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Dr. John A. Parmentola and Dr. Charles E. Kessel
In a recent Forum article (October 2016) Dr. Jassby identifies seven technical issues that a commercial fission and a future fusion reactor have in common. The theme of his argument is that as a result of these issues commercial fission is failing in the energy marketplace, and because these issues are shared it is likely that fusion will also fail.
Fission reactors have been adversely affected by the dramatic drop in the price of natural gas, largely due to fracking. The relatively low-risk premium for the financing and building of natural gas power plants, the very high efficiency of combined cycle natural gas plants (63%), and the substantial subsidies and government priorities afforded to electricity produced by renewables, have reduced the competitiveness of nuclear power. Fission has also relied on a technology whose efficiency (33%) has not changed over 60 years. Its demise is in large part a result of incremental improvements on 60-year-old concept and a poor investment strategy to improve its performance.
The goals of advanced fission concepts are to reduce the price of electricity, improve safety, reduce waste production and increase proliferation resistance. The vast majority of advanced fission reactor concepts involve higher operating temperatures, as high as 850 C, higher thermal conversion efficiencies, and fuel burnups 3 times that of current reactors. Research on new high temperature and radiation resistant materials, such as ceramic materials, for cladding, heat exchangers, and vessels, can have significant impacts on conversion efficiency, fuel burnup, safety margins, and waste production. Other concepts involve different coolants to increase operating temperatures, and still others offer unique core designs that enable higher fuel burnups. To reduce the risk premium of fission reactors, there are now several modular designs that allow manufacturing and assembly at a factory and then shipment by ground or barge to a reactor site for installation. Much of this is also relevant to the future of fusion reactors.
Abundant supplies of low cost energy are critical to coping with the challenges of world population growth and the demands that will be placed on a broad range of resources such water, food, materials, healthcare, communications, and transportation. The world cannot grow and the quality of life improved without abundant, affordable and reliable amounts of energy.
Utilities, taking advantage of the low costs of natural gas, are replacing aging coal plants at a higher than expected rate, resulting in electricity production from natural gas now surpassing coal. This will very likely continue as long as future government policies do not adversely affect the natural gas industry or the price of natural gas does not significantly increase.
If you now add to this, the aging of approximately 100 U.S. fission power plants, whose licenses are scheduled to end on a timetable from now until 2050, natural gas is again the likely replacement option.
The elimination of coal and fission plants amounts to a 53% reduction in our current total national electricity production. If electricity demand increased by just 1% per year from now to 2050, then that would require an additional increase in electricity production capacity by over 40%.
From now until 2050 the gradual shortfall of electricity capacity, whether 53% or 90% or if demand increases significantly, would have to come from only two sources, natural gas, which is an exhaustible resource, and renewables, which have significant scalability and operating capacity issues given the potential magnitude of the shortfall. This is not a viable energy security strategy for a major economic power like the U.S., because it could make our nation dependent upon others for our energy needs.
There are two scalable clean-energy options to consider, namely fission and fusion power. Nuclear fission and fusion power both have a common strategic advantage, large fuel reserves. For properly designed advanced nuclear reactors, proven uranium reserves (those reserves that today can be affordably extracted from the earth) have enough energy content to last the entire world well over 10,000 years based upon total world electricity consumption per year. Unfortunately, current nuclear reactors severely under-utilize the usable energy out of uranium fuel, which contributes to the larger nuclear waste volume.
For fusion energy, lithium is the key energy resource, which based upon proven reserves, could last well over 20,000 years assuming the same world electricity consumption rate per year. Such enormous amounts of energy do not include all known reserves of uranium and lithium, such as those that today are not economically extractable in the oceans.
1) Radiation damage imperils reactor integrity.
