Background of the Invention The solution deals with creating an environment where the very low concentration of radon and its decay products is achieved; by 100 - 10000 times lower than the common radon background in the open space. Additionally, such space may be equipped with technology enabling to achieve cleanroom parameters, it means a space with a very low concentration of aerosols. It can further be shielded against the external gamma radiation and neutrons. When located in an underground laboratory, it can also be shielded against the cosmic radiation. All this can result in comprehensive suppression of the above-mentioned components of external radiation.

Description of Prior Art

Radon is a ubiquitous radioactive gas, which is present in the outer space, it means the atmosphere, in concentrations of 5-10 Bq/m ³ . During inversion conditions in the atmosphere, its concentrations reach even higher values, tens to hundreds Bq/m ³ . In buildings where radon penetrates predominantly from the subsoil, building materials, or water, its concentrations are higher than in the atmosphere. Mean value of radon concentration in buildings in the Czech Republic is over 100 Bq/m ³ , but it can be found at concentrations above 1000 Bq/m ³ , and in extreme cases, the concentration values can even exceed 100 000 Bq/m ³ .

From the human health and safety point of view, the reference level of radon concentration in buildings according to the Czech legislation is currently 300 Bq/m ³ . Taking into account the harmful effects of radon, the World Health Organization (WHO) considers it the second major cause of lung cancer in population after smoking and therefore measures are adopted to reduce radon concentration in buildings aiming to achieve the lowest possible concentration in general, as far as technically and economically viable, below the reference value 300 Bq/m ³ . The methods for reducing the radon concentration in buildings are known and they are described for example in the publication M. Jiranek - OPATRENI PROTI RADONU VE STAVAJICiCH BUDOVACH, SLJJB (Radon Remedial Measures in Existing Buildings, The State Office for Nuclear Safety). The principles of such measures are, for example, the isolation against radon penetration from the subsoil, installation of active or passive ventilation of buildings, extraction of the subsoil, sealing of entries, increased ventilation rates using the outer air, etc. When such measures are applied, radon concentration values are reduced to tens of Bq/m ³ in the best case. It is virtually impossible to reach lower indoor concentration than the concentration in the outside air, 5-10 Bq/m ³ , since such concentrations are ubiquitous in the outer atmosphere and they enter into the buildings through unsealed openings. Traditional radon remedial measures are not focused on the ways to reduce the indoor radon concentration below the radon concentration in the open air.

The only method known so far how to reduce the radon concentration in a space/room below this value of 5-10 Bq/m ³ is to adsorb the radon directly inside the space/room using a sorbent. This method, however, due to the sorption rate and the sorbent capacity, does not enable to achieve low levels of radon concentration in a space/room.

The hypothetical alternative, though cumbersome to implement in practice, is to use the air stored for several weeks, where contained radon is allowed to decay significantly due to radioactive transformation. Considering the radon has a half-life of 3.8 days, implementation of such technology is difficult when large volumes of air are needed.

Very low radon concentration becomes required only recently in connection with the development of new technologies. Examples include nanoelectronics or sensors production. In the field of nanoelectronics, the problems in micro and nanoelectronic circuits are caused even by individual interactions of radioactive particles with high energy transmission, caused for example by alpha particles from radon decay products, in general so-called single event effects, SEE. The short-term decay products of radon, such as Po-218, Po-214, can deposit themselves on the surfaces of nanostructures and the newly formed decay product resulting from the radioactive transformation can penetrate into the internal structure. The long-term decay products of radon, such as Pb-210, Po-210, can cause accidental malfunctions over the long term. In the field of sensor manufacturing, the processes mentioned above cause radioactive contamination of their surfaces, which is hard to remove.

For manufacturing or operation of technologies where the radioactive contamination caused by the presence of radon must be monitored, it is necessary to minimize the concentration of radon and its decay products in the surrounding atmosphere. In future, it is expected that the required radon concentrations will be at the level of imBq/m ³ and concentrations of its decay products will be at the level of pBq/m ³ , while at present, there is no solution how to achieve it. In addition, there may be a request for such spaces to comply with the requirements for so-called cleanrooms, it means with minimum aerosol concentrations according to the standard CSN EN ISO 14644-1 (125301 ) "Cleanrooms and associated controlled environments - Part 1 : Classification of air cleanliness by particle concentration". The requirements for such cleanrooms, however, do not address the issues of concentrations of radon and its decay products, and radon concentration inside them reaches the same values as in the ambient environment.

