Boiling water reactor

The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor.

The two-phase fluid (water and steam) above the core enters the riser area, which is the upper region contained inside of the shroud.

The "wet" steam goes through a tortuous path where the water droplets are slowed and directed out into the downcomer or annulus region.

Differently from the PWR, in a BWR the control rods (boron carbide plates) are inserted from below to give a more homogeneous distribution of the power: in the upper side the density of the water is lower due to vapour formation, making the neutron moderation less efficient and the fission probability lower.

They are shielded by water several times their height, and stored in rigid arrays in which their geometry is controlled to avoid criticality.

The fact that the fuel rods' cladding is a zirconium alloy was also problematic since this element can react with steam at temperatures above 1,500 K (1,230 °C) to produce hydrogen,[4][5] which can ignite with oxygen in the air.

The Navy, seeing the possibility of turning submarines into full-time underwater vehicles, and ships that could steam around the world without refueling, appointed Captain Hyman Rickover to run their nuclear power program.

Rickover decided on the PWR route for the Navy, as the early researchers in the field of nuclear power feared that the direct production of steam within a reactor would cause instability, while they knew that the use of pressurized water would definitively work as a means of heat transfer.

In particular, Samuel Untermyer II, a researcher at Argonne National Laboratory, proposed and oversaw a series of experiments: the BORAX experiments—to see if a boiling water reactor would be feasible for use in energy production.

[6] Following this series of tests, GE got involved and collaborated with Argonne National Laboratory[7] to bring this technology to market.

The first, General Electric (GE), series of production BWRs evolved through 6 iterative design phases, each termed BWR/1 through BWR/6.

The ABWR incorporates advanced technologies in the design, including computer control, plant automation, control rod removal, motion, and insertion, in-core pumping, and nuclear safety to deliver improvements over the original series of production BWRs, with a high power output (1350 MWe per reactor), and a significantly lowered probability of core damage.

[9] The ABWR was approved by the United States Nuclear Regulatory Commission for production as a standardized design in the early 1990s.

The concept of passive safety means that the reactor, rather than requiring the intervention of active systems, such as emergency injection pumps, to keep the reactor within safety margins, was instead designed to return to a safe state solely through operation of natural forces if a safety-related contingency developed.

The tank containing the soluble neutron absorbers would be located above the reactor, and the absorption solution, once the system was triggered, would flow into the core through force of gravity, and bring the reaction to a near-complete stop.

prior to approval; still, the concept remained intriguing to General Electric's designers, and served as the basis of future developments.

Control rod withdrawal is performed slowly, as to carefully monitor core conditions as the reactor approaches criticality.

Due to the limitations of the manual control system, it is possible while starting-up that the core can be placed into a condition where movement of a single control rod can cause a large nonlinear reactivity change, which could heat fuel elements to the point they fail (melt, ignite, weaken, etc.).

Specifically, MFLCPR represents how close the leading fuel bundle is to "dry-out" (or "departure from nucleate boiling" for a PWR).

MFLCPR is monitored with an empirical correlation that is formulated by vendors of BWR fuel (GE, Westinghouse, AREVA-NP).

These mock fuel assemblies are put into a test stand where data points are taken at specific powers, flows, pressures.

Experimental data is conservatively applied to BWR fuel to ensure that the transition to film boiling does not occur during normal or transient operation.

Typical SLMCPR/MCPRSL (Safety Limit MCPR) licensing limit for a BWR core is substantiated by a calculation that proves that 99.9% of fuel rods in a BWR core will not enter the transition to film boiling during normal operation or anticipated operational occurrences.

To illustrate the response of LHGR in transient imagine the rapid closure of the valves that admit steam to the turbines at full power.

This rise in pressure effectively subcools the reactor coolant instantaneously; the voids (vapor) collapse into solid water.

When the voids collapse in the reactor, the fission reaction is encouraged (more thermal neutrons); power increases drastically (120%) until it is terminated by the automatic insertion of the control rods.

APLHGR, being an average of the Linear Heat Generation Rate (LHGR), a measure of the decay heat present in the fuel bundles, is a margin of safety associated with the potential for fuel failure to occur during a LBLOCA (large-break loss-of-coolant accident – a massive pipe rupture leading to catastrophic loss of coolant pressure within the reactor, considered the most threatening "design basis accident" in probabilistic risk assessment and nuclear safety and security), which is anticipated to lead to the temporary exposure of the core; this core drying-out event is termed core "uncovery", for the core loses its heat-removing cover of coolant, in the case of a BWR, light water.

So as to prevent this from happening, it is required that the decay heat stored in the fuel assemblies at any one time does not overwhelm the ECCS.

APLHGR is monitored to ensure that the reactor is not operated at an average power level that would defeat the primary containment systems.

The first is the inclusion of a thin barrier layer against the inner walls of the fuel cladding which are resistant to perforation due to pellet-clad interactions, and the second is a set of rules created under PCIOMR.