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ITER NEWSLINE 130
Most of the time, that is ...
The system will also be designed to transport hot water and hot gas for a "baking" operation that will be planned as part of regular maintenance operations for the ITER facility.
"In much the same way that the cleaning cycle of conventional ovens uses a high-heat setting to detach grease from the oven walls," explains Raphaël Mitteau, Mechanical Engineer in the Blanket Section, "we plan to 'bake' ITER to loosen impurities that have accumulated over time on the vacuum vessel and on components facing the plasma in the divertor and on the first wall."
During "baking," the cooling water system will supply water at high temperature and pressure to increase the temperature of the in-vessel components to 200-240 °C for 100 hours.
Isn't pumping hot water through the cooling water system a contradiction in terms?
"Planning for the cooling as well as baking operations in ITER is certainly the most challenging part of designing the cooling water system," says Giovanni Dell Orco, Senior Cooling Water System Engineer. "We'd bake to higher temperatures if we could, but we're limited by how much of a temperature and pressure differential the cooling water system components can take."
Molecules of all sorts will accumulate inside the ITER vacuum vessel, either buried in the near surface of the plasma-facing components, trapped in layers of plasma-deposited material, or "adsorbed" (deposited) on their surfaces: fuel molecules, particles from the erosion of materials, oxygen ... even trace quantities of material from the manufacturing process for the in-vessel components. Removing these contaminants is extremely important to achieving high performance in ITER, as too many impurities released from the walls during tokamak operation will pollute the plasma, causing dilution of the fusion fuels and plasma energy losses through photonic radiation.
Precious tritium can also accumulate. Tritium that goes unused during the fusion reactions makes its way to the plasma edge and can become trapped in "co-deposited" layers—that is, buried with the beryllium which the plasma erodes from the first wall and redeposits elsewhere. There are regulatory limits on how much tritium can be retained inside the vacuum vessel and control of this trapped inventory is extremely important.
"Baking does two important things," explains Richard Pitts, leader of the Plasma-Wall Interactions Group in the ITER Fusion Science & Technology Department. "It allows us to remove impurity species such as oxygen, which are lightly bound on material surfaces, improving plasma purity and performance. And it leads to the release of trapped hydrogenic species (outgassing) from both the near surfaces and the co-deposited layers. It will be an important mechanism for detritiation, especially from the divertor where we expect most of the tritium co-deposition with beryllium to occur."
Molecules which have been released from the "baked" surfaces move freely about the ITER vacuum vessel until they are removed by the ITER vacuum pumps. Spectroscopic analysis of the outgassed exhaust will allow scientists to determine exactly which molecules were removed, and in what quantities.
"In ITER," explains Raphaël Mitteau, "we're planning to bake the vacuum vessel at 200 °C, and the blanket, first wall and divertor at 240 °C, adjusting the frequency of these operations as necessary." The capability to drain water from the system from time to time and replace it with hot gas will also be provided by cooling water system engineers, allowing the ITER divertor alone—where most of the tritium is expected to be trapped— occasionally to be "baked" to higher temperatures (350 °C) than the first wall.
All tokamaks "bake" their vacuum vessels to keep the vacuum as clean as possible—each with its own in-house recipe. Some use electric heaters; others gas; still others a combination of both water heating and electric. The ITER systems have been designed to accommodate 500 "baking" events. During deuterium-tritium plasma operation, approximately 40 baking cycles are planned.
Story developed with Richard Pitts, Senior Scientific Officer; Raphael Mitteau, Mechanical Engineer in the Blanket Section; and Giovanni Dell Orco, Senior Cooling Water System Engineer.
NRG) in Petten, the Netherlands in order to explore possible common activities in support of the ITER Hot Cell.
Petten is the location of the High Flux Reactor and associated Hot Cell Laboratories, which have been serving the European fusion program for many decades and ITER for several years. A new reactor called Pallas is in the planning stages which will guarantee the production of medical isotopes and the continuation of nuclear research in the future. NRG is one of the two research units of the Euratom-FOM (Fundamental Research on Matter) association.
Topics of common interest and possible areas of support were identified, such as the handling of large components in a nuclear environment, maintenance for nuclear facilities, leak tightness in hot cell design, and tritium in irradiated materials. The visit of the Hot Cell Laboratories gave the ITER hot cell team the opportunity to see a variety of workstations where nuclear R&D experiments are performed on irradiated materials, in an environment that simultaneously serves the production of industrial radio-isotopes and facility waste management.
Engraved at the foot of a large rock that looms over the road, the text, in Latin, is still clearly legible: it says that a nobleman named Claudius Dardanus, the "Praetorian Prefect" of the Gauls, and his "distinguished and noble" wife Nevia Gallia, "carved out on both sides of the mountain" a road leading to "the place called Theopolis." The 18-line text explains that Theopolis was fortified "with walls and gates" and "established as being in their ownership to be shared for the safety of all."
The stone inscription, locally known as Pierre Écrite, includes no date, however it is generally agreed that it was carved around the year 425 AD at a time when the Western Roman Empire, crumbling under Barbarian pressure, was nearing its end.
