Advanced Boiling Water Reactor

The Advanced Boiling Water Reactor (ABWR) is a Generation III reactor boiling water reactor. The ABWR was designed by General Electric. The standard ABWR plant design has a net output of about 1350 MWe, however General-Electric Hitachi (GEH)has offered a design with greater electrical output.

Internal recirculation pumps inside of the reactor pressure vessel (RPV) are a major improvement over previous GE reactor plant designs (BWR/6 and prior). These pumps are powered by wet-rotor motors with the housings connected to the bottom of the RPV and eliminating large diameter external recirculation pipes that are possible leakage paths. Construction costs are also reduced. The 10 internal recirculation pumps are located at the bottom of the annulus downcomer region (i.e., between the core shroud and the inside surface of the RPV).

Even though BWRs can operate using only the available natural recirculation thermal pumping head without forced recirculation flow, forced flow is desirable in order to increase the available output from the reactor and as a convenient method to change the reactor output by changing the flow.

Prior to the ABWR, all large commercial nuclear steam supply systems provided by GE from the BWR/3 through the BWR/6 designs used jet pump recirculation systems. These systems have two large recirculation pumps (each up to 9000 Hp) located outside of the reactor pressure vessel (RPV). Each external recirculation pump takes a suction from the bottom of the annulus downcomer region through a large diameter nozzle and discharges through multiple jet pumps inside of the RPV in the annulus downcomer region. There is one nozzle per jet pump for the discharge back into the RPV and the external headers supplying these nozzles. Isolation valves are provided for each of the two external recirculation pumps. In the event of a pipe rupture close to the RPV, those isolation valves will be ineffective and the top region of the reactor may not be covered with water. With all of the jet pumps intact after this design basis accident (DBA)a minimum of two thirds (2/3) of the core will remain covered in water. Calculations indicate that fuel failure would be averted by "steam cooling" wherein the boiling of water in the lower core region will produce mixed quality steam that will absorb heat from the upper core region.

Consequently, internal recirculation pumps eliminate all of the jet pumps (typically 10), all of the external piping, the isolation valves and the large diameter nozzles that penetrated the RPV and needed to suction water from and return it to the RPV. This design therefore reduces the worst leak below the core region to effectively equivalent to a 2 inch diameter leak. The conventional BWR3-BWR6 product line has an analogous potential leak of 24 or more inches in diameter. A major benefit of this design is that it greatly reduces the flow capacity required of the emergency core cooling systems (ECCS). In the event of a fuel failure, a specially constructed basaltic floor with passive cooling features with terminate the flow of corrium before it breaches primary containment.

The first reactors to use internal recirculation pumps were designed by ASEA-Atom (now Westinghouse Electric Company by way of mergers and buyouts, which is owned by Toshiba) and built in Sweden. These plants have operated very successfully for many years.

The internal pumps reduce the required pumping power for the same flow to about half that required with the jet pump system with external recirculation loops. Thus, in addition to the safety and cost improvements due to eliminating the piping, the overall plant thermal efficiency is increased. Eliminating the external recirculation piping also reduces occupational radiation exposure to personnel during maintenance.

A nice operational feature in the ABWR design is electric fine motion control rod drives, first used in the BWRs of AEG (later Kraftwerk Union AG, now AREVA). Older BWRs use a hydraulic locking piston system to move the control rods in six-inch increments. Additionally the fine motion control rod design greatly enhances positive actual control rod position and similarly reduces the risk of a control rod drive accident to the point that no velocity limiter is required at the base of the cruciform control rod blades.

The ABWR is fully automated in response to a loss-of-coolant accident (LOCA), and operator action is not required for 3 days. After 3 days the operators must replenish ECCS water supplies. These and other improvements make the plant significantly safer than previous reactors.

As of December 2006, four ABWRs were in operation in Japan: Kashiwazaki-Kariwa units 6 and 7, which opened in 1996 and 1997, Hamaoka unit 5, opened 2004 having started construction in 2000, and Shika 2 commenced commercial operations on March 15, 2006. Another two, identical to the Kashiwazaki-Kariwa reactors, were nearing completion at Lungmen in Taiwan, and one more (Shimane Nuclear Power Plant 3) had just commenced construction in Japan, with major siteworks to start in 2008 and completion in 2011. Plans for at least six other ABWRs in Japan have been postponed, cancelled, or converted to other reactor types, but three of these (Higashidōri 1 and 2 and Ohma) were still listed as "on order" by the utilities, with completion dates of 2012 or later.

Several ABWRs are proposed for construction in the United States under the Nuclear Power 2010 Program. However these proposals face fierce competition from more recent designs such as the ESBWR (Economic Simplified BWR, a generation III+ reactor also from GE) and the AP1000 (Advanced, Passive, 1000MWe, from Westinghouse). These designs take passive safety features even further than the ABWR does, as do more revolutionary designs such as the pebble bed modular reactor. However, the US market incentive for construction of an ABWR is that the Nuclear Regulatory Commission (NRC) approved the ABWR design in 1997 and construction would have a smaller regulatory burden for approval; hence ABWRs could be constructed faster that other designs pending approval. There are no ESBWR design reactors in service world wide and the ESBWR design is pending approval by the NRC. The ESBWR is a natural circulation plant with features to be resolved such as the power oscillations expected to the local power induced thermal hydraulic instabilities during initial startup.

On June 19, 2006 NRG Energy filed a Letter Of Intent with the Nuclear Regulatory Commission to build two 1358 MWe ABWRs at the South Texas Project site. [http://www.neimagazine.com/story.asp?sectioncode=132&storyCode=2036890] On September 25, 2007, NRG Energy and CPS Energy submitted a Construction and Operations License (COL) request for these plants with the NRC. NRG Energy is a merchant generator and CPS Energy is the nations' largest municipally owned utility.

ee also

*nuclear power
*Nuclear safety in the U.S.
*Economics of new nuclear power plants
*pressurized water reactor
*Advanced Heavy Water Reactor

External links

* [http://www.nuc.berkeley.edu/designs/abwr/abwr.html Technical details and features of Advanced BWRs ]
* [http://www.engr.sjsu.edu/jrhee/me210/Table%20of%20Contents.pdf Advanced BWR General Description] (PDF, 272KB): Table of Contents, with active links to text.
* [http://www.ftj.agh.edu.pl/~cetnar/ABWR/Chapter12.pdf ABWR Plant Economics and Project Schedule (old)]