At present, the scope of issues of low radon concentration is being intensively dealt with at laboratories for fundamental physics experiments, especially the underground research laboratories, or in laboratories dealing with measuring very low radioactivity in material samples where very low background radiation, including very low radon concentration, is required in the environment. In these cases, however, the reduction of radon concentration is solved by shielding of the imminent surrounding of sensors and by delivery of air with low radon concentration only directly close to a sensor.

The scope of issues involving how persons entering and present in a radon-free environment affect the radon concentration has not been addressed yet.

The shielding against the external gamma radiation, neutrons, cosmic radiation is satisfactorily solved, for example by ultra-sensitive sensors in neutrino physics or in detection of dark matter, shielding with steel, lead or copper against the external gamma radiation, or possibly by shielding with low-level building material, see for instance patent CZ 305447, Shielding Composite Building Material. Also, the borated or lithiated polyethylene may be used as neutron shielding, or the sensors can be located underground to reduce the cosmic radiation effects. These cases, however, do not solve the issues of maintaining the low radon concentration at the level of imBq/m ³ and its decay products concentration at the level of pBq/m ³ during the presence of persons and while simultaneously maintaining a low concentration of aerosols.

Summary of the Invention

The setbacks of the state of art mentioned above are removed by the system for creating and maintaining radon-free and aerosol-free cleanroom, with radon concentration down at the level of imBq/m ³ and its decay products concentration at the level of pBq/m ³ with the option to reduce other components of radiation background, such as the external gamma radiation, neutron radiation or cosmic radiation, according to the presented invention.

The principle of the solution is that it consists of an enclosed space for the placement of examined objects, living organisms, production, and accessory evaluation devices, with all walls and ceiling made of a material with radon diffusion resistance D < 10 ^"12 m ² /s. The material used as well as all objects located inside the enclosed space are selected, in terms of the relevant radionuclides, so that the total emanation of radon in the enclosed space at the air exchange coefficient k = 1 lr ¹ is less than 1 imBq.h ^"1 . The enclosed space is equipped with the first entrance door, which opens towards the enclosed space. The enclosed space accommodates a delivery piping, the inlet of which is connected to the air outlet from the radon-free air delivery system with minimum overpressure 5 Pa relative to the surroundings. The capacity of this system is determined based on the enclosed space volume, and the radon concentration value at its outlet is less than 10 imBq/m ³ . Under the floor, the enclosed space is isolated from the subsoil against radon propagation from this subsoil. In order to simultaneously reduce the aerosol concentration in the enclosed space, the radon- free air delivery system leads to the enclosed space via an auxiliary circulation space and via a filtering ventilation aerosol system, which is located in the ceiling of the enclosed space and is sealed with a shielding cover. The outlet of the filtering ventilation aerosol system has multiple outlets to the enclosed space, for instance in the form of an air shower, and also to the auxiliary circulation space, while this is made via at least one inlet grille created in the wall common to this auxiliary circulation space and the enclosed space. Two other anterooms are connected to the enclosed space. The first anteroom is equipped with the second entrance door and it is connected to the second anteroom via the first connecting door and via the first one-way valve. The second anteroom is via the second connecting door and via the second one-way valve connected to the air shower. The air shower is connected to the enclosed space via the first entrance door and via the third one-way valve. An auxiliary fan is located in the ceiling of the second anteroom and it is connected by means of a connecting piece and via the fourth one-way valve to the space of the first anteroom. All the entrance doors and connecting doors have their edges sealed with a sealing featuring the radon diffusion resistance D < 10 ^"12 m ² /s. Any possibly untight spots, such as the entry of the delivery piping, connecting pieces, joints between panels, etc., are sealed against radon with a sealant or foil featuring also the radon diffusion resistance D < 10 ^"12 m ² /s. The isolation of the enclosed space from the subsoil can be implemented in several ways. One of them is a setup formed by a metal plate, which is put directly on the subsoil and a shielding foil, in the form of a tray for the floor laying, is placed onto the metal plate. Another option is that the enclosed space is equipped with supports below the floor and along its outer perimeter, which abut against the subsoil, while thus created space is isolated from the subsoil with a vented air gap formed between the subsoil and the enclosed space. Both ways can be combined to form an isolation where a metal plate is put on the subsoil with the use of the supports, and the space between the metal plate and the subsoil forms a vented air gap.

