Source terms: the new reactor safety debate.
Source Terms: The New Reactor Safety Debate
Though not widely publicized, there is a major reexamination re·ex·am·ine also re-ex·am·ine
tr.v. re·ex·am·ined, re·ex·am·in·ing, re·ex·am·ines
1. To examine again or anew; review.
2. Law To question (a witness) again after cross-examination. under way of a fundamental risk factor underpinning a number of nuclear power regulations. It involves the "source term'--a best-guess calculation of the radioactive releases that might escape a severely damaged nuclear power plant.
Two major studies addressing the adequacy of the research base that had been used to calculate source terms were published late last year, one by an industry group, the other by the American Nuclear Society The American Nuclear Society (ANS) is an international, not-for-profit 501(c)(3) scientific and educational organization consisting of approximately 11,000 engineers, scientists, educators, students, and others with nuclear-related interests. . Both studies concluded that source terms generally had been greatly overestimated when first calculated for the 1975 Reactor Safety Study, also known as WASH-1400, or the Rasmussen Report. Over the last decade, the Nuclear Regulatory Commission Nuclear Regulatory Commission (NRC), an independent U.S. government commission, created by the Energy Reorganization Act of 1974 and charged with licensing and regulating civilian use of nuclear energy to protect the public and the environment. (NRC NRC
1. National Research Council
2. Nuclear Regulatory Commission
Noun 1. NRC - an independent federal agency created in 1974 to license and regulate nuclear power plants ) has based many of its regulations in such important areas as reactor design, reactor siting and emergency planning (including evacuation zones) on those 1975 severe-accident source terms.
According to according to
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. a long-range research plan NRC published last year, the agency's policy is to revise regulations whenever research shows they are too stringent or lax. And at a November 1984 briefing before NRC's commissioners on the American Nuclear Society's two-year source term review, William Stratton William Grant Stratton (February 26, 1914–March 2, 2001), known as "Billy the Kid", was the Republican Governor of the U.S. state of Illinois from 1953 to 1961, succeeding Adlai Stevenson in that office. , the study's director, explained that research data developed during the last decade support a reduction in the source term of somewhere between a factor of 10 and 1,000. When NRC Commissioner James Asselstine asked how widespread this conviction was, Stratton responded that it was the virtually unanimous opinion of the entire nuclear research community.
Floyed Culler, president of the Palo Alto Palo Alto, city, California
Palo Alto (păl`ō ăl`tō), city (1990 pop. 55,900), Santa Clara co., W Calif.; inc. 1894. Although primarily residential, Palo Alto has aerospace, electronics, and advanced research industries. , Calif.-based Electric Power Research Institute (EPRI EPRI Electric Power Research Institute
EPRI European Parliaments Research Initiatives ), urged NRC to commence action immediately in this area. "Sufficient evidence now exists,' he said, "to initiate the process of establishing a new source term for regulatory purposes.' Moreover, he added, "We suggest that public safety is not increased by maintaining the excessively large source term,' because it forces NRC and the nuclear industry to spread their limited resources over too broad an area.
Stratton, Culler and others, however, may have overstated o·ver·state
tr.v. o·ver·stat·ed, o·ver·stat·ing, o·ver·states
To state in exaggerated terms. See Synonyms at exaggerate.
o the unanimity of opinion regarding this issue. On Feb. 21, an American Physical Society The American Physical Society was founded in 1899 and is the world's second largest organization of physicists. The Society publishes more than a dozen science journals, including the world renowned Physical Review and Physical Review Letters, and organizes more than twenty science study group briefed NRC's commissioners on yet a third analysis of research supporting source term calculations. Conducted under NRC contract, this study agreed that in most cases, new source term calculations indicate significantly smaller quantities of radionuclides could reach the environment than the Reactor Safety Study had indicated. However, it also found that for some accident scenarios, calculated radioactive releases "have not changed dramatically.' In fact, it said current research suggests there are even some types of accidents that might yield radioactive releases exceeding those in the Reactor Safety Study.
