Nuclear energy has been the key binding factor in early renewable energy enrichment with majority of nuclear fuel being accessible within technological reach. The various industrial purposes of the nuclear energy are under energy and military industries with main emphasis being undertaken in the defence advancement and technology. Nuclear propulsion is under active operational process in defence sectors over various machinery and vessel propulsions which includes heavy aircraft carriers, ice breakers and submersibles. The latest eye for technological advancement under military aircrafts is being brought under existence in Fuel capacitance and efficiency of the airborne vessels. These are being implemented in the best interests of the increase long range sorties with stealth capability being the most important aspect of the advancement in the military aircrafts. Nuclear fuel usage in aircrafts is considered to be one of the best advancements in addition to enhancements in avionics and aeronautical engineering design which form the backbone of manoeuvring tactics. Nuclear energy inclusion in various military class aircraft would be mobilised soon to increasing capacitance of the aircrafts to attain long sorties and stealth capabilities without the requirement of primary overhaul. This paper emphasizes on the need for a future security environment attributes such as rangeability ,dominance combined with ever rising fuel costs due to draining reserves which are to be compromised with the usage of Nuclear Reactor based power. This can be analysed through the technological analysis of a nuclear-powered aircraft in various roles within the established nuclear legal principles, wherein one can envision a concept of operations that allows future decision makers to effectively balance these risks while operationalising a nuclear-powered aircraft.
NUCLEAR POWER AND ITS EMERGENCE
Nuclear power is the resource which is capable of bringing about the needed change in lowering the dependence on oil and gas deposits that stands to be the highest contributor to the energy consumption on the planet. The military advancements in various nations across the world are part of the increasing international tensions for warfare-based engagements that are of critical importance on the planet. This non-renewable source of energy is part of the fuelling requirements for military technologies and enhancing advancement generations of aircrafts and airborne machinery. The requirement for major improvements to increase the efficiency levels of operation and controllability of the aircraft are of keen and costly interest. These advancements take high research costs that show upon in the form of manoeuvring capabilities to increase the sortie duration of the aircraft. These requirements are limited to and end point when the usage of fossil-based propulsion is integrated for the aircraft. The advancement integration of renewable energy usage in the airborne machineries to decrease the dependency and increase the efficiency of the aircraft increases with higher emphasis. These factors form the foundation for implementation of the nuclear fuel-based propulsion within the aircraft machinery
NUCLEAR PROPULSION AIRCRAFTS – EXPERIMENTAL
The scope of nuclear fuel-based aircrafts has seen a lesser emphasis and research due to the fact of much known infamous nuclear fuel-based reactor accidents in the energy sector. This was termed as line of production for radioactive elements-based fuel usage for electricity generation. These were part of domestic and commercial production in various countries alongside research integration for better and efficient reactors. The infamous nuclear accidents of 3-mile island, Chernobyl nuclear reactor and Fukushima Daichi nuclear disaster which involved negligent levels of vigilance towards providence of increased safe operational characteristics towards the reactor efficiency. Nuclear-powered aircrafts were brought into existence with major research and testing probations done by the United States Air Force (USAF)in the year 1955 under supervision and engineering analysis of General Electrics (GE Power). The technology of heat source production from the operations within a nuclear reactor were used and re designed for B-47 jet engine propulsion. These designed operations were undertaken onboard the X-39 Bomber Aircraft to achieve a theoretical value in rangeability of 30,000 miles at an airspeed of 460 miles per hour. These theoretical values were brought up into existence on the B 36 bomber of The United States Air Force (USAF) but reactors were never connected to the power and propellant grid of the aircraft. The nuclear reactor retrofitted B 36 bomber flew 47 sorties in 5 years with the engineered propellant onboard, thus displaying a possibility of working capability and operational capacity of flying the nuclear reactor at varied flying heights. The ground tests of the theoretical results from the nuclear test aircraft program were completely efficient with a huge implication of high mechanised costs in manufacturing.
