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By Charles Rhodes, P.Eng., Ph.D.

Elsewhere on this website Fast Neutron Reactors (FNRs) have been identified as the primary source of energy for meeting mankind's future energy needs. This web page focuses on the design of the FNR's primary sodium pool.

The 1000 MWt FNR described herein consists of a cylindrical primary liquid sodium pool 21.0 m diameter X 16.5 m deep, in which the liquid sodium depth is 15.5 m. The pool inside walls and bottom are formed from stainless steel sheets outside of which in the walls is a 1 m thick layer of ceramic fiber insulation which provides thermal insulation and which compresses to allow pool liner thermal expansion. Then in the walls there is a 1 m thick layer of dense fire brick which provides fiber ceramic insulation support and provides additional thermal insulation. On the pool bottom is a sheet steel layer which protects the underlying stainless steel pool bottom liner against accidental damage. Underneath the pool bottom is 2.0 m of fire brick.

Surrounding the outside of the fire brick is another sheet stainless steel liquid sodium containment wall. Outside this containment wall is a 1 m wide air gap for air cooling and maintenance access and outside that air gap is a 1 m thick reinforced concrete wall and floor which: excludes water, provides a third level of emergency sodium containment, provides emergency nuclear fuel containment and supports the roof and gantry crane structures.

On top of the pool bottom are 14 inch long X 6 inch high X 0.625 inch rectangular steel tubes which serve as the base for mounting the fuel bundle support tubes, control actuators and related liquid sodium hydraulic pressure tubes. On top of this base is a layer of solid B4C spheres whose function is to prevent melted reactor fuel forming a critical mass. Note that these B4C spheres are outside the neutron flux and hence will not form C-14.

There are no penetrations of the primary liquid sodium pool walls or floor. All reactor and heat exchange components are inserted into the primary liquid sodium pool via the top of the pool.

There is an upper limit on the thickness of the fiber ceramic filled region. In the event that the pool liner fails the spaces between the fibers will fill with liquid sodium. In that event the sodium level must not drop so much as to prevent fission product heat removal.

Within the air gap between the outer stainless steel pool wall and the concrete wall inner face are structural steel radial elements which maintain the 1 m air gap between the outer layer of stainless steel and the concrete. In the even of a pool liner failure this structural steel relieves hoop stress in the outer steel wall.

The structural steel I beams supporting the pool bottom must bear the entire weight of the FNR and the surrounding brick material and heat exchange bundles. The sheet steel outside the brick must be sufficiently strong and reinforced that it can withstand the liquid sodium hydraulic head in the event that there is a failure of the inner sheet stainless steel pool liner.

The top surface of the liquid sodium in the pool is nominally 1 m below pool deck level. The 1 m thick concrete walls extend straight upwards above the pool deck for 5 m before intersecting the reactor enclosure upper roof structure. Inside the concrete is the 1 m air gap which is used for circulating cooling air and which allows maintenance access to the ventilation space and inside roof structure when the pool gamma emission is sufficiently low. Above the pool deck there are both inner and outer sheet stainless steel walls separated by 1 m of compressible ceramic fiber thermal insulation. These sheet stainless steel walls must be continuously edge welded and bubble tested to be gas tight. The closed space containing the ceramic insulation is filled with low pressure argon. The ceramic fiber insulation prevents the inner and outer sheet metal ceilings collapsing together when the absolute pressure in the insulation space is below one bar. The inner sheet stainless steel wall is used to contain the argon cover gas and the neutron activated sodium vapor. The outer sheet stainless steel wall is used to exclude air. The inner sheet stainless steel wall normally operates at about 450 deg C and the outer sheet stainless steel wall normally operates at ambient temperature. The inner sheet stainless steel wall radius of curvature is 11.5 m. Hence the outer stainless steel sheet radius of curvature is 12.5 m. There is a partially evacuated low argon pressure between the inner and outer sheet stainless steel walls and ceiling that is used to detect leaks in either the inner or the outer sheet stainless steel pool wall/ceiling coverings.

The inside diameter of the liquid sodium pool must be 21.0 m to allow for a 11.2 m nominal diameter reactor core, a 1.6 m wide perimeter reactor blanket, perimeter clearance for moving equipment and a 2.6 m to 2.9 m wide ring for intermediate heat exchange bundles and related piping. Under the intermediate heat exchange bundles is an equal width liquid sodium guardband also functioning as a cool sodium return path. There is a 2.0 m thickness of brick and fiber ceramic insulation around the sides of the sodium pool and a 2.0 m thick layer of fire brick under the sodium pool. There is a 1 m wide air space outside the brick for service access and for circulated air heat removal. Hence the concrete enclosure inside diameter is 27.0 m.

