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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.

NRC Science 101 – About Spent Nuclear Fuel Part II

Greg Casto
Branch Chief
Division of Reactor Safety Systems

 
science_101_squeakychalkOur last post talked about the fuel that powers nuclear reactors. Today, we’ll talk about what happens to that fuel when it’s removed from a reactor.

You’ll recall that fuel becomes very hot and very radioactive as it is used in the reactor core to heat water. After about five years, the fuel is no longer useful and is removed. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In the U.S., every reactor has at least one pool on the plant site where spent fuel is placed for storage. Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away.  

These pools contain an enormous quantity of water—enough to cover the fuel by about 20 feet. The water serves two purposes: it cools the fuel and shields workers at the plant from radioactivity. Having 20 feet of water above the fuel means there is a lot more water than is needed for cooling and shielding the workers. Also, because of the extra water and the simple design of the pool, there is a lot of time for plant personnel to add water to the pool if needed for any reason.

fuelpoolThe pools are built to meet strict NRC safety requirements. They have very thick, steel-reinforced concrete walls and stainless-steel liners, and are protected by security personnel. There are no drains that would allow the water level to drop or the pool to become empty. The plants have a variety of extra water sources and equipment to replenish water that evaporates over time, or in case there is a leak. Plant personnel are also trained and prepared to quickly respond to a problem. They keep their skills sharp by routinely practicing their emergency plans and procedures.

When the plants were designed, the pools were intended to provide temporary onsite storage. The idea was for the spent fuel to sit in the pool for a few years to cool before it would be shipped offsite to be “reprocessed,” or separated so usable portions could be recycled into new fuel. But reprocessing didn’t end up being an option for nuclear power plants and the pools began to fill up.

In the early 1980s, nuclear plants began to look for ways to increase the amount of spent fuel they could store at the plant site. One way was to replace spent fuel storage racks in the pools with racks containing a special material that allowed spent fuel to be packed closer together. Another way was to place older, cooler and less radioactive fuel in dry storage casks that could be stored in specially built facilities at the plant site. We’ll talk more about dry spent fuel storage in future blog posts.

Most plants today use both re-designed storage racks and dry storage facilities to store spent fuel. All storage methods must be reviewed in detail and approved by the NRC before a plant is allowed to change storage methods.

Science 101 – What is Nuclear Fuel?

Kevin Heller
Reactor Systems Engineer, Division of Safety Systems

science_101_squeakychalkIn earlier Science 101 posts, we told you about nuclear chain reactions and how they are used to generate electricity in reactors. This post focuses on the fuel that reactors use to create those chain reactions.

You may recall that nuclear fuel rods get hot because of the nuclear reaction, and that heat is key to generating electricity. But what exactly are these fuel rods?

Nuclear fuel starts with uranium ore, which is found in the ground throughout the world. For now, we’ll just say that uranium ore goes through several steps to be processed and manufactured into nuclear fuel. In a future Science 101 post, we’ll talk more about the process of turning uranium ore into fuel pellets.

fuelpelletEach pellet is about the size of a pencil eraser. These pellets are stacked inside 12-foot long metal tubes known as fuel cladding. The tubes are sealed on each end to form a fuel rod, and between 100 and 300 fuel rods are arranged in a square pattern to form a fuel assembly. The number of fuel rods used to make a fuel assembly depends on the type of reactor the assembly will be used in and the company that makes the fuel.

fuelrodsWhile the assemblies are very long (about 12 feet), they are less than 1 foot wide. The assemblies have special hardware at the top and bottom and at intervals in between to keep the fuel rods firmly held and evenly spaced. Fuel assemblies are only slightly radioactive before they are placed into a reactor core. Typically, a reactor core will have between 150 and 250 fuel assemblies.

We talked before about the form of uranium that is important in commercial nuclear reactors. It is an “isotope,” or an atom with a very specific number of neutrons, known as U-235. Part of the process of turning uranium ore into nuclear fuel is enrichment—which increases the amount of U-235 relative to the other isotopes naturally found in uranium. Under the right conditions in a reactor, neutrons will cause U-235 atoms to fission, or split. This leaves two new, different atoms and a couple of neutrons. These new neutrons will then cause other U-235 atoms to fission, forming a chain reaction.

