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NRC Science 101: How a Nuclear Reactor Generates Electricity

Paul Rebstock
Senior Instrumentation and Control Systems Engineer
 

science_101_squeakychalkHow does a nuclear reactor generate electricity? Well — it doesn’t, really. Let’s begin at the end and see how it all fits together.

We begin by looking at an electric motor. A motor consists primarily of two major components: a stator, which stands still, and a rotor, which rotates within the stator. When electricity is applied to the motor, electromagnets within the stator and the rotor push and pull on each other in a way that causes the rotor to rotate. The magnets in the stator pull magnets in the rotor toward them, and then, as the rotor magnets pass by reverse themselves and push the rotor magnets away.

The parts are arranged so the pulling and pushing are all in the same direction, so the rotor spins inside the stator. The electrical energy applied to the motor results in mechanical energy in the rotor.

But that same machine can be used in reverse: If some outside force causes the rotor to spin, the interaction of the magnets causes electricity to be produced: the “motor” is now a “generator,” producing electrical energy as a result of the mechanical energy applied to its rotor. That’s the most common way to make large quantities of electricity.

So how do you make the rotor spin? That’s where the nuclear reactor comes in, although still indirectly. Recall that a nuclear reactor generates heat. The fuel rods get hot because of the nuclear reaction. That heat is used to boil water, and the steam from that boiling water is used to spin the rotor. As we have seen, when the rotor spins, electricity comes out of the stator.

When water boils, the steam that is produced occupies much more physical space than the water that produced it. So if you pump water through some sort of a heat source — like a nuclear reactor, or a coal‑fired boiler — that is hot enough to boil the water, the exiting steam will be travelling much faster than the water going in. That steam runs through a machine called a turbine, which acts something like a highly‑sophisticated windmill. The physical structure is vastly different from a windmill, and a large turbine can be far more powerful than any windmill that has ever been made, but the effect is somewhat the same: the steam, or wind, causes part of the machine to spin, and that spinning part can be connected to a generator to produce electricity.

The steam leaving the turbine is collected in a device called a condenser — essentially a metal box the size of a house, with thousands of pipes running through it. Cool water flows through the pipes, and the steam from the turbine is cooled and condenses back into water. Then the water is pumped back through the heater and the cycle continues.

Now, back to the nuclear reactor . . . We have seen how the reactor generates heat, and we have seen how heat is used to generate steam and how the steam then powers the turbine, which spins the generator that produces electricity. The final piece in the puzzle is how the heat from the nuclear reaction generates the steam.

bwrThe fuel rods are suspended in a water bath contained in a large metal container somewhat like a gigantic pressure cooker. A typical “reactor vessel” might be 15 feet in diameter and 20 feet high, and some are much larger than that. In some types of reactors, the water is allowed to boil, and the heat generated in the fuel rods is carried away in steam. These are called “boiling water reactors” (or “BWR”).

In others, the water is held at a very high pressure — on the order of 2000 pounds per square inch. (By the way, that is more than 60 times the pressure in the tires of a typical car.) In that situation, the water cannot expand and cannot boil. The water in that type of reactor carries the heat away while remaining liquid, and that heat is then transferred to another water system where the boiling occurs. This transfer takes place in a device aptly named a “steam generator.”

These are called “pressurized water reactors” (or “PWR”). A small PWR might have two steam generators. A large one might have four. Some have three. The steam from all of the steam generators is typically combined into a single “main steam line” that carries the steam to the turbine, so the reactor and all of the steam generators act together as a single steam source.

The water from the condenser is pumped directly into the reactor vessel for a BWR, or into the steam generators for a PWR.

So there you have it: the nuclear reaction heats the fuel, the fuel heats the water to make steam, the steam spins the turbine, the turbine turns the generator, and the generator makes electricity.

The author has a BS in Electrical Engineering from Carnegie-Mellon University.

Watts Bar – Making History In Yet Another Century

Jeanne Dion
Project Manager
Watts Bar Special Projects Branch
 

Unit 1 at the Watts Bar Nuclear Plant in Spring City, Tenn., has a claim to fame as the last U.S. commercial nuclear reactor to come online in the 20th century. Now, the Tennessee Valley Authority aspires to have its sister reactor (Watts Bar Unit 2) make its own historic claim.

Numerous cranes helped complete construction of the Watts Bar Nuclear Plant Unit 1 containment building in front of the plant’s cooling towers in 1977.

Numerous cranes helped complete construction of the Watts Bar Nuclear Plant Unit 1 containment building in front of the plant’s cooling towers in 1977.

If the NRC concludes that the reactor is safe to operate and approves its operating license next year, Watts Bar Unit 2 could become the first new commercial nuclear reactor to come online in the U.S. in the 21st century.

To understand a little of the history of Watts Bar Nuclear Plant, let’s rewind to a time when Schoolhouse Rock premiered and the first mobile phone call was made in New York City — a time predating the NRC. In 1973, the Atomic Energy Commission greenlighted construction of Watts Bar Units 1 and 2 under the “two-step licensing process,” where construction permits and operating licenses were issued separately.

