You still believe that the main sources of human radiation exposure come from the nuclear power plants?
Then, this series of articles are for you… We will tell you about natural ionising radiation sources, medical applications of radiation, as well as other interesting things.
The ionising radiation sources are conditionally divided into two groups – natural and artificial. The natural sources have always been around, but it was in 19th century when the artificial sources were first created by human civilisation. This is simple to explain through the example of the two prominent scientists, whose works were attributed to the discovery of radiation. Antoine Henri Becquerel discovered the Uranium radioactivity (the natural source), while Wilhelm Conrad Röntgen discovered the ionising radiation during deceleration of electrons, which were accelerated in the specifically designed device (the roentgen tube as an artificial source). Let’s make an analysis, in both percent and numerical equivalents, of exposure doses (a quantitative characteristic of effects of ionising radiation to a human body) an average Ukrainian citizen receives annually from various artificial and natural sources (Fig.1).
Fig. 1. Structure and averaged values of the annual effective exposure dose to the Ukrainian population
As we can see, the bulk of the exposure we receive from the natural sources of radiation. But have these natural sources remained exactly as they were at the earlier stages of civilisation? If it is so, then, there’s no worry since we have long before adapted to such exposure. Yet, unfortunately, this is not the case. Human activities lead to either concentration of the natural radioactivity sources or their potential impact on man increases.
One of such places where the potential impact of radiation on man increases is the outer space. The intensity of radioactive exposure depends on a height above the sea level. Therefore, the astronauts, pilots, and passengers of airline carriers, as well as population residing in mountain areas receive an additional exposure dose. Let’s try and figure out to what extent this is dangerous to man, and which “radiation” secrets are concealed in space.
Radiation in space: what is the danger to astronauts?
It’s all started with James Alfred Van Allen, the American physicist and astrophysicist, who decided to mount a Geiger-Muller counter onto the first satellite launched to the orbit. The readings of this instrument had officially confirmed the existence of intensive radiation belt around the globe. But, where did it come from in space? It is a known fact, that radioactivity in space existed for a very long time, even before the Earth, therefore, the outer space had been permanently filled and still is filling with radiation. After the relevant research, the scientists had come to the conclusion that radiation in space emerged either from the Sun during the solar flares, or from the space rays emerging as a result of a high-power energy occurrences in our and other galaxies.
It was established that radiation belts begin to occur at 800 km above the Earth, spreading up to 24 000 km. According to classification of the Federation Aeronautique Internationale, a flight is considered a space flight if its height exceeds 100 km. Respectively, the most vulnerable, in terms of getting a high dose of cosmic exposure, are the astronauts. The higher they go to the outer space, the closer they get to the radiation belts thus getting a greater risk of receiving considerable amounts of radiation.
Francis Cucinotta, a scientific supervisor of the USA National Aeronautics and Space Administration (NASA) research programme on radiation effects to man, has once mentioned that the most unpleasant consequence of the space radiation during the long-term flights was developing of the cataract, i.e. lens-form opacity. Moreover, there is a danger of coming down with cancer. However, Cucinotta points out that after the flight no specifically dreadful consequences were detected in the astronauts’ health. He just stresses the fact that there is a lot of unknown in terms of how exactly the cosmic radiation exerts its influence on the astronauts, and what are the real consequences of such an impact.
Nevertheless, protecting the astronauts against radiation in space has always been among the priorities. Back in the sixties of the last century, the scientists were at a loss and did not know how to protect the astronauts against cosmic radiation, especially when it was coming to a necessity of the spacewalk cases. In 1966 the Russian cosmonaut had finally dared a spacewalking but only in the heavily lead-coated spacesuit. With the advances made in technical progress, it became possible to create less weight and more safe spacesuits.
Development of outer space has always been an attraction for the scientists, researchers, and astronauts. The secrets of new planets as revealed could be in favour for further development of the mankind on planet Earth, but at the same time may be dangerous. That is why the Curiosity mission to Mars has had such a great importance. However, let us not wander from the focus of the article, and concentrate on the results of radiation exposure recorder by a respective instrument onboard the Mars rover vehicle. This instrument was located inside the spacecraft; therefore, its readings amount to the actual dose, which might be received by an astronaut in the manual craft. The scientists, who processed the results of measurements, have reported the deplorable data: the equivalent exposure dose was 4 times as higher from that maximum allowable for the personnel of nuclear power plants. In Ukraine there is an established exposure dose limit for those, who permanently or temporarily works directly with the ionising radiation sources, – that being 20 mSv.
In order to explore the outermost space it is required to launch the missions, which now cannot be technically possible using the traditional sources of energy. This issue has been resolved by employing the nuclear sources of energy, namely, the isotope batteries and reactors. These sources are unique in their kind since they have high energy potential, which significantly broadens the possibilities of missions to the outer space. For instance, it is now possible to send probes to the external borders of the Solar system. Since the duration of such flights is considerably long, the photovoltaic arrays cannot be used as a source of power supply to a spacecraft.
