What is nuclear energy?

Commercial nuclear energy is a major source of power generation in the US, France, Russia, Korea, China, and India.  Like coal and gas fired stations, nuclear power reactors generate electricity by creating heat to turn a steam turbine.  However, instead of burning fossil fuels, these reactors use nuclear fission reactions to produce heat.

How much power does nuclear energy generate in the US?

As of 2017, there are 99 nuclear reactors operating in 30 states in the United States.   These reactors provide about 20% or 800 terawatt hours (TWh) of the country’s total power mix.  As a baseload source, nuclear is operational 90% of the time.  The reactors are turned off every 18 to 24 months for refueling and maintenance.

How many nuclear reactors are there in the world?

Worldwide, there are around commercial 450 nuclear reactors today that provide 390 GW of electricity in 31 countries.  In 2015, these reactors produced 2410 TWh or 10.5% of the world’s electricity.

Regional Distribution of Nuclear Power Plants (Source: IAEA)

The capacity of these reactors range from a few hundred MW up to over one GW.  The world’s largest reactor with a capacity of 1.6 GW will commence operations in 2018 at Olkiluoto, Finland.

Who regulates nuclear power?

In the US, the Nuclear Regulatory Commission (NRC) licenses facilities before they can be operational part of the grid.  These licenses are issued for 40 years with a possible 20 year extension.

The NRC’s role extends beyond regulations of reactors and their associated fuel cycle facilities, but also the transportation, treatment, and management of spent nuclear fuel.

If you want to read more about the US licensing process, click here.

How long can a nuclear reactor last? Will any be decommissioned soon?

The average age of the US nuclear plant fleet in the US is about 35 years.  In the US, licenses for 22 reactors are due to expire by 2030.  This represents 27.7 GW of capacity or about 215 TWh (4 to 5%) of total electricity generation

These licenses can between renewed depending on the condition of the plant.  For example, in the state of California, Pacific Gas and Electricity will shut down the state’s last reactors in Diablo Canyon sometime between 2024 and 2025.  The two reactors at the power plant produces about 9% of the state’s total power today.  The plant is eligible for a 20 year license extension.  However, it is unclear how the utility will make up the loss in capacity, without significant investments in renewables and storage.   Additional capacity from natural gas may be necessary to meet the State’s energy needs.

How much does it cost to produce nuclear energy?

Among major US investor-owned utilities, the average cost of electricity (2015) from nuclear is about $26/MWh.  In contrast, the cost of electricity produced from coal is $37/MWh and from natural gas is $33/MWh.  It is important to note that the externalities (pollution and waste) associated with each source is not reflected in these values.

How many new reactors are under construction?

With respect to new reactors in the US, there are four reactors currently under construction: Georgia Power’s Vogtle Units 3 and 4 and South Carolina Electric & Gas Company’s Virgil C. Summer Units 2 and 3.

Worldwide there are 60 power plants under construction. 20 of them are in China and 6 in India.

How much fuel does a nuclear plant use?

From the point of view of fuel use, a nuclear power plant uses substantially less fuel than either a coal or gas fired plant. One kilogram of U-235 (3% enriched) fuel produces 2.5 million times more energy per kg than coal.  For example, a 1 GW coal fired plant, which powers about 750,000 homes, uses about 6750 tons of coal of day.  A nuclear power plant with the same capacity consumes only 3 kg of nuclear fuel per day.

How does nuclear reactors produce heat?

All commercial nuclear reactors use nuclear fission.  Nuclear fuel contains uranium-235 (U-235) the active form of uranium that fissions easily.

Schematic of fission reaction (Source: DOE)

To start up the reactor, the fuel is exposed to a neutron source like the element californium-252 (Cf-252).  Loose neutrons from the source destabilize the nuclei of the U-235 atoms, splitting it into two lighter elements, and more free neutrons.  Heat, as well as radiation, is released in this process.  The newly liberated neutrons go on to split other U-235 atoms in a sustained chain reaction that is referred to as “criticality”.

