Assessment of reliable performance

The process for assessing equipment capability to perform reliably under severe accident conditions involves the following:

 Surveying and evaluating of available information on assessment of reliable performance of equipment as described in international technical reports, codes and standards.

 Describing the assessment process that demonstrates the capability of the equipment to perform reliably.

 Evaluating the impact of specific environmental effects typical for severe accidents, such as temperatures spikes as a consequence of H2 combustion, high radiation levels caused by the release of active material from the melted core and atmospheric conditions in the containment after quenching of the molten core and corium concrete interaction.

 Evaluating the impact of specific environmental effects at individual equipment installation locations.

 Developing the general approach for assessing the reliable performance of the equipment. This approach may include equipment type testing, assessment of equipment survivability, comparison with previously tested equipment and evaluation of existing margins that may be available from previous qualification testing.

 Identifying alternative measures if the equipment performance is not sufficiently reliable.

The proposed sequence for assessing reliable performance under severe accident conditions may consist of the following steps:

 Performing reference functional tests in order to confirm the safety function under normal operating conditions.

 Conditioning of the equipment using applicable methods in order to simulate the consequences of thermal, radiological and mechanical aging under normal operation conditions (e.g. long lasting vibration and wear).

 Application of accident radiation dose (which may be higher than the dose under design basis condition)

 Application of the p-T profiles including humidity and chemical exposure simulating the accident phase.

 Application of conditions of the post-accident phase (may be up to one year or longer).

 Performing reference functional tests in order to assess the survivability of the equipment during the accident.

Seismic event testing is not described among the steps above, since it is assumed that the procedure regarding the seismic event testing is well established and therefore the equipment is already qualified.

Specific equipment performance acceptance criteria do not necessarily have to be established for the assessment of reliable performance. Rather, the objective of severe accident type testing is to document the expected equipment behaviour under simulated severe accident loads and to compare the performance of this equipment to the expected conditions in individual plant locations.

Qualitative acceptance criteria may be developed, however, it is more important to demonstrate that the equipment remains available, and is providing the required functionality. For example, a reduction in measurement accuracy may be acceptable provided that it can be demonstrated that a particular instrument is able to retain its functions under severe accident conditions for at least 100 days, albeit with degraded accuracy but still capable of providing information on the trends of designated parameters.

6.5.2.Formal assessment process (reports and deliverables) The formal assessment process may include:

 Development of a programme, which describes the methodology, applied for specific equipment or an equipment type series. It defines which assessment steps are needed to be performed and which methods are to be used for the assessment.

 Preparation of reports that provide a technical basis for performing assessment of equipment performance by analytical methods.

 Development a specific test specification that describes all steps belonging to type testing.

 Selection of appropriate testing laboratories that are able to reproduce the severe accident environmental conditions.

 Conducting the qualification tests in accordance with the test specifications. Each test has to be documented by suitable test protocols or test reports.

 Evaluation of the results gathered in the demonstration, and preparation of a qualification summary report for each equipment subjected to type testing.

 Preparation of a suitability analysis comprising the results of analysis and type testing to demonstrate the reliable performance of the equipment assessed.

If the assessed equipment is not suitable to perform reliably under severe accident conditions it is necessary to:

 Modify the equipment design or complete change of the used physical principles;

 Replace the equipment already installed in the plant;

 Change the installation location;

 Apply protection measures for the equipment in scope (e.g. additional thermal or radiological shielding);

 Propose alternative methods to gather required information for decision making (e.g. indirect measurements).

The quality assurance of the assessment of equipment capability process to perform reliably under severe accident conditions, and manufacturing processes of the electrical and I&C equipment needs to be conducted within the framework of a management system that meets the requirements of Ref. [14], and follows the recommendations of Refs [15] and [16].

