Clean Energy Prof

Thursday, July 2, 2009

The Fermi Chronicles - Part 20: Nuclear Events - Browns Ferry, Alabama, 1975

Browns Ferry In Alamaba Is the site of 3 BWRs each capable of generating more than 1.1 GW of electricity. It is the site of the first nuclear power plant to generate more than 1 GW. On March 22, 1975, a fire broke out in unit 1. Ironically, it was not a result of any malfunction, especially with the nuclear loop, but rather the result of what I can only call utter stupidity on the part of a single worker who was looking for leaks with a candle. The candle set fire to temporary cable seal and it took off from there. The fire was extinguished on one side of the wall where the check was being made for drafts, but no one realized that it had already spread to the other side and into the reactor building. By the time anyone had realized that anything had happened in the reactor building, extensive damage to the cabling had occurred. Also, the damage caused some of the safety systems in unit 1 to malfunction due to grounds and hot shorts as a result of teh melting, something that was thought to be impossible at the time. It took an entire year to fix all the damage! For reference, it costs a 1.1 GW plant about $1.5 million each day it is shut down in today's dollars.

Because of what happened at Brown Ferry, fire suppression is a separate group of personnel at every nuclear facility trained for only that one thing! Yes it costs a lot of money, but it costs a lot more to be shut down even for a short period of time. In addition, this fire caused the NRC to set tight standards for fire control that are still in effect to this day. Also, all nuclear plants now have the capability to totally shut down from outside the control room. Thus, they must have the equivalent of a battle bridge in the ST: TNG tv series.

Another interesting trivia fact about the Brown Ferry facility, specifically unit 2, is outlined in this Wikepdia entry:

Beginning in 2005 Unit 2 was loaded with BLEU (Blended Low Enriched Uranium) recovered by the DOE from weapons programs. This fuel contains quantities of U-236 and other contaminants because it was made from reprocessed fuel from weapons program reactors and therefore has slightly different characteristics when used in a reactor as compared to fresh uranium fuel. By making use of this fuel which would otherwise have been disposed of as waste the TVA is saving millions of dollars in fuel costs and accumulating a database recycled uranium reactions in LWR use.

Monday, June 29, 2009

The Fermi Chronicles - Part 19: Nuclear Events - Fermi 1, 1966

This one is pretty close to home. Fermi 1 still sits right next to Fermi 2. I walk right by the building every morning. As with all of the reactors up to this point in time that have experienced nuclear events, Fermi 1 was also an experimental 94-MW reactor. In fact, Fermi 1 was one of the first FBR reactors ever build. Also, it was cooled with molten sodium, making it a LMFBR (refer to my prior post on reactor types). The liquid metal would be its downfall, however, as no one was experienced in designing systems to contain and distribute liquid metal at the time. It is far more common now with many large-scale solar power generators using liquid metal (typically sodium) as the primary coolant.

There are no more FBRs in operation in the U.S. although they are quite common overseas. A primary advantage of FBRs is that they produce more fissile Pu-239 than U-235 that is burned. The Pu-239 can then be used for fission in nuclear power or for military applications. It is a very good design for countries that have very little indigenous uranium in the ground, thereby having to import it at the expense of monetary assets and national security. A downside of LMFBRs is that sodium reacts chemically with water*, and is known to leak no matter what is attempted. After all, sodium, Na, is a bit bigger than the oxygen atom, but far smaller than the H2O molecule.

