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B Reactor
The face of B Reactor during construction.
B Reactor is located in Washington (state)
B Reactor
B Reactor is located in the United States
B Reactor
LocationAbout 5.3 miles (8.5 km) northeast of junction of State Route 24 and State Route 240 on the Hanford Site
Nearest cityRichland, Washington
Coordinates46°37′49″N 119°38′50″W / 46.63028°N 119.64722°W / 46.63028; -119.64722
Area9.5 acres (3.8 ha)
Built7 June 1943[1] to September 1944[2]
ArchitectE.I. DuPont de Nemours & Company
NRHP reference No.92000245
Significant dates
Added to NRHP3 April 1992
Designated NHL19 August 2008[3]

The B Reactor at the Hanford Site, near Richland, Washington, was the first large-scale nuclear reactor ever built, at 250 MW. It achieved criticality on September 26, 1944. The project was a key part of the Manhattan Project, the United States nuclear weapons development program during World War II. Its purpose was to convert part of its natural uranium fuel into plutonium-239 by neutron activation, for use in nuclear weapons. Pure plutonium was then chemically separated in the site's T Plant, as an alternative to the Project's uranium enrichment plants. The B reactor was graphite moderated and water-cooled, via a contaminating open cycle with the Columbia River.

It was preceded by Clinton Laboratory's X-10 Graphite Reactor, a pilot plant for reactor production and chemical separation of plutonium, which by mid-1944 had reached a capacity of 4 MW. The B reactor thus represented a massive leap of two orders of magnitude in reactor design. Primarily constructed by DuPont, the operation was assisted by scientists including Enrico Fermi, John Archibald Wheeler, and Chien-Shiung Wu. Two identical reactors, the D Reactor and F Reactor, were launched in December 1944 and February 1945. The plutonium from the site was used in the Trinity test, the Fat Man bomb detonated above Nagasaki, the demon core, and thousands of US warheads during the Cold War. By the early 1960s, the reactors had been upgraded to capacities of 2000 MW. It is historically significant as the world's first large-scale reactor, the first to use water cooling, the first to experience xenon poisoning, the first employed for thermonuclear weapon tritium production, and the seventh critical assembly in total.

The reactor was permanently shut down in February 1968. It has been designated a U.S. National Historic Landmark since 19 August 2008[3][4] and in July 2011 the National Park Service recommended that the B Reactor be included in the Manhattan Project National Historical Park commemorating the Manhattan Project.[5] Visitors can take a tour of the reactor by advance reservation.[6]

Design and construction

[edit]

The reactor was designed and built by E. I. du Pont de Nemours and Company based on experimental designs tested by Enrico Fermi at the University of Chicago, and tests from the X-10 Graphite Reactor at Oak Ridge National Laboratory. It was designed to operate at 250 megawatts (thermal).

The purpose of the reactor was to breed plutonium from natural uranium metal. Enriching fissionable 235U from 238U, the dominant natural isotope, was extremely complex; plutonium's distinct chemistry made separation trivial by comparison. For example, the Y12 uranium enrichment plant in Tennessee required 14,700 tons of silver loaned by the Treasury Department for the windings in its calutrons, employed 22,000 people and consumed more electrical power than most states. Reactor B on the other hand needed only a few dozen employees and far fewer exotic materials required in much smaller quantities. The most important special material needed were the 1,200 tons of purified graphite for neutron moderation, and only enough electricity to run the cooling pumps.[7][8]

The reactor has a footprint of 46 by 38 ft (14 by 12 m) (about 1,750 sq ft (163 m2) and is 41 ft (12 m) tall, giving a volume of 71,500 cu ft (2,020 m3). The reactor core itself consists of a 36 ft-tall (11 m) graphite box measuring 28 by 36 ft (8.5 by 11.0 m) occupying a volume of 36,288 cu ft (1,027.6 m3) and weighing 1,200 short tons (1,100 t). It is penetrated horizontally through its entire length by 2,004 aluminum tubes containing fuel and vertically by channels housing the control rods.[4]

The core is surrounded by a thermal shield of cast iron 8 to 10 in (20 to 25 cm) thick weighing 1,000 short tons (910 t). Masonite and steel plates enclose the thermal shield on its top and sides, forming a biological shield for radiation protection. The bottom of the thermal shield was supported by a 23 ft-thick (7.0 m) concrete pad topped by cast-iron blocks. Based on the success of the first atomic pile, graphite was selected to moderate the nuclear reaction. This reaction was fueled by 200 short tons (180 t) of metallic uranium slugs approximately 25 mm (1 in) diameter, 70 mm (3 in) long (about as large as of a roll of quarters[4]), sealed in aluminum cans, and loaded into the aluminum tubes.[4]

The reactor was water-cooled. Its coolant was pumped from the Hanford Reach of the Columbia River, through the aluminum tubes and around the uranium slugs at a rate of 75,000 US gal (280,000 L) per minute. The water was discharged into settling basins. Water was held in the basins to permit the decay of short-lived radioactive waste, the settling out of particulate matter gathered from the reactor, and for the water to cool to within 11 °F of the river's temperature. It was then discharged back into the Columbia River.[9]

Map of the Hanford site with the B/C reactor site shown at the upper left.

