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Contents
Industrial nuclear
gauges
Nucleonic gauges or nucleonic control systems (NCS) have been widely used
in industries of developed countries to improve the quality of their
products, optimise processes and save energy and materials. It is
considered that NCS technology is by far the most requested among other
industrial nuclear techniques. Their economic benefits have been widely
demonstrated and recognised by industries. There are several hundred thousand nucleonic gauges
installed in industries all over the world. Simple, one parameter, static
measuring nucleonic gauges are commercially available from several
manufacturers. However, a significant number of NCS are not yet in the
market as standard products and the development of a new generation of
nucleonic devices is ongoing. The competition from alternative methods shows that NCS
have survived and prospered in the past because of their superiority in
certain areas to competitive methods. The success of NCS is due primarily
to the ability, conferred by their unique properties, to collect data
which cannot be obtained by other investigative techniques. Though NCS are
continually under pressure from alternative techniques, nevertheless, they
continue to make an increasingly important contribution to the better
management of natural resources, industrial efficiency and environmental
conservation. Relevant target areas for NCS applications are well
defined. Though the technology is applicable across a broad industrial
spectrum, the oil, gas and chemical industries, minerals and raw materials
exploration, mining and processing industries, iron, steel and metal
production plants, civil engineering, paper, pulp, plastics and cement
industries have been identified as the most appropriate target beneficiaries of NCS applications. These industries are widespread
internationally and are of considerable economic and social impact.
A radiation sealed source is
constituted of some radioactive mass stored in a container in order to
prevent the radioisotope from being in direct contact with the
environment.
As a general rule, gamma radiation emitters are enclosed in small holders
made out stainless steel whose volume is as low as a few cubic
millimetres. On the other hand, some beta sources may be stored in quartz
tubes and in some cases they may be used as metallic lames. In any alternative, the sealed source should be stored
in a source holder generally made out of lead. The holder plays two roles,
shield and collimator, focusing the radiation beam to a specific point
according to each particular instrument. Holders must include a mechanic, electromechanical or
pneumatic shutter whose purpose is to shield completely the radiation
source when the gauge is transported or when it is non-operative.
Thickness measurement
Different kind of papers, plastic films, fibreglass
sheets, rubber plates, adhesive layers, plastic or electrolytic coatings
and laminated metals are some examples of materials whose thickness are
usually
measured by nuclear gauges.
As a general rule
transmission instruments are preferred although their installation is
not always possible because of geometrical reasons. In the basic
configuration of a transmission gauge the media to be measured is
placed between the radioactive source and the detector so that the
radiation beam can be transmitted through it. The media attenuates the
emitted radiation (beta particles or photons) before reaching the
sensible volume of the detector. Both source and detector may be
collimated. The radiation intensity in the detector is a function of
several parameter of the tested material. Backscattering technique is used
every time transmission geometry is inadvisable for any reason. Whenever a
radiation beam interacts with matter a fraction of it is transmitted, a
fraction absorbed and a fraction is scattered from its original path. If
the scattering angle is greater than 90o some photons or
particles will come back towards the original emission point. The
measurement of this radiation is on the basis of the backscattering
method. In many processes, thickness must be
inside some tolerance limits and then comparative measurement is commonly
used. Both nuclear and electronic compensation techniques are employed. Measurement is always dynamic because the material to
be measured moves longitudinally in a continuous way while it is produced.
The nuclear measurement head may also move laterally in order to get
thickness profiles. The display system can be simply a plotter or an analog
instrument (such as a milliammeter) or something more complex such as a
CRT or LCD monitor showing graphic and digital information. In this case a
personal computer is generally used with a suitable software intended to
display information, store statistic data and calibration parameters and
control the production process. Ionization chambers are mainly used as detectors along
with beta or gamma radiation sources. Though the latter are in general limited to
measuring thickness in metal sheets. Nuclear gauges are often used to measure density in any kind of liquids flowing within pipes. For this purpose the transmission method is generally preferred. They are also frequently applied for soil density measurements where both transmission and backscattering techniques are used. Measurements can be absolute or relative, with nuclear or electronic compensation, and static or dynamic. In case of flowing liquids, the measurement is dynamic if the fluid velocity is "high" when compared with the system response time, otherwise it can be considered as static. Obviously this is the situation for a soil density gauge. Furthermore there are mobile devices designed for density liquid logging in vessels. The information can be displayed by means of any technique as in the case of thickness gauges. Soil density devices use to be portable and consequently simpler. However some instruments include microprocessors to make simple calculations combining soil density and moisture. Ionization chambers, Geiger-Müller (GM) tubes and scintillation counters are valid alternatives for detectors depending on each particular assembly. Regarding radioactive sources, gamma emitters are always used, especially 137Cs and 60Co. Portable soil density and moisture gauges need also a fast neutron source, usually 241Am-Be, and a thermal neutron detector.