If the sodium pool enclosure roof is structural steel. Its design is constrained by the use of prefabricated structural steel beams that are limited by road and rail transportation constraints to ~ 20 m in length.

It is anticipated that the reactor enclosure inner ceiling will have a maximum inside height above the pool deck of 3.0 m. Assuming that the gantry crane is supported by the concrete walls the gantry crane I beam tracks will be about 29 m long. These I beam tracks may need to be fabricated by field joining of two shorter I beam lengths. There is a further complication related to mounting the monitoring electronics package. The electronics package will consist of two parts, one which illuminates the fuel bundle indicator tubes and one which receives and processes data from the indicator tubes.

The concrete remains at ambient temperature unless both the inner stainless steel wall and the outer stainless steel wall rupture. The main function of the concrete is to attenuate gamma radiation emitted by sodium pool Na-24 decay while the reactor is operating. Other functions of the concrete include:
1) Exclusion of ground water;
2) Exclusion of rain water;
3) Exclusion of flood water;
4) Physical protection from grade level or air borne physical attack;
5) Reserve containment of liquid sodium;
6) Reserve fire containment;
7) Supporting the ceiling structure, the ceramic fiber insulation and the fuel bundle discharge temperature monitoring systems;
8) Supporting the gantry crane;
9) Supporting the gamma ray / neutron camera;
10) Guiding cooling air flow;
11) Reserve radio isotope containment.

The functions of the inner sheet stainless steel wall include:
1) Primary sodium vapor containment;
2) Primary argon cover gas containment;
3) Secondary air exclusion.

The functions of the outer stainless steel wall include:
1) Primary air exclusion;
2) Secondry argon cover gas containment;
3) Secondary sodium vapor containment;

The functions of the air gap include:
1) Space for circulation of cooling air;
2) Space for inspection and service access;
3) Space for emergency sump pumping of either liquid sodium or water.

The primary sodium pool can be thought of as being like four nested cups. This cup nesting is apparent on a FNR side elevation diagram.

The innermost cup, which contains the primary liquid sodium, is a stainless steel closed bottom vertical cylinder 21 m diameter X 16.5 m tall.

The next larger cup is a closed bottom vertical cylinder with a fire brick bottom 2.0 m thick and walls composed of compressible ceramic fiber insulation and fire brick. The inside dimensions of this cup are 21 m diameter X 16.5 m tall. The outside dimensions of this cup are 25 m diameter X 18.5 m tall.

The next larger cup is a closed bottom vertical cylinder fabricated from steel that is 25 m diameter X 18.5 m tall. Structural steel members connect the walls and bottom of this steel cup to the outer reinforced concrete cylinder.

The largest cup is a closed bottom vertical cylinder fabricted from reinforced concrete. The inside dimensions of this cup are 27 m diameter X 19.5 m tall. The outside dimensions are 29 m diameter X 20.5 m tall.

These nested cups face a number of engineering challenges including: a) Operating hoop stress;
b) Thermal expansion;
c) Field welding;
d) Sodium melting;
e) Volumetric constraints.

At the bottom of the inner most cup there is a 15.5 m head of liquid sodium.

The density of the liquid sodium is about 927 kg / m^3

The acceleration of gravity is about 9.8 m / s^2

Hence the static head pressure at the bottom of the pool is given by:
15.5 m X 927 kg / m^3 X 9.8 m / s^2 = 140,811 Pa

On a 1 m high strip of inner cup side wall this pressure exerts a force of:
1 m X 21 m X 140,811 Pa = 2,957,037.3 Newtons.

Let W = inner most cup wall thickness.

Then the hoop streess on the inner most cup wall material near the bottom of the primary liquid sodium pool is:
(2,957,037.3 Newtons) / (2 X W X 1 m) = (1,478,518.65 / W) Pa
where W is in m.

The maximum allowable working stress for stainless steel at 450 degrees C is:
10,000 psi X 101,000 Pa / 14.7 psi = 68,707,483 Pa

Hence the smallest allowable value of W is given by:
W = 1,478,518.65 / 68,707,483 = 0.0215 m
which indicates that near the bottom of the inner most cup the wall must be composed of (7 / 8) inch to 1.0 inch thick low carbon stainless steel.

