Yoshiro Asahi

New Non-Equilibrium Thermal-Hydraulic Model, (II)

A through analysis of LOFT L2-3 is made with the THYDE-P code, which is based on the new non-equilibrium thermal-hydraulic model. The various assumptions and correlations in use for the present analysis are explained. In order to investigate DNB (departure from nucleate boiling), rewetting and quenching at the pool condition, several parameters are defined. The interfacial heat transfer coefficient between the gas and the liquid is assumed to have the same pressure dependence as of nucleate boiling. The overall trends of the experiment are well reproduced by the calculation. Much effort was found necessary, however, to better understand DNB, rewetting and quenching at the pool condition, by taking into account the stored thermal energy and the stationariness of the temperature distribution in the fuel rod.

Improvement of Passive Safety of Reactors

A new subsystem called the passive safety and shutdown system (PSSS) is proposed to improve nuclear reactor safety. The subsystem can be added to the existing reactor coolant system (RCS) by a slight modification. To exemplify how the subsystem improves pressurized water reactor safety, anticipated transient without scram under loss of normal ac power is analyzed. The result indicates that the proposed subsystem is very effective in putting an end to the accident. The PSSS not only improves passive reactor safety, but also simplifies RCS and its control and surveillance systems. Items needing further investigation with regard to PSSS design are also discussed.

Theory of Omega-d, Modes

A theory is presented on the omega-d modes which are eigenfunctions of a particular type of neutron diffusion operator. First, a general discussion is given on the implicit eigenvalue problem, reality, boundedness and distribution of the eigenvalues, order of magnitude and closure of the eigenfunctions. It is shown that the regular and adjoint eigenvalue problems reduce to an identical implicit eigenvalue problem. No one has in the past explicitly shown that the eigenvalue is real. The large variation in the order of magnitude of the eigenfunctions gives rise to a large variation in the mode coupling. This implies existence of weakly coupled modes that can be ignored. Next, these properties are verified for a symmetric reflected slab reactor by numerical calculations. The results disprove the conjecture advanced by others that the neutron fluxes associated with a particular cluster of seven modes are approximately same. Finally, an equation is derived for the mode amplitudes which takes into account tempe...

Flooding and flow reversal of two-phase annular flow

Abstract The flooding and flow reversal conditions of two-phase annular flow are mathematically defined in terms of a characteristic function representing a force balance. Sufficiently below the flooding point in counter-current flow, the interface is smooth and the characteristic equation reduces to the Nusselt relationship. Just below the flooding point and above the flow reversal point in cocurrent flow, the interface is “wavy”, so that the interfacial shear effect plays an important role. The theoretical analysis is compared with experimental results by others. It is suggested that the various length effects which have been experimentally observed may be accounted for by the spatial variation of the droplet entrainment.

Conceptual Design of the Integrated Reactor with Inherent Safety

The Integrated Reactor with Inherent Safety (IRIS) has been designed with a primary objective of ensuring fuel integrity by passive means only. The steam generator is a once-through helical coil type. The steel reactor pressure vessel is submerged in an outer pool contained in a prestressed concrete containment vessel. The primary flow path, which has a double syphon structure with the main coolant pumps located at the outlet of the steam generator, is formed by concentric annuli. The various components required for steady-state plant operation are driven by a turbine or by on-site power so that they can be automatically shut down. Due to these passive features, not only are various systems simplified or eliminated, but constraints on the plant layout are also reduced

Some transient characteristics of PIUS

Transient characteristics of a process inherent ultimate safety (PIUS) reactor in typical accidental situations are investigated by using the plant dynamics analysis code THYDE-W. First, the modeling and steady-state adjustment for thermal hydraulics of a PIUS is described. Then the transient calculations are made for a pump coastdown, a steam generator feedwater trip, a flow blockage, and a boron dilution. They show that inherent safety of PIUS is ensured without control rod scram. The entrance of pool water to the primary loop, however, could give rise to a potential problem of thermal shock.

Intrinsically Safe and Economical Reactor (ISER)

Abstract The Intrinsically Safe and Economical Reactor (ISER) is designed based on the principle of a process inherent ultimate safe reactor, PIUS, a so-called inherently safe reactor (ISR). ISER has been developed jointly by the members of the Kanagawa Institute of Technology, the University of Tokyo, the Japan Atomic Energy Research Institute (JAERI) and several industrial firms in Japan. This paper describes the requirements for the next generation of power reactor, the safety design philosophy of ISR and ISER, the controllability of ISER and the results of analyses of some of the design-based accidents (DBA) of ISER, namely station blackout, accidents in which the pressurizer relief valve becomes jammed and stuck in open position and tube breaks in the steam generator. It is concluded that the ISER can ensure a wide range of controllability and fuel integrity for all the analysed DBAs.

Analysis of a BWR Turbine Trip Experiment by Entire Plant Simulation with Spatial Kinetics

Abstract An analysis of a boiling water reactor turbine trip was performed with the THYDE-NEU code. In spatial kinetics, reactivity was not used since the three-dimensional transient diffusion equation was solved with the implicit direct integration method. The plant was treated as a closed coolant system, and hence, it was necessary to cope with thermal-hydraulic behaviors at pressures as low as the atmospheric pressure. At low pressures, nonlinearity of the thermal-hydraulic equation is enhanced, and hence, a thermal nonequilibrium model is required. To satisfy the measured initial pressure distribution within the reactor, it was necessary to have the moisture separator model and to account for a reversible pressure drop at a junction with a flow area change. Among the parameters in THYDE-NEU is γ in the thermal nonequilibrium model in addition to C1 and C2 regarding the manner in which to express the coolant density used in the table look-up of cross sections. For a pair of C1 and C2, it is possible to find parametrically a value of γ, namely, γC, so that THYDE-NEU can reproduce the experimental fact that the core-averaged local power range monitor output RAPRM reached 0.95 at 0.63 s to generate a scram signal. One of the calculations with γC was compared with the experiment. It was shown that the spatial kinetics results are sensitive to the temporal behavior of the bypass valve opening. Among the assumptions in use, those to be scrutinized before further performing sensitivity calculations were indicated.