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Sunday, March 13, 2011

FACTS: Boiling Water Reactors

Special Report by Atomic Power Review. Illustrations from APRA library. Do not reproduce content or illustrations without permission.

The reactor plants that have been affected by the earthquake and tsunami in Japan are boiling water reactors - one of two main types used in the world. Let's look at a few details good for general information.

HEAT: The fuel elements in a nuclear reactor have one useful output -- heat. This heat must be transferred to something from which power can be derived by machines like turbines.

The earliest successful plans for nuclear reactors used water to remove heat from the fuel elements. This water then went through tubes in a heat exchanger and returned, cooler, to the reactor; electric pumps were used to circulate the water and the system was pressurized to make sure none of the water could boil into steam that would hurt the ability to transfer out heat. On the outside of the tubes in the heat exchanger was "secondary system" water that boiled into steam to turn turbines, which were connected to electric generators.

Speculative work on what would happen if boiling did in fact occur led fairly rapidly to design work and fuel development for reactors that actually DID allow the water to boil right in the core, and which would be sent to turn the turbines. After being condensed it was returned to the reactor. This theoretically simpler to build system actually included some complicated and serious design problems, and control problems but these were considered workable. It came time to build an actual test reactor.

The first serious hardware was BORAX 1, a test reactor built at NRTS Idaho Falls, Idaho which operated in 1953 and 1954; this was followed by BORAX II in 1954, BORAX III in 1955 and BORAX IV in 1956 all of which were used to prove out principles concerning boiling water reactor physics and behavior. In some cases the tests were deliberately so severe that damage occurred, and in fact the final test of BORAX I was a deliberate core destruction test resulting from deliberate control rod ejection in order to fully test and understand behavior of the core in extreme accident conditions.

While the BORAX tests, which essentially were just sections of a reactor without any real similarity to a power producing reactor, were in progress the design work was considered good enough to build, in parallel, an experimental prototype boiling water reactor of a much more workable design for power production. In 1954 Argonne National Laboratory, the chief reactor design bureau for the AEC proposed a design which was approved and on which construction began at Argonne in 1954. The new plant, designated EBWR, or Experimental Boiling Water Reactor, began test operations in 1956.

Meanwhile, after some early failures with sodium cooled reactor designs for the Navy, General Electric had become convinced that its future lay in commercial power generation and in the boiling water concept. In 1955, GE and Commonwealth Edison of Chicago made agreements to build an enormous (for the time) nuclear power station which would use an advanced design concept of boiling water reactor and which at an initial net output rating of 180 MWe would be the largest nuclear station in the world by some amount.

It was quickly decided that testing of a smaller plant was needed first, and GE built (very rapidly) a plant at its new Vallecitos Atomic Power Laboratory in California. This plant, the Vallecitos Boiling Water Reactor was funded by GE and included a steam power plant funded by Pacific Gas & Electric. This plant is shown below; click to enlarge.

The VBWR was completed on this site in June 1957 and the plant went online, with a rated electrical output to the PG&E system of 5000 KW on October 24, 1957. This was 17 months after ground breaking at the site. VBWR was granted Power Reactor Operating License No. 1 by the AEC. The core was rated 20 MWt initially but by 1958 was approved to operate at 30 MWt. Maximum plant pressure was 1000 psig.

At the same time, work on the enormous Dresden Nuclear Generating Station, 50 miles southwest of Chicago, continued with General Electric, as prime reactor contractor, employing Bechtel Corporation as construction contractor. (Both companies continue in the business; see our links for each in the "links" section.) Below is a relatively early artists' conception of the plant's final design which was drawn up by Bechtel Corporation.

The primary reactor plant was to be contained inside of a very large 190 foot diameter reinforced sphere, and the reactor was rated 626 MWt. The large rectangular building housed the turbine generator, rated 180 MWe, and support equipment for the plant. This was essentially how the plant was completed. The reactor was first taken critical in October 1959 and the plant went online as the world's largest nuclear power station in June, 1960. This reactor was a complicated 'dual cycle' plant that supplied steam to the secondary plant both directly from the reactor core, and also from small secondary steam generators. This feature made the plant able to load follow (adjust core output automatically with load changes) better than direct cycle boiling water designs, even if not as well as pressurized water designs. Below, a dramatic night view of Dresden 1.


While the whole GE Vallecitos / Dresden development program was going on, other companies that wished to enter the field as primary reactor vendors (like Westinghouse and GE) also decided to use various forms of the boiling water concept. One early example was ACF Industries' Nuclear Products - ERCO Division who began construction of the Elk River boiling water reactor plant in 1958. This plant, at Elk River Minnesota was an "add on" reactor to a power plant already in existence; it required the interposing of a coal-fired superheater to get steam quality to match that for which the existing turbines were designed. The plant was complicated, and troubled by developmental problems the small company could not deal with .. and in fact the division was bought wholesale from ACF Industries by Allis-Chalmers Manufacturing during construction. Below is a photo from our files.

Allis-Chalmers had already, separately taken a contract from Northern States Power to build a boiling water reactor plant along the Big Sioux River near Sioux Falls, South Dakota. This plant, the Pathfinder Atomic Power Station, was one of the most troubled in history because of the design of the reactor which was one of only two ever built here that attempted to use nuclear superheating of the steam in the reactor core. The plant was never successful and never passed full power tests, and was never put on the grid of Northern States Power for any measurable time before being terminated. However, being an early plant in a somewhat rural type of area it did get wide local press as the post card from the APRA collection below shows.


Aside from these and several other interesting but not really contributive sidelights General Electric had been rolling right along with further design work, a great deal of good press and further letters of intent and contracts. Its success in the field was assured, and many GE BWR plants would be built all over the world. As a close we will show one other relatively early but totally successful GE BWR plant. Below we show Consumers Power Company's Big Rock Point nuclear station on the farily remote coast near Charlevoix, Michigan. The plant operated successfully for many years; the site is now completely natural, the entire plant having been decommissioned and removed and the land is open for all uses.



Written by Will Davis for Atomic Power Review. Do not reproduce without permission.

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