Monticello Nuclear Generating Plant

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 [120].

Aerial View of Monicello Nuclear Generating Station
Aerial view of Monticello Nuclear Generating Station [29].


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 [120].

Nuclear Reactor

Uranium fuel assemblies
Uranium fuel assemblies [29].
Monticello Nuclear Generating Plant has a General Electric BWR-3 Mark 1 design for the reactor. In this particular reactor, water is continually circulated through the fuel rods to raise the temperature and produce steam. The reactor core consists of 484 fuel assemblies. Each fuel assembly has a 5.25x5.25 inch square cross section and a length of 14 feet containing multiple fuel rods [120]. 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.

Plant Operation

Overhead View of a Nuclear Reactor
Overhead view of a nuclear reactor [29].
To begin the power generation, 7,263,231 lbm/hr of feedwater is fed into the reactor to mix with heated water already circulating in the reactor. Jet pumps send the mixed water through a series of pipes to a sparger located above the reactor core. The feedwater sparger acts like a shower head, this is where the fresh feedwater and the water that has already been sprayed on the core is mixed and then sprayed down on the core. The water from the sparger produces a mixture of steam and liquid. This mixture is then sent to the steam separator, which separates the steam from the liquid water. The water is sent back to the reactor and the steam continues to a steam dryer, which further dries the steam to a 0.05% moisture content level, and then directs the steam to the high pressure turbine.

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.

Monticello High Pressure Turbine
Monticello high pressure turbine [29].
The turbine contains several stages, each designed to operate with specific pressures of steam. The high-pressure turbine (HPT) is designed the highest pressure steam which is at 959 psi. The steam expands through the turbine to 230.4 psi, losing a large amount of energy. The exhaust from the high pressure turbine is sent through a moisture separator. The resulting dry steam is then sent to two dual-flow low pressure turbines (LPT). The steam continues to lose energy through the LPT where it begins to condense. Because of this, LPTs are constructed with more robust material than HPTs, having increased corrosion and erosion resistance. Steam is consecutively extracted from different stages at 129 psi, 76.1 psi, 27.1 psi, 14 psi, 7.4 psi, and 3.3 psi. These steam lines are brought into different feedwater heaters or directly to the condenser.

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

Monticello Turbine Deck
Monticello turbine deck [29].
which then produces electricity. The rotor shaft may only rotate at 1800 rpm. This is due to generator design and the fact that the electrical output frequency needs to be at 60Hz in the United States.

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.

Control Room at Monticello Nuclear Generating Station
Control room at Monticello Nuclear Generating Station [29].
The hot water is sent from the condenser through five different feedwater heaters. The feedwater is approximately 382°F when it enters the boiler. Each of the feedwater heaters preheats the feedwater using steam that is diverted from the steam turbine. For example, steam is sent from the high pressure turbine exit to the feedwater tank closest to the boiler. Because the steam still contains excess energy after it exits the first feedwater heater it must be pumped into the next feedwater heater along with steam from the low-pressure turbine. This process continues until the steam from the turbines is returned to the condenser. Before it enters the boiler, feedwater is sent through a pump that increases the water pressure to approximately 1000 psig.

Emergency Equipment

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.

Outages

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

Dry Cask for Storing Spent Fuel Rods
Dry cask for storing spent fuel rods [29].
fuel has decayed enough to be stored safely in dry-fuel storage casks. Before the NRC allowed this plant to renew their license, they were required to build 10 additional dry storage casks. Scheduled outages are used as an opportunity for major maintenance projects and equipment upgrades.

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 [120].

Information current as of:
July-2013
Click here to see a line drawing of the Monticello Nuclear Generating Plant.
Want to know more on nuclear fission? Click Here.
This video explains nuclear fission.
The Monticello reactor actually contains two different spargers, a feedwater sparger and a core spray sparger.

The feedwater sparger is as described in the text, it mixes fresh feedwater with feedwater that has already been sprayed on the core and not turned to steam and then sprays it on the core in order to produce steam.

The core spray sparger is used as a backup for cooling incase the coolant system fails, it sprays water on the core to keep it from overheating in that situation.
What are wet turbines and how are they used in a nuclear power plant? Read this article to find out.
How does a feedwater heater inspection work? See this one by GE.
What is the future of spent fuel rods? Read this article by Scientific American.