3The AtomThe concept that things are made up of small particles dates back to ancient Greek philosophersIn fact, the term atom comes from the Greek word atomos which means indivisible.However as we’ll see that term is somewhat of a misnomer.
4Parts of an atom Nucleus Electron cloud Subatomic particles Neutron – no chargeProton – positive chargeElectron cloudElectrons – negative charge
5The concept of energy from atoms In 1904 Ernest Rutherford recognized the potential energy that could be generated from splitting atoms.On this point he wrote “If it were ever possible to control at will the rate of disintegration of the radio elements, an enormous amount of energy could be obtained from a small amount of matter.”
9The Neutron James Chadwick discovers the neutron. The discovery of the neutron was instrumental to work in fission because the neutron has no charge unlike the alpha particle which has a positive charge.As the alpha charge approaches the nucleus of a larger atom it is repelled like the two positive ends of a magnet (Coulomb barrier).Because a neutron has no charge it can enter the nucleus of larger atoms without being repelled and fission the atom.
11First Recorded Nuclear Fission In 1934 using still relatively newly discovered neutrons, an Italian physicist named Enrico Fermi began experiments bombarding uranium atoms with these neutrons which resulted in elements lighter than the initial uranium atoms.At the time Fermi did not know exactly what he had achieved through his experiments
13Further experimentation with nuclear fission and discovery of energy generation In 1938 German chemists Otto Hahn and Fritz Straussman bombarded uranium atoms with neutrons which produced lighter elements (barium) much like Fermi’s previous experiments.Hahn and Strausman contacted Austrian physicists Lise Meitner and Otto Frisch who proved that matter has been converted to energy proving Einstein’s theory.The resulting Barium was unique because it was only 60% of the atomic weight of uranium which means the nucleus had been cut almost in half.Before this time they were only able to chip away at the nucleus and the product of that chipping was the much heavier (than barium) radium.Initially Fritz was skeptical that is what was actually happening. He wrote that “A nucleus was not like a brittle solid that can be cleaved or broken.”1944 Otto Hahn is awarded the Nobel Peace Prize in Chemistry for “for his discovery of the fission of heavy nuclei”
15Nuclear PowerNuclear power is a man-made generation of energy by which a nuclear reaction is induced in order to convert the mass of the nucleus into energy in the form of heat which is used to power a turbine.
16What is a nuclear reaction? Great question so glad you asked.
17A nuclear reaction is defined as a process in which two nuclei or nuclear particles collide to produce products different from the initial particles.In nuclear power the nuclear reaction most often used is called induced nuclear fission.
18Gone FissionAll nuclei are held together by something called a strong nuclear force or strong interactionWhen bound a nucleus forms a closed system which has a definite, measurable mass.When a neutron collides with the nucleus the system is opened and some of the mass is lost in the form of binding energyThe process by which a nucleus is split releasing energy is called fissionThe sum of the remaining parts of the fission process will add up to a mass less than that of the original nucleus. The reason for this is that some of the mass has been converted to energy according to the formula e=mc2
20It’s Getting Hot in Here A nuclear fission reaction is an exothermic reaction meaning that the mass that released (i.e. binding energy) is in the form of heat.
21Generally neutrons are moving too fast to be absorbed by a nucleus. To overcome this problem moderators are introduced to the system to slow the neutrons down to the point where they will be absorbed by the nuclei causing instability and ultimately fission.One of the most common moderators used in nuclear energy production is water another is solid graphite
22Fissile vs. Fissionable A fissionable material is one capable of undergoing fission.A fissile material is one capable of undergoing a self-sustained chain reactionAll fissile materials are fissionable but not all fissionable materials are fissile235U, 239Pu,232Th = fissile238U, 240Pu = fissionableIn general, most actinide isotopes with an odd neutron number are fissile.
23Fissile vs. FertileA fertile material is a material that as is will not sustain a chain reaction but through neutron absorption and subsequent nuclei conversions the material will become fissile.
27So many choices Classifications of reactor types by varying criteria Classification by reaction typeNuclear fissionThermal reactorsFast neutron reactorsMore fissile materialNo moderatorLess transuranic wasteNuclear fusionRadioactive decayClassification by moderator typeGraphiteWaterLight elementOrganically moderated reactorsGolubev, V. I., et al. "Fast-Reactor Actinoid Transmutation." Atomic Energy = Atomnaia Energiia (1993): 83.
