Thursday, August 12, 2010

Zirconium and the Future: Why Research is Important


An Expected Ruling

On Friday, August 6, the Nuclear Regulatory Commission (NRC) turned down a request for enforcement against Vermont Yankee by the New England Coalition (NEC). In June, the NEC had asked that Vermont Yankee be required to operate its core at a much lower temperature. The NEC claimed that zirconium alloy (zircaloy) cladding is a fire hazard at Vermont Yankee's current operating temperatures.

Why all the concern about zirconium cladding? Fuel pellets are made of oxides (mostly uranium oxide) and can't burn. However, the fuel pellets are encased in zircaloy, which is metallic. Metals don't generally burst into flame. However, unlike oxides, metals can burn. So it is reasonable to be concerned about the temperature at which zircaloy will burn.

The NRC regulation for maximum temperature of light water reactor fuel cladding is 2200 F, while Vermont Yankee plant operates at 2060 F or below. However, NEC asked the NRC to rule that Vermont Yankee must operate with fuel cladding temperatures no higher than 1700 F. Naturally, the NRC turned down this request. No reputable regulatory agency is going to tell a plant that is in compliance with regulations: "Sorry. You think you are in compliance, but you aren't in compliance. We have changed the regulations and made them far stricter, just for you."

Besides being unfair, this would be a great way for a regulatory agency to get itself sued. Of course the NRC didn't do this. After several conference calls with people from NEC, the NRC sent NEC a letter basically saying: No. We're not going to tell Vermont Yankee to operate at a lower temperature. Your concerns are more generic than anything specific to Vermont Yankee. Your concerns will be forwarded to our area for general rulemaking.

You can download the letter the NRC sent NEC here, and also read the Brattleboro Reformer article on the ruling.

But Wait, There's More!

The request above was specific to Vermont Yankee. It was filed by NEC in June of this year, and turned down at the beginning of August, as described above.

However, there was an earlier request by Mark Leyse, a nuclear engineer and the son of Robert Leyse, also a nuclear engineer. Mark Leyse lives in New York city. Mark Leyse has made many requests to the NRC for re-evaluation of safety measures. He is particularly concerned with fuel cladding, apparently believing that the NRC has ignored his father's research results on this issue.

Mr. Mark Leyse's November 2009 request for lower temperature operation of all reactors has become an NRC rulemaking docket. Leyse's November 17, 2009 petition to the NRC is the first item in this docket. Leyse claims that oxidation of cladding became a self-sustaining reaction at around 1800 degrees, and that the NRC has ignored this situation for forty years. In the petition asking for rulemaking, Leyse quotes his father's test results from forty years ago. Mark Lesye includes a letter from his father Robert Leyse about tests dated 1970 on pae 116 of his petition. Throughout the petition, Leyse also extensively discusses the test, Flecht Run 9573, reported in 1973. (For simplicity, I am referring to these references as "forty-year old data." Strictly speaking, some of the quoted data is only thirty-seven years old.)

The consequences of all this, as always, are assumed to be dire. Here's a quote from the Rutland Herald article:

The NRC earlier in spring had already agreed to consider the matter raised by Mark Leyse of New York City, but in a review track that will take years, not months. The New England Coalition wants the margin of safety increased immediately.

Leyse and Raymond Shadis, senior technical adviser to the coalition, say Vermont Yankee’s peak cladding temperature of 1,960 degrees Fahrenheit only gives the plant operators 30 seconds to react during a loss-of-coolant incident scenario.

Thirty seconds. Sounds terrible, doesn't it? And "takes years, not months" sounds perfectly awful.

But Wait, There's Less!

I decided to review Mr. Leyse's letter, because I couldn't figure out some of the comments in the press. What does reactor operating temperature have to do with loss-of-cooling accidents (LOCA)? Of course, to some extent, a hotter reactor is slower to cool, but everything about a LOCA analysis is built with huge margins of error. And how could forty-year-old lab data be ignored by everybody, when zircaloy must be one of the most-studied compounds in the world? Zircaloy it has been used for cladding since the very first reactors, and people have been studying it forever. Just Google "Zircaloy" to see the endless lists of papers and references.

