(Transcript of the lesson commentary.)
Nuclear energy began with the Chicago Pile No. 1
With the discovery and subsequent study of nuclear reactions taking place when the nuclei of heavy elements are bombarded with slow neutrons, the era of human efforts to use nuclear energy for the production of electricity began. Nuclear fission became a phenomenon, and especially in connection with uranium and plutonium isotopes, it represented a potentially promising source of energy at the time. In the 1930s and 1940s, the prominent Italian experimental nuclear physicist Enrico Fermi, a Nobel Prize winner and a significant figure in the Manhattan Project in the USA, was intensively involved in the research of radioactive decay and neutron interactions leading to nuclear fission.
And it was Fermi who, in 1942, with a team of significant physicists and specialists, built a nuclear reactor using a fission chain reaction under the west stands of the Stagg Field football stadium of the University of Chicago. The first reactor in history was given the name Chicago Pile No. 1 due to secrecy, and its only purpose was to prove the possibility of controlling the fission chain reaction in uranium fuel and to experimentally verify physical calculations of reactor parameters and behaviour. The reactor structure consisted of a 7.5 m wide, 4 m high and 350-ton weight pile of graphite blocks, arranged in 57 layers and interspersed with enriched uranium blocks in the central part. The working core of the reactor had the form of an ellipsoid and contained 5.6 tons of uranium metal together with 36 tons of uranium oxide in the form of pressed spheres.
Fermi’s first reactor had no cooling due to its minimal power and its control and safety features were very simple. Cadmium rods manually inserted by an operator from the side of the reactor served to control the ongoing fission reaction. A single emergency rod, suspended above the reactor and controlled by a simple neutron flux evaluation system, ensured that the reactor did not go out of control under any circumstances. The backup method of immediately lowering the rod into the centre of the reactor was to cut its suspension by a man ready with an axe. Until today, the name SCRAM (Safety Control Rod Axe Man) has been used for rapid emergency shutdown of the reactor. The final safety feature of the first nuclear reactor were dedicated workers who were prepared to pour buckets of cadmium salt solution directly into the reactor.
Fortunately, during the experiment, which took place on 2nd December 1942 and lasted approximately 20 minutes, the reactor worked exactly according to the calculations and expectations of the present physicists. In the first phase, the functionality of all the components of the experiment was thoroughly verified while the reactor was still in a subcritical state, but a few hours later, the reactor reached a critical state and began to slowly increase its heat output. When the power reached about 0.5 W and the entire reactor heated up by several degrees Celsius, automation stopped the fission chain reaction due to the exponential increase in the intensity of the neutron flux. In another experiment, held ten days later, the reactor was started again and brought to a critical state. During more than half an hour of operation, it reached a maximum output of 200 W.
The first nuclear power plant producing electricity
The world’s first nuclear facility to produce electricity using nuclear fission was the now decommissioned EBR-1 Research Facility in Idaho, USA. The abbreviation EBR means that the reactor used in this power plant was an experimental breeder reactor (Experimental Breeder Reactor), originally known as Chicago Pile No. 4 or “Zinn’s Infernal Pile”. Above all, the main goal of the reactor designers was not the primary production of electricity, but the verification of Fermi’s theory that it is possible to construct a reactor that produces more fissionable material than it consumes. Fermi’s principle was confirmed by the very operation of the power plant and numerous experiments in 1953.
In the EBR-1 reactor, fission was caused by fast unmoderated neutrons and the reaction of neutrons with uranium 238U in the breeding core also produced plutonium. In order to not slow down the fast neutrons, the reactor used a coolant based on an alloy of sodium and potassium. Boldly using liquid metal cooling certainly had its benefits. On the other hand, due to the flammability of these metals when in contact with water or air, all primary circuit devices had to be perfectly tight.
The active core of the EBR-1 reactor was about the size of a football. 169 pencil-sized thin fuel rods were stacked in it. A further 192 rods of natural uranium were placed around the core, acting as a breeding core for the production of plutonium — another nuclear fuel. The maximum power output of the power station’s turbogenerator was only 200 kW but it was this turbogenerator that produced electricity for the four famous light bulbs, first lit on 20th December 1951. The very next day, the first nuclear power plant was able to supply electricity to the entire building.
During one of the experiments in November 1955, the EBR-1 reactor was damaged and the core even melted. Analysis of the event showed the high inherent safety of liquid metal-cooled fast nuclear reactors. During 13 years of operation, the EBR-1 reactor was able to reliably answer a number of operational questions and became an important milestone in the development of nuclear energy.
Another first belongs to the Russian nuclear power plant with the RBMK type AM-1 reactor, built in the 1950s in Obninsk, approximately 100 km southwest of Moscow. It was the first nuclear power plant in the world to be connected to the distribution network. Its output was already 5 MW and it started supplying the city of Obninsk with electricity on 27th June 1954. As with the EBR, this type of reactor was capable of producing plutonium in addition to electricity.
The Obninsk reactor was water-cooled and operated in a safe two-circuit connection. The reactor core with 128 enriched uranium fuel rods was surrounded by a graphite block reflector. Burned up fuel rods were replaced with fresh ones in the reactor during full operation once every 100 days using an overhead crane. The reactor of the first nuclear power plant to supply electricity to households has become an invaluable development step in the pursuit of the peaceful use of nuclear energy.
