In the case of the Heysham 2 AGR, the reactor gas instrument pipework varies in size between 3 mm and 22 mm outside diameter with larger sections of pipe being used for blowdown manifolds. The pipework is solid-drawn cold-finished seamless stainless steel to BS3605 and is to grades 316, 321 and 359 S18. Over the outside diameter pipe range between 6 mm and 22 mm, joints are made by use of socket welds with smaller pipes having silver alloy brazed sockets. Any pipes above the: range are butt welded. Some instru­ment connections are made by means of compression fittings.

All joints between the reactor penetration and the primary isolation valve of every circuit, and ten per cent of all others are subjected to surface crack de­tection. The examination is made by the use of a low halogen red dye penetrant process HF-P, HF-R and HF-D performed by qualified personnel, thus meeting the requirements of the ASME boiler and pressure vessel code Section 5, Article 6.

The pipework and associated components are built to a design pressure of 60 bar(g) and are pressure tested at 52.5 bar(g).

All pipework external to the reactor penetration up to and including the primary isolation valves is seismically qualified by analysis, as are the structures to which the pipework is attached.

The primary isolation valve for the burst can de­tection sample pipes forms part of the fuel standpipe closure unit and the 3 mm pipes, which are external to this, are clipped to the side of the pile cap upper cable trunking for routing off the pile cap. In addi-

lion to the self-sealing coupling, there is a secondary isolation valve at the instrument rack.

To prevent precipitation of tritiated water in the instrument pipework, it is maintained at a temperature well above the precipitation point associated with the reactor pressure by means of trace heating and lag­ging. At certain controlled points the temperature is allowed to reduce to enable any moisture that is m the gas to precipitate out and be collected and drained. This system of extraction is provided to remove excess moisture during, say, a boiler leak and not to remove the normal level of moisture that exists m the reactor gas.

halves are, in general, the preferred bellows seal vale with a secondary gland seal. They are stainless ясе! with socket weld connections.