- American Physical Society Sites
- Meetings & Events
- Policy & Advocacy
- Careers In Physics
- About APS
- Become a Member
Daniel L. Jassby
The proponents of fusion reactors claim that, when developed, such reactors will constitute the “perfect" energy source, and in particular will share none of the significant drawbacks of much-maligned fission reactors. That claim is contrary to fact. If fusion reactors are feasible, they would share seven serious disadvantages of fission reactors stemming from neutron production, tritium usage, coolant demands and operating costs. These issues are endemic to any type of MCF (magnetic confinement) or ICF (inertial confinement) fusion reactor that’s fueled with D-T (deuterium-tritium) or deuterium alone. The first five of the drawbacks discussed below have been considered individually for decades, but apparently never before compiled. The sixth and seventh, although critical, have been ignored.
1. Radiation damage to the structure imperils reactor integrity. In reactors fueled with D-T, eighty percent of the fusion energy consists of streams of 14-MeV neutrons. To produce usable heat, these neutron streams must be decelerated and thermalized by the reactor structure. The majority of reactor concepts use a solid first-wall and blanket structure, where the neutron radiation damage is expected to be worse than in fission reactors because of the higher neutron energies [1, 2]. Fusion neutrons knock atoms out of their usual lattice positions, causing swelling and fracturing of the structure. Also, large amounts of interstitial helium and hydrogen are generated, forming gas pockets that lead to additional swelling, embrittlement and fatigue.
In reactors with D-only fueling (much more difficult to ignite), the neutron reaction product has a lower energy (2.5 MeV) and the neutron streams are substantially less damaging on structures. However, a significant fraction of the tritium reaction product will unavoidably be burned to produce 14-MeV neutrons, and the deleterious effects on structures will still be ruinous on a longer time scale. If a practical source of He-3 can be found, neutron damage and activation can be reduced by an order of magnitude if the reactor utilizes the 3 He-D fuel cycle, where D-D reactions will comprise less than 1/4 of the total reaction rate.
The problem of neutron-degraded structures may be alleviated in those ICF and hybrid ICF/MCF concepts where the fusion fuel capsule is enclosed in a thick liquid lithium sphere or cylinder. But the fuel assemblies themselves will be transformed into tons of radioactive waste to be removed annually from each reactor. Molten lithium also presents a fire and explosion hazard, introducing a drawback common to liquid-metal cooled fission reactors.
2. Radioactive Waste. As noted above, bombardment by fusion neutrons knocks atoms out of their structural positions while making them radioactive and weakening the structure, which must be replaced periodically. That results in huge masses of highly radioactive materials that must eventually be transported offsite for burial. Many non-structural components inside the reaction vessel and in the blanket will also become highly radioactive by neutron activation. While the radioactivity level per kilogram of waste will be much smaller than for fission-reactor wastes, the volume and mass of wastes will be many times larger .
Fusioneers speculate that a low-activation structural alloy ("fantasium" or "miraculum") can be developed that will allow discarded reactor materials to qualify as low-level radioactive waste and disposed of by shallow land burial . Even if feasible, no municipality or county is likely to accept such a landfill.
3. Extensive radiation shielding is needed to reduce radiation exposure of plant workers, even when the reactor is not operating. In the intensely radioactive environment, remote handling equipment and robots will be required for all maintenance work on reactor components as well as for their replacement because of radiation damage, particle erosion or melting. Remote handling equipment must also be used for the disposal of radioactive waste.
4. Tritium Release. Corrosion in the heat exchange system or a breach in the reactor vacuum ducts could result in the release of radioactive tritium into the atmosphere or local water resources. Tritium will be dispersed on the surfaces of the reaction vessel, particle injectors, pumping ducts and other appendages. Preventing tritium permeation through solids remains a critical unsolved problem, so that some of this embedded tritium will eventually find its way into external cooling systems. Most fission reactors contain trivial amounts of tritium (< 1 gram) compared with putative fusion reactors (kilograms), but the release of even tiny amounts of tritium into the cooling water of fission reactors causes public consternation .
5. Nuclear Proliferation. The open or clandestine production of Pu-239 is possible in a fusion reactor simply by placing natural or depleted uranium at any location where neutrons of any energy are flying about, including appendages to the reaction vessel. With D-only fueling, tritium breeding is not required and all the neutrons will be available for Pu-239 production. The reactor mission could be dedicated to that purpose.
A reactor fueled with D-T or D-only will have an inventory of at least kilograms of tritium, and possibly tens of kilograms. This inventory will reside in the blanket, in tritium processing systems, and embedded in reactor components, providing opportunities for diversion of tritium for use in nuclear weapons .
Just as for fission reactors, IAEA safeguards will be needed to prevent plutonium production or tritium diversion.
6. Coolant Demands. A fusion reactor is a thermal power plant like one based on coal burning or nuclear fission, and would 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 pumping. A fusion reactor would require at least 30,000 gallons per megawatt-hour of "once-through" cooling, and must compete with agriculture and industry for often diminishing water resources. With drought conditions intensifying in many regions of the world, many countries could not support any fusion reactor, even with cooling towers to mitigate water demand.
7. Outsized Operating Expenses. Fission reactors are presently being shut down in the U.S. because the operational costs alone result in an uncompetitive cost of electricity . (The capital outlays have long been paid down or written down.) Fission plants typically require at least 500 workers over four weekly shifts. Fusion reactors will also need personnel heretofore peculiar to fission plants such as security experts for monitoring safeguard issues and specialty workers to dispose of radioactive waste. Additional skilled personnel will be required to operate a fusion reactor’s more complex subsystems including cryogenics, plasma heating equipment and elaborate diagnostics. Another intractable operating expense is the large amount of electrical power consumed by fusion reactor subsystems, associated facilities and buildings during inevitable downtimes. For example, the ITER facility will consume 110 MWe even when the tokamak plasma is not operating . There are also multiple recurring expenses including replacement of radiation-damaged components in MCF and fabrication of millions of fuel capsules for each ICF reactor. A corollary of extraordinarily high and irreducible operating costs is that the capital cost of a fusion reactor must be close to zero for economic competitiveness!
These seven drawbacks shared with fission reactors apply to any fusion energy concept. While radiation damage and waste production may be mitigated if the fusion source can be surrounded by thick lithium-metal blankets, or fueled with 3 He-D, the other detriments are irremediable. Fusion proponents constantly call for a “crash program” to develop a commercial reactor. (Presumably a crash program is one that’s shorter than the half-century ITER odyssey.) But even if a working fusion reactor could be demonstrated, these drawbacks would make deployment impossible wherever fission reactors face widespread public opposition or wherever their operating costs alone produce a non-competitive cost of electricity.
Daniel L. Jassby
Princeton Plasma Physics Lab (retired)
These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.