Instability of two-phase flow at low flow rate conditions is investigated analytically and experimentally. By applying the drift flux model, a linear analysis is conducted to study the effect of slip which is considered to play an important role at low flow rate conditions. Results obtained are compared with those of other analytical works and the reason for apparent variations is discussed. Some notes are also given on the lumped parameters which govern instability characteristics, and on the often-observed relation between the period of oscillation and the transit time.
Experiments are conducted using R-113 at the test fluid. The results are analyzed in terms of the stability boundary and the period of oscillation. In the analysis it must be assumed that the flow is laminar. It is found that the instability observed experimentally is typical in the gravity term of the heated section is the dominant destabilizing term in the momentum equation.

View full abstract

In liquid metal cooled fast breeder reactors, electromagnetic-flowmeters (EMFs) have been extensively used. To calibrate any performance degradation, it is desired to develop an appropriate method without taking the EMF out of the system. A correlation method is applied for calibration in in-service conditions. The flow rate is obtainable from the propagation velocity of the voltage fluctuation. However, the propagation velocity of the fluctua tion is larger than the mean flow velocity.
In this paper, the relationship between the flow velocity distribution and the fluctuation frequency is considered and a method of correcting the propagation velocity is proposed based on fluid mechanics. In the case of the 12 in. diameter EMF in "JOYO" reactor, the fluctuation propagates with the average velocity in the region from the pipe center to 0.79 times the inner radius. Then the calculated correction factor which transforms the propagation velocity of the fluctuation to the mean flow velocity is 0.937 and fairly agreed with the measured data. The calibration error of the correlation method does not exceed ±2%. The accuracy of 2% is comparable with that of the calibration by real flow in test loop.

View full abstract

Thermocouple fin effect on surface temperature measurement of a fuel rod has been studied at elevated wall temperatures under film boiling condition in a reactivity initiated accident (RIA) situation. This paper presents an analytical equation to evaluate temperature drops caused by the thermocouple wires attached to cladding surface. The equation yielded the local temperature drop at measuring point depending on thermocouple diameter, cladding temperature, coolant flow condition and vapor film thickness. The temperature drops by the evaluating equation were shown in cases of free and forced convection conditions. The analytical results were compared with the measured data for various thermocouple sizes, and also with the estimated maximum cladding temperature based on the oxidation layer thickness in the cladding outer surface.
It was concluded that the temperature drops at above 1, 000°C in cladding temperature were around 120 and 150°C for 0.2 and 0.3 mm diameter Pt-Pt•Rh thermocouples, respectively, under a stagnant coolant condition. The fin effect increases with the decrease of vapor film thickness such as under forced flow cooling or at near the quenching point.

View full abstract

Production conditions of ThO₂ sols suitable for gelling into crackfree microspheres in an external gelation process were studied. The sols were prepared under pH control and colloid size distributions of the resulting sols were determined. The gelation was carried out by using hexone as a drop formation medium and ammonia as a gelling agent. The crackfree gelation was achieved by the use of ThO₂ sols produced under favorable pH, which were large in colloid size and high in colloid fraction. "Preneutralization" preceding the pH control is also important for the good sols. Analyzing the colloid fraction, colloid size and crystallite size of sol, it was found that, under the favorable pH, colloid nuclei generating at early stages grow in the form of single-crystals with their number kept constant and, after cooling, they exist as polycrystalline colloids. The mechanism of cracking is also discussed.

View full abstract

Two simulation tests for a boiling water reactor large loss-of-coolant accident (LOCA), conducted in the two bundle loop, were analyzed using the current licensing code system. These tests were recirculation-pump suction-line double-ended break tests. One of these tests assumed failures for LPCS and 1 out of 3 LPCIs, and another test assumed HPCS failure. A main objective of these analyses is to confirm the conservativeness of the licensing analysis models. Conclusions reached from the analyses are as follows :
(1) Calculated heater surface temperature begins to rise much earlier than the measured temperature, due to the conservative GEXL model for a LOCA analysis.
(2) Calculated heat up rate is higher than the test data, mainly due to neglecting the steam cooling in the analysis.
(3) Calculated heater rewetting time is later than the test data, due to neglect of counter current flow limiting at the core inlet, when the measured ECCS flow rate is used in the analysis.
It has been confirmed that the current licensing analysis models give a conservative result for peak cladding temperature (PCT), due to the model conservativeness factors presented above, when the measured data are used in the analysis for the outflow from the system and the inflow to the system.

View full abstract

New core design and operating strategies have been proposed for daily load following of an improved BWR core with large power swing.
The core concepts were based on the WNS core which uses an axially two-zoned enrichment fuel. One principal design strategy utilized was to reduce power in the lower portion of the core by adjusting a division point of the axially two-zoned enrichment fuel. One operating strategy introduced is for controlling Xe distributions. This method, coupled with a direct power distribution control by control rods, could decrease the xenon induced power change in the lower part of core.
The BWR core designed and operated under the new strategies was shown to meet the daily load demand with large power swing: 1-h reduction in power from 100 to 50%; 8-h hold at 50% power; 1-h increase in power from 50 to 100% ; and 14-h hold at 100% power.

View full abstract