INTERNATIONAL DESIGN TESTING CONDITIONS
With major improvements being brought into existence of lightweight, heavy armament and manoeuvrable war planes, the technology for enhancing the nuclear power production under a lightweight reactor was part of a major research project. In the Development phases under 9 countries which were mandated to provide an international design of safety and operational compliance for its induction into various fighter aircrafts and bomber capabilities. These systems were adjusted and controlled within security and safety regulations. The design provisioned the countries with an operational probabilistic power output ranging from 40 megawatts to 200 megawatts at 100%-110% operating capacity of the reactor. The commercial usage capability was diagnosed during the theoretical tests that valued in range of 10-84 months, which is the highest range and timeframe possibility for a single sortie till date. Nuclear power poses unique risks to the health and safety which are to be controlled, prevented, and avoided. However, nuclear power also promises many significant benefits in a variety of current and future military applications which range from nuclear-powered ships and submarines which are part of active service and power to military installations in the United States, India and other locations in the future. Attack aircraft provide the capability to rapidly project both nuclear and conventional power irrespective of terrain, geography, and environmental conditions. This power can support ground forces in direct combat, prevent enemy reserves from entering the battle or disrupting strategic targets such as enemy leadership, communications nodes or industries used to develop future combat forces.
ENGINEERING DESIGN CAPABILITIES
The nuclear aircrafts were fuelled by re-engineered capabilities within the propulsion systems designed by aircraft engine propulsion manufacturers. There are two classes of propulsion systems inducted during the operational design of a nuclear-powered aircraft i.e. Direct Cycle Propulsion system and Indirect Cycle Propulsion system. The direct system of cycle operations used in nuclear reactors of the aircraft involved the inward gauging of air through the nuclear reactor, which is in turn heated by the fuel rods of the reactor. This thermal energy is transferred to the turbine that courses way for compressor driven propeller. The final stage of DCP system has the heated air expelled at high velocities through the exhaust nozzles that gives the turbine throttle and power. Whereas, in case of an Indirect Cycle was planned to be operationalised under the capabilities similar to nuclear power reactor in an atomic power station. The process involved usage of the thermal energy generated from the nuclear reactor, with absorption of heat by the liquid metal coolant placed in the buffer zonal regions around the reactor. The liquid metal coolant connected to an intermediate heat exchanger which converts the developed heat into a secondary loop leading the heated air to jet engines.
The jet engine contains radiators which acts as a medium of transfer for thermal energy to the air stream flowing through the engine on continuous sortie of aircraft. The propulsion projects were undertaken by series of operational procedures under the organisations involved in previous nuclear reactor constructions.This formed the foundation for development of the indirect propulsion system for turbines to reach a final credible research and development in the late 1950’s.
The airframe involved in protection and safety of the reactor onboard the aircraft was brought into due diligence for consideration after propulsion designs. These studies were limited to research and analysis based upon frameworks provided to the reactor in the atomic power station. Construction, operation, and testing of low-powered reactors with suitable shields, analysis of flight sortie requirements, and propulsion research studies were one of the critical factors involved in development stage of the nuclear aircraft propulsion in tactical reliability.
AIRCRAFT NUCLEAR POWER PLANT DESIGN AND REACTOR REQUIREMENTS
The Nuclear Power Plant in the Military Aircraft forms the heart to complete propulsion of equipment, machinery, and rotary operations in the aircraft. With the increasing requirements of flying heights and propulsion speeds in the military technology, A 3 Phased division was made in 1950’s to indicate the requirements of Military aircrafts in terms of warfare and long-range capabilities. The First division included the capability operational characteristic of Sea Level Operation at an air speed of 0.9 Mach followed by the second and third division with an Operational height of 45,000 Feet from GL at an airspeed of Mach 1.5 and Operational height of 65,000 Feet from GL at an airspeed of Mach 0.9, respectively. This brought upon the much-required military requirements into focus with compliance to be made to at least one of the aforementioned phases in order to have an operational capability and efficiency. The nuclear power plant design for the aircraft is to be made with due consideration given to other flight operational characteristics and performance efficiencies required by the military. As per the basic operational capability of Military Warfare, the nuclear power plant should have a power density within the core exceeding 1 kw/cm3with an optimal high efficiency power density as 5 kw/cm3.
