Protection from weapons of mass destruction in AFVs

Anti-radiation material usage in Armoured Fighting Vehicles.

The penetrating radiation of a nuclear explosion consists of 2 things, a stream of neutrons and gamma radiation, which both pose a danger to vehicle crews. One of the countermeasures against such threats is the usage of steel in armour. That is why streams of neutrons (a portion of which outside of the tank's armour exceeds the level of gamma radiation almost by 2-3 factors) pose the main danger when penetrating the tank's main armour. To prevent high-level doses of neutron radiation, the best solution is to use materials that can be enriched or are rich in hydrogen. 

Anti-radiation material in the form of additional shielding can primarily be mounted on the amour itself, which needs to have certain abilities to be used in the long run. The main factors are, being able to withstand high temperatures either from direct fire or combustible liquids and deactivating liquids against materials covered in radiation. Additionally, these materials should be cheap enough and technologically advanced. These materials are based on polyethene and polyisobutylene with the addition of lead, evenly incorporated in the material volume (used to improve the protection levels against gamma radiation) and also based on porous filler connections (for absorbing thermal neutrons, which produce secondary gamma radiation). While the last materials are known to increase the cost of creating ARM (Anti-radiation material), special layered materials are created, whereas the more expensive materials such as Boron are put into small thin layers, which justifies its more rational usage in protection. This allows to decrease the usage of Boron almost by 2 times and decrease the total cost by not degrading its protective capabilities. 

Taking 2 types of layered material composition: 21F/17F and 22F/17F. 
21F/17 containing 0% lead and 22F/17F (containing 50% lead (by weight)) includes 0,3%-0,5% Boron, at which around 60% of this material is contained in a 2,5mm thin layer, which is placed between the ARM and steel armour. Layered materials produced from 1981, replaced homogenous type materials, which contain twice as many porous filler connections.

Anti-radiation materials are mainly installed both inside and outside of tanks. For example, the T-72B and T-80 tank's anti-radiation packages weighed around 300kg-400kg, enough to dampen the total dose of penetrating radiation by around 3-3,5 times. 

T-72Bs external ARM plating

Internal ARM plates for T-72

Experimental testing showed that 30% of the penetrating radiation dose caused secondary gamma radiation, which occurred under the effect of thermal neutrons inside of the ARM and other tank parts. 

As mentioned previously, thermal neutrons are also part of the neutron spectre, which can interact with the tank's armour. These can be stopped by materials that contain Boron and also a rare earth metal called Gadolinium. If there are ARM plates mounted on the tank's external armour, that contain materials such as Boron and Gadolinium, thermal neutrons will be absorbed by the plating, preventing thermal neutrons from interacting with the tank's main armour. 
In the case of a tank not being equipped with ARM plating containing Boron and Gadolinium, EP-0016 coating can be used to lower levels of secondary gamma radiation. EP-0016 is an epoxy-based coating in combination with carbide boron. Depending on the thickness of the tank's main armour, the applied coating layer can vary from 3mm to 6mm, depending on the previously stated thickness of the tank's main armour. This coating is especially effective on thin armour plating, such as; hatches, turret roof and underside of the hull.

Although, in the current time, armoured ARM plates, and the increased technical difficulty of producing (layered coating, its poor operating time span, negative effects during production, the need to prepare vehicles for coating and other reasons) EP-0016 is practically no longer used. 

New materials.

During firing trials at armour plates covered with ARM, an interesting observation was made. In hotter climates, ARM plates did not cause additional spalling when a round would penetrate. What was more interesting was that, in temperatures where the internal ARM plating would drop to -15°C, after penetration, the plate would partially shatter, causing additional spalling inside of the vehicle. 
This issue was solved by adding belting fabrics. ARM plates would be covered by steel-belted plating or are installed in a way where the ARM plates don't make physical contact with the internal armour (which was common inside of T-64 tanks).

However, this wouldn't solve the problem effectively. Lower the resistant temperature of internal ARM plates, would require reconstructing the structural basis for the material, one way would be to use ethylene-propylene copolymer, a product with a high-molecular density, which is achieved in a combined polymerization of monomers with the usage of organometallic catalysts. The density and protective characteristics against radiation would be on par with the existing ARM. During firing trials, the new element showed increased resistance to kinetic effects and would not shatter at temperatures between -30/-40°C. However, the production would not become mainstream due to the complexity of production for this type of material. 

Construction solutions in Anti-radiation protection.

There are 3 Anti-radiation protection categories; collective, local and individual. 

Collective protection is classified as the protection of internally operated compartments by the crew, which can be protected with the usage of ARM, internal vehicle framing and functionality modules (motor-transmission bay, breach) and with the usage of depletable materials (fuel, ammo stowage). 

