Hee Cheon No

A condensation experiment in the presence of noncondensables in a vertical tube of a passive containment cooling system and its assessment with RELAP5/MOD3.2

A condensation experiment in the presence of non-condensable gas in a vertical tube of the passive containment cooling system of the CP-1300 is performed. The experimental results show that the heat transfer coefficients (HTCs) increase as the inlet air mass fraction decreases and the inlet saturated steam temperature decreases. However the dependence of the inlet mixture Reynolds number on the HTC is small for the operating range. An empirical correlation is developed, and its predictions are compared with experimental data to show good agreement with the standard deviation of 22.3%. The experimental HTCs are also compared with the predictions from the default and the alternative models used in RELAP5/MOD3.2. The experimental apparatus is modeled with two wall-film condensation models in RELAP5/MOD3.2 and the present model, and simulations are performed for several subtests to be compared with the experimental results. Overall, the simulation results show that the default model of RELAP5/MOD3.2 underpredicts the HTCs, and the alternative model over-predicts them, while the present model predicts them well throughout the condensing tube.

Design of an adaptive observer-based controller for the water level of steam generators

Abstract The water level contributions from the mass capacity, reverse dynamics, and mechanical oscillations are estimated by applying an adaptive observer. In an adaptive observer, both parameters and state variables of the system are estimated simultaneously. The cost function used in control design compensates the reverse dynamics so that the controller may be insensitive to the reverse dynamics. The time-varying problem is resolved by estimating the parameters at every time step. By estimating the flow errors along with the states and the parameters, a control algorithm is derived to treat the time-varying property, reverse dynamics, and flow measurement errors. The proposed algorithm is compared with the conventional P-I controller. It is found that the adaptive observer-based controller generates faster responses and smaller swell, shrink, and overshoot than the P-I controller does.

Simultaneous visualization of dry spots and bubbles for pool boiling of R-113 on a horizontal heater

A new experimental attempt was made to simultaneously observe the dynamic behaviors of bubbles and dry spots in the vicinity of boiling surface. Also, the two-dimensional bubble structures were obtained separately. From the visualization results, the formation of bubbles and dry spots occurs simultaneously. At critical heat flux (CHF), the surface rewetting is repeated by the local nucleate boiling around the large vapor film. At just after CHF, nucleate boiling at the locally wetted region is extinguished, resulting in the dryout of the whole heater surface. Therefore, we conclude that CHF is initiated from the locally limited nucleate boiling activity rather than any hydrodynamic instability.

Measurements of void fraction by an improved multi-channel conductance void meter

Abstract An improved multi-channel conductance void meter (CVM) was developed to measure a void fraction. Its measuring principle is based upon the differences in electrical conductance of a two-phase mixture due to the variation of void fraction around a sensor. The sensor is designed to be flush-mounted to the inner wall of the test section to avoid flow disturbances. The signal processor with three channels is specially designed so as to minimize inherent bias error due to the phase difference between channels. It is emphasized that the guard electrodes are electrically shielded in order not to affect the measurement of two-phase mixture conductance, but to ensure that the electrical fields are evenly distributed in the measuring volume. Void fraction is measured for bubbly and slug flow regimes in a vertical air–water flow and statistical signal processing techniques are applied to show that CVM has good dynamic resolution which is required to investigate the structural developments of bubbly flow and the propagation of void waves in a flow channel.

A REVIEW OF HELIUM GAS TURBINE TECHNOLOGY FOR HIGH-TEMPERATURE GAS-COOLED REACTORS

Current high-temperature gas-cooled reactors (HTGRs) are based on a closed Brayton cycle with helium gas as the working fluid. Thermodynamic performance of the axial-flow helium gas turbines is of critical concern as it considerably affects the overall cycle efficiency. Helium gas turbines pose some design challenges compared to steam or air turbomachinery because of the physical properties of helium and the uniqueness of the operating conditions at high pressure with low pressure ratio. This report present a review of the helium Brayton cycle experiences in Germany and in Japan. The design and availability of helium gas turbines for HTGR are also presented in this study. We have developed a new throughflow calculation code to calculate the design-point performance of helium gas turbines. Use of the method has been illustrated by applying it to the GTHTR300 reference.

Design of stability-guaranteed fuzzy logic controller for nuclear steam generators

A fuzzy logic controller (FLC) and a fuzzy logic filter (FLF), which have a special type of fuzzifier, inference engine, and defuzzifier, are applied to the water level control of a nuclear steam generator (S/G). It is shown that arbitrary two-input, single-output linear controllers can be adequately expressed by this FLC. A procedure to construct stability-guaranteed FLC rules is proposed. It contains the following steps: 1) the stable sector of linear feedback gains is obtained from the suboptimal concept based on LQR theory and the Lyapunovs stability criteria: 2) the stable sector of linear gains is mapped into two linear rule tables that are used as limits for the FLC rules; and 3) the construction of an FLC rule table is done by choosing certain rules that lie between these limits. This type of FLC guarantees asymptotic stability of the control system. The FLF generates a feedforward signal of S/G feedwater from the steam flow measurement using a fuzzy concept. Through computer simulation, it is found that the FLC with the FLF works better than a well-tuned PID controller with variable gains to reduce swell/shrink phenomena, especially for the water level deviation and abrupt steam flow disturbances that are typical in the existing power plants.