In 1985, construction quality issues at its plants caused TVA to stop work at both Watts Bar Units. Eventually, TVA resolved the issues and completed construction of Unit 1, and the NRC issued its operating license in 1996.

Fast-forward to more recent activities. TVA decided in 2007 to reboot the Watts Bar Unit 2 construction and licensing process. They submitted an update to their original license application to the NRC in 2009.

Other recent applicants have elected to use the combined license application process, where we issue a single license to both construct and operate a nuclear power plant at a specific site. However, because of the unique history of Watts Bar Unit 2, TVA chose to continue under the two-step licensing process. So, NRC staff developed a regulatory framework and established a licensing approach tailored specifically to the project.

We updated our construction inspection program associated with the two-step licensing process to provide guidance that reflects current NRC practices. For example, the NRC staff identified areas for further inspection at Unit 2 by screening applicable communications, allegations and other open items in the review.

The NRC staff also developed inspection guidance specific to TVA’s refurbishment program, which replaces or refurbishes systems and components at Watts Bar Unit 2. TVA’s resolution of key safety issues and the continued progress of construction inspection activities drive our review schedule.

If the operating license is issued next year, the NRC’s job doesn’t just end. We’d continue to inspect start-up testing required for power ascension and to oversee that Unit 2 transitions into the NRC’s Reactor Oversight Process before it can begin producing commercial power.

And, of course, the Resident Inspectors, the agency’s eyes and ears at the plant, would continue to carry out day-to-day inspection work to ensure safety and security is monitored and inspected during licensing and throughout the transition to commercial operation.

For more information about the Watts Bar Unit 2 project, visit the NRC’s website. There will be a Commission briefing Oct. 30 at 9 a.m. on the license application review. You get details about the briefing from the meeting notice. We’ll also do a live webcast.

Making Sure SAFER Resources Are Ready To Go

Jack Davis
Director, Japan Lessons Learned Division
 

mitigation_strategies_infographic_r4Part of the U.S. nuclear power industry’s response to the NRC’s post-Fukushima Mitigation Strategies Order involves emergency equipment centers in Memphis, Tenn., and Phoenix, Ariz. The centers have multiple sets of generators, pumps and other equipment. The centers would send needed equipment to a U.S. nuclear plant to maintain safety functions indefinitely if an event disabled that plant’s installed safety systems.

The NRC’s been reviewing how an industry group, the Strategic Alliance for FLEX Emergency Response (SAFER), can move equipment from the response centers to plants. We observed two demonstrations SAFER ran in July and reviewed SAFER’s equipment, procedures, and deployment strategy. Overall, the NRC staff concludes that having the response centers and the group’s plans and procedures in place will enable plants to comply with the final phase of the Order.

The group has contracted with Federal Express (for both truck and aircraft shipment) to get supplies to a plant within 24 hours of a request. SAFER’s documentation of FedEx’s capabilities included a proven ability to work with the Federal Aviation Administration to get proper access to otherwise restricted airspace in the event that equipment must be flown to a nuclear power plant site. 

One SAFER demonstration sent equipment by road from Memphis to the Three Mile Island plant in Pennsylvania. The NRC staff noted some areas for improvement, such as clarifying who’s responsible for unloading equipment at a site or where the equipment’s first tank of fuel will come from. SAFER responded by adding details to its plans and beefing up its training program.

The other demonstration simulated airlift of equipment from Phoenix to the Surry plant in Virginia. After the NRC shared its observations, SAFER gave our staff additional details on how it would obtain helicopters to bring supplies to a plant if area roads are impassable.

 We also reviewed a report on the Memphis center’s test of packing the equipment to efficiently load and fit onto FedEx’s planes. Although the test generated a delivery schedule a few minutes longer than the industry expected, the NRC is satisfied that SAFER has applied lessons learned to streamline its approach and ensure SAFER can meet its own deadlines.

 Our website’s Japan Lessons Learned section can give you more information about the mitigation strategy requirements and related guidance.

Science 101: How a Chain Reaction Works in a U.S. Nuclear Reactor

Paul Rebstock
Senior Instrumentation and Control Systems Engineer

 

science_101_squeakychalkThe primary active ingredient in nuclear reactor fuel is a particular variety, or “isotope,” of uranium, called U235. U235 is relatively rare — only about 0.7% of uranium as it exists in nature is U235. Uranium must be enriched to contain about 5% U235 to function properly as fuel for a U.S. commercial nuclear power plant.

U235 has 92 protons and 143 neutrons. Protons and neutrons are some of the almost unimaginably tiny particles that make up the nucleus of an atom — see Science 101 Blog #1. All other isotopes of uranium also have 92 protons, but different isotopes have slightly different numbers of neutrons.

Uranium is a radioactive element. Uranium atoms break apart, or disintegrate, into smaller atoms, releasing energy and a few leftover neutrons in the process. This happens very slowly for U235. If you have some U235 today, in about 700 million years you will have only half as much. You will have the remaining U235, plus the smaller atoms. The energy released will have gone into the environment too slowly to be noticed, and the extra neutrons will have been absorbed by other atoms.

