A situação está longe de estar sob controlo mas, entretanto, os ecotópicos europeus, com realce entre nós para o jornal "O Público", têm aproveitado a situação para lançarem freneticamente as mais mistificadoras campanhas alarmistas e mentirosas, falando em "apocalipse" e insinuando que se têm estado a verificar explosões nucleares na central de Fukushima I, a mais afectada das 50 existentes no Japão.
Acontece já ser possível encontrar informação objectiva, científica e rigorosa sobre o que se tem estado a passar, sendo de realçar o site do MIT que mantém actualizada informação fresca e pedagógica sobre o assunto, validada pelos profundos conhecimentos dos melhores especialistas americanos.
Vou, por isso, fazer uma transcrição da informação disponível e relevante sobre o ocorrido até ao momento.
O texto que se segue é da autoria de um eng.º nuclear americano reformado, redigido a pedido da filha (uma estudante de doutoramento do MIT), e foi depois adoptado pelo próprio MIT no seu site, como elemento pedagógico.
- As centrais nucleares de Fukushima
The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 2800 °C. The fuel is manufactured in pellets (cylinders that are about 1 cm tall and 1 com in diameter). These pellets are then put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1200 °C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form assemblies, of which several hundred make up the reactor core.
The solid fuel pellet (a ceramic oxide matrix) is the first barrier that retains many of the radioactive fission products produced by the fission process. The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.
The core is then placed in the pressure vessel. The pressure vessel is a thick steel vessel that operates at a pressure of about 7 MPa (~70 atmosferas), and is designed to withstand the high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release.
Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in (this is the part that was damaged in the explosions, but more to that later).
- Fundamentos sobre a reacção nuclear
It is worth mentioning at this point that the nuclear fuel in a reactor can never cause a nuclear explosion like a nuclear bomb. At Chernobyl, the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all structures, propelling molten core material into the environment. Note that Chernobyl did not have a containment structure as a barrier to the environment. Why that did not and will not happen in Japan, is discussed further below.
In order to control the nuclear chain reaction, the reactor operators use control rods. The control rods are made of boron which absorbs neutrons. During normal operation in a BWR, the control rods are used to maintain the chain reaction at a critical state. The control rods are also used to shut the reactor down from 100% power to about 7% power (residual or decay heat).
The residual heat is caused from the radioactive decay of fission products. Radioactive decay is the process by which the fission products stabilize themselves by emitting energy in the form of small particles (alpha, beta, gamma, neutron, etc.). There is a multitude of fission products that are produced in a reactor, including cesium and iodine. This residual heat decreases over time after the reactor is shutdown, and must be removed by cooling systems to prevent the fuel rod from overheating and failing as a barrier to radioactive release. Maintaining enough cooling to remove the decay heat in the reactor is the main challenge in the affected reactors in Japan right now.
It is important to note that many of these fission products decay (produce heat) extremely quickly, and become harmless by the time you spell “R-A-D-I-O-N-U-C-L-I-D-E.” Others decay more slowly, like some cesium, iodine, strontium, and argon.
- O que aconteceu em Fukushima em 11 de Março
When the earthquake hit, the nuclear reactors all automatically shutdown. Within seconds after the earthquake started, the control rods had been inserted into the core and the nuclear chain reaction stopped. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions.
The earthquake destroyed the external power supply of the nuclear reactor. This is a challenging accident for a nuclear power plant, and is referred to as a “loss of offsite power.” The reactor and its backup systems are designed to handle this type of accident by including backup power systems to keep the coolant pumps working. Furthermore, since the power plant had been shut down, it cannot produce any electricity by itself.
For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived (a very rare and larger than anticipated tsunami) it flooded the diesel generators, causing them to fail.
