Feed water (fdw) system

to prevent a coolant backflow from the reactor in an event, e.g., the feed water line ruptures on the turbine side.

The FDW system is the piping system, which returns the feed water from the turbine main condenser to the in-vessel (RPV) spargers. The FDW system in a 1,100 MWe BWR plant consists of two main feed water lines, two headers, six riser lines and, two pairs of isolation valves across the PCV wall penetrations (each pair being installed in series on each feed water line before and after the penetrations). The reactor water cleanup system piping is connected to the main feed water lines through a recombination-T and cleaned water is returned from the cleanup system to the reactor through the feed water lines.

2. Key Components

1. Reactor pressure vessel (rpv)

Design philosophies of the RPV and its ancillary equipment follow:

1. the RPV must withstand the normal loads combined with any transient loads and the loads generated during an accident;

2. the RPV must be designed to have provisions to prevent brittle fractures, with due consideration to neutron irradiation; and

3. the RPV must be designed to allow for in­service inspections over its service life.

Figures 2.4.2 and 2.4.3 illustrate the RPV and its ancillary equipment (see also Figure 2.3.4).

The RPV consists of a body, RPV support skirts and stabilizers.

1. RPV body

Hie RPV body is a welded vertical cylinder. The RPV is made of low alloy steel of high toughness at low temperatures. The inner surface of the vessel is coated, except for the top head and the main nozzles, with deposit welding of stainless steel or nickel-based alloy (Inconel). The top head of the vessel is stud-bolted with nuts to the vessel wall. The top head flange has a double O-ring seal structure and any leak across the inner O-ring seal can be detected. The core shroud, shroud head, peripheral fuel assemblies, neutron sources, core plate, top fuel guide and steam separator are supported by the shroud support, which consists of a support plate (welded

NSRA, Japan

2-40

Figure 2.4.2 Reactor pressure vessel (RPV)

Figure 2.4.3 Control rod drive mechanism and in- core monitor housing

to the vessel wall) and a series of support legs (welded to the bottom head of the RPV). The shroud support also provides lateral support for the fuel assemblies. Various brackets are welded to the vessel wall both inside and outside the vessel, for example, to support the core internals and to absorb the forces due to earthquakes and other phenomena.

The RPV has various welded nozzles: main steam nozzles, recirculation outlet and inlet nozzles, feed water nozzles, core spray nozzles, low-pressure core injection nozzles, top head spray nozzles and various instrumentation nozzles. All the main nozzles, which inject the cooling water into the RPV, have thermal sleeves. Stainless steel or Ni-based alloy (Inconel) safe ends are welded to the nozzles that are connected to the stainless steel piping.

The integrity and leak tightness of the RPV is assured by the pre-operational hydraulic tests. Hie RPV integrity is also checked over its lifetime by in-service inspections (according to the Japan Electric Association Code, JEAC-4205). In the meantime, the gradual degrading of material toughness due to the neutron irradiation effect is known. But in the BWRs, the annulus space (the downcomer) between the core shroud and the RPV keeps the RPV well apart from the core and keeps the neutron fluence sufficiently low to prevent brittle failures. Furthermore, the irradiation embrittlement of the welds and the RPV base metal in the belt line region of the core are examined using surveillance test specimens. The test results are used to determine the plant operating conditions to prevent RPV brittle fractures (Figure 2.4.4).

2 — 41

NSRA, Japan