On the occurrence of burnout downstream of a flow obstacle in boiling two-phase upward flow within a vertical annular channel

If a flow obstacle, such as a spacer is placed in a boiling two-phase flow within a channel, the temperature on the surface of the heating tube is severely affected by the existence of the spacer. Under certain conditions, a spacer has a cooling effect, and under other conditions, the spacer causes dryout of the cooling water film on the heating surface. The burnout mechanism, which always occurs upstream of a spacer, however, remains unclear.In a previous paper [Fukano, T., Mori, S., Akamatsu, S., Baba, A., 2002. Relation between temperature fluctuation of a heating surface and generation of drypatch caused by a cylindrical spacer in a vertical boiling two-phase upward flow in a narrow annular channel. Nucl. Eng. Des. 217, 81–90], we reported that the disturbance wave has a significant effect on dryout and burnout occurrence and that a spacer greatly affects the behavior of the liquid film downstream of the spacer.In the present study, we examined in detail the influences of a spacer on the heat transfer and film thickness characteristics downstream of the spacer by considering the result in steam–water and air–water systems. The main results are summarized as follows:

(1) The spacer averages the liquid film in the disturbance wave flow. As a result, dryout tends not to occur downstream of the spacer. This means that large temperature increases do not occur there. However, traces of disturbance waves remain, even if the disturbance waves are averaged by the spacer.

(2) There is a high probability that the location at which burnout occurs is upstream of the downstream spacer, irrespective of the spacer spacing.

(3) The newly proposed burnout occurrence model can explain the phenomena that burnout does occur upstream of the downstream spacer, even if the liquid film thickness t_F m is approximately the same before and behind the spacer.

For a better estimation of gamma heating in nuclear material-testing reactors and associated devices: status and work plan from calculation methods to nuclear data

The determination of gamma heating levels in material-testing reactors (MTRs) is of crucial importance as gamma heating affects both safety and performance parameters of MTRs [1 Amharrak H, Di Salvo J, Lyoussi A, Carette M, Reynard-Carette C. State of the art on nuclear heating in a mixed (n/gamma) field in research reactors. Nucl Instrum Methods A. 2014;749:57–67.[Crossref], [Web of Science ®] , [Google Scholar],2 Brun J, Reynard-Carette C, Lyoussi A, Merroun O, Carette M, Janulyte A, Zerega Y, Andre J, Bignan G, Chauvin J-P, Fourmentel D, Gonnier C, Guimbal P, Malo J-Y, Villard J-F. Numerical and experimental calibration of calorimetric sample cell dedicated to nuclear heating measurements. IEEE Trans Nucl Sci. 2012;59:3173–3179.[Crossref], [Web of Science ®] , [Google Scholar]]. The required accuracy (5% at one standard deviation) makes it necessary to calibrate bias and uncertainty associated with MTR gamma-heating calculations. Main steps of bias determination for gamma-heating calculations include, first, the development of a calculation methodology with the controlled use of physical approximations; second, the interpretation of gamma-heating measurements with reference calculations so as to determine bias supposed to be mainly due to nuclear data.     

The Role of the Collisions on the Weibel Instability Growth Rate in the Fast Ignition Scenario

In this paper, the effects of Coulomb elctron-ion collisions and plasma density gradient, \(\eta\), on the Weibel instability in inertial confinement fusion are investigated. The results are indicative that the corrected collision of Weibel instability growth rate of the relativistic region near the corona, \(\eta >0.3\), increases with increasing relativistic parameter, \(\upgamma\). Also, near the fuel core as \(\eta\) goes down, the corrected collisional growth rate decreases with increasing \(\upgamma\) for \(\mathrm{{\upgamma }}<6\) and and increases with increasing \(\upgamma\) for \(\upgamma >6\). Therefore, for \(\upgamma <6\), the effect of collision and fuel density gradient tend to stabilize the Weibel instability in fuel core, with the Weibel growth rate below the collisionless value. Also deposition condition of relativistic electron beam energy can be shifted to the fuel core for the suitable ignition.