FFR_chap15.txt

CHAPTER 15

EQUIPMENT FOR MOLTEN-SALT REACTOR HEAT-TRANSFER
SYSTEMS*

The equipment required in the heat-transfer circuits of a molten-salt
reactor consists of the components needed to contain, circulate, cool, heat,
and control molten salts at temperatures up to 1300°F. Included in such
systems are pumps, heat exchangers, piping, expansion tanks, storage
vessels, valves, devices for sensing operating variables, and other auxiliary
equipment.

Pumps for the fuel and blanket salts differ from standard centrifugal
pumps for operation at high temperatures in that provisions must be made
to exclude oxidants and lubricants from the salts, to prevent uncontrolled
excape of salts and gases, and to minimize heating and irradiation of the
drive motors. Heat is transferred from both the fuel and the blanket salts
to =~odium in shell-and-tube heat exchangers designed to maximize heat
transfer per unit volume and to minimize the contained volume of salt, es-
pecially the fuel salt.

Seaniless piping is used, where possible, to minimize flaws. Thermal ex-
pansion 1s accommodated by prestressing the pipe and by using expansion
loops and joints. Heaters and thermal insulation are provided on all com-
ponents that contain salt or sodium for preheating and for maintaining
the circuits at temperatures above the freezing points of the liquids and
to minimize heat losses. Devices are provided for sensing flow rates, pres-
sures, temperatures, and liquid levels. The devices inelude venturi tubes,
pressure transmitters, thermocouples, electrical probes, and floats. Inert
gasex are used over free-liquid surfaces to prevent oxidation and to apply
appropriate hase pressures for suppressing cavitation or moving liquid or
gas from one vessel to another.

The deviations from standard practice required to adapt the various
componients to the molten-salt system are discussed below. The schematic
diagram of a molten-salt heat-transfer system presented in Fig. 15-1 in-
dicates the relative positions of the various components. For nuclear op-
eration, an off-gas svstem 1s supplied, as described in Chapter 17. The
vapor condensation trap indicated in Fig. 15-1 1s required only on systems
that contain ZrF4 or a comparably volatile fluoride as a component of the
molten salt,

*By H. W, Ravage, W, F. Boudreau, 5. J. Breeding, W. G. Cobb, W. B, Mec-
Donald, H. J, Metg, and L. Storto.

661
662 MOLTEN-SALT REACTOR HEAT-TRANSFER EQUIPMENT [cHAP. 15

Pump (Includes Expansion Volume)

  
  
     
  

 

 

 

 

 

 

 

 

 

 

 

    
   

Liquid Level Vmp
indicating Device 1 I 39,
High Point
Heat
@—{Source Exchanger
Coolant System
Flow
Measuring Device
L ;
P;\:' (Venturi)
Pressure Measuring Devices
Shut-Off Yal
u alve \J? Vent Vent
. T Liquid Level
Fill and Drain Line Indicating Device
Yapor Trap
Equ-;Iizer
Dump Tank Valve

 

 

 

     
 

Pressure
~*——Regulators —

Inert
Gas Supply

F1G. 15-1. A molten-salt heat-transfer system.

15-1. PumMprs rOrR MOLTEN SALTS

Centrifugal pumps with radial or mixed-flow types of impeller have been
used successfully to circulate molten-salt fuels. The units built thus far
and those currently being developed have a vertical shaft which carries
the impeller at its lower end. The shaft passes through a free surface of
liquid to isolate the motor, the seals, and the upper bearings from direct
contact with the molten salt. Uncontrolled escape of fission gases or entry
of undesirable contaminants to the cover gas above the free-liquid surface
in the pump are prevented either by the use of mechanical shaft seals or
hermetic enclosure of the pump and, if necessary, the motor. Thermal and
radiation shields or barriers are provided to assure acceptable temperature
and radiation levels in the motor, seal, and bearing areas. Liquid cooling
of internal pump surfaces is provided to remove heat induced by gamma
and beta radiation.
15~-1} PUMPS FOR MOLTEN SALTS 663

  
   
  
    
  

Top Shaft Seal Assembly
~—Oil Inlet

Bearing {Upper)

3 Heated Gas Vent

 
   

{(Lower)
| Ring

  

XN

  
           

Drain

 

  

 

  
  

 
   

 

Ak_ - .
: z g
iy | A
o - 7 /
Impeller =5 i
- g‘;
3 3 - LNy
NN e
N oy .
gif _&\ mf‘&ff-’@? 772! level Indicator Float
'\ < SN
> 3 i-i %
_y ~ i;\‘
N
b oA
Fuel Inlet o

Fuel Discharge

Fig. 15-2. Sump-type centrifugal pump developed for the Aircraft Reactor
Experiment.

