WO2009138360A2

WO2009138360A2 - Radioactivity monitoring system

Info

Publication number: WO2009138360A2
Application number: PCT/EP2009/055582
Authority: WO
Inventors: Giuseppe Morandi
Original assignee: Technology Nuclear Electronics S.P.A.
Priority date: 2008-05-12
Filing date: 2009-05-08
Publication date: 2009-11-19
Also published as: WO2009138360A3; ITMI20080853A1

Abstract

A system of detection and monitoring of radioactivity caused by radionuclides at the gaseous state contained in a flow of fluid (107), comprising: a measurement chamber (127) comprising detector means (129) sensitive to the radiations emitted by the radionuclides, and feed means (103, 113, 115a, 115b, 121, 123, 125) for taking from said flow of fluid a fluid flow portion to be submitted to measure, and to feed it to the measurement chamber. The feed means are adapted to raise the pressure of the fluid flow portion to be submitted to measure with respect to an initial pressure (e.g., the atmospheric pressure) of the picked-up fluid, and to feed the fluid flow portion at raised pressure to the measurement chamber. Such pressure increase allows to have - in the same volume - a higher mass of sampled fluid to be exposed to the detector, resulting in an increase of sensitivity practically proportional to the ratio p/p₀ (where p is the actual pressure in the chamber, and p₀ is the atmospheric pressure.

Description

RADIOACTIVITY MONITORING SYSTEM

DESCRIPTION Background of then invention Field of the invention The present invention relates in general to the detection and monitoring of radioactivity, particularly to the detection and monitoring of radioactivity caused by radionuclides at the gaseous state (gaseous isotopes). Description of the related art

Systems of detection and monitoring of the aforesaid type are used in ventilation systems for radiogenic plants, such as for instance those installed in nuclear medicine laboratories, in radiodiagnostic and/or radiotherapy laboratories, in research or industrial plant laboratories and in general in any structure making use of accelerators of particles, to check the possible release of radioactivity in the atmosphere caused by nuclides in the form of gaseous isotopes that are not retained by the filters of the ventilation system.

The regulatory framework currently in force foresees two different approaches to the problem.

A first approach imposes the issuance, by a competent authority, of an operational license of the radiogenic plant. In this case, the admitted levels of release in atmosphere are set on the basis of an "integral" criterion, i.e. in terms of the maximum amount of radioactivity that can be released in predetermined periods of time. To achieve the issuance of an operational license, and to maintain it valid in time, complex and expensive procedures are nevertheless required.

A second approach, applicable to those plants that are characterized by release levels below an allowed maximum threshold (typically, equal to 1 Bequerel/gram), does not require the issuance of an operational license, but in this case the owners of the plant are required to be able to prove, through a system of continuous monitoring in time, that the release levels always result lower than the allowed maximum threshold. The level of sensitivity required to the detection and monitoring systems for this type of plants is extremely high, because, in order to reach the required level of confidence on the measured values, a widely applied "rule of thumb" asks for a sensitivity of about 1/10 of the allowed threshold.

As far as the Applicant is aware, the known monitoring systems can be classified as follows. A first class is that of systems exploiting proportional detectors, that make use of from one to four xenon-gas proportional detectors (each one fit with preamplifying electronic circuits, high- voltage management circuits and discrimination circuits), installed around the measurement chamber (which typically has a volume of about 11 liters) with lead screen and sampling with continuous flow pump (with typical delivery of 4 m³/hour). These systems only measure the total β radiation, and their sensitivity (and, consequently, their cost) depends on the number of detectors used. However, even in the optimal configuration with four detectors (one of which is used for compensation purposes) the sensitivities required for the plants without operational license are not achieved. A second class includes systems based on spectrometric methods with the employment of sodium iodide (NaI(TI)) detectors, that perform a sampling with a continuous flow pump in a measurement chamber having a predefined geometry (typically, a chamber with "Marinelli" type geometry, to optimize the detection efficiency), and in which the measure is performed with an NaI(TI) detector connected to a chain of electronic circuits comprising a preamplifier, an amplifier, an analog-to-digital converter (ADC) and a multichannel analyser (MCA). The calculation time of every single measure is typically of 10 minutes, and the data is processed through specific software. These systems have higher performances compared to the systems of the first class, because they not only allow the measure of β emitters, but also of other possible γ emitter contaminants; however, the sensitivity is acceptable only for plants subject to the issuance of the operational license.

