In BWR, enriched fuel is used. This reactor can be safely operated using natural convection within the core. The pressure in the forced circulation must be maintain constant irrespective of the load.
Light water, which acts as the coolant and moderator, passes through the core where boiling takes place in the upper part of the core.
The wet steam then passes through a bank of moisture separators and steam dryers in the upper part of the pressure vessel.
The water that is not vaporized to steam is re-circulated through the core with the entering feed water using two recirculation pumps coupled to jet pumps (usually 10 to 12 per recirculation pump).
The steam leaving the top of the pressure vessel is at saturated conditions of 7.2 mPa and 278 deg C.
The steam then expands through a turbine coupled to an electrical generator. After condensing to liquid in the condenser, the liquid is returned to the reactors as feed water.
Prior to entering the reactor, the feed water is preheated in several stages of feed water heaters. The balance of plant systems (Example: Turbine generator, feed water heaters) are similar for both PWR and BWRs.
The BWR reactor core, like that in a PWR, consists of a large number of fuel rods housed in fuel assemblies in a nearly cylindrical arrangement.
Each fuel assembly contains an 8×8 or 9×9 square array of 64 or 81 fuel rods (typically two of the fuel rods contain water rather than fuel) surrounded by a square Zircaloy channel box to ensure no coolant cross flow in the core.
The fuel rods are similar to the PWR rods, although larger in diameter. Each fuel rod is a zirconium alloy clad tube containing pellets of slightly enriched uranium dioxide (2% to 5% U-235) stacked end-to end.
The reactor is controlled by control rods housed in a cross-shaped, or cruciform, arrangement called a control element. The control elements enter from the bottom of the reactor and move in spaces between the fuel assemblies.
The BWR reactor core is housed in a pressure vessel that is larger than that of a PWR. A typical BWR pressure vessel, which also houses the reactor core, moisture separators, and steam dryers, has a diameter of 6.4 m, with a height of 22 m.
Since a BWR operators at a nominal pressure of 6.9 MPa, its pressure vessel is thinner that of a PWR.
Advantages
Heat exchanger circuit is eliminated and consequently there is gain in thermal efficiency and gain in lost.
Use of lower pressure vessel reduce the cost.
The metal temperature remains low for given output.
The pressure inside the pressure vessel is not high so thicker vessel not needed.
BWR is more efficient than PWR for given containment pressure.
Disadvantages
More safety precaution needed.
Wastage of steam resulting in lowering of thermal efficiency on part load operation.
Boiling limits power density, only 3 to 5% by mass can be converted to steam per pass through the boilers.
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