Standpipe penetrations

The details of the standpipe penetration through the concrete roof shield is shown in detail in Figure 8. The presence of the shield muff, the steps in the standpipe and roof rieeve and the shield plugs are such as to prevent any direct streaming lines-of-sight, even when allowances are made for expansion and/or distortion from concentric alignment due to hot conditions. 

Maximum Drilling Rate. In fast drilling operations (soft formations), the maximum penetration rate is limited by the maximum pressure available at the bit. This is the maximum allowable standpipe pressure minus the total losses in the circulating system. 

Double-liner systems are more prone to defects in the structural details (anchorage, access ramps, collection standpipes, and penetrations) than single-liner systems. 

Landfills have two LCRSs a primary LCRS and a secondary LCRS. Any leachate that penetrates the primary system and enters the secondary system must be removed. Vertical standpipes are used... 

The secondary leachate collection system is accessed by collection standpipes that must penetrate the primary liner. There are two methods of making these penetrations rigid or flexible. In the rigid penetrations, concrete anchor blocks are set behind the pipe with the membranes anchored to the concrete. Flexible penetrations are preferred since these allow the pipe to move without damaging the liner. In either case, standpipes should not be welded to the liners. If a vehicle hits a pipe, there is a high potential for creating major tears in the liner at depth. 

Fran an inspection standpoint the problon of access to the welds is considerable. The extension cones are of the order of 25 feet below pile cap level at the base of standpipes only 13 5/8" in dieuneter, these being the reactor vessel penetrations through v ich the remote inspection equipment must pass. The surface of the outer weld can only be reached by moving outside the envelope of the extension cone/GTA structures into a region of many obstructions Consequently, the coverage and quality of available photograph records are severely limited. An ultrasonic technique for examining the outer weld is attractive as it can, in principle, be applied from inside the cone. 

It has been assumed that prior to demolition of any part of the reactor vessel, the reactor Internals would have been cut up and removed, most likely through the larger permanent penetrations such as those for gas circulators and man access. This would allow the cutting and breaking up of activated material to take place within an enclosed and controlled environment. An alternative procedure which considers early removal of the standpipe region is discussed in Section 8.5. 

The standpipe assembly is contained within a 10m diameter circle of the pressure vessel top cap. It is central about the vertical axis of the reactor core and Is the means of access for fuelling and defuelling the reactor. Provision Is also made in the assembly for service penetrations used in monitoring and moderating the operational condition of the reactor. 

All the structures from the boron steel plate and above are penetrated by a vertical fuelling standpipe which allows fuel and control rods to be inserted into, and removed from, the reactor. Each fuelling standpipe serves 12 fuel channds clustered around a central control rod channel as shown in Figure 3. Multiples of the array shown in Figure 3 can be interlocked to cover the whole core completely. 

Fluxes and reaction rates were calculated in the channel pots, the boron shield, the pressure vessel, the pressure vessel nozzles, the concrete roof liner, the roof sleeve, the standpipe divided into several axial regions, the refuelling penetration muff shield and the concrete roof divided into several axial regions. In all, fluxes and reaction rates were calculated in 75 components. 

The type and layout of a reactor system including boilers, circulators, standpipes and control rods and penetrations required for instrumentations and other equipment can also affect the reinforcements. Substantial changes in the layout and design would be necessary if boilers, circulators and their closures were located within the walls rather that in the annular space. The same is true of the manner in which standpipes and control rods are arranged in the caps. 

The vessel is then taken to elasto-plastic and cracking conditions. As discussed later on, the relationships between gas load, prestressing strains, and deflections and cracks with and without the influence of creep have been established. These results (Fig. 5.29) show that where the influence of creep is not considered, the load-carrying capacity is overestimated by as much as 25%. With creep, considerable changes were discovered in the cracking pattern of the vessel and the overstress conditions in local areas (standpipes and boiler/circu-lator penetrations). 

The HTGCR vessel failure mode is moving more towards flexural criteria than shear criteria. The initiation and propagation of cracks and their positions are very different. The reason is purely the dominant role that the barrel wall/ cap slenderness ratio has played in producing flexural cracks. In plan around the boiler and circulator penetrations and in the standpipe area, most of the plastic zones are identical. 

Fig. 8.1. Sectional view of Wylfa magnox reactor (courtesy of U.K. Central Electricity Generating Board). 1, Reactor pressure vessel 2, fuel elements 3, graphite moderator 4, charge standpipes 5, guide tube assemblies 6, safety relief valve penetration 7, pile cap 8, charge machine on transporter 9, neutron shield 10, boiler 11, radial grid 12, gas circulator 13, gas circulation moter drives 14, pressure vessel prestressing cables 15, core gas inlet plenum 16, vessel man access 17, CO2 penetration 18, structural support columns 19, boiler steam and feed pipework.

The core void, of diameter 43 ft (13.1 m) is surmounted by the concrete top cap, of thickness 18 ft (5.5 m). Since the refueling system is one which permits individual access to every channel, the top cap is penetrated by a large number of standpipes, making it impracticable to pass prestressing tendons through the section above the core. Prestressing is therefore accomplished by circumferential tendons located in the outer region of the... 

A model of particular importance for the present analysis is concerned with the heatup and possible melting of the upper in-vessd structures (upper shroud head, standpipes, steam separators, and steam dryers). The shroud-head/steam-separator assembly consists of a domed base on top of whidi is welded an array of standpipes with a multi-stage steam separator located on the top of each standpipe. The entire assembly, made of stainless steel, rests on the top-guide grid and forms a cover for the core outlet plenum region. The steam dryer assanbly is mounted in the reactor vessel above the shroud-head/steam-separator assembly. Since, in tihe case of an accident, the upper shroud head may be directiy exposed to a high-temperature core, the combined effects of radiation from the core and convective/radiative heat transfer from the hot steam/gas mixture in the upper plenum, may increase the shroud temperature to failure point. When the weakened shroud head cannot support the mass above it, the upper structures may coUapse onto the core (except for the steam dryer which has a separate support system). The molten steel from these structures may penetrate the hot and partially molten core and flow into the lower plenum and, following lower head failure, into the containment. 

---