Nuclear energy is contributing to the long-term solution to stave off climate change. However, current nuclear fission technology accesses only about 1-3% of the nuclear energy content of natural uranium, which is inefficient, and also creates a radioactive waste disposal problem.
Combining nuclear fission technology with emerging nuclear fusion technology to create a fusion-fission hybrid would yield extra fusion neutrons to 1) convert much more of the uranium into fissionable material, which would increase efficient utilization of the nuclear fuel resource, and 2) significantly reduce (by fission) the most long-lived radioactive nuclear waste.
This book describes fusion-fission hybrid physics and technology. The first parts briefly review nuclear fission principles and describe design and safety of nuclear fission reactors; then the fundamentals of nuclear fusion and fusion reactor concepts are described, together with ongoing and future challenges and anticipated developments in this not-yet matured technology. Chapters cover the scientific basis of nuclear fission and the fission fuel cycle, advanced fission reactors, safety aspects, the scientific and technological basis of nuclear fusion power, future improvements expected, and then the fusion-fission hybrid (FFH) breeder and burner reactor concept principles, with illustrative FFH design concepts, safety analyses, and examples of the use of fusion neutrons for helping to achieve burning and breeding fission fuel cycles.
This concise work is essential reading for researchers and policy makers in nuclear energy research and engineering, including advanced students.
Inspec keywords: uranium; nuclear power; fission reactor fuel; neutron sources; radioactive waste processing; fission reactor design; fission reactor fuel reprocessing
Other keywords: fission reactor design; neutron sources; Tokamak devices; nuclear power; uranium; fission reactor fuel reprocessing; fission reactor fuel; radioactive waste processing; light water reactors
Subjects: Fission reactor fuel preparation and reprocessing; Neutron sources; Radioactive wastes from fission reactors; Fission reactor design; Nuclear reactors; Fission reactor fuel elements; Radioactive waste treatment and transmutation; Particle beam production and handling; targets
A fusion-fission hybrid (FFH) reactor is basically a copious fusion neutron source combined with a subcritical nuclear fission reactor application for those neutrons.
The annual world electric power production (in 2017) was 2.14 × 1013 kWh(e) (214 followed by 11 zeros; kWh(e) is kilowatt-hour electric) and is growing (doubling over the last 20 years) [1]. Most of this recent new power was provided by burning additional carbon-based fuels, unfortunately for mankind because of the associated atmospheric pollution leading to increased global warming with ultimately disastrous consequences, unless it is curtailed very soon.
The scientific basis for nuclear fission power is the conversion of a small amount of mass to an enormous amount of energy in nuclear fission reactions. For atoms of some isotopes of uranium and heavier transuranic (TRU) elements, the addition of a subatomic particle known as neutron to the nucleus results either in (1) the fission of the original nucleus into two or three nuclei plus two to three neutrons, less massive in sum, or (2) the capture of the neutron to create a nucleus one atomic mass unit heavier, or (3) the scatter of the original neutron to generally produce a more energetic nucleus and a less energetic neutron. (Neutrons may also scatter from an atom without entering the nucleus.) The different probabilities used for neutron capture, scattering, and fission happen to have the units of area and are thus colloquially known as "cross sections" for these neutron-nucleus reactions. In the case of neutron capture by a nucleus it is thereby transmuted into a nucleus one atomic mass unit (A) heavier but with the same nuclear charge (Z), the process being denoted [(A, Z + n ⇒ ((A + 1, Z)]. In the case of scatter, the nuclear constituents, hence the mass, remain unchanged, but the kinetic and internal energies of the nucleus are generally increased while the kinetic energy of the neutron is generally decreased.
The uranium nuclear fuel cycle is the sequence of industrial processes involved in the production of electricity from uranium and disposing of the waste. While different reactor types and reactors follow somewhat different processes involving somewhat different masses of different materials, the generic process and material balance provided by the World Nuclear Association [6] for a representative 1,000 MWe PWR using 4.5% enriched fuel to achieve 45 gigawatt-day/ton (GWd/t) burnup depicted in Figure 4.1 provides a representative case. However, we note that not all steps (e.g., reprocessing, disposal) are included in the present fuel cycles.