On the morning of October 5, 1966, as the reactor was being brought up to full power, the reactor coolant temperature became unusually high. The power increase was halted, and it was noted that two fuel assemblies were over 700F when they should have been at 580F. 10 minutes later, radiation alarms sounded. The reactor was shut down immediately in what is known as a reactor SCRAM. SCRAM** is an old acronym and is typically no longer used. Instead, the term trip is used, as in "the reactor has been tripped." But they called it SCRAM back in the 80's:

More than a month later, a microphone was inserted into the reactor core listening while the sodium was being pumped through the core. A metallic clapping sound was heard. Four damaged fuel assemblies were removed but it was well into 1967 already. Interestingly, this event cause the basic invention of the arthroscope which is now common in many medical procedures as it is minimally invasive - arthroscopic surgery. The Fermi 1 device certainly wasn't small, but was necessary to physically look into the reactor core. It was discovered that a metallic piece that looked like a crushed beer can was wedged and blocked a coolant nozzle. I can only imagine that every construction worker broke out in a sweat during this time. "Hey - who left a beer can in the reactor?" Actually, it was not a beer can at all, but part of a flow deflector that was put in during construction but was not in any blueprint, procedure or drawing whatsoever. Someone had installed it without any design consideration. Since the flow was a high-speed liquid metal, it simply got sheared off due to the viscous stress on it. I'm surprised it lasted as long as it did.

In any case, the plant was back in operation in June of 1970 and ran for a few years. It was refused a license renewal in 1972 by the Atomic Energy Commission and was shut down. Even after the nuclear shutdown, however, an oil-fired boiler was built and the Fermi 1 turbine generated electricity during peak demand in the summer. The oil-fired boiler was dismantled in the 1980's. Fermi 1 is still going through the decommissioning process today. At some point, there will be nothing left of it as the remaining pieces are sold off for scrap. I'm glad I got the opportunity to tour the Fermi 1 complex, albeit is is mostly gutted now. The biggest lesson learned from the Fermi 1 event is, of course, that design control must be strictly kept. Period. Nothing goes in that is not on paper. Also, the need was apparent for a way to monitor the core during and after an accident condition. This is now commonplace in the nuclear industry, but it took Three Mile Island to solidify this need in federal regulations under the NRC. During my training week here at Fermi 2, one of the modules that I was tested on was called the FME protocol. FME stands for Foreign Material Exclusion. When valves, pipes or pumps are opened up for replacement of maintenance, great care is written into tight procedures to make sure that foreign material does not get into the system.

* In 1970, a high-pressure sodium pipe burst. Right after that happened, a water pipe burst also. The two came into contact and an explosion ensued. Fortunately no one was hurt.

** SCRAM is an old acronym standing for Super-Critical Reactor Axe Man. This was literally a guy with an ax that would cut a rope holding up control rods in the first nuclear reactor - the reactor pile known as CP-1 in Chicago, IL. It was a safety measure written into the emergency procedure, albeit it was just one of several safety mechanisms. There are no ropes holding up control rods today. Thank goodness...

Trivia: Specialized operators that went into the top of the reator for maintenance wore special suits pressurized with air (so no infiltration, just exfiltration) and cooled with liquid nitrogen. The suits were very elaborate and these operators were called "reactornauts."

Previously:
The Fermi Chronicles - Part 18: Nuclear Events - SL-1 Event, Idaho, 1961
The Fermi Chronicles - Part 17: Nuclear Events - Windscale, UK, 1957
The Fermi Chronicles - Part 16: Nuclear Events - Chalk River, CAN, 1952
The Fermi Chronicles - Part 15: The Nuclear Business Model
The Fermi Chronicles - Part 14: Neutron Moderation
The Fermi Chronicles - Part 13: Nuclear Reactor Types
The Fermi Chronicles - Part 12: Generating Electricity
The Fermi Chronicles - Part 11: Worldwide Uranium Availability
The Fermi Chronicles - Part 10: Utilizing Nuclear Reactions To "Breed" More Fuel
The Fermi Chronicles - Part 9: Nuclear Fission
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post

Sunday, June 28, 2009

The Fermi Chronicles - Part 18: Nuclear Events - SL-1 Event, Idaho, 1961

This was an event that was unique in that it caused the first nuclear fatality. (at least in civilian nuclear power-generating) This event was the result of poor design and human error. Ironically, it occurred when the reactor had been shut down for almost 2 weeks. Since all 3 operators were killed in the accident, the event had to pieced by observing the evidence, versus eyewitness accounts. This was also the most rapid event in the nuclear industry (to my knowledge anyway), being completely done in all of 3 seconds.