As U-235 atoms fission, energy is released in the form of heat. That heat creates steam which turns a turbine to create electricity. After a few years, there is considerably less U-235 in the fuel. If the amount of U-235 were to drop too low, there would no longer be enough to keep a chain reaction going. So every 18-24 months about one-third of the fuel in a reactor core is removed and replaced with new, fresh fuel. The used fuel is often called “spent fuel.”

Spent fuel is very hot and very radioactive. The atoms created by the fission process are unstable at first and emit particles that create heat. Therefore, spent fuel must be handled and stored carefully, and under controlled conditions. We’ll talk more about spent fuel and how it is managed in a future Science 101 post.

OIG Audits NRC’s Scientific Research Program

Stephen Dingbaum
Assistant Inspector General for Audits

An Office of the Inspector General audit regarding the NRC’s process for ensuring integrity in scientific research is now available here. The audit set out to determine if the NRC has the controls is place to oigassure that scientific research is objective, credible, and transparent.  

The NRC’s regulatory research program conducts research in the areas of nuclear reactors, nuclear materials, and radioactive waste. Scientific information that supports research includes factual inputs, data, models, analyses and technical information, or scientific assessments. This scientific information often informs NRC regulations.

The OIG found that while the NRC has controls in place, the way it manages scientific information, including information associated with scientific research, needs to be strengthened. Specifically, the NRC must improve the internal controls associated with responding to public requests to correct scientific information and for designating it as influential scientific information. Additionally, the OIG audit states the NRC must adopt required guidelines on conducting peer review of its information products associated with scientific research.

The audit also states the NRC must have effective controls in place to ensure that its information products are objective, credible, and transparent. Without effective controls, an opportunity for maximizing the quality, objectivity, utility, and integrity of NRC scientific information is being missed and may result in compromising stakeholder confidence in NRC’s ability to regulate in an unbiased, trustworthy, and open manner.

The report makes five recommendations specific to the way the NRC handles scientific information, to ensure that the NRC adopts federal requirements on peer review, and to ensure that internal guidance that may be impacted by new or revised federal guidance is regularly reviewed to determine if revisions are necessary.

NRC management stated their general agreement with the audit findings and recommendations.

 

EXIT — A Good Sign of Radiation

Maureen Conley
Public Affairs Officer

refresh leafMost people know radioactive energy can be harnessed to provide electricity and even to diagnose and treat certain illnesses. But would it surprise you to learn that radioactive materials also perform an important safety function by lighting emergency EXIT signs?

Look for the EXIT sign the next time you go to work, school, a sporting event, religious service, the movies, or the mall. If the sign glows green or red, chances are it contains a radioactive gas called tritium. The tritium, a radioactive isotope of hydrogen, is sealed into glass tubes lined with a chemical that glows in the dark. Tritium emits low-energy radiation that cannot penetrate paper or clothing and even if inhaled, it leaves the body relatively quickly. As long as the tubes remain sealed, the signs pose no health, safety, or security hazard.

exit3We estimate there are more than 2 million of these signs in use in the United States. To ensure safety in handling and the manufacturing process, we and our Agreement State partners regulate the manufacture and distribution of tritium EXIT signs. Companies have to apply for and receive a license before they can manufacture or distribute one of these signs.

But because the signs are designed to be inherently safe, the NRC does not require any special training before a building can display the signs. Users are responsible for meeting the requirements for handling and disposal of unwanted or damaged signs and for reporting any changes affecting the signs.

exit2Proper handling and disposal is the most important safety requirement for these signs. A damaged sign could contaminate the immediate area and require an expensive cleanup. That is why broken or unwanted signs must be return to a licensed manufacturer, distributer, radioactive waste broker or radioactive waste disposal facility.

Tritium EXIT signs are one of several types of radioactive consumer products that we allow. These products can be produced and sold ONLY if they have a benefit that outweighs any radiation risk. See our earlier blog post for more information on how we regulate these products.

REFRESH is an occasional series where we revisit previous blog posts.

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