Leaker leaders look back3 November 2009
Problems with leaking fuel at four Exelon BWRs and four PWRs in Illinois, and at TVA’s Watts Bar PWR led to a major R&D effort with the Electric Power Research Institute and vendors. In 2009, the PWRs are scheduled to start their first cycle without any of the fuel assemblies from the time of the failure. In separate interviews, Exelon vice president of nuclear fuels Jim Malone and TVA nuclear fuel design manager James Lemons told NEI how they dealt with the problem.
Q. When did you first start to notice fuel failures in your PWRs?
Jim Malone: “What was happening to us, at the beginning of our second cycle of the fuel batch, we began to see fuel failures occurring as we went to 100% of full power. That was in 2004-2005. We have had a similar experience with our BWRs (at La Salle County and Quad Cities) where fuel failures were related to power changes. In that case we investigated the failures and sent fuel to Studsvik’s [Nyköping, Sweden] hot cell facility. They attributed the problem to missing pellet [cladding] surface. When we saw that behaviour in PWRs, we began to wonder if there was not a similar cause.”
Q. Are fuel failures more common in BWRs because they have more dynamic power ranges?
“In today’s environment, even BWRs tend to be run flat out, although after a period you have to change the control sequence. Typically after that event you see the damage. BWRs are more prone to that kind of fuel failure. At one point the vendors told us that the failure mechanism doesn’t occur in PWRs. It was common industry folklore. We were skeptical, because we had seen such similar behaviour, and visual examination of the discharged rods made us more suspicious. The failures didn’t look like debris fretting failures. Fuel failures present differently. They look like a kind of inward defect in the cladding metal like it caved in; the fret shows as a wearing away of metal surface.”
Q. How did you fix it?
“The solution for the BWRs was better pellet production quality. The vendor spent a lot of time and money improving it.”
Q. There must have been a delay before those improvements took effect. What did you do in the meantime?
“The plan was to be gentle. It’s a difficult comparison with PWRs. We used the rule of 85; we increased the margins of the thermal limits that could stress the fuel. We gave ourselves a 15% margin.”
Q. Did it work?
“It worked, but it wasn’t perfect. The problem was going to live as long as the fuel in the reactor might be susceptible. We operate on two-year cycles, so it would need to be six years before we have all the fuel out. The BWR problem really started with fuel manufactured in the 1999-2000 timeframe. We also had problems at our Braidwood and Byron PWRs; actually we had more problems at Braidwood. These are our only four Westinghouse PWRs. We have done a lot of work with Westinghouse to monitor the fuel manufactured to assure fuel quality. I am relatively confident that the quality of the fuel we are receiving is much higher than it was when we had the problem.”
Q. ‘Relatively confident’ does not sound very convincing.
“It’s nuclear power. Nothing is 100%. I’m not going to guarantee you anything; one has to be hyper-vigilant. We are pretty confident we have it right. But it would be foolhardy to say that we would never have anything happen again. One of the fundamental things is the entire industry’s effort working in the US, with great cooperation between the Institute of Nuclear Power Operators (INPO) and EPRI to drive the entire industry to reach the goal of no fuel defects by the end of next year. It is a great goal, there have been dramatic improvements. Everyone is taking it seriously and exchanging information. There is good cooperation from suppliers; there is a chance that this can be attained. Three or four years ago no-one thought it was possible.”
Q. How does fuel quality now compare with then?
“It’s better. If you look at the vendor data you can see that it has improved. We have 17 operating units, and we now have one defect, in one pin, in one reactor.”
Q. How many would you have had a few years ago?
“We would have had a few, spread out over a few reactors. We have had a lot of progress at Exelon. We were at zero earlier this year; we want to keep going until we get it right.”
James Lemons explains that utility the Tennessee Valley?Authority became suspicious that it was suffering from the same missing pellet surface problem when it started to have startup leaks at the same time as Exelon.