How does a nuclear reactor work?

The heat from sustained fission reactions drives a steam turbine to generate electricity.  Today’s reactors use ordinary (light) water as coolant to transfer heat from the nuclear core to the turbines, hence they are called light water reactors (LWRs).  Two types of reactor designs are used to power generation today.

In a boiling water reactor (BWR), the water is directly turned into steam in the reactor core and pumped directly to the turbines.  In the newer pressurized water reactors (PWRs), for example Westinghouse’s AP-1000 design, the primary loop of coolant flows through the nuclear core at very high pressure (2250 psi) so it remains a liquid.  The heat from this pressurized water is then transferred to a secondary loop of water that vaporizes and turns the turbines.

Commercial light water nuclear reactor designs (Source: NRC)

The PWR design ensures that any radioactive materials that contaminate the coolant remains within the reactor.

Does nuclear power produce greenhouse emissions?

Unlike fossil fuel-fired plants that burn coal, gas, or oil, nuclear reactors do not produce carbon dioxide greenhouse gas or particulate pollutions during its operation.  However, the construction of the facilities use large of volumes of concrete and steel, which is energy and pollution intensive.

Where does U-235 come from?

The primary sources of uranium are in the western United States, Australia, Canada, Central Asia, Africa, and South America.  Once mined, the ores are ground and the uranium is chemically extracted.

Enrichment process of U-235 (Source: NRC)

How is nuclear fuel prepared?

To produce nuclear fuel for reactors, the uranium from the ores undergoes a series of chemical and physical processes.  Following extraction from the ore, the uranium is enriched, converted to pellets, and assemble into a fuel rod.

Every ton of ore yields between one and four pounds of uranium oxide, a yellowcake powder.  Raw yellowcake consists of two isotopes of uranium: 99.3% of the heavier but inert uranium-238 (U-238) isotope and 0.7% of the lighter but fissile uranium-238 (U-238) isotope.

To obtain nuclear fuel grade uranium, the concentration of U-235 is enriched to 3% to 5%.  The isotope is enriched by chemically converting the yellowcake into uranium hexafluoride (UF6), which is gasified and separated in a centrifuge.

Once enriched, the uranium hexafluoride is chemically converted to ceramic uranium dioxide (UO2) pellets (1 cm in wide and 1 cm long).  To produce the fuel rods for the reactor, the pellets are clad in zirconium alloy tubes four to five meters long.

What happens when the nuclear fuel is used up?

As nuclear fuel is used to produce heat, fission products build up the rod cladding becomes exposed to damage.  Conventional LWR extracts 4% of the energy in fuel before being considered spent.  After about two years of use, the spent fuel is replaced with fresh fuel.  In the US, there are over 70,000 metric tons of spent nuclear fuel with about 22,000 tons added each year.  Globally, 6800 tonnes of spent fuel are produced each year.

Where does the spent fuel go today?

One of the challenges in nuclear energy is the disposal and management of nuclear waste, which remains radioactive and hazardous for 100,000s of years.  In the US, all spent reactor fuel is held in 40 feet deep pools made of steel-lined reinforced concrete several feet thick.  Co-located with the reactors, these pools cool the spent fuel and blocks the radiation.

After 10 to 20 years in the cooling pools, the plant owners move the older fuel into “dry cask” storage.  The Nuclear Regulatory Commission asserts that the spent fuel pool and the dry cask storage are sufficient for public health and safety.

Is there a long term solution for nuclear waste disposal?

There has yet to be a long term solution to the disposal of the spent reactor fuel and uranium mill tailings (the radioactive sludge left after uranium is extraction) in the US or elsewhere.  Nuclear experts believe the country needs geologic repositories to permanently separate these materials from the biosphere.

In the US, Nevada’s Yucca mountain was under consideration as a site for nuclear waste disposal since the early 1980s.  The federal government has spent at least $9 billion on the project, but licensing for the project was withdrawn in 2010 due to lack of political support and regulatory challenges.  The State of Nevada has pointed out that Yucca Mountain’s location near seismic and volcanic zones contributed to uncertainty in the safety of the facility.

