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(Slide 1) Question 10. What is the semi-mechanistic model? Answer In the last few years the semi-mechanistic model which is based on key embrittlement mechanisms of chemical elements influence is proposed. The model includes three basic mechanisms influencing to radiation embrittlement for RPV steels: matrix damage, irradiation induced precipitation and element segregation. (Slide 2) 1. As neutrons interact with the crystalline structure of steel, the ferrite matrix is directly damaged. In a given material and temperature the matrix damage can be assumed to be simply dependent on neutron fluence. 2. During matrix damage formation, copper together with other elements is known to lead precipitation of nano-precipitates also including matrix hardening and embrittlement. The precipitation effect is expected to saturate due to the progressive reduction of precipitable elements in solid solution. 3. In addition, other elements, like phosphorus, can segregate inside the grains, interacting with matrix defects (vacancies, interstitials, and dislocations) and be attracted to the Cu-type precipitates. The key embrittlement mechanisms taking place during irradiation of RPV steels are summarized in Table 1. Table 1.Embrittlement mechanisms considered Embrittlement mechanism Remarks Direct matrix damage Due to neutron bombarding Precipitation hardening in matrix Cu is leading element Segregation P is recognized segregating element (Slide 3) Based on the above mentioned partial effects, the total effect in term of transition temperature shift is: start / sat Tshift a n b1 1 e( ) c1 [1 / 2 1 / 2 Tangh( )] (2) c2 ( Slide 4) An example of primary radiation embrittlement calculated with the proposed model is given in fig. 1. 200 180 160 TOTAL DBTT SHIFT, C 140 ° 120 100 80 Precipitation (Cu lead) 60 segregation (P lead) 40 Direct matrix damage 20 0 1.00E+18 5.10E+19 1.01E+20 Fluence, n 1.51E+20 2.01E+20 cm-2 Figure 1. Example of primary radiation embrittlement calculated with the semi-mechanistic model.