Using waste to fuel future reactors3 June 2021
David Bradbury, George Elder, David Hebditch and Susan Hewish suggest how today’s nuclear waste can be used as future nuclear fuel and how this could affect management of existing and future waste streams.
LARGE LIGHT WATER REACTORS WITH a once through fuel cycle make up most of the world’s nuclear power fleet, but they have a low thermodynamic efficiency for electricity generation, have a legacy of spent fuel requiring long-term storage and only use 1% of the uranium mined to fuel them.
There have been technical solutions to those shortcomings since the beginning of the nuclear age. Fast neutron reactors, for example, can utilise a far higher proportion of the uranium than thermal neutron reactors. But any new plan must be more than just better reactors. Whether the reactors remain in-situ with the user or are returned to a host country at the end of life, the nuclear residues within them will need to be addressed.
There is no a-priori reason for nuclear power to be more expensive than other energy sources. If deployed efficiently it requires very little land, material or labour. The current cost barriers are primarily related to complex design, high initial investment, uncertainty over the costs of disposal of long lived-wastes, lack of vibrancy of the industry and the absence of learning by replication.
Internationally the focus of technical development is on new reactor designs, but the industry needs to remember that radioactive waste and its associated longevity are widely perceived as a critical disadvantage for the technology. That perception stifles investment in the technology as a whole.
We need nuclear power to meet a range of needs. It is well suited to complement other low carbon energy sources, for example to generate power where solar energy is inefficient. More generally, stored heat from reactors can be used to provide extra electricity to balance other intermittent electricity sources.
To achieve this it must not require sophisticated technical capability within the consumer’s country and must not leave behind waste management and guardianship issues.
We propose that spent nuclear fuel should be split into two streams, one that can be recycled as new nuclear fuel and the other that can conform to standards of low level waste suitable for immediate shallow burial.
Resolving the delayed waste management issue will allow a much wider variety of commercial companies to use nuclear reactors than at present. Wastes can be managed for these companies by specialist organisations that are equipped to recycle the reactors and their associated fuel.
Using fast neutron reactors and advanced fuel cycles it may be possible to significantly reduce the footprint of deep geological repositories for the disposal of ultimate waste. This Blueprint for Future Nuclear Power seeks to extend this approach. Meanwhile a new generation of small reactors use modular designs that could be built in large numbers in a factory and deployed elsewhere.
The new designs can incorporate inherent safety features, which simplify the design itself and the associated licensing and regulatory process. They include design for fast neutron reactors, which are far more efficient at “fissioning” their fuel, hence making better use of the fuel and causing less long-lived waste than LWRs. Fast neutron reactors will ultimately be the key to recycling their own fuel and legacy spent fuel from other reactors.
Transporting the reactors for installation also in principle allows the reactors to be returned after deployment. The user could thereby enjoy the benefits of nuclear reactors without having to deal with issues of long-term hazardous materials.
The aviation model
Nearly all nations safely enjoy the benefits of aviation, but the majority rely on just a few countries for special capabilities such as constructing aircraft. A similar pattern needs to develop for these small reactors. We call this “Hub and Satellite” nuclear power (Figure 1). A small number of organisations act as the hub, supplying specialist nuclear services and satellite nations can receive and benefit from the nuclear facilities — leaving no long-term hazardous legacy at the satellite stations.
There are three potential models to operate nuclear power in this way, depending on the size of the installation. The nuclear fuel is transported to and from reactors constructed and fixed at the satellite location, or the reactor is transported to and from the site with fuel inside, or for very small installations a fully integrated electricity generating plant can be transported to and from the satellite.
Each of these alternatives have manifestations established or planned — for example nuclear fuel reprocessing in the UK and France, Russia’s barge-mounted nuclear plant and a design study for small nuclear power plants for airlift to disaster zones.
The Hub and Satellite concept allows cost savings to be achieved by replication and mass manufacture, whereas the centralisation of the waste management means that the cost of the complex operations of recycling fuel and dispositioning wastes can be spread over a large number of reactors.
Managing waste as a resource
It is untrue to say, “Nobody knows what to do with nuclear waste”, because there are technically acceptable solutions (principally deep burial with engineered barriers in stable geology). However, because of residual uncertainties about the evolution of waste over very long periods of time, and heat generation in the short and mid-term, there is little current long-term disposition of spent fuel or high level waste (HLW). A large proportion of the world’s spent fuel is in surface storage in pools, which require human- supervised systems. Much of the rest is stored in air- cooled shielded storage casks, which still require a future permanent solution.
