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Hidaka, Akihide; Kudo, Tamotsu; Fuketa, Toyoshi
Transactions of the American Nuclear Society, 91, p.499 - 500, 2004/12
The radionuclides release from MOX under severe accident conditions was investigated in the VEGA program to prepare the technical bases for safety evaluation including PSA for LWR using MOX. The MOX specimen irradiated at ATR Fugen was heated up to 3123K in He at 0.1MPa. The Cs release started at about 1000K and was enhanced below 2200K compared with that of UO
. The possible reason is due to the formation of cracks connected to the high burn-up Pu spots. The total fractional releases were evaluated by alpha-ray, gamma-ray and ICP-AES and compared with the ORNL-Booth model. Although the model was prepared based on the tests with UO, the predictions are in reasonable agreement with the measurements. The VEGA test showed that the total releases from MOX are almost the same as those from UO under extremely severe accident conditions. This indicates that the consequences of LWR using MOX are mostly equal to those using UO. The effect of difference between MOX and UO on the consequences will be systematically investigated using the JAERI's source term code, THALES-2.
Yamamoto, Toshihiro; Miyoshi, Yoshinori
Transactions of the American Nuclear Society, 91, p.583 - 584, 2004/11
MOX powder and additives are mixed in the process of MOX fuel fabrication. A non-uniform mixing state of MOX powder and additives occurs during the homogenization mixing process. However, ordinary criticalit safety evaluations for mixtures assume that the mixtures have a uniform distribution of the mixing state. A non-uniform distribution of the mixing state in a sphere, which maximizes the effective neutron multiplication factor, was obtained using a concept of the fuel importance. As a result, the central portion of the sphere is composed of an optimal moderation region, and the surrounding region is composed of pure MOX powder. While keff is 0.545 for the uniform distribution, keff for the optimal non-uniform distribution is 0.590. That is, keff increases by 0.045.
Watanabe, Shoichi; Yamamoto, Toshihiro; Miyoshi, Yoshinori
Transactions of the American Nuclear Society, 91, p.431 - 432, 2004/11
Temperature effect is a main factor which affects the transient characteristics at a criticality accident. A series of reactivity effects due to changes in fuel temperatures were measured for two kinds of STACY heterogeneous lattice configurations. The core was composed of LWR-type fuel rod array and low-enriched uranyl-nitrate-solution concerning the dissolver of the reprocessing facility for LWR spent fuel. The critical solution heights at various solution temperatures were measured. From the change of the critical water height with fuel temperature, the reactivity effect was evaluated by a critical-solution-level worth method. The temperature effect was also calculated by using SRAC and the transport calculation code TWODANT. The experimental value was estimated to be -2.0 cent/
C for the case "2.1cm-pitch", and -2.5 cent/C for the case "1.5cm-pitch". The calculated results gave agreement with the experiments within 
10%.
Okawa, Tsuyoshi; Greenspan, E.*
ANS 2004 Winter Meeting, 1(1), 31 Pages, 2004/00
This paper describes a study of the feasibility of designing the ENHS (Encapsulated Nuclear Heat Source) reactor core to have a negative coolant void reactivity feedback along with a flat power distribution using a stepped geometry core.
Sakaba, Nariaki; Nakagawa, Shigeaki; Furusawa, Takayuki; Tachibana, Yukio
Transactions of the American Nuclear Society, 91, P. 377, 2004/00
Carbon deposition occurred occasionally in the graphite-moderated gas-cooled reactors was evaluated for the reactor pressure vessel, intermediate heat exchanger, etc. using the measured chemical impurity data for the initial condition of the safety demonstration test. By the evaluated result, it is confirmed that the high-temperature components keep their structural integrity during the any temperature transients in safety demonstration tests.
Kato, Masato; Kihara, Yoshiyuki; Konashi, Kenji*
Transactions of the American Nuclear Society, 91, p.463 - 464, 2004/00
The oxygen potentials of (Np
AmPu
U
)O
are determined depending on O/M ratio at temperatures from 1200 to 1350. The variation of the lattice parameter depending on O/M and the temperature of phase decomposition were also obtained.