A third class includes systems based on spectrometric methods with the use of hyperpure germanium detectors. These systems have a configuration similar to those of the second class, with the difference that the NaI detectors are replaced by germanium detectors, which significantly improve the performances, thanks to the high detector resolution. The main limitation of these systems resides in that the detectors needs to be cooled with liquid nitrogen, which substantially complicates the overall structure, in addition to the maintenance (periodic refills of the cooler need to be foreseen, for instance on a weekly basis), or through electric cooling, which requires permanently guaranteed electric supply. The initial costs and the operational costs are thus significantly high.

A fourth class of systems implements an on-line detection method, with anticoincidence detectors. These are innovative systems, designed to optimize the detection of β+ emitters by exploiting the annihilation specificity. The duct through which the air flow to be monitored flows is measured with four plastic detectors, arranged in a square and coupled in pairs with suitable coincidence circuits, so as to count only simultaneous events that take place at 180°. The systems so conceived are characterized by high efficiency (achievable thanks to the use of the plastic detectors) and by a very low ground noise (due to random coincidences), and do not require lead screenings. However, these systems, in addition to being extremely expensive, do not reveal γ emitters, and for measures of the duration of 10 minutes they do not reach the required sensibilities.

In summary, while for the detection and monitoring of radioactivity in ventilation systems for radiogenic plants subject to the issuance of an operational license by the competent authorities systems exist on the market with suitable performances, no systems are instead available capable of guaranteeing the detection sensibilities of β and γ emitters required by the authorities in the case of plants not subject to the issuance of an operational license. Summary of the invention

There is therefore the need of making available a system of detection and monitoring of gaseous radioactive isotopes, like for instance β and γ emitters, having high sensitivity, that allows complying with the regulations particularly in connection with the detection and monitoring of radioactivity in ventilation systems of radiogenic plants not subject to the issuance of an operational license by the competent authorities.

The Applicant has observed that a methodology that can form a good starting point for the achievement of an increased sensitivity is that based on the spectrometric method with the use of NaI detectors. This technique allows in fact to select the region of main interest ("Region Of Interest" or "ROI"), or a combination of two or more ROIs, and to maintain in time the relationships of the performed measures, in addition to the fact that the cost of the NaI detectors is lower compared to other types of detectors.

The Applicant has observed that a possible approach to the problem of increasing the performances of detection and monitoring systems, particularly of the type based on NaI detectors, consists of increasing the temporal duration of the individual measure over the limit of 10 minutes normally adopted. However, this approach is hardly practicable, because a significant increase in performance would require measurement times too large to be accepted, being the sensitivity dependent on the square root of time; moreover even the short life of the monitored isotopes shall be taken into account.

The Applicant has also observed that another possible approach to the problem of increasing the performances of a detection and monitoring system in systems based on NaI detectors consists of increasing the mass of the sample of fluid (air) exposed to the detector, by increasing the volume of the measurement chamber over the volume of 3 liters commonly adopted. However, the Applicant has noticed that in such a way satisfactory results are not achieved in terms of performance improvement.

The Applicant has found that a solution to the problem of increasing the sensitivity consists of increasing the mass of the sample of fluid (air) exposed to the detector by increasing the pressure of the flow of sampled air, i.e. the flow of air introduced into the measurement chamber, for a same (or substantially same) volume thereof. In other words, the sample of fluid to be exposed to the detector, rather than being introduced into the measurement chamber substantially at the atmospheric pressure, or however at the pressure of the ventilation system, is introduced into the measurement chamber at a higher pressure. An increase in the mass of fluid submitted to the measure can thus be achieved avoiding the negative side effects connected with the increase of volume of the measurement chamber.