And, of course, my major question. If zircaloy bursts into flame in water above 1700 Fahrenheit, and reactors routinely operate at 1900 Fahrenheit, how come they haven't all burned up already?

I reviewed the document to the best of my ability, and I welcome comments that will improve my review. Here's my analysis, with some quotes from Leyse's letter to NRC.

The Leyse request doesn't just refer to the temperature at which zirconium might burn in water. Instead, it also says that the modeling of loss of cooling accidents, the flood rate of water into the core, and the Baker-Just and Cathcart-Pawel equations non-conservative for calculating the temperature at which an autocatalytic (runaway) oxidation reaction of Zircaloy would occur in the event of a LOCA. .....Additionally, it can be extrapolated from experimental data that, in the event a LOCA, a constant core reflood rate of approximately one inch per second or lower (1 in./sec. or lower) would not, with high probability, prevent Zircaloy fuel cladding, that at the onset of reflood had cladding temperatures of approximately 1200F or greater, from exceeding the 10 C.F.R. § 50.46(b)(1) PCT limit of 2200 F.

In other words, this is not just about zircaloy. In this petition. Mark Leyse claims that just about every model used in the nuclear industry is inadequate. Zircaloy properties, LOCA calculations, reflood calculations--all wrong. In 2002, his father, Robert Leyse, said the same thing. The elder Leyse petitioned the NRC for rule-making about the Baker-Just and Cathcart-Pawel equations and LOCAs, and the NRC found his contentions to be not scientifically valid and wrote him to that effect in 2005.

This latest contention by Mark Leyse is a replay of this 2002 petition by Robert Leyse. Unfortunately, the NRC cannot just say: "Hey, we did this before. These contentions were made in a petition in 2002, and we settled these points in a ruling in 2005." No, they have to look at it again. This gives the opponents of nuclear power the opportunity to complain that it takes the NRC three years to look at anything.

Conspiracies Not Research

Robert and Mark Leyse seem to believe that all the research that has been done over the past forty years on LOCA modeling, zircaloy properties, and pretty much everything else....is part of a vast conspiracy to provide just the results the utilities want. The Leyses, and the Leyses alone, have the real facts, the true equations, the correct models. They discovered these "facts" forty years ago and have been ignored ever since.

I call this a conspiracy theory, decked out in scientific trappings.

Of course, once you buy into the idea that everything known about nuclear is wrong, then you can come up with scenarios where the entire core bursts into flame in thirty seconds and there is nothing anybody can do about it and so forth. This plays well with the anti-nuclear groups.

It doesn't play well with me. In science and technology, everything builds on what has gone before. Even the Einstein's famous breakthroughs were built on the past. He reviewed experiments that could not be explained without new ways of thinking about the speed of light. Einstein was attempting to explain the very latest experiments when he wrote his breakthrough papers.

I've done research, supervised research, and I deeply value research. Conspiracy theories that decide to ignore forty years of research should be called exactly what they are. Conspiracy theories.

I prefer science.

Update: NRC review of CORA tests, interim ruling now available here. Simplistic summary of NRC ruling: these results are in line with our other tests, and no rule changes are necessary.


Zirconium rod (not zircaloy) from Wikimedia.

21 comments:

Steve said...

There is just enough truth here to hide the baloney. When TMI had its meltdown, the zirconium did melt and did react with the water. However, it didn't burn -- it just underwent a replacement reaction with the water, releasing hydrogen. That would have burned as well, but it was in a place with no oxygen, so it couldn't.

Oh well. Let's hit the books so we can say something other than "it can't because it hasn't". How about "it's the passivation, stupid!"?

JD said...

The oxidation of zirconium in the cladding is definitely a concern during accidents. In fact, as I recall from classes, part of the design basis is to limit the total oxidation of cladding to about 1% of the whole core...something like that. There are two bad things that happen: one: hydrogen is formed from the water, which can lead to a hydrogen explosion (as at TMI-2), and two: oxidizing your cladding leads to cladding failure. Also, the reaction is exothermic, so it heats things up further, which sustains the reaction.

