NSE-1965.txt

NUCLEAR STRUCTURAL ENGINEERING 2 (1965) 224-232. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM

DESIGN CONCEPTS FOR THE CORE STRUCTURE OF
A MOSEL (MOLTEN SALT EXPERIMENTAL) REACTOR *

Paul R. KASTEN
Molten Salt Reactor Program, Oak Ridge National Laborvalory, Oak Ridge, Tennessee, USA

Uri GAT, S.SCHULZE HORN
Institut fiir Reaktorentwicklung, Kevrnforschungsanlage Jiilich, Jillich, Germany

Heinz W. VORNHUSEN
c/o Atomic Power Equipment Depaytment, Geneval Electric Company, San Jose, California, USA

Received 26 June 1965

General features of a MOSEL (MOlten Salt Experimental) reactor concept and a schematic flow dia-

gram are given,

A number of core designs appear capable of satisfying the requirements of such reactor. Interacting
parameters which need to be evaluated as a function of operating conditions are permissible tempera-
ture distribution, stresses, nuclear performance, and fabrication possibility as a function of core de-
sign. In general it appears that plate type designs facilitate low friction of structural material, achieve-
ment of modular units, and fuel flow control, Tube-type designs appear advantageous relative to stress

considerations and high fuel power density.

1. INTRODUCTION

General and structural features of a MOSEL-

reactor [1] are discussed. Thermodynamic rela-
tions of the core will be treated in a second arti-
cle. Although the possibility exists of cooling the
core with sodium in indirect contact, or lead in
direct contact, in this discussion .only cooling by
the blanket salt is considered.
- At the present time, nuclear power capacity
is not great enough to support large-scale, cen-
tral station processing and fabrication plants,
and the unit costs in low-capacity plants are
normally too high to permit bred fuel to be re-
cycled economically. The MOSEL reactor con-
cept attacks this problem by considering a fuel
cycle with such an extremely simple processing
and fabrication scheme, that on-site fuel recycle
in relatively small power plants (about 500 MWe)
appears economically feasible [1-3]. In addition,
use of the proposed fuel cycle should result in
breeding ratios greater than 1.05.

* Accepted by T. A.Jaeger.
** On leave from Kernforschungsanlage Jiilich.

2. GENERAL FEATURES OF THE MOSEL
REACTOR

Molten fluoride salts are promising reactor
fuels due to their ability to dissolve thorium and
uranium fluorides, their low viscosity, their low
vapor pressures at high temperatures, and the
ease with which uranium can be recovered from
the salt. The latter feature stems from the high
volatility of UFg (boiling temperature less than
100°C), and the low volatility of UF4 (boiling
temperature over 14009C). This remarkable
change in physical property which takes place
upon fluorinating UF4 forms the basis of the
fluoride volatility process, and is applied to the
present concept.

The MOlten Salt Experimental. (MOSEL) re-
actor concept concerns a two-fluid, two region
reactor utilizing thorium and uranium dissolved
in molten fluorides. The MOSEL concept con-
cerns a power reactor having a high core power
density and a breeding ratio greater than unity
under economic conditions.

Fig. 1 illustrates the basic flow diagram of
the MOSEL reactor. As shown, the power plant
includes the reactor, turbine generator, and fuel
processing facilities. Reactor operation is at low
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pressure and high temperature. The core fluid
consists of UF4 dissolved in NaF+BeFg (or a
similar salt mixture) and is contained in tubes
or between plates of a high-nickel alloy. The ba-
sic fissile fuel is U233, with U238 added to in-
crease the fast fission factor. The fluid fuel is
circulated primarily for purpose of fuel mixing,
addition, and fission product removal. The core
is internally cooled, with blanket fluid as the
core coolant. The blanket salt contains ThFy
dissolved in NaF +BeF9 (or a similar salt mix-
ture). The core coolant transfers energy to the
steam generator through an intermediate heat
transfer fluid. The steam generator portion of
the plant utilized boilers, superheaters, and re-
heaters to produce high-temperature, high-pres-
sure steam for use in the steam turbines, as in-
dicated schematically in fig. 1.

