Light-water reactor
were successfully reaching criticality, uranium enrichment began to develop from theoretical concept to practical applications in order to meet the goal of the Manhattan Project, to build a nuclear explosive.
[1] LOPO cannot be considered as the first light-water reactor because its fuel was not a solid uranium compound cladded with corrosion-resistant material, but was composed of uranyl sulfate salt dissolved in water.
[1] By the end of the war, following an idea of Alvin Weinberg, natural uranium fuel elements were arranged in a lattice in ordinary water at the top of the X10 reactor to evaluate the neutron multiplication factor.
[3] The purpose of this experiment was to determine the feasibility of a nuclear reactor using light water as a moderator and coolant, and clad solid uranium as fuel.
In 1946, Eugene Wigner and Alvin Weinberg proposed and developed the concept of a reactor using enriched uranium as a fuel, and light water as a moderator and coolant.
[5] For the design of this reactor, experiments were necessary, so a mock-up of the MTR was built at ORNL, to assess the hydraulic performances of the primary circuit and then to test its neutronic characteristics.
[7] Immediately after the end of World War II the United States Navy started a program under the direction of Captain (later Admiral) Hyman Rickover, with the goal of nuclear propulsion for ships.
It developed the first pressurized water reactors in the early 1950s, and led to the successful deployment of the first nuclear submarine, the USS Nautilus (SSN-571).
Though electricity generation capabilities are comparable between all these types of reactor, due to the aforementioned features, and the extensive experience with operations of the LWR, it is favored in the vast majority of new nuclear power plants.
The reason for near exclusive LWR use aboard nuclear naval vessels is the level of inherent safety built into these types of reactors.
Data from the International Atomic Energy Agency in 2009:[11] The light-water reactor produces heat by controlled nuclear fission.
This in turn affects the thermal power of the reactor, the amount of steam generated, and hence the electricity produced.
Operators of the BWR design use the coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps.
The cooling source, light water, is circulated past the reactor core to absorb the heat that it generates.
Although the coolant flow rate in commercial PWRs is constant, it is not in nuclear reactors used on U.S. Navy ships.
The use of ordinary water makes it necessary to do a certain amount of enrichment of the uranium fuel before the necessary criticality of the reactor can be maintained.
Although this is its major fuel, the uranium 238 atoms also contribute to the fission process by converting to plutonium 239; about one-half of which is consumed in the reactor.
The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement.
A uranium oxide ceramic is formed into pellets and inserted into zirconium alloy tubes that are bundled together.
The zirconium alloy tubes are pressurized with helium to try to minimize pellet cladding interaction which can lead to fuel rod failure over long periods.
This is primarily done to prevent local density variations from affecting neutronics and thermal hydraulics of the nuclear core on a global scale.
Therefore, if reactivity increases beyond normal, the reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat.
This "decay heat" will continue for 1 to 3 years after shut down, whereupon the reactor finally reaches "full cold shutdown".
If the temperature exceeds 2200 °C, cooling water will break down into hydrogen and oxygen, which can form a (chemically) explosive mixture.