This wall thickness requirement decreases with increasing height above the bottom of the inner cup. If we make the lowest 3 m out of 1 inch stainless steel plate, the next 3 m out of 7 / 8 inch plate, etc.
1st 3 m1 inch
2nd 3 m7 / 8 inch
3rd 3 m3 / 4 inch
4th 3 m5 / 8 inch
5th 3 m1 / 2 inch
6th 3 m3 / 8 inch


The thermal coefficient of expansion of stainless steel is:
17 um / m deg C

Thus the radial expansion of the innermost cup in transitioning from 15 deg C to 450 deg C is:
435 deg C X 17 X 10^-6 / deg C X 10.5 m = 0.0776 m ~ 8 cm.
This expansion must be absorbed by compression of the fiber ceramic insulation around the walls of the inner most cup. If we make the first meter of the ceramic insulation out of fiber material we can likely satisfy this condition, although we must watch out for the volumetric constraints.

The pool liner side walls will vertically expand:
435 deg C X 17 X 10^-6 / deg C X 16.5 m = 0.122 m = 12.2 cm

The walls above the pool deck will vertically expand by:
435 deg C X 17 X 10^-6 / deg C X 4 m = 0.029 m = 2.9 cm

Hence the fiber ceramic insulation between the pool enclosure metal ceilings must compress by:
12.2 cm + 2.9 cm = 15.1 cm or 15.1% insulation compression.

The pool deck is rigidly welded to the sodium pool inner liner. Hence the fiber ceramic in the walls above the pool deck must accommodate pool deck radial expansion of about 9 cm or 9% insulation compression.

In the event that the inner and outer steel cups fail the FNR geometry must be designed such that fission product decay heat can still be removed from the FNR. Hence a least 1 m of the heat exchange bundle tubimg must remain immersed in liquid sodium. Hence the maximum permissible drop in liquid sodium level is:
3 m + 6 m - 1 m = 8 m

The corresponding volume of liquid sodium is:
Pi (10.5 m)^2 X 8 m = Pi (882 m^3)

The liquid sodium disk under the reactor has a volume of:
Pi (13.5 m)^2 X 1 m = Pi (182.25 m^3)

The liquid sodium cylinder around the reactor when the maximum allowable amount of sodium has run out is:
Pi [(13.5 m)^2 - (12.5 m)^2][15.5 m - 8 m + 2 m]
= Pi [182.25 m^2 - 156.25 m^2] [9.5 m]
= Pi [247 m^2]

Thus the maximum liquid sodium volume available to fill in the ceramic insulation and brick work space is:
Pi (882 m^3) - Pi (182.25 m^3) - Pi [247 m^2]
= Pi (452.75)m^3

The gross volume of the brick work and ceramic insulation is given by:
Pi [(12.5 m)^2 (2 m)] + Pi [(12.5 m)^2 - (10.5 m)^2] (15.5 m - 8 m)
= Pi[312.5 m^3] + Pi [156.25 m^2 - 110.25 m^2](7.5 m)
= Pi [312.5 m^3 + 345 m^3]
= Pi [657.5 m^3]

Thus the fraction of the brick and ceramic insulation volume that must displace sodium is:
1 - [Pi (452.75) / Pi (657.5)]
= (1 - 0.68859)
= 0.3114

This condition is conservatively satisfied if we make the pool bottom out of solid brick and the pool side walls out of 50% brick.

Then the thermal expansion requirement is met if the 1 m thickness of ceramic fiber insulation in the pool side walls can readily compress by about 8% to accommodate inner cup thermal expansion.

The plan is to remelt the primary sodium by supplying heat to it from one or more of the intermediate sodium loops. These heat supply loops must be fitted with electric or fossil fuelled heaters.

The stainless steel inner cup liner must be field welded. Each wall ring can be assembled out of 17 3 m X 3 m stainless steel plates. Hence the side walls consist of 6 X 17 = 102 stainless steel 3 m X 3 m plates of varying thicknesses. The inner cup pool floor is fabricated from:
(7 X 7) = 49 3 m X 3 m stainless steel plates of uniform thickness. This number reduces to 45 floor plates when the corners are clipped.

The side walls of the outer steel cup are braced against the concrete walls using structural steel. Hence these outer steel cup side walls do not have to withstand large hoop stress. However, these side walls must be fitted with thermal expansion joints so that they do not buckle if hot liquid sodium penetrates the brick work behind these outer steel cup walls.

This web page last updated January 30, 2018.

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