Because of its longevity unresolved waste creates an institutional problem for organisations, which must take the responsibility for the waste in the long term. Few organisations can guarantee to last that long. In the United States, this problem was supposedly resolved by the Federal government taking on the long-term responsibility of spent fuel in return for a fee charged to the electricity utility. So far, the government has required payments from industry but not resolved the problem. By taking the responsibility for the problem the US government has not forced the providers of nuclear power to find other ways to resolve their waste issue, and has sustained a once-through fuel cycle which is tolerated despite its shortcomings.
The safest form of long-term management of nuclear materials is not to have them in storage at all. This can be achieved by separating waste for immediate disposal and recycling the rest through “just-in-time” procedures.
In order to demonstrate proper sustainability to the public, nuclear plants (and supporting activities) should be removed or decommissioned to completion soon after they stop operating. Besides being consistent with principles of intergenerational equity this makes best use of existing knowledge, expertise and resources.
Fuel components that cause a long-term hazard should be recycled into new nuclear fuel. Uranium (lightly enriched or depleted in the U-235 isotope) may be added or removed from the cycle for material balance purposes.
The irreducible waste of the nuclear process (principally fission products rather than minor actinides) is only a small fraction of spent fuel, and this fraction is also responsible for most of the troublesome heat generation. The fission product fraction itself can be allowed to stay where it is without further human intervention when the use is finished. That is because the hazard will decay to essentially harmless levels before the physical containment (eg canister) fails. If the heat-generating fraction is separated, the heat could potentially be used.
However efficient a recycling scheme is, it cannot eliminate all waste. Waste described as ‘low level’ in US terminology can generally be managed through existing shallow-burial facilities. Uranium originating from spent fuel for material balance purposes can be easily removed, and fed back in.
Nuclear physics requirements
The nuclear physics characteristics of fuel recycled in this way need to be carefully addressed, but it is possible in principle to combine recycled fuel with enriched or depleted uranium in schemes with thermal and fast neutron reactors. Meeting nuclear physics and material balance requirements is far easier in fast reactor than thermal reactor systems.
A primary requirement is to balance the ratio of fissile and fertile constituents of the fuel. For thermal reactors the fissile ratio will continuously fall as the reactors operate and therefore must be adjusted. For fast reactors the ratio may increase or decrease. Fast reactors can also be employed for the role of ‘incinerating’ problematic species that can build up in the fuel cycle.
It might be thought that the only way of increasing the fissile to fertile ratio is by adding fissile material, but the same effect can also be achieved by extracting uranium (Figure 2). That is far preferable for security considerations.
A key material to support the early stages of recycling schemes is high-assay low enriched uranium (HALEU). Internationally recognised standards place a limit of 20% enrichment of U-235 to protect against illicit use for nuclear weapons purposes. Most reactors are currently supplied with approximately 5% U-235, but HALEU (up to 20% enrichment) is extremely useful to achieve material balance for recycling schemes. The US Department of Energy has recently contracted with Centrus Energy Corporation for production of HALEU.
Separation to recover value
Spent nuclear fuel needs to be divided into two streams: principally fission products; and a mixture of uranium, plutonium and minor actinides. The objective is to separate fission products so that the hazard decay of the packages follows the green curve in Figure 3 (instead of the red curve for spent fuel itself). Much progress has already been made in developing separations of the type required.
The design of early nuclear reactors was generally not tightly integrated with the design of downstream plants for the processing of irradiated fuels and treatment of wastes. The PUREX process for separating spent fuel into uranium, plutonium and HLW achieved wide applicability and commercial maturity because it could be adapted to many different fuel types.
Over decades, many processes have been explored and developed for the separation of fissile and non- fissile materials from irradiated fuels, including solvent extraction, fluoride volatility, plasma flame, molten halide electrorefining, fractional crystallisation.
While earlier processes such as PUREX were designed for ‘pure’ uranium and plutonium products, more recent ones have concentrated on separating an actinide stream adequately partitioned for recycle to fast neutron reactors for power production or minor actinide incineration, with a fission product waste stream for near-term waste management.
The integration of irradiation, reprocessing, remote fabrication and recycle to fast reactor is complex, but it has been achieved by both the ANL integral fast reactor and its associated fuel cycle in the USA and by the RIAR BOR60 reactor and its associated fuel cycle in the Russian Federation. These operations have not yet reached the commercial maturity of PUREX reprocessing.