The only accessible fusion nuclear reaction is the combination of deuterium (D) and tritium (T) to produce a neutron and a helium nucleus, because there are ways to confine plasmas and heat them so that they generate thousands of mega-watts of energy to ultimately convert heat into electricity. For the D + T reaction, the neutrons stream out of the hot plasma and into the surrounding structures, heating them and providing the dominant part of the energy to make electricity. The differences between fission and fusion neutron exposure of materials has been recognized and addressed for some time now . In response to a desire to reduce the radioactivity of materials exposed to neutrons in fusion, alloy elements were replaced/minimized with much lower activity elements (e.g. Mo by W or V, Nb by Ta), creating the reduced activation ferritic-martinsitic (RAFM) steels, which are now the basis for all worldwide efforts to pursue fusion power plants. In addition, the materials community has identified and verified that precipitates in solids can be used to enhance strength and operating temperatures, and to trap the helium produced by fusion neutrons, thereby mitigating the degradation mechanisms associated with this gas produced in solids. Advanced RAFM steels are being developed at the laboratory level and activities to pursue industrial level production are being explored. Extending the lifetime and reducing the radioactivity of these materials in the harsh fusion nuclear environment is a task that the fusion research community is tackling. The issue is how long can these materials last in a fusion reactor designed for commercial use until they have to be replaced.
2) Radioactive waste
Fusion power plants will generate radioactive waste. However, all of this waste can be classified as low level waste (LLW), allowing its disposal in shallow burial repositories. In contrast, fission power plants generate LLW, intermediate level waste (ILW) and high level waste (HLW). Fission waste as spent fuel assemblies must be disposed of in deep geological repositories to guarantee no contact with the biosphere for many hundreds of thousands of years (e.g. Yucca Mountain). The volume of LLW and ILW generated per year from a PWR is ~ 200-350 m3 . For a 40 year plant life, the HLW volume is ~ 800 m3 and LLW/ILW volume is ~ 8000-14000 m3. Here we are quoting World Nuclear Association data  for operating fission power plants.
The estimated fusion waste volumes range from 1500-8000 m3  including the multiple fusion core blankets required during the plant’s lifetime, assumed to be about 50 years. These waste volumes are not significantly different from fission waste volumes, however efforts continue in the fusion community to minimize this waste  by 1) choosing/developing low activation materials, 2) controlling impurities in materials, 3) recycling/clearing plant materials with little to no induced radioactivity, 4) controlling the material choices where possible to allow recycling/clearance after short periods of time (1-10 years), and 5) guaranteeing that the highest radioactivity wastes will decay to low levels in a few decades after they are removed from the fusion core.
Nuclear analysis has shown how strongly the radioactivity of materials can be reduced when specific elements are controlled, and this is the reason the fusion program is pursuing improved materials so aggressively. These materials are not phantasm.
Contrary to the Jassby’s claim, there is a private company in West Texas, WCS Corporation, supported by its surrounding community, that already operates two LLW waste facilities, and has applied for NRC licenses to operate a consolidated interim storage facility to prepare used nuclear fuel for long term disposal at a geologic repository.
3) Radiation shielding
Radiation shielding is required in fusion power plants, and remote handling of all operations inside of the bio-shield (the barrier separating where humans can work with no impact on their exposure) in a fusion power plant is mandatory. We have a great deal of experience shielding, monitoring and handling of radiation sources through the use of neutron absorbing materials and remote handling equipment. The experience of remote handling on fission power plants, and at research hot cell facilities (going on for nearly 5 decades) is considered a very strong basis for fusion’s needs. A good example of our existing remote-handling capability is the fact that no human being has ever entered the H-Canyon facility at the Savannah River Site  over the 60-year span of its use in the handling, processing and disposal of special nuclear materials. The development of systems relevant to fusion is an important part of the fusion program.
4) Tritium release
Since tritium is not naturally occurring, a fusion power plant must generate its own tritium, by taking advantage of the neutrons produced through their interaction with lithium-based compounds in the blanket. Detailed calculations of the neutron behavior show that breeding enough tritium is achievable, and a recent JASON study  independently confirmed this conclusion.
Tritium WILL NOT BE RELEASED to the atmosphere in a fusion power plant from breaches in reactor vacuum ducts, heat exchangers, etc. Tritium permeation is NOT a critical unsolved problem in fact complex situations have been studied including tritium behavior in neutron irradiated samples while exposed to tritium plasma. Predicted total tritium levels in a fusion power plant can range from ~ 1-10 kg, which is much higher than the tritium levels produced in a fission power plant. For comparison, the Savannah River National Laboratory, which is the primary tritium and nuclear material handling laboratory for U.S. defense (and many other functions), has an inventory limit of 7 kg of tritium, and a processing rate of 2-3 kg of tritium per year. For ITER, the on-site inventory limit for tritium is 4 kg, and the targeted maximum loss is ~ 27 Ci/day. The containment and control of tritium is a fundamental design aspect of a fusion power plant, involving identifying all sources/inventories, all migration pathways, all forms (e.g. gas, water, dust), all materials interfaced, all environmental parameters both normal and off-normal (e.g. temperatures, pressures), all interfacing equipment (e.g. pumps, extraction, heat exchanger, purification, storage), and establishing a plant wide series of isolation rooms, regions, and buildings. Multiple barriers are utilized throughout the plant. All appropriate forms of de-tritiation will be present in a fusion power plant from the fusion core to the turbine building.
Tritium releases from fission power plants vary widely. For 2004-2005  gaseous releases range from 1 to 972 Ci (1 Ci = 10-4 grams of tritium), and the liquid effluent releases ranged from 142-2951 Ci. Comparing the fusion targeted limit of 3650 Ci/year, the releases would be similar, albeit the fusion power plant would have an extensive tritium sequestering and control system in place since its plant tritium inventory is much higher than fission. Tritium is one of the most critical quantities in a D-T fusion power plant, and this guides the fusion community’s emphasis on all tritium aspects.
5) Nuclear proliferation
Using a fusion reactor for proliferation is at least as impractical as using a commercial fission reactor to produce weapon grade plutonium. Proliferation cannot be described in any depth in a letter such as this, and so we refer to the several journals, papers and workshops on the topic [9,10]. The need for IAEA standards and safeguards for a fusion power plant does not appear to us to present a barrier to fusion power production, or even a discouragement, particularly since the IAEA has been involved in a wide range of fusion development activities for over 40 years already.
6) Coolant demands
Fusion power plant designs over the last 30 years have generally moved away from water as a coolant due to its limitations in the operating temperature (< 330C), high required pressures to avoid boiling (20 MPa), safety issue of water-lithium interactions, possibility of hydrogen explosion when water is dissociated during accidents (like Fukushima), and limitations in thermal conversion efficiency to make electricity. Helium is most often used as the coolant, and water is even eliminated as the secondary coolant in order to keep thermal conversion efficiency high (~45-60%) including the use of a bottoming cycle to utilize the waste heat to further improve efficiency. A great deal can be done to minimize water usage, these techniques are known, and they will not limit fusion’s potential.
7) Outsized operating expenses
Jassby’s reference to ITER electricity consumption whether during plasma operation or not, is not a relevant benchmark to judge fusion power plants. ITER’s mission is to demonstrate the sustained burning plasma for times long compared to plasma time scales and with sufficient gain (fusion energy produced by the plasma per energy injected into the plasma). It has no mission whatsoever to produce electricity, to minimize recirculating power, or to demonstrate efficient subsystems, all relevant goals of a power plant. The factors mentioned, recirculating power and replacement of fusion core components, have been included in conceptual power plant studies and viable power balance and net electricity production have been found.
The path we are on in eventually eliminating coal and nuclear fission reactors raises serious national security concerns. The real challenge we face is to create technically and economically feasible options over the next 35 years. This will require strong dynamic leadership with vision, serious efforts in long-term planning and sustained investment in high-risk/high-payoff R&D. The annual expenditure on energy in the US is ~ 1.2 trillion dollars, which comprises about 8.5% of the gross domestic product, and the 5 billion dollars spent on energy research per year appears to be utterly inadequate. Given the shortfall in electricity described above by 2050 and the limited options afforded by the current path we are on, more funding should be devoted to a portfolio of credible options for our long-term energy needs such as advanced fission reactors and fusion. Since we do not know what the future will bring, this would be a sensible hedging strategy to provide us with energy security options to adapt to a highly uncertain future.
 S. J. Zinkle and J. T. Busby, Materials Today, http://www.sciencedirect.com/science/article/pii/S1369702109702949
 N. P. Taylor, et al, http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.135.1187&rep=rep1&type=pdf
 Savannah River National Laboratory (F-Canyon and H-Canyon) http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.558.7021&rep=rep1&type=pdf
 JASON report on Tritium, http://fire.pppl.gov/jason_tritium_fusion_2011.pdf
 The Nonproliferation Journal, http://www.tandfonline.com/doi/full/10.1080/10736700.2013.852876?src=recsys
These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.