U.S. NRC Blog

Transparent, Participate, and Collaborate

Dry Casks 101: Managing Heat

CASK_101finalCaylee Johanson
Mechanical Engineer

In this series we’ve been talking about storing spent nuclear fuel in dry casks. One major function of these casks is to cool the fuel. Keeping the spent fuel from getting too hot is one way to ensure casks will be safe. As the fuel cools, heat is transferred from inside the cask to the outside.

Our experts look at how the cask will perform this function. We require the cask and fuel to remain within a certain temperature range. Our review looks at four main areas:

Spent fuel releases heat as a result of its radioactive decay. This is called decay heat. A key function of dry storage casks is to move the decay heat from the cask to the outside environment to ensure the fuel and cask components do not get too hot. Our experts look at how that heat will move through the cask and into the environment.

The method used to remove heat has to be reliable and provable. Heat must also be removed in a way that is passive—meaning no electrical power or mechanical device is needed. Casks use conduction, convection and radiation to transfer the heat to the outside.

Heat Radiation Transparent 2The graphic shows the three heat transfer methods. As you can see, conduction transfers heat from the burner through the pot to the handle. The process of heat rising (and cold falling) is known as convection. And the heat you feel coming off a radiator, or a hot stove, is known as radiant heat.

These methods work the same way in a storage cask. Where the canister or metal structure containing the fuel touches the fuel assemblies, heat is conducted toward the outside of the cask. Most casks have vents that allow outside air to flow naturally into the cask (but not into the canister) and cool the canister containing the fuel (convection). And most casks would be warm from radiant heat if you stood next to them. (The heat generated by a loaded spent fuel cask is typically less than is given off by a home-heating system.)

We limit how hot the cask components and fuel materials can get because we want to protect the cladding, or the metal tube that holds the fuel pellets. Limiting the heat is one important way we can ensure the cladding doesn’t degrade. The cask must  keep spent fuel cladding below 752 degrees Fahrenheit during normal storage conditions—a limit that, based on the material properties of the cladding, will prevent it from degrading. The fuel must also remain below 1058 degrees in off-normal or accident conditions (such as if a cask were dropped while it is being positioned on the storage pad, or if a flood or snow were to block the vents).

We also confirm the pressure inside is below the design limit to make sure the pressure won’t impact the structure or operations. Our experts review applications for new cask designs carefully to verify the fuel cladding and cask component temperatures and the internal pressure will remain below specified limits.

Each storage cask is designed to withstand the effects from a certain amount of heat. This amount is called the heat load. We look at whether the designer correctly considered how the heat load will affect cask component and fuel temperatures. We review how this heat load was calculated.

We also verify that the cask designer looked at all the environmental conditions that can be expected because these will also affect the cask component and fuel temperatures. These may include wind speed and direction, temperature extremes, and a site’s elevation (which can affect internal pressure). To make sure the right values are considered, we verify they match the historical records for a site or region.

We review all of the methods used to prove that the storage system can handle the specified heat loads. We also verify any computer codes used in the analysis and the values that were plugged in. For example, we look at the material properties for cask components used in the code. We look at calculations for temperatures and pressure. We make sure the computer codes are the latest versions.

And we only allow designers to use codes that have been endorsed by experts. We might run our own analysis using a different computer code to see if our results match the application.

The analysis and review allow us to see whether and how the dry cask will meet the temperature limits. Our review ensures the temperature is maintained and the cladding is protected. Finally, our review confirms the cask designer used acceptable methods to analyze or test the system and evaluate the thermal design. If we have any questions or concerns, we ask the designer for more information.

Only when we are satisfied that our requirements are met will we approve the thermal analysis in a cask application.

El Nino and NRC Preparedness

F. Paul Peduzzi
Branch Chief
Division of Preparedness and Response

elninoEl Niño is already making itself felt along the West Coast. This phenomenon occurs every two to seven years. It warms sea surface temperatures in the eastern-central Pacific Ocean, shifting average sea level pressure and tropical rainfall in dramatic fashion, and leading to weather pattern changes over parts of the northern and southern hemispheres.

Forecasters expect this year’s El Niño to be one of the strongest ever, based on changes in the sea surface temperatures of the Pacific.

No two El Niño’s are exactly alike, but the pattern generally has these effects:

  • Increased rain and snow across California and the southern United States, with less in the Pacific Northwest and in the Ohio and Tennessee valleys
  • Milder than normal winter across the northern United States
  • More hurricanes than normal in the eastern Pacific and fewer in the Atlantic during hurricane season (June 1 – November 30)

The NRC is alert to potential impacts on our licensees. Facilities such as nuclear power plants are designed to withstand much more severe weather than El Niño typically brings. Nuclear power plants are designed and built to withstand the most severe weather and floods historically reported for their area. Several plants experienced strong El Niño weather patterns in the ‘80s and ‘90s with no major problems.

Following the Fukushima events in Japan in 2011, the plants have enhanced their ability to deal with major floods. For example, additional portable safety equipment, such as pumps and generators, is now available both onsite and offsite.

However, El Niño’s storms could block roadways, making it difficult for plant staff to get to the site and impeding public evacuation routes. Plant operators can use other transportation means to get staff and equipment to the site, if needed. And emergency plans have provisions to clear evacuation routes or use alternate routes. These provisions have been tested before, such as during the Missouri River flooding of 2011

The bottom line? California may be unusually soggy this winter, but the NRC does not expect the current El Niño to cause any safety issues for the nation’s nuclear power plants. As always, we remain vigilant and continue to work with other federal agencies on emergency preparedness and incident response, just in case.

