Types of Nuclear Power Plants – Boiling Water Reactors (BWR)

Steam possesses immense power and we see that in a lot of applications in every day life, right from the pressure cooker which makes those stubborn pulses soft, to driving the heavy steam engine (although they are hardly seen these days). This steam can also be used to generate electricity by driving a turbine alternator arrangement with it. Nothing new, you might say, but then here water is not boiled by traditional heat sources but using the heat of the atom, in the form of exothermic heat from a nuclear fission reaction. BWR or boiling water reactor plants form an important variety amongst the commonly used types of power plants.

The main beauty of a BWR lies in the fact that the same water is used for all three purposes as a moderator, coolant, and the source for steam which drives the turbine blades. Water passes over the reactor core absorbing heat and turning into steam which is fed to the turbine blades. The used steam from the turbines is then fed into the condenser which coverts it into liquid state to be fed again into the reactor core, thus completing the closed loop cycle. The Fuel used in this type of reactor is enriched Uranium.

• Since there is no need for a separate boiler arrangement to produce steam unlike in some other types of nuclear reactors, it reduces the complexity of the arrangement and related costs. Moreover this leads to a reduced size of the reactor for a given amount of output.
• A BWR has self controlling characteristic which stems from the fact that an increase in reactivity causes increase of steam formation in the reactor. This increase of steam is accompanied by a decrease of density of the coolant which also acts as the moderator hence pushing the fuel towards sub-critical zone.
• One of the biggest drawbacks of a BWR reactor is its inefficiency to deal with sudden increase in load. This is due to the very fact of self balancing as explained in the previous point which is good for a fixed load but makes it a drawback when dealing with sudden increases of load.
• Since the same water is used for all purposes including moderation and cooling, it tends to be slightly radioactive. Hence when this water is fed to the turbines, they need to be shielded for this very reason. However the good news is that the half life of the radioactive steam is just of the order of 15 minutes which is manageable.
• The chances of fuel getting "burnt out" are significant in a BWR reactor mainly because water is in direct contact with fuel and when the same water gets converted to steam it may blank parts of the fuel surface from coming in contact with water thus leading to such a situation.

Hence we see that a boiling water reactor is pretty useful due to its good thermal efficiency, smaller size per unit power output and its characteristic to use water as coolant, moderator and steam generator.