ACCOUNTING FOR IMPACT OF FA GAP DEVIATIONS IN VVER-1000 TO ENGINEERING MARGIN FACTORS
24th Symposium of AER on VVER Reactor Physics and Reactor Safety (2014, Sochi, Russia)
Advances in spectral and core calculation methods
Abstract
ACCOUNTING FOR IMPACT OF FA GAP DEVIATIONS IN VVER-1000 TO ENGINEERING MARGIN FACTORS
Shishkov L.K, Gorodkov S.S., Mikailov E.F., Sukhino-Homenko E.A, Sumarokova A.S. NRC «Kurchatov Institute», Russia, Moscow
ABSTRACT
Jacket-free fuel assemblies of VVER-1000 and their western analogues change the shape in the process of operation. Typically, we note longitudinal bending. FA bendings threaten hazard, such as, first of all, reduced speed of control rod drop, however, alongside with induced bending there changes (also increases) moderator amount in the gaps between fuel assemblies, which results in additional jump of power release.
Nowadays, FA design has been supplied with amendments that ensure design speeds of CR drop. Nevertheless, FA shape changes and this effect shall be taken into account in calculation of power density. Gap values are of random character and their impact shall be accounted using engineering margin factors. In case of VVER-1000, application of margin factor leads to fuel cost increase in the cost of electrical power by (3 – 5) %, while the half of the cost increment is stipulated by changed gaps between fuel assemblies.
The paper discusses the mechanism of accounting gap influence on power density in tvels, gives numerical values of power coefficients of sensitivity and gap values calculated by selected computational models, and discusses the interference of gap influence and accounting for feedbacks.