It is generally known that flow-induced vibration at the nuclear fuel assembly structures may cause grid-to-rod fretting wear and subsequently rod failure. The flow-induced grid-to-rod fretting wear is found to initiate at a certain critical grid-to-rod gap that is strongly correlated with the extent of flow-induced vibration. In this paper, three vibration drivers acting on the grid-to-rod fretting wear are proposed, based on various fretting wear experience in commercial reactors as well as on various flow-induced fretting wear test results. The first active vibration driver is high turbulence-induced excessive fuel rod vibration with the combination of excessive grid-to-rod gap. The second active vibration driver is self-excited fuel assembly vibration in a low frequency range caused by hydraulically unbalanced mixing vanes of the spacer grid assembly. The third active vibration driver is self-excited spacer grid strap vibration in quite a high frequency range caused by some spacer grid designs. Each vibration driver on the grid-to-rod fretting wear damage is discussed. On the other hand, simplified three formulas for the grid-to-rod fretting wear progress such as constant work rate, constant stress rate and linear stress rate have been derived and used in predicting the rod failure time. It can be said that the linear stress rate is the most effective in predicting the rod failure time, while the constant work rate the least effective. In addition, spacer grids with the larger grid-to-rod contact area seem to generate the less fretting wear rate.
The fluid-induced fuel rod fretting wear in PWRs mainly proceeds in the grid-to-rod contact areas. This grid-to-rod fretting wear observed in various PWR fuel assembly designs may depend on external and internal vibration sources. The extent of the external vibration may be governed by reactor coolant flow velocity, non-uniform flow profiles caused by the reactor internals, and inter-fuel assembly gaps and fuel assembly-shroud gaps. The extent of the internal vibration may be controlled by spacer grid mixing vane pattern, irradiation-induced spring force relaxation and/or growth rate differences in fuel rod and guide tube. In this study, the internal vibration sources only are focused and its impact on the grid-to-rod fretting wear is evaluated, based on verification test results of a newly developed advanced fuel assembly as well as their operating experiences in various reactor conditions. In addition, key guidelines of fuel assembly design optimization are provided to prevent the grid-to-rod fretting wear-induced failure.
An advanced Pressurized Water Reactor (PWR) Fuel, PLUS7, for Optimized Power Reactors (OPR1000s) in Korea has been developed to achieve peak rod average burnup of 72 GWD/MTU, overpower margin increase of 10% with respect to a reference fuel used in the OPR1000s, fuel integrity maintenance even with 0.3 g of seismic load, zero defect against debris, and grid-to-rod fretting wear. All the mechanical and thermal hydraulic performances of PLUS7 had been verified through a wide spectrum of out-of-pile verification tests, whereas the burnup-dependent performances have been verified through in-reactor verification tests of four PLUS7 Lead Test Assemblies (LTAs) in one of the OPR1000s. During each refueling outage period, burnup-dependent parameters were measured, which include fuel assembly, fuel rod and spacer grid dimensional changes, fuel assembly bow and twist, fuel rod bow, and fuel rod oxide layer thickness. Based on the measured data, it is said that the PLUS7 fuel will maintain integrity at least up to a peak rod average burnup of 58 GWD/MTU. In addition, from the coolant activity analyses and ultrasonic tests, the PLUS7 fuel was found to be free from any fuel damage including debris-induced and grid-to-rod fretting-wear-induced fuel failures. This successful in-reactor operation of the PLUS7 LTAs has made the Korea Nuclear Fuel (KNF) initiate the supply of PLUS7 fuel on a region basis for all OPR1000s from the year 2006.
This study examines the likelihood of restatements for the financial statements disclosed before the initial public offerings (IPO) and the nature of these misstated financial statements. We find that before the IPO date, IPO firms are more likely to issue materially misstated financial statements than other public firms. Compared to the other IPO issuers, firms that restate their pre-IPO financials exhibit higher discretionary accruals and experience severe IPO underpricing. Also, the stock market reacts more negatively to the restatement announcements for pre-IPO financials than those of other public firms. Overall, our findings indicate that IPO firms are likely to issue misstated financial statements to succeed in their public debuts and equity financing.
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