The Monticello Nuclear Generating Plant is a 600 MWe boiling water reactor owned and operated by Xcel Energy. The plant is located in the town of Monticello, along the Mississippi River in central Minnesota and produces 10% of Xcel Energy's power in the Midwest .
History of the Plant
Construction of the Monticello Nuclear Generation Plant began in 1966, was commissioned in 1970, and began commercial operation in 1971. At that time, a 40-year license was granted by the Nuclear Regulatory Commission (NRC). In November of 2006, the NRC extended the plants operating license for an additional 20 years allowing it run through 2030 .
Nuclear Reactor. Each fuel rod contains pellets of uranium that create heat through a nuclear fission process. Nuclear fission is the process where a nucleus splits apart, typically uranium though other nuclear fuels are used as well, and releases a large amount of energy in the process. The fission process takes place when a stray neutron impacts the nucleus causing it to split. During the process, additional neutrons are released which cause further nuclear fission reactions to occur. This process is the heating process that drives the nuclear power plant. Due to the nature of the nuclear fission reaction, the rate of the reaction must be controlled in order to avoid a reactor meltdown. This is done with control rods which are made out of a neutron absorbing material such as boron or hafnium. These are raised and lowered in the reactor to absorb more or less of the neutrons which consequently controls the rate of the reaction. The feedwater is converted to steam by the heat from the reactor. The steam exits the boiler at approximately 539°F and 959 psi and then sent through a multistage turbine. Inside the reactor vessel, there are only stationary parts with the exception of the control rods. These can be moved in and out as necessary to control the rate of the reaction and for refueling.
The steam travels through a series of pipes where heat is lost and some of the steam is condensed into liquid water by manipulating drain valves to empty the condensate in the piping system. Some of the steam goes through the high pressure turbine then is routed through the low pressure turbine and sent to the condenser.
The steam eventually reaches a series of stop, control, and bypass valves that further direct the flow. Stop valves simply block the flow of the steam. They are generally used in the case of an emergency where they may shut off automatically to prevent contamination from reaching other equipment.
Bypass valves divert the steam away from the turbine and to the condenser. This is done when the steam is not pressurized or heated enough to be sent through a turbine. The bypass valve is usually used when the reactor is started up after an outage or during equipment maintenance. This allows for constant flow through the reactor to sufficiently cool the rods while maintaining a safe pressure level in the vessel.
Control valves regulate the steam flow into the high pressure turbine. This is necessary because an imbalance within the turbines can elevate the torque to levels that can cause fracturing of components and unnecessary outages. It is therefore essential that the control valves regulate the flow through the turbine.
As the steam expands through these various stages, it causes the turbine blades to spin, rotating the shaft. This shaft is connected to a generator
At the end of the multistage turbine process, most of the exhaust is sent to the condenser where it is converted back to saturated water. The steam in the condenser is cooled by a circulation system that draws water from the Mississippi River through pipes into the condenser. In the condenser, the water absorbs heat from the process water and is expelled back into the Mississippi at a higher temperature. The condenser also de-aerates the water as it flows through.
Environmental regulations prohibit the expulsion into the river of water higher than 94°F, so during the summer months, the cooling towers on site reduce the water temperatures to the required level. The temperature of the river in winter is much lower, reducing the need to use the cooling towers. However, some condenser water is passed through it to prevent ice buildup on the fan blades.
During an emergency shutdown, one backup system at Monticello provides power to the plant by use of two 20 cylinder, opposed piston, Fairbanks-Morse diesel generators. A second backup system uses two steam turbine-driven emergency pumps that extract steam and moisture from the reactor in order to rotate the turbines so that there is sufficient circulation to prevent the reactor from overheating. The robust blades in the turbine-driven pumps are designed to resist high moisture levels within the reactor.
About every 22 to 24 months, the Monticello Nuclear Generating Plant is shut down for refueling. During the refueling outages, plant operators replace approximately one-third of the fuel assemblies. The spent fuel rods are placed in a cooling pool until the
The plant employs over 500 full-time employees. During the scheduled outage in 2013, Xcel Energy hired close to 3000 contractors from around the country to undertake necessary updates and inspections .