The latter point, if accepted by the NRC staff and commissioners, could put on hold any immediate consideration of changes in nuclear regulations based on the current spate of new source term studies. It also sets the stage for a volatile new technical debate over nuclear safety.
Severe accidents of the type addressed in these studies are considered very unlikely events, emphasizes Richard Bogel, a senior scientific adviser with EPRI's source term program. "A high-probability accident,' he says, "would be in the neighborhood of 10(-7)--that would mean that an accident might occur [once in] every 10 million years for a single reactor.' Because consequences of such a low-probability event could be extremely serious, however, NRC's office of regulatory research made source term studies its top priority.
This whole reassessment of unclear source terms began immediately following the Three Mile Island (TMI TMI Too Much Information
TMI Three Mile Island
TMI TRMM Microwave Imager
TMI Transactions on Medical Imaging
TMI Texas Military Institute
TMI Teen Missions International
TMI Tauber Manufacturing Institute ) accident in 1979, explains Susan J. Niemczyk, a Washington-based nuclear safety consultant. Until last year she had been working under NRC contract on source term analyses at Oak Ridge Oak Ridge, city (1990 pop. 27,310), Anderson and Roane counties, E Tenn., on Black Oak Ridge and the Clinch River; founded by the U.S. government 1942, inc. as an independent city 1959. (Tenn.) National Laboratory. Many people in the safety community, she says, were surprised when radioactive releases from TMI turned out to be only a fraction of what the source term would have predicted for a presumedly less serious accident. Reasoning that the source term must be overly conservative to have been so far off in predicting what TMI emitted into the environment, NRC and the nuclear industry began reexamining the technical basis for source term calculations. What resulted are these three new studies.
Among their new findings:
Reactor containment buildings are stronger than assumed by the Reactor Safety Study, and therefore if they fail, it will be much longer into the accident than had been originally expected. That's important because in the first hours to days after an accident is initiated, many of the more biologically hazardous radioactive aerosols will settle out, adhering to surfaces in the reactor vessel--and, if the vessel breaches, onto structures within the surrounding containment building. Since these aerosols are the primary contributor to source terms, it's expected that the later a containment breaches, the less radioactive material radioactive material Radiation A substance that contains unstable–radioactive–atoms that give off radiation as they decay. See Radioactive decay. will escape.
Physical and chemical phenomena, previously neglected, can lead to a scavenging scavenging
of anesthetic. See anesthetic scavenging. or retention in the reactor of important radioactive materials--preventing their escape into the environment. For example, new data suggest that much of the iodine previously expected to escape as gaseous molecular iodine will in fact react with cesium cesium (sē`zēəm) [Lat.,=bluish gray], a metallic chemical element; symbol Cs; at. no. 55; at. wt. 132.9054; m.p. 28.4°C;; b.p. 669.3°C;; sp. gr. 1.873 at 20°C;; valence +1. to form a relatively nonvolatile salt that could either dissolve in water or condense con·dense
v. con·densed, con·dens·ing, con·dens·es
1. To reduce the volume or compass of.
2. To make more concise; abridge or shorten.
a. as an aerosol onto reactor building A reactor building is a general term for a building that houses a reactor of some type. In particular it often refers to a building containing a nuclear reactor. This can also be used to refer to pressure sealed buildings containing nuclear reactors, though it would be more correct surfaces. In fact, it was the near absence of the iodine in the releases from TMI that initially focused attention on the source term issue.
There may be additional potential fission-product retention sites within a reactor complex. These include the presure "suppression pools' (wells in boiling water reactors where steam can condense during an accident, but which might also filter out some fission products A general term for the complex mixture of substances produced as a result of nuclear fission. ); ice condensing con·dense
v. con·densed, con·dens·ing, con·dens·es
1. To reduce the volume or compass of.
2. To make more concise; abridge or shorten.
a. systems in some pressurized water reactors; and auxiliary buildings.