28Classifications cont’d Classification by coolant typeWaterLiquid metal (fast reactors)GasMolten saltClassification by generationGen I reactorsGen II reactors – most current reactors fall hereGen III reactors – improvements on existing technologyGen IV reactors – experimental technologies (including Pebble Bed reactors)Classification by phase fuelSolidFluidClassification by useElectricityPropulsionHeatTransmutation of elements
31gas-cooled nuclear reactor Pebble what?A pebble bed reactor isa high temperaturegas-cooled nuclear reactor(HTGR) that combines the fuel and moderator into small “pebble-sized” spheres which when piled in a critical geometry will allow for criticality.
32In 1944, Farrington Daniels had been working on a process of fixing nitrogen from air using a novel pebble bed heated furnace.Daniels proposed that a chain-reacting pile be constructed along similar lines. The pile would consist of uranium oxide and carbide pebbles whose heat of fission would be removed by a flow of a cooling gas. Daniels filed a patent on his idea in1945.In the patent, he calls the pile a "pebble bed reactor," claiming that the cooling gases be used to generate steam (to power a steam turbine), or "….the heated gases can be used directly in gas turbines." The next year, design work started on the Daniel’s power pile, a helium-cooled reactor based on Farrington Daniels' concept.The Atomic Energy Commission was also formed that year, and one of its first acts was to cancel the project.
33BMW, VW, PBR What’s the connection? Pebble Bed Reactor technology was first developed in the 1960s in Germany.Arbeitsgemeinschaft Versuchsreaktor (AVR) finished construction and went critical in in Jülich, West Germany.THTR-300 began operation in 1983.
34AVRTwo reactors named MERLIN and DIDO achieved criticality in 1962 after 4 years of construction.In 1967 the reactor was synchronized to the grid and began supplying electricity for consumer use.In 1985 the MERLIN reactor was decommissionedThe entire AVR facility was decommissioned in 1988During its operation the reactor was used to test several different technologies and safety measures.Perhaps best known in its lifetime for its 1970 demonstration of the passive safety measures built into the pebble bed design.
351970 AVR Reactor Safety Test All coolant was shut off and control rods were prevented from entering the reactor to slow down nuclear reactions.As the temperature rose to a peak of 1720o C, the chain reaction slowed due to the Doppler broadening until the temperature fell and stabilizedThe pebbles maintained integrity and the reactor itself suffered no adverse effects.
36THTR-300THTR-300 was under construction from when it first went criticalIt began producing electricity in 1985It had two unique features that separated it from AVRIt used pre-stressed concrete in the pressure vessel that held the pebbles (previously all pressure vessels were made of steel)The 670,000 fuel pebbles contained Thorium which is a fertile fuel not a fissile fuel in addition to Uranium.Thorium is 3x more plentiful than Uranium in the earth’s crustA lot of research and potential use of Thorium in India and RussiaIn 1985 a pebble became lodged in one of the feeding tubesIn 1989 amidst heightened scrutiny and fears caused by Chernobyl, THTR-300 was decommissioned.
39About 360,000 pebbles are placed in the core cavity. Burned pebbles are extracted from the bottom where they are examined for burn-up and reinserted in the top of the pileEach pebble can go through about 15 cycles before being completely used up
40Coolant Most PBRs use helium gas as a coolant Because helium is an inert gas it does not absorb neutrons as water does and thus it does not become radioactive when it is circulated throughout the reactor where it takes the heat away created by the fission.Helium has 3x the sonic velocity of air and 5x the thermal capacity of air meaning that it can turn turbines faster and allow more work to be done per mass unit than air.Additionally the multi-layer design of the pebble means that the helium never comes in direct contact with the radioactive material.The helium is then either uses directly to power a turbine or it is taken to a steam generator where it is forced into pipes where it heats water to the point of steam which then powers a turbineFinally the remaining heat can be used as process heat.
