The Use of Radioactive Isotopes and Nuclear Reactors in Space Application: Propulsion and Power concept
Hira High school and Science College Shahdherai
Swat, Khyberpakhtunkhwa, Pakistan.
As we know that men have insatiable thirst in exploration and discovery especially to unfold the mysteries of universe. There are lots of frontiers that could be covered in space exploration but unfortunately, there are some reasons which affect it. Firstly, the distances to be covered are extremely large, this large distance in outer space makes it very difficult to prospect for an astronaut involved in such a mission. Even a simple mission to far planets like Uranus, Neptune or even to nearest planet Venus would take a long time. Hence in case of Venus, it would need to take more than one year for such a mission. However, to explore outer space of solar system like to go to our nearest star “PROXEMA CENTAURI” it would take more than millions of years to reach there. Moreover, the second problem is greater amount of energy required, which need for an astronaut onboard for any outpost installation. Luckily, the availability of nuclear technology allows us to overcome these two problems. In this paper, it is demonstrated that by using nuclear reactors or Radioactive isotope decay like Americium-241 with half life of 432.7 years or Uranium -235 using Concept of propulsion and power can provide enough energy to travel vast distances in space and the energy need for an astronaut at ISS would be easily available.
Keywords: nuclear propulsion, nuclear rocket, space power application, nuclear power in space station.
The huge distance in space becomes very difficult for a space scientist and for astronaut to cover that. The reason is the fuel need for such a mission not finds easily because to goes out of solar system would takes million of year.
Even a simple mission in our solar system to the terrestrial planet like to Venus or to Mercury would also takes hundreds of years so, the one problem which effects our mission the most is the propulsion need for a space shuttle to covers such a distance,
And secondly we also know that energy requires for an astronauts onboard for any outpost Installation also becomes difficult.
Yeah it would be tempting to believe that all power in space could be supplied by solar means since the sun is available and free. However, in many cases the mission some time takes place in the dark place and solar panel is not always suitable for a mission (figure 1).
Figure 1 : regimes of possible space power applicability
Nuclear reactors have provided electrical power for some of the U.S. space program’s greatest successes, including the Apollo lunar landings and the Viking Landers that searched for life on Mars. RTGs made possible NASA’s celebrated Voyager explorations of Jupiter, Saturn, Uranus and Neptune, RTG Power sources are enabling the Galileo mission to Jupiter, the international Ulysses mission studying the Sun’s Polar Regions, and the Cassini mission to Saturn.
Pioneer 10 was launched from Cape Kennedy on 2
March 1972. It was the first interplanetary probe, successfully navigating the asteroid belt before
Making rendezvous with Jupiter and Saturn The probe was equipped with an array of instruments for measuring such phenomena as the solar wind and the magnetic and radiation fields surrounding Jupiter.
Nuclear reactors being a reliable and competitive source of energy they have some risk too and as well it also becomes difficult to do in some condition. The overcome of these problems are also demonstrated in this paper.
II. Space power concept:
To remove the waste, to lift a load and as well as for communication in International space station or on ground you will need sufficient power to do so for a long time. Moreover it would be very difficult for to generate electricity by thermal or chemical means in microgravity environment .
However with the availability of radioactive isotopes and as well as with the help of nuclear reactors the Electricity and all the other requirements in ISS would be easily available without any difficulty for several years 
For the production of electricity heat energy can be converted by many devices but the precise one is to use Thermo electric generator which typically works on Seebak Concept i.e
dV = SAB.(Kh - Kc)
dV is Voltage difference, SAB is two dissimilar metals, & Kh - Kc is temperature gradient.
In this way Nuclear reactors can provide infinite power for almost any time. However, they are not practicable for applications below 15 kilo watt (kW).
Radioisotopes are considered to be best used for continuous supply of low levels (up to 5 kW) of power or in combinations up to many times this value. For this reason, especially for long interplanetary missions, the use of radioisotopes for communications and the powering of experiments are preferred. The nuclear process shown in Fig. 2 can either be a critical reactor or
radioisotope fuel source such as plutonium oxide. In either case the heat can be converted to electricity either statically through thermoelectric or a thermionic converter, or dynamically using a turbine generator in one of several heat cycles (Rankin, Stirling, Brayton).. The nuclear workhorses for current space missions are the RTGs and the TEGs powered by radioisotopes in the Russian Federation that provide electricity through static
(and therefore reliable) conversion at power levels of up to half a kilowatt, or more by combining modules. And that energy can be used for to run machinery. .
III. Space Propulsion concept
Nuclear reactor can also be used in rocket propulsion system. Propulsion system in space is to change the position Or velocity (v), of space craft under the strong influence of gravity. 
When a space craft moves it generally term is momentum (mv) so, the basic concept of propulsion is to change the momentum (mv) of a space craft, and change in momentum is known as impulse (Imp). So, the basic aim of propulsion in space is to create impulse, to measure the impulse is often term is specific impulse (Isp).
Figure 2.The conversion of heat energy into kinetic or to mechanical energy
Specific impulse also known the exhaust velocity (ve) (OR) the impulse per unit weight ( ) but while discussing the engine in space there is no weight so it can become impulse per unit mass
The difference arises here only by the acceleration due to gravity (g).Where its value is 2 on earth.
However the value of g and reaction mass is not important while the vehicle discussing in space. .
The two parameter specific impulse and the mass of the rocket during launch and then in orbit is measure the efficiency of propulsion energy.
