Sub-headers

In February 1998, while readjusting the setting of SG safety valves in cold state, water leak was observed in one of the bosses in experimental thermo well in the steam sub header of one of the SG modules. The leaking boss and plug were replaced with dummy piece and all other similar welds were checked for any defects by liquid penetrant inspection. Investigation revealed that failure is due to lack of heat treatment during fabrication. 

A gap of 30 mm is provided between the two concrete to take care of differential thermal expansion. The biological shield concrete is cooled by circulating water through 180 coils embedded in concrete. The biological shield cooling (BSC) system has two distribution headers, each have 100% capacity. Each header has six sub-headers with individual isolation valves. 

Each sub-header cools 60° sector of the concrete. The coils from the sub headers are laid in the concrete in such a way that two adjacent coils are from different headers so that even if one sub-header is not available, concrete cooling is not affected. 

In August 2000, high inventory loss from the BSC system was observed. Also, water seepage was observed from the structural concrete manhole cover. When one of the sub-headers in header-A (A3), which cools 60° sector in the southwest portion of the concrete was isolated, the leak stopped. The leaky sub-header was isolated, water collected inside A1 cell (gap between steel vessel and biological concrete) was drained and reactor operation continued. There was no increase in biological shield concrete temperature. 

In May 2001, again, inventory loss in BSC system and water seepage was observed. Reactor, which was operating at low power, was shutdown and investigation was carried out. When the sub-header in header-B (B5), which again cools the same 60° sector in the southwest portion of the concrete, was isolated, the leak stopped. Detailed investigation revealed existence of minute leak in two more sub-headers in header B (B1 and B4). 

Isothermal tests conducted at higher sodium temperature of 375°C indicated that it is not possible to maintain the concrete temperature within limits as the first two leaky sub headers (A3 B5) cools the same l/6th region of the concrete. Hence reactor operation at high power was suspended. 

Injecting proprietary formulation sealants arrested the leak points in the coils of the two sub headers and the coils were tested for healthiness. For remaining coils, a global sealant treatment was carried out to arrest micro leaks and the system was normalized and power operation resumed. 

This repair consisted of replacing the circumferential welds on the hot sub-headers and the hot bends. In addition the longitudinal welds on the hot subheaders and the cold bends were also examined. This repair, currently underway, involves the following operations ... 

Mr Forbes Gower has expressed concern about flow starvation. The reduction in coolant flow in a few channels fed by the outermost sub-headers was only significant at zero power as power was raised and voidage developed the proportion of the total flow in these channels Increased. 

In the event of a loss of coolant, water is first injected from two sub-systems into the distribution headers between the common pump outlet header and the individual channel feeder pipes of the failed circuit from tanks pressurised to 10 MPa which are connected by fast acting valves. At the same time, a third sub-system... 

The System 80+ Standard Design is designed to preclude water spray from the fire protection system onto safety-related equipment. The sprinkler systems protecting the safety-related equipment is of the automatic sprinkler type. Actuation of these sprinkler systems requires the opening of the fusible link sprinkler heads and detection by combustible-products and/or heat detectors. In addition, the operator has the capability of isolating flow from the control room by isolating the Sub-sphere Building headers or, locally by manual isolation valves. 

In the Internet, each host is assigned a unique binary IP address. In the header of each IP packet there are fields for the IP addresses of the source and the destination nodes that are used to find a route through the network from the source to the destination. For scalability reasons, the address space is divided into domains and sub-domains. 

The CUSH code has been developed to provide a complete representation of a primary circuit containing.several hundred channels with appropriate pumps, steam drums, headers, tall pipes and other features associated with a practical arrangement. Each channel is represented one-dimensionally and such aspects as the appropriate single- and two-phase pressure drops and coolant density distribution are calculated. A three-dimensional specification of the nuclear power distribution is required as input to CUSH. Specifications of the feed-water flow for a given steam output are also needed to determine the sub-cooling at channel entry and may be derived from the JAVAN complete steam plant model. 

Special records are also used to specify defaults and audit data (date/time written etc). Each Section, Block, Sub-block etc. starts with a header line with a distinctive tag character, and ends with an explicit terminator hne. Some examples of SMD Format structure descriptions are given in the published draft specification. ... 

The event tree header, on the top of the picture, shows the sequence of subjects the events in the tree refer tb. The sutgects can be sub ems, components or fiinctions. In the header shown we have the following subjects ... 

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