One of the fundamental tenets of nuclear power plant design is “Defense in Depth.” This approach leads engineers to design a plant that can withstand severe catastrophes, even when several systems fail. A large tsunami that disables all the diesel generators at once is such a scenario, but the tsunami of March 11th was beyond all expectations. To mitigate such an event, engineers designed an extra line of defense by putting everything into the containment structure (see above), that is designed to contain everything inside the structure.
When the diesel generators failed after the tsunami, the reactor operators switched to emergency battery power. The batteries were designed as one of the backup systems to provide power for cooling the core for 8 hours. And they did.
After 8 hours, the batteries ran out, and the residual heat could not be carried away any more. At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event.” These are procedural steps following the “Depth in Defense” approach. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator.
At this time people started talking about the possibility of core meltdown, because if cooling cannot be restored, the core will eventually melt (after several days), and will likely be contained in the containment. Note that the term “meltdown” has a vague definition. “Fuel failure” is a better term to describe the failure of the fuel rod barrier (Zircaloy). This will occur before the fuel melts, and results from mechanical, chemical, or thermal failures (too much pressure, too much oxidation, or too hot).
However, melting was a long ways from happening and at this time, the primary goal was to manage the core while it was heating up, while ensuring that the fuel cladding remain intact and operational for as long as possible.
Because cooling the core is a priority, the reactor has a number of independent and diverse cooling systems (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and others that make up the emergency core cooling system). Which one(s) failed when or did not fail is not clear at this point in time.
Since the operators lost most of their cooling capabilities due to the loss of power, they had to use whatever cooling system capacity they had to get rid of as much heat as possible. But as long as the heat production exceeds the heat removal capacity, the pressure starts increasing as more water boils into steam. The priority now is to maintain the integrity of the fuel rods by keeping the temperature below 1200°C, as well as keeping the pressure at a manageable level. In order to maintain the pressure of the system at a manageable level, steam (and other gases present in the reactor) have to be released from time to time. This process is important during an accident so the pressure does not exceed what the components can handle, so the reactor pressure vessel and the containment structure are designed with several pressure relief valves. So to protect the integrity of the vessel and containment, the operators started venting steam from time to time to control the pressure.
As mentioned previously, steam and other gases are vented. Some of these gases are radioactive fission products, but they exist in small quantities. Therefore, when the operators started venting the system, some radioactive gases were released to the environment in a controlled manner (ie in small quantities through filters and scrubbers). While some of these gases are radioactive, they did not pose a significant risk to public safety to even the workers on site. This procedure is justified as its consequences are very low, especially when compared to the potential consequences of not venting and risking the containment structures’ integrity.
During this time, mobile generators were transported to the site and some power was restored. However, more water was boiling off and being vented than was being added to the reactor, thus decreasing the cooling ability of the remaining cooling systems. At some stage during this venting process, the water level may have dropped below the top of the fuel rods. Regardless, the temperature of some of the fuel rod cladding exceeded 1200°C, initiating a reaction between the Zircaloy and water. This oxidizing reaction produces hydrogen gas, which mixes with the gas-steam mixture being vented. This is a known and anticipated process, but the amount of hydrogen gas produced was unknown because the operators didn’t know the exact temperature of the fuel rods or the water level. Since hydrogen gas is extremely combustible, when enough hydrogen gas is mixed with air, it reacts with oxygen. If there is enough hydrogen gas, it will react rapidly, producing an explosion. At some point during the venting process enough hydrogen gas built up inside the containment (there is no air in the containment), so when it was vented to the air an explosion occurred. The explosion took place outside of the containment, but inside and around the reactor building (which has no safety function). Note that a subsequent and similar explosion occurred at the Unit 3 reactor. This explosion destroyed the top and some of the sides of the reactor building, but did not damage the containment structure or the pressure vessel. While this was not an anticipated event, it happened outside the containment and did not pose a risk to the plant’s safety structures.
Since some of the fuel rod cladding exceeded 1200°C, some fuel damage occurred. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started failing. At this time, some of the radioactive fission products (cesium, iodine, etc.) started to mix with the water and steam. It was reported that a small amount of cesium and iodine was measured in the steam that was released into the atmosphere.