The most efficient propulsion engines which can be retrofitted into the nuclear-powered aircraft are the Turbojet Engines and Compressor Jet Engines due to Inlet air-based circulation capabilities and lower handicapped properties which do not comply to the military requirements. The basic requirement of Nuclear power plant is to fulfil the provision of air inlet temperature into the turbojet engine as per the requirement of at least 1140 Degree Fahrenheit, which define the efficiency and operability levels of the engine to propel the aircraft. Based upon these characteristics of high heat requirements for inlet temperatures, the mechanical characteristics that decide the physical characteristics of size and weight of the Nuclear power plant in a manned aircraft are Specific Thrust and Specific Heat Consumption.
Specific Thrust in context to aircraft requirements refers to the amount of thrust that is generated or produced in pounds from pound of air being handled or propelled through the engine mechanisms which in layman terms, can be defined as the amount of downward force exerted from the aircraft with a pound of volumetric air being propelled through the engine mechanism of the aircraft. Whereas Specific Heat Consumption or Specific Fuel consumption in aircrafts refers to the loss of heat or fuel-based energy produced in the form of heat from the propulsion system into the atmosphere. The Temperature of the Fluid which propels the aircraft in the propulsion system stands to be the important factor affecting the aforementioned characteristics of Specific Thrust and Specific Heat Consumption, which was displayed in the TAB of ANP Program of USA in 1950.
In addition to the presence of Nuclear Fuel based reactor onboard the manned military bomber / aircraft, the presence of Chemical Fuel is to be present as a supplementary heat source. The Reactor based Nuclear Fuel and Chemical Fuel should not be accumulated or cross linked due to their high vulnerability to cause loss in process safety measures leading to a possible Radiation Explosion. The Chemical Fuel is to be kept as an auxiliary supplement to the Propulsion system of the manned aircraft in the systems. The major importance of presence of Chemical based propulsion as an auxiliary system includes its survivability role in powering the aircraft propulsion system in case of a Non-Destructive and Non-Damaging sequences of events. Its presence would be helpful in bringing upon the warming up conditions and quality assurance checks on board the aircraft with minimal risk being posed to ground personnel during operational repair and overhaul. A partial involvement of the chemical fuel in increasing the Specific thrust provided during take-off or high-speed operations in varying altitudes can be of immense efficient propulsions considering its role in increasing the air temperature above the required levels within the turbines of the turbojet engines.
The Various reactor types which were analysed to meet the requirements of size, structure, cooling capability and propulsion compliance included 7 to 8 categories of Aircraft reactor types designed with the basic foundation propulsion design. The Reactors included usage of Solid Fuel and Liquid Fuel based Reactors under the Stationary Fuel category of Aircraft reactor alongside the Homogeneous Fuel and Separated Moderator based Technology being used under the Circulating Fuel Aircraft Reactor category. The Basic categorisation of Fuel Flow based Reactor type is explained as shown below:
• The Stationery Fuel in aforementioned context refers to Fuel Source with minimal kinetic energy in terms of rotation throughout the nuclear power plant in the aircraft, wherein the Fuel remains intact in various enclosed cabins or containers.
• Circulatory Fuel refers to the fuel source which is placed and dispersed under continuous flow throughout the reactor containment with the fuel being completely in dissolved formats throughout the moderator which affects the flow of neutrons within a radioactive element being used as the source of fuel in the nuclear reactor. ( Representation Diagram for Power Plant Connectivity to Turbine Propulsion System )
Source : Forbes Magazine Excerpt
Within these reactor fuel types; A solid fuel can be used which is usually Sintered Uranium Dioxide which can be noticed to be used as a source of fuel in most of the Nuclear Power Plants in various nations. These are placed in a close compact pellet shaped enclosures or cermet’s to have high resistance and efficiencies. Liquid Fuel is used in some cases wherein the Fuel is placed in Tubular casings within the reactor. These form the Fuel component within the Stationery Fuel category of Aircraft Reactor.
