Engineers this week called for a rethink of how risk analysis for critical infrastructure is calculated following major earthquakes in Japan and New Zealand.
Reactor core meltdown fears
The threat of a reactor core meltdown at nuclear power stations faces the greatest scrutiny because of the ongoing crisis at Japan’s quake-and-tsunami-hit Fukushima power plant.
Engineers are still battling to prevent a major nuclear disaster at the plant which was severely damaged following the magnitude 9.0 earthquake and subsequent tsunami.
Christchurch in New Zealand was hit by a magnitude 6.3 earthquake two weeks earlier.
Cornell University School of Civil and Environmental Engineering professor Tom O’Rourke told NCE that engineers will have to reassess their designs against worst possible scenarios.
Recent earthquakes in Chile, Haiti, Christchurch and Japan have all produced beyond-design case effects, which means engineers must focus in on reassessing return periods, said O’Rourke.
“It requires re-examination of what we should be designing for,” he said. “When we design against probabilistic events it tends to be anywhere between 100 and 2,500 year return periods. But we are seeing events that are beyond this.”
“The ground movement was at predicted levels, but it was the tsunami that surpassed any design criteria”
Katsu Goda, Bristol University
Atkins technical director for geotechnics Robert May agreed that a rethink of natural disaster return periods for the UK would now be needed following the Japan disaster.
“Major tsunamis are very rare events but storm surges in the North Sea are a much greater hazard. Our east coast and Thames Estuary flood defences are designed for 1:1,000 year storm surge events. By contrast, Dutch coastal defences are designed for 1:10,000 year events,” he said.
“The original return period calculations should be revisited.”
O’Rourke and May added that recent events meant there could also be a case for re-zoning design codes and planning rules for areas with critical infrastructure.
May said that at-risk countries like Japan should consider introducing “tsunami zoning” along the lines of seismic zoning in design codes.
However, re-zoning has to account for socio-economic factors which sometimes limit the amount of protection that can be provided. This was the case in New Orleans following Hurricane Katrina, said O’Rourke. He helped to assess the impact on New Orleans’ critical infrastructure following Hurricane Katrina.
“In New Orleans, around $15bn (£9.1bn) has been spent on rebuilding levies but they are only able to withstand a 1 in 100 year flood because of the large footprint they protect,” O’Rourke said.
Meanwhile, further details have emerged about the design codes for the nuclear power plants in Japan, which confirm that this month’s earthquake and tsunami exceeded those anticipated in design standards.
“With the [Fukushima] nuclear power plant, it responded fine to the earthquake, but the tsunami knocked out its back-up systems”
Tom O’Rourke, Cornell University School of Civil and Environmental Engineering professor
According to Bristol University lecturer Katsu Goda, Japanese nuclear power plants are designed to withstand an earthquake between magnitude 7.0 to 7.9 depending on location. The 11 March quake was a magnitude 9.0 event.
“With the [Fukushima] nuclear power plant, it responded fine to the earthquake, but the tsunami knocked out its back-up systems,” said O’Rourke. A review by Japanese authorities could now demand that back-up generators are located at a previously unimaginable height, he suggested.
Past ICE President Jean Venables said that nuclear power plants, unlike communities that can be rebuilt elsewhere, cannot be moved and would likely need hard defences such as larger flood walls to protect theminstead.
Historical data lacking
The height of the protective wall at the Fukushima Daiichi nuclear power plant is understood to be 6m high. The tsunami wave was 8m high.
Fukushima nuclear plant owner Tokyo Electric Power Company (TEPCO) requires its plants to cope with a peak ground acceleration of 0.5g. This means the vertical forces exerted by the quake are 1.5 times gravity at their peak. The nearest available reading to the power plant recorded the ground shaking at 0.6g
“The ground movement was virtually at predicted levels, but it was the tsunami that surpassed any design criteria,” added Goda.
According to Japan Society of Civil Engineers guidance document Tsunami Assessment Method for Nuclear Power Plants in Japan, designs are to withstand the largest historical recorded tsunami with an additional factor of safety for statistical variation.
However, as historical records are poor for height of tsunamis, engineers estimate the height based on earthquake strength, along with geographical features.