The American Nuclear Society report says that such technological advances since the Reactor Safety Study in 1975 have eliminated concern that steam explosions and short-term overpressures might breach reactor containment buildings. Similarly, Tony Buhl, director of the four-year-old Industrial Degraded Core Rulemaking (IDCOR IDCOR Industry Degraded Core Rulemaking ) program, admits that there are still some "open issues' in the technical arena, but says, "I don't see any big unknowns.'
Regarding hydrogen generation, for instance, and its potential for detonation, he says, "This is an area that we [at IDCOR] spent a lot of time looking at'--even to the point of scrutinizing the results of large-scale experimental tests by the industry. IDCOR, a consortium supported by more than 60 organizations in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. , Finland, Sweden and Japan, now has its own severe-accident computer codes for generating source terms. Buhl says that these codes include hydrogen explosions --where the scenarios suggest they would occur--or the melting of fuel, even the ejection of melted fuel from a pressurized pres·sur·ize
tr.v. pres·sur·ized, pres·sur·iz·ing, pres·sur·iz·es
1. To maintain normal air pressure in (an enclosure, as an aircraft or submarine).
2. reactor vessel reactor vessel
The protective containment vessel surrounding the nuclear fission core in a nuclear reactor. onto a concrete mat. So these issues are no longer in the realm of the "big unknowns,' he says. Moreover, in an interview he suggested that even though uncertainties remain in some of these areas, source terms could probably be reduced somewhat.
Niemczyk disagrees. Having worked on the modeling of releases for nuclear accidents, she believes that the uncertainty associated with several of these technical issues is still quite high. It's her contention, in fact, that still-unknown factors associated with several potential accident effects might, in the worst case, actually contribute to source terms exceeding those calculated in the new industry studies. Among the potential problem areas she cites are hydrogen explosions, steam explosions, the high-pressure ejection of molten fuel from a reactor vessel, and interactions between molten fuel and the concrete base malt (floor) beneath the reactor vessel.
To gauge how much radioactive material might escape in an accident, researchers have to model that accident. What makes this problem so difficult, Niemczyk explains, is that there isn't just one type of accident that can occur, one type of plant that can be affected or one scenario for how an accident might develop; there can be hundreds of possible variables for each.
"Technically,' she says, "it's one of the most complicated problems we've had to look at.' Although better and better computer codes exist to model severe reactor accidents, each requires the inclusion of some critical numerical values that define the accident and initial condition of the reactor. However, Niemczyk points our, "in a lot of these things "These Things" is an EP by She Wants Revenge, released in 2005 by Perfect Kiss, a subsidiary of Geffen Records. Music Video
The music video stars Shirley Manson, lead singer of the band Garbage. Track Listing
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2. , our level of ignorance is such that we just don't know Don't know (DK, DKed)
"Don't know the trade." A Street expression used whenever one party lacks knowledge of a trade or receives conflicting instructions from the other party. what will happen. So we guess.' That's why, she says,
"if you talk to 10 different people you get 10 different answers.' Morever, she contends, severe-accident computer codes are so enormously complex that relatively few people are able to use them knowledgeably.
Authors of the Reactor Safety Study, and of the NRC regulations that have been based on its source terms, had been concerned by the degree to which their analyses had relied on this guessing. To account, in part, for the possibility that their best engineering judgment (these guesses) undervalued Undervalued
A stock or other security that is trading below its true value.
The difficulty is knowing what the "true" value actually is. Analysts will usually recommend an undervalued stock with a strong buy rating. or ignored critical factors, these authors deliberately wrote a conservative "fudge factor' into their calculations --an extra factor of 10 or 100 here and there to allow for errors in their original assumptions.
The American Physical Society (APS) study group shares some of these concerns about the computer codes used to model accidents. In academia, notes APS study chairman Richard Wilson There have been many people named Richard Wilson, including:
Harvard College, originally for men, was founded in 1636 with a grant from the General Court of the Massachusetts Bay Colony. physicist, for the many important computer codes used to model source term features, "that process has only just begun.'
