FFR_chap16.txt

CHAPTER 16
AIRCRAFT REACTOR EXPERIMENT*

The feasibility of the operation of a molten-salt-fueled reactor at a truly
high temperature was demonstrated in 1954 in experiments with a reactor
constructed at ORNL. The temperature of the fuel exiting from the core
of this reactor was about 1500°F, and the temperature of the fuel at the
inlet to the core was about 1200°F. The reactor was constructed before
the mechanism and control of corrosion by molten salts had been fully
explored, and therefore the experimental operation of the reactor was of
short duration. Since the work was supported by the Aircraft Reactors
Branch of the Atomie Energy Commission, the reactor was called the Air-
craft Reactor Experiment (ARE).T

The ARE was a thermal reactor in which moderation was accomplished
by BeO blocks through which the fluoride fuel was circulated in Inconel
tubes arranged in a symmetrical, heterogeneous matrix. The Inconel ves-
sel containing the core was essentially a right cylinder, approximately
52 in. OD and 44 in. in height, with 2-n.-thick walls. The fuel passages
consisted of l-in.-diameter Inconel tubes arranged in six parallel circuits,
and each circuit, by the use of reverse bends at top and bottom of the
core, made cleven passes through the core. The fuel passages did not
traverse the peripheral BeO blocks which served as a reflector around the
core of the reactor. A top view of the BeO blocks and the Inconel tubes
is shown in Fig. 16-1. The moderator and reflector blocks were cooled
by circulating liquid sodium from the bottom to the top of the pressure
vessel. The sodium permeated all interstices of the BeO and flowed rap-
idly through 1/2-in. vertical holes in the reflector sections of the BeO. An
elevation drawing of the reactor which illustrates these features is presented
in Fig. 16-2, and a photograph of the reactor vessel that was taken before
assembly of the thermal shield is shown in Fig. 16-3.

Since the purpose of the operation of this experimental reactor was to
study the behavior of the circulating-fluoride-fuel system and to identify
the problems associated therewith, the power output of the reactor was
not utilized but, rather, was simply dumped as heat. The heat-removal
svstem 1s shown schematically in Fig. 16-4. The fuel was circulated
through a finned-tube radiator type of heat exchanger. This radiator was
located within a sheet-metal housing of a toroidal shape. In another part
of the toroidal housing there was a second finned-tube radiator through

*By E. 8. Bettis and W. K. Ergen.
1R. C. Briant et al., Nuclear Science and Engineering, Vol. 2, No. 6, 795-853 (1957).
673
674 AIRCRAFT REACTOR EXPERIMENT [cHAP. 16

 

F1e. 16-1. Top view of the reactor core of the ARE. Hexagonal beryllium
oxide blocks serve as the moderator. Inconel tubes pass through the moderator
blocks to carry the molten-salt fuel.

which plant water flowed. A large centrifugal blower circulated the coolant
gas (helium) in the toroidal loop so that heat was picked up from the fuel
radiator and dumped into the water radiator.

An identical arrangement of radiators and blower was used for cooling
the sodium used as the moderator-reflector coolant. In the interest of
safety (for removal of afterheat in the event of a pump failure), the so-
dium circuit was installed in duplicate so that an entire sodium cooling
system was available as a spare. These two sodium loops were operated
alternately during the experiment in an effort to keep a check on the op-
erability of each loop. Had one loop failed to operate, the experiment
would have been terminated for lack of a spare cooling system.

The control system of the reactor was based on conventional practice.
The three safety shim rods were actuated by electrically driven lead screws
which moved electromagnets in a vertical plane. When these magnets were
driven to their lowest extremity, an armature was engaged to which the
shim (poison) rods were attached. Loss of current in the electromagnets
would allow the rods to fall under the action of gravity into thimbles in
the central region of the core. The regulating rod was a simple stainless-
steel pipe which was rigidly attached to a rack driven by a reversible elec-
CHAP. 16] AIRCRAFT REACTOR EXPERIMENT 675

Regulating Rod
Assembly

Safety Rod Assembly

 
 
  
    
    
  
 
 
 
  
    
  

Fuel Inlet Manifold

Reflector Coolant
Tubes

E 3
’ j BeO Moderator
nd Reflector

Fuel Tubes
| Thermal Shield

E?A/Assembly

Sooees . Fuel Qutlet
Manifold

04 81216
et

Scale in Inches

Fig. 16-2. Elevative section of the Aircraft Reactor Experiment.

tric motor through a pinion. Fission chambers located in the reflector, as
well as ionization chambers located outside the pressure shell of the reactor,
furnished the neutron and gamma-ray signals for the control system.

