Chapter 7 CONCLUSIONS
Use of coupled codes was applied successfully to the REA accident. It was demonstrated possible to conduct a detailed analysis of the core and of the whole plant behavior despite a transient involving sharp parameters variation (e.g. the reactor power and the fuel temperatures).
Thanks to the use of the coupled codes, it was possible to know with sufficient detail (0.1765 m of spatial resolution for a FA) the energy production (calculated by PARCS), the energy exchanged (calculated by RELAP5), and finally the maximum energy released. Calculations of the DNBR where also performed.
Furthermore, the use of the HELIOS code for cross section generation, allowed to perform calculations for a new different type of transient (REA with Xenon poisoned core) and with an higher neutronic resolution modeling.
After analyzing the reference cases and performed the sensitivity analyses, the following conclusions can be also highlighted for the safety of the NPP:
• HZP EOC resulted to be, among the reference cases, the case that caused the greater energy increase
• the particular Xenon poisoning of the core studied for the HZP EOC, showed clearly its dangerousness causing the greater energy increase
• having only two pumps in operation during this accidental event, could worsen the consequences, causing major stresses for the fuel element
• All values of the energy released calculated were well below the safety limits for WWER-1000 clad rupture (160 cal/g) and fuel fragmentation (220 cal/g). Thus, the NPP showed a safe response to this accident
• The fuel CL temperatures were also always below the melting point as well as the clad temperature did not reach the safety limit of 1204 °C.
• On the other hand, several phenomena of thermal crisis were found to happen during some transients (notably during the transients with decreased flow regime) and during the ‘worst possible’ scenario studied. This violates one of the safety criteria established in the IAEA guidelines [4] and it suggest that further studies, maybe with a sub-channel code, should be carried on to assure the safe behavior of the fuel elements
• It has to be noted, that, with increasing the fuel burnup in the future, further studies will be need. In fact, the trend of the regulatory agencies is toward a lowering of the safety limit value for the energy released
Concerning the methodology to perform such complex analysis the following achievements have been made:
• a procedure, involving BE codes and ‘ad-hoc’ created programs, was set up for the calculation of the energy released to the fuel for every FA layers
• modifications and improvements of the cross section interpolating and formatting programs were conducted
• software development for the automatic calculation of the main neutronic parameter and for the data processing (3D visualization and animation) was executed
Concerning the modeling used and the capabilities of the codes, it was also noted that:
• the thermal-hydraulic modeling as well as the neutronic modeling (for transient and cross section derivation) fulfill completely the resolution level required for the analysis
• RELAP5 stand alone and RELAP5/PARCS coupled codes calculated correct design values for the steady state conditions
• PARCS version 2.5 revealed its limits crashing during some transient calculations involving the presences of high void fraction in the channels. All the several tests performed later for these cases, modifying the code’s main numeric parameters, were unsuccessful. The use of a newest corrected version is therefore required for future calculations
Finally, the general conclusions of the activity performed for this thesis are resumed below: • capability to perform 2D neutron cross section libraries calculation has been acquired • capability to perform 3D neutronics/thermal-hydraulic analysis has been acquired
• knowledge of the state-of-the-art of the research activities and of the physical phenomena involved in the RIA has been acquired
• knowledge of an analytical procedure to perform a calculation of the energy released during a RIA has been acquired
• WWER-1000 NPP showed a safe response during the all transients simulated
• the established methodology will be useful for future calculations on the high burnup fuel that will be used in the next future (WWER-1000 mod. 392 and WWER-1500)
