Pressure Vessel Design

No. 1 reactor pressure vessel likely damaged immediately after quake

TOKYO (Kyodo) --The pressure vessel housing nuclear fuel at the No. 1 reactor of the Fukushima No. 1 power plant or its accessory piping is likely to have been partially damaged immediately after the March 11 earthquake, possibly allowing steam to leak out to the containment vessel encasing it, according to data made public by its operator.

At the reactor, the magnitude 9.0 quake registered an intensity smaller than envisaged under its quake-resistance design. But if the temblor actually caused damage to the critical reactor component, power suppliers across the country might be forced to reconsider the quake resistance designs for their reactors.

A diagram showing temperature changes at the reactor's containment vessel indicates that temperatures and pressure momentarily shot up immediately after the quake.

Mitsuhiko Tanaka, a former nuclear reactor design engineer, says high-temperature steam apparently leaked out to the containment vessel after either the reactor's pressure vessel or its accessory piping was partially damaged.

The operator, Tokyo Electric Power Co., admitted Wednesday that critical cooling piping at the same plant's No. 3 reactor may also have been damaged in the quake.

The utility, known as TEPCO, had earlier suggested that no major damage was done to the reactor, such as ruptures in the facility's main steam piping, until the arrival of massive tsunami after the quake.

"If we do our analysis on the premise that there was a leak in the piping, it matches (data) in reality," a TEPCO official said at a news conference, referring to possible damage at the No. 3 reactor. "We can't deny the possibility."

According to TEPCO, as soon as the reactor's emergency core cooling system was activated shortly after noon on March 12, pressure inside both the reactor's pressure vessel and containment vessel, which encases the pressure vessel, dropped, suggesting that steam was leaking from the cooling pipe.

While measuring devices may have malfunctioned, pressure readings corresponded to an analysis based on the hypothesis that steam did indeed leak from the piping, TEPCO said.

At the No. 3 reactor, the earthquake had an intensity greater than envisioned under resistance guidelines. The cooling piping is housed in a building designed to resist direct damage from tsunami.

"We must fully accept the fact that (the earthquake) has been cited as a possible" cause for the damage to piping, said Goshi Hosono, a special adviser to Prime Minister Naoto Kan, at a news conference.

Fabrication Of Boilers And Pressure Vessels | Thenigeriandaily.com

Depending on the laws of the country where boilers are sited, legal requirements on boiler design, construction and installation have facets in safety, explosion prevention and environmental contamination. Owners and operators must also regularly review their operations and maintenance practices to ensure compliance with local laws. Several codes and regulations for pressure vessel design and construction can be obtained from the American Gas Association, the American Society of Mechanical Engineers, American Boiler Manufacturers’ Association. Most pressure parts of boilers and pressure vessels are made from iron and iron-based alloys.  After extracting iron from ore, impurities are removed from the produced pig iron to alter its carbon content to the desired values. Further treatment with alloying elements (like carbon, manganese, nickel, chromium, molybdenum, boron, aluminium, silicon, tungsten and vanadium) will lead to the attainment of the desired physical, chemical, electrical, magnetic and thermal characteristics, which are not available in ordinary carbon-steel products. In the manufacture of boilers and pressure vessels, welding is the most popular method employed, though some are still riveted.  Using qualified and certified welders, the adoption of sound and well proven procedures for material and type of welding will produce good quality welds which can guaranty the safety and quality of the manufactured vessel.  For assembling boiler components, it is common practice to use threaded and/or expanded connections. Several types of welding exist which vary with what is being welded and for what purpose. After completion of fabrication works, it is important to check that there are no flaws in fabrication, that the welds are of good quality and that the materials of construction still retain their original mechanical and chemical properties. Non-destructive testing (or NDT) is very useful in this regard, starting with visual examination for leaks, cracks, bulges, dents, wear and loosening. A boroscope or fiberscope can be used for areas, which are difficult to see, by the naked eyes. Radiographic inspections, magnetic particle tests, liquid penetrant (dye) tests, ultrasonic inspections, eddy current testing, acoustic emissions tests, bubble emission tests, halogen leak detection, mass spectrometry and hydrostatic tests are other methods of non-destructive examinations to which pressure vessels can be subjected.

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