船用水冷却原子炉の熱限界と流力特性に及ぼすヒービング,傾斜等の影響とその対策

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The biggest difference between marine reactors and land reactors is that the former is greatly influenced by the rolling pitching and heaving motions of ship. Especially in the case of water cooled reactors, the quantity of influence and how to avoid or minimize it, must be fully analyzed, since the fluctuations of void and water velocity raised by ship motion can easily lower the burnout heat flux which is the special feature of water cooling. However, for the first Japanese nuclear ship, it is decided to have water cooled reactor, and moreover, Japan is surrounded by the typhoon zone and northern rough sea, so, the above influence should be considered seriously upon planning our first marine reactor. So far in Japan, the values of the acceleration and cyclic period of motions of ships have been cleared out by the efforts of ship engineers with many experiments on board actual ships. But, of the direct effect of such motions over the thermal and hydrauric performance of reactor is not so much reported yet. For these several years, we have been studying experimentally the influence of ship motions on the critical heat flux and void fluctuations of electrically heated atomospheric water cooled simple loops, similating rolling by big scale vibration stand, and heaving by small scale heaving apparatus which can generate cyclic heaving acceleration between 0 to 1.1 G_o. Recently, we have found that by this heaving apparatus, by which high cyclic heaving acceleration is applied to the loops, the critical heat flux q_<B_o> happens to decrease significantly accompanied by the explosional generation and diminish of void and violent fluctuation of inlet water velocity, and that, when listing is over-lapped, local values of q_<B_o> near walls fall down. Stimulated by the serious significance of these phenomena, we have carefully performed the measurement of q_<B_o> at single fuel-rod type loop of natural circulation, in the range of heaving acceleration 0 to 1.1 Go, listing angle 0° to 30°, and initial exit-void ratio 1.5 to 20%. The results are as followings ; When q_<Bomin> is a iocal minimum value pf q_<Bo> of a channel with heaving and listing applied, and q_<Bomin> is that of without any outer motion applied. The ratio q_<B_o>min<>/q_<B_o>min<_o>=f, which means the lowering factor of maximum heat flux (or safe power limit of a reactor) by the influence of ship motions is expressed in the shape [numerical formula]where, K_H expresses the decrease of mean value of q_<Bo> and has the shape that, [numerical formula]G^^- is the half amplitude of heaving acceleration, while G_o is the natural gravitational acceleration, and C_h is a constant anywhere between 0.3 to 0.7 which value is characteristic to the loop construction and hydrauric performances. The important fact referred is that, as long as void fluctuation happens to take place in channel, the value of initial void ratio, and the value of listing angle have no influence to the value of C_h. Since the value of K_H is mainly influenced by the heaving motion, K_H might be called "heaving factor". K_L expresses the decrease of the ratio of local minimun q_<Bo> to the mean valus of q_<Bo> over the channel section, and is mainly influenced by the listing. The value of K_L measured is between 0.95 to 0.92 in the range of listing angle 15° to 30°, and heaving acceleration 0.5 to 1.0 G_o. In parallel to these experimental researches, we have theoretically studied the fluctuation of void and flow, including experimental loop and some actual reactor loops of high pressure and complicated structures. Through many approximations and modifications as far as possible, simplified fundamental equations are developed, which solutions were shown to fit well to the results of above experiments and can be thought to predict the fluctuations of void and flow of actual reactors. The main interesting results of these theoretical analysis are those; any loop has a resonance frequency of heaving motion which maximizes the fluctuation of void and flow velocity and any heaving motion of enough higher frequency than that of resonance had almost no effect to those fluctuations, and that any loop has a critical subcooled tempereature (T_<s2cri>) at the exist, which can suppress the generation of bulk void against the given cyclic heaving motion. The values of calculated T_<s2cri> for usual reactors are within several centigrade degrees with heaving acceleration of 1.0 G_o. The methods and ways how to avoid and or reduce void fluctuations deduced by these experiments and theories are as followings; (1) to increase the subcooled temperature of exit water, more than the above critical subcooling T_<s2cri> if possible. (2) to increase the pressure of the system (3) to increase inlet water velocity by improved forced circulation.(4) to increase folded flow path number within core. (5) to decrease flow resistance in exit part of core and riser as possible. (6) to adjust the distribution of flow to the hot channel and parallel channels and to make flat the exit temperatures. (7) to decrease the length of risers, and to improve mixing of water and void with neighbouring risers. Parts of these methods are expected to be applied to the improvement of thermal and hydrauric design of the first Japanese marine nuclear reactor.
独立行政法人海上技術安全研究所の論文
1965-01-31

船用水冷却原子炉の熱限界と流力特性に及ぼすヒービング,傾斜等の影響とその対策

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