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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.

NRC Joins Five Other Agencies in Addressing Uranium Contamination on the Navajo Nation

Dominick Orlando
Senior Project Manager

 

Navajo coverLast year, after five years of work to reduce risks from uranium contamination on territory that is part of the Navajo Nation, the NRC, along with four other federal agencies, reported on our progress to Congress. This week, the five federal agencies issued a plan that spells out how we’ll continue coordinating that work for the next five years.

 The agencies’ second Five-Year Plan builds on lessons learned from the first five years. It reflects new information and defines the next steps to address the most significant risks to human health and the environment. The new plan commits us to working together to reduce these risks and find long-term solutions.

 In October 2007, Congress asked the agencies to develop a plan to address the contamination on Navajo land, which dates back to the 1940s when uranium was in high demand. The Navajo Nation had large uranium deposits but regulations were not what they are today and mining companies left extensive contamination requiring cleanup. Legislation and new regulatory provisions were put in place to address these issues.

 The 2013 report capped off a five-year program the agencies conducted, in consultation with Navajo and Hopi tribal officials, to address uranium contamination on their land. Part of this work was government-to-government consultations with the Navajo.

 The program was a joint effort among EPA, the NRC, the Department of Energy, the Bureau of Indian Affairs, the Centers for Disease Control and the Indian Health Service. It focused on collecting data, identifying the most imminent risks, and addressing contaminated structures, water supplies, mills, dumps, and mines with the highest levels of radiation. We also learned more about the scope of the problem and the work that still remains.

 The NRC’s role is to oversee the work done by DOE, which is the long-term custodian for three sites storing uranium mill tailings—a sandy waste left over from processing uranium—and one former processing site. We do that by reviewing and, if acceptable, concurring on DOE’s plans to clean up contaminated groundwater, visiting the sites to evaluate how DOE is performing long-term care activities, and reviewing DOE’s performance and environmental reports.

 We will work closely with EPA, DOE, the New Mexico Environment Department, and the Navajo during the cleanup of the Northeast Church Rock site—which EPA and Navajo officials identified as the highest priority site for cleanup. The NRC will also be part of outreach activities detailed in the plan, including participating in stakeholder workshops and contributing, as appropriate, to educational and public information activities.

 Five years from now, we look forward to being able to say that with close coordination among all the parties, we have continued to make major progress in addressing concerns about uranium contamination.

Checking the Links in the Nuclear Supply Chain

Mary Anderson
Vendor Inspector
Office of New Reactors

 

The NRC’s focus on nuclear power plant safety doesn’t stop at the plants. Since the 1970s (at that time under the Atomic Energy Commission), NRC inspectors have kept a watch on the companies that provide safety-related components and services to U.S. plants.

 magnifyingglassThe agency believes plants and vendors have effective quality assurance programs in place to proactively prevent the use of counterfeit, fraudulent and suspect items. These programs include careful supplier selections, effective oversight of sub-suppliers, and the authority to challenge a part’s “pedigree” when necessary. 

The NRC oversees these quality activities by inspecting nuclear power plants and their vendors. Vendor inspection can include site visits to production facilities. We create and share information and guidance for the nuclear industry to improve detection of counterfeit and fraudulently marketed products. We also incorporate this information into our inspection programs. The NRC has yet to see any instance of these items in safety-related systems in U.S. plants, but constant vigilance by the licensees and the NRC is essential to make sure it stays that way.

 These days our Vendor Inspection Center of Expertise operates out of the Office of New Reactors to cover both operating reactors and those under construction. NRC staff experts inspect vendors, and observe when plants audit their suppliers, to determine if the plants are properly overseeing their supply chain. Importantly, the NRC also verifies that the plants and their vendors comply with our quality assurance criteria and our “Part 21” requirements for reporting defects and noncompliance, as well as applicable codes and standards.

 The center’s staff also inspect companies applying for design certificates, early site permits or combined licenses. We check on whether the applicants have effective quality assurance processes and procedures for activities related to their applications.

 Right now, we’re working on several vendor-related issues, including evaluating the industry’s process for safely upgrading commercial products that aren’t specifically made for nuclear applications to be used in some plant systems. Common items such as gaskets, nuts and bolts, and electrical relays could be acceptable for nuclear plant use, for example.

 We’re updating and simplifying Part 21, the NRC regulation that covers counterfeit, fraudulent and suspect items. We’re also confirming effective controls are in place to prevent such items from making their way into the U.S. safety-system supply chain. We’re clarifying the processes for evaluating and reporting defects, and the acceptance criteria for off-the-shelf commercial products. The Center is developing regulatory guides so plants and vendors better understand these processes.

 The NRC’s vendor workshop in Portland, Ore., gave us a forum to put this issue in the spotlight. Among a range of vendor topics, this year’s workshop included an industry perspective on counterfeit, fraudulent, and suspect items.

 The NRC has also been actively involved with our international partners to address the risk of counterfeit and fraudulent items. We’ve collaborated with the International Atomic Energy Agency and the Nuclear Energy Agency to share best practices and recommend options to strengthen inspection programs and increase information sharing.

Be Aware, Take Action to Prepare

Patricia Milligan
Senior Level Advisor for Emergency Preparedness
 

Be Disaster Aware, Take Action to PrepareSeptember is National Preparedness Month, a time each year to reflect on the importance of knowing what to do before, during and after an emergency. The first step in preparing is to know your hazard. Once you do, FEMA has a wealth of resources to help you plan.

If you live near a nuclear power plant, you probably know it has operated safely and securely for decades. You should still be prepared in the unlikely event of a plant emergency. The two most important things to know are:

1) if you hear a siren or alert, tune in for instructions from state or local officials, and

2) follow those instructions.

A key part of the NRC’s mission is to make sure adequate plans are in place to protect the health and safety of the public. We require plant operators to develop emergency preparedness plans and regularly practice carrying them out in emergency exercises that include first responders and local and other federal government agencies.

These exercises test the skills of those who would respond in a real emergency and identify any areas that need to be addressed. We assess the operators’ performance during exercises. As part of our regular inspections, we also make sure the operators’ emergency plans meet our requirements and are capable of protecting the public.

While the NRC holds to operator to account for their on-site performance, FEMA evaluates how well the offsite response organizations perform during exercises to ensure that they are meeting FEMA requirements.

If you live near an operating nuclear power plant, you should already know whether you work or reside in the “Emergency Planning Zone.” This information would come from your state or local government. You could also receive an annual mailing from the plant. The exact zones and their configurations depend on a number of factors, such as specific site conditions, population and local emergency response.

In the event of an emergency, the plant operator will be in close contact with state and local officials, including emergency responders. Local officials, not the NRC, will make decisions regarding the best course of action. These decisions will factor in technical information about the plant and the weather, as well as other details regarding local emergency plans. That is why it’s important to tune in to their instructions.

It is important to keep in mind that evacuation is not always the best course of action. Depending on your location, you may or may not be advised to take potassium iodide as a way to protect your thyroid. State and local officials are in the best position to make these decisions, so do not take action until you receive instruction from them.

If you want more information on emergency planning, see our website. For more information on National Preparedness Month, check out this website. And don’t forget that FEMA has set aside Sept. 30 for America’s PrepareAthon, an opportunity for everyone to prepare for specific hazards that might affect them.

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