Editing Boiling water reactor (section)

== Comparison with other types ==
=== Advantages of BWR ===
* The reactor vessel and associated components operate at a substantially lower pressure of about {{convert|70|-|75|bar|psi}} compared to about {{convert|155|bar|psi}} in a PWR.
* Pressure vessel is subject to significantly less irradiation compared to a PWR, and so does not become as brittle with age.
* Operates at a lower nuclear fuel temperature, largely due to heat transfer by the latent [[heat of vaporization]], as opposed to [[sensible heat]] in PWRs.
* Fewer large metal and overall components due to a lack of steam generators and a pressurizer vessel, as well as the associated primary circuit pumps. (Older BWRs have external recirculation loops, but even this piping is eliminated in modern BWRs, such as the [[ABWR]].) This also makes BWRs simpler to operate.
* Lower risk (probability) of a rupture causing loss of coolant compared to a PWR, and lower risk of core damage should such a rupture occur. This is due to fewer pipes, fewer large-diameter pipes, fewer welds and no steam generator tubes.
* NRC assessments of limiting fault potentials indicate if such a fault occurred, the average BWR would be less likely to sustain core damage than the average PWR due to the robustness and redundancy of the [[Nuclear safety systems#Emergency core cooling system|Emergency Core Cooling System (ECCS)]].
* Measuring the water level in the pressure vessel is the same for both normal and emergency operations, which results in easy and intuitive assessment of emergency conditions. 
* Can operate at lower core power density levels using natural circulation without forced flow.
* A BWR may be designed to operate using only natural circulation so that recirculation pumps are eliminated. (The new ESBWR design uses natural circulation.)
* BWRs do not use [[boric acid]] to control fission burn-up to avoid the production of tritium (contamination of the turbines),<ref name="bonin"/> leading to less possibility of corrosion within the reactor vessel and piping. (Corrosion from boric acid must be carefully monitored in PWRs; it has been demonstrated that reactor vessel head corrosion can occur if the reactor vessel head is not properly maintained. See [[Davis-Besse]]. Since BWRs do not utilize boric acid, these contingencies are eliminated.)
* The power control by reduction of the moderator density (vapour bubbles in the water) instead of by addition of neutron absorbers (boric acid in PWR) leads to [[breeder reactor|breeding]] of U-238 by fast neutrons, producing fissile Pu-239.<ref name="bonin"/>
** This effect is amplified in [[Reduced moderation water reactor|reduced moderation boiling water reactors]], resulting in a light water reactor with improved fuel utilization and reduced long-lived radioactive waste more characteristic of sodium breeder reactors.
* BWRs generally have ''N''-2 redundancy on their major safety-related systems, which normally consist of four "trains" of components. This generally means that up to two of the four components of a safety system can fail and the system will still perform if called upon.
* Due to their single major vendor (GE/Hitachi), the current fleet of BWRs have predictable, uniform designs that, while not completely standardized, generally are very similar to one another. The ABWR/ESBWR designs are completely standardized. Lack of standardization remains a problem with PWRs, as, at least in the United States, there are three design families represented among the current PWR fleet (Combustion Engineering, Westinghouse, and Babcock & Wilcox), and within these families, there are quite divergent designs. Still, some countries could reach a high level of standardisation with PWRs, like [[Nuclear power in France|France]].
** Additional families of PWRs are being introduced. For example, Mitsubishi's [[Mitsubishi APWR|APWR]], Areva's US-[[European Pressurized Reactor|EPR]], and Westinghouse's [[AP1000]]/[[AP600]] will add diversity and complexity to an already diverse crowd, and possibly cause customers seeking stability and predictability to seek other designs, such as the BWR.
* BWRs are overrepresented in imports, when the importing nation does not have a nuclear navy (PWRs are favored by nuclear naval states due to their compact, high-power design used on nuclear-powered vessels; since naval reactors are generally not exported, they cause national skill to be developed in PWR design, construction, and operation). This may be due to the fact that BWRs are ideally suited for peaceful uses like power generation, process/industrial/district heating, and [[desalination]], due to low cost, simplicity, and safety focus, which come at the expense of larger size and slightly lower thermal efficiency.
** [[Nuclear power in Sweden|Sweden]] is standardized mainly on BWRs.
** [[Laguna Verde Nuclear Power Station|Mexico's]] two reactors are BWRs.
** [[Nuclear power in Japan|Japan]] experimented with both PWRs and BWRs, but most builds as of late have been of BWRs, specifically ABWRs.
** In the [[Central Electricity Generating Board|CEGB]] open competition in the early 1960s for a standard design for UK 2nd-generation power reactors, the PWR didn't even make it to the final round, which was a showdown between the BWR (preferred for its easily understood design as well as for being predictable and "boring") and the [[Advanced Gas-Cooled Reactor|AGR]], a uniquely British design; the indigenous design won, possibly on technical merits, possibly due to the proximity of a general election.{{citation needed|date=September 2021}} In the 1980s the CEGB built a PWR, [[Sizewell nuclear power stations|Sizewell B]].

