Nuclear Reactors

1. Nuclear Reactors How they work and what happened at Fukushima Daiichi Plant

2. Nuclear Power Reactors • The Nuclear Regulatory Commission (NRC) regulates commercial nuclear power plants that generate electricity. • 2 types of reactors are used in the U.S.

4. PWRs • In a typical commercial pressurized light-water reactor(1) the core inside the reactor vessel creates heat, (2) pressurized water in the primary coolant loop carries the heat to the steam generator, (3) inside the steam generator, heat from the steam, and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted in to the condenser where it condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the steam generators. The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need power. Pressurized-water reactors contain between 150-200 fuel assemblies.

6. BWRs • In a typical commercial boiling-water reactor, (1) the core inside the reactor vessel creates heat, (2) a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat, (3) the steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line, and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted into the condenser where it is condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the reactor vessel. The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need electric power. Boiling-water reactors contain between 370-800 fuel assemblies.

7. Where does the fuel come from? • Uranium is mined from the ground as uranium ore, often called yellowcake. • The yellowcake goes to a conversion plant, where chemical processes convert it to uranium hexafluoride. • Uranium hexafluoride contains 2 types of uranium, U-238 (>99%) and U-235 (< 1%). • U-235 is the kind that undergoes fission easily. • To make it stable as a fuel, the amount of U-235 is increased (enriched) to 3-5%. • However, that is too little U-235 to explode! • Uranium hexafluoride is converted into uranium dioxide powder and pressed into fuel pellets. The pellets are put into fuel rods!

8. Fuel Pellets

9. How big are fuel rods • http://www.nei.org/howitworks/nuclearpowerplantfuel/ • How spent fuel is stored • https://www.youtube.com/watch?v=G_h4mCCzsbs&list=PLECE49E00A076C7E9&index=7&feature=plpp_video

10. Fukushima Dai-Ichi Disaster • They are boiling water reactors. • Have a “fatal flaw” – when the electric pumps that pump the water into the reactor lose power the water inside the reactor isn’t replenished. The water inside boils away to steam. If all the water boils away, the fuel rods are overexposed and overheat (MELTDOWN) • Tons of uranium melting on the floor of the reactor can eat through the reactor!

11. Of course there are secondary pumps to come online if this happens. • And there is a backup battery as well. They were only designed to last for a couple of hours. • BUT, guess what? • When it’s all underwater, IT DOESN”T WORK!!!!!!

12. When the fuel rods overheated, the pellets cracked and allowed water inside. The fuel pellets reacted with the water and produced HYDROGEN gas – which is very, very explosive! • A lot of gas built up, and pressure built up, which needed to be vented. It vented so fast, hydrogen exploded!!!! • It didn’t explode the reactor vessel, just the surrounding concrete and steel buildings.

13. To cool off the fuel rods, they started pumping a mixture of seawater and boron into the reactors. The boron absorbs neutrons which start the chain reaction to help cool down the fuel rods.