Operation

[edit]

The B Reactor had its first nuclear chain reaction in September 1944, the D Reactor in December 1944 and the F Reactor in February 1945. The initial operation was halted by a problem identified as neutron absorption by the fission product Xe-135, first identified in a research paper of Chien-Shiung Wu that was shared with Fermi.[10] It was overcome by increasing the amount of uranium charged. The reactor produced plutonium-239 by irradiating uranium-238 with neutrons generated by the nuclear reaction. It was one of three reactors – along with the D and F reactors – built about six miles (10 km) apart on the south bank of the Columbia River. Each reactor had its own auxiliary facilities that included a river pump house, large storage and settling basins, a filtration plant, large motor-driven pumps for delivering water to the face of the pile, and facilities for emergency cooling in case of a power failure.[4]

Emergency shutdown of the reactor, referred to as a SCRAM, was attained either by rapidly fully inserting the vertical safety rods or, as a backup method, by the injection of borated water into the reactor. In January 1952, the borated water system was replaced by a "Ball-3X" system that injected nickel-plated high-boron steel balls into the channels occupied by the vertical safety rods.[4]

The plutonium for the nuclear bomb used in the Trinity test in New Mexico and the Fat Man bomb dropped on Nagasaki, Japan was created in the B reactor. The B Reactor ran for two decades, and was joined by additional reactors constructed later. It was permanently shut down in February 1968.[4][11]

Current status

[edit]
Exterior of the Hanford B Reactor as of 2018
Hanford B Reactor Control Station as of 2018
Hanford B Reactor Tubes and Elevator as of 2018

The United States Department of Energy has administered the site since 1977[12][13] and offers public tours on set dates during the spring, summer, and fall of the year, as well as special tours for visiting officials.[6][14]

As of 2014 six of the nine production reactors at Hanford were considered to be in "interim safe storage" status, and two more were to receive similar treatment. The exception was the B Reactor, which was given special status for its historical significance.[15]

In a process called cocooning or entombment, the reactor buildings are demolished up to the 4 ft-thick (1.2 m) concrete shield around the reactor core. Any openings are sealed and a new roof is built.[16] Most auxiliary buildings at the first three reactors have been demolished, as well. The C reactor was put into operation in 1952 and was shut down in 1969.[17] It was cocooned as of 1998.[18] The D reactor operated from 1944 to June 1967, and was cocooned in 2004. The DR Reactor went online in October 1950,[19] and was shut down in 1964. It was cocooned in 2002.[20] The F reactor was shut down in June 1965 and cocooned in 2003.[21] The H Reactor became operational as of October 1949 and was shut down as of April 1965. It was cocooned as of 2005.[22] Cocooning of the N-Reactor, which operated from 1963 to 1987, was completed as of 14 June 2012.[23] The decommissioned reactors are inspected every five years by the Department of Energy.[18]

The K East and K West reactors were built in the 1950s and went into use in 1955. They were shut down in 1970 and 1971, but reused temporarily for storage later.[24] Preliminary plans for interim stabilizing of the K-East and K-West reactors were underway as of 30 January 2018.[16]

The B Reactor was added to the National Register of Historic Places (#92000245) on 3 April 1992. A Record of Decision (ROD) was issued in 1999, and an EPA Action Memorandum in 2001 authorized hazards mitigation in the reactor with the intention of allowing public tours of the reactor.[25] It was named a National Historic Landmark on 19 August 2008.[3][4]

In December 2014, passage of the 2015 National Defense Authorization Act (NDAA) made the B reactor part of the Manhattan Project National Historical Park, which also includes historic sites at Oak Ridge, Tennessee and Los Alamos, New Mexico.[26][27] The park was formally established by a Memorandum of Agreement on November 10, 2015, which was signed by the National Park Service and the Department of Energy. Museum development at Hanford may include the B Reactor, Bruggemann's Warehouse, Hanford High School, Pump House, and White Bluffs Bank.[28]