Q. What causes the leaks?
James Lemons: “What is happening is that the fuel pellet is expanding more than the cladding during power increases. When the pellets begin contacting the clad it induces stress in the clad, the cladding can withstand the stress up to a certain point. Fuel duty limits (ramp rates) are supposed to prevent the stress from exceeding the capability of the cladding. As you begin the power increase, there is a percentage of power per hour, about 3%, in which the stress builds up in the cladding, and the cladding creeps out in response, but this takes time. If you power up too fast, you will get a crack. If you go slow, the cladding creeps to relieve the stress, and you won’t get a failure. The ramp rate provided by the manufacturer was supposed to be sufficient. But leaks occurred anyway, and Watts Bar and Exelon PWRs had radiation leaks. Exelon and EPRI sent the fuel to the Studsvik hot cell in Sweden for further analysis.
“What they found in the hot cell was that there was a primary defect, a pellet-cladding interaction (PCI) crack. They did metallurgy on the pellets and found chips on the surface at the cracks. There has been a lot of work in the industry that has shown that surface defects cause a stress riser in cladding. If you have a pellet with cladding, and take a chunk out, as it expands you can see that the piece of cladding right next to the chip is not being supported, it is not being pushed out, yet the cladding is expanding everywhere else, and that puts a bending moment on the unsupported area, and significantly increases stress on that area. So this can cause cladding to fail at much lower ramp rates than were believed to exist.”
Lemons says that rods that failed at Exelon were Westinghouse rods that contain an integral fuel burnable absorber (IFBA) neutron poison, an isotope of boron-10, applied after the uranium fuel was pelletised but before the pellets were put in rods. Watts Bar also receives fuel from Areva and Global Nuclear Fuel.
“Our failures were also IFBA rods. That was a pretty good correlation there. We contracted with a company called Anatech, through EPRI, to calculate fuel cladding stresses during the startup manoeuvre. The analyses (using the EPRI Falcon software) concluded that cladding failure was possible using the actual power escalation rates if a defective fuel pellet with a significant surface chip was present at the failure site. [Lemons adds that Westinghouse has since modified its fuel fabrication and inspection process].
“We had them model our Watts Bar cycle 7 second burn failure assuming the same chip size as in the hot cell tests that Exelon used. They determined that if the pellet with the chip was in the worst axial location when it failed, it could violate the cladding. But if it were in the best spot it wouldn’t have. Based on that the company went through a calculation of protective ramp rates to protect the fuel in the next cycle of startup, assuming a pellet defect. I don’t know what the frequency of the bad pellets was, or whether they were any in the fuel, but we had to assume there would be a bad pellet in a bad spot.
“The power ramp rates that came back were really restrictive. For the Watts Bar cycle 8 startup regime it was 3% per hour from 40-55%, and above 55% power it was half a percent per hour, with two 24-hour holds, and two 48-hour holds. Before the restriction, the recommended ramp rates were 3% per hour from 40% to 100%. Approximately two days of generation were lost due to the startup limitations (and cost the utility roughly $2 million).
“Where there becomes a risk for failure is not when the fuel is new; then there is a gap between the pellet and the coating. The cladding isn’t contacted by the pellet. Then, after one cycle of operation, that gap is closed due to swelling from radiation, and the cladding creeps down and becomes a smaller diameter, due to the system pressure outside of the cladding. What turns out to be the riskiest place is in the start-up of the second cycle. The third cycle fuel tends to be moved in positions in the core where the power is so low that it is generally not a problem for the cladding to withstand the pressure. The greatest risk is when fuel goes from relatively low power at the end of the first cycle to a location of higher power at the second cycle.
“The wild card is that you don’t know where the bad pellet is. You assume that the worst pellet is in the worst place. There are just so many pellets in the core that the risk of a rogue pellet is relatively high. Assuming the fuel batch is a third of the core, that’s 10 million pellets in every reload. It is a big problem to get every one perfect.”
Q. So what is the plan now?
“Cycle 10 starts this fall. I haven’t done the ramp-up rates yet, but they will be pretty close to the vendor recommendations, and near 3% per hour. We will have removed all the susceptible fuel from the core, the vendors have reduced the allowable chip size and there haven’t been any repeat failures, which is a good data point.”
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