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Suzuki, Motoe
no journal, ,
The RANNS code has been developed for the analysis of LWR fuel rod behavior in accident conditions. Progress of analysis is described with test IDs of the NSRR experiments and main features of results. The present stage of model development is illustrated in a map, and a trial evaluation of failure condition is placed in the map as an approximate method prior to the failure prediction function which will be attained by introducing a microscopically designed failure criterion. In the trial evaluation, the stress intensity factor KI is estimated for an incipient crack which is generated in the outer region of cladding. This method is applied to the OI-11(high burnup PWR fuel) test in the NSRR, and temporal change of KI in plane strain condition is obtained from cladding geometry, predicted temperature and hoop stress, to determine an apparent critical KI (=KIC) value at the failure moment in the actual experiment. This KIC value is compared with the value obtained in the analysis of another PWR experiment. The result is that the analysis is consistent with the actual failure in terms of KIC value on the assumption that the incipient crack size is equal to the thickness of hydride rim region. Also, applying this method to the BWR cladding suggests that the size of hydride in BWR cladding is so large that the incipient crack size is to be set much larger than that of the PWR cladding. On these results, next step and perspectives are considered.
Ota, Akira
no journal, ,
The cladding stress-strain model is essential to analyze fuel behaviors in the anticipated transients or the accidents, and it becomes especially important for evaluating the high-burnup fuel under RIA condition. Recently the material property tests have been conducted for the fuels of burnup is up to 60 GWd/t (pellet). Using these data, a new model is investigated that is applicable to this data range and up to much higher burnup. To determine the strength coefficient, K, and the strain hardening exponent, n, describing plastic deformation, a least-square fit is performed. On this occasion, the existence of intermediate region is assumed between the elastic and plastic regions. From the fittings, following results have been obtained; the effect of irradiation on the n-value is insignificant in the data range of burnup= 26-58 GWd/t. The effect of irradiation on the K-value is also insignificant after burnup exceeds 20 GWd/t.
Nagase, Fumihisa
no journal, ,
JAEA conducts experimental studies to evaluate high burnup fuel behavior under LOCA conditions. This paper presents results from recent quench tests with irradiated cladding sampled from 70 GWd/t PWR fuels.
Sugiyama, Tomoyuki
no journal, ,
LWR fuel behaviors during a reactivity initiated accident (RIA) are being studied in the NSRR test program. Results from recent three tests are presented. The test with a ZIRLO sheathed UO
rod resulted in fuel failure. Despite of high burnup of 79 GWd/t and thick oxide layer of 
70
m, the enthalpy at failure was approximately 230 J/g, which is slightly lower than 250 J/g obtained for rods with 40m-oxide at 50 GWd/t. The result shows that the cladding with improved corrosion-resistance increases safety margin against fuel failure, and that the decreasing rate of fuel enthalpy at failure becomes low with corrosion increase.
Udagawa, Yutaka
no journal, ,
With the RANNS code, analyses of PCMI behavior were performed for high burnup BWR fuel series with identical specification and base irradiation history in order to investigate mechanical conditions at PCMI failure in pulse-irradiation tests in the NSRR. The target of analysis is FK-series consisting of FK-6, 7, 8, 9, 10 and 12. The FK-series rod is a typical BWR 8
8 step II fuel and was subjected to base irradiation in Fukushima-Daini power plant for 8 years up to pellet average burnup of 61 GWd/tU before pulse-irradiation in the NSRR. Heat removal from pellet is significant in FK-8 due to relatively wide pulse. Therefore, for a similar amount of energy deposition in fuel rod, wide pulse tends to cause small pellet thermal swelling, which explains small hoop strain in cladding periphery in the FK-8 considering its energy deposition. Based on calculated stress, strain and measured failure time, cladding stress and strain at PCMI failure were estimated, which revealed the following features. Estimated failure stress and strain in cladding periphery distribute in narrow ranges and definite thresholds exist between failure and non-failure cases. This clear trend of grouping suggests the importance of cladding periphery in PCMI behavior of BWR fuel. Magnitude of failure stresses and strains macroscopically represents the degree of resistance of the FK-series cladding against crack propagation by PCMI, while deviation reflects randomicities which are dependent on local mechanical conditions, distribution of crack in pellet, orientation of zirconium-hydride in cladding, and so on.
Hamanishi, Eizo*
no journal, ,
JAEA introduces analytical method to obtain true stress - true strain curve from the ring tensile test results. In the method, Finite Element method Analysis is performed iteratively to reproduce the experimental load - displacement curve. The strain hardening exponent and the strength coefficient, representing the true stress - true strain obtained by this analysis, were in agreement with reference data. Accordingly, the evaluation method was successfully verified.