That said, the NRC and reactor designers/operators are well aware of this. It is one of the factors in determining the safe thermal margin. For BWRs, the most important margin involves making sure the coolant isn't completely turned to steam by the time it reaches the top of the fuel element.

If the NRC took a look at these claims and found them invalid, that oughta settle things. It's not in their interest to be setting unsafe limits.

Zirconium alloy as a fire hazard at legally safe operating temperatures? Well..there are 34 or so BWRs in the US that don't seem to have caught on fire despite operating at those temperatures for decades.

Meredith Angwin said...

Thank you so much for the comments.

When I first read about this set of petitions to NRC to lower plant operating temperatures, I thought...I'll blog about it. The temperatures at which zircaloy oxidizes are well known, though it might take me a little digging to find out. (My research was about alloy 600 and 690 and steels, not zircaloy.)

But when I looked into the petitions to the NRC, I realized that they were about many topics, not only zircaloy. LOCA calculations, reflooding rates, you name it. I came to the conclusion it would probably take anybody three years to analyze all the various contentions.

But without taking three years, I can say that an author who picks only limited data from the 70s, and bases everything on that, is not doing credible science.

Howard Shaffer said...

Technical: Loss of Coolant Accident (LOCA) calculations, and designs based on them, were always meant to be VERY conservative-with a lot of fat, on the safe side, becasue of unknowns.
Then there was an accident at a full sized reactor plant that proves beyond a shadow of a doubt how much safety margin there really is. This plant had a small to medium sized leak that went on for a few hours with NO Emergency Core Cooling (ECCS) coming on to put water back into the core. Most of the core melted. The Zirconium reacted with water to produce hydrogen. The hydrogen had an explosion. The ECCS came on. The core melting stopped. The molten material had melted a little way into the bottom of the reactor vessel. Fission products escaped from the piping system.
The fission products and hydrogen stayed in the Containment. The solid lump of the core remains was removed and sent to safe disposal. It took years to design special robotic tools to cut up the solid core lump, but it was done.
The plant was Three Mile Island Unit 2. The year was 1979.

Political: The NEC petition was an attempt to keep Vermont Yankee in the news, negatively. Mr. Shadis 30 seconds is plucked from the report for scare purposes. The event discussed is a LOCA. The ECCS comes on automtically, and is designed and tested so that Operators don't have to do anything for TEN MINUTES after an accident begins.

The Grannies getting arrested at VY again is obviously a coordinated attempt to keep VY in the news, negatively, of course.

Charles Bell said...
This comment has been removed by the author.
Charles Bell said...

At nuclear facilities, security forces are armed and authorized to use deadly force against intruders. Signs are prominently posted and entrances guarded. These trespassers should be treated the same as the angry airline passengers who end up in federal court and serve jail time. Its not funny. Nuclear power is serious business. Not for amateurs.

Charles Bell said...

The five crieria are specified in 10 CFR 50.46 Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors.

http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-0046.html

)(1) Peak cladding temperature. The calculated maximum fuel element cladding temperature shall not exceed 2200º F.

(2) Maximum cladding oxidation. The calculated total oxidation of the cladding shall nowhere exceed 0.17 times the total cladding thickness before oxidation. As used in this subparagraph total oxidation means the total thickness of cladding metal that would be locally converted to oxide if all the oxygen absorbed by and reacted with the cladding locally were converted to stoichiometric zirconium dioxide. If cladding rupture is calculated to occur, the inside surfaces of the cladding shall be included in the oxidation, beginning at the calculated time of rupture. Cladding thickness before oxidation means the radial distance from inside to outside the cladding, after any calculated rupture or swelling has occurred but before significant oxidation. Where the calculated conditions of transient pressure and temperature lead to a prediction of cladding swelling, with or without cladding rupture, the unoxidized cladding thickness shall be defined as the cladding cross-sectional area, taken at a horizontal plane at the elevation of the rupture, if it occurs, or at the elevation of the highest cladding temperature if no rupture is calculated to occur, divided by the average circumference at that elevation. For ruptured cladding the circumference does not include the rupture opening.

