Tokyo s'engage à fermer des réacteurs nucléaires
Le Monde.fr avec AFP et Reuters | 17.03.2015
Privé depuis des mois d'énergie nucléaire, le Japon se lance dans le démantèlement d'installations atomiques vieillissantes, précipité par le désastre de mars 2011 qui a déjà condamné les six réacteurs de la centrale accidentée Fukushima Dai-ichi et occasionné l'arrêt des 48 réacteurs nucléaires de l'archipel.
Mardi, la compagnie d'électricité Kansai Electric Power a opté pour la démolition de deux de ses réacteurs trop vieux et dont la prorogation de durée de vie serait trop coûteuse. Elle prévoit de déconstruire les unités Mihama 1 et 2 respectivement mises en exploitation en 1970 et 1972 dans la région de Fukui (ouest) où se trouvent plusieurs centrales nucléaires.
La société Japan Atomic Power Company a également décidé le démantèlement de Tsuruga 1 (datant de 1970), toujours dans l'ouest. Deux autres compagnies régionales, Chubu Electric Power et Kyushu Electric Power, pourraient sous peu annoncer des résolutions similaires pour leurs unités respectives Shimane 1 (1974) et Genkai 1 (1975).
PAS PLUS DE QUARANTE ANS DE MISE EN SERVICE
Depuis le drame de Fukushima, les réacteurs nucléaires au Japon ne doivent pas fonctionner plus de quatre décennies. Mais, moyennant des contrôles poussés et modifications techniques, ils peuvent en théorie obtenir une dérogation pour vingt ans supplémentaires.
Le gouvernement a demandé l'an passé à tous les producteurs d'électricité du pays de déclarer rapidement leurs intentions au sujet des installations atteignant la limite d'âge, à savoir les unités lancées dans les années 1970, au nombre d'une douzaine en plus des six de Fukushima. Pour les cinq dont le démantèlement a été décidé ou est en voie de l'être, le coût d'une éventuelle prolongation serait trop élevé au regard de leurs capacités : il s'agit de modèles de faible puissance (moins de 600 mégawatts pour chaque). A l'inverse, trois autres tranches, d'un âge voisin, Takahama 1 et 2 et Mihama 3, pourraient bénéficier d'un prolongement d'activité.
La date butoir pour une demande de prolongation est fixée à juillet 2015. Pour l'instant, seule la centrale de Sendai et deux réacteurs sur le site de Takahama ont passé avec succès les premiers tests de sécurité.
Lire : A Fukushima, l’interminable décontamination:
Le gouvernement souhaite depuis longtemps que l'industrie nucléaire abandonne les réacteurs considérés comme les plus vulnérables en cas de catastrophe naturelle, dans l'espoir de rallier l'opinion publique à la réouverture progressive des autres réacteurs.
Si tous les réacteurs étaient définitivement stoppés au bout de quarante ans, la capacité potentielle nucléaire de l'archipel chuterait à 15 % de l'électricité produite à horizon 2030, selon les calculs des organismes spécialisés, soit deux fois moins qu'avant le sinistre de mars 2011. D'où la volonté de proroger l'usage des plus puissants à défaut d'en construire de nouveaux.
Depuis cette page, voir la vidéo "Comprendre la situation à Fukushima en deux minutes
Lire aussi: Les autorités locales japonaises réticentes face à la relance du nucléaire
Three Japanese reactors to be retired
World Nuclear News, March 17, 2015
Kansai Electric Power Company and the Japan Atomic Power Company (JAPC) have decided to decommission three nuclear power plants shut down after the Fukushima accident of 2011 rather than applying to restart them.
Kansai Electric officially announced its decision to decommission units 1 and 2 at its Mihama plant, while JAPC will decommission Tsuruga unit 1. Both companies have said they had assessed the work needed to ensure the units meet requirements introduced by the Japanese Nuclear Regulation Authority (NRA) announced in July 2013.
Tsuruga-1, a 341 MWe boiling water reactor, started up in 1970 and is one of Japan's oldest nuclear reactors. Independent studies completed in 2014 confirmed that geologic faults running under the site are inactive. However, JAPC said that while it would have been technically feasible to bring Tsuruga-1 up to the standards required, the size of the project and the degree of capital investment required underpinned the decommissioning decision.
JAPC's announcement does not include Tsuruga-2, a pressurized water reactor (PWR) which started up in 1987, although the company has yet to apply for permission to restart that reactor.
Mihama-1 and -2 are both PWRs and have also been in operation since the early 1970s, unit 1 since 1970 and unit 2 since 1972. A third unit at the site started up in 1976, and was not included in Kansai's statement today.
Japan's entire fleet of 48 operable nuclear reactors have been taken out of service since the Fukushima accident of 2011. To date, the NRA has received restart applications for 19 units under the revised regulatory regime, triggering a review process to ensure that plants meet the revised criteria before being permitted to restart. Those criteria include measures such as the provision of alternative power supplies, multiple sources of cooling water, back-up control rooms and venting to prevent hydrogen escape.