Two principal products arise, one suitable for nuclear fuel and another that is a potentially useful source of heat but with no long-term hazard associated with it.
The aim of the separation should be to receive, process and export materials using ‘just in time’ practices to avoid building up large amounts of material in storage. The physical amount of nuclear fuel consumed in a nuclear reactor is very small by industrial standards. The processing of it should be appropriately nimble and small scale.
Figure 2 shows how existing spent nuclear fuel and depleted uranium can potentially be incorporated into the process of recycle. The vast majority of material in SNF has potential value as recycled fuel – only a tiny proportion of the SNF is truly “waste”.
The potential locked up in the world’s existing spent fuel (together with stocks of depleted uranium) represents at least 100 years of mankind’s total energy needs.
Sustainable use of fission product waste
If use is to be made of the materials or heat generation from fission product waste, then enough demand must be identified.
Russia uses Sr-90 for radioisotope electric generators but it is doubtful that gamma-emitting isotopes can be used in electric generators. Gamma irradiators based on Cs-137 are also established, but not on a large scale. Neither of those applications could fully utilise the supply. (It should be noted that the processing plant would potentially provide access to Np-237, which is a component of SNF. By neutron irradiation in a reactor Np-237 can be converted to Pu-238 — the superior radionuclide for radioisotope generators used in long-distance space exploration.
The most flexible use would be district heating in a remote location. The source would be perhaps up to about 10MW, suitable for a small community of a few hundred people. If fission product canisters were placed in an underground pool with heat exchange to the surface, the resulting output of hot water could supply reliable and continuous district heating to homes in those remote communities, akin to supplying geothermal energy but at a location of choice rather than being limited by geographical constraints of availability.
For safety the underground pool would need a passive heat sink path to the surrounding ground in the event of the heat exchange system failing.
Constant heat output would be maintained, compensating for radioactive decay by adding extra fission product canisters to the pool on a periodic basis. At walk- away time, the facility would be allowed to move to passive heat exchange to the ground immediately surrounding the fuel pool. After a hundred years or so the facility would no longer be significantly heat generating and after a few hundred years any radioactive hazard would be effectively gone.
The ideas presented here are not new, but focus objectives that have been there almost since the inception of the nuclear industry.
In the USA, there has been only intermittent development of nuclear fuel recycling since the days of President Carter. Whilst there is re-invigoration in the USA as a result of the strength of the National Laboratories, other countries (particularly Russia and China) are further developing partitioning initiatives of this type.
The UK has historic capabilities in this field, but these have become depleted with the shutdown of facilities at Dounreay and Sellafield. France has very significant capability, as has The Siberian Chemical Complex in Tomsk, the RIAR in Dimitrovgrad and various surrounding Russian institutes are active as a centre of development of some of these ideas, and they have the skilled people and facilities to do the relevant work.
France’s nuclear research agency, Commissariat a` l’e´nergie atomique (CEA), indicated in September 2019 that “industrial development of fourth-generation reactors is not planned before the second half of this century.” Significant R&D, development and design work is required to enable full Gen IV reactor and fuel cycle facilities, including those for waste management with adequately short radioactive decay period, to be introduced. This period of development will allow the time for such an industry to be created using fast neutron reactors that can meet the technical requirements and specifications for the whole cycle. Nevertheless, there is nothing to stop this timescale being accelerated if there is an initiative to supply sufficient development resources and industrial sponsorship.
Incremental improvements in full-scale Gen III reactors (sometimes referred to as Gen III+ type) are expected in the shorter term and there may be early deployment of small and medium scale reactor types of new concept design such as Moltex, GE-Hitachi, Terrestrial Energy, ARC, Leadcold, NuScale, Holtec, etc.
The new concept designs based on fast neutron flux are normally intended to reduce unit costs and may be leaders in establishing the principles of this Blueprint for Future Nuclear Power.
Edit of Blueprint for Future Nuclear Power. Full version at:
Authors: David Bradbury, Associate, TU¨V UK Ltd (TU¨V NORD); George Elder, Associate, TU¨V UK Ltd (TU¨V NORD); David Hebditch, Associate, TU¨V UK Ltd (TU¨V NORD); Susan Hewish, Director Nuclear, TU¨V UK Ltd (TU¨V NORD)