INVERSIONES SUBSIDIARIAS

6. INTEGRITY ASSESSMENT OF STRUCTURES,

and the method is non-dispersive. Shear waves can be generated even through surface coatings of thicknesses of up to 1.5 mm.

— Magnetostrictive sensors, which are used to inspect piping. These are devices that launch guided waves, which are more sensitive than the shear waves used in the electromagnetic acoustic transducer system. These waves can follow the contour of a structure, can accommodate thickness discontinuities, can travel considerable distances and can be picked up at a remote location. This method can be used to characterize corrosion damage and to inspect structures whose surfaces are not directly accessible, as in the case of painted surfaces.

— Radiographic techniques, which are used in the detection of both surface and deep flaws in materials. These techniques use penetrating gamma or X ray radiation. Flaw detection is based on the differential absorption of radiation. Radiation is routed through the material and is captured on film on the opposite side of the source.

Two dimensional defects are indicated as the density changes (i.e. various shades of grey) on the film, which remains as a permanent record. Gamma ray sources are portable, while X ray machines are usually bulky, so the choice depends on the application. Instead of a film, gamma radiometry uses a radiation detector and a counter. The count or count rate is used to characterize the geometry. The following is a list of limitations of this testing technique:

● Cracks are hard to size and shape because the sizing capability is highly dependent on the angle of incidence of the rays with respect to the crack orientation;

● Results are not immediate;

● Access from both sides is mandatory;

● Accessibility to the test specimen may be a problem;

● Heavy biological shielding is usually necessary and even total area evacuation may be required.

— Acoustic emission inspections, which are useful for the detection of crack propagation. This technique uses small amplitude stress waves that are emitted by kinetic energy, as the material is locally strained beyond its elastic limit. Piezoelectric transducers placed on the material surface detect stress waves as small displacements. Triangulations can be used to identify the exact location of the emerging crack, even in very large structures. This NDE method is best used as a continual monitor of critical SSCs for first detection of crack propagation. The system is extremely sensitive and fast, but also obstructive. Applications include SCC, hydrogen cracking and evolution of gas inclusions. Conducting the test and interpreting results requires considerable experience, as background noise often interferes with signals.

— Thermographic inspections, which are recommended for the detection of property variations at interfaces of layered materials. The materials need to be thermally conductive, because a pulse of thermal energy is applied. A thermographic scanning camera with infrared spectrum detection capability monitors the thermal state of the object. The time dependent temperature gradient to the internal condition of the material is used to produce the results. It is less effective in the detection of subsurface flaws in thick objects and resolution is lost at the edges. The higher the maximum temperature difference, the better the flaw detection capability.

— Electrochemical corrosion monitoring, which is used to measure corrosion rates. The direct current is directed from a counter electrode to the working electrode and the change in potential is measured between the two electrodes. Alternatively, alternating current impedance changes can be used and converted into corrosion rate information.

6.1.2. Destructive testing

Tests altering the shape, size or structure of a material are considered destructive. Two types of destructive methods are applied:

— Planned destructive tests. The most common planned destructive test is that of the RPV surveillance programme, in which specimens are inserted into the RPV, periodically withdrawn and destructively tested;

these tests serve for periodic evaluation of RPV material embrittlement and are a necessary part of PSRs.

Some special cases, such as a rule in the Russian Federation [40], still require one weld from the primary piping in each plant to be cut out after 100 000 operating hours to produce specimens to determine any potential changes in the mechanical properties of the material that has seen all actual stressors during operation. Of particular interest are the tensile properties. These tests are usually omitted nowadays, as other

— Unplanned tests (e.g. material ageing, integrity, etc.). These tests are usually conducted when some information about material properties is missing or it is necessary to obtain new data.

Destructive tests require that some material be removed from the component surface, by mechanical or electroerosive machining, to be used as a test sample. The removed material can be either negligible (e.g. taking scraps for chemical analysis or for neutron dosimetry measurements) or substantial in size (e.g. samples for metallographic tests, mechanical tests, tensile tests, subsize Charpy impact specimens, fracture toughness tests, small punch tests for determination of tensile properties and/or transition temperature tests, etc.). In all these cases, stress analysis of the component with the material removed (e.g. effect of sampling on stresses and potential notch effect on fatigue) is necessary to confirm that the safe operation of the component will not be affected. In all such cases, previous authorization/approval from the regulatory body needs to be obtained.

Not all destructive tests require complete destruction of the specimen. A test is called semidestructive when it has negligible effects on the component integrity, shape and size. Examples of tests of this nature are hardness tests, instrumented hardness tests, automated ball indentation tests and replicas for metallographic examination.

6.1.3. Leak before break

The LBB criterion allows the relaxation of design requirements of safety related pipes and headers.

The concept has been used to implement safety upgrades of existing plants that do not meet current regulatory requirements. When restrictive regulatory rules are issued to operating plants, applying the LBB concept allows these plants to obtain regulatory approval without the implementation of large design changes that would have not been economically feasible.

Such is the case of postulated double ended guillotine breaks (DEGBs). Where DEGBs are postulated and the LBB concept can be justified, an LTO case can become licensable. The LBB criterion, when applied to the main primary circuit piping, suggests that DEGBs in the primary circuit piping are extremely unlikely. This conclusion has been based on several tests which have shown that the failure mode of large pipes and headers has a high probability to begin with leaks through small cracks before they break, and that the crack grows only gradually until it reaches its critical crack length. Another important point is that the leak does not immediately challenge the pipe’s capability to withstand its design loads. The LBB concept also relies on the fact that leaks can be easily detected, by using either existing or newly added monitoring techniques.

The LBB concept applied to DEGBs has been interpreted by some as a proactive and preventive approach to large LOCA issues. In order to validate the concept and turn it into a design criterion, fracture mechanics methods have been applied and models validated by experiments that have been proven capable of predicting the evolution of postulated DEGBs, under all operational conditions, including accident loads. This capability has allowed operators to use the time interval between the first leak and the critical crack length to control, inspect and monitor their critical piping systems. These defensive measures, together with conservatively selected critical crack parameter thresholds, has allowed the development of administrative procedures for the prevention of catastrophic failures, without having to introduce massive design changes and the risks that these involve.

If a leak grows large enough to start producing a void in a pipe, a power pulse may follow. To avoid such an occurrence, mitigating action should be taken before voiding starts; this needs to be long before a crack has grown to the critical crack length.

The probability of leaks that would propagate quickly to DEGBs has been estimated for CANDU reactors and gave failure rates of 7.7 × 10⁻¹⁰ per reactor-year for 460 mm pipes and 8.8 × 10⁻¹⁰ per reactor-year for 560 mm headers. Failure rates for smaller pipes were much higher. By crediting LBB, all pipes greater than 180 mm in diameter would have extremely low failure rates. By not crediting LBB, only pipes greater than 460 mm would have extremely low failure rates [69].

Other concepts somewhat similar to LBB, notably the low break probability and break preclusion concepts have also been used. Implementation of the break preclusion concept ensures that the probability of piping breaks and/or leakage cracks, when occurring for unknown and/or hard to predict reasons, is lowered to an acceptable level. This concept is used, for example, in PWRs in France and Germany to prove the quality of the primary coolant piping systems through fracture mechanics. The break preclusion concept originally applied to the main piping has been developed into an integrated concept for the whole pressure boundary within the containment and is also used in the PSR of operating nuclear power plants.