Fuel processing consists of uranium recovery
by fuel fluorination, and is applied to both the
blanket and core fluids. In the blanket cycle,
uranium-free salt is returned to the blanket re-
gion, while the recovered UFg is reduced to UFy
and fed to the core region. Core processing con-
sists of fissile fuel recovery and discard of fis-
sion-product-containing salt.

The fluid nature of the fuel, the simple pro-
cessing scheme, and the use of blanket fluid as
the coolant permit reactor power to be increased
without significant change in nuclear perfor-
mance. This is done by extending the core in one
direction, perpendicular to coolant flow, which
also permits modular-type construction. Based
on present information, the MOSEL reactor con-
cept discussed here appears to have economic
breeding ratios in the range of 1.05 - 1.1, fissile-

Fig. 2. Integral core and container:
DESIGN CONCEPTS FOR THE CORE STRUCTURE OF A MOSEL REACTOR 229

fuel doubling times between 15 - 60 years, fuel
cycle costs less than 1.5 mills/kWh and power
costs under 6 mills/kWh in power plants of 500
MWe rating, utilizing on-site processing and fuel
recycle, and employing capital charges of 15 and
10%/year for depreciating and non-depreciating
items, respectively. Compared with other reac-
tor types, the MOSEL concept appears superior
for combining on-site fuel recycle, breeding ra-
tios above unity, and low power costs in rela-
tively small nuclear plants.

3. CORE DESIGNS

To secure breeding the core region must be
surrounded by a blanket of molten salt carrying
fertile material. The core is separated from the
blanket by a wall. The thickness of the separat-
ing wall is dependent upon pressure difference
between core and blanket and the structural de-
sign of the core. For reasons of neutron econo-
my this wall should be as thin as possible to
lower parasitic absorption of neutrons and in-
crease the breeding rate.

The blanket salt removes the heat generated

Fig. 3. Core and container separable.

in the core by passing through the core before
flowing through the blanket region. The pressure
difference between core and blanket thus is de-
pendent on the pressure drop of the coolant in
passing through the core; the core length is lim-
ited to that corresponding to a tolerable thick-
ness of the wall separating core and blanket.
The flow of the coolant ensures mixing of the
salt in the blanket region.

It is possible to design the core and vessel as
a unit, as indicated in fig. 2. In such an arrange-
ment the replacement of core parts would involve
appreciable difficulties. A soiution to this prob-
lem consists of connecting the core to a flange
as seen in fig. ®. Going further in this direction
and utilizing a rectangular type core leads to the
design indicated in fig. 4. Here the core is di-
vided into several parts, each of which can be
removed and replaced separately. This type of
structural layout also provides freedom in the
choice of the volume to surface ratio while main-
taining a constant core length. By controlling the
leakage, the breeding ratio may be optimized to
give the most economical design. An arbitrary
volume to surface ratio is also possible with an
annular type design, but fluid flow through the
inner cylinder would be required and give rise to
design problems.

4, POSSIBLE SHAPES OF FUEL CHANNELS

Two basic fuel-channel designs for the MOSEL
core are considered here. One considers use of
pipes and the other of plates. Although a number
of other possibilities appear to exist, they can,
with some modification, be roughly classified
under either category. Although the core is
cooled internally, it is advantageous to control
fuel flow for mixing purposes. This places some
restrictions on the core design.

4.1. Plate design

Fig. 5 shows a unit cell based on a plate de-
sign. As shown, a is the half width of the fuel
channel, b is the thickness of the structural ma-
terial, and ¢ is the half width of the coolant
channel. The proportions of fuel, material and
coolant in the core are in the ratio of a:b:c. (In
reality a correction must be made to account for
support structures about every 20 mm.)

Plate type core designs can involve a) concen-
tric rings, b) spirals, c) involutes, d} plates.
Some features of these type designs are dis-
cussed below.

a) Concentric rings, shown in fig. 6, are ad-
228 P.R.KASTEN et al,

AN

Fig. 4, Core built in parts, enables different combinations of basic elements.

vantageous in view of resistance to forces in-
duced by pressure, but the diameter of the core
must be kept small, because otherwise rather
thick walls are needed for the outer rings. An
important difficulty is the required arrangement
of the separate fuel and coolant streams with re-
spect to inlet and outlet.