With the exception of JAPC's Tokai-2, which started up in 1978, restart applications have not yet been submitted for any Japanese reactor that has been in operation for over 30 years. As today's announcements would suggest, utilities will consider carefully the economic prudence of undertaking major work on older units. More decommissioning announcements could well be on the way.
So far, four reactors - Kansai's Takahama-3 and 4 and Kyushu's Sendai-1 and -2 - have restart approval from the NRA, with preliminary permission to make the changes to the basic design of the reactor installations needed to meet the regulatory requirements. Local and prefectural government approval is also required before they can restart.
Two more power companies to scrap aging nuclear reactors
Japan Times, JUI, March 18, 2015
Kyushu Electric Power Co. and Chugoku Electric Power Co. decided Wednesday to decommission two aging reactors, following a similar move the previous day by the operators of two other nuclear power plants, amid safety concerns in the wake of the 2011 Fukushima nuclear crisis.
Kyushu Electric’s board decided to scrap the No. 1 reactor at the Genkai plant in Saga Prefecture, and Chugoku Electric Power Co. decided to decommission the No. 1 reactor at the Shimane plant in Matsue, Shimane Prefecture.
A regulation brought in following the March 2011 nuclear disaster at Tokyo Electric Power Co.’s Fukushima No. 1 plant forbids nuclear reactors from operating for more than 40 years in principle, but they may be allowed to continue operating for another 20 years if the operators make safety upgrades and the unit passes the regulator’s screening.
Demolishing the five aging nuclear reactors would generate more than 20,000 cu. meters of radioactive waste, but no decision has been made on possible disposal sites.
The waste that needs to be buried underground would fill more than 100,000 barrels, said officials at the Agency for Natural Resources and Energy. Unless sites are chosen, the waste will need to be temporarily stored within nuclear power plant sites after the reactors are dismantled.
Operators of aging plants are facing a tough decision as huge amounts of additional investment are needed to meet the new safety requirements to keep reactors operating beyond 40 years.
On Tuesday, Kansai Electric Power Co. and Japan Atomic Power Co. decided to scrap a total of three [additional] old reactors.
Kansai Electric will decommission the No. 1 and No. 2 reactors at its Mihama power plant. Japan Atomic Power Co. plans to decommission the No. 1 reactor at its Tsuruga power station. Both plants are in Fukui Prefecture.
According to the agency, low-level waste generated during demolition of the five reactors would total 21,788 cu. meters, based on estimates by the power firms as of the end of March 2014.
So-called L1 category items, the most radioactive, such as the control rods inserted into the fuel to halt criticality, must be buried at least 50 meters below ground. A total of 533 cu. meters of such waste is expected to be generated by the five reactors.
L2 category items, such as filters and waste liquid, would total 5,105 cu. meters. L3 category items, which are the least radioactive, would amount to 16,150 cu. meters, according to the estimates.
Background: March 20 2015
It has long been known that three of the reactor cores at the Fukushima Dai-ichi nuclear station suffered meltdowns. In Units 1, 2 and 3, the nuclear fuel inside the reactor core, -- made mostly of uranium and plutonium oxides, and the fuel cladding, made of zirconium metal -- over-heated and melted at a temperature of 5000 degrees F (2800 degrees C).
The resulting mass of molten core material is called "corium". The question in many people's minds has been, how far did this molten material go? Did it collect and resolidify at the bottom of the reactor vessel? Or did it melt right through the reactor vessel and resolidify at the bottom of the containment vessel? Or did it melt even further, perhaps right through the floor of the reactor building into the ground below -- as the core of the Chernobyl reactor did in 1986?
Muon tomography is a technique developed in 2003 for obtaining a three-dimensional image of the internal distribution of very heavy elements (like uranium and plutonium) inside several thick-walled and otherwise impenetrable containers. This technique is well-suited to get a picture of where the fuel in a melted-down nuclear reactor might have gone after a major meltdown accident like the Fukushima disaster.
Muon tomography is a slow process requiring months of effort, harnessing extremely the energetic muons that are produced when cosmic radiation from outer space collides with air molecules in the Earth's atmosphere, and directing those muons through the core of the reactor, painstakingly building up a picture, in sections, of where the fuel is. Muons are much more penetrating than x-rays, and they are particularly good at picking out the heaviest elements while ignoring the lighter elements found (for example) in the walls of the containment structures.
Well the results are in for Unit 1, and the answer is -- there is no fuel in the core at all. The reactor vessel is devoid of fuel. It has all melted or dripped through the bottom of the reactor vessel, and it is still unknown where it has all gone to. We are talking about 150 tonnes of very heavy, intensely radioactive material, that has melted like candle wax and simply dripped away.... Time will tell whether the same is true for Units 2 and 3.
At Chernobyl, where the core of the reactor melted right down into the ground almost 30 years ago, workers have only now finished building the largest mobile structure every constructed -- an enormous containment shell that will slide on concrete rails to fit tightly over the damaged Chernobyl reactor so that now, three decades after the accident, workers in Ukraine can begin the long and difficult job of dismantling the reactor building and recovering and packaging the many tonnes of highly radioactive corium from its underground cavity.
