Research and Development of Advanced Nuclear System

2-4

For Enhancement of the Reliability of SFR Pipe Designs=Creep Strength Assessment of Mod.9Cr-1Mo Steel Welds Taking
Type-IV Damage into Account=

Mod.9Cr-1Mo steel(welded joints)

Integrated pump-IHX component

equivalent to 550 㷄×60 years

Weld (a)Primary hotleg pipe(in red)Weld (b)Reactor vessel

Primary cold-leg pipe (in yellow)

ElongationǬ(%)

30

Sold symbols meandata obtained understress smaller thanhalf of 0.2% proof
stress of base metal2010ٌ٤‫غ‬٠

025000500 ͠550 ͠600 ͠650 ͠

30000

35000

Time-Temperature parameterFig.2-7 Schematic view of the JSFR RV and primarysystems
Employing Mod.9Cr-1Mo steel, a simple piping layout will beachieved. On the basis of results of creep damage evaluations
for welds (a) and (b), it was confirmed that the stress at thewelds was less than the allowable stress, and that the SFR
pipe was designed with enough reliability.

Fig.2-8 Observed creep rupture elongation for Mod.9Cr-1Mo
steel welds
Significantly small ductility was observed in long-term creep
tests, which were performed with a stress less than half of the0.2% proof stress of the base metal, and the failure
mechanism shifted from ductile creep to Type-IV failure.

For Sodium cooled Fast Reactors (SFR), a compact plantdesign will be achieved by shortening the pipe length andemploying Mod.9Cr-1Mo steel (Fig.2-7), which has a low
thermal expansion rate and excellent creep strength.
However, it is believed that the creep strength of welds ofhigh-chromium ferritic steels, such as Mod.9Cr-1Mo steel,degrade compared to those of the base metals, particularly
after long-term use and in high-temperature regions. In theJapanese technical standard for thermal power plantcomponent design the allowable stress for these steel welds
has been reduced because of Type-IV damage. In this study,
available creep test results for welds were analyzed
considering the possible failure mechanisms, and the validityof the SFR pipe designs with acceptable margins was verified.
As shown in Fig.2-8, significantly small ductility wasobserved in the long-term creep tests, which were performed
with a stress less than half of the 0.2% proof stress of thebase metal. In addition, the failure position observed in thecreep tests shifted from the base metal to the heat-affectedzone with an increase in the creep rupture time. As a result,in Mod.9Cr-1Mo steel welds, it was clarified that the failure
mechanism shifts from ductile creep to Type-IV failure withan increase in the creep rupture time. Therefore, creep
characteristic equations for Mod.9Cr-1Mo steel welds wereproposed on the basis of the individual polynomial
regressions for the long-term and short-term regions.
In addition, the distribution of the ratio of observed creep
lives to calculated creep lives in the long-term region wasremarkably smaller than that in the short-term region. Thus,
individual safety factors were proposed, and 60 year creep
damage was evaluated for two welds in the hot-leg pipe of anSFR, as shown in Fig.2-7. As a result, it was confirmed thatthe stress at the welds was smaller than the allowable stress,
and that the SFR pipe was designed with enough reliability.

Reference
Wakai, T. et al., Creep Strength Evaluation of Welded Joint Made of Modified 9Cr-1Mo Steel for Japan Sodium Cooled Fast Reactor (JSFR), Proceedingsof the ASME 2010 Pressure Vessels & Piping Division/K-PVP Conference, Washington, DC, USA, PVP2010-26014, 9p., in CD-ROM.

40 JAEA R&D Review 2012