The principles used in the design of pumps for normal liquids are applic-
able to the hydraulic design of a molten-salt pump. Experiments have
shown that the cavitation performance of molten-salt pumps can be pre-
dicted from tests made with water at room temperature. In addition to
stresses induced by normal thermal effects, stresses due to radiation must
be taken into account in all phases of design.

The pump shown in Fig. 15-2 was developed for 2000-hr durability at
very low irradiation levels and was used in the Aircraft Reactor Experi-
ment for circulating molten salts and sodium at flow rates of 50 to 150 gpm,
at heads up to 250 ft, and at temperatures up to 1550°F. These pumps
have been virtually trouble-free in operation, and many units in addition
to those used in the Aircraft Reactor Experiment have been used in devel-
opmental tests of various components of molten-salt systems.

The bearings, seals, shaft, and impeller form a cartridge-type subassembly
that is removable from the pump tank after opening a single, gasketed
064 MOLTEN-SALT REACTOR TEAT-TRANSFER EQUIPMENT  [crar. 15

joint above the liquid level. The volute, suction, and discharge connec-
tions form parts of the pump tank subassembly into which the removable
cartridge is inserted. The upper portion of the shaft and a toroidal area
in the lower part of the bearing housing are cooled by circulating oil. Heat
losses during operation are reduced by thermal msulation.

In all the units built thus far nickel-chrome alloys have been used in
the construction of all the high-temperature wetted parts of the pump to
minimize corrosion. The relatively low thermal conductivity and high
strength of such alloys permitted close spacing of the impeller and bearings
and high thermal gradients in the shaft.

Thrust loads are earried at the top of the shaft by a matched pair of pre-
loaded angular-contact ball bearings mounted face-to-face in order to pro-
vide the flexibility required to avoid binding and to accommodate thermal
distortions. Either single-row ball bearings or a journal bearing can be
used successfully for the lower bearing.

The upper lubricant-to-air and the lower lubricant-to-inert-gas seals
are similar, rotary, mechanical face-type seals consisting of a stationary
graphite member operating in contact with a hardened-steel rotating mem-
ber. The seals are oil-lubricated, and the leakage of oil to the process side
is approximately 1 to 5 ce/day. This oil is collected in a cateh basin and
removed from the pump by gas-pressure sparging or by gravity.

The accumulation of some 200,000 hr of relatively trouble-free test op-
eration in the temperature range of 1200 to 1500°F with molten salts and
liquid metals as the circulated fluids has proved the adequacy of this basic
pump design with regard to the major problem of thermally induced dis-
tortions. Four different sizes and eight models of pumps have been used
to provide flows in the range of 5 to 1500 gpm. Several individual pumps
have operated for periods of 6000 to 8000 hr, consecutively, without main-
tenance.

15-1.1 Improvements desired for power reactor fuel pump. The basic
pump described above has bearings and seals that are oil-lubricated and
cooled, and in some of the pumps elastomers have been used as seals be-
tween parts. The pump of this type that was used in the ARE was de-
signed for a relatively low level of radiation and received an integrated
dose of less than 5 X 108 r. Under these conditions both the lubricants and
elastomers used proved to be entirely satisfactory.

The fuel pump for a power reactor, however, must last for many years.
The radiation level anticipated at the surface of the fuel is 105 to 10° r/hr.
Beta- and gamma-emitting fission gases will permeate all available gas
space above the fuel, and the daughter fission products will be deposited
on all exposed surfaces. Under these conditions, the simple pump described
above would fail within a few thousand hours.
15-1] PUMPS FOR MOLTEN SALTS 665

Considerable improvement in the resistance of the pump and motor to
radiation can be achieved by relatively simple means. Lengthening the
shaft between the impeller and the lower motor bearing and mserting addi-
tional shielding material will reduce the radiation from the fuel to a low
level at the lower motor bearing and the motor. Hollow, metal O-rings or
another metal gasket arrangement can be used to replace the elastomer
seals. The sliding seal just below the lower motor bearing, which prevents
escape of fission-product guses or inleakage of the outside atmosphere,
must be lubrieated to ensure continued operation. If oil lubrication is used,
radiation may quickly cause coking. Various phenyls, or mixtures of them,
are much less subject to formation of gums and cokes under radiation and
could be used as lubricant for the seal and for the lower motor bearing.
This bearing would be of the friction type, for radial and thrust loads.
These modifications would provide a fuel pump with an expected life of the
order of a vear. With suitable provisions for remote maintenance and re-
pair, these =simple and relatively sure improvements would probably
suffice tor power reactor operation,