Timeline of major events

[edit]
Year Date Event
1943 October U.S. Army Corps of Engineers breaks ground to build B Reactor[29]
1944 13 September First uranium fuel slug loaded into B Reactor[29]
1944 26 September Initial reactor criticality achieved[29]
1945 3 February B Reactor plutonium delivered to Los Alamos[29]
1945 16 July B Reactor plutonium used in world's first nuclear explosion. (Trinity Test Site, New Mexico) [29]
1945 9 August B Reactor plutonium used in Fat Man bomb dropped on Nagasaki, Japan[29]
1946 March B Reactor operations suspended[29]
1948 June B Reactor operation resumed[29]
1949 March B Reactor begins production of tritium for use in hydrogen bombs[29]
1954 1 March First use of B Reactor tritium in a test detonation of a hydrogen bomb at Bikini Atoll[citation needed]
1968 January 29 Atomic Energy Commission directs shutdown of B Reactor[29]
1976 B Reactor declared National Historic Mechanical Engineering Landmark by American Society of Mechanical Engineers[29]
1994 B Reactor declared National Historic Civil Engineering Landmark by American Society of Civil Engineers[29]
2008 B Reactor declared National Historic Landmark by U.S. Department of Interior and National Park Service[29]
2009 U.S. Department of Energy announces public tours[30]
2011 July National Park Service recommends B Reactor be included in a national historic park commemorating the Manhattan Project.[5]
2014 December 2015 National Defense Authorization Act (NDAA) includes B reactor in Manhattan Project National Historical Park[26]
2015 10 November Manhattan Project National Historical Park formally established by Memorandum of Agreement[28]

See also

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References

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Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

B Reactor

View on Grokipedia
from Grokipedia
The B Reactor at the Hanford Site in southeastern Washington state was the world's first full-scale plutonium production reactor, constructed as part of the Manhattan Project during World War II to generate fissile material for nuclear weapons. Completed in September 1944 after construction began the previous October, the graphite-moderated, water-cooled reactor achieved criticality on September 26, 1944, and rapidly scaled to full power of 250 megawatts by February 1945. It produced plutonium-239 through neutron irradiation of uranium-238, yielding the fissile cores for the Trinity test detonation on July 16, 1945, and the Fat Man implosion-type bomb dropped on Nagasaki on August 9, 1945, which contributed decisively to Japan's surrender. Designed and built by the DuPont company under wartime secrecy, the B Reactor exemplified unprecedented engineering feats, including the rapid assembly of over 2,000 process tubes within an 11-month timeline, marking the onset of industrial-scale nuclear technology. Decommissioned in 1968 after producing plutonium for subsequent Cold War arsenals, it was designated a National Historic Landmark in 2008 and now forms a core site of the Manhattan Project National Historical Park, offering public tours to illustrate its pivotal role in atomic history.

Historical Context

Role in the Manhattan Project

The B Reactor served as the cornerstone of plutonium production within the Manhattan Project, the United States' classified program to develop atomic weapons during World War II. Established at the Hanford Site in Washington state, it was engineered to irradiate uranium fuel in a sustained nuclear chain reaction, yielding plutonium-239 as a fissile material alternative to scarce uranium-235. This approach addressed the project's urgent need for bomb-grade material, leveraging graphite-moderated reactor technology scaled from Enrico Fermi's experimental Chicago Pile-1 to industrial levels capable of processing thousands of tons of uranium annually. Construction of the B Reactor, the first of three planned plutonium facilities at Hanford's 100 Area, began in October 1943 under the direction of the U.S. Army Corps of Engineers and E.I. du Pont de Nemours & Company, which managed operations to maintain secrecy and efficiency. The reactor's design incorporated a 36-foot aluminum process tube lattice housing 2,004 slugs cooled by the , enabling power outputs up to 250 megawatts thermal. It achieved initial criticality on September 26, 1944, just 13 months after , validating the feasibility of continuous, high-volume breeding essential for weaponizing the element. In its early operations, the B Reactor supplied the plutonium core for the test detonation on July 16, 1945—the world's first nuclear explosion—and for the implosion-type bomb dropped on , , on August 9, 1945, contributing directly to the war's end. By February 1945, it had reached full production power, yielding approximately 250 grams of daily and demonstrating the Manhattan Project's capacity to transition from laboratory-scale experiments to wartime industrial output under extreme secrecy constraints.