During a holiday shutdown, core maintenance was to be performed by 3 operators. The SL-1 reactor was a small (3 MW) experimental boiling water reactor (Fermi 2 is rated at 1,100 MW). Back in 1961, the manual lifting of control rods was common. During maintenance, one of the control rods was to be lifted a total of 3 inches so as to connect it to the control rod drive mechanism. It was lifted 20 inches instead. No one knows why it was lifted out so much, but because of the poor reactor design, it went prompt critical instantly vaporizing the surrounding water and causing a steam explosion. This had the effect of shooting the control rod fully out of the core and impaling one of the operators to the ceiling (nasty!). The steam explosion lifted the entire reactor head 9 feet off the ground, killing the other two operators.

Theories as to why the control rod was lifted so far out has been purely conjecture. The media ran with a murder/suicide storyline (so sensationalism in the media is not a new thing after all). The far more likely theory is that the rod became stuck and you just know what a guy with two of his buddies watching has to do - get it unstuck. Yeah - tug on it with all your strength. Unfortunately, once it became loose, it was quite impossible to stop at just 3 inches. It's a guy thing. Wikipedia (don't always trust them!) has an entry on the aftermath of the accident:

There were no other people at the reactor site. The ending of the nuclear reaction was caused solely by the design of the reactor and the basic physics of heated water and core elements vaporizing, separating the core elements and removing the moderator.

Heat sensors above the reactor set off an alarm at the central test site security facility at 9:01pm, the time of the accident. The first response crew, of firemen, arrived nine minutes later and initially noticed nothing unusual, with only a little steam rising from the building, normal for the cold (−20 °F or −30 °C) night. The control building appeared normal. On approaching the reactor building, their radiation detectors jumped sharply to above their maximum range limit, and they withdrew, unable to know whether they could safely proceed or for how long they could remain.

At 9:17 p.m., a health physicist arrived. He and a fireman, both wearing air tanks and masks with positive pressure in the mask to force out any potential contaminants, approached the reactor building stairs. Their detectors read 25 Roentgens per hour (R/hr) as they started up the stairs, and they withdrew.

Some minutes later, a health physics response team arrived with radiation meters capable of measuring gamma radiation up to 500 R/hr—and full-body protective clothing. One health physicist and two firefighters ascended the stairs and, from the top, could see damage in the reactor room. With the meter showing maximum scale readings, they withdrew rather than approach the reactor more closely and risk further exposure.

Around 10:30 p.m., the supervisor for the contractor running the site and a contractor health physicist arrived. They entered the reactor building and found two mutilated men: one clearly dead, the other moving slightly. With a one minute and one entry per person limit, a team of five men with stretchers recovered the operator who was still breathing; he did not regain consciousness and died of his head injury at about 11 p.m. Even stripped, his body was so contaminated that it was emitting about 500 R/hr. They looked for but did not find the third man. With all potential survivors now recovered, safety of rescuers took precedence and work was slowed to protect them.

On the night of 4 January, a team of six volunteers used a plan involving teams of two to recover the second body. Radioactive Gold 198Au from the man's brass watch buckle and Copper 64Cu from a screw in a cigarette lighter subsequently proved that the reactor had indeed gone supercritical.

The third man was not discovered for several days because he was pinned to the ceiling above the reactor by a control rod. On 9 January, in relays of two at a time, a team of eight men, allowed no more than 65 seconds exposure each, used a net and crane arrangement to recover his body.

The bodies of all three were buried in lead-lined caskets sealed with concrete and placed in metal vaults with a concrete cover. Richard Leroy McKinley is buried in section 31 of Arlington National Cemetery.