Since the reactor’s cooling capability was limited, and the water inventory in the reactor was decreasing, engineers decided to inject sea water (mixed with boric acid – a neutron absorber) to ensure the rods remain covered with water. Although the reactor had been shut down, boric acid is added as a conservative measure to ensure the reactor stays shut down. Boric acid is also capable of trapping some of the remaining iodine in the water so that it cannot escape, however this trapping is not the primary function of the boric acid.
The water used in the cooling system is purified, demineralized water. The reason to use pure water is to limit the corrosion potential of the coolant water during normal operation. Injecting seawater will require more cleanup after the event, but provided cooling at the time.
This process decreased the temperature of the fuel rods to a non-damaging level. Because the reactor had been shut down a long time ago, the decay heat had decreased to a significantly lower level, so the pressure in the plant stabilized, and venting was no longer required.
- Explicação das explosões nas unidades 1 e 3 (15 de Março)
Explosions at units 1 and 3 occurred due to similar causes. When an incident occurs in a nuclear power plant such as a loss of coolant accident or when power is lost, usually the first response is to depressurize the reactor. This is done by opening pressure relief valves on the reactor vessel. The water/steam mixture will then flow down into the suppression pool, which for this design of a reactor is in the shape of a torus (technical term for the shape of a donut). By blowing the hot steam into the suppression pool some of the steam is condensed to liquid phase, which helps keep the pressure low in the containment.
The pressure in the reactor vessel is reduced by venting the water/steam mixture. It is much easier to pump water into the vessel when it is at a reduced pressure, thus making it easier to keep the fuel cooled. This procedure was well underway after the earthquake. Unfortunately, because of the enormous magnitude of the earthquake, an equally large tsunami was created. This tsunami disabled the onsite diesel generators as well as the electrical switchyard. Without power to run pumps and remove heat, the temperature of the water in the reactor vessel began to rise.
With the water temperature rising in the core, some of the water began to vaporize and eventually uncovered some of the fuel rods. The fuel rods have a layer of cladding material made of a zirconium alloy. If zirconium is hot enough and is in the presence of oxygen (The steam provides the oxygen) then it can undergo a reaction that produces hydrogen gas. Hydrogen at concentrations above 4% is highly flammable when mixed with oxygen; however, not when it is also in the presence of excessive steam.
As time went on, the pressure in the containment rose to a much higher level than usual. The containment represents the largest barrier to the release of radioactive elements to the environment and should not be allowed to fail at any cost. The planned response to an event like this is to vent some of the steam to the atmosphere, just to keep the pressure under control.
Also known as the torus, this large doughnut-shaped structure sits in the centre of the reactor building at a lower level than the reactor. It contains a very large body of water to which steam can be directed in emergency situations. The steam then condenses and reduces pressure in the reactor system.
The pressure in the pool was seen to decrease from three atmospheres to one atmosphere after the noise, suggesting possible damage. Radiation levels on the edge of the plant compound briefly spiked at 8217 microsieverts per hour but later fell to about a third that.
A close watch is being kept on the radiation levels to ascertain the status of containment. As a precaution Tokyo Electric Power Company has evacuated all non-essential personnel from the unit. The company’s engineers continue to pump seawater into the reactor pressure vessel, in an effort to cool it.
Prime minister Naoto Kan has requested that everyone withdraw from the ten kilometer evacuation zone around the nuclear power plant and that people that stay within remain indoors. He said his advice related to the overall picture of safety developments at Fukushima Daiichi, rather than those at any individual reactor unit.
- Explosão na unidade 2 e incêndio na piscina de resíduos da unidade 4
It was reported earlier today that the explosion at Unit 2 of the Fukushima Daiichi plant damaged the suppression chamber. As discussed in the previous paragraph, the suppression chamber/torus (i.e. donut shape vessel containing water) is used to depressurize the reactor. The suppression pool is designed to condense the hot steam from the reactor, but can only do so as long as sufficient cold water remains in it. It should also be noted that the suppression pool is part of the primary containment.
