Design and experiment of hot gas duct for the HTR-10

Abstract

A horizontal coaxial double-tube hot gas duct is a key component connecting the reactor pressure vessel and the steam generator pressure vessel for the 10 MW High Temperature Gas-cooled Reactor—Test Module. Hot helium gas from the core outlet flows into the steam generator through the liner tube, while helium gas after being cooled returns to the core through a passage formed between the inner tube and the duct pressure vessel. Thermal insulation material is packed into the space between the liner tube and the inner tube to resist heat transfer from the hot helium to the cold helium. The thermal compensation structure is designed in order to avoid large thermal stress because of different thermal expansions of the duct parts under various conditions. According to the design principal of the hot gas duct, the detailed structure design and strength evaluation for it has been done. A full-scale duct test section was then made according to the design parameters, and its thermal performance experiment was carried out in a helium test loop. With helium gas at pressure of about 3.0 MPa and a temperature over 900 °C, the continuous operation time for the duct test section lasted 98 h. At a helium gas temperature over 700 °C, the cumulative operation time for the duct test section reached 350 h. The duct test section also experienced 20 pressure cycles in the pressure range of 0.1–3.4 MPa, 18 temperature cycles in the temperature range of 100–950 °C. Thermal test results show an effective thermal conductivity of the hot gas duct thermal insulation is 0.47 W m⁻¹ °C⁻¹ under normal operation condition. In addition, a hot gas duct depressurization test was carried out; the test result showed that the pressure variation occurred on the liner tube was not more than 0.2 MPa for an assumed maximum gas release rate.

Introduction

The modular high temperature reactor (HTR-module) concept developed by Siemens/Interatom in 1981 was the first small, modular-type reactor concept to be proposed worldwide; the thermal output of the pebble bed HTR-module is 200 MW (Lohnert and Reutler, 1983, Lohnert, 1990).

The hot gas duct is a unique component exclusively found in this type of reactor where both the nuclear core and the power conversion unit are placed into two pressure vessels, which need a connecting duct. Passing through the HTR-module core, hot helium gas is conveyed via the liner tube of the horizontal hot gas duct to the steam generator. After being cooled down, the cold helium gas is returned to the lower section of the reactor pressure vessel via a passage between the coaxial inner tube and pressure vessel of the hot gas duct.

The 10 MW High Temperature Gas-cooled Reactor—Test Module (HTR-10) is the first high temperature gas-cooled reactor developed at the Institute of Nuclear Energy Technology (INET) of the Tsinghua University, China (Wang et al., 1991). Its basis is the HTR-module design. The pebble type fuel elements pile naturally in a cavity built by laying graphite blocks, forming a cylindrical pebble bed core. Helium gas is used as the coolant. The helium gas pressure in the primary loop is 3.0 MPa. The inlet helium temperature of the core is 250 °C and the outlet helium temperature of the core is 700 °C (the corresponding inlet/outlet temperatures are 300 and 950 °C in the secondary project stage of the HTR-10). The reactor core is located inside the reactor pressure vessel, while the steam generator and the helium circulator are in the adjacent vessel, i.e. the steam generator vessel. The two vessels are connected together by a hot gas duct (Fig. 1). The cold helium gas of 250 °C flows out of the helium circulator, passes through the annual space between the inner tube and the duct pressure vessel, goes up along the inner surface of the reactor pressure vessel, then flows through the core from the top to the bottom. After being heated up in the core to the 700 °C, the hot helium gas flows into the steam generator through the liner tube of the hot gas duct.

The hot gas duct has to operate at high temperature and medium pressure condition for a long time. Its performance of thermal insulation between hot and cold helium flow, its ability to resist temperature cycles and pressure cycles, and to sustain total structure integrity at high temperature and medium pressure conditions will play an important role in the long-term safe operation of the HTR-10.

Detailed structure design and strength evaluations of the hot gas duct have been carried out. A hot gas duct test section with the same structure and dimensions as the prototype was fabricated, and an engineering simulating experiment was carried out on helium gas test loop.