F. Aglar, A. Tanrikut

Turkish Atomic Energy Authority, Turkey

Abstract. This paper gives a general insight on activities of the Turkish Atomic Energy Authority (TAEA) concerning passive cooling applications and simulation of innovative nuclear power plant design. The condensation mode of heat transfer plays an important role for the passive heat removal application in advanced water-cooled reactor systems. But it is well understood that the presence of noncondesable (NC) gases can greatly inhibit the condensation process due to the build up of NC gas concentration at the liquid/gas interface. The isolation condenser of passive containment cooling system of the simplified boiling water reactors is a typical application area of in-tube condensation in the presence of NC. The test matrix of the experimental investigation undertaken at the METU-CTF test facility (Middle East Technical University, Ankara) covers the range of parameters; Pn (system pressure) : 2-6 bar, Rev (vapor Reynolds number): 45000-94000, and Xi (air mass fraction): 0-52%. This experimental study is supplemented by a theoretical investigation concerning the effect of mixture flow rate on film turbulence and air mass diffusion concepts. Recently, TAEA participated to an international standard problem (OECD ISP-42) which covers a set of simulation of PANDA test facility (Paul Scherrer Institut-Switzerland) for six different phases including different natural circulation modes. The concept of condensation in the presence of air plays an important role for performance of heat exchangers, designed for passive containment cooling, which in turn affect the natural circulation behaviour in PANDA systems.

1. INTRODUCTION

Nuclear energy is one of the options presently available to cope with energy needs along the forthcoming century. This challenge is requiring a tremendous effort to assure nuclear energy competence in terms of economics and safety with respect to the other potential sources of energy. In the case of water cooled power reactors, new advanced designs have been proposed of either an evolutionary or a passive type, the latter being particularly appealing for using natural forces to carry out safety functions under the most adverse conditions posed by hypothetical accidents. In this regard containment of passive reactors is to be equipped with what has been called Passive Containment Cooling Systems (PCCS).

PCCS’s features depend on specific designs. However, most of them share their reliance on steam condensation to mitigate long term pressure rise in containment. New boundary conditions and device geometries prompted renewed to investigate steam condensation to eventually demonstrate PCCS’s capability to meet their goals. As a result, experimental and analytical programs were launched worldwide, often on the basis of a fruitful international co­operation [1].

Concepts of passive safety systems with no active components have been investigated for new generation light water reactors [2]. The primary objectives of the passive design features are to simplify the design, which assures the minimised demand on operator, and to improve plant safety. To accomplish these features the operating principles of passive safety systems should be well understood by an experimental validation program. Such validation programs are also important for the assessment of advanced computer codes, which are currently used for design and licensing.

In an application, the proposed advanced passive boiling water reactor design, simplified boiling water reactor (SBWR), utilises as a main component of the passive containment cooling systems (PCCS) the isolation condenser (IC). The function of the IC is to provide a passive heat exchanger for the removal of the reactor coolant system sensible heat, and core decay heat to a reservoir of water within the containment. In performing this function, the IC must have the capability to remove sufficient energy from the reactor containment in order to prevent the containment from exceeding design pressure shortly following design basis event and to significantly reduce containment pressure in the longer run [3]. However, it is well established that the presence of noncondensable (NC) gases in vapors can greatly inhibit the condensation process. The mass transfer resistance to condensation results from a build-up of NC gas concentration at the liquid/gas interface leading to a decrease in the corresponding vapor partial pressure and thus the interface temperature at which condensation occurs [3]. As a result, reduction in heat transfer rate is unavoidable with respect to the pure condensation case.

A part of the long-term research and development efforts of the Turkish Atomic Energy Authority (TAEA) is planned to concentrate on passive cooling systems. In this paper, a general insight on activities of the TAEA concerning condensation in the presence of air is given. Moreover, TAEA participated to an international standard problem (OECD/NEA ISP — 42) which covers a set of simulations of PANDA test facility, which is the scaled model of SBWR for different phases of natural circulation modes. The concept of condensation in the presence of air plays an important role for the performance of heat exchangers, designed for passive containment cooling, which in turn affect the natural circulation behaviour in such innovative systems.