What are fission and fusion? What fuels a nuclear reaction? Nuclear Powerplants What are fission and fusion? What fuels a nuclear reaction?

So how do we use the heat of fission to boil water, make steam, spin a generator, and make electricity? The Harnessed Atom Atoms and Isotopes

Today’s Topics Nuclear Reactors Using heat to generate electricity Fueling the reaction Controlling the reaction Moderator and coolant Pressure vessel Using heat to generate electricity Heat transfer Types of nuclear powerplants Safety is built in to nuclear powerplants Layers of containment Engineered safety systems Monitoring system Workers contribute to safety Nuclear Regulatory Commission The Harnessed Atom Atoms and Isotopes

The Reactor The heart of the nuclear powerplant is the reactor. The reactor is made up of 4 parts: Fuel assemblies Control rods Coolant/moderator Pressure Vessel These 4 parts are all contained in the containment structure The Harnessed Atom Atoms and Isotopes

Fuel Assemblies Uranium pellets Fuel Rods Fuel assemblies are stacked together into… Fuel Rods which are bundled together into… Fuel assemblies The Harnessed Atom Atoms and Isotopes

Balancing the Reaction Reproduction constant (K). The average number of neutron released from each fission event (the splitting of one atom). K = 1: For a chain reaction to sustain itself, at least one neutron from each fission event must strike another nucleus and cause a new fission event. The reactor is considered critical. The reaction rate and the temperature will remain stable. The Harnessed Atom Atoms and Isotopes

Balancing the Reaction K < 1: The chain reaction will fade out. Reactor considered subcritical K > 1: More than 1 neutron per event are causing subsequent fission The reactor is supercritical The rate of reaction and the temperature in the core will increase The Harnessed Atom Atoms and Isotopes

Chain Reactions In a chain reaction, each uranium atom that splits releases 2 or 3 additional neutrons. The Harnessed Atom Atoms and Isotopes

Chain Reactions Inside a reactor, control rods capture the additional neutrons to control the reaction and maintain a steady rate of fission. The Harnessed Atom Atoms and Isotopes

Control Rods Main purpose: to absorb neutron and slow down the chain reaction Made of cadmium or boron The atomic structure of these elements allow them to work like sponges to absorb neutrons The Harnessed Atom Atoms and Isotopes

Control Rods Lowering the control rods into the reactor allows them to absorb more neutrons Raising the control rods out of the reactor keeps them from absorbing as many neutrons The Harnessed Atom Atoms and Isotopes

Coolant and Moderator Water slows down the neutron, so that they can be “caught” by other atoms or by control rods Water also absorbs the heat generated by the reactions The Harnessed Atom Atoms and Isotopes

Pressure Vessel Encases fuel, control rods, and coolant/moderator Massive steel walls 23 cm (9 in.) thick, and it often weighs more than 270,000 kg (300 tons). Tremendous strength to withstand high temperature and high pressure The Harnessed Atom Atoms and Isotopes

Heat Transfer Heat produced by fission reaction is absorbed by water Water has high specific heat It can absorb more heat than other materials without its temperature changing as much The Harnessed Atom Atoms and Isotopes

Heat Transfer Heat absorbed in water is used to create steam Steam is used to spin a turbine… The Harnessed Atom Atoms and Isotopes

Pressurized Water Reactor Loop 2 Loop 3 Loop 1 PWRs keep water in the reactor under pressure so that it can not boil. Three loops of water keep the water from the reactor and water in the steam generator from ever mixing. The Harnessed Atom Atoms and Isotopes

Boiling Water Reactor In BWRs a single loop both delivers steam to the turbine and returns water to the reactor core to cool it. Another loop contains the cooling water. The Harnessed Atom Atoms and Isotopes

Cooling the Water Water is cooled so that it can be recycled through the powerplant to absorb heat again The Harnessed Atom Atoms and Isotopes

Cooling Towers Not all cooling towers look like tall hyperbolic cylinders: Mechanical Draft cooling towers: The Harnessed Atom Atoms and Isotopes

Layers of Safety Ceramic fuel pellets provide a stable solid form for fuel. Fuel rods made of strong “zircaloy.” This helps fission products from leaking. Pressure vessel, with its 9-inch-thick steel walls, holds radioactive materials within the reactor core. The Harnessed Atom Atoms and Isotopes

Layers of Safety Containment building, a massive concrete and steel structure that encases the reactor Prevent radioactivity from being released into the environment if an accident should occur Reactor containment buildings are among the strongest structures in the world The Harnessed Atom Atoms and Isotopes

Monitoring Systems The Harnessed Atom Atoms and Isotopes

NRC Nuclear Regulatory Commission Regulates civilian use of nuclear materials Nuclear powerplants Medical and industrial uses Research sites Nuclear waste All facilities must be licensed by this agency The Harnessed Atom Atoms and Isotopes

Other Types of Nuclear Powerplants The Harnessed Atom Atoms and Isotopes

Words to know Boiling‑water reactor Chain reaction Condenser Control rods Convective cooling Coolant Cooling tower Critical Fission Fission event Fuel assemblies Fuel rods Heat transfer Heavy water Laws of Thermodynamics Light water Moderator Monitoring Multiplication factor Nuclear Regulatory Commission Pressurized‑water reactor Reactor vessel Redundancy Specific heat Subcritical Supercritical The Harnessed Atom Atoms and Isotopes

Reactor Designs

The Reactor The reactor is the heart of the nuclear powerplant. These designs are innovative variations on fuel and coolant Oddly, they aren’t new—they were developed in the 1940s and 50s by the clever young physicists who worked out the first designs

Light Water Reactors

Pressurized Water Reactor Use light (regular) water as a moderator & coolant Use slightly enriched uranium … 4% U-235

Boiling Water Reactor Also a LWR (regular water), but with only 2 loops Otherwise, pretty much the same technology

Breeder Reactors

Breeder Reactor U-238 Blanket Blanket of U-238 surrounds U-235 or Pu-239 fuel to “breed” plutonium using neutrons Coolant is liquid metal (sodium) so neutrons stay “fast”

High Temperature Gas-Cooled Reactors

High Temperature Gas Cooled Reactor Uses U-235 or Pu-238 fuel in a graphite moderator Instead of water for coolant, it uses helium gas

Very High Temperature Gas Cooled Reactor (Advanced Pebble Bed) Uses U-235 dioxide or carbide fuel pebbles instead of rods Uses helium gas as coolant at temperature of 1000°C Can be designed as a breeder reactor

Heavy Water Reactors

CANDU Canadian Deuterium reactor uses natural un-enriched uranium fuel (0.7%). Sub-critical, so it needs heavy water Du2O to reflect neutrons to sustain reaction

Research and Special Purpose Reactors

MIT Research Reactor MITR-II Actually a LWR, uses high-enriched uranium fuel But also sub-critical, so it uses heavy water reflector around the outside to reflect neutrons back in to sustain reaction

Molten Salt Reactor Uses dissolved UF4 in molten fluoride salt or sodium—the coolant and the fuel are mixed together Circulates the “liquid” fuel to the graphite core where it becomes critical

Naval Propulsion Reactor Miniature PWR Uses high-enriched U-235 fuel

Production Reactor Hanford B Reactor Graphite core, breeds Pu-239 Weapons production for national defense

Graphite Reactor Graphite Reactor Oak Ridge 1943-60s Built of blocks of graphite with U-235 fuel Used for research and plutonium production

Swimming Pool Reactor Reactor sits in a pool of water High Flux Isotope Reactor HFIR Oak Ridge Reactor sits in a pool of water Used for research and medical isotope production