Hanford Site Selection and Establishment

The selection of the for plutonium production in the began in spring 1942, driven by the need for a large-scale facility to irradiate fuel in nuclear reactors and separate chemically. Scientists at the in , led by , established key criteria including a minimum of 25,000 U.S. gallons per minute for cooling, at least 100,000 kilowatts of , and extensive isolation buffers such as a 4-mile radius around chemical separation plants and a 225-square-mile exclusion area to mitigate unknown hazards. These requirements assumed the construction of three to four helium-cooled reactors and two separation plants, prioritizing sites distant from population centers, railroads, and highways to ensure secrecy and . By December 14, 1942, a meeting in , finalized these criteria, leading to Hanford's recommendation on December 31 after evaluating multiple locations. Hanford, a remote desert valley along the in southeastern Washington, was favored for its abundant cooling water from the river, reliable hydroelectric power from the nearby and Bonneville Dams, stable geology suitable for containment, and an uninhabited tract of approximately 500,000 acres with existing railroad access and regional labor availability. It outperformed alternatives like sites near —deemed too proximate to urban areas for the risks of reactor operations—and other candidates in or that lacked comparable water and power resources or sufficient isolation. General , head of the , personally inspected the site and endorsed it on January 16, 1943, with final War Department approval on February 9, 1943. Site establishment proceeded rapidly in , with the U.S. government acquiring about 780 square miles of land for $5.1 million (in 1943 dollars), displacing fewer than 1,500 from the small farming communities of Hanford, White Bluffs, and Richland, who received compensation based on appraised property values excluding improvements like crops and equipment. The acquisition, managed by Colonel Franklin T. Matthias of the Army Corps of Engineers, involved 90-day notices, though some pursued legal challenges settled out of ; the process also impacted Native American groups like the Wanapum by restricting access to traditional Columbia River fishing grounds. was contracted to design and construct the Hanford Engineer Works, emphasizing secrecy through compartmentalization and rapid wartime mobilization, which enabled groundbreaking for initial facilities like the B Reactor's water-cooling plant by August 1943. This establishment positioned Hanford as the primary site for full-scale plutonium production, complementing uranium enrichment at Oak Ridge.

Design and Engineering

Technical Specifications

The B Reactor was a graphite-moderated, light-water-cooled nuclear reactor designed for large-scale plutonium production as part of the Manhattan Project. Its core consisted of a graphite stack measuring 36 feet by 36 feet by 28 feet, comprising approximately 2,200 tons of high-purity graphite arranged in about 75,000 machined blocks. The reactor was designed for a thermal power output of 250 megawatts. Horizontal process tubes numbering 2,004 pierced the moderator, each 44 feet long with an outer diameter of 1.73 inches and inner diameter of 1.61 inches, fabricated from 2S aluminum alloy coated with 72S . These accommodated stacks of 32 to 35 cylindrical uranium fuel slugs per tube, each slug 1.44 inches in diameter and 8.7 inches long, clad in aluminum with a 0.086-inch annular gap for flow. Cooling water from the , demineralized and pumped at an initial rate of 30,000 gallons per minute (20 gallons per minute per tube at 200 psi inlet pressure and 18-19.5 feet per second velocity), passed through the tubes in a single pass before discharge. Control and safety systems included 9 horizontal control rods and 29 vertical safety rods inserted through dedicated channels in the stack. gas circulated through the at 2,600 cubic feet per minute to facilitate and remove impurities, with later modifications incorporating CO2 to manage swelling from exposure. The reactor was enclosed in a 105-B building measuring 120 feet by 150 feet by 120 feet high, featuring a 10-inch shield and additional biological shielding of , steel, and concrete totaling around 10,000 tons.
ParameterSpecification
Thermal Power (Design)250 MW
Graphite Mass~2,200 tons
Process Tubes2,004 (aluminum, 44 ft long)
Fuel Slugs per Tube32–35 (uranium, aluminum-clad)
Coolant Flow (Initial)30,000 gpm total (20 gpm/tube)
Control Rods9 horizontal, 29 vertical safety rods

Key Innovations and Theoretical Foundations

The B Reactor's theoretical foundations derived from Enrico Fermi's (CP-1), the first artificial achieved on December 2, 1942, which validated the feasibility of a controlled fission process using fuel moderated by . In CP-1, served as an efficient , slowing fast fission neutrons to thermal energies to enhance the fission probability in the 0.7% U-235 isotope present in , while avoiding significant absorption that would preclude a sustained (k_eff > 1). Diffusion theory and exponential pile calculations from the at the informed the distribution, critical size, and material ratios needed to scale this concept from a low-power experimental assembly (power level ~0.5 watts) to an industrial plutonium production facility. These principles emphasized a lattice geometry of uranium- blocks to optimize neutron economy, enabling transmutation of U-238 to Pu-239 via followed by two beta decays, with minimal parasitic losses. A primary innovation was the shift to light water cooling for high-power operation, as air cooling—adequate for prototypes like the at Oak Ridge—proved insufficient for the B Reactor's targeted 250 megawatts thermal output, based on calculations showing inadequate removal rates without risking fuel meltdown. Water, drawn from the at 75,000 gallons per minute, was circulated through 2,004 horizontal aluminum process tubes (each 8 feet long, 1.5 inches inner diameter) embedded in a 36-by-36-by-36-foot stack, directly contacting aluminum-clad slugs to extract efficiently via . This required novel mitigation, including thin aluminum canning (0.035 inches thick) on cylindrical elements (1.1 inches diameter, 8 inches long, ~113 grams each), preventing oxidation and generation that could displace or poison the chain reaction. Further design advancements included a modular lattice (over 200,000 blocks, density ~1.68 g/cm³) with horizontal channels for and vertical ones for control rods ( and ), allowing precise reactivity management via differential insertion. The front-face loading mechanism, using a shielded crane to insert and extract slugs without full shutdown, supported continuous plutonium harvesting—critical for wartime production—while rear-face discharge tubes facilitated irradiated removal into water-filled canyons for chemical processing. These features, engineered by DuPont under specifications, represented the first integration of production-scale neutronics with robust thermal-hydraulics, overcoming theoretical scaling challenges like flux depression and limits (~1 MW/m²) through empirical validation from CP-1 and X-10 data.

