LEARNING FROM FISSION | SPECIAL REPORT
machine at the Culham Campus in Oxford. This uses a centrifuge to spin a chamber filled with molten lead and lithium to open a cavity in the liquid metal, where the plasma sits. A piston system pumps more liquid metal into the chamber, compressing the plasma to initiate fusion. The process is repeated in pulses. TAE Technologies, a California start-up, plans to abandon D–T fuel and instead to fuse ordinary hydrogen with boron-11. Washington state start-up, Helion aims to generate electricity directly from fusion, without using the process to heat fluids and drive turbines. Helion plans to fuse a mixture of deuterium and helium-3. However, helium-3 would need to be produced by D–D fusion.
While these private companies build on decades of
state investment in larger projects some, such as Tokamak Energy, CFS and GF, are also benefitting directly with investment from the UK government and the US DOE.
Fusion challenges As research continues, there is a growing realisation that the claimed advantages fusion has over fission are not so clear cut. Undoubtedly, fusion reactors are more intrinsically safe
than fission. As the fusion process is difficult to start and maintain, there is no risk of a runaway reaction or core meltdown. Fusion can only occur under strict operational conditions and in the case of an accident or system failure, the plasma will naturally terminate, lose its energy very quickly and extinguish before any sustained damage is done to the reactor. However, critics of fusion have indicated a number of
potential problems that need to be addressed: the limited supply of lithium-6, beryllium and tritium; the problem of power drain; large amounts of intermediate- and low-level wastes (ILW and LLW) generated; and cooling. They also question the claim that there is no proliferation risk. Tritium is another key concern. While the hydrogen
isotope deuterium can be extracted inexpensively from seawater, tritium is not at all readily available given a half-life of just over 12 years. It exists naturally only in trace amounts and nuclear reactors are required to produce it. Fusion scientists aim to solve this problem by breeding the tritium they need. The high-energy neutrons released in fusion reactions can split lithium into helium and tritium if the reactor wall is lined with a lithium blanket. However, a working fusion reactor is first needed to breed tritium in this way. Currently, the world’s only commercial sources are the 19 Canada Deuterium Uranium (Candu) reactors, which each produce about 0.5 kg a year as a waste product. Half of these reactors are approaching the end of their design life. Furthermore, unless new production methods can be developed the available tritium stockpile will steadily decline as fusion reactors like ITER begins burning the fuel. To date, tritium breeding has never been tested in a
fusion reactor and its efficiency is unknown. In a recent simulation, nuclear engineers at the University of California found that, in a best-case scenario, a power-producing reactor could only produce slightly more tritium than it needs to fuel itself. Tritium leakage or prolonged maintenance shutdowns could further reduce this margin. Daniel Jassby, who was a principal research physicist working on fusion at the Princeton Plasma Physics Lab until 1999, has detailed the problems facing fusion in several recent articles, noting that less than 10% of the
injected fuel will be burned before it escapes the reacting region. “The vast majority… must therefore be scavenged from the surfaces and interiors of the reactor’s myriad sub-systems and re-injected 10-to-20 times before it is completely burned,” he says, adding that, in the two magnetic confinement fusion facilities where tritium has been used (Princeton’s Tokamak Fusion Test Reactor, and JET), approximately 10% of the injected tritium was never recovered. As to other drawbacks, Jassby says the neutron streams
produced by fusion lead directly to four problems: radiation damage to structures; radioactive waste; the need for biological shielding; and the potential for the production of weapons-grade plutonium 239 – thus adding to the threat of nuclear weapons proliferation. In addition, fusion reactors share many of the problems that also face fission, “including tritium release, daunting coolant demands, and high operating costs”. Problems unique to fusion devices include unavoidable on-site power drains that drastically reduce the electric power available for sale as well as the limited availability of tritium.
Power play Fusion reactors consume much of the power they produce, leading to “parasitic power drain, on a scale unknown to any other source of electrical power”. Essential auxiliary systems external to the reactor must be maintained continuously even when the fusion plasma is dormant and when the fusion output is interrupted for any reason, this power must be purchased from the regional grid. Power is also needed to control the fusion plasma in magnetic confinement fusion and to ignite fuel capsules in pulsed inertial confinement fusion. If the fusion power is 300 MWe, the entire electric output of 120 MWe barely supplies on-site needs, he adds. “To have any chance of economic operation… the fusion power must be raised to thousands of megawatts so that the total parasitic power drain is relatively small”. A fusion reactor also places place immense demands
on water resources for the secondary cooling loop that generates steam, as well as for removing heat from other reactor subsystems such as cryogenic refrigerators and pumps. Parasitic electric power drain places additional demands on water resources for cooling not faced by any other types of thermoelectric power plant. To produce usable heat, the neutron streams carrying 80% of the energy from deuterium-tritium fusion must be decelerated and cooled by the reactor structure, its surrounding lithium-containing blanket, and the coolant. The neutron radiation damage in the solid vessel wall is expected to be worse than in fission reactors. A number of components, such as the blanket and divertor, are expected to need to be replaced every 5–10 years. In addition, while the radioactivity level per kilogram
of waste would be much smaller than for fission-reactor wastes, the volume would be many times larger. There is growing international recognition that this is a problem that needs to be addressed. A study published in Nuclear Fusion in May 2022 concluded: “A waste strategy needs to be developed to mitigate the impact that the large waste volumes could have on the public perception of fusion as a viable and clean alternative source of energy.” Also, this year the UK’s Committee on Radioactive Waste Management (CoRWM) produced a preliminary position paper on the U
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