In a preferred embodiment, the radon-free air delivery system is formed by blocks connected in series. The first block in the chain is a compressor with the ambient air suction, which is via an air receiver, dryer, cooling system, and at least one cylinder with absorbent, connected to the inlet of a temperature and pressure adjustment block. The outlet of the temperature and pressure adjustment block is the outlet of the radon-free air delivery system. In order to achieve a higher class of cleanliness of the enclosed space, at least one additional anteroom may be inserted between the first and second anterooms. In order to increase comfort and to enable monitoring of the processes going on inside the enclosed space, the system may also be equipped with a continuous detector of radon located inside the enclosed space or its vicinity, and/or other continuous detectors of radon located in the adjacent areas formed by the anterooms, air shower, and the surroundings, and/or a differential pressure sensor enabling to check the pressure difference between the enclosed space and the surroundings, and/or a humidity regulator, and/or a humidifier, and/or a system functionality checking device, and/or a CO2 content monitor with an alarm located in the space where the safety of present persons is monitored, and/or security cameras located in the space where the safety of present persons is monitored. As for the wired connections, these are led outside from the enclosed space and their entries are sealed against radon with a sealant or sealing featuring the radon diffusion resistance D < 10 ^"12 m ² /s.

Another improvement is that the interior of the enclosed space is equipped with a shielding against the external gamma radiation, and/or neutron radiation. If the enclosed space is a part of an underground laboratory, it is automatically protected against cosmic radiation and can also be additionally equipped with shielding against the external gamma radiation, and/or neutron radiation.

The advantage of the presented system is that it enables to achieve and maintain low radon concentration down at the level of imBq/m ³ and its decay products concentration at the level of pBq/m ³ in sufficiently large space, larger than 10 m ³ . Such concentrations can be maintained even when persons are present inside. Moreover, its improvements enable it to maintain low radon concentration and simultaneously low aerosols concentration, also even when persons are present inside, which is something the systems known so far are not capable of, and it is not known that anybody has been working on such solution up to now. The enclosed space can be additionally equipped with combinations of shielding against the external gamma radiation, neutrons, cosmic radiation, all this again even when persons are present inside.

Explanation of Drawings The presented system for creating and maintaining radon-free and aerosol-free cleanroom at the radon concentration down at the level of imBq/m ³ and its decay products concentration at the level of pBq/m ³ with the options for reduction of other components in the radiation environment will be further described by means of the attached drawings.

Fig. 1 illustrates how the solution for creating only the radon-free environment would look like, with the option to enhance it with an entrance anteroom for the case when a person will be present in the enclosed space. Fig. 2a and Fig. 2b show side views of the ways how to isolate the enclosed space from the subsoil. Fig. 3a shows the plan view of the enclosed space with two anterooms and the air shower without the ceiling part, which is designed as both radon-free environment and aerosol-free cleanroom, while its side view is shown in Fig. 3b. Fig. 4 shows the ventilation system of the first and the second anteroom, and it refers to Fig. 3a and 3b. Fig. 5 shows an example of a block diagram of the radon-free air delivery system.

Detailed Description of the Preferred Embodiment

As the first illustration, the example in Fig. 1 will be described, which solves the creation of a radon-free environment with the option that persons are present inside the enclosed space. The system is formed by the enclosed space 3, where the examined objects, living organisms, production and accessory devices are placed. All walls and ceiling of the enclosed space 3 are made of a material with radon diffusion resistance D < 10 ¹² m ² /s.

The material used as well as all objects located inside the enclosed space 3 are selected, in terms of the relevant radionuclides content, so that the corresponding total emanation of radon, it means the rate of radon supply from the whole inside area of the enclosed space 3, was appropriate, it means for example at the air exchange coefficient k = 1 lr ¹ it should be less than 1 imBq.lr ¹ .

