Nuclear Safety Research

6-4

Research on Thermal Aging Embrittlement of Cast Stainless Steels－ Study of Thermal Aging Embrittlement in “FUGEN” Nuclear Reactor －Injection valve body251510 nm（b）
Charpy impact energy (J/cm²)

Steel Grade:
SCS13
Operating temp:
275 ℃
Period of service: 138000 h

Injection
valve body

As collected

Recovery heated

As collected

Recirculation pump casing

35

Cr concentraion (at%)

Steel Grade:
SCS13
<40 ℃
Operating temp:
Period of service: 138000 h

Recirculation
pump casing
Recovery heated（a）

1801601401201008060
40200

Recirculation
pump casingInjectionvalve bodyFig.6-9 Sampling point schematics and service conditionof test specimens collected from “FUGEN” reactor
Specimens of cast stainless steels were collected fromrecirculation pump casing and injection valve body withdifferent service temperatures in “FUGEN” reactor.

Fig.6-10 Results of analyses of samples collected from
“FUGEN” reactor(a) Cr concentration distribution map obtained using
3DAP. (b) Result of Charpy impact test. Phase separationand thermal aging embrittlement have progressed inrecirculation pump casing.

Cast stainless steels are extensively used in light water
reactors to fabricate valve bodies, pump casings, and primarycoolant piping. Previous studies have shown that thermal
aging around 300 °C causes embrittlement. Predictions of thedegree of loss in toughness suffered during extended reactor
service life would be valuable for assessing the reliabilityof cast stainless steel reactor components. Therefore, somemodels for predicting changes in the mechanical properties
because of thermal aging have been developed. However, tomaintain the toughness of the components in service within areasonable time period, the aging process in the test materialmust be accelerated at high aging temperatures. Thus, it
is very important to examine the thermal embrittlement
mechanism using data obtained from materials that have aged
on site for a long period.
A program is currently underway for a decommissioning
plan that considers the progress and dismantling of the“FUGEN” reactor. The reactor contains valuable structural
materials that have been used for 25 years. In this study, thedegree of toughness and the microstructural characterizationof cast stainless steel components dismantled from the“FUGEN” reactor were investigated to determine the long-term
thermal aging effect at low temperatures (Fig.6-9). We studied
the microstructural changes due to thermal aging using thethree-dimensional atom probe (3DAP) technique, which issuitable for nanometer-scale analysis of the materials. The
analysis of the sample material, which was collected from the“FUGEN” reactor, under thermal aging at 275 °C shows thatthe initial homogeneous distribution of the Cr concentration
becomes modulated as the structure progresses into spinodal
decomposition (Fig.6-10(a)). This microstructural change isprobably related to hardening, and it causes the thermal aging
embrittlement. The results of Charpy impact tests ensure thereliability of the injection valve body. On the other hand,
the results from the recirculation pump casing show earlysigns of an ongoing thermal aging effect (Fig.6-10(b)). We
are planning to investigate the underlying mechanism of theembrittlement process to validate and improve the accuracyof the prediction model by using the valuable material dataobtained from the “FUGEN” reactor.
The present study was sponsored by the Japan Nuclear
Energy Safety Organization (JNES).

Reference
Nogiwa, K. et al., Influence of Thermal Aging on Cast Stainless Steels Used in JAEA’s Nuclear Reactor Fugen, Journal of Nuclear Science and Technology,
vol.50, no.9, 2013, p.883-890.

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JAEA R&D Review 2013