There are nearly 100 nuclear power plants across the United States, all of which rely on concrete containment vessels (NCVS) to provide the last layer of safety in case of accident.
Some of these vessels exhibit serious signs of aging associated with a slow, but irreversible, degradation of concrete known as alkali-aggregate reaction, or AAR. This degradation may prevent these structures from passing the strict seismic requirements of the Nuclear Regulatory Commission (NRC), according to a study led by Victor Saouma, professor of civil, environmental and architectural engineering at the University of Colorado Boulder.
In the wake of incidents like the Fukushima Daiichi nuclear disaster and an existing nuclear structure known to suffer from AAR, Saouma – a world leader in AAR – and his team set out to investigate this problem.
They found that relatively low levels of AAR can decrease the structural resilience by about 20 percent, which could be a major safety concern should an anticipated earthquake occur.
Funded by a grant from the NRC, the study involved complex static testing in the Center for Infrastructure, Energy and Space Testing, combined with computer simulations to assess risks in similar structures.
“Post-Fukushima, reactors must show resistance/resilience to even higher amplitude earthquakes than originally designed for,” Saouma said. “But at least one nuclear reactor in the U.S. suffers from AAR, and with a potential additional life of over 20 years, there is a justifiable concern for its long term structural integrity. This should be thoroughly investigated with utmost scientific rigor”
The team’s multifaceted approach focused on material characterization (prediction of future degradation), structural testing and computer simulation of a NCVS damaged by AAR and hit by an earthquake.
Saouma encourages more research using dynamic testing that accurately simulate seismic activity, as well as further inquiry into topics like the distribution of damage inside the structure coupled with truly state-of-the-art analyses before final safety assessments can be made.
“While most nuclear reactors were designed to last for 40 years, requests are coming in for many of them to be pushed to 60 and some even talk of a total of 80 years,” Saouma said. “Our study points to the need for thorough safety assessments before reactors are relicensed, especially if they are known to suffer from AAR.”