The stack has the purpose of exhausting gas from the standby gas treatment system and main exhaust duct and it stands near the reactor building on a bedrock foundation. In a multi-unit plant, it is often made as a common facility. The height of the stack is determined based on the results of meteorological research and wind tunnel tests.

2. Advanced boiling water reactor (abwr) plant arrangement

Figure 2.2.8 illustrates the building arrangement of a twin-unit l,350MWe class plant with Ldype slide- along layout

The main features of this example are as follows.

1. The reactor building is not a combined structure and the radioactive waste treatment building is also an independent separate building.

2. The PCV is a reinforced concrete structure with a steel lining inside.

3. The reactor water recirculation pumps are internally located in the RPV to get a compact PCV and reactor building.

4. The main steam turbine utilizes 52-inch last­stage buckets and moisture separator re­heaters for vastly improved performance.

5. The main steam piping enters the turbine building from the side of the high pressure turbine to make the building shorter and more compact

6. The moisture separator/ re-heaters are located

on the turbine operating floor and a common radioactive waste treatment system in an independent building is located between the two turbine buildings. This provides for the turbine building volume reduction further, the floor space of the common radioactive waste building may be used as a temporary turbine parts lay-down area during the outage.

7. The main control room is common for two units and located in an independent building between the two reactor buildings. The emergency electrical panel room is with the main control room.

(li) The service building is common for two units and is located on one side of the control building to make access to both units easy.

2- 15

NSRA, Japan

radwaste building 2F

unit No.6 turbine building 2F

about

A—A section

Figure 2.2.8 Advanced boiling water reactor (ABWR) plant arrangement

NSRA, Japan

2- 16

3. Nuclear Reactor and Core

1. Fuel Rod and Fuel Assembly

1. Structure of fuel rod and fuel assembly

1. Fuel rods

The material used for BWR fuel is uranium dioxide (UO^. It is compatible with water upon accidental contact and can be used at high power densities owing to its high melting point (2800°C). Its density remains stable even for long-term neutron irradiations. A fuel rod is a long tube containing sintered pellets of slightly-enriched UO₂ stacked in a Zircaloy-2 cladding. The ends of the cladding are sealed by cylindrical Zircaloy-2 end plugs welded to the cladding. Zircaloy-2, a zirconium alloy, has been chosen as it meets all the nuclear and physio-chemical requirements for nuclear fuel cladding: sufficient mechanical strengths at elevated temperatures, low neutron absorption properties, low interactions with coolant, and good stability.

In recent years, a new type of cladding, zirconium liner cladding, has been developed and is now widely used in BWRs. The zirconium liner cladding*⁴ eliminates the need for a pre­conditioning operation (Section 4.1.3) and facilitates the load following operation without restriction. The zirconium liner tube is a Zircaloy-2 tube with a pure zirconium liner (a thin layer of approximately 0.1mm in thickness) on its inner surface.

Gadolinium loaded fuel rods have sintered pellets of uranium-gadolinium mixed oxide with very low gadolinia (GcbO_:t) contents (cf. Section 2.3.2.(3)). Figure 2.3.1 shows a typical fuel rod and its structure.

2. Fuel assembly

Figure 2.3.2 shows, as an example, the structure of a 9x9 fuel assembly (Type A) which has 74 fuel rods and two water rods. Eight fuel rods are short-sized, being approximately two- thirds of a standard rod length.

Prior to development of 9x9 fuel assemblies, reload fuel of high burn up 8x8 assemblies with 60 fuel rods and one water rod of large diameter were in use. MOX fuel assemblies with the same structures as the 8x8 assemblies are being planned. In the latest ABWRs, the 9x9 fuel

assemblies are loaded from the very beginning. Figure 2.3.3 shows fuel rod arrangements. The fuel rods are encased in a Zircaloy channel box. Eight fuel rods (tie rods) on the periphery act as the structural members in both 8x8 and 9x9 types, joining the top and bottom support plates (tie plates). Partial-length fuel rods are fixed to only the bottom tie plate.

Water rods, besides their function as the structural support for seven axial spring spacer grids, facilitate flattening the power distribution in each fuel assembly. They are called water rods because the openings on their top and bottom surfaces allow for the coolant to flow through their center cavity.

Spacer grids maintain fuel rod spacing. The upper ends of the fuel rods and water rods are axially supported by expansion springs, so that the rods can move freely inside the top tie plate openings. This support structure and the optimum design of spacer spring constants allow the fuel rods and water rods to expand freely in the axial direction. BWR fuel assemblies have special design features in order to avoid any mistakes in their orientation when loading the core, such as handle orientation of the upper tie plate and the position of the channel fastener.

The fuel assemblies are vertically loaded to form an overall cylindrical configuration.

With irradiation experiences obtained over more than a quarter century, BWR fuels have been improved in design to provide high performance and high reliability which are associated with increased fuel burn up. The 9x9 fuel assemblies were developed for higher burn up, up to about 45,000 MWd/t from conventional burn up of 39,500 MWd/t in 8x8 fuel assemblies with the aim of reducing the amount of spent fuel and the fuel cycle cost.

*⁴⁾ Zirconium liner cladding has a pure zirconium layer on its inner surface which has better ductility than the conventional Zircaloy-2. It is effective to reduce the pellet­clad mechanical interactions (PCIs) by preventing the development of excessive cladding stresses caused by thermal expansion of fuel pellets.

2- 17

NSRA, Japan

2.6m

— 0.26m

(Upper plenum)

— 0.15m

(Lower plenum)

— 2.2m

(Fuel effective length)

Part-length fuel rod