Muon scans confirm complete reactor meltdown at Fukushima Reactor #1
The Tokyo Electric Power Company (TEPCO) has announced that its muon tomography scanning efforts at Fukushima have borne fruit, and confirmed that nuclear plant’s Reactor #1 suffered a complete meltdown following the earthquake and tsunami that struck Japan on March 11, 2011.
Thus far, the muon tomography scans haven’t revealed anything that scientists and cleanup crews working at Fukushima didn’t expect. But that doesn’t make the work any less important. The only way to safely clean the site and dispose of the highly radioactive slag that’s now believed to fill the bottom of the Pressure Containment Vessel, or PCV, is to first map out what melted within the core and where the flow went afterwards.
Fukushima’s before and after
Muon tomography was used to scan the damaged reactor because muons can penetrate materials that absorb other imaging wavelengths, like X-rays, in their tracks. Muons have also been used to image buildings and structures like the Great Pyramid in a search for secret chambers, and to examine volcano magma chambers for evidence of imminent eruptions. Superman’s X-ray vision is actually more like muon vision, except for that whole can’t-see-through-lead restriction.
The expected position of Reactor #1’s corium
What today’s findings confirm is that nuclear fuel rods inside the reactor underwent complete meltdown. The image below shows a before-and-after shot of what a reactor looks like in normal operation and then after partial meltdown has begun. Note that the water level inside the Reactor Pressure Vessel (RPV) has dropped and the rods are melting as a result. This began to happen in Reactor #1 within hours of the tsunami. Subsequent analysis over the past few years has confirmed that there seemed to be very little nuclear fuel remaining inside the RPV. Maybe.
Did Fukushima suffer a melt-through at Reactor #1?
After first denying that a melt-through had occurred, TEPCO later changed its tune and said that it most likely had, at least at Reactor #1. This means that molten corium flowed completely through the RPV and into the PCV before being stopped by the several meters of concrete within the base. This wasn’t an entirely settled question, however, since radiation measurements and water testing have not found the isotope levels that would be expected if the majority of the corium were in direct contact with the concrete layer beneath the PCV. One alternate theory is that the seawater that was pumped into Reactor #1 after the disaster may have cooled the corium before it finished burning through the reactor pressure vessel.
Scans like the above appear to support TEPCO’s position that melt-through occurred, but the organization’s trustworthiness and understanding of the conditions at Fukushima Daiichi have been called into question multiple times since the accident. Conditions at the facility have been repeatedly misrepresented (or were simply inaccurate), and the company ignored multiple safety reports and warnings that the plant was vulnerable to a tsunami in the first place.
What happened to the fuel rods is more than an academic question. Reactor #1 contained an estimated 125 tons of uranium dioxide, zirconium, steel, boron carbide, and inconel, and finding out where the corium flowed is critical. TEPCO has announced that unlike Chernobyl, which is slowly being sealed inside a layer of concrete, they intend to scrap reactor Daiichi 1, 2, 3, and 4. This makes it particularly critical to understand where the corium is in order to facilitate its eventual removal. The scrapping process is a long one — it’ll take an estimated 30-40 years to finish, and the company won’t start removing reactor fuel until ten years after the accident.
In a very recent e-mail about the Muon Scan at Fukushima showing the Complete Meltdown of Unit 1, I incorrectly stated that "workers have only now finished building the largest mobile structure every constructed -- an enormous containment shell that will slide on concrete rails to fit tightly over the damaged Chernobyl reactor".
In fact the construction is not yet finished, but it was expected to be finished and fitted over the Chernobyl reactor in the summer of 2015. Apparently, according to World Energy News, that target date has now been pushed forward to the end of 2017. Sorry for the error.
The enormous and highly sophisticated steel structure is called the "New Safe Confinement". It stands 110m high, and it is 250m wide and 150m long, weighing in at 30,000 tonnes. It is currently being assembled 600m away from the damaged reactor. This allows the workers to work for 40 hours a week without exceeding the radiation exposure limits for atomic workers. Once it is finished the entire massive structure will be "slid" along concrete tracks to fit over the old crumbling sarcophagus.
It is called a "Confinement" structure rather than a "Containment" structure because it is not designed to hold in hot pressurized gases as reactor containments are required to do, but simply to confine radioactive solids and dusts and vapours that may be stirred up during an inadvertent collapse of the existing sarcophagus or of the damaged reactor building itself, or by the radioactive demolition work that will be taking place inside the New Safe Confinement structure.
Chernobyl New Safe Confinement (Image: EBRD)
Chernobyl Confinement reaches final stage, but funds need boost
World Nuclear News, March 17, 2015
Construction of the Chernobyl New Safe Confinement on the site of the 1986 nuclear accident is entering its final stage, the European Bank for Reconstruction and Development (EBRD) said yesterday.
Building Chernobyl's New Safe Confinement