PSI - Issue 28

Casper Versteylen et al. / Procedia Structural Integrity 28 (2020) 1918–1929 Versteylen/ Structural Integrity Procedia 00 (2020) 000–000

1927

10

Table 5. Cumulative probability for cleavage fracture according to equation (1). All cracks are presented for the standard case, and in the case of crack 9 the other load cases are also presented. The probability for cleavage for normal forged irradiated material and for irradiated weld material are presented in columns 3 and 4.

�

�

� for Unirradiated forged material

Crack ID

for Irradiated forged

for Irradiated weld

material (40 year)

material (40 year)

#4 (standard) #5 (standard) #6 (standard) #7 (standard) #8 (standard) #9 (standard)

7.87E-07 3.98E-06 3.19E-05 1.13E-05 2.58E-06 4.57E-05 3.92E-05 4.97E-05 3.35E-05 4.11E-05

6.09E-05 3.07E-04 2.43E-03 8.61E-04 1.99E-04 3.48E-03 2.98E-03 3.78E-03 2.55E-03 9.82E-04

5.15E-04 2.58E-03 2.00E-02 7.15E-03 1.67E-03 2.86E-02 2.45E-02 3.10E-02 2.11E-02 4.65E-03

#9 (-10%) #9 (+10%)

#9 (AR 0.67)

#9 (SA-508gr.4N)

4. Discussion Given the restrictive

ASME postulated cracks in typical RPV material undergoing pressurized thermal shock seem to lead to a non negligible probability of cleavage crack formation. The master curve approach based on the Weibull distribution is fitted to experimental data for SA-508gr.3, see Heerens and Hellmann (2002). The shift in � due to irradiation of weld material compared to unirradiated base material is approximately 3 orders of magnitude. The radiation embrittlement, particularly of weld material can lead to risks during a LOCA. For the crack directly underneath the nozzle, the probability for cleavage crack fracture can reach almost 3% for the ASME postulated crack. When reviewing cracks in RPV material, special attention should be paid to weld material. The probabilities for cleavage fracture is highly dependent on the value of � , and thereby on the irradiation and weld material. The probability of cleavage for unirradiated SA-508gr.3 and SA-508gr.4N should be very similar according to the master curve approach. However the resistance to irradiation for is SA-508gr.4N is theoretically much higher, due to different chemical composition. The irradiation resistance depends heavily on the Sulphur, Phosphorus and Copper contents. Those solute elements lead to radiation embrittlement due to segregation to grain boundaries and the formation of precipitates. The binding energy of vacancies in bcc-iron is much higher for Copper than it is for Nickel and Manganese Domain (2006) and Versteylen et al. (2017). It therefore conceptual sense that SA-508gr.4N is therefore more radiation-resistant than SA-508gr.3. In fact Kim et al. (2016) indicate that the value of � is lower for SA-508gr.4N than for SA-508gr.3. The use of SA-508gr.4N steel could be very promising since it can reduce the probability of cleavage fracture significantly. It should be noted that more research is needed regarding both the reference temperature and its shift under influence of radiation and its chemical composition. The interaction of plastic deformation before the occurrence of cleavage also plays a vital role. These factors are taken into account for SA-508 gr. 3 material by Tanguy et al. (2006). In order to obtain those results for many different alloys, more targeted experimental research is needed on the models linking ductile deformation and cleavage probability (Beremin model), such as performed by Hojo et al. (2016).