The Coolant which acts as the source of cooling the reactor from high heat generation capabilities and protect the reactor from all possible damages in relation to the loss of reactor flow. Coolants used are dependent upon the Type of moderator being used and the Fuel Source being used within the reactor with due consideration being given to Stationery and Circulatory requirements of the fuel. The Stationery and Circulation Fuel systems in the reactor can be operated on various propulsion majorly upon Turbojet Propulsion system, with an exception of Turbo Compressor Jet Engines being compatible within the Stationery Fuel Type of Sintered Uranium Dioxide alongside a Boiling Coolant or Gas Based Coolant within the reactor design.
OPERATIONAL RADIATION DAMAGE AND DOSAGE RATES ANALYSIS
The Operational Radiation damage refers to the deterioration and inefficiency caused to the Operational Crew and the Mechanism installed within the aircraft due to excessive exposure than the permissible limits of radiation from fuel source. Shielding is the mechanism which stands to be the permanent mechanical and chemical protection which allows the compartment of Reactor to be isolated from the rest of the compartments in the aircraft. This allows lower chances of Radioactive emissions, thereby leading to lower levels of Radiation dosage and loss of operational equipment on board the aircraft.
The Shielding based weight imparted within the aircraft is of major consideration, as it involves the levels of permissible dosage for the crew and other organics / mechanical equipment within the aircraft during its operation .Gamma radiation doses have been expressed in roentgens (r) for many years, 1 r being the gamma radiation dose giving an energy deposition of83.8 erg/g of air. On the same basis, the “rep”(roentgen equivalent physical) was devised to serve as a measure of both neutron and gamma radiation doses. Thus 1 rep in gammas i s equal to 1 r, and 1 rep in neutrons is a dose giving an energy deposition of 83.8 erg/g of tissue. This stands to be the basic conversion and dosage measurements that were used to accurately measure the shielding and permissible limits for the crew designed to operate the nuclear propelled aircraft. The Living tissue within the human body stands to be of major importance and is most affected by high-speed neutrons within the nuclear fuel, thereby to correlate the damage caused by gamma and neutron-based rays being emitted from the nuclear fuel, “rem” was established which stands for roentgen equivalent man.
On an overview, “rep” represents the damage caused by neutron and gamma rays emitted from nuclear fuel to the organic material in the aircraft whereas, “rem” represents the damage caused by neutrons and gamma ray emission from nuclear fuel to the human tissues of the aircraft crew.
The usually accepted permissible limits in the laboratory facilities is 15 rem/ year which is usually deviated by a rate of 7 times the permissible rates due to due negligence by the operating personnel. Considering the same, a permissible limits of 100 rem was established as the regulation of operability within the aircraft for the crew, A shielding protection designed to contain 1 rem / hr would permit 100 hours of operational flying hours for the crew. If a better shielding is provisioned within the aircraft which involves a protection limitation of 0.1 rem / hr, this would increase the flying operational hours by 10 times i.e. 1000 hours per crew.
The radiation damage induced in the aircraft not only applies on the operational crew but is effective on the mechanism and mechanical operating parts of the aircraft which are organic in nature. The amount of maintenance work required to maintain the aircraft in proper conditions completely depends on the reliability and service life planned for the specific equipment in the aircraft. The organic materials embedded or transferred in flight during a sortie are highly vulnerable to deterioration and damage from nuclear radiation in the operating environment , thereby bringing up the requirement of placing upper limits on the shielding levels in the aircraft and the degree of division required.
A Practical engineering-based experimentation was done on various materials used in the aircraft as part of the analysis to test for its strength and agility towards the radiation emissions from the power plant. The various minor parts used within the aircraft were tested against the nuclear radiation, wherein the best available quality of equipment was used for testing purposes against the emissions.
It was noticed that the highest quality of materials when exposed to 30,000 rep / hr which is equivalent to 3,00,000 rem / hr would be completely damaged and deteriorated leading to loss of aircraft equipment and major fluctuation in operational stability of the aircraft. Based upon this foundation of exposure test and engineering analysis , it was devised of having a nuclear-powered airplane truly operational, to have a desirable and essential limitation on radiation dose from there actor to a value such that elastomers and greases would have an operational life of at least 300Hrs , if located 10ft from the reactor. To satisfy this condition the reactor shield should be designed to emit a dose of not more than 1000 rep/hr which is equivalent to 10,000 rem/hr at a distance of 50 feet from the centre of the reactor.
