Shroud support

Of course whenever science and religion meet, there are controversies. That is certainly the case with the Shroud. A heated debate continues today. One of the scientists, perhaps in exasperation, declared that anyone who believed the shroud was genuine was a flat-earther. The shroud supporters raised hundreds of questions about the validity of the AMS results, claiming that burned cloth was dated, that there was varnish on the cloth, that the samples came from later repairs to the shroud, that mold growing on the cloth produced a younger date, among many others. Scientists pointed out that any contaminants would have had to be present in amounts three times the weight of the sample to cause such an error. 

The RPV is a vertical, cylindrical pressure vessel of welded construction, with a removable top head, and head flanges, seals and bolting. The vessel also includes penetrations, nozzles, shroud support, and venturi shaped flow restrictors in the steam outlet nozzles. 

The jet pump diffuser is welded to the core shroud support plate and designed to recover static head from the available kinetic energy. 

The shroud support plate in jet pump plants contains two access holes (manways that provide access to the lower plenum) that are covered by welded-in or bolted to the plate. The welded-in cover plates rest on a small ledge near the bottom edge of the access hole. Access hole covers are fabricated from Alloy 600 plate and are welded to the shroud support plate with Alloy 182 or 82 as shown in Figure 2-16. Some plants have replaced welded-in cover plates with those fixed with Alloy X-750 bolts and nuts to avoid the IGSCC of Alloy 182. In Siemens BWRs, the core shroud support is made of stabilized austenitic stainless steel. The shroud support is welded to the bottom head of the RPV. It is supplemented by the... 

Shroud Support Plate Control Rod Drive Housing... 

In GE type vessel internals, AISI Type 304 and 304L stainless steel are used in various product forms in most of the internals components, as for example, core plate. Jet-pump, core shroud, steam dryer and top guide. Alloy 600 is used in shroud support. 

Fatigue thermal cracking due to rapid cycling has been discovered in several plants. For example, at Oskarshamn 1 the core shroud, shroud support and feed water skirt have experienced thermal fatigue cracks. 

IGSCC of the shroud support has been reported in several plants and has initiated in Alloy 182 welds due to tensile residual weld stress as discussed below. 

The shroud support of the Japanese BWR/2 plant showed about 300 cracks in the weld lines. These cracks were found during the core shroud replacement and initiated in Alloy 182 welds. 

Fluence levels at the shroud support are too low to facilitate lASCC. 

The shroud support in a Swedish plant showed cracks in the flanged coimection to the core shroud. These cracks were due to rapid thermal fluctuations. 

In 1999 about 300 cracks were found on the shroud support weld lines of one Japanese BWR/2 plant during the core shroud replacement. Most cracks were found in the horizontal weld to the RPV bottom (HIO weld) and perpendicular to the weld lines. These cracks initiated in Alloy 182 welds due to tensile residual weld stress. The shroud support upper than HIO weld was replaced. The cracks in HIO weld were removed and the weld was reinforced with Alloy 82 welding. Shot peening treatment was applied to remaining part of Alloy 182/Alloy 600 to decrease residual stress. 

GE SIL 462S1 - Shroud Support Access Hole Cover Cracks, February 22, 1984. 

NRC Information Notice 88-03 - Cracks in Shroud Support Access Hole Cover Welds, US. NRC (Office of Nuclear Reactor Regulation), February 2, 1988. 

CFR 50.46 Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors Provides requirements to address ECCS performance and coolable geometry with respect to various RPVIs (core shroud, support plate, etc.) [6.6]... 

GE SIL 462R1 (03/22/01) Shroud Support Access Hole Cover Cracks Discusses cracking (SCC) of the shroud support access hole cover to shroud support weld. Makes recommendations regarding susceptibility, inspection, and structural margin determinations and repair options if cracking is detected. [6.211... 

Shroud Support BWRVIP-38/-104 BWRVIP-03 BWRVIP-52 BWRVIP-62/-79... 

IGSCC of the shroud support has been observed in several operating plants. The inspection of the shroud support has the difficulty due to limited access. Although the shroud support has substantial structural margin with cracks, it is recommended to perform a planned inspection because Alloy 182 has the potential of IGSCC and ageing management of the shroud support is required. 

The BWRVIP has developed guidelines [6.42, 6.43] for inspection and flaw evaluation of core shroud supports. 

In Sweden, during the execution of the modernization project FENIX at Oskarshamn 1, ABB Atom replaced the core shroud support and core spray riser pipes. The follow up project (MAX) targets replacement of the following internals ... 

BWR Vessel and Internals Project, BWR Shroud Support Inspection and Flaw Evaluation Guidelines (BWRVIP-38), EPRI Report TR-108823, September 1997. 

BWRVIP-104 BWR Vessel and Internals Project, Evaluation and Recommendations to Address Shroud Support Cracking in BWRs, EPRI Technical Report 1003555, September 2002. 

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