The isotope 235U92 has a large fission cross section for thermalized (very low energy) neutrons (Figure 3.2), but 235U92 constitutes only 0.72 atom% (0.71 wt%) of natural uranium, the remaining 99.3% of which is 238U92, which has a negligibly small "thermal" fission cross section and a small but non-negligible fast fission cross section. However, neutron capture in the more plentiful 238 U 92 initiates a variety of transmutation/radioactive decay chains that convert the almost non-fissionable 238 U 92 into various transuranic isotopes [5], some of which have quite large fission cross sections, notably 239 Pu 94 and 241 Pu 94 , the creation process of which is illustrated in the second decay/transmutation chain as shown in Figure 3.6.
The original man-made (there is evidence of a prehistoric nuclear reaction in an African cave) nuclear reactor constructed in Chicago during the WWII Manhattan Project, under the leadership of Enrico Fermi, was able to maintain a constant neutron flux level without an external neutron source ("go critical") using natural uranium of the composition of the mined uranium ore by making use of a number of design strategies such as lumping the fissionable isotopes rather than a uniform dispersion. However, it was determined at the time that such reactors were impractical and that there would be great advantage in operational capability and flexibility if the uranium was "enriched" in the highly fissionable isotope 235U92 and/or if some fissionable plutonium could be bred from 238U92 (hence the rationale for the path taken in the Manhattan Project).
The fission of a 235U92 (or plutonium or other transuranic atom) nucleus with atomic mass A uranium = 235 within a fuel element converts the mass of that nucleus to the lesser masses of the (usually two) "fission product" nuclei and the mass of two to three neutrons plus about 200 MeV of kinetic energy (energy of motion) of these particles, plus the energy of one or more "gamma rays" (similar to X-rays). Most of this energy is in the form of kinetic energy of the two recoiling intermediate-mass "fission product" nuclei, plus the considerable kinetic energy of the two to three neutrons, gamma rays, and neutrinos.
Advanced generation-3 versions of the original PWR, BWR, HTGR, and CANDU pressure tube reactors have been developed, and a series of generation-4 reactors are now under development, including the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), the molten salt reactor (MSR), the supercritical water reactor (SCWR), the sodium-cooled fast reactor (SFR), and the very high temperature reactor (VHTR) to produce high-temperature process heat.
While this book is intended to be self-contained with respect to the science, engineering, and mathematics needed to understand the subject matter at an advanced introductory level, those who might wish to carry out more in-depth investigations/calculations will benefit from some familiarity with the literature of the fields of science and technology involved.
In more than 17,000 cumulative reactor-years of nuclear power reactor operation (as of June 2019), in 33 countries, there have been only three major reactor accidents, only one of which caused fatalities. Contrary to the scare propaganda promulgated by various antinuclear groups that were formed following the end of the Vietnam War to continue the profitable "protest movement" business, nuclear fission power has been a very safe industry, with one exception - Chernobyl.
The nuclear fission fuel cycle is illustrated for present nuclear reactors in Figure 4.1 [6]. First, the mined natural uranium is processed in centrifuges to enrich the 235U92 concentration of uranium from 0.72% in natural uranium to 4-6%; this enriched fuel is fabricated into metal-clad fuel rods (or pins), which are loaded into nuclear reactors, leaving behind as residue slightly depleted (in 235U92) uranium and the "tails," consisting basically of 238U92 and less than 0.2-0.3 wt% 235U92. The tails are retained by governments for future use as fuel or in other applications where heavy atomic mass material is needed.
The nuclear fusion energy release process is different than the nuclear fission energy release process discussed previously, although the basic process of conversion of mass to energy is the same. As discussed above, the nuclear fission energy release results from the neutron-induced fission of a heavy mass atomic nucleus (e.g., uranium) into a few intermediate mass atomic nuclei and a few basic nuclear particles (e.g., neutrons, protons, alpha particles), with a net reduction in total mass, which is converted to energy via ΔE = - Δmc 2 (c is the speed of light, Δm ≤ 0).
