The Hanford B Reactor

February 13, 2023, 11:49AMNuclear NewsJeremy Hampshire
Front face of the B Reactor at the Hanford Site. (Photo: DOE)

In remote southeastern Washington you will find the sprawling Hanford Site, which was constructed to produce plutonium for the Manhattan Project. Within this complex is the first plutonium production reactor, the Hanford B Reactor. The DuPont Corporation was responsible for construction and operation of the B Reactor. Due to the urgency of the Manhattan Project, construction was completed in just over a year, and The B Reactor went critical on September 26, 1944. After the needs of the Manhattan Project were satisfied, the reactor was briefly shut down and then restarted to produce plutonium for roughly another 20 years, supporting Cold War efforts. In addition to plutonium production, the B Reactor also pioneered the process to produce tritium for the first-ever thermonuclear test.

Construction of the B Reactor. (Photo: DOE)

Construction of the reactor pile consisted of the following materials: 2.5 million square feet of Masonite, 2,200 tons of graphite, and 221,000 linear feet of copper tubing, just to name a few. The pile was built with a biological shield approximately four feet thick. Inside this biological shield was a pile of graphite blocks that measured 36 feet tall by 36 feet wide by 28 feet deep. More than 2,000 horizontal process tubes carried an average of 59 uranium fuel slugs as well as cooling water that allowed the reactor to run as high as 2,000 MW. The uranium fuel was enclosed in aluminum containers for transportation through the process tubes, which would stop the fuel from coming into contact with the cooling water, thus preventing oxidation. New fuel was pushed into the front of the reactor through the process tubes. As new fuel was pushed into these process tubes, previously irradiated fuel would be pushed out the back of the reactor into a cooling pit. From the cooling pit, the hot fuel would be loaded into a rail car and submerged in water for cooling purposes while being transported to the 200 Area for processing. The face of the reactor had an elevator that allowed for easy fuel loading; there was another elevator at the rear of the pile that allowed personnel to uncap the fuel channels prior to refueling. The rear elevator was never used to transport irradiated fuel, since the irradiated fuel was allowed to fall into the cooling pit below.

The B Reactor had a total of 29 boron-containing safety rods that entered the reactor vertically from the top. Each 35-foot safety rod was held above the reactor by a steel cable attached to an electromagnetically driven winch. If power to the reactor was lost, the electromagnets would lose power and the safety rods would automatically drop into the pile. Reactivity control was managed with nine 75-foot-long control rods. Of the nine control rods, two were used as regulating rods and seven were used as shim rods. Interestingly, the control rods were inserted into the core horizontally rather than vertically.

Heat was removed from the reactor via a single-pass design with cooling water drawn from the Columbia River, which was treated at a max flow rate of 38,000 gallons per minute to remove any impurities. After filtration, the water was sent to two underground holding tanks, each of which held 5 million gallons. Next, the cooling water was pumped to a deaeration facility to remove the oxygen and carbon dioxide introduced during filtration. After deaeration, the cooling water was pumped to four holding tanks, each with a capacity of 1.75 million gallons, then pumped directly to the reactor for cooling at a pressure of 350 pounds per square inch and a flow rate of 3,000 gallons per minute. The cooling water passed through the core, cooling the fuel, and ultimately traveled to a 7.2-million-gallon retention basin. After approximately four hours in the retention basin, radionuclides with a short half-life would have decayed substantially, and the cooling water would be returned to the Columbia River. A secondary cooling system operated by steam-driven pumps was available as backup. And if needed, a third cooling method composed of two towers of cooling water could be employed.

The pile employed a thermal shield made of cast iron blocks that absorbed gamma rays and slow neutrons, heating up as it did so. Cooling of the thermal shield was also accomplished using water. The pile also utilized the previously mentioned biological shield to slow fast neutrons in addition to absorbing slow neutrons and gamma radiation. One key component of the biological shield was Masonite with a hydrogen content of 6 percent, which aided in slowing neutrons. The atmosphere inside the reactor consisted of helium, an inert gas that allowed the reactor to run more efficiently because the standard atmosphere contained nitrogen, a known neutron absorber.

The Hanford B Reactor control room. (Photo: DOE)

During operation, as the reactor reached 9 MW, intervention was needed to keep the reactor’s power stable and prevent a decrease in power—an initial problem that baffled operators. The control rods needed to be removed constantly to reduce neutron absorption and increase power in the core. After a while, the control rods were completely withdrawn, but the reactor’s power was still falling, contrary to normal operation. Many potential causes of the mysterious decrease in reactor power were evaluated, including a loss of helium atmosphere in the reactor. John Wheeler and Enrico Fermi tried to solve this mysterious puzzle and in time determined that the fission byproduct xenon-135 was the culprit. Xe-135 happened to be a large neutron absorber—enough so that it effectively killed the fission chain reaction. To combat this issue, additional process tubes were added, which created enough reactivity to ward off the neutron-absorbing effects of the Xe-135.

On January 29, 1968, the Atomic Energy Commission issued an order to shut down the Hanford B Reactor. On February 12 of that year, the reactor was powered down for the last time, and it is currently listed as a National Historic Landmark. As the world’s first production reactor, it proved useful in its wartime purpose and helped propel the nuclear industry with early lessons learned. The Hanford B Reactor’s construction started just months after Enrico Fermi’s Chicago Pile-1 experiment in December 1942. The unprecedented rate of innovation, speed of construction, and the relatively few issues encountered during construction and reactor start-up were simply amazing.

Jeremy Hampshire is an ANS member whose avocation is writing about nuclear science and technology’s history. His experience includes time as a lead nuclear quality assurance auditor and a senior nuclear technical advisor.

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