It should be noted that unit 4 was under a 105-day long outage and that the fuel in the reactor had been moved to the spent fuel pool. Reports throughout the day indicated that the temperature of the spent fuel pool was increasing.
Current reports also indicate that the temperatures in the spent fuel pools of units 5 and 6 are also increasing.
- Acerca das piscinas de arrefecimento de resíduos
To accomplish these goals, SNF is stored in water pools and large casks that use air to cool the fuel rods. The pools are often located near the reactor (in the upper floors of the containment structure for a BWR Mark-1 containment). These pools are very large, often 40 feet deep (or larger depending on the design). The pools are made of thick concrete, lined with stainless steel. SNF assemblies are placed in racks at the bottom of these pools, so almost 30 feet of water covers the top of the SNF assemblies. The assemblies are often separated by plates containing boron which ensure a neutron chain reaction cannot start. The likelihood of such an event is further reduced because the useful uranium in the fuel has been depleted when it was in the reactor, so it is no longer capable of sustaining a chain reaction. The water in the pool is sufficient to cool the SNF, and the heat is rejected through a heat exchanger in the pool so the pool should stay at fairly constant average temperature. The water depth also ensures the radiation emitted from the SNF is shielded to a level where people can safely work around the pools.
If there is a leak in the pool or the heat exchanger fails, the pool temperature will increase. If this happens for long enough, the water may start to boil. If the boiling persists, the water level in the pool may fall below the top of the SNF, exposing the rods. This can be a problem as the air is not capable of removing enough heat from the SNF so the rods will begin to heat up. If the rods get hot enough, the zirconium-based cladding will oxidize with the steam and air, releasing hydrogen which can then ignite. These events would likely cause the clad to fail, releasing radioactive fission products like iodine, cesium, and strontium. It is important to note that each of these occurrences (cooling system failure, pool water boiling, fuel rod overheating in air, zirconium oxidation reaction) would each have to last sufficiently long in order to cause an accident, making the total likelihood of a serious situation very low.
The most significant danger if such an event were to occur is that there is no robust containment structure (like the one housing the reactor,) surrounding the SNF pool. While SNF pools themselves are very robust structures, the roof above each pool is not as strong and may have been damaged, meaning the surface of the pool may be open to the environment. As long as the water covers the fuel, this does not pose a direct threat to the environment, however it does allow for a possible dispersion of these fission products if a fire were to occur. But if the water level stays above the fuel, the threat of a large dispersion event is low.
- Ponto da situação (0h00 de 16 de Março, hora de Lisboa)
Yesterday there appears to have been a fracture in the wetwell torus (see diagram: that circular structure below and to the side of the reactor vessel) in Unit 2, caused by a hydrogen explosion, which led to a rapid venting of highly radioactive fission product gases (mostly noble [chemically unreactive] gases, the majority of which had a half-life of seconds to minutes). It also caused a drop in pressure in the supression pool, which made the cooling process more challenging. However, despite some earlier concerns, it is now clear that containment was not breached. Even under this situation of extreme physical duress, the multiple containment barriers have held firm. This is an issue to be revisited, when the dust finally settles.
Units 1 and 3, the other two operating reactors at Fukushima Daiichi when the earthquake struck, continue to be cooled by sea water. Containment is secure in both units. However, like Unit 2, there is a high probability that the fuel assemblies have likely suffered damage due to temporary exposure (out of water), as the engineers struggled over the last few days to maintain core coolant levels. Whether there has been any melting of the clad or rods remains unclear, and probably will continue to be shrouded in a cloud of uncertainty for some time yet.