Three additionad improvements, now beig studied, should make pos-
=ihle w fuel pump that will operate trouble-free throughout a very long life.
The first of these ix a pilot bearing for operation in the fuel salt. Such a
bearing, whether of hydrostatic or hydrodynamie design, would be com-
pletelv unattected by radiation and would permit use of a long shaft so
thut the motor could be well shielded. A combimed radial and thrust
bearing just below the motor rotor would be the only other bearing required.
The ~econd mprovement is a labyrinth type of gas seal to prevent escape
of fission gases up the shaft. There are no rubbing surfaces and hence no
need for Tubricants, 2o there ¢an be no radiation damage. The third inno-
vition 1= o hemispherical gas-cushioned bearing to act as a combined thrust
and radind bearing. Tt would have the advantage of requiring no auxiliary
lubrication <upply, and it would combine well with the lubyrinth type of
gos ~eal. It would, of course, be unaffected by radiation.

15-1.2 A proposed fuel pump. A pump design embodying these last
three features is shown in g, 15-3. [t 18 designed for operation at a tem-
peruture of 1200°F, a flow rate of 24,000 gpm, and a head of 70 ft of fluid.
The lower bearing iz of the hydrostatic type and 1z lubricated by the
molten-=alt tuel. The upper bearing, which is also of the hydrostatic type,
i< cu=hioned by helium and serves also as a barrier against passage of gaseous
fission products into the motor. This bearing is hemispherical to permit
accommodation of thermally induced distortions m the over-all pump
structure.

The principal radiation shielding is that provided between the source
and the area of the motor windings. Layers of beryllium and boron for
666 MOLTEN-SALT REACTOR HEAT-TRANSFER EQUIPMENT [cHAP. 15

Shield Retainer

Sealing Head @

   
 
 
  
  
 
 
  
  
  
  
  
  
  
  
     

Handling |

Electrical Connection ‘
|

Motor Coils

Low Temperature

Coolant Connection

Helical Coolant Passages
Rofor
Gas Circulation Laminations

Stator

Hydrostatic

Gas Connection !
Gas Bearing

Nuclear Radiation Shield

Gamma Shield
Support Ring

Neutron Shield

Blanket
Purge Gas Fiow
Maximum Fuel Level

Z

Coolant
Blanket Salt Passage

By-Pass Flow Channel

Fuel Expansion Tank

Minimum Fuel Level

Yolute (Discharge to Rear) o

{i A, -

Fuel Inlet

   

Hydrostatic Bearing

Impeller 0 1 2 3
O e W W T T 7 ]
Scale—Feet

Fre. 15-3. Improved molten-salt fuel pump designed for power reactor use. Op-
erating temperature, 1200°F; flow rate, 24,000 gpm; head, 70 ft of fluid.
15-3] VALVES 667

neutron shielding and a heavy metal for gamma-radiation shielding are
proposed. The motor is totally enclosed, to eliminate the need for a shaft
seal. A coolant 1s circulated in the area outside the stator windings and
between the upper bearing and the shielding. Molten-salt fuel is eirculated
over the surfaces of those parts of the pump which are in contact with the
giseous fission produets to remove heat generated in the metal.

15-2. HeaT ExXcHANGERS, ixpansioN TANKS, AND DrRAIN TANKS

The heat exchangers, expansion tanks, and drain tanks must be especially
designed to fit the particular reactor system chosen. The design data of
item= suitable for a specific reactor plant are deseribed in Chapter 17.
The spectal problems encountered are the need for preheating all salt-
wind =odium-containing components, for cooling the exposed metal surfaces
in the expansion tank, and for removing afterheat from the drain tanks.
It hus been found that the molten salts behave as normal fluids during
pumping and flow and that the heat-transfer coefficients can be predicted
from the physical properties of the salts.

15-3. VALVES

The problems associated with valves for molten-salt fuels are the con-
~i~tent alignment of parts during transitions from room temperature to
120071, the selection of materials for mating surfaces which will not
tusion-bond in the salt and cause the valve to stick in the closed position,
and the provision of a gastight seal. Bellows-sealed, mechanically operated,
poppet valves of the type shown in I'ig. 15—4 have given reliable serviee in
test SVsTes,

A number of corrosion and fusion-bond resistant materials for high-
temperature use were found through extensive screening tests. Molyb-
dennm negainst tungsten or copper and several titanium or tungsten carbide-
mekel cermets mating with each other proved to be satistactory. Valves
with very accurately machined cermet scats and poppets have operated
<uti=fictorily in 2-in. molten-salt lines at 1300°F with leakage rates of less
than 2 ¢ hr. Consistent positioning of the poppet aund seat to assure
leaktightiuess 1s achieved by minimizing transmission of valve-body dis-
tortions= to the valve stem and poppet.