=== Disadvantages of BWR ===
* BWRs require more complex calculations for managing consumption of nuclear fuel during operation due to "two-phase (water and steam) fluid flow" in the upper part of the core. This also requires more instrumentation in the reactor core.
* Larger reactor pressure vessel than for a PWR of similar power, with correspondingly higher cost, in particular for older models that still use a main steam generator and associated piping.
* Contamination of the turbine by short-lived [[activation product]]s. This means that shielding and access control around the steam turbine are required during normal operations due to the radiation levels arising from the steam entering directly from the reactor core. This is a moderately minor concern, as most of the radiation flux is due to [[Nitrogen-16]] (activation of oxygen in the water), which has a half-life of 7.1 seconds, allowing the turbine chamber to be entered within minutes of shutdown.  Extensive experience demonstrates that shutdown maintenance on the turbine, condensate, and feedwater components of a BWR can be performed essentially as a fossil-fuel plant.{{citation needed|date=September 2021}}
* Though the current BWRs are considered<ref name=":0" /> to be less likely to suffer core damage from the "1 in 100,000 reactor-year" limiting fault than the present fleet of PWRs (due to increased ECCS robustness and redundancy), there have been concerns raised about the pressure containment ability of the as-built, unmodified Mark I containment – that such may be insufficient to contain pressures generated by a limiting fault combined with complete ECCS failure that results in extremely severe core damage. In this double failure scenario, assumed to be extremely unlikely prior to the [[Fukushima I nuclear accidents]], an unmodified Mark I containment can allow some degree of radioactive release to occur. This is supposed to be mitigated by the modification of the Mark I containment; namely, the addition of an outgas stack system that, if containment pressure exceeds critical setpoints, is supposed to allow the orderly discharge of pressurizing gases after the gases pass through activated carbon filters designed to trap radionuclides.<ref>KEIJI TAKEUCHI [http://www.asahi.com/english/TKY201103140243.html COMMENTARY: Crucial vents were not installed until 1990s] Asahi.com</ref>

=== Control rod issues ===
* Control rods are inserted from below for current BWR designs. There are two available hydraulic power sources that can drive the control rods into the core for a BWR under emergency conditions. There is a dedicated high-pressure hydraulic accumulator and also the pressure inside of the reactor pressure vessel available to each control rod. Either the dedicated accumulator (one per rod) or reactor pressure is capable of fully inserting each rod. Most other reactor types use top-entry control rods that are held up in the withdrawn position by electromagnets, causing them to fall into the reactor by gravity if power is lost. This advantage is partially offset by the fact that hydraulic forces provide much greater rod insertion forces than gravity, and as a consequence, BWR control rods are much less likely to jam in a partially inserted position due to damage to the control rod channels in a core damage event.  Bottom-entry control rods also permit refueling without removal of the control rods and drives, as well as testing of the control rod systems with an open pressure vessel during refueling.