential to decarbonize power generation and contribute billions to the UK economy within a decade. As was the case for the Large Hadron Collider (LHC) enabling the full industrialization of HTS and other technologies such as big data handling, it is anticipated that practical fusion power will have a similar, if not greater impact than the LHC within the first decade of successful demonstration of practical nuclear fusion.

Radiation shields based on cemented carbides (cWCs) and reactive sintered borides (RSBs) have the potential to fulfil generic thermo-mechanical properties and be safe in terms of activation and transmutation. A key roadblock in the development of practical fusion power is the lack of regulation concerning nuclear safety for fusion power since there is no legislation specific to nuclear fusion to date. Silicon-coated cWCs have also been demonstrated to have oxidization kinetics, orders of magnitude slower than tungsten metal between 900C - 1200C in air, a strong safety factor when considering a loss of coolant and exposure of hot radiation shield to air. Experimental compact spherical tokamaks (cSTs) have limited space for shielding due to geometry, particularly the central column. The extreme environment of cST will serve to demonstrate the safety and utility of the cWC-RSB shielding concept for other applications including but not limited to conventional nuclear power. Data on their thermo-mechanical properties and interactions with parts within a fusion reactor will prove their practicality outside nuclear power.

Key first stage beneficiaries will be those in academia but this research will have a direct impact on the nuclear industry and related beneficiaries including the powder metallurgy and refractory materials industries. The links between academic and industrial partners from the beginning of this fellowship will make rapid industrialization of cWC-RSB and power generating fusion reactors possible within the decade. The strong industrial basis that underpinned the development of non-activating cWCs and RSB materials will demonstrate the scalability of these materials in an industrial context, a major issue when considering any new concept in functional materials.
Second stage beneficiaries will include academic and industrial users of radioactive materials and irradiating environments and industrialization of cWCs-RSBs materials. This is a wide field covering medicine, industrial isotopes, mining, and minerals. Within the UK, the MoD, UKSA/ESA, and the NHS will see the benefits of compact radiation shielding materials in terms of safer handling of radioactive materials. Overall, if the cWC-RSB concept is successfully demonstrated within cSTs, this will enable the rapid adoption of power generating fusion reactors in the UK and the world-wide decarbonization of power generation within the next 10-20 years.