Boiling Water Reactor

• In 1939, Otto Hahn and Strassman discovered Nuclear Fission when they found that a slow-moving neutron collides with a uranium nucleus; it breaks into two smaller nuclei of comparable masses with the release of energy. 
• In simple words, Nuclear Fission means breaking up the heavier nucleus into two smaller nuclei and releasing an enormous amount of energy.

• Reactant total mass is more than product total mass & EXCESS mass is converted to energy (using Einstein’s Mass – Energy Relation (E = mc^2)). The energy released from 1 nucleus of Uranium (235) is nearly 93 Mega Electron Volt. When one Uranium nucleus undergoes fission, the energy released might be small. But from each fission reaction, three neutrons are released. These three neutrons can cause further fission in three other Uranium nuclei. This process is called a chain reaction.
• The energy produced in the nuclear reaction can be used to convert water into steam, which can be converted into electricity using Steam Turbine and Generator.

Most of the Nuclear Reactors found in India are PHWRs.

Information at Glance

Details of PHWRs

The primary reaction which leads to the generation of energy while using Uranium as fuel is

• The most crucial point in Uranium Fission Reaction is the release of an extra 2.5 (average) Neutrons, which leads to the possibility of a chain reaction. If controlled, it can be used to produce energy called Nuclear Energy. At the same time, if it remains uncontrolled, it can result in an Atomic Bomb.
• But the biggest hurdle, in this case, is the fact that neutrons liberated in the nuclear reaction are fast-moving & will not cause fission (instead, they will escape without causing any collision). To ensure a Fission reaction, these neutrons must be slowed. For this purpose, Moderators are used, which in this case are D2O (Heavy Water) & Graphite.
• The reaction rate can be controlled by Control Rods, which are made up of neutron-absorbing material like Cadmium. 
• The energy released in fission is continuously removed by a suitable Coolant which transfers heat to a working fluid which in turn may produce steam to drive the turbine & generate electricity. 
• Pressuriser is used because when heavy water is heated, it expands & becomes less dense. As a result probability of collision between neutrons and heavy water molecules to reduce the speed of neutrons decreases. It is crucial to decrease the speed of neutrons to ensure fission. The Pressuriser ensures the suitable density of the heavy water.

• It is the oldest type of Nuclear Reactor.
• Fuel Used: Enriched Uranium
• Working: Energy released during the fission reaction directly heats the (light) water. The same water is used to turn the turbine and then recycled back, to be used again in the cycle.
• Moderator: No Moderator is used. The probability of neutron colliding with U-235 is achieved by using Enriched Uranium.

• BWRs are the second most widely used reactors in the world. But in India, we don’t use them on a large scale. Tarapur Atomic Power Station was constructed initially with two boiling water reactors (BWR) under the 1963 Agreement between India, USA & International Atomic Energy Agency (IAEA).

Issue with BWR

• The use of enriched Uranium increases cost and complexities.
• Light water is directly heated by radioactive material. Hence, nuclear radiation fallout in case of an accident is maximum in such reactors. For example, Japan’s Fukushima Nuclear reactor, which caused great damage after a nuclear accident, was BWR.