Where the equation for the measure of specific impulse as seen as (1)
Where represent mass of propellant and represent mass flow.
Here it is seen that mass of propellant is inversely proportional to specific impulse, So, In chemical propulsion there is used hydrogen and oxygen which provides a specific impulse about 4400 m/s with mass ratio of earth escape of 15.
However americium OR hydrogen heated with fission reactor can achieves twice the impulse with mass ratio of 3.2. And with different core it can be generate much more impulse i.e. seven times having mass ratio of only 1.2 .
IV. Nuclear rocket
Nuclear rocket is the rocket which use nuclear reactors usually fission concept instead of chemical propulsion to generate thrust it include nuclear thermal rocket (NTR), Nuclear electric rocket (NER) or Hybrid (NTR/NER). 
These rocket are propelled by the force of nuclear explosion, usually liquid hydrogen is heated via nuclear fission which subsequently can be expanded in nozzle and thus accelerated at a high ejection velocity (6,000 to 10,000 m/s) to generate thrust. There are also many design are find for nuclear thermal propulsion i.e. liquid, solid or gas core rocket. 
They use solid, liquid or gas core reactors respectively.
Electro thermal propulsion rocket have been use in many orbital mission in this type of rocket the propellant is heated electrically (heat energy of nuclear reactor are converted into electricity) and then it accelerate the ionized gas via electrostatic force at supersonic velocity having range from 1,000 to 5,000 m/s and thrust range 0.01 to 0.5 N. But it has low Specific impulse therefore it can be use only for interplanetary missions, however, it can be use for long time instead of chemical rocket 
The nuclear rocket has many advantages which overcome on chemical rocket. Like it can be use in much complicated mission even it can be travels to the edge of our solar system or even to our nearest cluster, nuclear space craft can be also used for to carry heavy payload or to carry flyby, rover etc to the far planet in our solar system, because it provides a large specific impulse as a result faster travel and the probability for complicated missions can be met, secondly nuclear reactor has low molecular weight which can increase the propulsive force per unit of propellant flow and allowing us for increase the proportion of total weight of payload .
V. Microgravity effect on core reactor
In microgravity environment every mass tends to be in state of motion (try to revolve from heavenly bodies near them due to centripetal force) So, The big challenge of core reactors in microgravity environment is to control the flow of fission. It seems like very difficult but under certain design it is possible to keep the reaction self sustain. So the level of energy would be maintained. In core reactors usually the flow is turbulent due to the randomness motion of molecule. And hence it becomes difficult to keep flow within specified boundaries. For this reason Reynolds number flow are preferred for microgravity environment.
It must be needed to be have a controlled chain reaction (neutron produce = neutron used) If it is not controlled or not under the specified boundaries then the chain reaction times would be increases due to uncontrolled condition in the core reactor. And some parts of the core reactor are exposed and hot spot would be appear on that as well some parts of the core reactors would be damage due to high temperature because the distribution of heat is not possible in such circumstance.
Figure 3 Precise core reactors for microgravity environment
1, Reflector moderator (usually made of beryllium) 2, Gaseous fissile zone 3, Working medium flow zone, 4, Fissile material diminution replenishment 5, working medium Inlet.
To overcome on this problem a special core is design (shown in figure 3) to sustain fission reactor under control especially with this core the possibility of environmental hazard would be reduce. And various high temperature fuels are possible with this design. in The fissile material for example Uranium or americium would be located in the center of cavity enclosed by the neutron moderator reflector. The working gas flow is close to the cavity walls and it is heated by the high temperature plasma radiation which causes the fission reaction to be self sustained under microgravity conditions. 
VI. Radiation hazards of Nuclear rocket
Expose of a person to radiation does not mean that the person would get cancer. People are exposing to radiation on daily basis like on lesser extent from human activities i.e.-rays. And radiation comes out from natural environment i.e. in the earth cosmic rays and radon. The radiation expose is measures in unit of Dose called millirem. Over the course of year the average person would expose to a total of 360 millirem to natural radiation .In nuclear thermal rocket some of the radioactive element would be release into the atmosphere The particles which are hazardous to people would remain high in the atmosphere for long course of time gradually these particles being spread thinly across the world. And eventually making their way on the surface especially on ocean since these materials are Insoluble once it reaches to the surface most of it would become trapped by the soil or by the ocean and not pose a health hazard. Thus most of the release materials won’t be breath by people. But the small amount of released material would be breath by the people and as well this small amount will be also distributed across the world. So, In this way the amount to be breath will be much less the person may receive it less than one millirem in 50 years. This small radiation is negligible compare to the 15,000 millirem a person will get it from natural resources over the period of 50 years. 
VII. Result and conclusion
In this paper we have emphasize on the importance of nuclear reactors in space based application, where the nuclear reactors and radioactive isotopes would provides infinite heat energy for long durations during journey of the space craft and that energy can be converted into other form of energy and the need for power and propulsion would be met. But in microgravity environment it becomes difficult to do but however with the help of gas core reactors and other special core design would allow us to do so in microgravity environment too. And the space exploration would be become more precious and peaceful.
I would like to thank national space agency SUPARCO Space awareness society including Mr. Qamar Abbas and Mr. Tahir hussain and My School principal Mr. Sami ullah for to guide me as well for their support. I also like to express my gratitude with SUPARCO propulsion team for did my conceptual analysis.
I am also grateful to Mr. Rahman u din and Mr. Zulfiqar Ali for Insatiable support and guide.
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