42Safety MeasuresMost reactor systems are enclosed in a containment building designed to resist aircraft crashes and earthquakes.The reactor itself is usually in a two-meter-thick-walled room with doors that can be closed, and cooling plenums that can be filled from any water source.The reactor vessel is usually sealed.Each pebble, within the vessel, is a 60 mm (2.6") hollow sphere of pyrolytic graphite.A wrapping of fireproof silicon carbide;Low density porous pyrolytic carbon; andHigh density nonporous pyrolytic carbonPyrolitic graphite is generally thought to be incapable of burning when exposed to air (in the event of a breach) absent some hydroxyl radical (usually from water).The fission fuel is in the form of metal oxides or carbides
43Operated at 250C above annealing temperature of graphite to prevent accumulation of Wigner energy Continuous refueling prevents excessive reactivity and also allows for the individual pebbles to be examined for defectsGraphite does not experience phase transitions
44BenefitsModular configuration combines the benefits of customizability with the benefits of uniformity. Additionally modular configuration makes the reactor expandable so utility companies can add modules as demand increases. This reduces the initial capital investment and risk that the utility companies must undertake.Uniform design means potential for less regulatory oversight to ensure compliance.Higher efficiency than other modes of electricity generation (up to 45% as opposed to 32% in other nuclear reactors)Modular design allows the plant to come on in stages producing revenues faster.High operating temperatures allows for thermo-chemical production of methanol by combining hydrogen molecules separated from oxygen molecules at such high temperatures with recycled CO2Modular design reduces costs of construction, maintenance, replacement of components, and even decommissionMIT design allows for entire reactor to be made assembly-line style and shipped to reactor site on a few railcars.PBMR can refuel while online whereas normal light water reactors have to shut down every months for 45 days to refuel
45Benefits cont’dPassive safety features reduce the risk of human error causing meltdown (Think The Simpsons)Higher efficiency means that there is almost no possibility of proliferation from spent pebbles.No corrosion of components because irradiated water will not be circulating throughout the reactor.Passive safety measures allow for smaller staff size because it obviates the “defense in depth” requirementThe pebble is designed for long term disposal without reprocessingLow cost/kilowatt hour which reduces the longer the reactor is onlineHigh operating temperatures provide use of process heat beyond electricity generation.The online fuel replenishing eliminates the time the reactor must go offline for refueling.Almost nonexistent GHG emissions (from generation)
46Conceptual depiction of modular components of a PBMR
47Problems/CriticismsThere is a learning curve in PBR technology that will have to be overcome before PBR technology becomes mainstreamCan the US wait until the learning curve is overcome or will current needs make the wait impossible?A new design means that the Nuclear Regulatory Commission will have to be educated on the new technology and new standards will have to be developed and implemented before large scale production can commence.The pebbles are larger in volume than current nuclear fuel technology which means a higher volume of waste materialIt is impractical to reprocess the fuel in spent pebbles (which means over 80% of the fuel will be wasted).There are other technologies that are currently being advanced that compete with PBR technology that show greater potential.
48The Reactor: Several different models to chose from Both the genius of the concept and a curse
50Chinese ReactorHTR-10 (High Temperature Reactor capable of 10 Megawatts heat output) was built by Tsinghua University in China.Construction began in 2000 and it went critical in 2003It is virtually a replica of the AVR plantIn 2005, China announced plans to construct another two 250Mwt pebble bed modular reactors (HTR-PM) in Rongchen City in the Shandong province capable of generating 200 Mwe.Construction is set to be finished in 2010.The HTR-PM reactor will employ a steam generator but with the capability of switching to a gas turbine as the technology matures.
51South African ReactorA South African utility company, Eskom, began investigating pebble bed technology in 1993.In 2000, Eskom began looking for investors and eventually gained the support of Industrial Development Corporation of South Africa, British Nuclear Fuels (BNFL), and the US utility Exelon.Since then Exelon has dropped out and the BNFL role has been taken over by Westinghouse, which BNFL sold to Toshiba.Eskom moved forward with its modular design partnering with Tsignhua University to help with design and sent its proposed fuel pebbles to Russia for testing.However, in September of this year the South African government who partially owns Eskom reported record losses and cited tough economic times as the reason for indefinitely suspending plans to finish the PBMR demonstration plant.Eskom hinted at the possibility of building a smaller plant for purposes other than electricity generation such as coal gasification and liquefaction; oil extraction from tar sands; or desalination of seawater.
52PBMR Work in the United States MIT has been studying pebble bed reactor technology since It has developed models for a modular reactor design.In 2003, the DOE began funding research for advanced gas reactors.The Idaho National Laboratory was chosen to do this research because it is the DOE’s lead nuclear technology R&D site.The INL has been testing the pebble fuel by subjecting it to high levels or radiation simulating years of exposure in a few months.Currently after 9% burn up of the fuel there seems to be no damage to the fuel cellsThe next milestone that the INL hoped to achieve was 12-14% in 2008Earlier this year the INL surpassed its goal of 12-14% and achieved 19% burn up. The higher the burn up the less fuel is required to produce each unit of energy.
53Future uses for PBMR technology beyond electricity generation Extracting gas from oil sandsIndustrial heatingCoal to gas operations