Tomonari Koga

Prediction method for thermal stratification in a reactor vessel

Abstract The prediction method for thermal stratification phenomena in a fast breeder reactor is described. The focus of attention is placed on the applicability of water test results to predict thermal stratification phenomena in a real plant. The basic feature of thermal stratification was examined in a cylindrical plenum, using water and sodium as test fluids. The similitude relationship between a small-scale test and a real plant is discussed in order to understand the experimental results. The scale-model experiments for LMFBRs (liquid metal-cooled fast breeder reactors) were also performed to see the effects of a reactor configuration and reactor-trip operation condition. Then the magnitudes of the temperature gradient and the ascending speed of stratified interface in the hot plenum of LMFBRs were predicted, based on the results of the water scale-model.

Heat removal characteristics of vault storage system with cross flow for spent fuel

Abstract Since the amount of spent fuel to be stored is expected to steadily increase in Japan, a use of large-scale dry storage facilities is considered to be a promising method in practice at reasonable economic cost. The present study is concerned with the heat removal experiment of vault storage system adopting cross flow with passive cooling, using a 1/5 scale model. The results show that the flow pattern of air in the storage module strongly depends on the ratio of the buoyancy to the inertia force. A simple method to estimate air flow patterns in the storage module was proposed, where the Ri (Richardson) number was considered as the most representative parameter. Then the heat transfer rate from a storage tube to cooling air was estimated, which could apply to the design of a full-scale vault storage system with cross flow, in which dozens of storage tubes were placed. The acquired information was also used to optimize heat removal design of the vault storage system in the present study.

Quantitative prediction of natural circulation in an LMFR with a similarity law and a water test

This paper is devoted to logical derivation of a similarity law for single-phase natural circulation expected in a liquid metal fast reactor (LMFR). Though the way of the derivation is principally conventional, this paper shows that explicit definition of a representative velocity and a representative temperature difference, as used in previous studies, is generally inappropriate in formulating a similitude law. The paper also presents formulae which allow to directly convert water test results into actual plant values. Using the conversion formulae, it is demonstrated that data acquired in a small scale water experiment can be converted and the results are comparable with results of actual LMFR computation. The accuracy of the experimental prediction is discussed.

Experimental and computational simulation for natural circulation in an LMFBR

This study has been conducted to establish a simulation method for the natural circulation phenomena in the reactor vessel of an LMFBR. The principal results obtained in the present study are as follows: 1. (1) Two dimensionless numbers, the Gr′ number and the Bo′ number, are derived and found the most influential on the natural circulation. 2. (2) A large velocity fluctuation is observed even in the steady state as if a mean velocity field does not exist in natural circulation. 3. (3) A higher order numerical scheme seems to be appropriate to simulate natural circulation.

A New Concept of the 4S Reactor and Thermal Hydraulic Characteristics

CRIEPI (Central Research Institute of Electric Power Industry) has been developing the 4S reactor (Super Safe, Small and Simple reactor) for application to dispersed energy supply and multipurpose use, with Toshiba Corporation [1,2,3,4]. Electrical output of the 4S reactor is from 10MW to 50MW, and burn-up reactivity loss is regulated by neutron reflectors. The reflector that surrounds the core is gradually lifted up to control the reactivity according to core burn-up. 30year core lifetime without refueling can be achieved with the 10MW 4S (4S-10M) reactor. All temperature feedback reactivity coefficients, including coolant void reactivity, of the 4S-10M are negative during the 30year lifetime. A neutron absorption rod is set at the center of the reactor core with the ultimate shutdown rod. The neutron absorption rod used during the former 14 years is moved to the upper part of the reactor core, and the operation is continued through the latter 16 years. The pressure loss of the reactor core is lower than 2kg/cm2 to enable effective utilization of the natural circulation force, and the average burn-up rate is 76GWD/t. To suppress the influence of the scale disadvantage, loop-type reactor design is one of the candidates for the 4S-10M. The size of the reactor vessel can be miniaturized by adopting the loop type design (4S-10ML). In the 4S-10ML design, integrated equipment which includes primary and secondary electromagnetic pumps (EMPs), an intermediate heat exchanger (IHX) and a steam generator (SG) is adopted and collocated by the reactor vessel. The decay heat removal systems of 4S-10ML consist of the reactor vessel air cooling system (RVACS) and SGACS (a similar system to the RVACS, with air cooling of the outside of the integrated equipment vessel). They are completely passive systems. To decrease the construction cost of the reactor building, a step mat structure and the horizontal aseismic structure are adopted. 4S-10ML has unique features in the cooling systems such as integrated equipment and two separate passive decay heat removal systems which operate at the same time. To evaluate the design feasibility, the transition analyses were executed by the CERES code developed by CRIEPI [5]. In this paper, the design concept of 4S-10ML, and the results of the plant transition analyses are described.© 2004 ASME

An application of a higher order finite difference method to a natural convection experiment in the hot plenum of an LMFBR

Abstract A two dimensional thermal-hydraulic analysis of a natural circulation experiment has been performed to evaluate the effectiveness of a higher order finite difference method for solving the Navier-Stokes and the energy equations. In the method, the convection terms appearing in each equation are solved by the Method of Characteristics using the third order Lagrange type polynomial as the interpolation function, and an iterative procedure is applied to solve the time derivative terms of each equation stably with second order accuracy. The analytical results have been compared with an experiment in which the temperature and the velocity distributions in the plenum region were measured with their fluctuations, and it was shown that the higher order finite difference method could simulate natural convection phenomena involving fluctuations well.

ICONE15-10603 HEAT REMOVAL CHARACTERISTICS OF THE 10MWE SODIUM COOLED SMALL FAST REACTOR (4S)

The 4S is a sodium-cooled small fast reactor designed to supply energy and hot water on isolated islands or in remote locations. Small reactors are often said to have disadvantages in terms of economies of scale. This problem can be overcome by making the structure simple, doing away with the need for maintenance, and designing a core that requires no refueling during reactor lifetime. Taking into account the nature of current demand from the economy and the market, there are two options for the electrical output of the 4S reactor: 10MWe and 50MWe. The core of the 10MWe 4S reactor (4S-10M) has a 30-year lifetime without refueling. Metallic fuel is employed. Burn-up reactivity loss of the 4S is regulated by neutron reflectors which surround the core. All temperature reactivity coefficients including void reactivity are kept negative during core life-time. The 4S-10M is a tall pool-type reactor. It has an intermediate heat exchanger (IHX) in the annulus space inside the reactor vessel (R/V). There are two primary electro-magnetic pumps (EM pumps) in serial under the IHX, and an air flow path on the surface of the guard vessel (G/V) as a decay heat removal system (RVACS; Reactor vessel auxiliary cooling system). The secondary sodium heated in the shell part of the IHX flows to the steam generator (SG) via the tubes of the air cooler of the intermediate reactor auxiliary cooling system (IRACS). The decay heat removal systems of 4S-10M consist of a RVACS and IRACS. Both are passive systems. To clarify the safety margin of the 4S-10M, it is important to confirm the characteristics of the two decay heat removal