As with prior events in the early days of nuclear energy, much was learned. Probably the most important nugget of information learned here was that reactors shouldn't be designed to go prompt critical if a single control rod is removed. All modern reactors are designed with that criterion, called the "one stuck rod" criterion. Also, only highly-trained operating personnel may dictate what occurs on or around the reactor core. None of the 3 men killed had such training. They were basically maintenance workers. Very specific procedures are now in place for any maintenance whatsoever.

For the sake of nostalgia, here is the SL-1 documentary video from the Atomic Energy Commission back in the day:

Saturday, June 27, 2009

The Fermi Chronicles - Part 17: Nuclear Events - Windscale, UK, 1957

As with the Canadian NRX reactor (and subsequent nuclear event in 1952), the Windscale reactor in Cumberland, England (now Sellefield) was an experimental reactor commissioned in the mid-1940s not to investigate the possibility of civilian uses for nuclear power, but rather to manufacture plutonium for nuclear weapons (the cold war was being waged after all). This experimental reactor was gas-cooled (GCR), which is definitely my least favorite design. It was also graphite moderated. Unfortunately, graphite can react to neutron flux at low levels by storing potential energy that at some point could be spontaneously released as heat. This is now known as the Wigner Effect. To release the potential energy stored in the dislocation of its crystalline molecular structure, an annealing process must be initiated periodically. Basically, the material has to be heated to a certain temperature, allowing the crystalline structure to expand and realign, and then cooled very slowly. This process was initiated at Windscale but the reactor, and monitoring instrumentation, simply were not designed for such a process.

To initiate the annealing process at Windscale, the fan speed was reduced through the graphite. Then, the reactor power was increased, but temperature sensors were misplaced in the core. Since the reactor was not designed for the process, damage to the fuel resulted (the fuel was metallic uranium, which has a lower melting temperature than uranium oxide), but was not picked up by any instrumentation. When control rods were inserted back to reduce the power, the energy release decreased faster than anticipated. The thought was that the annealing process was not completed, so the control rods were withdrawn once again. The annealing process began on October 7, 1957. By October 10, operators knew something was awry. Core temperature continued to increase rather than decrease. Radiation sensors at the top of the discharge stack were pegged out. Air samples a half-mile away were 10 times normal! Two operators donned protective clothing and removed an inspection plug where a thermocouple was reading an unusually high temperature.

Through the plug hole they could clearly see to their horror that several channels of fuel were glowing cherry red, and that the core had been on fire for at least 48 hours! Turns out, the uranium from the damaged fuel during the annealing process reacted with the graphite and caught fire. Increased air flow simply fueled the fire more. Finally, the core was flooded with water, even though there was a danger of hydrogen generation that could have led to an explosion.

For good reason, the GCR design was never built again. A lot of radioactive material, about 10,000 Curies worth (mostly radioactive iodine), was released into the atmosphere. However, no one was injured or killed in the incident. As a precaution though, radiation levels in the surrounding area as well as food and milk were monitored closely. Also, milk was banned in the area for almost a month as that is where the radioactive iodine would be most likely to accumulate.

As with the NRX reactor in Canada, much was learned from this event. For one, no more GCRs! (that's one most websites don't list in the "things learned" section). Liquid is far more controllable than gas. In addition, the second annealing process was done without a procedure once it was noted that the core didn't behave as expected. Today, when something occurs unexpectedly, everything stops and evaluations take place. Also, much was learned about proper instrumentation. Also, graphite was seen not to be a good candidate as a moderator, although the Russians still used it (Chernobyl was graphite-moderated and the graphite fire there contributed the majority of contamination released into the atmosphere).

Some lessons are hard to learn I guess. In addition, the need for an emergency plan for the plant and surrounding area was clearly seen. This is standard at every plant now. The sharing of information between plants was not done back in 1957 as there was still a cloak of secrecy surrounding each nuclear plant. Lessons learned at Windscale could have prevented the 3-mile island incident 20 years later. Again, some lessons were tough to learn...