Construction

Project Timeline and Milestones

Construction of the B Reactor began with groundbreaking for its water-cooling plant on August 27, 1943, as part of the development under the . The following month, on October 10, 1943, crews drove the first stakes for the reactor's pile building, initiating the core structural work. By early 1944, the 120-foot-high pile building—a massive, radiation-shielded —had been completed. Assembly of the pile commenced in February 1944, advancing rapidly despite the unprecedented scale and untested design. The cast-iron base and shielding layers were finalized by mid-May 1944, with the structure approaching operational readiness by July 1944. On September 13, 1944, the first uranium fuel slugs were loaded into the reactor, overseen by physicist . The B Reactor achieved initial criticality on September 26, 1944, successfully sustaining a controlled at production levels and becoming the world's first industrial-scale -producing reactor. It escalated to full power output of 250 megawatts thermal in February 1945, enabling plutonium shipment for the Trinity test on July 16, 1945. This timeline reflects an extraordinarily compressed schedule—spanning roughly 11 months from major structural start to criticality—driven by wartime imperatives, DuPont's engineering leadership, and mobilization of over 17,000 workers at peak for the reactor alone. The first irradiated fuel slugs were discharged on December 25, 1944, yielding initial plutonium batches for weapons production.

Workforce, Secrecy, and Logistical Challenges

The construction of the B Reactor was managed by E.I. du Pont de Nemours and Company under contract with the U.S. Army Corps of Engineers as part of the , requiring the rapid recruitment of a massive to meet wartime deadlines. conducted nationwide recruitment efforts, interviewing 262,040 applicants and hiring 94,307 workers to staff the , where the B Reactor was the centerpiece. The workforce peaked at approximately 45,000 to 50,000 personnel by mid-1944, with an average of 22,000 employed throughout the project duration. Workers faced initial hardships in temporary accommodations at Hanford Camp, including tents, barracks, trailers, and Quonset huts, which contributed to high turnover rates until improvements such as recreational facilities and higher wages were implemented. Despite these efforts, labor shortages persisted due to the project's scale and competing wartime demands, necessitating continuous recruitment drives. Secrecy was paramount, enforced through compartmentalization where most workers operated on a strict need-to-know basis; fewer than 500 individuals at Hanford understood the site's true purpose of production. The project used codenames like "" and "Site W" to obscure its nature, with the 500,000-acre site isolated from public view—reactors and facilities spaced miles apart and distant from highways and towns—to minimize leaks. Access was tightly controlled, including restrictions on nearby Native American fishing areas, and all personnel underwent FBI background checks to prevent . Logistical challenges arose from the remote desert along the , requiring to construct supporting infrastructure from scratch, including housing for over 45,000 workers, roads, rail lines, and utilities, all while adhering to untested designs. occurred on August 27, 1943, with the reactor achieving criticality just 13 months later on September 26, 1944, demanding unprecedented speed amid wartime material constraints and secrecy protocols that disguised shipments and prohibited open advertising. These factors compounded labor and supply issues, as equipment and blocks for the reactor's core had to be transported covertly without revealing the project's atomic objectives.

Operation

Startup and Initial Criticality

Fuel loading into the B Reactor's 2,004 aluminum process tubes began on September 13, 1944, following the completion of construction earlier that month. Workers manually inserted approximately 64,000 fuel slugs, totaling around 500,000 pounds, into the core under strict secrecy and safety protocols. The startup process culminated in the initiation of a self-sustaining , known as achieving criticality. On September 26, 1944, at 10:48 p.m., operators, under the guidance of , withdrew the reactor's control rods to allow multiplication to reach a . This marked the world's first successful operation of a large-scale production reactor, validating the design based on 's earlier experiment scaled up for industrial production. Initial operations proceeded without immediate complications, confirming the reactor's ability to produce through in fuel. The event represented a critical milestone in the , enabling the subsequent production of for atomic weapons.

