Safety and Risk of Light Water Reactors - The Risks of Nuclear Energy Technology - page 113

reactor pressure vessel during reactor operation. In this way, it is also possible to
determine in advance the point when the maximum permissible neutron fluence is
reached for the wall of the reactor pressure vessel.
The NDT is influenced, above all, by the existence of small fractions of copper,
phosphorus, and sulfur in the steel alloy. As a consequence, these elements must be
kept below preset concentration levels (Cu

< < <

0.01 %) [ 2 , 30 ].
Precisely defined notch impact specimens of the base metal of the reactor
pressure vessel must exhibit not less than 68 J notched bar impact work at a
temperature of NDT + 33 K [ 4 , 30 ]. The prescribed minimum temperature for the
reactor pressure vessel under nominal pressure during reactor operation or under
accident conditions is 50 C.

0.1 %, S 0.01 %, P

5.6.4.2 Quality Assurance and In-Service Inspections (Basis Safety)

In addition to the design conditions and rules referred to above, quality assurance
throughout the manufacturing process must ensure that all components have the
required toughness of the base metal and the welds. For purposes of fracture
mechanics the size of any small cracks must be below critical lengths [ 2 , 4 , 28 - 30 ].
Non-destructive test methods (such as surface crack inspection and ultrasonic
inspection) are used to determine crack size and crack distribution. Figure 5.9
shows the manipulators for ultrasonic inspection within the reactor pressure vessel
and the manipulators for ultrasonic inspection of the coolant inlet and coolant outlet
nozzles in the upper region of the reactor pressure vessel.

5.6.4.3 Hydrostatic Test of the Reactor Pressure Vessel

As an integral test, a hydrostatic water test is carried out at 1.3 times the design
pressure (22.8 MPa). In this pressure test, the temperature is not more than 55 C
above the NDT temperature. This is far below the operating temperature. As the
fracture toughness decreases with temperature, such a pressure test is close to the
design limits of the reactor pressure vessel. After this pressure test, the ultrasonic tests
must be repeated. These in-service inspections must be repeated—as an example in
Germany—during plant life time at regular intervals of 8 years [ 2 , 4 , 28 ].

5.6.4.4 Leak-Before-Break Criterion

In accordance with the fracture mechanics findings, small through-cracks far below
the critical crack length already would give rise to leakages of the reactor pressure
vessel (leak-before-break criterion) [ 4 , 29 , 30 ]. These can be detected in reactor
operation.