Nuclear Engineering and Design 139 (1993) 221-233 221 North-Holland Neutronic and thermal hydraulic design of the graphite moderated helium-cooled high flux reactor Peng Hong Liem and Hiroshi Sekimoto Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan Received 6 April 1992, revised version 22 June 1992 A new design concept for a high flux reactor was investigated, where a graphite moderated helium-cooled reactor with pebble fuel elements containing (Z35U,23Su)o2 TRISO coated particles was taken as its design base. The reactor consists of an annular pebble bed core, a central heavy water region, and inner, outer, top, and bottom graphite reflectors. The maximum thermal neutron flux in its central heavy water region as high as 10is era -2 s -1 with thermal neutron spectral purity of more than two orders of magnitude and a large usable volume of more than 1,000 liters can be achieved by (1) diluting the fissile material in the core and (2) optimizing the core-reflector configuration. The in-core thermal-hydraulic analysis was done to obtain adequate information about the coolant flow pattern and pressure distribution, core temperature profile, as well as other safety aspects of the design. To protect the reactor during off-normal or accident events, a large margin of temperature difference between the operating fuel temperature and the fuel limit temperature is confirmed by lowering the coolant inlet and core rise temperatures. 1. Introduction An alternative design concept for a high flux reactor (HFR) has been proposed by P.H. Liem et al. [1] and its feasibility is being investigated. Instead of a water- cooled reactor, a graphite moderated helium-cooled reactor with pebble fuel elements containing (Z35U, 23SU)Oe TRISO coated particles was taken as the design base. To provide higher values of thermal neu- tron flux with a better spectral purity and larger usable volume, further calculation and refinement of the de- sign have been continuously worked, Those efforts are also accompanied by the in-core thermal-hydraulic analysis to provide adequate information of the coolant flow pattern and pressure distribution, core tempera- ture profile as well as other safety related aspects of the design. This paper presents the methods, procedures and results of the above design calculation and refinement. Section 2 reviews briefly the fundamental physics be- hind the design concept as well as objective and proce- dures for refinement and optimization of the HFR design. In Section 3, the thermal-hydraulic properties of the core containing pebble fuel elements and related experimental data are discussed, and numerical meth- ods for solving the steady-state thermal-hydraulic and flow problems inside the core are presented. In Section 4, the results of the calculation and refinement of the design are presented and discussed. Section 5 provides the conclusion of the present work. 2. Neutronics 2.1. Design objectives and procedures The main purpose of the high flux research reactor is to provide a neutron flux density as high as possible having an energy spectrum, space dependence and usable volume which are optimized to meet the user's or experimenter's needs. Furthermore, the reactor fa- cility must be able to be operated easily, safely and uninterruptedly. The high flux reactor design objectives, procedures and constraints are tabulated in table 1. The design procedures are divided into neutronic and thermal-hy- draulic parts, where for each part, the design objectives and constraints are defined. However, in the present work, rather than concluding with only one optimal design, we search optimal ranges of various design parameters which give high values of figure of merit (FOM) and desirable thermal design results. The re- 0029-5493/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved