Power supply for high-frequency heating

[Heating]

Pipe external surface

Pipe internal surface

[After cooling]

ripe external surface Pipe internal surface

’«=Yield stress

Tensile

Compression

Stress distnbution

Defomation ^bution

Figure 4.3.7 Principle of IHSI

(b) Corrosion resistant cladding (CRC)

This is a technology to mitigate SCC susceptibility by covering the wetted pipe inner surface in a sensitized region near a weld with sensitization-resistant metal deposit. There are various methods for this technology, and a representative example is shown in Figure 4.3.8 which secures a high-ferrite layer in the wetted surface of a weld.

CRC to the first lever

CRC to the second layer

(As-welded)

(Stress improvement etc. with solution heat treatment or surface polishing)

Figure 4.3.8 Principle of crc

3. Solution heat treatment (SHT)

Cr carbide which has precipitated in the grain boundary will, when heated to an elevated temperature (1000 °C or more), decompose and dissolve uniformly into a base metal, eliminating the Cr-deficient layer near the grain boundary. This heat treatment is called SHT. It can remove a sensitized region of material due to welding and also reduce residual stress at the same time. This technology must be applied to joints produced in a factory since it requires a heat-treating furnace.

4. Inner surface polishing

This technology reduces SCC by polishing the pipe internal surface for the purpose of mitigating internal surface residual stress, and thereby shifting the stress on and very close to the surface to compression stress.

5. Heat sink welding (HSW)

In HSW, the pipe internal surface is welded while cooling it with flowing water or spray after making a barrier of up to 2 or 3 layers by air welding as shown in Figure 4.3.9. In this way, residual tensile stress near the internal surface of the pipe weld, which is an SCC factor, makes use of the thermal stress due to a temperature difference in the through-thickness direction

Heat sink welding

(remaing layer after air welding)

Figure 4.3.9 Principle of HSW

Air welding (up to 2nd or 3rd layer)

Water cooling of inner surf.

NSRA, Japan

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produced by HSW between the outside and internal surfaces. HSW has been confirmed to reduce the residual tensile stress better than conventional air welding.

b. Repair technologies

1. Internal grinding

This is a technology to prevent crack propagation by removing the crack from the internal surface by using an internal grinding device (e.g., vacuum manipulator). For the area where the crack was removed, recurrence of cracking there is prevented by removing the region with high susceptibility to SCC by surface finishing.

2. Weld-overlay (WOL)

This is a repair method to weld-overlay the outside surface of a pipe weld with SCC by using a weld metal of high-ferrite content with excellent resistance to SCC; i.e. just like wrapping the pipe (Figure 4.3.10). The strength lost by the pipe part with the repaired cracks is compensated for by the belt-like weld-overlay (Figure 4.3.11).

Figure 4.3.10 Conceptual diagram of WOL

WOL portion with structural strength

Axial cross section of pipe with WOL

Figure 4.3.11 Conceptual diagram of wol pipe cross section

ii) Preventive maintenance technologies against pipe wall thinning

For piping design of BWR plants, a corrosion allowance is taken into account in selecting pipe wall thickness, and wall thickness measurements are periodically conducted. However, prompted by the pipe rupture accident due to wall thinning at Unit 2 of the Sally Nuclear Power Plant in December 1986 in the US, and the secondary system pipe rupture accident due to the wall thinning event at Unit 3 of the Mihama Nuclear Power Plant in August 2004 in Japan, pipe wall thinning management was systematized taking into account new knowledge of pipe wall thinning phenomena, and was standardized as a code by the Japan Society of Mechanical Engineers. This code was endorsed by Ministry Order No. 62, and pipe wall thinning management is conducted now according to this code. Pipe wall thinning phenomena subject to this code, places to be inspected for wall thinning, and their inspection frequencies have all been clarified. Wall thinning phenomena and management methods in BWR plants are shown in the following.

1. Pipe wall thinning phenomena subject to the JSME code

1. Flow accelerated corrosion (FAC)

Corrosion predominantly governed by chemical action accelerated by turbulence of flow, although the fluid impulsive force is small, is called FAC. A characteristic of pipe wall thinning due to this phenomenon is that uniform wall thinning occurs widely in a region where flow turbulence is produced. A schematic diagram of FAC is shown in Figure 4.3.12.

2. Liquid droplet impingement (LDI)

This is a phenomenon that when liquid droplets of wet steam impinge on the pipe wall surface at high speed in a piping system containing wet steam, the produced impulsive force causes pipe wall thinning. A characteristic of pipe wall thinning due to this phenomenon is that wall thinning occurs only in a region of a pipe where wet steam impinges, and it appears as pin hole­like wall thinning. A schematic diagram of LDI is shown in Figure 4.3.13.

2. Pipe wall thinning management method for

BWR plants

Pipe wall thinning management for BWR plants

4-33

NSRA, Japan

f Iron becomes iron ion due to oxidizing reaction,; 1 then, dissolves out. _ )

J A part of ion becomes Fe(OH)₂, and then, s are-precipitates to form oxide film (magnetite).: •- _"W As flow velocity becomes larger, i

; i the oxide film will get thin and iron ion ■ concentration will al sodr op. ■■

With accelerated dissolution of iron ion 1 metal dissolution gets accelerated. !

Figure 4.3.12 Schematic diagram of FAC

Figure 4.3.13 Schematic diagram of LDI (example, wall thinning downstream from an orifice due to liquid droplet impingement)

according to the Code established by the Japan Society of Mechanical Engineers is performed by dividing plant piping into management ranks as shown in Figure 4.3.14 and Figure 4.3.15 for FAC and LDI, respectively.

Each management rank is controlled by the control methods as shown in Figure 4.3.16.

3. Preventive pipe maintenance technology against high-cycle thermal fatigue cracking

In high-cycle thermal fatigue cracking, damage is produced due to small locally-cyclic thermal stresses in a mixing region of high temperature water and low temperature water or fluctuation of cavity flow going into a branch line with a dead end. For BWRs, measures are taken against this phenomenon at the design stage and include introduction of recombination tees to mitigate the temperature difference at a joint etc. between feedwater piping and reactor coolant clean up system piping. However, since damage events due to high-cycle thermal fatigue cracking were confirmed in BWRs inside and outside of Japan, research on this subject was performed in the private sector and an evaluation guide was established by the Japan Society of Mechanical Engineers. The selection of locations to be inspected and their inspections have been conducted according to the guide.

FAC affecting factor (dissolved oxygen concentration), management rank to be determined by actual plant data

Figure 4.3.14 Wall thinning management rank for FAC

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