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Category Archives: General

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.

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.

Throwback Thursday — The First Regulatory Information Conference

Crowd ImageThe NRC’s first Regulatory Information Conference was held at the Mayflower Hotel in Washington D.C. on April 18-20, 1989. It began as a small conference (some 500 attendees at the first one) on nuclear safety regulation. Today, it is a large public meeting with more than 3,000 attendees from some two dozen nations. In 2015, it will be held at the Bethesda North Marriot Hotel and Conference Center in North Bethesda, Md., from March 10 through 12th. Registration will open early in 2015.

Now for our history question: Which Executive Director for Operations made introductory comments at that first RIC?

REFRESH: In Nuclear Power Plants – Behavior Is Under Observation

Mark Resner
Access Authorization Program Coordinator

 

refresh leafThe NRC requires that all nuclear power plants follow strict access authorization regulations that are intended to make sure only trusted individuals have the OK to be in the most sensitive areas of the plant. These access authorization regulations require fingerprint checks, drug and alcohol screening, psychological testing and other hurdles when employees are first hired, and must be periodically updated if the individuals are to continue to have access to these areas.

But even once a worker has been granted so-called unescorted access, they are still subject to a “behavioral observation program.” In other words, the NRC requires that every plant have a program in which all employees and supervisors are trained in detecting problems such as drug or alcohol abuse or other impairments of employees.

As part of the program, all employees are required to report to their supervisors any suspicious behavior they see among their coworkers. Suspicious behavior could be a worker observed in an area of the plant where they don’t have authorization to be, or if a worker made threatening statements about harming people or plant equipment.

The NRC regulations even require workers to report on themselves or “self-disclose” if they, for whatever reason, believe they are no longer mentally and physically fit to safely perform their duties. An example of this is an employee undergoing marital problems that are causing them stress that interferes with their duties. Such an employee may be referred to an Employee Assistance Program or their assigned duties may be changed until the person is deemed fit for duty.

If a determination is made to deny the person unescorted access for any reason, their name and that fact is entered into an information sharing database that NRC requires all U.S. nuclear power plants to use. Should that person attempt to enter (or get a job at) another nuclear plant, the information about their access status would be available for review by the plant they were attempting to access.

Ultimately, a determination that an employee is not trustworthy or reliable – based on behavior observation or self reporting — has serious implications for that person maintaining their access authorization but such determinations are necessary to keep nuclear power plants operating safely in their communities.

REFRESH is an occasional series where we revisit previous blog posts. This one originally ran in May 2012.

Part II: Ensuring Safety in the First Temple of the Atom

Thomas Wellock
NRC Historian
 

https://www.lib.ncsu.edu/specialcollections/digital/text/engineAs noted in Part I of this story on the NC State research reactor, the Atomic Energy Commission (AEC) was very anxious to promote the world’s first civilian reactor. But its enthusiasm was tempered by the challenge of placing a reactor safely on a busy college campus and developing an approval process for non-AEC reactors.

The AEC turned to its Reactor Safeguard Committee, the forerunner of today’s Advisory Committee on Reactor Safeguards. The Committee was formed in 1947 to evaluate the safety of new reactors proposed by AEC laboratories and contractors.  “The committee was about as popular—and also necessary—as a traffic cop,” recalled Safeguard Committee Chairman Edward Teller.

The Committee’s most significant contribution was establishing a conservative approach to safety given the engineering uncertainty of that era. “We could not follow the usual method of trial and error,” Teller said. “The trials had to be on paper because the actual errors could be catastrophic.” The Committee developed a “simple procedure” of challenging a reactor designer to write a “hazard summary report” that imagined the worst “plausible mishap”—soon known as a “maximum credible accident”—and demonstrate the reactor design could prevent or mitigate it.

Five NC Stte physics professors designed the reactor. Here, in the reactor control room (left to right front row) are Clifford K. Beck and Arthur C. Menius, Jr. Standing is Newton Underwood, three unidentified students, Arthur Waltner and Raymond L. Murray.

Five NC State physics professors designed the reactor. Here, in the reactor control room, (left to right front row) are Clifford K. Beck and Arthur C. Menius, Jr. Standing is Newton Underwood, three unidentified students, Arthur Waltner and Raymond L. Murray.

The Committee focused on several hazards, including a surge in the chain reaction called a reactor “runaway,” a catastrophic release of radioactive material from fire, sabotage, or an earthquake, and hazards from routine operation that might result from leaks or inadvertent exposures. The Committee asked NC State to address these concerns in a “hazards summary report.”

To meet the Committee’s desire for inherent safety, NC State proposed a “water boiler” reactor, which was believed to have “student-proof” safety margin given its strongly “negative coefficient” of reactivity that limited greatly the possibility of a runaway. NC State also developed interlocks and an extremely dense concrete shielding to discouraged sabotage.

In order for NC State to commit the funds to such a long-term project, it needed an early approval. This created a dilemma since the college did not yet have a detailed, complete design.  The AEC used a two-step conditional approval that was similar to its later construction permit/operating license process. In step one, construction did not begin until NC State addressed the most important design safety issues. When it did, the AEC agreed by contract to supply enriched fuel. The fuel was not delivered, however, until NC State resolved all outstanding safety questions and a final inspection took place. With that, the first civilian reactor in history went critical in September 1953.

The AEC approach to safety at NC State foreshadowed many later regulatory practices. As important as the 1954 Atomic Energy Act is to current regulatory practice, it is interesting to see that many of the critical elements have even deeper roots back toward the beginning of the atomic era.

 

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