While this happens very slowly, the disintegration of each individual atom happens very quickly, and the fragments are ejected at a very high speed. Those high-speed fragments are the source of the heat generated by the reactor. Under the right man-made conditions, the number of U235 atoms that disintegrate each second can be increased.

When a U235 atom disintegrates, it releases some neutrons. Some of those neutrons can be made to interact with other U235 atoms, causing them to disintegrate as well. Those “target” atoms release more neutrons when they disintegrate, and then those neutrons interact with still other U235 atoms, and so on. This is called a “chain reaction.” This process does not work well for other isotopes of uranium, which is why the uranium needs to be enriched in U235 for use as nuclear fuel.

Most of the energy released when a U235 atom disintegrates is in the form of kinetic energy — the energy of physical motion. The fragments of the disintegrated atom collide with nearby atoms and set them vibrating. That vibration constitutes heat. The fuel rods get hot as the reaction progresses. The faster the chain reaction — that is, the larger the number of U235 atoms that disintegrate each second — the faster energy is released and the hotter the fuel rods become.

The uranium in a U.S. commercial nuclear reactor is thoroughly mixed with neutral material and formed into pellets about half inch wide and three-quarters of an inch long. The pellets are stacked tightly in metal tubes, forming “fuel rods” that are several feet long. Each fuel rod is just wide enough to hold a single column of pellets. The fuel rods are sealed, to keep all of the radioactive materials inside. There are thousands of these fuel rods in a typical reactor. They contain around 60 tons of uranium – but only about three tons are U235. (The majority of the uranium in the reactor is in the form of the most abundant naturally occurring isotope of uranium, U238, which cannot sustain the fission process without the help of an elevated concentration of the isotope U235.)

The people in charge of the reactor can control the chain reaction by preventing some or all of the released neutrons from interacting with U235 atoms. The physical arrangement of the fuel rods, the low U235 concentration, and other design factors, also limit the number of neutrons that can interact with U235 atoms.

The heat generated by the chain reaction is used to make steam, and that steam powers specialized machinery that drives an electrical generator, generating electricity. Science 101 will look at how that works in more detail in a later issue.

The author has a BS in Electrical Engineering from Carnegie-Mellon University.

 

The NRC Commission Has Held 5,000 Meetings—Give or Take

Annette Vietti-Cook
Secretary of the Commission

 

After one of our commissioners noted a milestone in July – the 5,000th meeting of the NRC’s Commission – we thought it might be useful to share what the Secretary of the Commission does behind-the-scenes in planning Commission meetings. There is much more planning than you might think.

The NRC Commissioners conduct a public meeting. Annette Vietti-Cook is on the left.

The NRC Commissioners conduct a public meeting. Annette Vietti-Cook is on the left.

First some background. The “Commission,” in NRC-speak, means the presidentially-appointed, Senate-confirmed Commissioners acting together. At full-strength there are five Commissioners. The Commission sets policy for the NRC, develops regulations on nuclear reactor and nuclear materials safety, issues orders to licensees and adjudicates legal matters.

The federal Sunshine Act requires that any time the Commissioners meet to conduct agency business, the meeting must be public. Exceptions to this requirement are made when the Commission discusses matters such as security or confidential legal, personnel, personal or proprietary information. Our regulations lay out how we will meet the Sunshine Act requirements.

Public Commission meetings are held at NRC headquarters in the Commissioners’ Conference Room, with planning starting months in advance. This is where the staff members in the NRC’s Office of the Secretary (we call it SECY) come into play.

To prepare for the meeting, SECY works with NRC staff to plan agendas for proposed public meetings, including lists of potential internal and external contributors, which are intended to provide the Commission with a range of perspectives.

In the weeks ahead of a meeting, the NRC staff and other presenters send background materials and slides to the Commissioners. This advance information allows the Commissioners to come prepared to get their questions answered. Meanwhile about a half-dozen people in SECY are making sure of the details— arranging parking and pre-registration for external participants, getting relevant information posted on our public website, creating a seating chart for those who will brief the Commission.

As meeting day approaches, SECY ensures other logistics are in order. They make sure the room is set up properly, with name tags, microphones, and water pitchers placed on the conference table, chairs arranged, flags properly positioned. On meeting day, these preparations probably won’t be noticed by the 50-60 people who may come to the meeting and the untold number tuning into the webcast. (Incidentally, the room holds 155). The Chairman opens the meeting and turns the meeting over to the presenters. Following, the presentations, the Commissioners have an opportunity to ask questions.

Even after the meeting ends, SECY has more to do. All public Commission meetings are webcast, recorded and transcribed. The transcript must be validated and posted to the NRC website. The webcast is archived. And following most every meeting, SECY develops a memo to give the staff direction (we call this an SRM, or staff requirements memorandum), which must be approved by the Commission.

So you see, a lot of work goes into organizing the 5,000 or so Commission meetings we’ve held since the inception of the NRC almost 40 years ago – not just in my office. We hope you’ll tune in or attend a Commission meeting in the future. You can find the Commission’s meeting schedule here and a complete schedule of NRC public meetings here.

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