Have there been nuclear accidents?

In normal operations of a nuclear power plant, heat is transferred away from the fuel at a consistent rate that prevents overheating.  However, if the reactor core loses coolant or insufficient coolant pressure, the reactor core overheats, melting the reactor fuel.

The accidents at Three Mile Island in 1979 and Chernobyl in 1986, and more recently at Fukushima Dai-ichi in 2011 resulted in the reactor core melting down.  In the case of Fukushima, the melted fuel breached the containment of one of the nuclear reactors, releasing radioactive materials into the environment.

Can nuclear fuel be used for military applications?

Nuclear fuel cannot be directly used to make nuclear weapons.  The uranium that is used to make weapons are enriched to at least 90% U-235.  However materials along the nuclear fuel cycle from yellowcake to spent fuel can be reprocessed to obtain weapons grade uranium.

The International Atomic Energy Agency (IAEA) plays a role in ensuring the peaceful use of nuclear power.  The organization sends inspectors to verify compliance to the Treaty on the Non-Proliferation of Nuclear Weapon (NPT) among member states.  The chairman of IAEA has stated that the agency’s budget, which was at €361 million in 2016, has not kept up with the demand for these services.

What is fast breeder reactor (FBR) technology?

During the early stages of the nuclear power industry, the US government was concerned about the supply of uranium.  The government originally pursued fast breeder reactor (FBR) technology as a method to extract more energy from the fuel.  Experimental FBRs were built in the US, but those efforts were abandoned in 1984 due to proliferation risk and the cost effectiveness of the existing LWR fleet.

Japan, Russia, China, and India still continue to develop FBR to this day.  Over a period of 31 years, Japan spent $10 billion on Monju, its prototype FBR reactor, which was decommissioned at the end of 2016.  Today, the country continues to develop FBR through the Astrid reactor in collaboration with France and the PRISM reactor with the US.

Why are new nuclear innovations needed?

The US and several other countries are now pursuing advanced designs to improve the affordability and safety of nuclear power.  The deployment of these reactors can contribute to the use of clean energy sources that are economically competitive in both established and emerging economies.

In the West, more than 30 reactor designs have been pursued since the 1990s.  Most of these projects were funded privately with one estimate showing $1.3 billion being invested.

What is small modular reactor (SMR) technology?

Small Modular Reactors (SMRs) are a type of advanced nuclear reactor based on PWR designs.  In contrast to recently built LWRs that are exceeding 1 GW, SMRs can be scaled down to 50 MW.  These reactors take a modular approach in construction that allows for compact designs but also lower capital costs and faster construction.  SMRs are of interest to developing countries that have a growing need for electricity but are limited by their financial capacity.

What kinds of nuclear innovations are being pursued today?

Launched in 2002 by the Nuclear Energy Agency of the OECD, the Generation IV International Forum (GIF) provides a framework for international cooperation in R&D for advanced nuclear energy systems that overcome the limitations of LWR.  The organization has identified six types of technologies for commercialization in the 2030s:

China and Russia are aggressively aiming to be leaders in exporting advanced nuclear designs. Among them, the sodium-cooled fast reactor (SFR) BN-800 in Russia is already producing power.  In China, the construction of the High Temperature Reactor Prototype Module is nearly complete.  China has also identified molten-salt reactor (MSR) as a high design priority and plans to start prototyping by 2018.

What is nuclear fusion?

In addition to the aforementioned fission-based technologies, there is active research in nuclear fusion.  Instead of releasing energy through the splitting of the atom, heat is produced by combining the nuclear of two lightweight elements like hydrogen.

What is LENR?

The field of low energy nuclear reactions (LENR), formerly referred to as “cold fusion”, is receiving some renewed attention in the private sector.  Independent research groups in US, Japan, EU, and China are investigating the phenomena of anomalous heat from metal catalyzed hydrogen reactions at temperatures, well below that needed for conventional fusion reactions.