The enclosed space 3 is equipped with the first entrance door 3J_, which opens towards this enclosed space 3 because it is desirable that the overpressure inside the enclosed space 3 pushed it and helped create higher tightness. Along all its edges, the entrance door 3J_ is equipped with a sealing featuring the radon diffusion resistance D < 10 ^"12 m ² /s that prevents both radon diffusion and convection. The enclosed space 3 accommodates a delivery piping 2, the inlet of which is connected to the air outlet from the radon-free air delivery system 1 with minimum overpressure 5 Pa relative to the surroundings. The capacity of the radon-free air delivery system 1 is determined based on the volume of the enclosed space 3 and can be modified based on the requested final radon concentration value in the enclosed space 3. Radon concentration value at the outlet of the radon-free air delivery system 1 is less than 10 imBq/m ³ . Under the floor 8, the enclosed space 3 is isolated from the subsoil 5 against radon propagation from this subsoil 5. Any possibly untight spots, such as the entry of the delivery piping 2, connecting pieces, joints between panels, etc., are sealed against radon with a sealant or foil featuring also the radon diffusion resistance D < 10 ^"12 m ² /s. The dashed line in Fig. 1 shows an addition to the enclosed space 3, it is an entrance anteroom 4 for cases when somebody will work inside the enclosed space 3. The entrance anteroom 4 is attached in front of the first entrance door 3J_ and it is equipped with the entrance door 4.1 , which opens towards the entrance anteroom 4. Along all its edges, the entrance door 4J_ is equipped with a sealing featuring the radon diffusion resistance D < 10 ¹² m ² /s. A damper 4,2 is installed in the wall common to the entrance anteroom 4 and the enclosed space 3, which delivers radon-free air from the enclosed space 3 into the entrance anteroom 4.

As for the isolation of the enclosed space 3 from the subsoil 5, the examples of options are shown in side views in Fig. 2a and 2b. According to Fig. 2a, the enclosed space 3 is isolated by a setup formed by a metal plate 6, which is put directly on the subsoil 5. On top of the metal plate 6, there is a shielding foil 7 in the form of a tray, in which the floor 8 is laid. Fig. 2b shows an example where the enclosed space 3 is equipped with supports 9 below the floor 8 and along its outer perimeter, which abut against the subsoil 5, and it is isolated from this subsoil 5 by thus created vented air gap 9J_. These two approaches can be advantageously combined.

For completeness, an example of the used radon-free air delivery system 1 in Fig. 5 will be described. This radon-free air delivery system 1 is formed by blocks connected in series. The first block in the chain is a compressor 1_J_ with the ambient air suction, which is via an air receiver 1^2, which is in fact a pressure cylinder connected to the compressor λ_Λ_, via a dryer 1J3, cooling system λ Λ_, and at least one cylinder 1_J5 with absorbent, connected to the inlet of a temperature and pressure adjustment block U3. The outlet of the temperature and pressure adjustment block U3 is the outlet of the radon-free air delivery system 1 and it leads into the enclosed space 3 via the delivery piping 2, which is sealed against radon.

In general, it is a system that enables to achieve and maintain low radon concentration down at the level of imBq/m ³ and its decay products concentration at the level of pBq/m ³ , while persons may be present inside the system. The system operates as follows.

The compressor λ_Λ_ sucks the ambient air and compresses it. The air is led to the air receiver \ ≥, in order to stabilize the pressure. Next, the humidity is removed in the dryer 1J3. The output of the dryer 1J3 is the air with the dew point below the temperature in the cooling system 1_ The cooling system A_ at its outlet provides the air cooled down to the temperature, which depends on the required reduction of radon concentration in the delivered air. Typically, these temperatures range from -30 to -80°C. The cooled air is led to the cylinder 1_J5 with absorbent, which can be for instance an activated carbon, where the radon is absorbed. After the air is processed, its temperature and pressure are adjusted in the temperature and pressure adjustment block 1_J5 according to the requirements for the output radon- free air. The outlet of the radon-free air delivery system 1 is led by the delivery piping 2 into the isolated and sealed enclosed space 3. The entry of the radon-free air into the enclosed space 3 is sealed with a sealing against radon. In the case of large spaces such as halls, the air delivery is designed in such a way to provide its distribution in the whole space. In order to prevent protrusion of the outer air to the inside by convection, overpressure is created in the enclosed space 3, minimum 5 Pa relative to the surroundings. As already mentioned, the walls of the radon-free space/room are made of material with high radon diffusion resistance D < 10 ^"12 m ² /s and are constructed either as one-piece, without breaks, or joint by sealing, sealant, etc., with high radon diffusion resistance to prevent radon propagation from the outside both by diffusion and convection through untight spots. All possible entries to the enclosed space 3, joints between panels, cracks, mounts, cable entries, etc., are duly sealed with a sealant featuring high radon diffusion resistance D < 1 0 ¹² m ² /s. Another reason for proper sealing is the need for overpressure creation. Also, the walls may be overall and additionally sealed with radon-resistant sealing foil, such as TROPAC, also featuring the radon diffusion resistance less than 10 ^"12 m ² /s.

