Tufts University Logo Engineering

Search  GO >

this site tufts.edu people
 
Tufts University Engineering    
 
Tufts University
Print

Vannevar Bush Dean's Medal Recipient Speaks about Fukushima

Richard A. Meserve, President of the Carnegie InstitutionView the lecture and Q&A

Richard A. Meserve, president of the Carnegie Institution, international expert on nuclear energy and Tufts University alumnus spoke about the impact of the Fukushima nuclear plant accident and also received the first Vannevar Bush Dean's Medal from Tufts School of Engineering on Monday, April 4, 2011.

Meserve, A'66, addressed the failure and stabilization of the Fukushima reactors; health effects; cleanup challenges; likely impacts on nuclear engineering, construction and regulation; and the need for planning for extreme events of all kinds.

Meserve chaired the U.S. Nuclear Regulatory Commission (NRC) prior to becoming president of the Carnegie Institution in April 2003. He is a member of President Obama's Blue Ribbon Commission on America's Nuclear Future and chairman of the International Nuclear Safety Group, which is chartered by the International Atomic Energy Agency.

On March 11, 2011 a 8.9 magnitude earthquake triggered a massive tsunami to hit northeastern Japan where the Fukushima Daiichi nuclear power plant is located.

At the time of this extreme event, the Fukushima nuclear power plant had three of six nuclear reactors in operation. The other three nuclear boiling water reactors had been closed for maintenance, inspection, and refueling, "which was very fortunate," Meserve said.

In a boiling water reactor system, the nuclear fuel assembly contains uranium oxide fuel rods surrounded by a non-reactive zirconium alloy. The fission of uranium fuel produces an enormous amount of energy, as well as 200-300 different fission products, such as Iodine-131 and Cesium-137, which also continue to fission to produce additional energy.

"The problem you confront is that you can turn off the nuclear reaction, but there are these fission products that are there that are going to continue to produce energy and you need to be able to deal with that energy getting somehow out of the reactor," said Meserve.

The energy, as heat, converts water flowing through the reactor core into steam that powers turbines and that also creates enormous amounts of pressure in the system that must be released.

Even though the earthquake triggered the immediate shutdown of the reactors, they were still were giving off about 6 percent of their full power of approximately 2000 megawatts, said Meserve.

"If you were able to convert all of that 140 megawatts into electricity, you could have enough power for a city of over 100,000 people," he said. "So, even if you've turned off the reactor, you've still got a big problem of being able to get the heat out of the system."

If this heat isn't removed from the system, which normally operates at about 280°C, the previously nonreactive zirconium alloy surrounding the uranium fuel rods begins to react.

"At about 1000°C, there are really two reactions that you need to worry about: If you're in air, zirconium and oxygen is going to produce zirconium oxide and a lot of energy. And if you're in steam, you're going to produce zirconium oxide and a lot of hydrogen, as well as a lot of energy."

These reactions then cause the uranium fuel to burn, compromising the integrity of the system, releasing radioactive noble gases and the fission products. As the temperature climbs higher still, the fuel begins to melt and additional radionuclide fission products are released.

According to "inconsistent stories" and news reports coming out of the plant, said Meserve, the earthquake may have damaged the backup power supply systems to keep the safety systems running properly to ensure that the uranium fuel would continue to be kept cool after the shutdown.

Then, an hour after the earthquake, the tsunami hit. "You had a wall of water 46 or 47 feet high that inundated all that area where all the intake structures are that were bringing sea water into the plant," said Meserve.

"The plant was designed to deal with a tsunami of 5.7 meters, so they were completely overwhelmed with water. This was truly a six sigma event."

As systems failed and the uranium fuel burned, the buildup of hydrogen from the reacting zirconium alloy wasn't properly vented. The hydrogen eventually reached explosive levels and blew off the top of the reactors.

The nuclear power plant operators attempted to cool the burning reactors with additional water, both salt and fresh water, but in doing so "there's a lot of contaminated water all over the place," said Meserve.

"And that's where we are today. The only way to cool these reactors is pouring cold water over them," Meserve said. "And every day this proceeds, they're contaminating the environment more."

"In one sense, their job is getting easier," Meserve said, adding that the more time passes, the more energy and heat will have dissipated. But dissipating energy also means an increase in radioactive fission products. "The problem is with all this contamination, the area is getting increasingly radioactively contaminated, so it's very hard for humans to get in there to work."

Clean up crews can only work for a certain amount of time before reaching a radiation exposure threshold.

In the United States, said Meserve, the NRC radiation dose limit for a worker in a nuclear power plant is 50 millisievert (mSv). For comparison, Americans receive an annual average background radiation of about 6 mSv and person undergoing a full body CT scan receives 10 mSv.

According to reports Meserve has read, the highest radiation dose for a worker in Fukushima is 200 mSv, "which is obviously four times the NRC annual dose limit."

"The doses are substantial," said Meserve, "But not yet at the stage, for short periods of time, you have to worry."

Meserve concluded his talk by touching a number of points beyond health effects and contamination that will need to be reassessed in the wake of the Fukushima accident.

"One of the things that's going to come out of this activity in Fukushima is there's going to be a huge assessment of lessons learned," said Meserve, adding that disaster planning and communication are key, especially in the face of the increased likelihood of extreme events.

Following the lecture, an audience member asked, given the consequences of the Fukushima incident, whether Meserve supports nuclear power.

Meserve responded affirmatively. "I do – but I am one of the few people who believe in climate change," he said. "You need to not take anything out of your weaponry to deal with the [energy] challenge, and it's my view that nuclear has to be part of the picture, but you have to do it safely."

—Julia C. Keller is Communications Manager at Tufts University School of Engineering
[This story was posted on April 6, 2011]

Lecture

Loading the player...

Q&A

Loading the player....