In 1978-88, the fusion scientists and engineers of the United States, USSR, Europe, and Japan collaborated through the International Atomic Energy Agency's INTOR Workshop [35,36,37] to (1) determine if magnetic confinement tokamak fusion was ready to move forward to the experimental power reactor (EPR) stage; (2) if so, identify a conceptual design of an EPR that combined reactor-relevant physics and technology; and (3) identify and prioritize additional required R&D. Based on the positive results of the INTOR Workshop, Sec. Gorbachev suggested to President Reagan at the 1985 Geneva Summit meeting that the two countries join together to construct and operate the INTOR EPR. This led to the restructuring of the INTOR Workshop into the ITER project [35], today involving the EU, Russia, United States, Japan, China, South Korea, and India, which, after years of negotiations, R&D, and detailed design, is building such a tokamak EPR in France to begin operation in the early 2020s. The design objectives of the superconducting ITER shown in Figure 13.1 are input energy multiplication Q ≥ ≈ 10, P fus = 400 MWth. The toroidal plasma chamber is indicated by the two D-shaped open yellow spaces in the central part of Figure 13.1, with the central solenoidal magnet that induces the plasma current between them. Successful ITER operation will lead to the introduction of fusion power reactors.
While it is generally believed that ITER will achieve its objectives to explore the joint operation of a thermonuclear plasma and thermonuclear reactor-relevant technologies in a fusion tokamak environment, it is also generally believed that both the plasma operational parameters and the technology must subsequently be improved relative to that available to support the ITER design basis, based in part upon new knowledge from the operation of ITER, but also based in part on new physics and technology insights which will lead to more technically practical and economically competitive fusion reactors in the future.
It is clear from the foregoing discussion that a major limitation on the extraction of nuclear energy from uranium (and thorium) fuel ores is the relative scarcity in fission reactors of neutrons, in addition to those necessary to maintain the fission chain reaction, which could be used to transmute the (almost) non-fissionable U238, which constitutes 96% of uranium ore, into fissionable Pu239 and Pu241 or to transmute the non-fissionable Th232 into fissionable U233 or U235. This situation can be remedied by operating fission reactors with a neutron source to provide enough additional neutrons to sustain the neutron fission chain reaction and to capture neutrons in non-fissionable U238 to transmute it into fissionable Pu239 and Pu241 (see the second transmutation chain in Figure 3.6) or capture neutrons in non-fissionable Th232 to transmute it into fissionable U233 and U235 (as shown in the first transmutation chain in Figure 3.6).
The fusion-fission hybrid (FFH) reactor concept is basically a subcritical nuclear fission reactor supported by an additional D-T fusion neutron source to sustain the neutron chain fission reaction for additional energy production, destruction of "nuclear waste," breeding of fissionable material, and/or other neutron applications of the nuclear fission reactor. Such a subcritical FFH reactor would almost certainly be more complex and expensive than an "equivalent" critical fission reactor (CFR). Thus, an FFH reactor would be justified only if its overall benefits were sufficiently greater than those of a CFR for a given nuclear mission. In the technically informed opinion of the author (and others), they would seem to be.
There have been a number of scoping "designs," at various levels of detail, of fusion-fission hybrid (FFH) reactors published in the literature over the years. Many of these have been individual design studies, but there have been at least two collected sets of studies of particular reactor types for different FFH missions. We will illustrate the genre (1) with a discussion of the recent substantial Georgia Tech subcritical advanced burner reactor (SABR) (TRU burner) [12,25] and SABrR (Pu breeder) [48] tokamak reactor FFH design studies and (2) with a brief summary of an earlier, but recently updated, comprehensive series [10,13] of magnetic mirror FFH studies from a few years back. The tokamak is the leading fusion reactor concept in terms of achieved performance, level of development, and level of effort world-wide, while the magnetic mirror is probably the geometrically simplest magnetic fusion reactor concept, which encountered particle and energy confinement problems several years ago, but has more recently achieved more encouraging confinement results.
Nuclear fission power has an unresolved, but not unresolvable, problem which fusion can at least in large part solve the disposal of spent nuclear fuel (SNF) containing radioactive transuranics (TRU) with extremely long half-lives of 100,000+ years. While disposal of this "spent" fuel by burial in secured repositories appears to be technically feasible, it is wasteful of an enormous nuclear fuel resource and is adamantly, if irrationally, opposed politically, at least in Nevada in the United States. A more efficient, but more technically difficult, solution of the SNF issue is to separate the long-lived TRU in SNF, the most long-lived of which is mostly fissionable minor actinides (MA), and fission them, thereby not only diminishing the SNF "nuclear waste" problem but also extracting about 33% more nuclear energy from the nuclear fuel in the process. What is needed is the neutrons with which to fission these actinides, and these can come from a fusion neutron source, i.e., a fusion-fission hybrid burner reactor.
SABR was modeled in ERANOS (European Reactor ANalysis Optimized calculation System), a fast reactor code system developed to model the Phénix and SuperPhénix reactors. ERANOS employs the European Cell COde (ECCO) to collapse 1968-group JEFF2.0 cross sections within each reactor lattice cell to the 33 groups used in core transport calculations, varying from 20 MeV down to 0.1 eV. The core geometry was described in R-Z cylindrical geometry and the core calculations performed in the ERANOS discrete ordinates transport module BISTRO using an S8 quadrature with 132 radial and 216 axial mesh points. The fuel was depleted for 100 days in each burnup step in the EVOLUTION module before reperforming the core neutron flux calculations.
Some insight into the physics and engineering constraints on FFHs can be obtained from relatively simple physics models of global neutron and plasma power balance computational models and models of technological constraints [19-21,26,28-30,32,33] that serve to illustrate how (tokamak) physics and technology limits/constrains the fusion neutron source strength and the level of auxiliary power needed for the neutron sources. We illustrate this by calculating the total neutron population in a nuclear reactor with an external fusion neutron source S using the so-called "point kinetics" model describing the dynamics of the total neutron population "n" in a nuclear reactor with an external source S. Extensions to more sophisticated models are readily envisioned.
Substantial progress has been made in recent years in achieving the plasma conditions required for a tokamak fusion neutron source. Using DT fuel, fusion powers exceeding 10 MW have been produced in both TFTR and JET. DT plasmas in these devices have approached the conditions required for Q p = 1. Operating with deuterium plasmas, JT60-U has reached parameters exceeding those required for Q p = 1 in DT plasmas.
As discussed previously, the availability of neutrons ultimately limits the amount of the nuclear energy in the uranium and thorium ores that can be extracted in critical nuclear fission reactors operating on "breeding" fuel cycles. More neutrons are needed to "breed" fissionable 239Pu94, 241Pu94, and higher fissionable transuranics by neutron capture in the majority isotope in uranium ore, 238U92 (following the neutron transmutation/decay chain of Figures 3.1 and 3.6), and at the same time more neutrons are needed to fission enough fissionable atoms to maintain the fission neutron chain reaction in larger reactors.
At the present rate of nuclear power generation in the United States, enough spent fuel will soon have accumulated to fill a Yucca Mountain-type high-level waste repository (HLWR). The forecast for increased power generation by nuclear power in the next 30 years and over the coming century magnifies the issue of spent nuclear fuel (SNF) disposal. Between 2007 and 2010, the US Nuclear Regulatory Commission (NRC) accepted applications for 26 new light water reactors (LWRs), and the NRC expected applications for another five reactors in 2011 (Ref. [3]). These 31 reactors would increase the current nuclear power output of the United States by 30%, increasing the amount of discharge fuel needed to be stored in geological repositories by a similar amount.