It rupid valve operation is not required, a simple “freeze” valve may be
used to en=ure n leaktight seal. The freeze valve consists of o section of
pipe. u=ually flattened, that is fitted with a device to cool and freeze a salt
plug and another means of subsequently heating and melting the plug.
668 MOLTEN-SALT REACTOR HEAT-TRANSFER EQUIPMENT [cHAP. 15

   

Cermet Seat
and Poppet

 

Bellows
Protector

Bellows

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

=

Fra. 15-4. Bellows-sealed, mechanically operated poppet valve for molten-salt
service.

15-4. SysTeMm HeATING

Molten-salt systems must be heated to prevent thermal shock during
filling and to prevent freezing of the salt when the reactor is not operating
to produce power. Straight pipe sections are normally heated by an elec-
tric tube-furnace type of heater formed of exposed Nichrome V wire in &
ceramic shell (clamshell heaters). A similar type of heater with the Ni-
chrome V wire mstalled in flat ceramic blocks can be used to heat flat
surfaces or large components, such as dump tanks, etc. In general, these
heaters are satisfactory for continuous operation at 1800°F. Pipe bends,
irregular shapes, and small components, such as valves and pressure-
measuring devices, are usually heated with tubular heaters (e.g., General
Electric Company “Calrods’™) which can be shaped to fit the component
or pipe bend. In general, this type of heater should be imited to service at
15-3] JOINTS 6G6Y

1500°F. Care must be exercised in the installation of tubular heaters to
avold failure due to a hot spot caused by insulation i direct contact with
the heater. This type of failure can be avoided by installing a thin sheet of
metal (shim stock) between the heater and the insulation.

Direct resistance heating in which an electric current 1s passed directly
through a section of the molten-salt piping has also been used successfully.
Operating temperatures of this type of heater are limited only by the
corrosion and strength limitations of the metal as the temperature is
increased. Experience has indicated that heating of pipe bends by this
method 1s usually not uniform and can be accompanied by hot spots caused
by nonuniformity of liquid flow in the bend.

15-5. JoINTs

Failures of some system components may be expected during the de-
sired operating life, say 20 years, of a molten-salt power-producing reactor;
consequently, provisions must be made for servicing or removing and re-
placing such components. Remotely controlled manipulations will be
required because there will be a high level of radiation within the primary
shield. Repair work on or preparations for disposal of components that
fail will be carried out in separate hot-cell facilities.

The components of the system are interconnected by piping, and flanged
connections or welded joints may be used. In breaking connections between
a component and the piping, the cleanliness of the system must be pre-
served, and In remaking a connection, proper alignment of parts must be
re-established.  The reassembled system must conform to the original
leaktightness specifications. Speecial tools and handling equipment will
be needed to separate components from the piping and to transport parts
within the highly radioactive regions of the system. While an all-welded
syvstem provides the highest structural integrity, remote cutting of welds,
remote welding, and inspection of such welds are difficult operations.
Special tools are being developed for these tasks, but they are not yet
generally available. Flanged connections, which are attractive from the
point of view of tooling, present problems of permanence of their leak-
tightness.

Three types of flanged joints are being tested that show promise. One
is a freeze-flange joint that consists of a conventional flanged-ring joint
with a cooled annulus between the ring and the process fluid. The salt that
enters the annulus freezes and provides the primary seal. The ring provides
a backup seal against salt and gas leakage. The annulus between the ring
and frozen material can be monitored for fission product or other gas leak-
age. The design of this joint is illustrated in Fig. 15-5.
670 MOLTEN-SALT REACTOR HEAT-TRANSFER EQUIPMENT [cHAP. 15

—.fl--Gap {(~1/168in.)

P

   

" 1~Soft-Iron or Copper Seal Ring

+— Air Channel for Cooling

 

Frozen-Salt Seal

 

Weld of Flange to Loop Tubing

Salt Flow

 

; : Ring Insert to Provide Labyrinth
9 : for Salt Leakage

g \!\Frozen—Sulf Seal

fi [ Narrow Section to Reduce Heat
x Transfer from the Molten Salt
in the Loop Tubing

1 B 0 % o
Inches

 

 

 

Air Inlet to Cooling Channel
[XXXX] Between Flange Faces Indicate Region of Frozen-Salt

Seal; Indicate Region of Transition from Liquid
to Solid Salt

Fia. 15-5. Freeze-flange joint for 1/2-in.-OD tubing.