For equipment that has already been installed in the plant, the following provisions may be considered when assessing the equipment capability under severe accident conditions:

 Identify potential changes in the plant design to limit the impact of consequence of severe accident conditions on the existing equipment;

 Select the equipment by considering the following aspects:

o Whether the equipment design follows quality development processes;

o Whether the equipment installed in containment/fuel storage building or in locations where multiple stressors could occur in case of severe accident has already been qualified for LOCA conditions.

o Whether the equipment has been installed in a location with only radiation stress or only temperature stress in case of a severe accident.

o Whether the equipment is already qualified to required seismic loads with sufficient margin or does not contain material that could be vulnerable to the radiation and high temperature degradation

 Justify that the existing qualification programme covers new requirements or, if it is not the case, perform the qualification programme for addressing the change in the anticipated environments.

When existing equipment does not have a sufficient margin to cover severe accident environmental conditions then an assessment of survivability of the equipment with in the new conditions needs to be performed or additional tests are needed on equipment of the similar representative type (typically same technology, same suppliers, same materials).

The irradiation dose expected during a severe accident includes both γ and β radiation. When existing equipment qualification is being analysed to credit to previous qualification results it is important to note that previous radiation qualification may have been performed solely using γ-radiation. Severe

accident conditions may have significantly greater levels of β-radiation that may cause additional degradation. A common practice is to irradiate the equipment by using γ sources to simulate the energy deposition of both γ and β radiation effects. Justification of the equivalence of relative levels of β radiation compensated by additional γ radiation needs to be provided.

6.5.3.Simulating the impact of degradation on equipment function

Electrical and I&C equipment are subjected to degradation due to high temperature, accident radiation combined with humidity (or submergence), and chemical exposure. Following the basic concept set out in Section 6.1, a simulation of the degradation may be covered by the following three demonstration steps:

Step 1: Accident radiation exposure: This step can be performed applying a dose rate that represents the average of the dose rate vs. time function during the accident. Since the severe accident dose rate is typically very high early in the severe accident mission time, and then exponentially decreasing over time following the initial stages of the severe accident, it might be more practical to perform the accident irradiation exposure for a long term mission time using a lower dose rate (simulating the average dose rate over the entire expected mission time) than that which would otherwise be applied to simulate the first days after the onset of the accident, followed by a significantly longer exposure at a decreasing lower rate. Since the average dose rate over the severe accident mission time is also much higher than the dose rate applied under design basis conditions, it is perhaps less important to simulate the effects of the initially high dose rate, and more important to observe instrument performance in handling the effects of total accumulated dose.

Step 2: Chemical exposure: In order to reduce the pressure and the temperature within the containment, spraying is an applicable method. Sprays usually contain boric acid and chemical agents and may contribute to the equipment degradation. The equipment under test may be exposed to spraying during accident simulation (p-T curve) and during the post-accident phase simulation.

Carefully selected equipment protective materials, such as stainless steel of appropriate thickness, glass or ceramics can limit or avoid the effect chemical exposure to the equipment.

Step 3: Pressure and temperature loads: Pressure and temperature are applied in the time scale identical to the actual expected event scenario. This is valid for early and middle stages of the accident simulation (e.g. up to 100 or 150 h). In contrast, the equipment that is needed for operation in the long term (e.g. up to one to three years), for example the containment pressure boundary related parts, needs to be tested using an accelerated aging procedure. This procedure might be similar to the simulation of the in service ageing for design basis accident qualification.

Duration of accident conditions covering a period of one month to one year of accident conditions need to be applied, in order to account for approximation uncertainties. The uncertainties in development of those profiles are relatively large; that is why the use of the approximated environmental profiles for test purposes is reasonable. The test profiles are therefore developed to consider these uncertainties.

In some cases, the accident profile may be adapted for the test purposes. For example, using the method of energy deposition (i.e. calculated for bounding cases), a simplified bounding profile can be created. This method is acceptable because the degradation of the equipment is approximately proportional to the energy deposition. The calculated temperature profile used for equipment testing is shown in Fig. 3. and Fig. 4.

FIG. 3. An example of simulated temperature behaviour in the containment during severe accidents (figure courtesy of VUJE, a.s.).

FIG. 4. An example of adapted temperature profile during hydrogen burning in the containment (figure courtesy of, a.s.).