UPDATE (11/1/2010): YouTube videos of the 'disaster:'

Friday, June 26, 2009

The Fermi Chronicles - Part 16: Nuclear Events - Chalk River, CAN, 1952

This incident occurred at the Chalk River nuclear facility on December 12, 1952. The Canadian government built the Chalk River facility in cooperation with the U.S. and Britain back during WWII. It was an experimental reactor (weren't they alll back then?) called the NRX. This accident is an eerie forshadowing of the Chernobyl disaster in that a combination of mechanical and human errors led to the accident. From the Canadian Nuclear FAQ:

On December 12, 1952 a combination of mechanical failure and human error
led to a now-famous power excursion and fuel failure in the NRX reactor at AECL Chalk River Laboratories. At the time NRX was one of the most significant
research reactors in the world (rated at that time for 30 MW operation), in its
sixth year of operation.

During preparations for a reactor-physics experiment at low power, a defect in the NRX shut-off rod mechanism combined with a number of operator errors to cause a temporary loss of control over reactor power. Power surged ultimately to somewhere between 60 and 90 MW over a period of about a minute (the total energy surge is estimated to be approximately 4000 MW-seconds). This energy load would normally not have been a problem, but several experimental fuel rods that were at that moment receiving inadequate cooling for high power operation ruptured and melted. About 10,000 Curies of fission products were carried by about a million gallons of cooling water into the basement of the reactor building. This water was subsequently pumped to Chalk River laboratories' waste management facility, where the long-term ground water outflow was monitored thereafter to ensure adherence to the drinking water standard. The core of the reactor was left severely damaged.

This accident is historically important, not only because it was the first of its type and magnitude, but also because of its legacy to Canadian and international practice in reactor safety and design. Nobody was killed or hurt in the incident, but a massive clean-up operation was required that involved hundreds of AECL staff, as well as Canadian and American military personnel, and employees of an external construction company working at the site. In addition the reactor core itself was rendered unusable for an extended period.

More detail on the human error is present over at Hyperphysics:

First, four valves which kept air pressure from raising the control rods were opened in error by an operator. The supervisor noted warning lights and rushed to the basement to close the valves. Once he had closed them, he assumed that the rods had dropped back, but they hadn't dropped fully - they had dropped only far enough to shut off the warning lights.

The supervisor, realizing that the reaction was still on, called the control room to order the operator to push buttons 4 and 3 to stop the reactor, but mistakenly said 4 and 1! The operator rushed off to do it before he could correct his mistake. Button 1 raised 4 banks of control rods, causing the reaction rate to double every 2 seconds. This buildup was noted after about 20 seconds and the reactor was scrammed. Because of the air pressure problem, the control rods didn't go all the way down. After about 44 seconds, the plant physicist dumped the heavy water to kill the moderation and stop the reaction. This dumped tons of radioactive water into the basement.

More detail on the reactor itself can be found over at a site run by Peter Jedicke for his students:

NRX was contained in an aluminum cylinder, called the calandria, with a diameter of about 8 m, a height of about 3 m and its axis oriented vertically. The calandria was fitted with about 175 vertical calandria tubes arrayed in a hexagonal lattice, was filled with up to 14 000 litres of heavy water (D2O) and helium gas and sealed. The purpose of the helium gas was to prevent chemical reactions with air. The helium was held at a constant pressure of about 3 kPa above atmospheric, even as the fluid level in the calandria changed, by connection to an external helium gas holder with a capacity of approximately 40 m3. As helium was moved back and forth from the gas holder to keep the pressure constant in the calandria, the volume of the gas holder changed by means of a domed cylinder that slid up and down on sealed tracks. Thus the changing height of this dome gave an indication of gas flow to and from the calandria.

