Question: Which reactor complexes were the only ones constructed during the Manhattan Project
About the article "Manhattan Project", section "Hanford reactors".

Candidate passages:
Although an air-cooled design was chosen for the reactor at Oak Ridge to facilitate rapid construction, it was recognized that this would be impractical for the much larger production reactors.
Initial designs by the Metallurgical Laboratory and DuPont used helium for cooling, before they determined that a water-cooled reactor would be simpler, cheaper and quicker to build.
The design did not become available until 4 October 1943; in the meantime, Matthias concentrated on improving the Hanford site by erecting accommodations, improving the roads, building a railway switch line, and upgrading the electricity, water and telephone lines.
As at Oak Ridge, the most difficulty was encountered while canning the uranium slugs, which commenced at Hanford in March 1944.
They were pickled to remove dirt and impurities, dipped in molten bronze, tin, and aluminum-silicon alloy, canned using hydraulic presses, and then capped using arc welding under an argon atmosphere.
Finally, they were subjected to a series of tests to detect holes or faulty welds.
Disappointingly, most canned slugs initially failed the tests, resulting in an output of only a handful of canned slugs per day.
But steady progress was made and by June 1944 production increased to the point where it appeared that enough canned slugs would be available to start Reactor B on schedule in August 1944.
Work began on Reactor B, the first of six planned 250 MW reactors, on 10 October 1943.
The reactor complexes were given letter designations A through F, with B, D and F sites chosen to be developed first, as this maximised the distance between the reactors.
They would be the only ones constructed during the Manhattan Project.
Some of steel, of concrete, 50,000 concrete blocks and 71,000 concrete bricks were used to construct the high building.
Construction of the reactor itself commenced in February 1944.
Watched by Compton, Matthias, DuPont's Crawford Greenewalt, Leona Woods and Fermi, who inserted the first slug, the reactor was powered up beginning on 13 September 1944.
Over the next few days, 838 tubes were loaded and the reactor went critical.
Shortly after midnight on 27 September, the operators began to withdraw the control rods to initiate production.
At first all appeared well but around 03:00 the power level started to drop and by 06:30 the reactor had shut down completely.
The cooling water was investigated to see if there was a leak or contamination.
The next day the reactor started up again, only to shut down once more.
Fermi contacted Chien-Shiung Wu, who identified the cause of the problem as neutron poisoning from xenon-135, which has a half-life of 9.2 hours.
Fermi, Woods, Donald J. Hughes and John Archibald Wheeler then calculated the nuclear cross section of xenon-135, which turned out to be 30,000 times that of uranium.
Fortunately, DuPont engineer George Graves had deviated from the Metallurgical Laboratory's original design in which the reactor had 1,500 tubes arranged in a circle, and had added an additional 504 tubes to fill in the corners.
The scientists had originally considered this overengineering a waste of time and money, but Fermi realized that by loading all 2,004 tubes, the reactor could reach the required power level and efficiently produce plutonium.
Reactor D was started on 17 December 1944 and Reactor F on 25 February 1945.
The first artificial nuclear reactor, Chicago Pile-1, a graphite-moderated device that produced a microscopic amount of heat, was constructed by a team led by Enrico Fermi in 1942.
The construction and testing of this reactor (an "atomic pile") was part of the Manhattan Project.
This work led to the construction of the X-10 Graphite Reactor at Oak Ridge National Laboratory, which was the first nuclear reactor designed and built for continuous operation, and began operation in 1943. 
Belgium has a long industrial history in the nuclear sector.
With Biraco in Olen, which regularly hosted Marie and Pierre Curie, it was at the start of theindustrial production of radium in 1922.
The Uranium ore used was discovered in 1913 in Katanga in then Belgian Congo by UMHK.
The ore found in the Shinkolobwe mine was exceptionally rich.
Even before the second world war the United States expressed an interest in it.
However it wasn't until 1942 when the United States required uranium for the Manhattan Project, and Belgium was one of the few countries with an appreciable stock of uranium ore, that Edgar Sengier struck a deal.
For the following decade Belgium through its colony was one of the main suppliers of uranium to the United States.
This trade relationship resulted in Belgium being granted access to nuclear technology for civil purposes.