The Applicant has observed that thanks to this solution, the sensitivity of the measure can be improved, substantially proportionally to the amount, in weight, of the sample of fluid present in the chamber, and therefore proportionally to the pressure of the fluid itself.

According to an aspect of the present invention, there is provided a system of detection and monitoring of the radioactivity caused by gaseous radionuclides contained in a flow of fluid, comprising:

- a measurement chamber comprising detector means sensitive to radiations emitted by the radionuclides; and

- feed means for taking from said flow of fluid a fluid flow portion to be submitted to measure, and to feed it to the measurement chamber, characterized in that said feed means are adapted to raise the pressure of the fluid flow portion to be submitted to measure with respect to an initial pressure of the fluid flow portion taken, and to feed the fluid flow portion at increased pressure to the measurement chamber. The pressure increase allows to have - in the same volume - a higher mass of sampled fluid to be exposed to the detector, resulting in an increase of sensitivity practically proportional to the ratio p/po, where p is the actual pressure in the measurement chamber, and po is the initial pressure of the taken fluid flow portion, e.g. the atmospheric pressure. According to another aspect of the present invention, there is provided a method of detection and monitoring of radioactivity caused by gaseous radionuclides contained in a flow of fluid, comprising the phases of:

- taking from said flow of fluid a fluid flow portion to be submitted to measure, and feeding it to a radiation measurement chamber, and - measuring the radiations emitted by the radionuclides contained in the fluid flow portion fed to the measurement chamber, characterized in that said phase of feeding comprises raising the pressure of the fluid flow portion to be submitted to measure with respect to an initial pressure of the fluid flow portion taken, and feeding the fluid flow portion at increased pressure to the measurement chamber.

Brief description of the drawings

These and other features and advantages of the present invention will be made evident by the following detailed description of an embodiment thereof, provided merely by way of non- limitative example and shown in the attached drawing, in which a scheme of a radioactivity detection and monitoring system according to an embodiment of the present invention is depicted.

Detailed description of an embodiment of the invention

With reference to the drawing, in Figure 1 there is shown a scheme of a radioactivity detection and monitoring system according to an embodiment of the present invention. The system, denoted overall with 100, is associable to a ventilation system of a radiogenic plant, like for instance a plant installed in a laboratory of nuclear medicine, or in a laboratory of radiodiagnostic and/or of radiotherapy, or in a laboratory of a research center or an industrial plant, or in a structure making use of accelerators of particles, to check the possible release in the atmosphere of radioactivity caused by nuclides in the form of gaseous isotopes that are not retained by the filters of the ventilation system.

The system includes an aspiration duct 103, in fluid connection with a ventilation conduit 105 of the ventilation system, in which there flows a flow of air 107 whose content of gaseous radioactive isotopes has to be detected and monitored. Preferably, the aspiration duct 103 is placed along the ventilation system downstream of filters 109 of the ventilation system, and even more preferably in proximity of a discharge stack 111 of the ventilation system, through which the ventilation air is released to the external environment, typically in the atmosphere. The aspiration duct 103 has the function of picking up, from the flow of air 107 that flows in the ventilation conduit 105, an air flow portion to be submitted to detection and monitoring; since the air picked up has already been submitted to filtration by the filters 109, and having to check the presence of gaseous radioactive isotopes, no particular solution is necessary in order to pick up the air flow portion to be submitted to measure (like for instance solutions for guaranteeing isokinecity conditions of the picked-up flow of air), being however preferable that the pick up takes place in correspondence of the central zone of the conduit 105, avoiding the flow limit layer in correspondence of the wall of the conduit 105.

