5Ws Activity Features of Nuclear Reactors

The nuclear reactor Control rods Moderator and coolant (water) Steel vessel Fuel pins Pump Concrete shield Water from turbines Water to turbines

Control rods: Contain elements like Boron or Cadmium The atoms of boron or cadmium can absorb neutrons. Lowering the control rods in the right position between the fuel pins ensures that an average of 2-3 neutrons are absorbed for every nucleus of U-235 that fissions, keeping the fission reaction going at a steady speed, rather than in an uncontrolled way They can be lowered down in the core of the reactor between fuel pins. Their position is automatically controlled by sensors that measure the flux of neutrons. When the flux of neutrons becomes too high, the bars are lowered down into the core. When the flux is decreasing, the rods are lifted up. For safety reasons; the bars control the flux of neutrons in the core of the reactor. This ensures that the chain reaction is controlled. The control rods are fully inserted in the core to shut the reactor down. Stuck? Click on the titles to reveal the content

Fuel pins: They are the nuclear fuel. They generally contain natural or enriched uranium. Natural uranium contains less than 1% U-235, which can fission when it absorbs a thermal neutron. This fission releases different fragments, among which other neutrons. These will be absorbed by atoms in the control rods, U-238 (without fission) or U-235, causing the reaction to continue. Situated in the core of the reactor, between the control rods and surrounded by the moderator and coolant. When the fission reaction starts, the fission products recoil carrying a lot of KE. These products collide with surrounding atoms, leaving the pins very hot. Often the uranium is enriched with additional U-235 to enhance the fission reaction. The pins in the reactor must have critical mass, i.e. the minimum mass able to produce a self sustaining chain reaction. Stuck? Click on the titles to reveal the content

Moderator/Coolant: Usually water, but it could be made of carbon too. When the neutron produced in the fission reaction enter the moderator, they transfer their KE to the atoms in the moderator. In this way the moderator gains energy. The neutrons slow down until their average KE is the same as the KE of the moderator atoms. At this point, the neutrons are called thermal neutron, as they don’t transfer any more KE to the moderator. It surrounds the fuel pins and if it is water it also act as a coolant, so it is circulating inside a closed circuit made of steel (steel vessel). The water always circulates in the steel vessel, gaining energy in the core and transferring its KE to the water from the turbines to turn it into steam. The moderator is enclosed in a steel vessel capable of resisting the high pressures and temperatures inside the reactor and also because the coolant becomes radioactive inside the core. Water is a good moderator because if the temperature rises too much it turns into steam and forms bubbles, so the neutrons cease to collide with the atoms in the moderator and the chain reaction stops. Stuck? Click on the titles to reveal the content

Nuclear waste: Contain fission products and U- 238 and U-239 atoms Nuclear waste becomes radioactive due to the continuous bombardment of thermal neutrons. This causes the U-238 to absorb neutrons and turn into U-239, which disintegrates by radioactive emission. Also the fission products are made of isotopes rich in neutrons and therefore are beta emitters. Waste nuclear material is stored in sealed containers for many years. Sometimes is then buried underground, or under the sea. The fuel pins are removed from the core after use and they are placed in cooling ponds until they get cooler. The used fuel is removed from the fuel cans using remote control systems, because of the intense radioactivity. This fuel is also reprocessed to recover any nuclear fuel which didn’t get used in the reactions. Stuck? Click on the titles to reveal the content