In order to evaluate the water chemistry in the irradiation field during IASCC irradiation test, a water radiolysis code for the irradiation loop system was developed. In the water radiolysis code, a multiple node model was introduced since the irradiation loop system has a wide rage temperature distribution as well as dose distribution. To investigate applicability of the developed water radiolysis code, water chemistry at the water sampling point of the irradiation loop system was measured and compared with analytical results under several water chemistry conditions. Further, water chemistry distribution in the in-pile region as well as in the out-pile region was calculated by the developed water radiolysis code.

View full abstract

Neutronics of a reactor core with SiC cladding and structure was compared with that with steel cladding and structure analytically for small lead-cooled fast reactors. Uranium nitride fuel was used for this reactor. U²³⁵ enrichment was 11% in inner core and 13% in outer core for relatively flat neutron flux distributions and power density distribution. The core design was optimized using natural uranium blanket and nitride fuel for long life-core with reshuffling interval of 15 years. The analytical result indicated that neutron energy spectrum was slightly softer in the core with the SiC cladding and structure than that with steel cladding and structure. The SiC type reactor was designed to have criticality at the beginning of cycle (BOC), although the steel type reactor could not have criticality with the same size and geometry. In other words, the SiC type core can be designed smaller than the steel type core. The peak power densities could remain constant over the reactor operation. The consumption capability of uranium was quite high, i.e. 10% for 125 MWt reactor and 18.4% for 375 MWt reactor at the end of cycle (EOC).

View full abstract

In order to know the effects of reduced surface tension on void fraction, adiabatic experiments were conducted for both air-water and air-water with surfactant systems at room temperature and pressure. Void fraction data were obtained for bubbly, slug, churn and annular flows in a vertical channel with two subchannels simplifying a triangle tight lattice rod bundle. The void fraction was found to be lower in air-water system than air-water with surfactant one. In addition, the void fractions for both systems were found to be lower than those calculated by various correlations in literatures for circular pipe flow.
In order to study the cause of the above data trend, for annular flows, the void fraction has been calculated by a subchannel analysis using wall and interfacial friction correlations in literatures as constitutive equations, and by assuming the liquid film to be uniform over the wall perimeter. The best agreement between the calculation and the experiment has been obtained when NASCA correlation for wall friction force and modified RELAP5/MOD2 correlation incorporating reduced surface tension effects for interfacial friction force were used.

View full abstract

Series of out-of-pile experiments to obtain the knowledge on the transient void behavior during reactivity initiated accidents are in progress at JAEA. In the present series of experiments, the transient void behavior in a test section of 2 x 2 bundle geometry under atmospheric pressure condition was measured using an impedance technique. The measuring areas and the arrangement of electrodes for the impedance technique were defined on the basis of numerical analyses and scaled model experiments. The comparison was made between the impedance and differential pressure techniques for steady boiling experiments to estimate the accuracy of the impedance technique. The impedance technique showed a good agreement with the void fraction estimated from the differential pressure. The transient void behavior in the bundle geometry was measured using the impedance technique. The void fraction distribution in the bundle cross-section could be quantitatively obtained by the impedance technique. It could be properly confirmed that the transient void behavior depended on both the subcooling of inlet water and the heat generation rate of simulated fuel rods.

View full abstract

To decrease the fluid drag force on the surface of a specified object subjected to unsteady flow, under a constant volume condition, the adjoint variable method is formulated by using FEM. Based on the Lagrange multiplier method (a conditional variational principle), this method consists of the state equation, the adjoint equation and the sensitivity equation. To solve the equations effectively using the steepest descent method, a parallel algorithm is constructed. The shape optimization code for solving a 3D problem using a parallel algorithm was implemented on PC cluster using the HEC-MW library⁽¹⁾. Results show that, by using shape optimization, the fluid drag force located in Reynolds number 250 can be reduced by about 38.1%.

View full abstract

In this study, the steady-state and transient critical heat fluxes (CHFs) were measured on 1.0 mm diameter horizontal cylinders of gold and platinum heaters under saturated conditions due to transient heat inputs, Q₀exp(t/τ), in a pool of Fluorinert FC-72. Heaters were heated by electric current with the periods, τ, were ranged from 10 ms to 20 s, and the pressures ranging from atmospheric up to around 1.2 MPa. The steady-state CHFs are dependent on pressure and almost agree with the values obtained by Kutateladze's correlation based on hydrodynamic instability (HI) model. It was considered that the boiling inception and the direct transition during the steady-state period occur by the pre-pressure of = 1.2 MPa. The trend of typical transient CHFs were clearly divided into the first, second, and third groups for long, short, and intermediate periods, respectively. The direct transition process to film boiling without nucleate boiling for the short period was confirmed due to the heterogeneous spontaneous nucleation (HSN) in originally flooded cavities on the cylinder surface. It was observed that the CHFs for direct transition process during short periods show a difference in the dependency of pressures between both heaters.

View full abstract