A filter 113 is preferably provided along the aspiration duct 103, for further filtering the picked-up samples of air, and to avoid the presence therein of possible aerosols. The filter 113, where provided, is preferably dimensioned to have a capacity much higher than the air flow portion flowing through the aspiration duct 103, with the purpose of minimizing the load losses and to ensure a long period of operation without interventions (cleaning, substitution of the filtering cartridge), for instance at least one year. The filter 113 can be of whatever type commonly employed in systems of radioactivity monitoring in flows of air, and can be for instance dimensioned for a delivery of 24 m /hour (about seven times the capacity of the aspiration duct 103) and a filtering power of 4 μ.

At the end of the aspiration duct 103, downstream of the filter 113 (where provided), a compressor 115a is inserted, having the function of raising the pressure of the flow of air picked up from the conduit 105 with respect to the pressure of the flow of air 107 in the ventilation system (pressure Pin), bringing the pressure to the desired value (pressure Pm); for instance, in case the flow of air 107 in the ventilation system is substantially at the atmospheric pressure, the compressor 115a is for instance capable of raising the pressure of the picked-up flow of air to a value in the range from 0.3 to 0.4 MPa (i.e. 3-4 bar); the pressure value of the air flow after the compression is preferably adjustable upon setting of an operator.

The compressor 115a preferably has to ensure the absence of contamination of the picked-up air, caused by residues of oil, deposits deriving from the mechanism and/or from the seals, a high reliability for prolonged periods of continuous exercise (preferably at least one year of continuous operation without maintenance interventions), the constancy of the performances in the considered time period and the possibility of working in closed circuit, without losses in the atmosphere. An "oil-less" compressor type can for example be used, guaranteed for at least 10,000 hours of continuous operation.

A cooling fan 117a is preferably associated to the compressor 115a. In a preferred embodiment, with the purpose of increasing the reliability of the system, in parallel to the compressor 115a (normally in operation), a second compressor 115b can be provided, normally kept in "stand-by", and which automatically starts working in case the compressor 115a normally in operation goes off duty; a respective cooling fan 117b is also associated to the second compressor 115b. The two compressors 115a and 115b are for instance fed through respective ducts branching off from the aspiration duct 103, the branch being preferably downstream of the filter 113, where provided. Preferably, at the entrance of the compressor(s) 115a, 115b there is provided a respective cut-off valve 119a, 119b, for instance a manual valve, to allow, where necessary (for instance, in order to perform system maintenance interventions, e.g. for the substitution of the compressor), isolating the compressor to be submitted to maintenance, without having to stop the system operation. In output from the compressor in operation (115a or 115b), a duct 121 conveys the picked-up air flow portion to a filter unit 123, wherein possible impurities or debris released by the compressor are retained, and then reaches a cooling-drying unit 125. In fact, the picked-up flow of air, in consequence of the compression process operated by the compressor 115a or 115b, is significantly heated, reaching relatively high temperature values that, in addition to decreasing its density, and therefore the mass of air that would be monitored, could also result incompatible with the operational conditions of the radioactivity sensor and the related electronics directly mounted thereon.

The cooling-drying unit 125 functions as well to remove the humidity from the air sample, avoiding possible deposits of humidity in the measurement chamber 127 (described subsequently), and along the downstream circuit, which may hinder the correct operation of the pressure regulator unit 139.

Alternatively, the filter unit 123 and the cooling-drying unit 125 may be replaced by a refrigerant unit, for instance an air-air heat exchanger, adapted to cool the compressed air flow portion so as to practically bring it back to the environment temperature, followed by a condense filtration/separation unit 125 that allows avoiding that, in the measurement chamber (described subsequently), possible deposits of humidity take place.