The NRX calandria tubes were sleeves, 6 cm in diameter, into which various rods could be inserted from above into the reactor. A few calandria tubes were sometimes used to insert other materials into the reactor to be irradiated as part of various experiments. Most of the tubes held uranium fuel rods, 3.1 m long, 3.1 cm in diameter, with a mass of about 55 kg. The fuel rods were originally clad in aluminum, which was chosen because it absorbed less neutrons than steel or other materials available at the time. There was actually a double-walled jacket of aluminum around the fuel rod, with ordinary water flowing between the walls as a coolant. This also left a ring of air between the outer aluminum sheath and the calandria tube. Up to 250 litres of water taken from the Ottawa river passed through the coolant sheaths of the fuel rods each second; and an airflow of about 8 kg per second was maintained through the calandria tubes. These coolant systems included considerable plumbing outside the calandria which could be altered as part of the experiments. In particular, there were storage tanks for the water and a 61 m stack for the air.

More detailed analysis of the accident at the above link. As with other nuclear incidents, a whole lot was learned. Redundant safety systems were implemented. Here's what was learned from the accident, as detailed by the Canadian Nuclear FAQ:

the need for an independent, reliable, fast-acting shutdown system, separate from routine reactor control;
the need for shutdown capability even in a reactor that is already shutdown (i.e., the safest reactor configuration may not be one with all neutron absorbers in the core);
the need for a reactor trip on rate of change in power, in addition to a high power threshold;
the importance of written and thoroughly reviewed procedures for every operational and experimental activity;
the importance of an efficient human-machine interface in the control room;
the need to balance thorough safety coverage with simplicity that does not interfere unduly with operations.

The accident also demonstrated that, due to a combination of redundant safety features, emergency procedures, and a level of inherent "forgiveness" (or robustness) in the technology, a major fuel-melt accident in a nuclear reactor can occur without significant environmental effects and radiation exposure to the surrounding population.

The NRX core was completely rebuilt, improved, and restarted within 14 months following the accident (the first time something like this was attempted), and the reactor continued to perform for another four decades before being retired.

So a lot of good came from this. I should note that no one was injured or killed in this event. Also, as a bit of trivia, one of the officers of the U.S. nuclear navy that was involved in the cleanup was one Lt. Jimmy Carter, who later would become the 39th President and, unfortunately, would be largely responsible for holding back nuclear power in the U.S. with policies some of which are still in effect to this day. Another bit of trivia - the current reactor at Chalk River, in addition to one in the Netherlands, produce about 80% of material for nuclear medicine - in the world!

UPDATE: 7/24/2009 - From the New York Times via Instapundit: Radioactive Drug for Tests Is in Short Supply. Both of the above reactors that produce nuclear material for medicine are currently shut down, leading to a shortage. Maybe they just need to dedicate a few more reactors for that purpose?

A global shortage of a radioactive drug crucial to tests for cardiac disease, cancer and kidney function in children is emerging because two aging nuclear reactors that provide most of the world’s supply are shut for repairs.

The 51-year-old reactor in Ontario, Canada, that produces most of this drug, a radioisotope, has been shut since May 14 because of safety problems, and it will stay shut through the end of the year, at least.

Some experts fear it will never reopen. The isotope, technetium-99m, is used in more than 40,000 medical procedures a day in the United States.

Loss of the Ontario reactor created a shortage over the last few weeks. But last Saturday a Dutch reactor that is the other major supplier also closed for a month.
The last of the material it produced is now reaching hospitals and doctors’ offices. The Dutch reactor, at Petten, is 47 years old, and even if it reopens on schedule, it will have to be shut for several months in 2010 for repairs, its operators say.

Tech-99m, as it is abbreviated, emits a gamma ray that makes its presence obvious. It has a half-life of six hours, meaning that it loses half its strength in that period. Thus it does its job quickly, without lingering to give the patient a big dose. But it also means the isotope must be produced and used faster than most other drugs.

Tech-99m is the product of another isotope, molybdenum-99, which also has a short half-life, 66 hours. Thus a week after it is made, less than a quarter of the molybdenum-99 remains. Stockpiling is not practical.