There is near-universal agreement that more experimental work could be carried out in the areas Niemczyk mentioned, and that peer review should continue on the computer codes to which Wilson refers. Where agreement breaks down is over the potential in these areas for increasing source terms above the levels estimated by the American Nuclear Society and by the nuclear industry's IDCOR program.
Like Niemczyk, Marshall Berman Marshall Berman (born 1940) is an American Marxist Humanist writer and philosopher. He is currently Distinguished Professor of Political Science at The City College of New York and at the Graduate Center of the City University of New York, teaching Political Philosophy and Urbanism. is among those who challenge the assertion that enough is known to claim that source term codes account adequately for several of the more important potential accident sequences. At Sandia National Laboratories Sandia National Laboratories, which is managed and operated by the Sandia Corporation (a wholly owned subsidiary of Lockheed Martin Corporation), is a major United States Department of Energy research and development national laboratory with two locations, one in Albuquerque, New in Albuquerque, N.M., Berman supervises NRC-sponsored work in the experimental investigation of both steam and hydrogen explosions, and the writing of nuclear-accident computer codes that model these for use in source term analyses.
Modeling of the events leading up to hydrogen detonation and of detonation itself "is almost absent,' Berman says. "And there aren't any models of it at all in the reactor safety codes.' Similarly, he contends that the modeling of steam explosions "is very primitive, or absent.' In fact, he says, "there are no mechanistic models that have been experimentally verified of steam explosions in any code. Either the code user assumes that they don't occur, or there's some branch point [in the logic] and some assumptions made.' For instance, Berman is familiar with an IDCOR code, called MAAP MAAP Movement Activist Apprenticeship Program
MAAP Minnesota Association of Alternative Programs
MAAP Master Air Attack Plan
MAAP Maritime Academy of Asia and the Pacific (Bataan, Philippines) , which deals with steam explosions; "that model assumes that large steam explosions never occur,' he says. Berman concedes IDCOR may be correct in assuming that these won't occur. But right now, he says, "we don't know that.'
The term "steam explosion' is a bit of a misnomer misnomer n. the wrong name.
MISNOMER. The act of using a wrong name.
2. Misnomers, may be considered with regard to contracts, to devises and bequests, and to suits or actions.
3.-1. . It's not a chemical process, but a physical one. "In fact, it's just boiling,' Berman explains, "but boiling at such a fast rate that it's similar to an explosion, in the sense that shock waves can form.' These melt/water interactions, occurring when melted fuel drops into relatively cool water, or vice versa VICE VERSA. On the contrary; on opposite sides. , create a reaction somewhat analogous to the violent interaction that occurs between hot oil and water.
Berman believes that "uncertainty concerning the nature and violence of such [steam] explosions is very high.' On the one hand, he explains, "We have shown that at the scales at which we study these phenomena--20 to 25 kilograms--explosions occur. And [computer] models have been developed that in fact explain observations at this scale.' But these same models predict that as the scale increases --that is, as the mass of material involved increases--the energetics en·er·get·ics
n. (used with a sing. verb)
1. The study of the flow and transformation of energy.
2. The flow and transformation of energy within a particular system. of the explosion will go down. If that is true, Berman says, "then there's very little threat to reactors, because reactors are big and could easily take the kinds of explosions that we see here.'
"My concern,' he says, "is that fluid mechanics fluid mechanics, branch of mechanics dealing with the properties and behavior of fluids, i.e., liquids and gases. Because of their ability to flow, liquids and gases have many properties in common not shared by solids. is very complicated, and people have frequently been wrong in the past.' Therefore, he refuses to accept the models' predictions until there has been experimental validation of the phenomena at higher scales. "We have proposed some experiments up to the 2,000-kg scale-- that's about 2 tons,' he says. "Most researchers believe that's large enough to demonstrate the validity or lack of it in the models.' To date, however, NRC has not provided funds for these tests.