Plutonium Production and Wartime Contributions

The B Reactor achieved initial criticality on September 26, 1944, marking the first successful operation of a large-scale production reactor and initiating the industrial-scale production of for nuclear weapons. Following a period of testing and adjustments to ensure stable operation, the reactor began full-scale fuel irradiation, with uranium slugs processed through adjacent chemical separation facilities to extract . By early 1945, Hanford shipped its first batches to Los Alamos Laboratory, enabling the fabrication of cores for atomic devices. During , the B Reactor's output directly supplied the for the test device, detonated on July 16, 1945, in , which confirmed the viability of implosion-type fission weapons. It also provided the core for the bomb, dropped on , , on August 9, 1945, contributing to Japan's surrender and the war's end. These contributions stemmed from the reactor's design capacity to irradiate thousands of aluminum-clad fuel elements in a graphite-moderated pile, cooled by the , yielding weapons-grade through and processes. The wartime production underscored the Manhattan Project's emphasis on rapid scaling: from construction start on June 7, 1943, to operational plutonium yield within 15 months, overcoming challenges like poisoning that briefly halted operations in January 1945 before supplementation restored output. This enabled the U.S. to deploy -based weapons ahead of adversaries, with B Reactor's success validating Enrico Fermi's theoretical pile concepts adapted for under DuPont's engineering oversight.

Operational Challenges and Resolutions

The B Reactor's initial operation, beginning with criticality on September 26, 1944, was disrupted by poisoning, a fission byproduct with a high absorption cross-section that accumulated in the reactor core and halted after approximately 18 hours at low power levels. This unexpected issue, unforeseen in pre-startup models due to the reactor's novel scale and enrichment effects, reduced reactivity to near-shutdown conditions despite intact fuel and moderator integrity. engineers, led by Crawford Greenewalt, resolved it by overriding Met Lab recommendations and loading additional fuel slugs into the reactor's excess process tubes—expanding from an initial 1,500 to 2,215 aluminum-clad cylinders—enabling power escalation to 2,650 megawatts, sufficient to burn out the xenon inventory and restore sustained operation by early October 1944. This empirical fix validated the design's conservative overcapacity, originally incorporated by to accommodate uncertainties, and informed subsequent production reactors at Hanford. Sustained high-flux irradiation induced the in the moderator, causing atomic displacements that led to anisotropic dimensional growth—up to several inches in stack height—and potential stored energy release risks if temperatures exceeded annealing thresholds. Operators mitigated this through periodic annealing via controlled power reductions and tube realignments, as well as inserting additional fuel channels to compensate for expansion without compromising economy; the reactor's modular lattice, designed with spare positions, facilitated these adjustments without full shutdowns exceeding routine maintenance cycles. Cooling system demands, involving 75,000 gallons per minute of Columbia River water pumped through 2,215 horizontal aluminum tubes to dissipate 250 megawatts of heat, strained pumps, pipes, and electrical infrastructure, resulting in leaks from corrosion, galvanic effects, and vibration-induced fatigue. Fuel slug integrity failures, occurring in about 0.1% of slugs due to canning defects or radiation embrittlement, risked contaminating coolant with fission products; these were detected via downstream radiation monitors, isolated by tube plugging, and managed through diluted discharge into the river after verifying low activity levels below operational thresholds. Reinforcements, redundant pumps, and process tube coatings—developed iteratively in on-site labs—ensured continuous plutonium output, with no criticality excursions or core damage reported during wartime runs.

Shutdown, Decommissioning, and Post-War Use

Cold War Restart and Final Shutdown

Following , the B Reactor was shut down on March 15, 1946, primarily due to concerns over moderator expansion that threatened long-term operational integrity. It entered a standby status as demands temporarily declined after wartime production goals were met. With the onset of the and heightened needs for expanding the U.S. nuclear arsenal—particularly in light of Soviet advancements—the Atomic Energy Commission authorized the reactor's restart. Operations resumed on July 15, 1948, enabling renewed production at a thermal power level of 250 MW. Over the subsequent two decades, the B Reactor processed uranium fuel slugs to yield for approximately 20 nuclear weapons annually, contributing to the buildup of the American stockpile amid escalating tensions with the . This period saw incremental upgrades, including enhanced cooling systems, to sustain output despite the reactor's aging pile and accumulating fission products. By the mid-1960s, newer, more efficient reactors at Hanford and other sites had assumed the bulk of plutonium production, rendering the B Reactor economically and technically obsolete. The U.S. Atomic Energy Commission directed its permanent shutdown on February 12, 1968, after 19 years of intermittent and then continuous service, totaling over 30 distinct operating campaigns since 1944. Post-shutdown, the facility entered monitored cold standby until 1978, when decommissioning preparations began, marking the end of its role in production.