Molybdenum-99 is made when uranium-235 is split, but only about 6 percent of the fission fragments are molybdenum. Purification has to be done in a heavily shielded “hot cell.”

The common method is to put a uranium target into the stream of neutrons produced in the reactor as uranium is split. But the preferred material is a high-purity uranium-235, which is also bomb fuel.

The reactors’ poor condition has been obvious for a while. In 2007, Canadian safety regulators said the Ontario reactor should not restart, but the Canadian Parliament overruled them.

In 1996, the company that purifies the molybdenum from the Ontario reactor, MDS Nordion, contracted with Atomic Energy of Canada Ltd., which owns the reactor, to build two new ones. MDS Nordion paid more than $350 million for them.

But when the new reactors were started up, both showed a problem: as the power level increased, the reactors had a tendency to run faster and faster, a condition called positive coefficient of reactivity. That is a highly undesirable characteristic in a reactor, one that contributed heavily to the Chernobyl disaster in 1986. So Atomic Energy of Canada Ltd., which is owned by the Canadian government, said it would not open them.

Thursday, June 25, 2009

The Fermi Chronicles - Part 15: The Nuclear Business Model

In my years as an engineering Professor, I have worked mostly with issues involving the auto industry, albeit my research has been more fundamental in nature and applicable to many applications. Grant money, however, came mostly from suppliers of the auto industry. In doing so, I was exposed to the business model in southeast Michigan, which can be summed up in one word - proprietary. It is a very competitive business. Nothing is shared. Everything is protected. In fact, several grants that I worked on specifically stated that I was not to publish any results whatsoever for X number of years. It made sense in that industry.

Nuclear power, however, is a different animal altogether. Within the industry itself, nothing is secret. Even the most minor glitches of any kind are communicated throughout the nuclear power industry. Not next week. Right now. Daily updates. This is known as the Operational Experience (OE). After the Three Mile Island accident back in the 1970's, the NRC gave INPO (Institute of Nuclear Power Operations) reign over nuclear power plants in the U.S. in seeing to it that information is shared so that one incident need not be repeated again. In a capitalistic society, this was initially resisted by some, although there is wisdom in this in the nuclear industry particularly. In essence, one accident makes the entire industry look bad. This is very different from the auto industry where the Ford/Firestone rollover fiasco of years ago gave certain Ford cars a bad name, but had no ill effect on the rest of the Ford fleet or on Chrysler or GM. From the INPO website:

Our mission at the Institute of Nuclear Power Operations (INPO) is to
promote the highest levels of safety and reliability – to promote excellence –
in the operation of nuclear electric generating plants.

We work to achieve our mission by:

Establishing performance objectives, criteria and guidelines for the nuclear
power industry

Conducting regular detailed evaluations of nuclear power plants

Providing assistance to help nuclear power plants continually improve their
performance

INPO employees work to help the nuclear power industry achieve the highest
levels of safety and reliability – excellence – through:

Plant evaluations

Training and accreditation

Events analysis and information exchange

Assistance

These are the four cornerstones of INPO.

INPO has teeth, and INPO visits here remind me of ABET accreditation visits at OU. Not a lot of fun, but necessary. Although at OU no one gets hurt. It is far more serious here. After Chernobyl back in the 1980's, WANO (World Association of Nuclear Operators) was formed to share information globally. From the WANO website:

After the accident at the Chernobyl nuclear power plant in 1986, nuclear operators world-wide realized that the consequences had an effect on everynuclear power plant and international cooperation was needed to ensure that such an accident can never happen again.

WANO was formed in May 1989 by nuclear operators world-wide uniting to exchange operating experience in a culture of openness, so members can work together to achieve the highest possible standards of nuclear safety.

The culture of openness allows each operator to benefit and learn from others’ experiences, challenges and best practice, with the ultimate goal of improving nuclear plant safety, reliability and performance levels for the benefit of their customers throughout the world.

