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Droning On Over Nuclear Power Plants

Monika Coflin
Technical Assistant
Division of Security Policy

Drones, or unmanned aerial vehicles, have been in the news lately. Last fall, unidentified drones breached restricted airspace over 13 of France’s 19 nuclear power plants in a seemingly coordinated fashion. In January, a drone crashed onto the lawn of the White House. And this week, a drone was found on the roof of the Japanese prime minister’s office.

PrintDrones may be fun toys, but they pose a number of concerns. They can be used to conduct surveillance to gather intelligence about facility security. They can also be used to deliver payloads that could include explosives. While the majority of drones currently in use are relatively small, larger ones are becoming available that could possibly deliver payloads capable of causing damage to facilities that are not hardened.

Security experts haven’t yet identified who was responsible for the French flyovers, but with the prices of drones falling and their popularity rising, the potential threat will likely continue to grow.

There are ways to detect and intercept drones, such as jamming radio signals or using helicopters to pursue encroaching drones. Chinese scientists are developing a laser weapon that can detect and shoot down small, low-flying aircraft, and interception drones have the ability to drop nets over intruding drones. However, there are many legal issues that challenge the use of these techniques.

The Federal Aviation Administration (FAA) has a long-standing “Notice to Airmen” warning pilots not to linger over nuclear power plants. The FAA has also issued guidelines on where users should not fly drones, but the industry is largely unregulated as more companies look to use the relatively new technology in their businesses. The FAA has been working to craft a comprehensive regulatory framework for drones, following calls from Congress and the President, and recently issued draft regulations for the commercial use of drones.

PrintPresident Obama likened the drone industry to cyberspace, which has brought new technologies that U.S. laws are still trying to catch up to.

“These technologies that we’re developing have the capacity to empower individuals in ways that we couldn’t even imagine 10-15 years ago,” the President said, pledging to work to create a framework that “ensures that we get the good and minimize the bad.”

Given the evolving nature of technology and the need to balance the threat with the potential benefits of drones, the NRC is actively engaging with the departments of Homeland Security, Energy, and Defense to move this government collaboration effort forward. For example, we have reached out to the FAA to examine available legal and regulatory options, and attended inter-agency meetings to learn about how other agencies are addressing potential impacts from drones.

In addition, NRC will participate in a U.S.-initiated drone working group under the nuclear counterterrorism umbrella with the governments of France and the United Kingdom. The NRC has provided, and will continue to provide, pertinent information on this topic in a timely manner to its licensees to ensure continued safe and secure operations.

REFRESH — What is a Reactor Trip and How Does it Protect the Plant?

Samuel Miranda
Senior Reactor Systems Engineer

Note: Last week, the Prairie Island nuclear power plant “tripped.” So, it seemed like a good time to revisit a blog post we did two years ago on the subject.

refresh leafOn occasion, a nuclear power plant will “trip,” meaning something happened that caused the reactor to automatically shut down to ensure safety. In other words, a trip means a plant is doing what it’s supposed to do. Let’s look at the term a bit more closely.

Key operating parameters of a nuclear power plant, such as coolant temperature, reactor power level, and pressure are continuously monitored, to detect conditions that could lead to exceeding the plant’s known safe operating limits, and possibly, to damaging the reactor core and releasing radiation to the environment.

If any of these limits is exceeded, then the reactor is automatically shut down, in order to prevent core damage. In nuclear engineering terms, the automatic shutdown of a nuclear reactor is called a reactor trip or scram. A reactor trip causes all the control rods to insert into the reactor core, and shut down the plant in a very short time (about three seconds).

How do control rods do their job?

pwr[1]The control rods are composed of chemical elements that absorb neutrons created by the fission process inside the reactor. They are placed methodically throughout the nuclear reactor as a means of control. For example, as the control rods are moved into the reactor, neutrons are absorbed by the control rods and the reactor power is decreased. Inserting them all at the same time shuts down the reactor. Control rods can also be inserted manually, if necessary.

The plant operator then determines the reason for the trip, remedies it and, when it’s determined to be safe, restarts the reactor. So, while not common, a reactor trip is an important way to protect the components in a nuclear power plant from failing or becoming damaged.

REFRESH is an occasional series where we re-run previous blog posts. This post originally ran in December 2012.

 

 

Western U.S. Reactors are Completing Their Seismic Picture

Lauren Gibson
Project Manager
Japan Lessons-Learned Division

An ongoing lesson from 2011’s Fukushima Dai-ichi accident involves U.S. reactors better understanding their earthquake hazard. Reactor owners in the Western parts of the country have had to assemble a particularly complex jigsaw puzzle of seismic information. They’ve just sent the NRC their detailed re-analysis.

seismicgraphicThe graphic shows the three pieces of information U.S. reactor owners have used to analyze their specific hazard:

  • Where quakes are generated (seismic source)
  • How the country’s overall geology transmits quake energy, (ground motion/attenuation) and
  • How an individual site’s geology can affect quake energy before it hits the reactor building (site amplification).

Central and Eastern U.S. reactors benefitted from region-wide updated earthquake source information and a model of quake energy transmission for the first two pieces. Plants west of the Rockies, however, had to deal with the West’s more active and interconnected faults.