The other ongoing serious issue is with managing the heat dissipation in the spent fuel ponds. These contain old fuel rods from previous reactor operation that are cooling down, on site, immersed in water, which also provides radiation shielding. After a few years of pond cooling, these are transferred to dry storage. The heat in these rods is much less than those of the in-core assemblies, but it is still significant enough as to cause concern for maintaining adequate coverage of the stored fuel and to avoid boiling the unpressurised water. There have been two fires in Unit 4, the first tentatively linked to a failed oil pump, and the second, being of (currently) unknown cause, but the likelihood is that it was linked to hydrogen gas bubbling.
There appears to have been some exposure of this spent fuel, and radiation levels around this area remain high — making access in order to maintain water levels particularly troublesome. Note that apart from short-lived fission product gases, these radiation sources are otherwise contained within the rods and not particularised in a way that facilitates dispersion. Again, the problems encountered here can be linked to the critical lack of on-site power, with the mains grid still being out of action. As a further precaution, TEPCO is considering spraying the pool with boric acid to minimise the probability of ‘prompt criticality’ events. This is the news item we should be watching most closely today.
E finalmente, para romper um pouco a barragem terrorista de alguns media sobre a radioactividade emitida no processo, transcrevo um artigo fresco da Scientific American sobre isso.
- De como a radioactividade afecta a saúde
Though radioactive steam has been released to reduce pressure within the plant's reactors and there has been additional radiation leakage from the three explosions at the plants, the resulting spikes in radiation levels have not been sustained. The highest radiation level reported thus far was a pulse of 400 millisieverts at reactor 3, measured at 10:22 am local time March 15. The level of radiation decreases dramatically as distance from the site increases. Radiation levels in Tokyo, about 220 kilometers to the southwest, have been reported to be only slightly above normal.
"We are nowhere near levels where people should be worried," says Susan M. Langhorst, a health physicist and the Radiation Safety Officer at Washington University in St. Louis.
According to Abel Gonzalez, vice-chairman of the International Commission on Radiological Protection who studied the 1986 Chernobyl disaster, current information coming from Japan about levels of radiation leakage are incomplete at best and speculations about "worst-case scenarios" are as-of-yet irrelevant.
The health effects of radiation depend on its level, type and the exposure duration.
Level of radiation:
The average person is exposed to 0.2 to 0.3 millisieverts of background radiation per year, a combination of cosmic radiation and emissions from building materials and natural radioactive substances in the environment.
The U.S. Nuclear Regulatory Commission recommends that beyond this background level, the public limit their exposure to less than an additional 1 millisievert per year. The U.S. limit for radiation workers is 50 millisieverts annually, though few workers are exposed to anything approaching that amount. For patients undergoing medical radiation, there is no strict exposure limit and it is the responsibility of medical professionals to weigh the risks and benefits of radiation used in diagnostics and treatment, according to Langhorst. A single CT scan, for example, can expose a patient to more than 1 milliSievert of radiation.
Radiation sickness (or acute radiation syndrome) usually sets in after a whole-body dose of 3 sieverts, that is, 3,000 times the recommended public dose limit per year, says Langhorst. The first symptoms of radiation sickness—nausea, vomiting, and diarrhea— can take mere minutes or up to days to manifest, according to the Centers for Disease Control. A period of serious illness, including loss or appetite, fatigue, fever, gastrointestinal problems, and possible seizures or coma, may follow and last from hours to months.
Type of radiation:
The type of radiation of concern in the current situation is ionizing radiation, which is produced by spontaneously decaying heavy atoms, such as iodine-131 and cesium-137. Ionizing radiation is so-called because it has sufficient energy to ionize atoms (change the charge on them, usually by knocking out electrons), giving them the potential to tamper with the atoms and molecules within living tissues.