The volume of the radon-free air delivered from the radon-free air delivery system 1, which removes radon from the air, depends on the volume of the enclosed space 3, requested radon concentration inside, and on the radon concentration in the outside. For example, if the room volume is 100 m ³ , the requested radon concentration inside is 10 imBq/m ³ , and the radon concentration in the outside is 10 Bq/m ³ , the required capacity of the radon-free air is minimum 690 m ³ /h. The entrance of a person or persons highly contaminated with radon into the enclosed space 3 is either prohibited, or it is necessary to wait before entering until the radon content in a body reduces; or it is necessary to significantly increase the capacity of delivered radon-free air. The entrance anteroom 4, marked by a dashed line in Fig. 1 , is optional when the presence of persons is expected and it serves to reduce the radon intrusion from the outside into the clean space.

The radon-free air, which is supplied into the space/room, is delivered with slight overpressure to ensure that the enclosed space 3 is overpressurized minimum 5 Pa relative to the surroundings, and it is led inside in such a way to ensure that it will blend sufficiently fast in the space, for instance using several inlets, while simultaneously it must not interfere with installed technology due to fast air streams near the inlets. Further, in case the design contains also aerosols removal by means of the filtering ventilation aerosol system 4, the radon-free air is taken to the filter of the filtering ventilation aerosol system 14, which will intercept impurities possibly contained in the supplied radon-free air by this filter. The arrangement described above can be enhanced in such a way that in addition to radon concentration reduction, it will reduce the aerosol content in the enclosed space 3, which is the subject of this invention. It involves a system of spaces in a cascade that are mutually separated in order to reduce the radon propagation into the next space, which is always cleaner than the preceding one. In the most simple configuration, Fig. 1 , it consists of the radon-free enclosed space 3 as such with one entrance anteroom 4, which enables to achieve and maintain low radon concentration down at the level of imBq/m ³ and its decay products concentration at the level of pBq/m ³ , while persons may be present inside the system and low concentration of aerosols is maintained.