NRC Finishes Review of Vermont Yankee Decommissioning Planning Report

Neil Sheehan
Public Affairs Officer
Region I

More of a marathon than a sprint, the decommissioning of a nuclear power plant can in some cases take decades. But central to the successful completion of that process is careful planning and vigilant oversight.

vyIn December of 2014, the Vermont Yankee nuclear power plant embarked on that phase of its life after being permanently shut down. As required by the NRC, Entergy, the plant’s owner, submitted a Post-Shutdown Decommissioning Activities Report, or PSDAR, on Dec. 19, 2014.

What exactly is a PSDAR? It is a report designed to provide the NRC and public with a general overview of the company’s proposed decommissioning activities. The report includes estimated costs for decommissioning and an affirmation that the decommissioning can be completed consistent with the site’s environmental statement.

Since the PSDAR only provides information and is not a federal action, it does not require NRC approval. However, the agency does review such submittals to confirm they meet regulatory requirements.

Besides performing an evaluation of the nuts-and-bolts aspects of the decommissioning plans, the NRC staff also reviewed public comments regarding the report. Along those lines, the agency held a public meeting on Feb. 19, 2015, in Brattleboro, Vt., for the purpose of receiving comments. Those remarks and others submitted separately in writing were all considered as the report was being prepared.

The NRC staff has now completed its review of the report and has determined the planned decommissioning activities, schedules and other information described in it are consistent with the agency’s requirements in this area. A copy of the NRC’s letter to Entergy regarding the PSDAR review results will be made available in the agency’s electronic documents system, ADAMS.

Also on the topic of Vermont Yankee’s decommissioning, as of Feb. 1, 2016, the responsibility for Vermont Yankee has been transferred within the NRC from the office responsible for operating reactors to the office responsible for decommissioning nuclear power plants.

Going forward, the Office of Nuclear Material Safety and Safeguards’ Division of Decommissioning, Uranium Recovery and Waste Programs will oversee licensing activities involving Vermont Yankee.

The NRC will continue to perform inspections at Vermont Yankee, with the intention of being on-site anytime a major activity is taking place.

 

Crossing the Finish Line at Watts Bar

Joey Ledford
Public Affairs Officer
Region II

Watts Bar Unit 2, the nation’s first new commercial nuclear unit in a generation, received its NRC operating license last October and is closing in on its first nuclear chain reaction. (Power production is still a ways off.) The NRC is still on the job as the staff transitions to operational inspection duties.

An NRC inspector looks on as TVA workers install components at Watts Bar Unit 2.

An NRC inspector looks on as TVA workers install components at Watts Bar Unit 2.

The agency’s Region II-based construction inspection staff, supplemented by headquarters staff, have booked more than 127,000 hours making sure the new unit has been built according to its design specifications. More than 350 agency inspectors and other staff have been involved in the inspection and project management effort, which geared up in earnest in 2008 when the Tennessee Valley Authority committed to completing the unit it had initially started building in 1973 and later suspended.

The Watts Bar plant, located about 50 miles northeast of Chattanooga, Tenn., has a unique history. Unit 1, which also traces its roots to 1973, was the last U.S. plant to come on line when it was finally licensed in 1996 after a similarly lengthy construction hiatus.

When work resumed on Unit 2, the NRC recalled a handful of staffers who had been involved in inspecting work on the sister unit to ensure “knowledge transfer.”

“Our goal is to verify the design is accurate,” said James Baptist, who was a team leader for several years during Watts Bar 2 construction and has recently become chief of the Region II branch overseeing the transition from construction to operation. “We want to ensure Unit 2 looks and operates just like Unit 1. It greatly assists the effort when you have a working model right beside you.”

As is the case with most NRC inspection efforts, the corps of construction resident inspectors led the way, reporting to the site daily and amassing a big percentage of those 127,000 hours.

“Everything came through the residents in terms of what was going on at the site,” said Chris Even, who recently transitioned from senior construction project manager to senior project inspector in the new branch overseeing the transition. “We always relied on the residents for knowing exactly what was going on.”

The workload was huge from the beginning, with more than 550 construction inspection items to be inspected and closed. And Baptist noted that even though the plant was designed in the 1970s, it’s built to today’s standards.

“They purposely built Unit 2 to be a mirror image of Unit 1 while including all the updated safety enhancements that have accrued over the last 25 or 30 years,” he said.

For example, Watts Bar is the first plant in the nation to comply with all the NRC’s post- Fukushima upgrades as well as the newest cybersecurity requirements.

One might think that with the license issued and the plant about to start up that the NRC inspection effort would be winding down. Baptist said that is not the case.

“We still have our foot on the gas,” he said.

Just as the NRC inspectors were dedicated to make sure Watts Bar Unit 2 was constructed and tested according to the design and NRC regulatory requirements, they will continue to maintain that vigilance as the plant begins and continues to operate.

 

 

Throwback Thursday – The Smithsonian Welcome Center and the NRC

welcome centerSeen here, under construction, is the now-open Welcome Center at the Smithsonian’s National Museum of American History. It is named for a previous NRC Chairman and his wife. Question: What is the name of the Chairman?

Photo courtesy of the Smithsonian Institution

Follow

Get every new post delivered to your Inbox.

Join 1,795 other followers

%d bloggers like this: