Nuclear reactor is one integrated part of every nuclear fission power plant where nuclear fuel is made to undergo nuclear fission reaction and the same is allowed to continue in a controlled chain reaction. Thermal energy (heat) derived from the exothermic nuclear fission is transferred to the coolant within this reactor. This coolant, in turn, drives the steam turbine (either directly or indirectly). Bases on the type of neutron used to initiate fission, the nuclear reactors can be broadly classified as thermal reactors and fast reactors. Thermal reactors, the most common one, are such reactors where thermal neutrons (having 0.025eV energy and 2.2km/s velocity at 20°C) are bombarded on the nuclear fuel to initiate fission. On the other hand, fast reactors employ fast neutrons (having 1 – 10MeV energy and about 50,000km/s velocity at 20°C) to initiate fission. Thermal reactors can again have various derivatives, namely Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), Pressurized Heavy-Water Reactor (PHWR), Advanced Gas Cooled Reactors (AGCR), Light Water Graphite Reactor (LWGR), etc. Although the PWR and BWR both utilize regular water as coolant as well as moderator, they have different working principle in driving the turbine to generate electricity.
In PWR, the thermal energy derived from nuclear fission is first transferred to the coolant (water). The coolant pressure is maintained in such a way that it does not boil, rather it remains in liquid phase even at very high temperature. A dedicated pressurizer is employed for such purpose. This high temperature coolant then transfers heat in a heat exchanger to another working fluid (again water) without physically mixing. This working fluid is allowed to change its phase to rotate the turbine. The coolant, after transferring heat to the working fluid, is returned back to the reactor to complete the cycle. Thus PWR power plants consists of two different loops – the primary loop where heat is taken from the reactor and is transferred to the working fluid, and the secondary loop where turbine is rotated. In the primary loop, the water is maintained at high pressure to restrict it from boiling, and thus the name “Pressurized Water”. On the other hand, the coolant (water) is allowed to boil (or change its phase from water to steam) in the Boiling Water Reactor (BWR). Thus the steam can be directly fed to the turbine without utilizing an intermediate heat exchanger. Hence BWR consists of only one loop. Although it offers higher thermal efficiency owing to elimination of intermediate heat exchanger, it is associated with the risk of radioactive contamination in case of any leakage. Various similarities and differences between Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) are given below in table format.
Similarities between PWR and BWR
- Both Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) employ nuclear fission reaction to generate thermal energy, which, in turn, is utilized to drive the turbine for generating electricity.
- Both PWR and BWR are thermal reactors, which indicate that the nuclear fission reaction is initiated by the thermal neutron (it has energy of 0.025eV and corresponding speed of 2.2km/s at 20°C). On the contrary, fast reactors utilize fast neutrons (1 – 10 MeV energy).
- Both PWR and BWR require 3 – 5% enriched uranium fuel. An enriched fuel has higher percentage of U-235 isotope. In the naturally available uranium, U-235 isotope is only about 0.7%, and the rest is U-238 isotope. But this U-238 isotope is not fissile material (thus cannot be used as nuclear fuel). Thus only 3 – 5% U-235 isotope available within the entire fuel can undergo nuclear fission reaction to generate thermal energy, rest remains intact.
- Both PWR and BWR employ only normal water or light water (H2O) as moderator, as coolant and also as working fluid. On the contrary, heavy water reactors, gas cooled reactors and graphite reactors can employ other materials (like heavy water, carbon dioxide, graphite) for such purposes.
Differences between PWR and BWR
| Pressurized Water Reactor (PWR) | Boiling Water Reactor (BWR) |
|---|---|
| Pressurized Water Reactor (PWR) power plants consist of two loops—(i) primary loop or coolant loop that takes away heat from reactor, and (ii) secondary loop or working fluid loop that drives the turbine. A heat exchanger (HE) is employed to transfer heat from primary loop to the secondary loop. | Boiling Water Reactor (BWR) power plants consist of a single loop where the coolant that takes away heat from the reactor is directly fed to the turbine. Thus no heat exchanger is desired. |
| In the primary loop, normal water (H2O) acts as coolant-cum-moderator. In the secondary loop, the normal water acts as working fluid. However, water from one loop is not allowed to mix with the water of other loop. | Since it has only one loop, so normal water (H2O) serves all three purposes – cooling, moderation, and working fluid. |
| Normal water in the primary loop, that acts as moderator-cum-coolant, is not allowed to boil. That means the water remains in liquid phase throughout the cycle of primary loop. However, the water in the secondary loop is allowed to boil. | Here the normal water (H2O) is allowed to change its phase. Thus the water (liquid phase) is first converted into steam (gaseous phase) within the reactor, and then the steam is again condensed to water before pumping back to reactor. |
| Here steam is generated in a heat exchanger outside the nuclear reactor. | Here steam is generated within the reactor itself. |
| Here the water in the primary loop is maintained at high pressure (15 – 17 MPa) to avoid boiling at reactor exit. | Here water pressure remains comparatively low (7 – 8 MPa) as it is allowed to boil. |
| A pressurizer is required to use mandatorily to maintain water pressure in such a way that it does not evaporate even at very high temperature. | No such pressurizer is employed as evaporation of the water is desired. |
| The temperature of the water at the reactor exit is kept around 310°C (corresponding to the working pressure to avoid boiling). | Steam temperature at reactor exit remains comparatively low (around 285°C). |
| PWR has comparatively low thermal efficiency owing to two different loops. | BWR offers higher thermal efficiency. |
| In PWR, the control rods are inserted from the top of the nuclear reactor. | In BWR, the control rods are inserted from the bottom of the nuclear reactor. |
| Since the fluid is maintained at high pressure, so the PWR core volume is less. | For the same power generation, core volume of the BWR is comparatively larger. |
| Since the working fluid loop is separated from the primary loop, so PWR is less risky in spreading of radioactive materials owing to leakage. | Since same fluid passes through the reactor and turbine in BWR plants, so any leakage in the turbine can spread radioactive elements into the atmosphere. |
References
- Introduction to Nuclear Reactor Physics by R. E. Masterson (2017, CRC Press).
- Fundamentals of Nuclear Reactor Physics by E. E. Lewis (2008, Academic Press).