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Home , Fusion power

Physics The Power of the Future

Gareth Roberg-Clark ‘14

ow will human civilization topes of elemental , which power itself as time goes on? typically has no . The reaction HThis question has pervaded products are -4 (also called an scientific debate and research for years ) and a fast-moving neu- as humanity has continued to consume tron (4). Fig. 2 depicts this reaction. massive quantities of fossil fuels. One The reaction’s output energy is alternative to fossil fuels, nuclear fu- granted to the product , which sion, was viewed in the 1950’s as a in a fusion reactor collides with a blan- means of completely replacing fos- ket material covering its walls. The sil fuel usage without generating any atoms in the blanket material vibrate, greenhouse gas emissions. Though heating up and boiling water that sur- Image retrieved from: http://2.bp.blogspot.com/_7rUC6j6LDl8/TNQFC8ywc6I/ research into fusion reactors began in AAAAAAAAAI0/c-xbFXnmbak/s1600/curve+of+binding+energy+raster+big.png rounds the chamber. When converted earnest in the following decades, sci- Fig. 1: Binding energy and number of nucleons to steam, the water vapor will turn entists began to realize the enormous (3). a turbine and generate . obstacles they faced in getting fusion The D-T reaction, however, has a light elements (smaller mass number reactions, which normally occur in the downside. It requires , a radio- than iron, Fe) fuse or when heavy ele- intense heat and of , to active that has a half-life of 12.6 ments (higher mass number than iron) successfully occur in confined spaces years. This means that in 12.6 years, split apart. Mass number refers to the on earth (1). A fusion reactor capable half of a tritium sample’s atoms will sum of an element’s and neu- of powering humanity to this day has send out potentially damaging particles trons, which make up atomic nuclei. not been developed despite numerous and photons. The process continues Another quantity called binding energy and expensive attempts at creating until the tapers off. Though refers to the potential energy used to controlled fusion. David Montgomery, 12.6 years is a relatively small half life, keep an atom’s mass compacted into the Eleanor and A. Kelvin Smith Pro- tritium is still radiotoxic, meaning its a nucleus (3). Fig. 1, which compares fessor of Physics at Dartmouth College, radiation could damage exposed living average binding energy (BE) and num- believes that fusion research could tissues over time (5). Tritium is also ex- ber of nucleons (A), shows the divide have had much greater success by now ceedingly rare in nature and has to be at iron between fusion and fission. had the underlying nature of matter at bred in a reaction with metal. One promising fusion reaction is extremely high and pres- This lithium will most likely reside in the D-T reaction between sures been more thoroughly investigat- the blanket material of the reactor, al- (a hydrogen atom with a neutron in ed before delving into these projects (2). lowing neutron collisions to constantly its nucleus, A=2) and tritium (a hy- However, he also believes that, if create fuel for fusion reactions to occur. drogen atom with two neutrons in its properly executed, fusion would be an Fig. 3 describes the breeding process. nucleus, A=3). Both are nuclear iso- amazing asset to humanity. Fusion has Breeding complicates the process fantastic long-term benefits. Its fuel is abundant, it generates very little waste, and the chance and potency of its disas- ters are minimal. Fusion has plentiful fuel, it is sustainable, and it is very safe. For these reasons, nuclear fusion could be the answer to humanity’s energy prob- lems thousands of years into the future. Nuclear reactions are reactions between atomic nuclei. Fusion is one type of nuclear reaction in which two nuclei collide with each other, fusing into a larger mass. The other type, fis- sion, is the splitting apart of a nucleus into smaller nuclei. If a nuclear reaction is to be used as a power source, it has to release more energy than was put into Image by Chen Huang ‘12, DUJS Staff. its reactants. Energy is released when Fig. 2: Tritium reacting with Deuterium.