The shim and control rods which entered the hot reactor core had to be
cooled to prevent overheating from neutron capture and gamma-ray ab-
sorption. This cooling was effected by circulating helium in a closed loop
that included a water-cooled radiator, as in the case of the fuel and sodium
circuits. This helium circuit was integral with a helium-filled monitoring
annulus which surrounded all fuel and sodium piping in the system. This
annulus was formed by putting a continuous stainless-steel sleeve around
all hot piping, and the helium circulated in the annulus performed two
functions: (1) it kept the hot lines at essentially an even temperature dur-
ing the warmup period when the system was heated by means of electrical
heating units placed on the outer surface of the annulus, and (2) the helium
was monitored to ensure that the fuel and sodium piping was leaktight.

Large, heated reservoir tanks were connccted to the system through
isolation valves so that the sodium and the fused fluoride mixture could
676 AIRCRAFT REACTOR EXPERIMENT fcaap. 16

 

Fra. 16-3. View of the ARE Vessel before addition of the thermal shield. The
external strip heaters with their electrical leads are shown in place.
CHAP. 16] AIRCRAFT REACTOR EXPERIMENT 677

 

     
     
 

 

 

 

      

 
    
    

 

 

 

 

 
     

- Rod TN
Actuator
. i .
Helium Pump | Helium
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; e . : | / - i ": Y i {7 . — T
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Reflecter Coolant

Fig. 16-4. Schematic diagram of the heat-removal system for the ARE.

be pressurized from the tanks into the system and could be drained back
into the tanks after the experiment was over. Dry helium was used for
operating penumatic instruments and for pressurizing the liquids into the
system from the tanks.

Pumps for both the sodium and the molten-fluoride mixture consisted
of sump-type centrifugal pumps with overhanging shafts. The pumps were
mounted vertically, and a gas space was provided between the liquid level
and the upper bearings of the pump. The pumps were located so that the
free-liquid surface in the sump tank was the high point in both the fuel
and the sodium eircuits. The sump tank of the pump also served as an
expansion tank for the liquid. The isometric drawing of the fuel system
presented in Fig. 16-5 indicates the relative levels of the components.

Both of the liquid systems, fuel and sodium, were fabricated entirely of
Inconel, and all closures were made by inert-gas-shielded electric-arc
(Heliare) welding. The welding procedure was adopted after extensive
experimental research and developmental work, and meticulous care was
exercised in all welding operations. The entire reactor system, that is, the
reactor vessel, heat exchangers, pumps, dump tanks, piping, and auxiliary
equipment (with the exception of control rod drives), was located in con-
crete pits below ground level. After the reactor was brought to criticality
by manual fuel injection, concrete blocks were placed on top of the pits
to complete the shielding of the system as required during power operation.

Fuel was added as a molten mixture of Nal” and UF4 (enriched in U2%)
after the sodium system had been heated and filled with sodium and the
fuel system had been heated and filled with fuel carrier—a molten mixture
of Nal’ and ZrFs. The fuel additions were made into the sump of the fuel
pump through the use of a temporary enrichment system that was capable
of injecting (by manual operation) a few hundred grams of fuel mixture
678 AIRCRAFT REACTOR EXPERIMENT [cHAP. 16

Standby Fuel Pump £E1Ed

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e Fill Tank No. 2

Fia. 16-5. Layout of the fuel system components for the ARE.

at a time. This method of fuel addition was laborious and time-consuming,
but it effectively and safely enriched the reactor to a critical concentration.