In output from the cooling-drying unit 125, the air flow portion to be monitored reaches a measurement chamber 127, in which a radioactivity sensor 129 is inserted, for instance a NaI sensor or an hyperpure germanium sensor (HPGe), or an LaBr sensor, or other types of sensors suitable for the purpose. Preferably, the measurement chamber 127 is a chamber with "Marinelli" geometry (from the name of its inventor); it is a classical geometry that allows the insertion of the sensor 129 in optimal conditions for the measure efficiency, particularly the sensor 129 is positioned in a central tubular portion of the measurement chamber 127, so that substantially the whole inner volume of the measurement chamber is exposed to the sensor. For instance, the measurement chamber 127 can have an inner volume of about 3 liters, and it is preferably realized in aluminum league (Anticorodal), welded, being designed to sustaining a sufficiently high maximum pressure, for instance equal to about 7 bar.

In correspondence of the measurement chamber 127 a pressure transducer 131 is installed, and preferably also a temperature transducer 133.

The measure data produced by the sensor 129 (and by the related signals pre- processing electronics) and the signals produced by the pressure 131 and temperature 133 transducers are sent to a data processor 135, for instance a Personal Computer (PC) that controls the system and that is programmed for processing the measure data. The knowledge of the conditions of pressure and temperature of the dynamic sample of air submitted to measure allows to appraise with great accuracy the actual mass of the sample of air submitted to measure, improving the performances of the system and making less critical the constancy of the pressure value of the air within the measurement chamber 127.

The measurement chamber 127 and the sensor 129 are housed inside a lead screen that has the purpose of minimizing the natural radiation always present in the environment, which, on the contrary, would produce a background signal that could result incompatible with the achievement of the required sensitivity and precision. The geometry of the screen is such as to contain therewithin both the measurement chamber 127 and the sensor 129 and the related electronics; for instance, the lead screen can have a thickness of about 10 cm around the measurement chamber 127, and of about 5 cm in its lower section that screens the electronics of the sensor 129.

In output from the measurement chamber 127, a duct 137 conveys the monitored air flow portion to a pressure regulation unit 139. The regulation of the pressure can be realized either with feedback regulation systems (i.e. controlled by a pressure transducer that acts - through a "Proportional-Integral-Derivative" or "PID" regulator - on a regulation valve), or with a manual system not in feedback, in which a regulation valve acts so as to introduce a fixed load loss (preferably adjustable by the operator). The pressure regulation unit 139 allows keeping, within the measurement chamber 127, the desired value of pressure of the dynamic sample of air that is submitted to measure; a continuous flow of air is present in the measurement chamber, and the time of permanence of the air in the measurement chamber is negligible compared to the decay time of the radioactive isotopes to be monitored. Immediately up-stream the valve, a further filtration unit 138 provides an additional protection against possible micro-occlusions of the valve of the pressure regulation unit 139, that could impair its proper operation. Downstream of the pressure regulation unit 139, the flow of air returns to the initial pressure (i.e. to the pressure present in the ventilation conduit 105) and, preferably after having passed through a flow capacity measurer 141, for instance of the piston meter type, the air (brought to the initial pressure Pout) is then conveyed back, through a discharge duct 143, into the ventilation conduit 105, preferably at a point sufficiently downstream of the pick-up point (aspiration conduit 103) to avoid any possible blow-by. The air is then discharged into the external environment through the stack 111.

Preferably, a safety system is foreseen, comprising for instance:

- a pressure switch 145 adapted to actuate a discharge electrovalve 147 located along a duct 149 branching off from the duct 121 and terminating into the discharge duct 143;

- a safety valve 151, located at the exit of the compressor(s) 115a and 115b and activatable to discharge into the discharge duct 143;

- a pressure measurer (manometer) 153, placed at the exit of the measurement chamber 127, to allow the visual control of the pressure in the chamber;

- a low flow alarm device 155, associated with the flow measurer 141.

