Introduction to Nuclear Reactor for A-Level Students

Nuclear Reactor:- Definition:- A machine which has a controlled and sustainable nuclear reaction for any of several purposes, to produce power, to supply neutrons, to induce nuclear reactions, to prepare radioisotope, or to breed fissionable material from certain fertile materials is called nuclear reactor. Principle:- The environment of fissioning nuclei is controlled is such a way that only neutron out of 2.5 neutrons released on the average is used to induce fission in another atom. In this way, the rate of energy generation is maintained at a constant level. Thus controlled chain reaction is the principle of nuclear reactor. Explanation:- The reactors are divided in to two types on the basis of neutron energy. v Thermal Reactor v Fast Breeder Reactor Thermal reactors use thermal neutrons, whose kinetic energy is nearly equal to the kinetic energy of the molecules at room temperature. On the other hand fast breeder reactors use fast neutrons. Enrichment of uranium:- Natural uranium contains about 1% (.7%) of 92U235 and 99.3% of 92U238. the rare isotope 92U235 takes the effective part in building up the chain reaction. the nuclear reactor (thermal) uses the uranium as fuel. Natural uranium is a very low grade fuel. First of all natural uranium is enriched by increasing the percentage of 92U235 before its use in the nuclear reactor. Enriched uranium is used as nuclear fuel in a nuclear reactor. Enriched uranium means percentage of 92U235 becomes greater than its original percentage in natural uranium. Parts of a Nuclear Reactor:- A nuclear reactor consists of the following parts: Core:- This is the most important part of the nuclear reactor. It contains fuel which is cylinders of Uranium Moderator:- The function of moderator is to slow down the neutrons produced during th fission process. As the neutrons produced during the fission process are very fast and are not suitable for producing fission in the reactor fuel. Control Rods:- These rods keep the speed of the fission reaction under control. Usually these are Cadmium or Boron rods, which have the property of absorbing neutrons. By lower the control rods into the reactor, the speed of the fission process reduces and can be stopped when the rods fall in during power failure etc. Cooling system:- It consists of pipes along which the coolant is pumped in the form of gas and water. The coolant (gas) is heated while passing through the core. Heat exchanger:- This is the part of the nuclear reactor where the heat of the nuclear fission is transported to ordinary water. The water is converted to steam, which in turn runs the turbine to produce electricity. Biological shield:- It is a wall of concrete which absorbs radiations emitted due to fission of the fuel and protects the workers from their dangerous effects. Working:- The fissionable fuel is enriched uranium. The uranium fuel elements are in the shape of thin rods about 1 meter long and 2 cm in diameter. They are placed in a medium known as moderator. The moderator is made up of nuclei of comparable mass to that of the neutron and of material which does not absorb the precious neutrons. Graphite is usually used a moderator. The fast neutrons produced in a fission process undergo several collisions with the nuclei of the moderator and are slowed down to thermal range. A slow neutron can induce another fission event in the fuel element and can thus cause a chain reaction. The energy liberated by the fission process yields a huge amount of heat. The temperature of the system rises or falls as the rate of chain reaction rises or falls. In order to maintain a steady rate of chain reaction the temperature of the system is regulated to a determined value with the help of control rods of boron or cadmium, which are effective neutron absorbers. These rods can be moved in and out of the reactor from the control room. If the fission chain starts to go too fast, the control rods can be inserted a bit deeper and neutrons can be absorbed. As the reaction dies off, the control rods can be slightly with drawn. A charge machine is used to start the reactor. The uranium fuel rods are lowered in to fuel channels in the graphite core. The control rods are then raised slowly and placed at certain position as soon as the chain reaction proceeds at the required rate. The reactor is now said to gone critical and heat is produced steadily. In a power reactor, a liquid is circulated through the shielded reactor and heat exchanger as shown in the figure. The hot liquid flowing through the heat exchanger vaporizes a more volatile liquid like water, the resulting hot gas or steam under pressure drives a turbine. The turbine in turn drives an electric generator. Draw backs:- The main draw back of nuclear reactors is the nuclear waste products. The uranium rods after usage are converted in to radioactive elements which have a long half life. This waste cannot be drop in to sea or dumped in