(3) Maximum hydrogen generation. The calculated total amount of hydrogen generated from the chemical reaction of the cladding with water or steam shall not exceed 0.01 times the hypothetical amount that would be generated if all of the metal in the cladding cylinders surrounding the fuel, excluding the cladding surrounding the plenum volume, were to react.

(4) Coolable geometry. Calculated changes in core geometry shall be such that the core remains amenable to cooling.

(5) Long-term cooling. After any calculated successful initial operation of the ECCS, the calculated core temperature shall be maintained at an acceptably low value and decay heat shall be removed for the extended period of time required by the long-lived radioactivity remaining in the core.

Charles Bell said...

10 CFR 50.46 Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors.
The five limits are specified in paragraphs (b)(1) through (b)(5)

Charles Bell said...

(b)(1) Peak cladding temperature. The calculated maximum fuel element cladding temperature shall not exceed 2200º F.

(b)(2) Maximum cladding oxidation. The calculated total oxidation of the cladding shall nowhere exceed 0.17 times the total cladding thickness before oxidation. As used in this subparagraph total oxidation means the total thickness of cladding metal that would be locally converted to oxide if all the oxygen absorbed by and reacted with the cladding locally were converted to stoichiometric zirconium dioxide. If cladding rupture is calculated to occur, the inside surfaces of the cladding shall be included in the oxidation, beginning at the calculated time of rupture. Cladding thickness before oxidation means the radial distance from inside to outside the cladding, after any calculated rupture or swelling has occurred but before significant oxidation. Where the calculated conditions of transient pressure and temperature lead to a prediction of cladding swelling, with or without cladding rupture, the unoxidized cladding thickness shall be defined as the cladding cross-sectional area, taken at a horizontal plane at the elevation of the rupture, if it occurs, or at the elevation of the highest cladding temperature if no rupture is calculated to occur, divided by the average circumference at that elevation. For ruptured cladding the circumference does not include the rupture opening.

Charles Bell said...

(b) (3) Maximum hydrogen generation. The calculated total amount of hydrogen generated from the chemical reaction of the cladding with water or steam shall not exceed 0.01 times the hypothetical amount that would be generated if all of the metal in the cladding cylinders surrounding the fuel, excluding the cladding surrounding the plenum volume, were to react.

(b) (4) Coolable geometry. Calculated changes in core geometry shall be such that the core remains amenable to cooling.

(b)(5) Long-term cooling. After any calculated successful initial operation of the ECCS, the calculated core temperature shall be maintained at an acceptably low value and decay heat shall be removed for the extended period of time required by the long-lived radioactivity remaining in the core.

Charles Bell said...

Furthermore Appendix K to 10 CFR Part 50--ECCS Evaluation Models
provides all the design criteria for evaluating compliance with 10 CF 50.46
The following NRC webpage contains all the information and reference information. Everything is solidly based on experimental and theoretical studies of the Zirconium-Water reaction, thermal hydraulics of departure from nucleate boiling phenomena, well established critical heat flux correlations, reactivity and reactor kinetics, well known data on decay heat from the radioactive decay of actinides, including neptunium and plutonium generated during operation, as well as isotopes of uranium, cladding swelling and rupture mechanical studies. Everything is clearly specified in how analyses are done. Each plant maintains its UFSAR which details how it complies with design criteria.


http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appk.html

Charles Bell said...

June 29, 2005 DENIAL OF A PETITION FOR RULEMAKING TO REVISE APPENDIX K TO 10 CFR PART 50 AND ASSOCIATED GUIDANCE DOCUMENTS (PRM-50-76)

http://www.nrc.gov/reading-rm/doc-collections/commission/secys/2005/secy2005-0113/2005-0113scy.pdf

The NRC commissioners formally denied the petitioner, Mr. Robert H. Leyse.

The NRC concluded that Appendix K of 10 CFR Part 50 and the existing guidance on best-estimate ECCS evaluation models are adequate for assessing ECCS performance for US LWRs using Zircaloy-clad UO2 at burnup levels authorized in plant licensing bases.

donb said...

Steel burns too. Just take a pad of steel wool and touch a match to it.