The pressure switch 145, in combination with the electrovalve 147, implements a first safety level against excessively high pressures; the safety valve 151 implements a second safety level against excessively high pressures; the manometer 153 allows a prompt visual check of the operating conditions by an operator. For instance, in case of malfunctioning the following can be foreseen:

- in case of high pressure (an event possible in case of clogging of the circuit, something that is extremely unlikely, thanks to the precautionary solutions from the filtration viewpoint): stop of the compressor 115a or 115b currently in operation and generation of an off-duty alarm; in case of missed intervention of the pressure switch 145 and of the electrovalve 147, the safety valve 151 in any case intervenes. The malfunctioning is however also signalled by the low-flow alarm generated by the alarm device 155;

- in case of stop of the working compressor 115a the low- flow alarm commands in the first place the start of the stand-by compressor 115b; if the latter is correctly started, the pressure in the circuit returns to the preset value, in a relatively short time, for instance about 40 seconds, and therefore no appreciable problems are encountered, even if a measure cycle was in progress (the measure cycle, having a duration of 10 minutes, would result affected for slightly more than 7% of its duration); only if after a preset delay (for instance 1 minute) the normal operating conditions are not restored (permanence of the low- flow signal), the system off-duty alarm is generated.

Suitable conditions for the operation of the system are for instance the following: - delivery: about 3.6 Nm /hour (equal to about 1 liter per second); this capacity involves that the mass of the sample of air submitted to measure is totally refreshed every 9 - 12 seconds (depending on the working pressure, that by way of example is assumed to be adjustable between 3 and 4 bar); therefore the measures, even if longer than the usual 10 minutes, provide valid results also for isotopes with decay times shorter than the duration of the individual measure;

- pressure: as already mentioned, the system can be designed to operate at values of pressure between 3 and 4 bar, but nothing prevents from increasing the pressure values to higher values; however, some considerations suggest to stay in this range of values, since lower pressures could reduce and even frustrate the advantages deriving from performing the measure on a pressurized mass of air (the mass of air submitted to measure could excessively reduce, no longer guaranteeing that the measure has the required sensitivity), while increasing the pressure over the suggested values might not give significant advantages, because the higher pressure would require to increase the thickness of the measurement chamber, something that, on the other hand, would increase the absorption of the radiations emitted by the gaseous isotopes by the walls of the measurement chamber, and the components (particularly of the compressor) would be more stressed, in detriment to the general reliability of the system.

In operation, the measure data produced by the sensor 129, as well as the signals from the pressure 131 and temperature 133 transducers are sent to the PC 135. On the latter, a program is executed (pre-installed in advance) adapted to perform the calculation of the concentration, in the sample of air submitted to measure, of each radionuclide of interest, based on the count provided by the detector 129 (corrected in view of the detector efficiency value), compared to the real mass of the sample of air submitted to measure. The precision of the calculated value of concentration depends on the correct evaluation of the mass of the sample of air submitted to measure; the accuracy of this evaluation is obtainable by inputting to the program that runs on the PC 135, in addition to the volume of the measurement chamber 127, also the measured values of pressure and temperature of the sample of air submitted to measure, obtained through the transducers 131 and 133.

The PC 135, properly programmed, also allows supervisioning the whole cycle of measure, the analysis, the alarms and the transfer of the information. The program preferably foresees a simplified user interface, for instance of graphic type. Preferably, locally to the PC 135 a data base of the acquired spectra and of the results of the measures is taken, for instance using a database management application. Through the program that is run by the PC 135, the operator can preferably configure the ROIs and the related alarms in case predetermined values of radioactivity are exceeded.

The present invention provides a method and a system adapted to perform a pressurized dynamic sampling of a fluid to be submitted to monitoring in order to detect the presence and the concentration of radioactive isotopes, using a continuous flow of fluid through a measurement chamber, with time of permanence of the fluid in the measurement chamber sufficiently short to result substantially negligible in comparison to the decay time of the isotopes to be monitored. The increase of the 5 pressure of the flow of fluid fed to the measurement chamber allows to have - in the same volume - a higher mass of sampled fluid to be exposed to the detector, resulting in an increase of sensitivity practically proportional to the ratio p/po, where p is the actual pressure in the measurement chamber, and po is the initial pressure of the fluid, e.g. the atmospheric pressure.

