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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 an FNR's dome shaped roof and its related overhead protective net structure.

The FNR Roof provides a combination of structural, weather protective and missile protective functions.

The structural functions include:
a) Support of the sodium pool space side walls;
b) Support of the sodium pool space ceiling;
c) Support of the overhead monitoring system;
d) Support of the overhead polar gantry crane;
e) Physical support of a roof containing sufficient weight of overhad fill to withstand a tornado;
f) Physical support of a dome of sufficient strength to withstand the air coupled nearby explosive blast of a bomb;
g) Physical support of the weather protective surface;
h) Physical support of the required overhead net.

The weather protective functions include:
a) A water tight membrane;
b) Sufficient membrane slope for natuaral drainage;
c) Sufficient rigidity for shape maintenance;
d) Sufficient distributed weight to withstand a tornado;
e) Sufficient translational support to withstand a hurricane;
f) Sufficient translational stability to withstand a severe earthquake
g) Natural missile protection sufficient to withstand impacts by tree trunks and utility poles thrown by hurricane and tornado winds. Note that natural missiles generally have impact velocites less than 100 m / s, are seldom more dense than steel and typically have averarge densities comparable to water.

Practical experience during the Russian invasion of Ukraine has demonstrated that an irrational high altitude military attack with a laser guided armor penetrating bomb is a potential and credible future threat to FNRs. This military threat issue is shared by other types of nuclear power plants and by major hydro-electric dams.

Military Missile Protection:
The FNR military missile protective functions include:
a) Ability to withstand low angle attacks such as an impact by a descending airplane or a direct tank shell or anti-tank (shaped charge) rocket attack;
b) Ability to withstand a high angle attack such as an impact by a falling artillery shell or a falling anti-tank bomb witha two stage shaped charge;
c) Ability to absorb a determined high angle attack with an armor penetrating bomb dropped from a high altitide without significant danger to the public;
d) Ability to withstand a high angle attack such as by a military rocket with a chemical explosive warhead that explodes on impact.

A ten ton armor penetrating bomb typically consists of a tall thin pointed bomb casing with a hard substance nose made from a few tons of tungsten carbide, tail fins that steer the falling bomb trajectory to home on a laser designator and the balance of the bomb weight consisting of a stable explosive. The bomb is typically dropped from an altitide of about 20,000 feet and attains an impact velocity greater than the speed of sound. The hard nose allows it to cut through thick steel armor or through a 5 m thickness of reinforced concrete. The bomb has a time delay fuse which causes detonation after the bomb has fully penetrated its target. During WWII this type of bomb, without laser guidance, was succesfully used to attack the German U-boat pens at Lorient and Brest in France and to sink the heavily armored German battleship Tirpitz while it was moored in Norway.

The point is that it is impractical to build a rigid dome over a FNR that by itself is of sufficient thickness to resist an attack by such an armor penetrating bomb. However, it is possible to make a rigid dome and to install above that dome a protective structure layer consisting of:
a) A hard surface layer to distribute the initial point impact force;
a b) Multiple layers of polyester ribbon netting embedded in fill that can absorb and distribute much of the bomb's initial kinetic energy via fill compression and polyester ribbon tension. The fill might take the form of expanded polystyrene, which has demonstrated suitable shock absorbing qualities when embedded in vehicle bumpers.
c) Cause the bomb's explosive to detonate at a point well above the dome, thus causing the explosive energy of the bomb to dissipate over a wide area.

The general approach is for the FNR to have robust multi-layer concrete walls and a rigid dome roof over which is constructed a protective structure. The concrete walls absorb low angle falling missile attacks, the dome shape deflects low angle rising missile attacks and the nets embedded in the protective structure cause the kinetic energy and forces from high angle attacks to be widely distributed. The dome plus its protective structure has sufficient weight per unit area to withstand a tornado.

Assume that the dome upper surface is part of a sphere. Assume that the slope of the dome upper surface at its perimeter is 45 degrees. Then a bit of geometry shows that the ratio of dome height to dome diameter is: 0.2071.

Hence if the dome diameter is 34 m the dome height above the top of the side walls is:
0.2071 (34 m) = 7.04 m

The bottom inside of the dome should extend below the top of the side walls to provide the dome lateral stability.

A protective net has the property that if it is deformed by a point impact by a projectile much of the projectile's momentum and kinetic energy are absorbed by the net material which deforms and compresses the surrounding fill. The projectile energy and momentum are transferred onto the dome and its supporting concrete sidewalls and to net perimeter supports, guy wires and a net perimeter hoop ring. This same principle has been used for safely catching falling high wire circus performers. A variation of the same concept is used by bullet resistant vests.

The top surface of the protective structure should consist of ceramic tiles epoxy bonded onto steel plates. The ceramic plates provide a hard layer which resists point penetration by a hard bomb nose material such as tungsten carbide. The steel layer enlarges the area over which the initial point impact force is distributed.

The underlying fill further distributes the impact force over an even larger area of polyester netting. Multiple layers of this alternating fill-netting-fill structure will likely be required.

It is likely impossible for a FNR power plant to absorb a determined military attack without some damage. However, in that case the object is primarily to prevent a major FNR explosion, not to expect the FNR to be operable immediately after the attack.

This web page last updated June 28, 2022.

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