Local protection is classified as plating or arrays of plates mounted near biological points (crew), for example, plates integrated into the crew seats. 

Individual protection is classified as the direct usage of protective plates on crew members. 

Main collective protection is in practice the usage of main vehicle armour with additional ARM plating, either placed outside of the vehicle on main armour points or mounted ARM plates inside of the vehicle behind the main physical barrier. 
Special testing was conducted and showed that the installation of ARM plates externally would be significantly worse at protecting against neutron streams compared to ARM plates being mounted internally. Additionally, the placement of ARM plates against gamma radiation did not play a significant role, due to both ways providing identical values. 

The goal to efficiently lower the overall radiation levels would be, for 30%-60% of ARM to be installed internally, while the remains of ARM are mounted externally. On the other side, the received recommendations in regards to the ways of the mounting would always be corrected due to structural vehicle limitations, in this case, there would be other solutions to improve protection against neutron streams. 

As previously mentioned, depletable materials, like fuel which is stored in the frontal fuel tanks. The effectiveness of this collective category heavily depends on the vehicle's compartment configuration. 

T-72 fuel tank arrangement

With the placement of fuel tanks in the front of the vehicle on both sides and the driver-mechanic centre-lined with the vehicle, full fuel tanks can increase the protection against neutron radiation by around 40% in comparison to empty fuel tanks. Taking into account the usage of fuel over time, the protection characteristics will decline, due to fuel consumption. This problem can be solved by segmenting the fuel tanks. A relatively slim segment of around 150-200 mm is created in the fuel tanks, which is the last one to be depleted. 
With this, the relative protection against neutron radiation will only drop once the main fuel compartments have been emptied and the last segmented compartment is used for fueling the engine. This allows for maximizing the overall protection of the vehicle against neutron radiation. 

The advantages of collective protection are that it is always ready for operation during nuclear fallout; preventing the degradation of the crew's health and their ability to fight, and the lack of theoretical limitations to the maximum protection levels based on the vehicle structural configuration. 
Collective protection also affects the weight that is added on when using protective plating. The standard crew placement in the hull and turret requires a large portion of the armour to be covered in protective elements, which is interlinked with the overall weight that is added when utilizing ARM. A solution to this problem would be the creation of a crew capsule placed in the frontal part of the hull, which would lower the required amount of ARM. 

Local protection is more of a rational way to improve the levels of anti-radiation protection by using minimal amounts of ARM. Structural local protection can be in the form of fixed or movable plating of varying thicknesses, from 20mm to around 40mm. In the case of increasing the thickness, even more, the effectiveness of slowing down a penetrating dose slows down. With a thickness of around 60mm, the effect of protecting against doses is practically 0%. The maximum optimal effectiveness can be achieved with the usage of local plating with ARM and can be lowered to around 1.5x. 

One of the main issues when creating local protection is the lack of dedicated space for integrating it. One of the suggested solutions was the usage of flexible protective plating, which would be filled with hydrogen-rich liquids. A rubber-style container was created and filled with diesel fuel. Sadly such a suggestion didn't get past the testing phase and was never adopted. 

Another example of local protection would be using ARM plates on protective artillery shields. These would be made out of fixed and movable components. With a thickness of around 20mm, the protection levels against neutron waves would be increased by around 1,2x. These protective shields were commonly used in T-72A production models. 

The crew is protected by hatches along with a turret and turret roof. In modern tanks, around 70% of neutron doses are on head level height and are prevented with the usage of the aforementioned protective segments. 

Due to compartment size restrictions, it's impossible to continuously increase the ARM thickness, which would result in affecting crew ergonomics, visibility and elevation in a negative way. 

Another solution that was thought to be a possibility to lower head exposure to neutron radiation was the use of adjustable local protection. In one of the prototype variants, a movable plating system was used. With the lack of an active radioactive threat, the plates would be folded and fit closely to the internal turret roof. In the case of a radioactive threat, the plates would be turned around on a horizontal axis and would align perfectly to create a semi-spherical plating array in the form of an umbrella, which would protect the crew's heads without lowering ergonomics levels. 

Individual protection as stated previously is the idea of placing ARM on the crew, in the form of vests, and helmets. To protect the upper body against neutron radiation as a special vest was created and also the same idea was applied to the helmet. The total mass was around 7kg-8kg and would lower neutron radiation levels by around 1,35 times and the total dose of penetrating radiation by around 1,2 time. 

The advantages and disadvantages of individual protection are similar to local protection. 

Materials used: 

• "Защита Танков. Москва. Издательство МГТУ им. Н.Э. Баумана 2007"
• NEUTRON SHIELDING PERFORMANCE OF WATER-EXTENDED POLYESTER