Decommissioning Process

Following its final shutdown on February 12, 1968, the B Reactor underwent initial deactivation procedures, including the removal of assemblies and the draining of coolant systems to transition the facility to a non-operational state. These steps aligned with standard post-shutdown protocols for Hanford's production reactors to mitigate immediate radiological risks and prepare for long-term management. Unlike the other eight surplus production reactors at Hanford, which were slated for full decommissioning involving decontamination, dismantlement, and placement into Interim Safe Storage (ISS) with concrete cocooning, the B Reactor avoided such measures due to its pivotal role in the Manhattan Project. In the 1993 Environmental Impact Statement and Record of Decision (EIS-0119) for decommissioning those eight reactors, the U.S. Department of Energy (DOE) excluded the B Reactor, citing its potential for preservation as a national historic site amid growing recognition of its engineering and historical value. This decision was influenced by advocacy efforts starting in 1991, which highlighted the reactor's status as the world's first full-scale plutonium production facility and argued against demolition to retain it as a tangible link to atomic history. Instead of dismantlement, the B Reactor entered a surveillance and (S&M) regime governed by DOE/RL-2001-68, encompassing regular inspections, structural monitoring, and minimal interventions to ensure radiological safety while preserving the facility's integrity. This included a Removal Action Work to address specific risks without altering , allowing for ongoing oversight rather than irreversible decommissioning. Preservation activities have continued, such as planned structural upgrades in 2024-2026 to stabilize the site for public access and historical tours, reflecting a shift from disposal-oriented decommissioning to long-term stewardship. By , its designation as a further solidified this approach, prioritizing interpretive use over waste-generating demolition.

Safety, Environmental Impact, and Controversies

Radiation Safety Measures and Incidents

Radiation safety at the B Reactor was overseen by a program established in 1944 by Herbert L. Parker, who drew on prior experiences with hazards to implement structured protections for workers handling radioactive materials during production. Key principles emphasized time, distance, and shielding to minimize exposure, coupled with administrative oversight to ensure compliance. Engineered controls included concrete shielding around the reactor core, locked access doors to restricted areas, and extended-handled tools to keep personnel at a safe distance from sources. Administrative measures involved Special Work Permits (SWPs) for all tasks in radiological zones, which specified permissible exposure times, required breaks for recovery, and mandatory training on procedures; these permits were developed by health physicists and reviewed by workers prior to entry. consisted of gloves, respirators, coveralls, hooded suits, and goggles to prevent , , or from fission products or activated materials. Monitoring protocols required workers to carry radiation survey meters, wear special badges to track cumulative doses, and undergo body scans upon exiting contaminated areas to detect internalized radionuclides. No major radiation overexposure incidents specific to B Reactor operations are documented in historical records from the Manhattan Project era, reflecting the effectiveness of these layered controls in preventing acute worker accidents during its initial wartime plutonium production phase from 1944 to 1945. However, broader Hanford Site practices during early operations allowed low-level releases of fission products and into the via cooling water, with dilution deemed sufficient to limit environmental and downstream human exposure risks at the time, though later assessments questioned initial assumptions about long-term . Worker dose monitoring via film badges and area surveys helped maintain exposures below contemporary standards, though epidemiological studies of Hanford personnel have indicated elevated cancer risks even at sub-regulatory levels, underscoring limitations in early accuracy.

Hanford's Broader Environmental Legacy

The , encompassing the B Reactor and associated facilities, generated extensive radioactive and chemical contamination during plutonium production from 1944 to 1988, primarily through tank farm leaks, deliberate discharges into cribs and trenches, and inadvertent releases from reactor cooling systems. Single-shell tanks, numbering 149 and holding legacy waste, have leaked an estimated 1 million gallons of high-level radioactive liquid into the soil since the , with at least 67 confirmed leakers contaminating underlying vadose zones and aquifers. These releases introduced isotopes such as , cesium-137, , and into the environment, with persisting in unlined disposal sites across the 580-square-mile reservation. Groundwater contamination spans over 70 square miles above regulatory standards, affecting a primary plume in the 200 East and West Areas with billions of gallons polluted by , nitrate, , , and other radionuclides. Pump-and-treat systems have processed more than 2 billion gallons since 2012, extracting approximately 600 tons of contaminants, yet migration toward the continues, with plumes advancing at rates up to 280 feet per year in some areas. The river itself received historical effluents from Hanford's nine production reactors, including B Reactor, resulting in detectable sediment contamination, though direct impacts have diminished since operations ceased. Cleanup efforts, managed by the U.S. Department of Energy under the Tri-Party Agreement with Washington State and the EPA, focus on vitrification of 56 million gallons of tank waste, vadose zone remediation, and groundwater restoration, but face delays from technical challenges like grout failures and waste complexity. Projected costs exceed $589 billion through 2060 or beyond, with 177 underground tanks—most single-shell—requiring retrieval and treatment amid ongoing leaks risking further aquifer and river pathway contamination. Despite progress, such as emptying 18 tanks since 2011, institutional risks from funding shortfalls and shifting priorities persist, underscoring the site's enduring legacy as one of the most contaminated nuclear facilities globally.