6 Dartmouth Undergraduate Journal of Science the D-D reaction (or other reactions sion fuel, causing the samples to blow that utilize different elements) will up rapidly. Inertial confinement seems take time and considerable research. more promising as a means of weap- Fusion occurs constantly in the ons development as opposed to power . The sun’s environment has the generation (8). No current project in- ideal conditions that allow nuclei to corporates all of the elements of a suc- fuse and radiate energy. On earth, a cessful fusion reactor, and this is why sustained environment has to be cre- fusion power, if successful at all, can ated to allow nuclei to interact in this only be a viable energy source decades manner. Often the environment is a into the future. Once the power gen- , a fluid composed of negatively eration hurdle is overcome, however, and positively charged . The fusion humanity could continue to reap the fuel is heated to temperatures on the benefits of this plentiful energy source. order of millions of degrees Celsius, One reason nuclear fusion could forcing the and nuclei of each remain a viable energy source long atom apart and creating the plasma. into the future is the widespread and The atomic nuclei, now isolated from consistent availability of its fuel, re- negatively charged electrons, have gardless of which reaction is carried enough energy at high temperatures out. Deuterium (in the form of D2O to overcome the electrostatic repulsion molecules) can be extracted from sea- between positively charged protons (7). water on a continual basis. Tritium is Once nuclei are close enough, the at- not present in nature and will be ob- tractive weak can attract tained from breeding reactions with the two nuclei and force them to fuse. lithium metal or from reactions in Unfortunately, plasmas are exceeding- plants. Lithium metal, ly difficult to contain. The constituent though present in small concentrations ions of the plasma tend to spread out- in seawater, will likely be extracted ward rapidly, dissipating heat by knock- from the earth’s crust (7). Without tak- ing into nearby neutral particles and ing into account the lithium present eventually recombining into atoms (8). in seawater, a report by four research- A few methods of confining plas- ers of the European Fusion Develop- mas have been devised. One is magnetic ment Agreement (EFDA) asserts that confinement, in which applied magnet- there is enough lithium to last a fusion ic fields corral the ionized plasma, mak- power economy for 1000 years (10). ing sure that it does not knock into the This leaves ample time for research- reactor walls. Magnetic confinement is ers to develop reactors that utilize the Image by Chen Huang ‘12, DUJS Staff. used in devices, an example of D-D reaction. Once that technology is Fig. 3: Generating lithium via breeding of which is ITER, an international proj- mastered, seawater could be harvested reactions. ect based in the south of France and run to provide fusion power indefinitely. of sustaining fusion fuel supplies con- by Russia, the U.S., India, China, Ja- An issue present in any energy siderably because the blanket mate- pan, and the European Union. ITER’s source is the production of harm- rial has to constantly flush out newly goal is to reach ignition – a state in ful waste, be it radioactive, polluting, made tritium. Since tritium’s radioac- which the fusion fuel burns without ad- or greenhouse-gas-effect-enhancing. tive and rare nature is problematic, it ditional supplied energy – by the year Since the D-T reaction produces non- would be better to eliminate it from the 2026. ITER’s ignition will last at most radioactive Helium-4 and a neutron, reaction altogether. In fact, two deu- for 400 seconds (7). , which fusion will produce no direct radioac- terium nuclei can fuse in a D-D reac- also use magnetic confinement, may be tive waste and will not create green- tion. There are three sets of products able to harness the power of fusion re- house gas emissions. The same goes this reaction can create depending on actions over sustained periods of time. for the D-D reaction, which will not how the deuterium fuses: tritium and This is the goal of Wendelstein 7-X, a produce regardless a , Helium-3 and a neutron, or project at the Max Planck Institute for of which specific reaction takes place, Helium-4 and a high-energy gamma Plasma Physics in Germany that is to be provided any tritium produced is ab- ray (6). The only radioactive product finished by 2015 (7). Wendelstein 7-X’s sorbed as fuel and rapidly consumed is tritium, which could immediately reactor will use a complicated “twisted to make Helium-4 (6). such fuse with deuterium to produce more ” shape to confine its plasmas (9). as ITER, however, will produce two energy. At the moment, however, fu- Inertial confinement, another indirect sources of radioactive waste: sion reactors are incapable of sustain- method, is under development at the tritiated dust and activated materials. ing the temperatures required for the National Ignition Facility in the United Tritiated dust results from interac- D-D reaction to occur (7). Designing States. In inertial confinement, vast ar- tions between hot plasmas and their a fusion reactor that could facilitate rays of lasers are aimed at pellets of fu- containing walls in tokamaks. A study FALL 2011 7 done by H. Maubert of the ITER orga- supplies would be cut immediately if References nization and L. Di Pace of ENEA FUS/ any malfunction in the reaction cham- 1. R. B. White, Theory of tokamak plasmas TEC in Rome analyzed the output and ber occurred. Similarly, in the event of (North-Holland, Amsterdam, 1989). possible impact of this radioactive dust an in-plant accident, power to the re- 2. D. Montgomery, Personal Interview, 21 May on reactor workers and nearby civil- action chamber would be cut off (7). 