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Researchers worked with especial emphasis during the early 90's They took particularly into account the existence of some radium isotopes in formation water as calcium, barium and strontium salts and also the presence of radon-222 in some gas reservoirs. These natural radioisotopes are known by the acronym NORM (Naturally Occurring Radioactive Materials). Scales, sludges, depositions, tools, downhall tubing, tanks, vessels, separators, heaters and industrial waste stores have been analysed in order to evaluate the impact in the environment of NORM as well as the dose on workers and public members due to external and internal irradiation. The radioactive isotopes usually stay with the water phase and may either incorporate themselves in pipe scale or precipitate into sludges. With regard to radon, it has a boiling point between that for ethane and propane. Therefore, the highest radon concentrations are found in gas processing units related with ethane/propane treatment. Research was extended to treatment plants and distilleries and produced important recommendations and, in some cases, regulations of obligatory observance. Among them: periodic monitoring using specialised instruments, classification of areas according to its dose level, decontamination of tools, materials and vessels, dosimetric control of workers, evaluation of environmental impact and effects on the public.
Contents
Radioactivity
The atomic nucleus is constituted mainly by two types
of particles:
protons and neutrons. The number of protons is called
atomic number and determinates the chemical behaviour of the
element which the atom belongs to. The sum
protons and neutrons
is the mass number. There are atoms
having identical atomic number but different
mass number, they are called isotopes of the considered element. For some values of atomic and mass
numbers the nuclei are stables, that is to say that they do not change as
time goes by unless an external action takes place. When the ratio
neutron to protons differs from the value for a stable state the nucleus
starts the emission of alpha or beta particles in order to
regain stability. This mechanism is known as radioactive disintegration
and may additionally produce electromagnetic radiation originated in
the nucleus (gamma
radiation). For a radioactive material, its
disintegration velocity is proportional to its mass and is called
activity, being the becquerel its measurement unit (1 Bq = 1
d/s). In practise is very common to use another unit, the curie (1 Ci = 3,7 x 1010 Bq = 37 GBq).
The time for a radioactive mass to
be reduced at a half as a consequence of radioactive decay is the
half-life of the radioisotope enclosed in this material. There are 68 radioactive isotopes in the Earth's crust being
the most
important potassium-40 (40K), rubidium-87 (87Rb) and those that make part of the three natural
chains: uranium-235 (235U) that includes 17 radioisotopes, uranium-238 (238U) that includes 19 radioisotopes and thorium-232 (232Th) that includes 12 radioisotopes.
They are, in conjunction with cosmic radiation, the largest source of
radiation to which human beings are exposed.
Exposure to
radiations
If some piece of matter is exposed to a radiation beam
such a material absorbs a portion of the radiation energy. The ratio
between the absorbed energy and the irradiated mass is called dose.
For radiation protection purposes, exposure to ionising radiation is most
often measured in terms of effective dose. This is based on the
energy deposited in tissue by radiation, taking into account the
type of radiation and the sensitivity of the affected tissue. It is thus a
measure of the overall risk arising from the exposure. The unit is called
sievert (1 Sv = 1 J / Kg). Radiations may induce changes at
cellular level that could be hazardous for a living being. There are two
mechanisms able to
produce biological effects: irradiation and contamination. In the former a
human being is exposed to a radiation beam while in the latter the person
is in physic contact with the radioactive material including inhalation
and ingestion. The 88% of the effective dose to which an average person is exposed come from natural sources and only the 12% are due to artificial sources as it is shown in the picture bellow.
The 94.5% of the artificial radiation to which an average human being is exposed have their origin in medical applications (diagnosis and therapy). That is to say that just 5.5% of the artificial radiation dose come from other sources (fall-out, nuclear power stations, non-nuclear industries, consumer goods, occupational).
Control of
operating units
According to what was said above, periodic surveillance
of operating units has become a standard practise in many industrialised
countries. This periodic control includes a comprehensive monitoring using
specialised instruments, classification of areas according to their dose
level, decontamination of tools, materials and vessels, dosimetric control
of workers, evaluation of environmental impact and effects on the public.
All these routines are suitable and recommended not
only for upstream petroleum facilities but also for downstream sites which
are fed by natural gas such as refineries and chemical and petrochemical
installations.
External control
External control implies measuring natural
radioactivity in points close to the external surface of operating units,
pipes or equipment that make part of the installation under surveillance. Special care should be taken in
points in which flow direction changes abruptly such as pump discharges,
control valves and pipe bends because there the highest concentrations of
radioisotopes are usually found. Once the monitoring has been completed, the areas
should be classified according to their radioactivity level. It is
strongly recommended to tag with an appropriate sign those areas whose
radiation levels are high enough to demand that action. Internal control refers to measuring natural radioactivity inside processing units, equipments or piping. There are two types of internal NORM: removable and fixed materials. Removable NORM can be inhaled by workers, becoming an internal source, therefore its effects has to be added to the total exposure coming from fixed and removable radioactive materials. Hence internal monitoring is extremely important any time equipment or processing units are opened. Radon-222 makes part of one of the natural radioactive chains but, being a noble gas, can easily break such a chain and move freely through porous materials with no chemical reactions. Although radon is a non-reactive gas, its daughters (mainly lead-210, polonium-210 and bismuth-210) are chemically actives and able to react with materials in contact with them. For that reasons, as it was said above, radon-222 is responsible for the 48% of the total dose received by an average person. Consequently, as an additional task in any installation monitoring, it is convenient and recommended to take air samples from strategic points in order to measure radon-222 later in laboratory Air samples should be also taken whenever maintenance tasks implying chances of generating radioactive aerosols are performed. Brushing, hydroblasting and polishing are a few examples of those activities. Radioisotopes coming from industrial installations may pass to surroundings water bodies where they are likely to incorporated themselves into the food chain by means of different mechanisms. Because of that it is a good idea to periodically control surface and underground water for radioisotope contamination. The results of activity measurements in water samples will be on the base of any further study related with the environmental impact of radioactive materials. In November 2005 NOLDOR S.R.L. and Pluspetrol S.A. submitted a paper about NORM measurements to the "6as Jornadas de Preservación de Agua, Aire y Suelo en la Industria del Petróleo y del Gas" (Workshop on Water, Air and Soil Preservation in Oil and Gas Industries), held in Neuquén, Argentina, organised by the Argentinean Institute for Oil and Gas. This report gives an account of the results of fourteen NORM measurement campaigns in different oil fields belonging to the Argentinean oil company Pluspetrol. It can be downloaded (in Spanish) clicking here: NORM_IAGP_2005.pdf (433 KB). NOLDOR S.R.L. offers the following specialised services in relation with NORM:
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