GROUND HANDLING , EMISSION AND AIRCRAFT MAINTANENCE ANALYSIS
A proper examination is done to analyse the risks and hazards from operating a nuclear-powered aircraft under stall position or post and pre sortie positions. These stages pose the highest risk of contamination in a societal area rather than in a flying sortie. An aircraft requires regular maintenance and overhauls to have better efficiency in the operational mechanism of the sortie. During the maintenance and operational repairs / overhauls of the aircraft, three phased categories of works are possible to be operated within the reach of the personnel involved. The Range of 50 ft from the core of the reactor releases a radiation emission of 1000 rem/hr considering direct human intervention. This is considered as the full power dose with the limitation of shielding from emission to be the maximum in engineering capabilities.
The Three phased working requirements as part of handling and maintenance of nuclear aircraft can include the following :
• Regular Maintenance operations – Usage of Auxiliary Shielding can be provisioned to enhance ground resistance capability from radiation emissions to the working / operating personnel.
• Post-Sortie / Pre-Sortie Handling and Maintenance –Usage of Auxiliary ground shielding not possible due to its presence in taxiway or runway regions / tarmac area.
• Unscheduled Activity of Emergency –Auxiliary Shielding cannot be available due to the unprecedentedestimation involved in the emergency requirements and operational emergencies of the aircraft.
With the Testing flight program initiated to operate a nuclear based power plant within a B 36 bomber of the United States Air Force under Aircraft Nuclear Propulsion Program, it was noticed that the aircraft requires approximately 2000 Personnel working hours for Regular Maintenance per flight. The 2000 working hours has been estimated with consideration being given to one sortie operations in a week.With consideration given to the maintenance operation under Regular Maintenance, the aircraft is placed under shutdown mode for the 1st phase maintenance operations. The emission levels from the radioactive source falls by a factor of 20 when the reactor onboard is shutdown. The Fission products which are generated as part of the operations in nuclear fuel when nearing to decay reduce the emission levels factor further by 2 times in addition to the 20 Times reduction on shutdown.
A proper decontamination and protection schedule must be planned for the maintenance and operations team personnel, with 1/3rd of the maintenance hours devoted being at a distance of minimal effect from the reactor staging in the aircraft and the remaining 2/3rd within the vicinity of the aircraft. The Auxiliary shielding for the reactor is arranged in the 1st phase of maintenance operations by replacing the fuel or water in the shielded region of the reactor with Zinc bromide , mercury, or oil metal shot mixture. Since these materials are induced into the shielding , the shielding material needs to be strong enough to sustain the dead load and operating load due to these mixtures being replaced as an when required in the reactor.
The Below mentioned infographic gives an idea of the Shielded Nuclear Reactor with fuel elements representation which can be retrofitted within the aircraft as per ORNL Research Document , United States of America
This basis of foundation to develop nuclear propulsion in the fighter aircrafts and bomber / air surveillance warplanes of future generations can be made with increasing technological advancements. These can lead to low-cost nuclear reactors which are durable, lightweight, and cost efficient based upon the 3D printing capabilities. These make the strategic goal and availability of long-range military and commercial aircrafts possible in the near future with continuous advancements. Nuclear energy is determined to stand strong and efficient in the face of decreasing fossil fuel-based propulsion, which are limited and are struck with depleting reserve levels. The planet’s dependency on renewable sources will bring the most possible and efficient output, of which nuclear energy stands to be part of the future technology of long commutation, higher military control and economic warfare.
- Report on Manned Aircraft nuclear Propulsion program by Comptroller General of United States – Department of Defence – The United States of America
- Air Command and Staff College Report – Major Brian JG (United States of Air Force)
- Application of Atomic Engine in Aviation – Military Press of MoD for USSR
BY Pasagada Pardha Sai, CNSP (P), M. Tech (HSE),B. Tech (Civil Engg.)