I think I need to increase the safety margin for the nails that hold my house together by cranking up the air conditioning. Nothing like cool air on a hot day...

Meredith Angwin said...

I just want to thank everyone who answered. Charles Bell took the time to give us complete chapter and verse about all the factors and all the thought and all the calculations that go into deciding the temperature at which to run a nuclear plant. Thank you to everyone!

Anonymous said...

"And, of course, my major question. If zircaloy bursts into flame in water above 1700 Fahrenheit, and reactors routinely operate at 1900 Fahrenheit, how come they haven't all burned up already?"

Actually, during normal operation at BWRs fuel rod temperatures are about 550 Fahrenheit and at PWRs fuel rod temperatures are about 600 Fahrenheit.

It is during a loss-of-coolant accident that fuel rod temperatures would substantially increase. In some cases, maximum fuel rod temperatures could reach over 1800 Fahrenheit.

Anonymous said...

The NRC is considering one rulemaking petition, PRM-50-84, that Mark Leyse submitted in its rulemaking process.

PRM-50-84 is on a different fuel-cladding safety issue than the one NEC raised at Vermont Yankee:

http://edocket.access.gpo.gov/2008/E8-27938.htm

Meredith Angwin said...

Anonymous

Thanks for the comments. I spent about ten minutes hitting myself on the head and calling myself stupid after I read the first one. Of course, primary side water is around 600 F in a PWR, and the reactor cooling water is somewhat less hot in a BWR. I knew that. I DID, I KNEW THAT. 600 F was the basis number for so many tests on how fast pitting corrosion might grow or crack propagation or....Okay. I'll stop with the self-hitting.

However, in self-defense, I will say that the 1700 was presented very much like...WHY do they let the reactors run this way! As opposed to: In a LOCA, the cladding might overheat.

My main point (salvaging small shreds of dignity here) was that the matter was presented as info-about-zircaloy, as in "please remember the test about runaway oxidation that was supposedly ignored by the regulators." However, as I said in my main post, it was clearly more complicated than that, with most of the issue being about LOCA calculations. The zircaloy test was almost as a red herring.

Best,
Meredith

Anonymous said...

Meredith

I think the way the papers have reported on NEC's enforcement petition has been very confusing: they basically state that Vermont Yankee is operating with fuel rod temperatures of 1960 F.

So I think it's easy to get confused about what the fuel rod temperatures are during typical operation at Vermont Yankee--especially when the centerline temperature of the UO2 fuel pellets sheathed by the fuel rods is over 1800 F during operation.

But, as I said before, the fuel rods at Vermont Yankee would be around 550 F during operation. It's sad the papers left out that important fact when reporting on NEC's enforcement petition and thereby confused their readers.

Thank you for posting my comments.

Anonymous said...

There are recent developments to this story. After denying the 2.206 petition that you discuss--regarding lowering the licensing basis peak cladding temperature at Vermont Yankee--NRC turned it into a rulemaking petition (PRM-50-95). Here is a link to the docket folder:

http://www.regulations.gov/search/Regs/home.html#docketDetail?R=NRC-2009-0554

PRM-50-95 is the fourth item in the docket folder.

Howard Shaffer said...

When I post comments, write to media etc. I put my name on it.

Testimony without a name is not admissible in court.

These concerns focus on a few lab tests and ignore the design basis for the tests. The Design Basis LOCA is a double ended rupture of the largest pipe connected to the reactor. When I was at Yankee Atomic Electric Nuclear Services Division this double ended break was referred to as "the Battleship in the desert" problem. You had to design for it, but you weren't allowed to ask how it got there. When "Anonymous" can credibly explain how a double eneded break might happen, I'll be happy to listen. Until then I'll consider the results of an actual full scale test of a small break LOCA bounding (TMI-2). This along with the Pennsylvania Department of Health 20 year follow up study.
And please don't tell us that Insurance companies paid, therefore guilt is admitted. We all know that if its cheaper to pay than fight, they'll pay. Our country has come to that- and the situation where someone can buy hot coffee at a drive through, burn themselves, sue and get paid off.

kevin144 said...

still think so after Japan this weekend?