The reactor was taken to criticality essentially without incident. The
total amount of U235 added to the system to make the reactor critical was
approximately 61 kg, but small amounts of fuel were withdrawn from the
system for sampling and in trimming the pump level. The uranium con-
centration at criticality was 384 g/liter of fluoride mixture. The calculated
volume of the core was 38.8 liters at 1300°F, and thus the clean critical
mass of the reactor was 14.9 kg of U235,

It was demonstrated that the reactor had an over-all temperature co-
efficient of reactivity of —6 X 1075 (Ak/k)/°F. As was anticipated, the
fast negative temperature coefficient of reactivity (associated with the fuel
expansion coefficient) served to stabilize the reactor power level. From a
power lever of 200 kw upward, the temperature coefficient controlled the
system so precisely that the reactor responded to load demands in a
thoroughly reliable manner.

The response of the reactor was demonstrated in a number of experi-
ments, one of which is described in 1Mig. 16-6. The abscissa, to be read from
right to left, is the time in minutes, and the print-outs from recorders
giving the reactor inlet and outlet temperatures are the ordinate. Initially,
in this experiment, the reactor was operating at low power. Then the heat
CHAP, 16] AIRCRAFT REACTOR EXPERIMENT 679

Reactor Inlet and Qutlet Tube Temperatures
hundreds of degrees F

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

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Fic. 16-6. Chart of inlet and outlet temperatures for the ARE as influenced by
various experimental procedures.

extraction from the fuel was slowly increased and there was, first, a re-
sultant decrease in the temperature of the fuel which reached the reactor
inlet from the heat exchanger. This increased the reactivity and the re-
actor power, as indicated by the temperature rise at the reactor outlet.
The spread of inlet and outlet temperatures corresponds to a power level
of 2.5 Mw. When the heat extraction was reduced, the inlet temperature
080 AIRCRAFT REACTOR EXPERIMENT [cHaP. 16

rosc and the outlet temperature fell until the two temperatures became
nearly coincident. As may be seen, the control rods did not determine the
power output; they only mfluenced the average temperature. Insertion
of the shim rods decreased the temperature. Another rapid increase in the
power demand on the fuel system again spread apart the inlet and outlet
temperature recordings, and full insertion and full withdrawal of the
regulating rod depressed and then raised both temperatures simultaneously.
Next, the power extraction was stopped and the regulating rod wus in-
serted to make the reactor suberitical.

The third spread of the temperatures in I'ig. 16-6 was a result of a
demonstration which showed that the reactor could be brought to eriticality,
without use of the rods, by the power demand alone. Power extraction
from the sodium system cooled the reactor to make it eritical, and power
extraction from the fuel again caused the spread of inlet and outlet tem-
peratures.

The remarkable stability of the system made it unexpectedly possible
to demonstrate that no more than 59, of the Xel'39 was retained in the
molten fuel. It had been computed that the xenon poisoning after 27 hr
of operation at full power would amount to 2 X 1072 in AL/k if all the
xenon formed stayed in the fuel until it decayed. This level of poisoning
was less than would be expected from the usual equations, partly because
the fuel spent only one-fourth of the time in the core and was thus effec-
tively only subjected to one-fourth of the flux, and partly because many
of the neutrons had energies above the large Xe!'3 absorption resonance.
As little as 59 of this computed poisoning would have been detectable,
but none was found.

There was a small leakage from the gas volume above the liquid surface
of the fuel pumps which made operation at a high power level somewhat
awkward, but danger to operating personnel was circumvented by operat-
ing with the reactor pit at a subatmospheric pressure and remotely ex-
hausting the pit gases to the atmosphere at a location where they were
adequately dispersed.

The entire program of experiments that had been planned for the reactor
was completed satisfactorily. The reactor was shut down after a total
power production of 96 Mwh, and it was later dismantled. The fuel and
sodium systems had been in operation for a total of 462 and 635 hr, re-
spectively, ineluding 221 hr of nuclear operation, with the final 74 hr of
operation in the megawatt range.