Debates on Necessity Versus Ethical Concerns

The plutonium produced by the B Reactor played a pivotal role in the "Fat Man" bomb detonated over on August 9, 1945, which contained approximately 6.2 kilograms of derived from Hanford's operations. Advocates for the reactor's necessity emphasize that this production capability enabled the to deploy atomic weapons that compelled Japan's on August 15, 1945, averting —an Allied invasion projected to incur 400,000 to 800,000 U.S. casualties and up to several million Japanese deaths based on military planning documents. Japanese intercepts and leadership statements prior to the bombings indicated no intent to capitulate despite the Declaration's July 26, 1945, demand for surrender, with militarists preparing for homeland defense involving civilian mobilization. Critics of this view, including revisionist historians, contend the bombs were unnecessary as Japan was already militarily defeated and diplomatically inclined toward peace following Soviet entry into the war on August 8, 1945, arguing the bombings served more to demonstrate U.S. power to the than to secure victory. However, declassified documents reveal persistent Japanese military resistance plans and a failed coup to reject surrender terms even after , underscoring the bombs' causal role in overriding internal opposition to capitulation. Ethical debates surrounding the B Reactor intensified among Manhattan Project scientists, who grappled with the moral implications of weaponizing for urban targets. In 1945, the Franck Committee, comprising prominent physicists, recommended a non-combat demonstration explosion to avoid setting a for tactics, warning of an and the erosion of international norms against indiscriminate destruction. and others later reflected on the profound ethical weight of unleashing such power, with post-war petitions from project alumni urging against further use absent clear existential threat. Long-term ethical concerns extend to the environmental and health legacies of B Reactor operations, which discharged radioactive effluents into the and generated waste now stored in 177 underground tanks holding 56 million gallons of material, with ongoing leaks documented since the posing risks to and indigenous communities. Preservation efforts for the reactor as a have drawn criticism for potentially sanitizing these costs, as public tours have been accused of minimizing the bombings' civilian toll—estimated at 200,000 deaths—and Hanford's contribution to a nuclear arsenal that perpetuated proliferation. Proponents counter that acknowledging the reactor's dual legacy of technological triumph and moral quandary fosters informed discourse on nuclear restraint, without excusing wartime exigencies.

Legacy and Preservation

Scientific and Technological Achievements

The B Reactor marked a pioneering achievement in as the world's first full-scale production reactor, designed to generate through the irradiation of in a sustained . Constructed by under the , it achieved initial criticality on September 26, 1944, scaling up experimental designs from Enrico Fermi's and the in Oak Ridge to an industrial capacity of 250 megawatts thermal power. Its core consisted of approximately 2,000 short tons of moderator surrounding 2,214 horizontal aluminum process tubes loaded with uranium fuel slugs, cooled by up to 30,000 gallons per minute of water, enabling operation without moving parts in the reactor vessel itself. A critical technological innovation emerged in addressing , a neutron-absorbing fission product that caused unexpected reactivity shutdowns shortly after startup on September 13, 1944, when fuel loading began. Theoretical predictions by physicist John Wheeler had anticipated this buildup from iodine-135 decay, but the effect exceeded initial models; the solution involved incrementally increasing fuel elements and power output to overwhelm the poison through higher , restoring full operation by late November 1944 and validating predictive reactor physics for large-scale systems. This adaptive engineering not only ensured output—beginning with the first processed batches in early 1945—but also established protocols for managing transient poisons in graphite-moderated reactors, influencing subsequent designs worldwide. Beyond wartime plutonium yields for the test and Nagasaki device, the B Reactor demonstrated the viability of mass-producing weapons-grade isotopes, processing over 36,000 fuel slugs per batch by full-power attainment in February 1945. It later pioneered tritium production techniques by irradiating lithium-6 targets, supplying material for the 1952 thermonuclear test and advancing fusion weapon concepts. These feats underscored causal mechanisms of neutron economy and in megawatt-scale fission, laying empirical foundations for commercial and isotope applications in and .

National Historic Recognition and Public Access

The B Reactor was recognized as a National Historic Mechanical Engineering Landmark by the in 1976 for its pioneering role in industrial-scale nuclear production. It was listed on the in 1992, reflecting community-led preservation efforts to prevent decommissioning and demolition amid post-Cold War site cleanup. On August 19, 2008, the U.S. Department of the Interior officially designated the B Reactor a , acknowledging its status as the world's first full-scale plutonium production reactor and its contributions to the . In December 2014, authorized the , which includes the B Reactor as a key component alongside sites in , and ; the park was formally established in 2015 through a memorandum of agreement between the , Department of Energy, and other partners. This designation emphasizes the reactor's historical significance in advancing and ending , while supporting ongoing DOE stewardship for preservation and . Public access to the B Reactor is managed by the U.S. Department of Energy in collaboration with the , primarily through free guided tours that provide interpretive programs on its history and operations. These tours, lasting approximately four hours including bus transport from a visitor station, have historically operated from mid-April to mid-November on weekdays, accommodating limited groups with advance registration required due to security protocols at the . However, tours were temporarily suspended in 2025 to facilitate major preservation projects, including roof replacement and structural upgrades to maintain the facility's integrity. Virtual tours and panoramic views remain available online via resources for broader public engagement.

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