The reverse of the coin are the potential risks posed by the use of radioactive sources in space. Mainly, it is the danger of unforeseen circumstances or accident situations. That is why the states launching the space facilities with nuclear sources of energy onboard take a maximal possible effort to protect individuals, public, and biosphere against radiological threats. Such conditions were stipulated in the principles relating to the use of nuclear sources of energy in the outer space, and adopted in 1992 by Resolution of the General Assembly of the United Nations Organisation (UN). These principles also establish that any state launching a space facility with nuclear sources of energy onboard shall in a timely manner inform the interested countries in case there is a damage to the space facility causing a threat of radioactive materials returning to Earth.
Moreover, the United Nations Organisation jointly with the International Atomic Energy Agency (IAEA) has developed the framework of ensuring safe use of nuclear sources of energy in the outer space. They are intended to supplement the IAEA regulations on safety by the high-level management, which takes into account the additional measures of ensuring safety when using nuclear sources of energy at the space facilities throughout all the stages of missions: launch, operation and decommissioning.
Should we be threatened by radiation when using an airline service?
The space rays bringing radioactivity penetrate practically all corners of our planet, however, distribution of radiation is not occurring evenly. The Earth’s magnetic field pushes aside a considerable quantity of charged particles from the equatorial zone, which leads to more radioactivity concentrations in the North and the South Poles. Moreover, as it was mentioned before, the cosmic exposure depends on the height. Thus, those who live at the sea level receive from the cosmic radiation approximately 0.003 mSv annually, while those residing at the 2 km level may receive a twice as much dose of radiation.
As we know, under the cruise speed of airliners being 900 km/h, with the account taken of correlation between windage and ascentional force, the optimal height for a flying aircraft normally constitutes 9-10 km. Therefore, when a plane reaches this height, the level of exposure may increase practically by a factor of 25 as from that at the elevation of 2 km.
The transatlantic passengers are mostly exposed in the course of one flight. When flying from the United States of America to Europe a person may receive the additional 0.05 mSv. The thing is that the Earth atmosphere has the corresponding shielding protection from the cosmic radiation, while when an airliner ascends up to the mentioned optimal height this protection partially disappears, which leads to an additional exposure. That is why frequent over-the-ocean flights increase the risk for a body to receive an increased dose of radiation. As an example, 4 such flights may cost a person to receive a dose of 0.4 mSv.
If we speak about the pilots, the situation here is somewhat different because they fly transatlantic quite frequently. The exposure dose for the airliner pilot may exceed 5 mSv per year. In the Ukrainian terms, when receiving such a dose a person would be categorised as a personnel not directly handling the ionising radiation sources, but, due to workplaces located in rooms and on industrial sites with radiation hazardous processes may additionally be exposed. For such persons the established exposure dose limit is 2 mSv per year.
The International Atomic Energy Agency pays a great deal of interest to this matter. The IAEA has developed a number of safety regulations. The issue with exposure of the aircraft crew teams has also been reflected in one of such documents. According to the Agency’s recommendations, the authority responsible for establishing the reference exposure dose level for aircraft crew teams is the national regulatory body or other responsible and competent authority. In case the postulated dose is exceeded, the employers of an aircraft crew team shall carry out the corresponding measures to dose assessment and registration. Moreover, they are obliged to communicate information to the female crew members on the risk to an embryo or a foetus from the cosmic exposure, and on the necessity of an early notification of pregnancy.
Is it possible to consider the outer space as a place for burial of radioactive waste?
We have already made sure that the cosmic exposure however being not a carrier of catastrophic consequences to the mankind, may increase the level of exposure to man. Through assessing the effects of the space rays on man, many scientists also study a possibility of using the outer space for the needs of the humankind. In the context of the present article, the idea of burial of radioactive waste in space looks ambiguous and intriguing.
The thing is that the scientists of the countries where the nuclear power engineering is employed actively find themselves in a permanent search for places where the ever stockpiling radwaste could safely be localised. The outer space has also been considered by some scientists as one of the potential places where the hazardous waste could be located. For instance, the experts from the “Pivdenne” State design bureau, which is based in Dnipropetrovsk, jointly with the International Astronautics Academy are studying the engineering aspects of implementing the idea of burial of waste in a far space.
On the one hand, sending radioactive waste to space is very convenient because it could be carried out anytime and in unlimited amounts, which puts the future of these waste in our ecosystem off the agenda. Moreover, according to experts, such flights would not require a pinpoint precision. Yet, on the other hand, this method has its weaknesses. The main issue here is to ensure that launching a space launch vehicle at all stages would be safe for the biosphere of the Earth. The probability of an accident during the launch is considerably high, and is estimated to be at 2-3%. Fire or explosion of a space launch vehicle at its launch, in flight, or its crash and impact may become a cause of a considerable spread of hazardous radioactive waste. That is exactly why when studying this method the main attention has to be focused right on the matter of safety under any possible accident situations.
Olga Makarovska, Deputy Chairperson of the State Nuclear Regulatory Inspectorate of Ukraine; Dmytro Chumak, leading engineer to the information Support Section of the Information-and-Engineering Department of SSTC for Nuclear and Radiation Safety, 10/03/2014