Columbia, Diablo Canyon Part I and Part II and Palo Verde used the Senior Seismic Hazard Analysis Committee (SSHAC) approach to develop site-specific source models and ground-motion models. This group of independent seismic experts develops guidance on major seismic studies such as this. The group has met several times the past few years to ensure the Western plants properly conduct and document their seismic activities.

The NRC carefully considers SSHAC comments and recommendations before the agency comes to its own conclusions on seismic issues. We’re currently evaluating the Western plants’ reports and will issue our short-term screening and prioritization review later this spring.

As for the Central and Eastern U.S. plants’ March 2014 submittals, we screened them to determine what other actions the plants might have to take. Plants that have more to do were grouped into three priority groups with staggered deadlines. Many of those plants submitted additional analyses in December 2014, and the NRC continues reviewing both that information and the March 2014 submittals.

Photo Friday — The NRC Operations Center

fotofridayopcenterVisitors got a rare glimpse of the NRC’s Operation Center last week when tours were offered as part of the annual Regulatory Information Conference. Here, NRC officials show off the Executive Team room, from where an NRC response effort would be managed. Other sections of the center include the Reactor Safety team, the Protective Measures team, the Liaison team and the Public Affairs team, among others. The Op Center is staffed 24/7 by specially trained Headquarters Operations Officers.

Understanding Nuclear Power Plant Risk

Mark Caruso
Senior Risk Analyst
Office of New Reactors

When it comes to the safety of using nuclear power to generate electricity, the NRC mission is protecting people from health risks by licensing and regulating nuclear power plant design and operation. In a perfect world there would be no risk at all. In the real world, we focus on managing and reducing risk below its already very low levels.

bikeridingFor instance, you can reduce the risk of a bicycle accident by ensuring you have working brakes and reflectors/lights. Wearing a helmet and leaving your headphones in a pocket while riding also reduce risk, but wrapping yourself in bubble wrap is probably going too far!

We all understand things in our lives that we consider “risks,” like riding a bicycle, by looking at how severe a bad outcome is and how likely that outcome is. The NRC asks three questions when considering risk:

  1. What can go wrong?
  2. How likely is it to go wrong?
  3. What are the consequences?

These three questions are called the risk triplet. Let’s apply the risk triplet to lifting a piano. What can go wrong? A crane could drop the piano while lifting it to a building’s upper floors. How likely is a piano drop? Since crane workers take lots of precautions that’s very unlikely. What could a falling piano do? If the piano did fall and you were unlucky enough to be underneath it…you can imagine the consequences! This event has a low likelihood and a high consequence. There are also high likelihood/low consequence events and high likelihood/high consequence events.

The NRC’s risk-management effort starts by identifying and eliminating high likelihood/high consequence events at U.S. nuclear power plants before moving to less-likely events.

Engineers use a method called probabilistic risk assessment (PRA) when analyzing risk at nuclear power plants. These assessments use engineering and math to find the answers to the risk triplet questions and create tools called the event tree and the fault tree. These trees map out possible ways and likelihoods of reaching a desirable or undesirable outcome in an organized way. Engineers use these maps to understand and manage nuclear power plant risk. An event tree starts with a trigger (initiating) event and then tracks the different possible resulting events that either reach or prevent an undesirable outcome.

In the sample PRA below, a skydiver jumping from a plane is the initiating event. The event tree follows what could normally occur next and then considers what happens if those events succeed or fail. For example, these events include attempting to deploy the main and reserve parachutes  

The desirable outcome occurs if either parachute opens successfully. The undesirable outcome occurs if both chutes fail to open. Since a skydiver would not normally start with the reserve parachute, this event tree contains three event sequences:

  1. Main parachute opens — desirable outcome
  2. Main parachute fails, reserve parachute opens — desirable outcome
  3. Both parachutes fail to open — undesirable outcome

Fault trees help determine a percentage between zero (outcome never occurs) and one hundred (outcome always occurs) for the outcome of each event sequence in the tree.

faulttreeA fault tree shows all the combinations of things that must go wrong to “fail” an event in an event tree. The diagram shows the ways a reserve parachute can fail to open. Think of a fault tree as a sort of family tree. Rectangles represent either “parent” or “child” events and circles represent pure “child” events. The “and” symbol between parent and child events indicates all child events must occur for their parent event to occur. The “or” symbol indicates any child event can cause their parent event. Engineers use the tree to identify the different combinations of child events leading to the event at the top of the tree. Historical parachute performance data helps provide a numerical value for the likelihood of each pure child event (e.g., dead battery). A mathematical formula combines individual event likelihoods to provide the numerical value of the likelihood of each combination of child events.

Event trees and fault trees are two basic parts of risk assessment, just like the brakes and gas pedal are basic parts of a car. In the same way all the other parts under the hood make the car work, risk assessments have lots of other moving parts that we could discuss in the future. The bottom line, however, is that risk assessments help the NRC and nuclear power plant engineers properly reduce already very small health risks, resulting in safely produced electricity at nuclear power plants.

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