Overview of Deterministic Safety Analysis: Areas of Analysis and Computer Codes (Part. 1)

1. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making Overview of Deterministic Safety Analysis:Areas of Analysis and Computer Codes (Part. 1) Workshop Information Lecturer Lesson IV 2_1 IAEA Workshop City , CountryXX - XX Month, Year

2. Methodologies of DSA • Designer and/or operators of nuclear plants develop EVALUATION MODELS in order to perform Deterministic Safety Analyses. • An Evaluation Model (EM) is the calculational framework for evaluating the behavior of a reactor system during a postulated transient or DBA. • The name was originally applied to loss-of-coolant (LOCA) analyses (see 10 CFR 50.46) but has been generalized to all AOO and Accident (Safety Analysis Report) Chapter XV events. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

3. Methodologies of DSA • An EM may include one or more computer codes, other calculational devices (analytical tools, calculational procedures), special models, and all other information necessary for application of the calculational framework to a specific event, such as: • Procedures for treating the input and output information. • Specification of those portions of the analysis not included in the computer codes for which alternative approaches are used. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

4. Methodologies of DSA • A Methodology comprises several types of calculations: • Thermohydraulic, simulating the behavior of the coolant in the plant. • Neutronic, simulating the fission processes in the reactor core. • Structural, simulating the behaviour of structures against the loads, stresses,… • Radiological. • All these types of calculations are “deterministic” (no probability involved, conservative assumptions). But this is not exactly true for best-estimate methodologies. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

5. Methodologies of DSA • Thermohydraulic calculations: • Performed with fluid-dynamics codes • Simulating coolant behaviour in primary and secondary systems, the containment… • Neutronic calculations: • Performed with neutronic codes • Simulating fission process in the core • Structural calculations • Radiological calculations IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

6. Methodologies of DSA • The methodologies can be conservative or best-estimate: • Conservative: use pessimistic or worst-case assumptions and models. Most of the analysis presented to regulatory bodies follow this approach. • Best-estimate or realistic: most of assumptions and models are realistic (some conservatisms are maintained), include uncertainty analysis. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

7. Methodologies of DSA • EMDAP: EM Development and Assessment Process • Six basic principles (as stated in DG-1096 of USNRC): • Determine requirements for the EM: i.e. Mathematical models, components, phenomena, physical processes,etc, needed to evaluate the event behaviour relative to adequate figures-of-merit. • Develop an assessment base consistent with the aforementioned requirements: experimental data. Sometimes performance of new experiments is required. • Develop the EM: the calculational devices are selected or developed. For a particular plant and event, it is necessary to select proper code options, boundary conditions and the temporal and spatial relationship among the component devices. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

8. Methodologies of DSA • Assess the adequacy of the EM: by comparing requirements and capabilities. Some of this assessment is best made during the early phase of code development, to minimize posterior corrective actions. It is important to assure that the calculational devices are used within the range of their assessment. • Follow an appropriate Quality Assurance protocol during the EMDAP. • Provide comprehensive, accurate, up-to-date documentation. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

9. Computer Codes and Applications • Computer codes: main tools to perform the Deterministic Safety Analysis, by determining the response of the reactor to such transients. • Such computer codes use to be a collection of models needed to simulate the behaviour of a nuclear plant (or of a part of it) during a transient or accident. • Models: thermohydraulic (TH), neutronic, thermomechanical, structural, radiological, etc. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

10. Computer Codes and Applications • Codes used in conservative DSA use to be different from that used in best-estimate DSA: • Conservative DSA: conservative codes (i.e., codes with pessimistic models, used with pessimistic assumptions). Use to be proprietary codes. • BE DSA: BE or realistic codes (those with realistic models). They are mainly system codes. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

11. Computer Codes and Applications • System codes: large codes that can model the entire reactor system to simulate certain accidents: • COBRA-TRAC, RELAP5, TRAC-P, TRAC-B, CATHARE, RETRAN… • They are realistic, state-of-the-art codes. • Control volume approach, big nodalizations. • State-of-the-art TH models (two-fluid models). • Core model: relatively simple, represented by an average channel and/or a hot channel. The output can be input to a subchannel analysis code. • Simple neutronic models (e.g. point kinetics). IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

12. Computer Codes and Applications • There are codes specifically designed for licensing analysis of AOOs: simpler than the system codes. • There are “component codes” which are designed for representing some parts of the reactor: e.g. the core, a fuel rod or a core subchannel. Their models can be more complex or detailed that those of the system codes. • There are codes designed to analyze beyond design basis accidents and severe accidents: MAAP, MELCORE. Sometimes, a degraded core analysis core. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

13. Computer Codes and Applications • There are codes designed to analyze beyond design basis accidents and severe accidents: MAAP, MELCORE. Sometimes, a degraded core analysis code can be linked to a system code to provide an integrated view of the entire accident (e.g. RELAP/SCDAP). IAEA Training Course on Safety Assessment of NPPs to assist Decision Making

14. Computer Codes and Applications • Codes which take part on Safety Analysis must undergo: • A development and assessment process, performed by the methodology developer. • A parallel verification and validation process. • Analysts and users of the codes should be suitably qualified, experienced and trained. • Codes should draw on operating experience that can be derived from similar nuclear plants and relevant experimental data. There is an accumulated basis of operating experience and data for AOOs. IAEA Training Course on Safety Assessment of NPPs to assist Decision Making