Seal Material Insert
Shown Before Being
Fused to Form Seal

 

 

 

 

 

 

 

 

 

 

0 Ya 1
Weld of 1 Inches

Flange to Loop Tubing

Fia. 15-6. Cast-metal-sealed flanged joint.
15-6] INSTRUMENTS 671

   
      

Weld of Flange to
Loop Tubing

Raised Tooth

Annealed Copper Ring e e e —
Inches

Fic. 15-7. Indented-seal flange.

A cast-metal-sealed flanged joint is also being tested for use in vertical
runs of pipe. As shown in Fig. 15-6, this joint includes a seal which is cast
in place in an annulus provided to contain it. When the connection is to
be made or-broken the seal is melted. Mechanical strength is supplied by
clamps or bolts.

A flanged joint containing a gasket (Fig. 15-7) is the third type of joint
being considered. In this joint the flange faces have sharp, circular, mating
ridges. The opposing ridges compress a soft metal gasket to form the seal
between the flanges.

15-6. INSTRUMENTS

Sensing devices are required in molten-salt systems for the measurement
of flow rates, pressures, temperatures, and liquid levels. Devices for these
services are evaluated according to the following criteria: (1) they must
be of leaktight, preferably all-welded, construction, (2) they must be cap-
able of operating at the maximum temperature of the fluid system, (3) their
accuracies must be relatively unaffected by changes in the system tem-
perature, (4) they should provide lifetimes at least as great as the lifetime
of the reactor, (5) each must be constructed so that, if the sensing element
fails, only the measurement supplied by it is lost. The fluid system to
which the instrument is attached must not be jeopardized by failure of the
sensing element.

15-6.1 Flow measurements. Flow rates are measured in molten-salt
systems with orifice or venturi elements. The pressures developed across the
sensing element are measured by comparing the outputs of two pressure-
measuring devices. Magnetic flowmeters are not at present sufficiently
sensitive for molten-salt service because of the poor electrical conductivity
of the salts.
672 MOLTEN-SALT REACTOR HEAT-TRANSFER EQUIPMENT [cHAP. 15

15-6.2 Pressure measurements. NMeasurements of system pressures re-
quire that transducers operate at a safe margin above the melting point of
the salt, and thus the minimum transducer operating temperature is usually
about 1200°F. The pressure transducers that are available are of two types:
(1) a pneumatic force-balanced unit and (2) a displacement unit in which
the pressure 1s sensed by displacement of a Bourdon tube or diaphragm.
The pneumatic force-balanced unit has the disadvantages that loss of the
instrument gas supply (usually air) can result in loss of the measurement,
and that failure of the bellows or diaphragm would open the process system
to the air supply or to the atmosphere. The displacement unit, on the other
hand, makes use of an isolating fluid to transfer the sensed pressure hydro-
statically to an isolated low-temperature output element. Thus, in the
event of a failure of the primary diaphragm, the process fluid would merely
mix with the isolating fluid and the closure of the system would be
unaffected.

15-6.3 Temperature measurements. Temperatures in the range of 800
to 1300°F are commonly measured with Chromel-Alumel or platinum-
platinum-rhodium thermocouples. The accuracy and life of a thermocouple
in the temperature range of interest are functions of the wire size and, in
general, the largest possible thermocouple should be used. Either beaded
thermocouples or the newer, magnesium oxide-insulated thermocouples
may be used.

15-6.4 Liquid-level measurements. Instruments are available for both
on-off and continuous level measurements. On-off measurements are made
with modified automotive-type spark plugs in which a long rod is used in
place of the normal center conductor of the spark plug. To obtain a con-
tinuous level measurement, the fluid head is measured with a differential
pressure instrument. The pressure required to bubble a gas into the fluid
is compared with the pressure above the liquid to obtain the fluid head.
Resistance probe and float types of level indicators are available for use in
liquid-metal systems.

15-6.5 Nuclear sensors. Nuclear sensors for molten-salt reactors are
similar to those of other reactors and are not required to withstand high
temperatures. xisting and well-tested fission, lonization, and boron tri-
fluoride thermal-neutron detection chambers are available for installation
at all points essential to reactor operation. Their disadvantages of limited
life can be countered only by duplication or replacement, and provisions
can be made for this. It should be pointed out that the relatively large,
negative temperature cocfficients of reactivity provided by most circulating-
fuel reactors make these instruments unessential to the routine operation
of the reactor.