The Haigerloch atomic pile was a German nuclear research facility. It was built in a rock cellar in Hohenzollerischen Lande, Haigerloch early in 1945 as part of the German nuclear program during World War II.
In this last large-scale experiment of the uranium project with the name B8 or B-VIII, as in previous piles, a finite nuclear chain reaction was induced via bombardment of the fuel by a neutron source and measured. Natural uranium was used as fuel and heavy water, graphite, and light water were used as moderators. The criticality of the chain reaction was not achieved; the plant was also not designed for operation in a critical state, and the term reactor often used for it today is therefore only applicable to a limited extent. Later calculations showed that the reactor would have had to be about one and a half times the size to become critical.
The American Special Alsos unit found the facility on April 23, 1945, and dismantled it the following day. The scientists involved were captured and the materials used were flown out to the United States. Today, the Atomic Cellar Museum is located at the former site of the reactor.
Previous history
Previous reactor tests
The main objective of the German uranium project during the Second World War was the technical utilization of nuclear fission, which had been experimentally researched by Otto Hahn and Fritz Straßmann in 1938 and theoretically explained by Lise Meitner and Otto Frisch. In a series of reactor experiments, known as "large-scale experiments", the aim was to test the theoretical considerations for generating energy from uranium in practice. For this purpose, natural uranium was bombarded with neutrons in heavy water as a moderator and the resulting increase in neutrons was observed. The researchers of the uranium project did not refer to their development goal as a reactor, but as a "uranium machine" or "uranium burner".W. Heisenberg, K. Wirtz:Large-scale tests in preparation for the construction of a uranium burner. In: Naturforschung und Medizin in Deutschland 1939–1946. Für Deutschland bestimmte Ausgabe der FIAT Review of German Science, Vol. 14 Part II (ed. W. Bothe and S. Flügge), Wiesbaden: Dieterich. Also reprinted in: Stadt Haigerloch (ed.): Atommuseum Haigerloch, Self-published, 1982, pp. 43-65.
Under the direction of the Nobel Prize winner Werner Heisenberg, a total of seven large-scale experiments called B1 to B7 were carried out at the Kaiser Wilhelm Institute for Physics (KWIP) in Berlin-Dahlem from 1941 to 1944. The physicists investigated the reactivity of plates of uranium metal of various thicknesses with increasing success. At a second location, the Physics Institute of the University of Leipzig, four further experiments L-I to L-IV with concentrically arranged layers of uranium powder and heavy water were carried out by Heisenberg and his colleagues in 1941 and 1942. In experiment L-IV, when neutron multiplication had already been detected, the uranium powder ignited after the formation of an oxyhydrogen gas and the entire facility burned. No persons were injured. The event represents - sensu lato - the first recorded "reactor accident" in history - years before the commercial use of nuclear fission was even conceivable. The experiments in Leipzig were discontinued and only compact metallic uranium was used from then on. At the same time, another group was working on similar experiments at the Gottow Experimental Station near Berlin under the direction of Kurt Diebner. In their three experiments G1 to G3 in 1942 and 1943, uranium cubes were used instead of plates with good results; in addition to heavy water, kerosene was also used as a moderator. The Heisenberg and Diebner groups competed for the scarce materials. In kerosene wax, both the hydrogen and the carbon of the hydrocarbons it contains act as moderators. Even today, neutrons from smaller neutron sources are often slowed down with kerosene. However, attempts to cool/moderate nuclear reactors with organic substances have not yet progressed beyond the experimental stage.
Relocation of research
In 1943, all major German cities were under threat from Allied bombing raids. As a result, it was decided to relocate the KWIP to a more rural area. The suggestion to use the Hohenzollerische Lande for this probably came from the head of the Physics Division in the Reichsforschungsrat, Walther Gerlach, who had studied at the University of Tübingen and was a professor there in the late 1920s, making him familiar with the area. Another reason for selecting southern Germany was that it had been largely spared from air raids up until then. Additionally, the scientists involved favored this location to reduce the risk of being captured by the Soviets in the event of defeat.Dahl: Heavy Water and the Wartime Race for Nuclear Energy, p. 254.
Subsequently, the KWIP was relocated to Hechingen, approximately 15 km from Haigerloch. It was housed in the Grotz and Conzelmann textile factories and in the brewery building of the former Franciscan Monastery of St. Luzen. The relocation occurred in stages: by the end of 1943, about a third of the institute moved to Hechingen, with Carl Friedrich von Weizsäcker and Karl-Heinz Höcker from Strasbourg joining in 1944, and finally Heisenberg relocating there. At the same time, the Kaiser Wilhelm Institute for Chemistry, was relocated to nearby Tailfingen (now part of Albstadt), with prominent scientists such as Otto Hahn and Max von Laue involved in the move.
By January 1945, only Karl Wirtz, Kurt Diebner, and a few technicians remained in Berlin from the Uranverein. Wirtz was setting up the largest German reactor test to date in the still-intact Dahlem institute bunker when the Red Army advanced to within 80 km of Berlin. As a result, Gerlach decided on January 27, 1945, to abandon the almost completed test setup. He immediately traveled to Berlin to evacuate the scientists and materials to southern Germany.
