Download (Slide 1) Question 10

(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.