The Claudius Dardanus that is mentioned in the inscription was no ordinary citizen. As Praetorian Prefect of the Gauls he was second only to the Emperor himself. A converted Christian, his correspondence with such spiritual luminaries as Saint Augustine and Saint Jerôme has been preserved.
What brought the prominent statesman and his wife to this remote and almost inaccessible corner of the Empire is clearly stated in the text at Pierre Écrite: Claudius and Nevia came to establish a community which, in reference to Saint-Augustine, they named Theopolis, the City of God.
There are other examples of Christian communities being established by patrician Roman families in the 5th century. All of them were large estates, more like a village than a villa. What has baffled generations of historians and archaeologists however is that no trace of Theopolis has ever been found. Except for the Pierre Écrite, not a single text mentions Dardanus' City of God, nor has a single stone or artefact ever been linked to it.
Only toponymy, the scientific study of place names, has provided a tentative clue to where Theopolis might have been located. At some distance from Pierre Écrite the mountain hamlet of Théous seems to echo the name of Dardanus' estate.
While historians and archaeologists cannot provide an explanation of the fate of Theopolis and its founders, mystery buffs, amateur explorers and paranormal enthusiasts have written thousands of pages in an attempt to solve "the enigma of the Lost City."
The eerie landscape north of Pierre Écrite; the strange, unnatural-looking rock formations that surround the nearby villages; and the legends that are attached to some neighbouring chapels have provided them with just the right setting to feed their imagination.
A similar system is being installed at Réacteur Jules-Horowitz (RJH)—the research reactor that is presently under construction at CEA-Cadarache.
Seismic pads are twenty centimetre-thick "sandwiches" made of six alternate layers of rubber and metal plate. Placed atop concrete columns that rise 2.2 metres from the lower raft of the structure (the groundmat), these 90 by 90 centimetre pads support the upper raft (the basemat) that is the actual "floor" of the installation.
Each of RJH's 195 pads will each bear a weight of 550 tonnes ... and could bear considerably more. The pads are arranged in such a way that the 110,000 tonnes of the installation are uniformly distributed. The same principle will be applied to ITER, where the facility's 350,000 tonnes will be distributed over approximately 600 pads.
Seismic pads are the key to what engineers call "aseismic base isolation." Their flexible structure filters the frequency of the shake, rattle and roll—or more appropriately, the accelerations—that a hypothetical earthquake would cause. "The system is simple, robust and requires little maintenance," says Lionel Germane, a civil engineer at RJH. "It can reduce a potential acceleration of 0.70 G to a mere 0.13 G."
Despite the cost of the pads themselves, and the necessity of having two reinforced concrete rafts instead of one, there is economic benefit attached to implementing seismic pads. "Had we not chosen this solution, structural modifications to withstand the effects of an earthquake would have been much costlier."
In order to ascertain whether seismic pads can retain their quality and characteristics throughout the installations' lifetimes, a joint qualification program was implemented by ITER and RJH in 2005. "The buildings' mechanical properties rely necessarily on those of the seismic pads," explains Laurent Patisson who managed the qualification program on CEA side before joining ITER as Nuclear Building Section Leader. "It is therefore extremely important to know their variability over time and to integrate this data into the building design."
Seismic pads do age—but slowly. Accelerated aging tests and various mechanical "torture" show that they lose 20 to 25 percent of their flexibility after 75 years, which is longer than the planned lifespan of both installations, including final shutdown and dismantling phases. "In any case," says Lionel, "we must be able to replace any pad if need arises."
In both RJH and ITER, the seismic pad system is sized to withstand an earthquake whose hypothetical intensity is based on data from historical and geological seismic events.
Parameters from the 1708 Manosque earthquake, from the 1909 Lambesc earthquake, and from a "paleo-earthquake" that occurred in the Middle Durance Valley some 9,000 to 26,000 years ago have been computed and their intensity increased to define a "maximum historically-plausible" seismic event.
It is against that ghost of an earthquake that the standing army of pads will protect both installations.
The Japanese minister was welcomed at the Visitors Centre by ITER Director-General Kaname Ikeda, who later gave a presentation of the project and its status. Mr Kawabata stressed the importance of ITER for Japan and expressed his appreciation and respect to Director-General Ikeda "for having brought the project to its present stage."
When questioned by Japanese journalists about the ongoing review of Japan's science budgets, Mr Kawabata insisted that "[his] government would carry out its commitment as planned. There are two major programs that we intend to push," he said, "one of them is the next-generation fast breeder reactor, and the other one is ITER."
Mr Kawabata's trip to Europe also included a meeting in Brussels with Máire Geoghegan-Quinn, the European Commissioner for Research, Innovation and Science and in Paris with Valérie Pécresse, the French Minister of Higher Education and Research.
RJH, a nuclear research reactor that will also produce much-needed radio-elements for medical use, is partially funded by the EUR 35 billion stimulus package.
On his visit to Cadarache Prime Minister Fillon was accompanied by Valérie Pécresse, Minister of Higher Education and Research; Roselyne Bachelot, Minister of Health and Sports; and Christian Estrosi, Minister of Industry.