Nuclear fuel studies look to boost, safeguard reactors

Studies into Accident Tolerant Fuels (ATFs) and high burn rate fuels will help safeguard reactors in the case of accidents and lend a significant boost to existing reactor efficiency and economic performance.

Used experimental light water reactor fuel arrives at Idaho National Laboratory (Source: INL)

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Nuclear power potentially offers unlimited electricity with a manageable waste stream for an increasingly electrified world, but it takes time to build.

While Silicon Valley and other technological centers look to new, advanced reactors, such as small modular reactors (SMRs), many in the industry are examining how to get more out of the over 440 reactors already generating power worldwide.

More for less

Part of that effort is being poured into upgrading the fuel that goes into existing reactors so that they may generate more electricity for longer periods more economically.  

Nuclear fuel for light water reactors (LWR) has been steadily increasing in efficiency since the first commercial reactors were installed in the 1970s.

Average burnup rate, measured in gigawatt days thermal per metric ton of uranium (GWDt/MTU), has risen from between 13 GWDt/MTU and 22 GWDt/MTU for boiling water and pressurized water reactors in the 1970s, to around 46 GWDt/MTU by 2017, according to the U.S. Energy Information Administration (EIA). 

Researchers at the Idaho National Laboratory (INL) are working on next-generation fuel that will raise that bar even further.

Used experimental light water reactor fuel from a commercial plant was delivered to INL for the first time in two decades in December in order to conduct research and testing to improve safety features and extend the fuel’s use.

The 25 fuel rods were developed and manufactured by Westinghouse Electric Company, with help from national laboratories, and are designed to deliver cost savings for consumers and increase a nuclear power plant’s resilience under potential accident conditions, says INL.

The shipment, consisting of 12-foot (4 meter) long rods each of which contain 100 pounds (45.4 kilos) of mostly uranium, includes high burnup fuel and accident tolerant fuels (ATFs).

“There's always been an interest in increasing burnup, so you can extract more energy from those rods before you pull them out of the reactor,” says National Technical Director for the Department of Energy’s (DOE’s) Advanced Fuels Campaign at INL Daniel Wachs.

“We can look at how the fuel behaves and subject it to safety testing, so that you can develop the safety case necessary to allow the reactors to work harder.”

Reference Scenario for uranium supply and demand, tU

(Click to enlarge) 

Source: World Nuclear Association

Burning hotter and longer

Nuclear fuel can be designed to burn hotter, so producing more energy in a shorter period of time, known as facility uprates, or it can be designed to burn longer, or burnup extension.

INL’s focus for the moment is on burnup extension, Wachs says, potentially moving reactors from an 18-month cycle to a 24-month cycle.

The next step will be to uprate plants to generate more power consistently over those 24 months, he says.

The regulatory burnup limit has moved from the original near 30 GWDt/MTU to the current level of 62 GWDt/MTU, though vendors have submitted topical reports to the Nuclear Regulatory Commission (NRC) to increase that to 68 GWDt/MTU.

“Now we want to push to 75 or maybe even 85 GWDt/MTU,” says Wachs.

Increased electrical output for plants in the United States could be the equivalent of adding new reactors to the fleet, he says.

Due to the accelerated speed of the program and the NRC already having safety data in hand to review, any U.S. light water reactor should be able to use the new fuel by around 2027-2028.

The Framework for Irradiation Experiments (FIDES) program, run out of the OECD’s Nuclear Energy Agency, means once the research and regulatory specifications have been finalized in the United States, the fuel will be available for nuclear power stations worldwide.

Keeping it safe

Borne out of the accident at the Fukushima Daiishi nuclear plant, the ATF program aims to design a set of new technologies that have the potential to enhance safety at nuclear plants by offering better performance during normal operations, transient conditions, and accident scenarios, according to the NRC.

INL has been tasked with finding fuel technologies that are more resilient under severe conditions such as the combined earthquake and tsunami that hit the Fukushima plant in 2012.

Such technology includes rupture- and heat-resilient cladding, so-called doped fuel, which reduces fuel pellet rigidity and is easier on cladding, and tougher materials such as silicon-carbide plating.

The research has found that ATFs not only improve the fuel’s resilience under extreme conditions, but performance is also better under normal operating conditions.

“We would expect the existing fleet and water-cooled small modular reactors to benefit from ATF, but we're actually seeing cases where the technology potentially spills over to support fast reactors and gas reactors,” says Wachs.

“There's a lot of near term R&D work that’s also paying forward into the longer term, so we really feel like it's been a big win for the Department of Energy and for the research in general.”

By Paul Day