2011. ians. They concluded that the minimal Emergency scenarios for fusion 3. R. Nave, Nuclear Binding Energy (n.d.). Available at http://hyperphysics.phy-astr.gsu. amount of dust created (0.25 grams plants still need to be considered, how- edu/hbase/nucene/nucbin.html (20 May 2011). each year) would even in a worst- ever. If an in-plant emergency occurred 4. M. Moyer, Sci. Am. 302, 50-57 (2010). case scenario have a negligible impact in which tritium fuel inventories leaked Available at http://www.nature.com/ on these two groups of people (11). outside the plant to the public, civilians scientificamerican/journal /v302/n3/full/ scientificamerican0310-50.html (22 May 2011). The other source of radioactive would be exposed to an amount of ra- 5. Radiotoxicity (n.d.). Available at http:// waste, activation, occurs as the walls diation roughly equal to that of natu- www.euronuclear.org/info/encyclopedia/r/ of a fusion the reactor are continually ral radiation sources (10). In an even radiotoxicity.htm (02 June 2011). bombarded with high-energy neutrons. worse scenario in which an earthquake, 6. S. Atzeni, J. Vehn, The Physics of Inertial Fusion: Beam Plasma Interaction, The blanket material progressively de- the likes of which has never been re- Hydrodynamics, Hot Dense Matter (Clarendon grades until it has become brittle and corded by humanity, struck a finished Press, Oxford, 2004). radioactive (7). According to Cook et ITER plant, about 1 kg of radioactive 7. - a al., the quantity of degraded material tritium could be released to the public, Resource (n.d.). Available at http://www.world- nuclear.org/info/inf66.html (22 May 2011). produced is comparable to that of a exposing civilians to about 0.4 8. B. Rogers, Personal Interview, 19 May 2011. fission reactor (10). However, the ma- of radiation. To give some comparison, 9. D. Fisher, Personal Interview, 19 May 2011. terial produced by fusion and fission the Fukushima Daiichi compound re- 10. I. Cook et al., Safety and Environmental are inherently different: activated fu- leased 0.4 sievert per hour after a mas- Impact of Fusion (2001). Available at http:// www.efda.org/eu_fusion_programme/ sion materials have a “maximum decay sive earthquake struck the plant (12). downloads/scientific_and_technical_ heat intensity nearly a hundred times Therefore, a destroyed fusion plant publications/SEIF_report_25Apr01.pdf (22 May lower and a lower long-term radiotox- similar to ITER could release a fraction 2011). icity than fission materials” (10). This of the total radioactive material that a 11. H. Maubert, L. Di Pace, Radioprotection 43, 13-22 (2008). Available at http://www. means that, compared to fission-ac- top-tier nuclear fission disaster would. radioprotection.org/index.php?option=com_ tivated waste, fusion-activated waste Thus, fusion reactors could re- article&access =standard&Itemid=129&url=/ requires less maintenance and en- lease fast-spreading and potentially articles/radiopro/pdf/2008/01/rad200720.pdf (22 ergy to store and poses a smaller po- harmful tritium into the environment. May 2011). 12. R. Buerk, Japan earthquake: Radiation tential health risk to factory workers However, the probability of such disas- levels fall at Fukushima (2011). Available and civilians in the surrounding area. ters occurring is low, especially con- at http://www.bbc.co.uk/news/world-asia- The EFDA suggests that with ade- sidering that even more sophisticated pacific-12749444 (22 May 2011). quate research and incentive the major- defense mechanisms and containment 13. I. Sample, Flagship Iter fusion reactor could cost twice as much as budgeted (2009). ity of stored activated material could be methods could be developed as time Available at http://www.guardian.co.uk/ recycled, reused, or at worst be placed goes on. As such, fusion power’s po- science/2009/jan/29/nuclear-fusion-power-- in a repository, allowing its radiotoxic- tential to drastically cut the sever- funding (22 May 2011). ity to taper off over the course of a few ity and likelihood of future emergen- hundred years after an initial storage cies makes it an even more appealing period of fifty years (10). If proper re- long-term alternative energy source. search into appropriate recycling meth- With the right investment of re- ods is done, fusion technology could sources and time, nuclear fusion could produce very little lasting radioactive become an ideal power source long waste whose threat to humanity and the into humanity’s future. Fusion excels environment virtually disappears over in terms of fuel availability, waste pro- short time scales. In essence, fusion duction, and safety, though developing would be a sustainable energy source. fusion as a power source will no doubt Fusion would also be a safe power cost colossal amounts of money, as the source. The danger of nuclear-power- steep cost of the ITER project (€10 bil- related emergency situations, all too lion and rising) attests (13). One must clear after the recent disaster at the bear in mind that any alternative en- Fukushima Daiichi nuclear plant in ergy source that effectively cuts green- Japan, would be diminished greatly if house gas emissions will have an unde- fusion power replaced fission power sirable and weighty price tag. However, in the future. Radioactive meltdown if researchers and governments take would be a non-issue in fusion reac- the initiative to fully understand the tors for two reasons. Sustained fusion behavior of plasmas and harness the reactions require a continual supply of best reactions available, nuclear fu- fuel, much like wood fires. “Runaway” sion could supply much needed power reactions are not possible because fuel for the rest of humanity’s existence.

8 Dartmouth Undergraduate Journal of Science