Nuclear is the only “baseload” low-carbon energy source, meaning it can provide a consistent level of electricity to the grid — unlike solar or wind, which are subject to fluctuations in daylight or wind patterns. This distinction is critical because the highest demand on our electrical grid often comes when wind and solar are not generating power, such as at nighttime, when demand ramps up as people cook and use lights in their homes. According to the World Nuclear Association, in 2018, the world’s nuclear power plants supplied 2,563 billion kw of electricity and emitted 74 million tons of carbon dioxide. Using fossil-fuel coal to generate that power would have emitted 2,276 million tons of CO₂, and fossil-fuel natural gas would have discharged 1,278 million tons of CO₂.
But no one is building nuclear in the United States because of the economics. A new commercial nuclear power plant like those currently operating in this country, with a 1,100-megawatt generation capacity, costs upward of $6 billion to construct. Utility companies can’t tie up that much capital in a 10-plus year construction project before they see income from the plant.
One of the biggest contributors to that price tag is the reactor pressure vessel. This is a steel cylinder more than 20 feet long and some 15 feet in diameter, with walls more than 1 foot thick. The U.S. Nuclear Regulatory Commission (NRC) requires to be forged in one piece. This mandate evolved because in the 1960s, weld metals were found to become more brittle during reactor operation — primarily due to the effects of irradiation, which caused nanometer-scale anomalies in the metal that compromised its integrity and therefore threatened power plant safety.
It costs around $150 million to forge a pressure vessel, and only a handful of companies in the world — none of which are in the United States — can do it. If reactor manufacturers could make pressure vessels in smaller pieces and weld those pieces together, they could bring the cost down to $1.5 million — for a factor-of-100 reduction! That would be in addition to strengthening U.S. manufacturing and security, as well as our supply chain infrastructure.
Modern manufacturing and joining/welding methods remove a lot of the embrittlement concerns from the 1960s. For example, powder metallurgy with hot isostatic pressing (PM-HIP) is a heat-treatment process that bonds powdered metals into a desired shape. PM-HIP produces equally shaped (equiaxed) grains, which — unlike the highly deformed or elongated, and therefore weakened, grains found nearest the worked surface from the forging process — increase corrosion resistance and mechanical load-bearing capability. PM-HIP can produce very complex component geometries without additional machining and shaping, dramatically reducing the number of welds, for tremendous cost and time savings.
The separate parts can then be joined via electron beam (EB) welding, which uses a high-energy electron beam as the heat source to cause local melt and flow of the pieces. The advantage of this method is that the microstructure changes and residual stresses generated often are not detectable, or present at all, unlike with conventional tungsten arc welding.
These new processes need to be “qualified” — that is, receive approval from the NRC — for use in commercial reactors. Nuclear reactor designers must ensure that the material maintains sufficient structural and mechanical integrity to withstand an accident scenario during an anticipated reactor lifetime of 40 to 60 years.
Our ongoing project will qualify the novel processes by irradiating samples to their projected end-of-life conditions in the Advanced Test Reactor (ATR) at the Idaho National Laboratory. This step will be followed by mechanical testing — nanoindentation, shear punch and fracture testing, plus scanning electron microscopy (SEM) and transmission electron microscopy (TEM) — to understand how the microstructures evolve during irradiation and impact mechanical integrity.
This testing data will help us make the case to the NRC that it is safe and responsible to qualify these materials for commercial nuclear applications in the United States. This will significantly increase the affordability of the new “small modular reactors” (SMRs) and make large, gigawatt-scale plants more cost-attractive. As a result, the economics will turn to favor relying on nuclear plant construction to reduce climate-warming greenhouse gases.
By Janelle P. Wharry
School of Nuclear Engineering School of Materials Engineering (by courtesy)
Purdue University College of Engineering
- Project Partner: EPRI: www.epri.com
- Project Partner: Nuclear AMRC namrc.co.uk
- A project summary in the DOE-NE AMM Newsletter (PDF)