Preparations The Felsenkeller
As early as July 29, 1944, the accidentally discovered potato and beer cellar of the Haigerloch Schwanenwirt was rented for 100 Reichsmark per month as the new location of the Berlin research reactor. The Felsenkeller (rock cellar) had been excavated at the beginning of the 20th century as a rail tunnel for the Hohenzollerische Landesbahn. In the narrow Eyachtal valley, it had been driven into the mountain under the Schlosskirche (castle church), with a 2030m top layer of shell limestone protecting it against air attacks.Dahl: Heavy Water and the Wartime Race for Nuclear Energy, p. 255.
The tunnel was about 20 meters long and three meters high, with a trapezoidal cross-section that meant it was about four meters wide at the ceiling and five meters at floor level. It was supported along its entire length by wooden support beams placed two meters apart. A small two-part porch concealed the entrance.
A three-meter-deep cylindrical pit was excavated for the reactor in the rear part of the rock cellar, a transport crane was installed on the cellar ceiling and a diesel generator was set up in the abandoned beer house on the opposite side of the street. By the end of 1944, the conversion work in the rock cellar, disguised as a "cave research station", had progressed far enough for the construction of the actual reactor to commence.
Transportation of materials On January 31, 1945, Gerlach, Wirtz and Diebner left the capital at the head of a small road convoy. They were followed by a number of trucks loaded with several tons of heavy water, uranium, graphite and technical equipment. After a night-time journey over the icy Autobahn, the convoy stopped the following day about 240 kilometers south of Berlin in the small town of Stadtilm in Thuringia where Diebner's working group had been relocated the previous summer. Gerlach believed that Diebner's laboratory was more advanced than Heisenberg's and decided without further ado to unload the materials there. Very annoyed at the change of plan, Wirtz contacted Heisenberg in Hechingen, who immediately set off for Stadtilm together with von Weizsäcker and arrived there three days later after an adventurous journey by bicycle, train and car.Dahl: Heavy Water and the Wartime Race for Nuclear Energy, p. 256.
On site, Heisenberg tried to convince Gerlach to take the materials to Haigerloch after all. The two went to Hohenzollern on February 12, 1945 to inspect that site. Wirtz, meanwhile, remained in Stadtilm to ensure that the materials were not used in Diebner's experiments. After Gerlach had ascertained in Haigerloch that the Felsenkeller was better suited as a new location for the reactor, he agreed to the relocation again. Trucks were once again procured and on February 23, 1945, the physicist Erich Bagge set off from Haigerloch with a new convoy to collect the materials from their storage site in Stadtilm.
Four weeks after leaving Berlin, 1.5 tons of uranium, 1.5 tons of heavy water, 10 tons of graphite and a small amount of cadmium finally arrived in Haigerloch at the end of February 1945. The uranium had been mined in Sankt Joachimsthal in the Sudetenland and supplied by the German Degussa corporation.Klaus Mayer et al: Uran aus deutschen Nuklearprojekten der 1940er Jahre – eine nuklearforensische Untersuchung. In: Applied Chemistry. Volume 127, 2015, The heavy water had been produced by Norsk Hydro in Norway. In addition, the physicist Fritz Bopp from Berlin had flown in a 500 milligram radium beryllium sample as a neutron source. Over ten tons of uranium oxide as well as small quantities of uranium metal and heavy water remained at Diebner's lab in Stadtilm.
The research reactor The system
Once the materials had arrived in Haigerloch, work began immediately on rebuilding the test facility. Von Weizsäcker and Wirtz played a leading role in the construction and experiments. Heisenberg himself managed the project from Hechingen, often cycling back and forth between the two towns. In addition to Bagge and Bopp, other scientists involved in the project on site included Horst Korsching and Erich Fischer.
The outer shell of the reactor consisted of a concrete cylinder fitted with a boiler made of aluminum with a diameter of 210.8 centimeters and a height of 216 centimeters. The boiler rested on wooden support beams on the floor, and the space in between was filled with normal water. A second, smaller boiler sat inside. Made of a very light magnesium alloy, it had a diameter of 124 centimeters and the same height as the outer boiler. The magnesium boiler had already been used in the large-scale test B6, the aluminum boiler had been used for the first time in the large-scale test B7. Both had been manufactured by the Berlin company Bamag-Meguin.Dahl: Heavy Water and the Wartime Race for Nuclear Energy, S. 257.
Between the two boilers, ten tons of graphite formed a 43-centimeter layer serving as a neutron reflector and shielding. Graphite had been used as a reflector for the first time in the previous large-scale experiment B7; it had not been used in even earlier experiments because Walther Bothe had estimated in 1941 that graphite would absorb too many neutrons.W. Bothe und P. Jensen: Die Absorption thermischer Neutronen in Elektrographit. Forschungsbericht 1941. Zeitschrift für Physik Bd. 122 (1944) S. 749. The lid of the inner boiler consisted of two magnesium plates which also held a graphite layer sandwiched between them.