Results 1 - 4 of 4
Results 1 - 4 of 4. Search took: 0.015 seconds
|Sort by: date | relevance|
[en] Considering the fuel assembly ( FA ) load history and the nonuniformity of energy release for the fuel elements ( FEs ) contained in this FA , a method for forecasting of probability of WWER-1000 FE cladding failure due to FE cladding deformation damage accumulation, has been developed. In the case of four - group damage distribution model, the maximum value of damage parameter accumulated in a four - year fuel campaign is approximately two times greater compared with the one - group model. A FA rearrangement algorithm characterized by exceeding of the limiting value for cladding damage parameter and the safety margin for WWER-1000 operation, has been found. The thesis that the value of FE cladding damage parameter plays an important role in estimating the limit state of claddings, has been proved. The condition of FA rearrangement algorithm admissibility and the criterion of minimum cladding failure probability which can be used in a control system insuring the hermeticity of WWER-1000 FE claddings, have been derived
[en] A method for calculating the engineering margin factor (EMF) in calculations of the energy release in the core of VVER - 1000 reactors is proposed in the paper. The analysis of various approaches in the calculation of EMF is carried out and various factors influencing EMF and the ways of their consideration — deterministic and statistical — are determined. The main attention is paid to the influence of gaps between the fuel assemblies on the energy release of fuel rods and the contribution of this factor to the EMF. The limitations and conservatism of two - dimensional small - scale calculations of the energy release of fuel rods in case of deviation of the gap size between the fuel assemblies from the design one are shown. A three - dimensional approach to calculating the contribution of gaps to the EMF is proposed. The approach is based on detailed measurements of the shape of fuel assemblies removed from the core performed at Zaporizhzhya NPP ; simulation of the distribution of gaps in the reactor core  using measurement data; two - dimensional calculations of the energy release of fuel rods in separate fuel assemblies, surrounded by gaps of different widths, with mirroring boundary conditions; three - dimensional calculations of energy release of fuel rods in fuel l assemblies in the reactor core. Two - dimensional and three - dimensional calculations are performed by the well - known ALPHA - H/PHOENIX - H/ANC - H codes. The proposed approach allows considering not only the change in the fuel rod power, particularly of the peripheral rods, which is inherent in the currently used methods of calculating EMF, but also takes into account the change in the power of the fuel assemblies in the core, which makes the proposed method more realistic and removes the excessive conservatism o f EMF calculations and, thereby, allows improving fuel efficiency. For fuel assemblies produced by Westinghouse, it is proposed to use full EMF: for fuel rod power (F ΔH ) 1.111 and for fuel rod linear power (F Q ) 1.173. The use of the BEACON TM monitoring system makes it possible to further reduce the EMF: for fuel rod power (F ΔH ) - up to 1.084 and for fuel rod linear power (F Q ) - up to 1.121.
[en] Highlights: • Fuel cladding failure forecasting is based on the fuel load history and the damage distribution. • The limit damage parameter is exceeded, though limit stresses are not reached. • The damage parameter plays a significant role in predicting the cladding failure. • The proposed failure probability criterion can be used to control the cladding tightness. - Abstract: A method for forecasting of VVER fuel element (FE) cladding failure due to accumulation of deformation damage parameter, taking into account the fuel assembly (FA) loading history and the damage parameter distribution among FEs included in the FA, has been developed. Using the concept of conservative FE groups, it is shown that the safety limit for damage parameter is exceeded for some FA rearrangement, though the limits for circumferential and equivalent stresses are not reached. This new result contradicts the wide-spread idea that the damage parameter value plays a minor role when estimating the limiting state of cladding. The necessary condition of rearrangement algorithm admissibility and the criterion for minimization of the probability of cladding failure due to damage parameter accumulation have been derived, for using in automated systems controlling the cladding tightness.
[en] The safety of fuel loading of VVER reactors is justified by calculations of the neutronic characteristics of the forthcoming campaign. These calculations are based on the design parameters of fuel assemblies (FA) — fuel enrichment, materials, design features, etc. However, during operation, some parameters change in an uncontrolled manner. In particular, FA can deform — bend or twist, this leads to the appearance of increased gaps between the fuel assemblies. These regions filled with a moderator lead to an increase in comparison with the calculations for the generation of thermal neutrons and, as a result, to a surge in the power of the fuel rods surrounding these regions. Safety requirements limit the power of fuel rods. Therefore, design capacities are increased by means of the so-called engineering margin factor to account for random outbursts. The deviation of the size of the water gaps between the fuel assemblies from the design ones should be known to calculate this coefficient, for example, the size distribution function of the, gaps. This information is most often obtained by modeling the mechanical state of fuel assemblies in the, reactor core. Other approaches are based on experimental data. Measurements in the core during discharging campaign are not possible. Therefore, the geometric parameters of the fuel assembly after the the, from the core are measured. The presented paper uses the data of such measurements obtained after 24th fuel campaign of ZNPP unit mechanical It is assumed that the fuel assemblies tend to retain the form they have in the “free” state, and analyzed., interaction with neighboring fuel assemblies leads to a certain equilibrium state that can be easily proposed In contrast to similar calculations, the elastic energy functional of interacting fuel assemblies is were, whose minimum gives the required size distribution function of the gaps. 24 and 25 campaigns studied., modelled; the role of inter-sector FA shuffling was the, The distribution of the gaps between the fuel assemblies in VVER-1000 core is calculated based on of, measured deformations of the fuel assemblies discharged from the core and the elastic characteristics not, the fuel assemblies. It was demonstrated that 95 % of gaps in the cores both with FA-A and FA-WR do the, exceed 7.6 mm. The results can be used to calculate the engineering margin factor in determining release., peaking factors of energy release.