High Temperature Test Facility Description

The High Temperature Test Facility (HTTF) is an electrically heated, helium cooled, experimental facility located at Oregon State University (OSU). Designed as a scaling facility for the Modular High Temperature Gas Reactor (MHTGR), the HTTF produces thermal hydraulic conditions expected in advanced gas-cooled nuclear reactors. The HTTF consists of a prismatic ceramic core which accommodates 516 coolant channels, 42 bypass channels, and 210 electric heating elements. Coupled to the core is a steam generator for full thermodynamic analysis of a gas-cooled reactor system. To provide a radiative boundary for core heat loss quantification, the HTTF core is surrounded by the Reactor Cavity Cooling System (RCCS). The RCCS is a square array of steel panels accommodating water flow which surrounds the reactor pressure vessel (RPV) of the HTTF. Outside the RCCS water panels is a layer of fiberglass insulation which serves as the final thermal boundary between the HTTF system and ambient conditions of the room.

Coolant is provided to the RPV via the inlet portion of the combined inlet/outlet duct. Entering coolant travels upwards around the core in the upcomer. The upcomer leads into the upper plenum where the coolant makes a 180° turn and flows downward into the core coolant channels. After heating in the coolant channels, the coolant exhausts into the lower plenum, takes a 90° turn, and exits the RPV via the outlet portion of the combined inlet/outlet duct. A detailed description of the HTTF can be found here

Instrumentation in the HTTF core provides gas concentration, ceramic core temperature, coolant temperature, and system pressure. A detailed description of the HTTF instrumentation can be found here

PG-26 Transient Description

Tests have been conducted at the HTTF to provide experimental data for hypothetical transients in gas-cooled nuclear reactors. Specifically, the PG-26 transient aimed to simulate a Depressurized Conduction Cooldown (DCC). The DCC transient emulates a reactor which undergoes a simultaneous power excursion, loss of forced convective coolant flow, and depressurization of the primary system. To accomplish this, the HTTF underwent the following transient progression:

1) t = 0-180,000 s

a) Heat up

2) t = 180,000 s

a) Turn off primary loop gas circulator b) Depressurize primary loop to atmospheric conditions

3) t = 180,000-213,000 s

a) Increase electric heater power to emulate reactor power excursion b) Slowly decrease electric heater power to emulate decay heat curve of nuclear reactor core

4) t = 213,000-270,000 s

a) Shut off electric heater power, monitor temperatures

Of note, the transient had a few anomalies which affected the results. During part 1 of the transient, a slow coolant leak occurred in the primary system which caused a decrease in primary coolant inventory. HTTF operators were able to fix the leak during the test, however, the amount of coolant lost, amount of make-up coolant added, temperature of the make-up coolant, and the time of the make-up coolant addition were unknown during the making of this model.

During parts 2 and 3 of the transient multiple thermocouples temporarily failed due to the high temperatures experienced in the HTTF core. This resulted in experimental data sets which include instantaneous changes in temperature which are not realistic.

At approximately t = 213,000 s, one of the electric heater banks failed and lost power. To preserve core symmetry HTTF operators turned off the other operating heater bank. This resulted in a premature termination of part 3 of the transient.