We have investigated two types of fuel cycle for a SABR consisting of an annular, Na-cooled fast reactor surrounding a tokamak fusion neutron source. The first fuel cycle type is one in which all of the TRUs in LWR SNF are transmuted in a SABR, and the second fuel cycle type is one in which some of the plutonium in LWR spent fuel is set aside for future use and the remaining plutonium plus the MAs are transmuted in a SABR. In both fuel cycle types the fuel residence time between reprocessing steps was set by clad radiation damage limits (200 DPA reference value), and the separation of TRUs from fission products was assumed to be only 99% efficient. We found that, by repeated recycling of the TRU fuel discharged from SABR with a blend of fresh TRUs discharged from LWRs, the decay heat of the repository content could be reduced by a factor of ∼10 at 100,000 years relative to the decay heat if the discharged fuel from LWRs was buried directly (Figure 24.1). Noting that decay heat load was the limiting design factor for Yucca Mountain capacity, this reduction in decay heat implies a corresponding reduction by a factor of 10 in HLWR capacity requirement. This result is based on the conservative assumption that the actinide-fission product separation efficiency is only 99%. We note that there are other measures (e.g., Sr and Cs management and cooling before storage) for reducing the required repository capacity, and they are not incompatible with the transmutation solution proposed here. A 3,000-MW(thermal) SABR operating on such fuel cycles, with 75% availability, would be capable of burning all of the TRUs discharged annually from 3 LWRs of 1,000 MW (electric), or burning all of the MAs and some of the plutonium discharged from 20 to 25 LWRs of 1,000 MW (electric). Thus, one could envision a nuclear fleet with 75% of the energy produced by LWRs and 25% of the energy produced by SABRs that burned all of the TRUs discharged from the LWRs. Alternatively, one could envision a nuclear fleet with 95% of the energy produced by LWRs and 5% produced by SABRs that burned the MAs (primarily) and some of the plutonium discharged from LWRs, while plutonium was accumulated to start up fast reactors.
Closing the nuclear fuel cycle is an important step in advancing the prospects of nuclear energy in both the near and far terms. The once-through cycle largely employed today uses a very small percentage of the potential energy content of natural uranium ore and produces high-level waste, for which we have yet to implement a long-term solution. A solution to the overall fuel cycle problem would have the dual benefits of extending the uranium resources of earth by a factor of 10-100 over the once-through cycle and of greatly reducing the volume, decay heat, and longevity of repository-bound waste. Various fast reactor technologies and designs have been developed with the intent of closing the front end (breeder reactors), the back end (burner reactors), or both, of the fuel cycle.
It is standard practice to analyze the dynamic response of nuclear fission reactors to unanticipated malfunctions. A set of "design basis accidents" have been developed for this purpose: loss-of-coolant accident (LOCA), loss-of (coolant)-flow accident (LOFA), loss-of-power accident (LOPA), etc. We have examined the consequences of such design basis accidents in SABR#1 (Na loop cooling), in SABR#2 (Na pool cooling), and in SABR#3 (10-node calculation model of SABR#2).
The subcritical advanced burner reactor #2 (SABR#2; Figure 27.1) reactor is a Na-cooled, pool-type, metal-fueled fast transmutation reactor, described previously.
In order to impact climate change over the next century, a massive source of clean, carbon-free energy is needed. In the authors' opinion, nuclear power provides the only technically and environmentally credible option. A major technical problem preventing the widespread expansion of nuclear power is spent nuclear fuel (SNF). The US government currently has no plan to deal with SNF generated by commercial nuclear power plants. Plant operators have been forced to stockpile SNF on-site. The common suggestion to deal with SNF is the high-level waste repository (HLWR). A repository would have to store the SNF on the order of a million years until it is no longer significantly radioactive. Various organizations have proposed several HLWRs, but none of them are close to becoming a reality in the United States.
Why not just harvest free green energy? Isn't it much simpler?
An appealing, but in large part misleading, argument has been made that "green" energy freely existing in nature - wind, solar, rivers, waves - can just be harvested to replace the power produced by burning carbon-based fuel. Many people argue wouldn't it be simpler, and somehow purer, to just use this "free" energy than to develop all of this nuclear energy?
About 4.17 trillion kWh(e) of electricity were generated at utility-scale electricity generation facilities in the United States in 2018. Of this, 64% was from fossil fuels (coal, natural gas, petroleum, and gas), 19% was from nuclear energy, and about 17% was from solar, wind, hydro, and other "renewable" energy sources. The task before us is to displace this 64% of the electricity from fossil fuels with electricity that does not put more carbon in the atmosphere and that minimizes negative environmental impact, and to make similar displacements elsewhere in the developed and developing world.