Prize for Nuclear Safety Model

Will a nuclear power station’s pressure vessel be able to deal with the strain? That’s what every nuclear engineer and every safety expert worried about at Three Mile Island, Fukushima, and Chernobyl.

Now, there’s a new mathematical model for assessing pressure vessel structural integrity assessment – which last year won a top prize at the American Society of Mechanical Engineers (AMSE) during their Pressure Vessels and Piping conference held in Anaheim, California.

Nuclear mini pressure vessel. Photo Nuclep/FAPESP

Nuclear mini pressure vessel. Photo Nuclep/FAPESP

The model was deigned by Brazilian scientist Claudio Ruggieri from the University of São Paulo’s Engineering School (POLI-USP) and Robert H. Dodds from the University of Illinois. The paper, “A Weibull Stress Approach Incorporating the Coupling Effect of Constraint and Plastic Strain in Cleavage Fracture Toughness Predictions” by Ruggieri Dodds resulted from the recently completed research project “Further developments in a micromechanics model for brittle fracture and applications to the master curve methodology”, supported by the São Paulo Research Foundation (FAPESP).

Pressure vessels are important to Brazil because they house the reactor core of the pressurized water reactor (PWR) used in the Angra dos Reis nuclear power plants as well as in the Brazilian Navy’s future nuclear submarine.

In a PWR, water is pumped under high pressure to the reactor core, where it moderates and is heated by fission of uranium atoms in the primary coolant loop. The heated water then flows to a secondary system with a steam generator that drives turbines, which in turn generate electricity. The reactor vessel is made of extremely tough ductile steel to withstand the high pressures and temperatures of the water flow in the primary loop.

“The main factor in pressure vessel degradation is exposure to neutron radiation from the nuclear reaction, which significantly affects the characteristics of the material on the nanometric scale, causing brittleness, flaws and potential fracture,” Ruggieri told Agência FAPESP.

The chain reaction from the release of nuclear energy for conversion into electricity begins when a neutron collides with an atom of fissile material, typically uranium 235 (U-235).

To assess the effect of free neutrons on the reactor’s structural components, several small test specimens are encapsulated and placed inside the reactor, where they receive high doses of radiation.

The specimens are then submitted to fracture toughness trials. With our new model, the imprecisions inherent in experimental measurements can be corrected, making the assessment of neutron-induced degradation more accurate and reliable,” Ruggieri said.

“The main aim was to understand the role of plastic strain in the cleavage fracture process by means of a probabilistic fracture parameter and gauge its contribution to the assessment of potential catastrophic pressure vessel failure,” Ruggieri said.


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