EXPERIMENTAL AND NUMERICAL STABILITY INVESTIGATIONS ON NATURAL CIRCULATION BOILING WATER REACTORS

MARCEL CHRISTIAN PABLO

IOS Press

Lugar: Delft, Países Bajos; Año: 2007 p. 160

978-1-58603-803-8

<!-- /* Font Definitions */ @font-face {font-family:"Palatino Linotype"; panose-1:2 4 5 2 5 5 5 3 3 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-536870009 1073741843 0 0 415 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0mm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} h1 {mso-style-next:Normal; margin-top:12.0pt; margin-right:0mm; margin-bottom:3.0pt; margin-left:0mm; mso-pagination:widow-orphan; page-break-after:avoid; mso-outline-level:1; font-size:16.0pt; font-family:Arial; mso-font-kerning:16.0pt;} p.MsoHeader, li.MsoHeader, div.MsoHeader {margin:0mm; margin-bottom:.0001pt; mso-pagination:widow-orphan; tab-stops:center 220.95pt right 441.9pt; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES; mso-fareast-language:ES;} p.MsoFooter, li.MsoFooter, div.MsoFooter {margin:0mm; margin-bottom:.0001pt; mso-pagination:widow-orphan; tab-stops:center 220.95pt right 441.9pt; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES; mso-fareast-language:ES;} @page Section1 {size:210.0mm 842.0pt; margin:70.85pt 30.0mm 70.85pt 30.0mm; mso-header-margin:35.45pt; mso-footer-margin:35.45pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> In the design of novel boiling water reactors (BWRs) active systems are replaced by passive ones in order to reduce the risk to failures. For that reason natural circulation is being considered as the main cooling system in next generation BWRs. The dynamics of natural circulation BWRs involve complex interacting phenomena which differ from those existing in conventional forced  circulation BWRs. For that reason predicting the stability characteristics of these novel reactors is a difficult task which has to be carefully investigated. The stability of natural circulation BWRs mechanism has been investigated in this thesis with a strong emphasis on experiments. The prototypical Economical Simplified BWR (ESBWR) design from the General Electric Company has been taken as the reference design. In order to simulate as good as possible the ESBWR at rated conditions, a downscaled facility was constructed based on a fluid-to-fluid scaling approach especially derived for that purpose. The scaling rules were then used to design the so-called GENESIS test facility in which the void reactivity feedback mechanism was artificially implemented. The use of GENESIS allowed investigating the thermal-hydraulic and the neutronic-thermal-hydraulic stability performance of the ESBWR for a wide range of conditions. From the analysis of the results it was found that GENESIS (representing the ESBWR at nominal conditions) is very stable exhibiting a large margin to instabilities. From the numerical experiments performed by using the sophisticated TRACG and ATHLET codes important discrepancies were observed in the results. This finding indicates that important limitations still exist in the numerical estimation of the stability performance of nuclear reactors involving complex two-phase flows. The GENESIS test facility was also used to perform a parametric study which helped to further investigate the instability mechanisms. As a result it was observed that the characteristic resonance frequency of the thermal-hydraulic mode is found to be much lower (~0.11 Hz) than in forced-circulation BWRs (~1 Hz) indicating a static head dominated phenomenon since it corresponds well with typical frequencies of DWOs traveling through the core-plus-chimney sections. In addition it was experimentally observed that the position of the feedwater sparger inlet affects the stability of the thermal-hydraulic oscillatory mode. The stability of natural circulation BWRs at startup conditions was investigated by using both experimental and numerical tools. Detailed experiments allowed observing the axial temperature and void fraction evolution taking place in the CIRCUS test facility for two different configurations: the single channel case and the two parallel channels case. Such a characterization helped understanding the instability mechanisms present during the observed flashing-induced oscillations. In addition, the effect of the friction distribution in the stability of flashing-induced oscillations occurring in a single channel was studied. The CIRCUS test facility was modified in order to investigate the occurrence of flashing-induced oscillations in two-parallel channels. As a result a detailed experimental investigation was conducted which allowed determining the main characteristics of the mechanisms driving those oscillations. It was found that reverse flow plays an important role in determining the evolution of the partial flows within the parallel channels. In addition a-periodical oscillations were which were found to be attributed to multifractal deterministic chaos. The work presented in this thesis shows that the complex phenomena which determine the stability performance of natural circulation BWRs can be well reproduced by a downscaled test facility. In addition, these facilities allow investigating the physical interaction between the thermal-hydraulics with the neutronics which greatly influence the reactor stability. As a result of this thesis it is found that the natural circulation BWRs can be safely operated with large margins to instabilities. The results performed at start-up conditions predict that vapor can be produced while the reactor remains stable. To what extent this vapor is enough to pressurize the reactor vessel in a reasonable period is still unclear.