Civil Engineering Reference
In-Depth Information
to turbine
from feedwater system
Emergency core
cooling and
residual heat
removal system
Fig. 5.7
Reactor protection system of a PWR [
1
,
4
]
and a neutron fluence of 10
19
nvt for neutrons with a kinetic energy
0.1 MeV. The
wall of the pressure vessel is 25 cm thick in the cylindrical part [
2
,
4
].
The mechanical stresses in the pressure vessel wall are caused by
>
- loading as a result of dead weight,
- internal pressure,
- thermal stresses due to temperature gradients.
The mechanical stresses produced must be determined in accordance with the
ASME Boiler and Pressure Vessel Code [
18
] and the RSK Guidelines [
13
]. A
distinction must be made between primary and secondary stresses. Primary stresses
are caused by the internal pressure and dead weight and cannot be relieved by
plastic deformation. Secondary stresses are relieved by plastic deformation (ther-
mal stresses). One important criterion in the ASME codes [
18
] and RSK Guidelines
[
13
] is that the primary stresses in the undisturbed part, e.g., the cylindrical wall,
must not exceed the value of 0.33
∙
σ
B
or 0.67
σ
0.2
at operating temperatures
(
σ
0.2
-stress at 0.2 % strain). This must be demonstrated
by stress and fatigue analyses for all load cases occurring.
Compliance with these criteria is able to exclude so-called failure by ductile
fracture or brittle fracture (failure by crack growth with limited leakage) in a reactor
pressure vessel. This conclusion is supported, inter alia, by the fact that higher
internal pressures of approx. 24-26 MPa cause the seal of the top shield to leak as a
result of straining of the top shield screws. The leakage area then would correspond
to up to a 69 cm
2
leak.
Compliance with the provisions of the ASME code [
18
] must always be verified
by several independent expert consultant organizations. Those provisions were laid
down internationally between 1970 and 1980 after the basic principles had been
σ
B
¼
compressive stress,