Ionizing radiation takes different forms. In gamma and X-ray radiation, atoms release energetic light particles that are powerful enough to penetrate throughout the body. Alpha and beta radiation are of lower energy and can often be blocked by just a sheet of paper. However, if radioactive material is ingested or inhaled into the body, it is actually the lower energy alpha and beta radiation that becomes the more dangerous. That's because a large portion of gamma and X-ray radiation will pass directly through the body without interacting with the tissue (considering that at the atomic level, the body is mostly empty space), while alpha and beta radiation, unable to penetrate tissue, will lose all their energy by colliding with the atoms in the body and likely cause more damage.
In the Fukushima situation, the radioactive materials detected, iodine-131 and cesium-137, emit both gamma and beta radiation. These radioactive materials are products of the nuclear fission reactions that generate power in the nuclear power plants.
The Japanese government has evacuated 180,000 people from within a 20-kilometer radius of the Fukushima Daiichi plant. They are urging people within 30 kilometers of the plant to remain indoors, close all windows, and to change clothes and wash exposed skin after coming in from the outside. These measures are mainly aimed at reducing the potential for inhaling or ingesting beta-emitting radioactive material. Duration of exposure:A very high single dose (acquired within minutes) of radiation can be more harmful than the same dose accumulated over time. According to the World Nuclear Association, a single dose of 1 sievert is likely to cause temporary radiation sickness and lower white blood cell count, but does not cause death. A single dose of 5 sieverts would likely kill half of those exposed within a month. At 10 sieverts, death occurs within a few weeks.
The effects of long-term, low-dose radiation are much more difficult to gauge. DNA damage from radiation can cause mutations that lead to cancer, especially in tissues with high rates of cell division, such as the gastrointestinal tract, reproductive cells and bone marrow. But the increase in cancer risk is so small as to be difficult to determine without studying a very large population of people exposed to radiation. As an example, according to Langhorst, 10,000 people exposed to a 0.01 seivert whole-body dose of radiation would potentially increase the total number of cancers in that population by eight. However, the normal prevalence of cancer would predict 2,000 to 3,300 cancer cases in a population of 10,000, so "how do you see eight excess cancers?" says Langhorst. Lessons from Chernobyl:According to Gonzalez, some of the emergency workers at Chernobyl received several sieverts of radiation, and many were working "basically naked" due to the heat, allowing contaminated powder to be absorbed through their skin. In comparison, the Japanese workers are most likely very well-equipped and protected at least from direct skin doses. TEPCO, the company that owns the plant, has evacuated most of its workers, but 50 remain at the site to pump cooling seawater into the reactors and prevent more explosions. These workers are likely exposing themselves to high levels of radiation and braving significant health risks. "As a matter of precaution, I would limit the workers' exposure to 0.1 sievert and I would rotate them," says Gonzalez. The workers should be wearing personal detectors that calculate both the rate and total dose of radiation and that set off alarms when maximum doses are reached. "If the dose of the workers start to approach 1 sievert then the situation is serious," he says.
The thousands of children who became sick in the aftermath of the Chernobyl disaster were not harmed from direct radiation or even from inhalation of radioactive particles, but from drinking milk contaminated with radioactive materials. Cesium-137 released during the Chernobyl explosion contaminated the grass on which cows fed, and the radioactive substances accumulated in cows' milk. Parents unaware of the danger served contaminated milk to their children. "Certainly this will not happen in Japan," says Gonzalez.
When it comes to radiation exposure, professionals who frequently work with radioactive materials, whether in a hospital or a nuclear power plant, abide by the ALARA principle: "As Low as Reasonably Achievable." Radiation exposure limits are conservatively set well below the levels known to induce radiation sickness or suspected of causing long-term health effects. Temporary exposure to levels many times these limits is not necessarily dangerous.
News of the U.S. Navy repositioning its warships upwind of the reactor site, the distribution of potassium-iodide pills by the Japanese government, and images of officials in hazmat suits using Geiger counters to measure radiation levels among babies may stoke the public's fears—but for now these measures are ALARA in action, or "good extra precautions," says Gonzalez. The idea here is to always err on the side of caution.