An example of a more complex arrangement is shown in Fig. 3a, 3b and 4. In this arrangement, the radon-free air delivery system 1 leads to the enclosed space 3 via an auxiliary circulation space 3 and via a filtering ventilation aerosol system 4, which is installed in the ceiling of the enclosed space 3 and is sealed with a shielding cover 1_5. The outlet of the filtering ventilation aerosol system 14 has multiple outlets to the enclosed space 3, for instance in the form of an air shower, and from here it is taken to the auxiliary circulation space 13 via at least one inlet grille 13.1 created in the wall common to this auxiliary circulation space 13 and the enclosed space 3. In the given example, two anterooms are connected to the enclosed space 3. The first anteroom J_0 is equipped with the second entrance door 10.1 and it is connected to the second anteroom V_ via the first connecting door 1 1 .1 and via the first one-way valve 10.2, and the second anteroom H is via the second connecting door 12.1 and via the second one-way valve 1 1 .2 connected to the air shower 12. The air shower 12 is connected to the enclosed space 3 via the first entrance door 3J_ and via the third one-way valve 12.2. The second entrance door 10.1 , the first connecting door 1 1 .1 , and the second connecting door 12.1 have their edges sealed with a sealing featuring the radon diffusion resistance D < 10 ^"12 m ² /s. An auxiliary fan 16 is located in the ceiling of the second anteroom JJ_ and it is connected by means of a connecting piece 17 and via the fourth one-way valve 10.3 to the space of the first anteroom 10. The first anteroom 10 and the second anteroom V\_ serve to reduce the radon propagation into the radon-free enclosed space 3 when persons enter inside, and to reduce the aerosol concentration. Into the anterooms 10 and JJ_, the air is led from the enclosed space 3 via the fourth one-way valve 10.3 and via the first oneway valve 10.2 enabling the air supply from the enclosed space 3, and from here the air is led via the second one-way valve 1 1 .2 to the air shower 12, and via the third one-way valve 12.2 to the enclosed space 3, and not vice versa. This way the circuit is closed in order to reduce the radon propagation from the outside into the system. The air shower 12 serves to reduce the aerosol concentration by cleaning the entering persons in special clothes and to reduce the radon propagation into the radon-free enclosed space 3 when persons are entering it. Otherwise, everything that was mentioned in connection with the basic version of the system applies also to this alternative. The entrance of persons highly contaminated with radon into the room is either prohibited, or it is necessary to wait before entering until the radon content in a body reduces; or it is necessary to significantly increase the capacity of delivered radon-free air. In practice, a person enters into the first unclean anteroom 0 where it takes its clothing off, proceeds to the second clean anteroom H where it takes on special clothing, then proceeds to the air shower 12 where it is cleaned from aerosols, and finally it enters into the clean enclosed space 3. In addition, the enclosed space 3 is permanently cleaned by high-level air filtering by means of the filtering ventilation aerosol system 14, in this example via HEPA filters with laminar flow inside the space so that it meets the requirements for a so-called cleanroom. The filtering ventilation aerosol system 14 is located above the cleanroom in sealed shielding cover 15 isolated from the surroundings in order to prevent the radon propagation into the filtering ventilation aerosol system 14.

If a system that creates both the radon-free environment along with the aerosol concentration reduction is required, it is not necessary to increase the ventilation capacity above 690 m ³ /h.

Further, the enclosed space 3 can be shielded against the external gamma radiation, neutrons, and when located in an underground laboratory, also against cosmic radiation.

Apart from the versions described above, the system may also be equipped with additional elements, such as measuring, regulating and safety devices. It is possible to add a continuous detector of radon for measuring very low levels of radon at the level of units of imBq/m ³ , which samples the air from the clean enclosed space 3 or can alternatively be placed directly in this enclosed space 3. Other continuous detectors of radon may be used in other adjacent spaces, such as the anterooms, air shower, the outside, where they check radon concentration in the surroundings. Another option is to use a differential pressure sensor for checking the pressure difference between the cleanroom and the surroundings, a regulator of humidity in the inside air, a humidifier, which uses controlled water with a very low concentration of radionuclides, in particular processed to contain no radon. Other optional accessories include a system functionality checking device, in particular checking the supplied air capacity, pressure differences between the rooms, measuring the radon concentrations in individual parts of the system, measuring the humidity and temperature, etc. The sensors use either wireless data transfer or wired data transfer. In the latter case, the cable entries shall be sealed against radon and led to the central control and data unit.

As safety elements, it is suitable to add CO2 content monitors with alarms located in the space where the safety of present persons is monitored, and security cameras located in the space where the safety of present persons is monitored.

Industrial Applicability

The system may be used for both scientific and industrial applications, which require extremely clean space with a maximum reduction of radon in the air, including suppression of radioactive and non-radioactive aerosols, and possibly all other components of ionizing radiation.

These applications may include the electric industry, in particular in the field of development and production of nanotechnologies where the radon decay products may cause so-called single event effect, which may affect transportation, as it may affect the control elements for aviation, or on the contrary, the control systems located underground. In the sphere of research, development, and production of new biotechnology products, it may be used to help reveal so far unsolved issues of small doses of radiation and their influence on the DNA, so-called ZERO DOSE. The system may find applications in medicine, pharmacological industry where it may solve issues related to extremely clean compounds. The system is also suitable for laboratories, for example for protection against radiation, as it creates an extraordinarily protected environment in case of radiation and nuclear accidents. In general, the system may be used in any industry where the production requires extremely clean materials.