FINAL REPORT on Experimental Validation of Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accidents

The U.S. Department of Energy is performing research and development that focuses on key phenomena that are important during challenging scenarios that may occur in the Next Generation Nuclear Plant (NGNP)/Generation IV very high temperature reactor (VHTR). Phenomena Identification and Ranking studies to date have identified the air ingress event, following on the heels of a VHTR depressurization, as very important. Consequently, the development of advanced air ingress-related models and verification & validation are of very high priority for the NGNP Project. Following a loss of coolant and system depressurization incident, air ingress will occur through the break, leading to oxidation of the in-core graphite structure and fuel. This study indicates that depending on the location and the size of the pipe break, the air ingress phenomena are different. In an effort to estimate the proper safety margin, experimental data and tools, including accurate multidimensional thermal-hydraulic and reactor physics models, a burn-off model, and a fracture model are required. It will also require effective strategies to mitigate the effects of oxidation, eventually. This 3-year project (FY 2008–FY 2010) is focused on various issues related to the VHTR air-ingress accident, including (a) analytical and experimental study of air ingress caused by density-driven, stratified, countercurrent flow, (b) advanced graphite oxidation experiments, (c) experimental study of burn-off in the core bottom structures, (d) structural tests of the oxidized core bottom structures, (e) implementation of advanced models developed during the previous tasks into the GAMMA code, (f) full air ingress and oxidation mitigation analyses, (g) development of core neutronic models, (h) coupling of the core neutronic and thermal hydraulic models, and (i) verification and validation of the coupled models.

Design of stability and performance robust fuzzy logic gain scheduler for nuclear steam generators

A robust fuzzy logic gain scheduler (robust FLGS) is designed, based on the synthesis of fuzzy logic inference and the H/sub /spl infin// technique. The robust FLGS has a set of robust control gains in a gain pool. The fuzzy inference engine chooses one of the most desirable control gains considering the steam generator (S/G) operating condition. The robust gains in the gain pool are obtained by the time-domain H/sub /spl infin//-control technique such that both of the weighted H/sub /spl infin// norms of two transfer functions are minimized: the transfer functions between the steam flow disturbance and the mass capacity portion of S/G level, and the reverse dynamics of S/G. The FLGS rules are automatically generated using a genetic algorithm (GA). The GA gives a set of the gain scheduling rules that reflect given performance design specifications well. Computer simulation confirms that the proposed controller shows both guaranteed stability and good performance in terms of small overshoot and fast settling time in spite of S/G parameter variations and steam flow disturbances.

Bubble Lift-off Diameter and Nucleation Frequency in Vertical Subcooled Boiling Flow

A series of experiments were carried out to investigate phenomena from bubble nucleation to lift-off for a subcooled boiling flow in a vertical annulus channel. A high-speed digital video camera was used to capture the bubble dynamics. The bubble lift-off diameter and bubble nucleation frequency were evaluated in terms of heat flux, mass flux, and degree of subcooling. The fundamental features of the lift-off diameter and nucleation frequency (i.e., the variations across nucleation sites and the dependence on the flow and heat flux conditions) were addressed based on the present observation. A database for the bubble lift-off diameter was built by gathering and summarizing the data of Prodanovic et al., Situ et al., and the present experiments. We evaluated the predictive capabilities of Unals model, Situ et al.s model, and Prodanovic et al.s correlation against the database. We obtained the best prediction results by modifying the wall superheat correlation in Unals model. In addition, we suggested a new correlation for a combined parameter of the bubble nucleation frequency and bubble lift-off diameter.

Assessment of RELAP5/MOD3.2.2γ against flooding database in horizontal-to-inclined pipes

Abstract A total of 356 experimental data for the onset of flooding are compiled for the data bank and used for the assessment of RELAP5/MOD3.2.2γ predictions of counter-current flow limitation (CCFL) in horizontal-to-inclined pipes simulating a PWR hot leg. RELAP5 calculations show that higher gas flow rates are required to initiate the flooding compared with the experimental data if the L/D is as low as that of the hot legs of typical PWRs. Based on the present data bank, the new CCFL correlation is derived, which shows the L/D effect. The present correlation agrees well with the database within the prediction error, 8.7% and it is implemented into the RELAP5 and validated against the data bank. The predictions of the flooding limit by the modified version lie well on the applied CCFL curve if the L/D is lower than 22, which is the case of the hot legs of typical PWRs.