Berman is also trying to make modeling of hydrogen combustion more realistic. Of the several forms of hydrogen combustion that could occur during a reactor accident, detonation--the type of combustion that takes place so fast that shock waves form--poses the most serious source term risk. A large pressure spike initiated by the detonation could weaken or rupture a reactor's containment building, allowing radioactive aerosols to spew into the environment. What's more, the violence of these events means that there's always a possibility that high-velocity missiles--from piping to shards of concrete --might be hurled at the containment building's walls.
Combustion experts once scoffed at the idea that major detonations might occur under the conditions that could be present in a nuclear accident. (Although it's now generally conceded that such detonations are physically possible, many safety engineers still discount the possibility that the specific conditions necessary to cause them will ever develop). "But here's where the conventional wisdom was quite wrong,' Berman says. Through a series of combustion experiments at Sandia, he says, "we showed that there's a close link between the probabilities of detonation and the size of the gas cloud that you're dealing with.' As a result, under the severe conditions that might be present in a core-melt accident, he says detonations in large volumes--like a reactor containment building--may be possible. In fact, Berman admits that experiments in this area fuel his skepticism over large-scale effects that the models predict for steam explosions. "If there's an analogy between combustion and steam explosions,' he explains, "then the opposite of the conventional wisdom might occur.'
Dana Powers, also at Sandia, supervises NRC-sponsored experiments and computer-code building in three other areas of severe-accident effects influencing source terms: fission-product revaporization, the high-pressure ejection of melted fuel from a reactor's pressure vessel Pressure vessel
A cylindrical or spherical metal container capable of withstanding pressures exerted by the material enclosed. Pressure vessels are important because many liquids and gases must be stored under high pressure. , and interactions between melted fuel and concrete. Because there is still so much uncertainty about the science governing these effects, he, like Niemczyk, feels it is probably too soom to advocate a factor of 10 reduction in source terms.
Take revaporization, a phenomenon that's been recognized only since the Reactor Safety Study. Aerosols that form when fission products vaporize va·por·ize
To convert or be converted into a vapor.
To dissolve solid material or convert it into smoke or gas. will, as they decay, deposit heat onto whatever they've become attached to. Their decay heat Decay heat  is the heat released as a result of radioactive decay. This is when the radiation interacts with materials and the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms. , explains Powers, raises the temperature to the point where vapor pressure vapor pressure, pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid. is sufficient to revaporize--and, therefore, remobilize--aerosols late in the accident sequence.
In some calculations, most of the source term is composed of this revaporized material. "But nobody has actually observed this process, Powers notes. "So while we've identified and named the phenomenon, we don't have experimental verification of its magnitude or importance. We only have computer codes. And if you've never seen something,' he asks, "how good can your modeling of it be? I have to be a little suspicious.' He notes, however, that experimental verification of this phenomenon is currently under way, both at Sandia and Argonne National Laboratory Argonne National Laboratory, research center, based in Argonne, Ill., 27 mi (43 km) SW of downtown Chicago, with other facilities at the Idaho National Engineering Laboratory, 50 mi (80 km) W of Idaho Falls, Idaho. Founded in 1946 by the U.S. , near Chicago.
Another concern recognized only recently, he says, is the possible high-pressure spritzing of melted fuel through small holes in the bottom of a reactor vessel --a phenomenon that could spray radioactive material throughout the containment building much the way a carbonated drink will brupt from a shaken bottle. A lot of high-temperature aerosols would form. If zirconium zirconium (zərkō`nēəm), metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.22; m.p. about 1,852°C;; b.p. 4,377°C;; sp. gr. 6.5 at 20°C;; valence +2, +3, or +4. , a metal in the fuel's cladding, were exposed to air, it would oxidize oxidize /ox·i·dize/ (ok´si-diz) to cause to combine with oxygen or to remove hydrogen.
1. To combine with oxygen; change into an oxide.
2. , creating a tremendous amount of additional heat. "Such high temperatures in an enclosed volume means you get high pressure that might break containment,' Powers explains.
