|Fig. 1: Diagram of a BWR containing: (1) Reactor Vessel, (2) Fuel Core Element, (3) Control Rod Element, (4) Circulation Pumps, (5) Control Rod Motors, (6) Steam, (7) Inlet Circulation Water, (8) High Pressure Turbine, (10) Electric Generator, (11) Electrical Generator Exciter, (12) Steam Condenser, (13) Cold Water for Condenser, (14) Pre-Warmer, (15) Water Circulation Pump, (16) Condenser Cold Water Pump, (17) Concrete Chamber, (18) Connection to Electricity Grid. (Source: Wikimedia Commons).|
The boiling water nuclear reactor (BWR) is a simpler version of a pressurized water reactor (PWR) and, as a result, is more efficient than its predecessor. The layout of the reactor differs from other reactors such as PWRs because steam is generated inside the reaction chamber itself instead of being transferred to an external reaction pump. BWRs are best used for situations in which a constant supply of energy is needed because it does not react well to sharp increases in demand. 
The first patent for a "Boiling Water Nuclear Reactor" was filed in 1968 and describes the reactor as relating to "pressurised vapour generators of a kind in which liquid to be vaporised undergoes forced recirculation in a circuit having disposed therein a heater means. Generators of this kind are employed for raising steam to drive, for example, a steam turbine."  Since then, boiling water reactors have been used extensively throughout the energy industry, and are among the most most popular types of nuclear reactors. Unfortunately, the most famous BWRs since their invention were the reactors that overheated at Fukushima. 
Some of the most important early improvements in BWR design involved their energy efficiency, their raw energy output, and their safety.
Researchers in the early days of boiling water reactors tried a few different ways of improving BWRs energy efficiency, one of which was to find a better way of delivering steam from the core to the turbine. Many of the first reactors experimented with complex recirculation pumps, in which excess water that didn't reach the turbine cycled back into the reaction chamber. This mechanism of recirculation remains one of the principle ways in which different companies attempt to tweak the BWR design. 
Another innovation in nuclear reactor design related to BWRs concerned raw energy output. A Russian reactor, the RBMK, used distinct "pressure tubes" that caused steam to collect in a collection of distinct chambers rather than one central one. It is important to note that these are not classified as classical BWRs because they use graphite as a moderator (the chemical that facilitates fission), and water as a coolant (which cools the thermal system).  That being said, they are quite similar to boiling water reactors because they also do not have external steam pumps. Having many pressure tubes in a reactor has two principal advantages. The first is that operators can take single tubes offline at a time to refuel them, meaning the system as a whole never has to be shut down. On the other hand, BWRs with just one pressure tube have to be shut down once every year for maintenance. The second is that engineers can monitor individual pressure tubes in order to isolate where a system was failing and fix it as quickly as possible. 
Arguably more important than increases to power output are increases to the safety of these systems. These include the banked position withdrawal sequence, which gives operators a framework to abide by when they are withdrawing or inserting the control rods that help keep the reactivity of the system in check. This framework mandates that the rods be aligned in a checkerboard pattern, ensuring that the power in the core is safely distributed across each rod. As a result, the drop of a single control rod would have a minimal effect in increasing the reactor's power levels.  Another major safety feature added onto the original BWR design was a series of pressure suppression contaminants. When too much pressure builds up, this system opens safety valves in the chamber containing the steam, which relieves the pressure to a safe value. 
Learning from these advancements, the Advanced Boiling Water Reactor (ABWR) started development in the 1970s, and was first used commercially in 1996. Its most important innovation was the implementation of reactor internal pumps.
The reactor internal pumps control recirculation flow, which consists of water that has passed through the turbine system and water that comes back from the steam separators and dryers (Separators and dryers exist both to make sure that the steam is light enough to enter the turbine and to decrease the reactivity of the steam since it's so close to the core). [4,5] Keep in mind that this water functions as both a moderator and a coolant, so it is vital that it generates enough flow for a sufficient amount of power, but not enough to cause dangerous reactivity or excess hydrogen. As a result, any improvements in the reliability of circulation are quite impactful. Hitachi highlighted a few ways in which using reactor internal pumps rather than a combination of external circulation loops, large diameter pipes, and internal jet pumps (which are what ABWR's predecessor used) bettered the system. These included that less power is required for the recirculation system and that the water is exposed to less radiation. 
Conversely, the Simplified Boiling Water Reactor found a way to avoid using forced propulsion techniques at all. It used exclusively natural circulation of its water to maintain the activity of the reactor core, and was proposed by engineers at General Electric in 1988. Their goals at the outset of the project were to create cheap power, with simpler safety systems, and a short construction time (to avoid major regulatory changes between the beginning of construction and the end). As a result, they came up with a system that is less expensive to run and requires less maintenance due to its core cooling system only relying on gravity and its containment cooling system being run by the natural flow of water. 
Overall, these advancements to the already simplified design of the BWR have established it as one of the more efficient nuclear reactor designs. Although it lacks a high degree of thermal efficiency (the steam that drives the turbine does so at 286Â°C on average, compared to 317Â°C for PWRs), it achieves the same amount of steam cycle efficiency (32%) as the PWR design at half of the pressure.  According to the World Nuclear Association, BWRs built in 2015 were the most efficient reactors that year. They featured "capacity factors" of 88.2% on average, compared to 81.3% for PWRs produced in the same timeframe. Capacity factor is calculated by the association to be the total amount of electricity delivered to the grid as a % of the maximum amount of electricity possible to deliver.  Furthermore, by the time the ABWR was established in the market in 2003, it cost between $50 and $55 per MWh to operate, compared to between $43 and $49 per MWh for coal plants. Much of the cost was not even related to operations of the reactor. Instead, a substantial portion of the costs were related to capital expenditures and maintenance. 
Boiling water reactors have transformed the nuclear power industry, from their inception in the 1960s, when they were outputting only a few hundred MWe, to now, when their output is in the thousands of MWe.  The features of these reactors, and reactors that BWRs inspired, have changed in countless ways over the past 50 years, and these evolutions led to more efficient, cost-effective, and safe reactors than ever before.
© Christopher Barry. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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