thick zirconium-alloy core vessel has a 
much lower pressure rating, since the 
core-to-blanket differential pressure is 
controlled and is also limited by rup- 
ture discs. (Use of a thin core vessel 
minimizes absorption of neutrons pass- 
ing into the blanket region.) 
Key Control Loops 
Key loops for HRT, shown in Fig. 3, 
control the following: 
Core pressure is controlled at 2,000 
psi from sensed pressure by the propor- 
tioning of power to the pressurizer elec- 
tric heaters; the blanket pressure is 
similarly controlled by a core-to-blan- 
ket differential-pressure signal. 
Pressurizer liquid level is controlled 
from sensed level by pneumatic control 
of the letdown valve. (Electric control 
actions from the signals from pneu- 
matic transmitters and transducers are 
derived from pressure switches. Sole- 
noid-actuated pilot valves provide elec- 
tric interlock control of the pneumatic 
signals to final control elements.) 
Reactor power is controlled by tur- 
bine-governor (or manual) signal to a 
valve throttling steam from the core 
heat exchanger. Because of the large 
negative temperature coefficient and 
the low heat capacity, this reactor sys- 
tem cannot produce more heat than is 
being removed, except for very short 
times. Heat is removed by opening 
the steam throttling valve, which 
causes a decrease in the temperature of 
the fuel leaving the heat exchanger. 
Because of the negative temperature 
coefficient of the system, this cooler 
fluid entering the core causes an in- 
crease in reactivity and the fuel is re- 
heated until it overcomes the excess 
reactivity. 
Average core temperature is con- 
trolled by varying the concentration of 
the fuel solution. That is, the cireulat- 
ing fuel solution can be concentrated 
or diluted by pumping water or more 
concentrated fuel to the reactor core. 
The nominal core outlet temperature 
when the design power of 5 Mw of 
heat is being produced is 300° C, and 
the inlet temperature is about 250° C. 
The inlet and outlet temperature will 
vary with the power extraction, while 
the average temperature is a function 
only of fuel concentration. 
Blanket temperature is controlled 
by a signal, derived from the difference 
in average core and blanket tempera- 
ture, that operates the steam valve of 
the blanket heat exchanger. 
inc clases IN 
SESES wake me mm ey Rm “ 
FIG. 2. Main control board and console for HRT 
Pressure 
switches 
Pressure 
Switches 
Electric - 
Recorder 
Controller 
Control 
Circuit 
pneumatic 
converter 
Pressure 
Recorder 
Controller 
separator 
FIG. 3. 
Key control loops 
Plant Layout 
The reactor equipment exposed to 
the radioactive fuel and blanket solu- 
tions is located in a 54 X 31 X 25-ft- 
deep steel tank lining an excavation. 
The top of this tank is flush with the 
building floor and ground level for 
economy in shielding. Conduits and 
piping from this reactor cell enter un- 
derground auxiliary-system galleries 
through a 515-ft side shielding tank 
filled with aggregate and water. How- 
ever, the main control room and the 
steam equipment are above ground. 
The reactor cell tank is normally held 
Price Tag 
Approximately $400,000 was spent for 
labor and materials for the HRT _ instru- 
mentation and control systems; this was 
about 15% of the plant costs. Materials 
costs were about $100,000 for valves and 
valve operators and $100,000 for instruments 
and electrical control equipment. 
Circulating pump 
Pulsafeeder 
pump 
at a pressure of 0.5 atm, so that, in the 
very remote possibility of a brittle fail- 
ure of the reactor vessel, the pressure 
rise would be limited to that which can 
be contained by the tank. The 5-ft- 
thick concrete roof shielding plugs are 
removable for remote maintenance 
operations. 
All process and instrument lines 
through the shield wall are blocked by 
valves on signal of high shield pressure 
to prevent the escape of radioactivity. 
Electrical leads are sealed by glass-to 
metal seals. 
Thermocouple, electrical, and air 
lines can be disconnected by remotely 
operated tools for equipment repair or 
removal. 
All critical core and blanket system 
transmitters, except electrical level 
transmitters and thermocouples, are 
located in two shielded instrument 
cubicles located adjacent to but out- 
27 