the earth. Because due to its radio active nature, it will damage any living present in its vicinity. Although, nuclear energy is very cheap compared to other sources of energy but proper dispose of the waste products is expensive too much. Liquid Metal Fast Breeder Reaction (LMFBR):- Definition:- A type of reactor which breeds itself as well as uses liquid metal for transformation of heat in to the heat exchanger is called LMFBR. Explanation:- U235 being less abundant cannot be sued for ever as a fuel. So a way out was needed to use the abundant isotope as a fuel in the nuclear reactor. This would increases the life span of the present Uranium reserves by 100. Breeding process:- In this type of reactor fast neutrons are used to convert 92U238 in to 94Pu239 as follows: 92U238 + on1 → 93Np239 + β- 93Np239 → 94Pu239 + β- 94Pu239 can be used as fuel as efficiently as 92U235 but the only difference is that fast neutrons can carry out the following fission process. on1 + 94Pu239 → X + Y + 2on1 + 200 MeV Where X and Y are fission fragments. As it is emitting two neutrons per fission, therefore it can be used for nuclear bombs as well. The Liquid Metal Coolant:- The 94P239 breeder reactor is called LMFBR because a liquid metal like sodium is used as a coolant instead of water, which slows down the the neutrons and hinders the process of breeding. Sodium is solid at room temperature but becomes liquid at 98C and is therefore a good heat transfer liquid. Working:- The core of the reactor (the region containing the fissionable material) is made up of 15 to 30% U235 which is surrounded by blanket of U238. since fast neutrons are more efficient in converting U238 to 94Pu239, there is no need for a moderator to slow down the neutrons produced in the fission. Cadmium rods control the chain reaction in the blanketed core, contained in a primary vessel. The flow diagram of the sodium coolant and the intermediate heat exchanger are shown in the fig. the feed water circulating in the outer heat exchanger is converted in to steam to drive a turbine, which may operate an electric generator to produce electricity. Advantages and Disadvantages:- In comparison to thermal reactors, the following are the advantages and disadvantages of breeder reactors: Advantages:- v The life span of the present uranium. Uranium sources are increased by 100 times by using 92U238 instead of 92U235. v The 92U238 left over while enrichment of 92U235 for thermal reactor can be used. v Ores with a very small concentration of 92U235 which is useless for conventional reactors can be used for the as blanket of 92U238 in LMFBR. v Energy produced is of low cost due to readily availability of 92U238(with out enrichment). Disadvantages:- v Sodium explodes if exposed to air or water. Therefore any leakage can be dangerous. v The coolant used in LMFBR and the heat exchanger is opaque. Therefore it is not possible to see inside the core, what is going on there while repairing during maintenance. v The high energy neutron makes sodium radioactive because: 0n1 + 11Na23 → 11Na24 + γ-ray ( T1/2 = 15 hrs) To be safe, it is advised to wait at least twenty half lives which is 300 hours or 12.5 days. So it is important to wait 12.5 days after shutting down the reactor in order to carry some repair work. v Long switching off the reactor, increases the cost significantly and also reliability of LMFBR. v Highly riched sodium coolant can cause a serious accident in LMFBR and then there is a chance that 94Pu239 would be emitted in to atmosphere which can result in devastating results. Uses:- Nuclear reactors can be used for following purposes: v Nuclear reactors are widely used as sources of great power. They are gradually replacing the more conventional energy sources for the production of electricity. Karachi nuclear power plant (KANUP) is an example of converting nuclear energy to electrical energy. Reactors power some ocean going ships and submarines. v Reactors are used to supply neutrons, which are of immense importance in nuclear studies and research. Neutron diffraction is a powerful tool for studying crystals structure. v Reactor are used to prepare a large variety of artificial radioisotopes which are widely used in industry, medical diagnose and therapy, and agriculture. In fact the tracer isotope techniques are widely used for the study of number of processes in biology, chemistry, physics, archaeology and other branches of science. v Production of plutonium which is used in atomic bombs as a fuel. v Nuclear reactors can be used to make fissionable material fro certain non fissionable but fertile materials. v It is used for the production of atomic energy for industrial and peaceful purposes. Thorium Breeder Reactor:- The above difficulties with LMFBR have led to water moderated slow neutron breeder reactor using 90Th 232to breed 92U238in spite of lower breeding ratio. Such a breeder would be safe and have a less concentrated core, thus avoiding safety problems with LMFBR.