In 1952 this led to establishing SCK•CEN, a study center for nuclear research.
The first reactor BR1 (Belgian Reactor 1) became critical in 1956.
Construction of BR2 started the following year.
The BR2 reactor is one of the five main reactors producing molybdenum-99 which decays into technetium-99m, the radioisotope used in more that 80% of diagnostic imaging procedures in nuclear medicine.
In 1954 Belgium was one of the founding members of Cern, and three years later it was one of the original signatories of the Euratom Treaty.
In 1957 A site in Dessel Belgium, a stone's throw away from SCK•CEN was chosen to be the location for Eurochemic.
Thirteen OECD countries (Germany, France, Belgium, Italy, Sweden, The Netherlands, Switzerland, Denmark, Austria, Norway, Turkey, Portugal, Spain) joined forces to built a pilot nuclear reprocessing installation.
This made the "atoomwijk", a housing project for workers on private land near the site a unique scientific village in Europe at the time.
In 1958 the world fair Expo 58 in Brussel was to be powered by BR3.
The reactor was to be built in Brussels, and open to visitors.
Ultimately safety concerns and administrative problems moved the reactor to the SCK•CEN site.
The other iconic symbol of the belief in nuclear technology at the time the Atomium housed a photo exhibition.
In 1959, the Trico Center was established in Kinshasa its TRICO I research reactor was the first nuclear reactor on the African continent.US built BR3 was connected to the grid in 1962.
Making it the first pressurized water reactor in Western Europe.SCK•CEN played a large role in developing MOX fuel.
In 1960 near the same site NV Belgonuclaire, a joint venture between SCK•CEN, Electrabel and Tractebel received its first plutonium from the United States, with the goal of industrially producing MOX fuel.
In 1963 BR3 was loaded with MOX fuel and was the first reactor in the world to generate electricity this way.
In 1967 the commercial Chooz-A plant in France, close to the Belgian border, was connected to the grid.
It had been constructed by a French-Belgian joint venture Sena (Société d'énergie nucléaire Franco-belge des Ardennes) to house a French-Belgian prototype pressurized water reactor, the first one built in Western Europe.
It was jointly operated and delivered electricity to both countries.
In 1972 Belgium participated with The Netherlands and Germany in the failed SNR-300 fast breeder reactor.
In 1973 Belgium and four other European countries formed Eurodif.
The first commercial nuclear reactor in Belgium, Doel-1, was taken into service in 1974.
Six more were connected to the grid during the following ten years.
Plans for an eighth reactor were scrapped instead utilities Electrabel and SPE took a 25% participation in the French Chooz-B nuclear power plant.
In 1974 Eurochemics stopped its reprocessing activities.
In 1987 BR3 was the first pressurized water reactor to be shut down in Europe.
Decommissioning BR3 and Eurochemic has given Belgium significant expertise in the decommissioning of nuclear sites.
Currently Belgium is preparing to contribute to Generation IV reactor research through the MYRRHA project. 
Red Gate Woods is part of a forest preserve within the Palos Division of the Forest Preserve District of Cook County, Illinois.
Located within the preserve is the original site of Argonne National Laboratory and the Site A/Plot M Disposal Site, which contains the buried remains of Chicago Pile-1, the world's first nuclear reactor.
After the initial tests, the reactor at Stagg Field at the University of Chicago was removed and reassembled at the Manhattan Project site in Red Gate Woods.
Local residents reported encountering US Army MPs guarding the area during World War II but no one was aware of the true nature of the activities until long after the war.
After further experiments and the shutdown of Pile 1, a huge hole was dug and the 2-story high reactor was pushed into it and buried.
Other reactors were also built at the site and nuclear waste was buried there.
The site is monitored by the United States Department of Energy and is open to the public.
There is signage in the parking lot showing Albert Einstein and Enrico Fermi at the Red Gate Woods site during the Manhattan Project.
Concrete markers designate historic sites and the foundations of Manhattan Project labs still exist. 
The "Thin Man" nuclear bomb was a proposed plutonium gun-type nuclear bomb which the United States was developing during the Manhattan Project.
Its development was aborted when it was discovered that the spontaneous fission rate of nuclear reactor-bred plutonium was too high for use in a gun-type design.