b) Spiral heat exchangers are used in chemi-
cal industry. They are easy to manufacture, but
when pressure is applied, they tend to distort.
Control of flow can be applied relatively easy, if

the fuel stream is fed in at the center it may flow
out at the edge. Coolant flow in this scheme
would be axial and perpendicular to fuel flow.

c) Involute type designs, as indicated in fig. 7,
permit a constant distance to be maintained be-
tween curved surface; also they can carry part
of the forces induced by pressure. Difficulties
are associated with the proper design of inlet
and outlet flow streams from section to section.

d) Plates arranged in square or other regular
geometries have certain advantages. Plates suit
DESIGN CONCEPTS FOR THE CORE STRUCTURE OF A MOSEL REACTOR 229

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230 P.R.KASTEN et al.

regular polygonal forms perfectly, as illustrated
in figs. 8 and 9, in addition, they are easy to ar-
range. An important advantage of plates lies in

their ability to absorb any bending energy on the -

container walls induced by pressure differen-
tials, thus avoiding a thick container wall. The
plates can be arranged perpendicular to the con-
tainer wall and forces tending to bend the con-
tainer wall outwards are taken up in the plane of
the plates. Tensile stress induced thereby are

Fig. 9. Plates arranged hexagonally.

easier to handle than bending stresses. An ar-
rangement of plates permits low volume fraction
structure in the core. Also, as indicated in
fig. 4, outer plates may be used as a fuel dis-
tributor (fig. 11).

4.2. Tube designs

Tubes are more stable than plates and are
thus more able to withstand forces induced by
pressure differentials. A difficulty with the use
DESIGN CONCEPTS FOR THE CORE STRUCTURE OF A MOSEL REACTOR

231

Fig. 10, Pipes connected in a tube sheet (fuel entry and exit not shown).

of tubes is associated with proper arrangement
of the streams of fuel and coolant. It seems rea-
sonable to contain the fuel inside the tubes to en-
able flow control and allow a possible shut off of
a single tube in case of a leak. If the tubes are
connected by means of a tube sheet, as shown in

fig. 10, proper flow of coolant appears difficult
fo obtain. If the coolant stream is introduced
perpendicular to the fuel tubes, as it is done in
ordinary heat exchangers, this leads to coolant
flow distribution favouring the outer regions of
the core. Since power density in general is high-
232 P.R.KASTEN et al,

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Fig. 11. Section showing container plate arranged as fuel distributor.

est in the core center, the associated high tem-
peratures may require a non-uniform tube spac-
ing and/or size in the core region.

Another possibility of arranging the tubes
would be toconnect several tubes to a distributor
pipe, which acts as a manifold (as in fig. 2). Such
an arrangement would also permit various fuel
concentrations to be present within the core re-
gion.

6. CONCLUSION

There appear to be a number of core designs
which are capable of satisfying the requirements
of a MOSEL-type reactor. Some of these would
permit the core to be built up from modular
units, thus enabling exchange of part of the core
in case of failure.

The choice between the use of plates and tubes
depends primarily upon fabrication ability and
the mechanical and physical properties of the
core structure. In a following article it is shown
that plate designs lead to some thermodynamical
and nuclear advantages. Tube designs appear
advantageous relative to stress considerations.
Plate designs appear advantageous with regard

to arrangement of the flow of the fuel and cool-
ant. A disadvantage of tube designs is associated
with the use of a tube sheet or some other ar-
rangement requiring relatively large amounts of
structural material. Also, use of tubes requires
a wall between core and blanket that is capable
of carrying pressure differential between the two
regions. Additional construction material in the
core leads to increased neutron losses and lower
breeding ratios.

It appears that a plate-type design involving
regular polygons facilitates fluid flow patterns
and flow control. At the same time, the amount
of structural material required is relatively low,
which is an important factor when criticality is
achieved in the heat exchange region.

REFERENCES

1. P.R.Kasten, The MOSEL-reactor-concept, Third
Intern. Conf. on the Peaceful Uses of Atomic Ener-
gy, Geneva 1964, A Conf. 28/P /538,

2. P.R.Kasten, Eine Bewertung von Thorium-Brenn-
stoffkreisliufen, Kernforschungsanlage Jilich, Sep-
tember 1964,

3. P.R.Kasten, Das MOSEL-Reaktor-Konzept, Atom-
praxis 10 (1964).