The diagram below depicts how a pressurized water reactor operates. The energy of a nucleus (Uranium) is released through fission by a chain reaction occurring in the core. The Uranium undergoing fission releases neutrons which result in more fission reactions. The released energy heats pressurized water which is being pumped through the reactor core. The water is pressurized in order to raise its boiling point to a sufficient level necessary to avoid evaporation in the reactor core.
This high pressure water will then flow into the steam generator, where it will transfer its thermal energy to a second system, which consists of non-pressurized water. The transfer of energy from system one will heat the water in system two until it boils, evaporates and generates steam. This steam will then turn a turbine connected to a generator which in turn, spins the generator and creates electrical energy. Meanwhile the steam used to turn the turbine is then condensed back into water (by the condenser) which is then re-funnelled back into the steam generator to receive the heat transfer once again and continue the cycle. The pressurized water from system one also gets cooled after transferring its thermal energy to the second system, and it is then pumped back into the reactor to core to be reheated. Both of these processes continuously repeat which is what generates continuous, efficient and long term electricity. Now that we’ve discussed the basics of how the reactor works we can take a closer look at the actual reactor core and how it operates.
The nuclear reactor core contains the nuclear fuel, the moderator/s and the control rods. Sourced from: https://www.clpgroup.com/NuclearEnergy/Eng/power/power4_1_2.aspxThe fuel in the reactor consists of 96.
8% Uranium-238 and 3.2% Uranium-235 (on average) The U235 can: not absorb a neutron and stay the same, absorb a neutron becoming U236 yet remain stable and not fission or fission into a multitude of different products The other 96.8% of Uranium-238 some can absorb a neutron and fission, some may stay the same, some can beta decay into neptunium and then maybe plutonium and then that can fission into other more fission fragments also releasing energy and neutronsThe energy used to heat the water is created by an induced fission reaction in the core: uranium-235 absorbs a neutron, changing into an excited uranium-236* nucleus.
With the excitation energy provided by the kinetic energy of the neutron as well as the forces that bind the neutron. The uranium-236 then fissions fission fragments, Kinetic Energy as well as 2-3 neutrons (on average 2.45) and 1 or more gamma photons. The emitted neutrons are slowed down by moderators (in this case the water), giving them more chance of colliding with uranium-235, and undergoing fission again. The energy released from a fission reaction is all due to the mass defect which is the difference in mass of the products and the reactants. Essentially, in the fission of Uranium-235, the total mass before fission is greater than the total mass after the fission event. In the reaction below it appears that the atomic mass is the same, but this is a rounded value.
Without rounding the added up total mass of the reactants ends being slightly larger than the total mass of the products. The mass difference is what is converted into thermal energy, in the form of Kinetic energy and 1 – 2 gamma photons. This energy released is equivalent to the binding energy which is the energy needed to hold the nucleus together. When U236 fissions the binding energy is released and this is equivalent to the difference in mass (mass defect). We can obtain the value of the binding energy released by using the mass defect in the equation E=mc2. On average the binding energy released or mass defect converted into energy is around 200MeV. 1 0 n+235 92 U236 92 U92 36 Kr+141 56 Ba+31 0 n+200MeVThis process continues repeatedly in a chain reaction and is maintained at a controllable rate by Control Rods and moderators.
The number of neutrons produced by fission events must be equal to the number of fission-producing neutrons in order to sustain a nuclear reaction. Sometimes the reaction might ‘run away’ (overproduce neutrons and begins destabilising). Then control rods containing neutron poisons (like boron-10) are lowered into the fuel to decrease the number of neutrons in the substance. The control rods are then removed when the chain reactions begins to produce insufficient neutrons to continue the reaction.The fuel needed for this chain reaction to occur is either plutonium-239 or Uranium-235 of which both are very rare. Plutonium can obtained only be obtained from Uranium (Uranium-238 beta decays into neptunium-239 which will then beta decay into plutonium-239).
Often we find Uranium in its 238 form and it must be enriched, in order to attain the Uranium as Uranium-235. To get sustained fission, the amount of uranium-235 must be increased, this is done by separating uranium-235 out of naturally occurring uranium ore. Then by putting this back into a certain amount of naturally occurring uranium. This increases (enriches) the proportion of uranium-235 in the quantity. Usually a sample of Uranium fuel with have about 0.9% uranium-235, but for it to be sufficient to sustain a controlled chain reaction it’s enriched until about 1-4% is uranium-235 content.For fission to occur frequently in Uranium-235 the initial neutron must be a ‘slow’ neutron.
Neutrons produced in fission events are ‘fast’ neutrons. The probability of these causing new fission events in uranium-235 is very small. Thus Moderators are used to slow down the neutrons produced through the fission process. A moderator is a material/substance with nuclides that have a slightly larger masses than the neutrons. The neutrons then share their energy with these nuclides through multiple collisions.
This results in a rapid loss of energy (they become slower). Which further increases the probability of neutrons entering a uranium-235 nucleus and resulting in more frequent fission reactions. In this reactor it is water which acts as the moderator.
