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Category Archives: Operating Reactors

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.

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.

Back To Where We Were For North Anna New Reactor Environmental Review

Tamsen Dozier
Project Manager
Office of New Reactors

It’s not often we have to say “never mind,” but that applies to what had been potential changes to our environmental review for a new reactor in Virginia. Multiple changes in the proposed design for the North Anna application have eliminated one reason to supplement our work.

naDominion Virginia applied in November 2007 to build and operate a new reactor at the company’s North Anna site, northwest of Richmond. This would be the third North Anna reactor, co-located with the two that have operated safely since 1978 and 1980. Dominion’s original application proposed building General Electric-Hitachi’s Economic Simplified Boiling Water Reactor.

The NRC’s review includes examining issues as required by the National Environmental Policy Act. Dominion had addressed many of these environmental questions by successfully obtaining an Early Site Permit for the North Anna site before applying for a reactor license. The NRC does an additional review when considering a reactor license application. We held meetings with the local community and also got input from state officials and other federal agencies. We examined all this information before publishing a supplement to the permit’s environmental review, evaluating the possible construction and operation of the GE design at North Anna.

In 2011, Dominion amended its application to reflect the company’s change to a different reactor design. At that point we decided we’d need another supplement to the EIS to evaluate any changes in the previously evaluated impacts. In 2013 Dominion changed course again, returning to GE’s design. Since we’ve already documented our environmental review for the GE design, there’s no longer a need for a supplement for any design changes. We just published a notice to this effect in the Federal Register (Jan. 29, 2015).

Beyond these design changes, NRC regulations require more reviews for additional substantial application changes or if significant new environmental information comes to light. We’ll keep looking for project changes or new information and we’ll prepare a supplement if one’s needed.

Additional Scrutiny at Pilgrim Nuclear Power Plant Set to Continue

Neil Sheehan
Public Affairs Officer
Region I

Update: As a follow-up, the NRC is launching a Special Inspection today (Monday, Feb. 2) at the Pilgrim nuclear power plant in response to the shutdown that occurred at the Plymouth, Mass., facility on Jan. 27. The six-member team will review equipment issues experienced during the shutdown, including the partial loss of off-site power. The results of the NRC inspection will be made publicly available within 45 days of the inspection’s completion.

Last fall, a team of NRC inspectors was tasked with evaluating whether issues at the Pilgrim nuclear power plant that triggered increased agency oversight had been satisfactorily addressed. That team has now returned its findings in the form of a newly issued inspection report.

pilgrimAnd the answer – at least at this point in time – is that Entergy, the Plymouth, Mass., plant’s owner, still has some more work to do.

Specifically, although the eight-member team has determined that, in general, the company’s problem identification, root cause evaluation and corrective action plans were adequate, it has identified deficiencies in the implementation of corrective action plans, as well as in understanding of the issues’ causes.

In its report, the team cites several examples where fixes were not completed as intended or were closed prematurely.

As a result, per agency protocols, the NRC is assigning two “parallel” “white” (low to moderate safety significance) inspection findings to Pilgrim. The findings will administratively replace two “white” performance indicators that initially led to the plant receiving additional scrutiny.

The net effect will be the plant will continue to receive heightened attention until the NRC can perform a follow-up team inspection and is satisfied the concerns have been resolved. The NRC will conduct that additional inspection once Entergy notifies the agency of its readiness for it.

To back up for a moment, the Pilgrim plant’s performance indicator for Unplanned Scrams (shutdowns) with Complications crossed the threshold from “green” to “white” following the third quarter of 2013. Then, in the fourth quarter of last year, the performance indicator for Unplanned Scrams per 7,000 Hours of Operation also changed to “white,” something that occurs if a plant has more than three such shutdowns during the designated period.

This placed Pilgrim in the Degraded Cornerstone Column of the Action Matrix used by the NRC to assess plant performance.

Pilgrim has not had any unplanned scrams since October 2013, and the performance indicators discussed above are currently “green.” But the assignment of the white findings will keep the plant in the Degraded Cornerstone Column pending successful completion of the supplemental inspection.

The NRC intends to discuss the inspection results during the Annual Assessment meeting for the plant. That meeting will likely take place in March near the plant, but a date, time and location have not yet been firmed up.

 

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