WANO seeks to assist members through its programmes of work;

Peer Reviews

Operating Experience

Technical Support and Exchange

Professional and Technical Development

When a WANO member participates in a WANO activity, they know that they will have highly experienced teams of experts from other nuclear power plants to help them improve safety and reliability at their own power plant.

One of the interesting aspects as I mentioned above is that even minor glitches are shared throughout the nuclear industry. I see daily emails detailing OE summaries from all over, including glitch title, date, location, significance, description, what was learned, etc. All in a great deal of detail. All of this is, of course, confidential as you just know someone outside the nuclear industry (read - the biased press) would get a hold of this and paint the nuclear industry a disaster-in-waiting. That would be quite the opposite of reality, but why ruin a long-standing, biased, media-driven narrative?

Wednesday, June 24, 2009

The Fermi Chronicles - Part 14: Neutron Moderation

Not moderation in the political sense, as that word is equivalent to 'waffling,' or 'flip-flopping.' In the nuclear industry, moderation of neutrons necessary for fission to occur is extremely important in controlling the core reaction rate. If left to its own devices, nuclear fission would not only be self-sustaining, but would chain-react to consume all the fuel in a very short time period releasing such a large amount of energy that the core would melt down, or, depending on design, would halt the nuclear reaction as some reactors are dependent upon slower neutrons. Thus, some means of control must be available. As a backdrop, recall from earlier that there are two types of neutrons - fast and thermal. When fission occurs, all neutrons are fast (about 10% of the speed of light!), but slow down as they bump into other atoms (neutron scattering). Once a few bumps occur, the fast neutrons have lost pretty much all of their kinetic energy and have the same speed as the ambient atoms floating around. They have been thermalized, or slowed down to thermal neutrons. Also, recall that fast neutrons can cause fission in all fissionable material, while thermal neutrons can only cause fission in fissile material (U-235, P-239), of which there is very little in a typical nuclear reactor core. The material used to slow down or thermalize the neutrons is called a moderator.

A good moderator should slow down fast neutrons in very few collisions, and should not absorb them to any significant extent. If it took too many collisions to thermalize the fast neutrons, then there would be a good chance of what is termed 'neutron leakage.' The ability of a material to moderate, absorb, detect, etc. neutrons is quantified in terms of a cross-section, and has units of area. For instance, in the case of a moderator, ideally it should have a large scattering cross-section, meaning that their is a very good probability that a majority of fast neutron is thermalized. A small cross-section here would be undesirable since it would certainly lead to neutron leakage. Other aspects of the moderator, however, such as the absorption cross-section, should be absolutely minimal (thus maintaining a constant neutron flux). A moderator should ideally scatter the neutrons elastically, as I wrote about in a prior entry. The most ideal moderator would be one with equal mass to the neutron - the hydrogen atom (not practical).

As it turns out, H2O ('light' water) is an excellent moderator and is used in BWRs. Heavy water, D2O is used in some PWRs, including the CANDU reactor types.

Russian reactors use graphite, which was the moderator of the infamous Chernobyl reactor. Helium has been used in GCRs (gas-cooled reactors don't seem to be ideal candidates in my book). Taking standard water as an example, it takes on average about 19 collisions to thermalize a neutron, and the H2O was given a great deal of energy in the process. The only downside is that water has a greater absorption cross-section than say heavy water, D2O, but D2O has a smaller scattering cross-section. So there is typically a trade-off with most moderators. This causes the water to increase in temperature. Water is also a good moderator from the standpoint of safety. In a PWR, if a loss of coolant accident were to occur, moderation would be lost and the reaction shut down simply from the fact that thermal neutrons would no longer be available. Loosing coolant, however, would negate the cooling of the hot fuel rods and lead to loss of geometry rendering the fuel rods useless.

Unlike the moderator, control rods have a high absorption cross-section as they are not meant to moderate, but rather to stop. I'll talk about control rods, typically made of mostly boron (I'll try to make the discussion more lively), with some halfnium, in a later installment.