At this time, the primary unknowns are what the heat load might be and whether any support structures in the containment building might serve either as a heat sink A material that absorbs heat. Typically made of aluminum, heat sinks are widely used in amplifiers and other electronic devices that build up heat. Small heat sinks are the most economical method for cooling microprocessors and other chips. or as an obstruction to limit the aerosols spraying up from under the reactor vessel. "In fact,' Powers told SCIENCE NEWS, "it should be a relatively easy phenomenon to model once we understand what it is we're trying to model and what kind of parameters to put into our models.' Obtaining the data to do that, he says, "is constrained right now only by the fact that our researchers have to sleep and that all our equipment has not arrived.'
Finally, regarding that interactions between melted fuel and concrete, Powers notes that "one of the things we've observed fairly recently is that there is a release of radioactive material occurring during the melt/concrete interaction that people had not anticipated. They had thought that the releases would be short in duration and not very large in magnitude. In fact, we find quite the opposite.' The interaction would also release a class of very hazardous radioactive materials that, in terms of source terms, had previously been all but discounted. Called nonvolatile fission products, they do not vaporize except at extremely high temperatures. Moreover, as the high-temperature melted fuel interacted with the water in the concrete, it would generate copious amounts of steam--carbon dioxide too, if there had been much limestone in the concrete.
"So we not only have the temperatures necessary to vaporize materials, but we also have the force to carry them up out of the melt and into the reactor containment,' Powers says. Yet to date, he adds, few analyses have recognized the radioactive releases that could result from this phenomenon in their source term calculations --not because their authors haven't looked at it, but because "they used models that were just incorrect.' Sandia's models are based on some of the few actual experiments conducted in this area. Typically involving about 200 kg of melted uranium dioxide uranium dioxide
A black, highly toxic crystalline powder, UO2, once used in ceramic glazes and gas mantles, now used primarily to pack nuclear fuel rods. and zirconium dioxide, "our experiments are not small,' Powers says. They show that temperatures of the melt, its composition and the types of concrete used can all have a profound effect on both the type and amount of gases and fission products formed.
According to Powers and Berman, their research could resolve uncertainties associated with the phenomena they are investigating within two years. Some, like Niemczyk, argue that NRC should wait out the results of these studies before considering changes to many important power plant regulations. Others, like Stratton and Culler, have suggested that the wait isn't necessary. In the next few weeks, NRC is expected to begin deciding which tack it will take.
Photo: Generation of flammable gas during concrete /molten-core material interactions.
Photo: Schematic of one boiling water reactor and its containment.
Photo: Left, Sandia's steam explosion facility is used to conduct not only steam explosions but also hydrogen combustion tests. The latter focus on studies of deflagrations, a relatively slow form of combustion, involving mixtures of steam, hydrogen, air and water droplets. Below, a steam explosion: Left frame records 5 kg of melted fuel falling into a tank of room-temperature water. Three remaining frames (l to r) record their interactions at 0.05-second intervals beginning at 0.15 second. Among recorded measurements is the conversion of heat into the kinetic energy kinetic energy: see energy.
Form of energy that an object has by reason of its motion. The kind of motion may be translation (motion along a path from one place to another), rotation about an axis, vibration, or any combination of imparted to exploded debris.
Photo: Above, series of photos (l to r) recording high-pressure (600 lb/in(2)) ejection of melted reactor-core material onto a concrete slab Concrete slab
A shallow, reinforced-concrete structural member that is very wide compared with depth. Spanning between beams, girders, or columns, slabs are used for floors, roofs, and bridge decks. , beginning (far left) at 0.05 second into the experiment. Second frame, at 0.1 second, shows vapor condensation. Third frame, at 1.15 seconds, captures the maximum aerosol cloud. In the final frame, at 1.95 seconds, the ejection is completed. These experiments quantify production of aerosols and flammable gases. Right, Martin Sherman measures irregular diamond-shaped detonation cells (barely detectable in photo) left on the surface of a sooted metal sheet that had lined the inside of Sandia's Heated Detonation Tube. The larger the cells, the less detonable det·o·na·ble
That can be detonated: detonable warheads; detonable bombs. the mixtures.