10 The invention has been here described making reference to a possible embodiment thereof; however, those skilled in the art, based on the teachings provided in the present description, will be able to make several variations to the embodiment here described, as well as to devise alternative embodiments, without departing from the scope of protection set forth in the claims that follow.

I K * * * * *

Claims

1. A system of detection and monitoring of radioactivity caused by radionuclides at the gaseous state contained in a flow of fluid (107), comprising:

- a measurement chamber (127) comprising detector means (129) sensitive to the radiations emitted by the radionuclides; and

- feed means (103,113,115a,115b,121, 123,125) for taking from said flow of fluid a fluid flow portion to be submitted to measure, and for feeding it to the measurement chamber (127), characterized in that said feed means are adapted to raise the pressure of the fluid flow portion to be submitted to measure with respect to an initial pressure of the taken fluid, and to feed the fluid flow portion at raised pressure to the measurement chamber.

2. The system according to claim 1, in which said feed means are adapted to raise the pressure of the fluid flow portion to be submitted to measure of about 3-4 bar with respect to the initial pressure.

3. The system according to claim 1 or 2, in which the initial pressure is substantially the atmospheric pressure.

4. The system according to any of the preceding claims, in which said feed means include at least one compressor (115a,115b) inserted between a pick-up point (103) of the fluid and the measurement chamber.

5. The system according to claim 4, further comprising pressure regulation means (139) associated with the measurement chamber adapted to keep in the measurement chamber the fluid at the raised pressure.

6. The system according to claim 5, comprising a discharge duct (137,143) in fluid connection with the measurement chamber for the discharge of the fluid submitted to measure, said pressure regulation means being inserted in said discharge duct.

7. The system according to claim 6, in which said pressure regulation means include a manual valve.

8. The system according to claim 6, in which said pressure regulation means include an automatic feedback regulation system.

9. The system according to any of the claims from 5 to 8, in which said pressure regulation means are adapted to determine in the measurement chamber a continuous flow of fluid at the raised pressure, with time of permanence of the fluid substantially negligible in comparison to the decay time of the radionuclides.

10. The system according to any of the preceding claims, in which said detector means include one or more detectors of NaI, HPGe - Hyperpure Germanium - and LaBr type.

11. The system according to any of the preceding claims, further comprising a fluid pressure measurement device (131), associated with said measurement chamber.

12. The system according to claim 11, further comprising a fluid temperature measurement device (133) associated with said measurement chamber.

13. The system according to any of the preceding claims, further comprising a data processor (135) operationally coupled to said detector means for receiving measure data.

14. The system according to claim 13 when dependent from claim 11, in which said data processor is operationally coupled to said pressure measurement device.

15. The system of claim 13 when dependent from claim 12, in which said data processor is operationally coupled to said pressure measurement device.

16. The system of claim 14 or 15, in which said data processor is programmed for processing the data provided by the detector means and to derive values of concentration of radionuclides keeping track of the measurement pressure and, possibly, of the measure of temperature of the fluid in the measurement chamber.

17. The system according to any of the preceding claims, further comprising at least one safety device against overpressure of the fluid in the measurement chamber.

18. The system according to any of the preceding claims, further comprising at least one signaling device of low pressure in said measurement chamber.

19. The system according to any of the preceding claims, in which said feed means include one or more among a refrigeration unit of the fluid in pressure, a condense filtration/separation unit, and a fluid drying unit.

20. A method of detection and monitoring of radioactivity caused by radionuclides at the gaseous state contained in a flow of fluid (107), comprising the phases of:

- taking from said flow of fluid a fluid flow portion to be submitted to measure (103, 113, 115a,115b,121, 123,125), and feeding it to a radiation measurement chamber (127), and - measuring the radiations emitted by the radionuclides contained in the fluid flow portion fed to the measurement chamber, characterized in that said phase of feeding comprises raising the pressure of the fluid flow portion to be submitted to measure with respect to an initial pressure of the picked-up fluid flow portion, and feeding the fluid flow portion at raised pressure to the measurement chamber.