Level measurement
Level gauges are usually installed in large tanks containing liquids and
can be classified as level detectors (on / off) or continuous gauges. The
former is used just to know whether the liquid is above or bellow a
reference level while the latter is suitable to determine the actual
liquid level inside a working interval. The most important nuclear gauge advantage, not only in
this case but in many other situations, is their ability to measure the
parameter of interest with no contact with the system under study which
could operate at very high temperature or pressure or contain corrosive
materials making it difficult to use conventional techniques. Generally the transmission configuration is preferred
using 60Co and 137Cs gamma sources and Geiger Müller
or scintillation
detectors. However the backscattering technique is sometimes used with
241Am-Be neutron sources and thermal neutrons detectors such as
boron trifluoride (F3B) tubes or chambers.
A continuous level gauge is based on a linear sealed source and a radiation detector located in both sides of a vessel containing some liquid. The schematic assembly shown bellow is intended to measure the liquid level inside an interval determined by the source length. It is also possible to work with an inverse configuration, that is to say a punctual source and one or more GM detectors in a parallel connection but in a linear assembly. In case of working with a linear source, this is made out of an irradiated wire that is coiled on a metal bar with a wide space between consecutive spirals. Geiger- Müller tubes are almost always used.
Concentration measurement
There are two kinds of nuclear concentration gauges:
those in which the sample thickness is constant and those in which it is
variable. The former refers to instruments that works submerged in a
liquid media in which the knowledge of the concentration of some impurity
or pollutant is needed. The sample thickness is the gap between source and
detector through which the liquid flows freely. This technique is also
used in laboratory devices where the sample must satisfy especial
conditions related with shape and thickness. On its side variable
thickness gauges are used as on-line meters to determine concentration in
solid materials. In this framework, nuclear turbidy-meters
are primarily intended to measure sediment concentration in water bodies.
Compared with conventional gravimetric instruments they have the typical
NCS advantages, that means that no samples are required and the supplied
information comes from an average area rather than from isolated points
and for these reasons is more representative of the zone under study.
Nuclear instruments are a complement for optical and ultrasonic turbidy-meters
that have low sensibility at high concentrations. Another application field for
nuclear techniques is coal ash concentration measuring. Coal consists of
coal matter and mineral matter. Coal ash is the oxidized incombustible
residue from the coal combustion and its concentration is closely
correlated with mineral matter content. The knowledge of the coal ash is
very important not only for the coal industry (in the control of coal
washeries and blending operations) but also in metallurgical industry. NOLDOR S.R.L. provides the following duties related with nuclear industrial gauges:
Radiotracers
Tracer is a detectable substance added to a chemical, biological, physical or industrial
system by means of some injection or
labelling technique in order to study and evaluate the characteristics and
the dynamic behaviour of such a process. Pollutant dispersion in water bodies, aquifer
behaviour, mechanism of vegetal nutrition, human tissue dynamic,
industrial production processes are all fields where tracers can play an
important role. Tracers give a comprehensive knowledge of all these
systems and allow to better understand their evolution and dynamic. Examples of tracers are solids in suspension, dyes, salts, alcohols and radioisotopes. The major advantage of radiotracers
is that they can be detected without physical contact and therefore without affecting the system. The selection of the most suitable
tracer for a given process is extremely important.
In any case it should
satisfy the following conditions:
to behave the same way that the material
under investigation,
to be detected easily and unambiguously,
to follow
the behaviour of the material without being removed from the process by
any way,
to be injected and detected by mechanisms that not disturb the
process, its residual concentration must be as low as possible. In case of
using radiotracers, they must also satisfy conditions regarding the type
and energy of the emitted radiation and their half-life. On the other hand
they have the following advantages: Identity:
all isotopes of a given element have identical chemical properties,
therefore a radioisotope of some element is an ideal tracer for such an
element.
Specificity: the emission of radiation is a specific property of
radiotracers and it is not affected by interferences from other materials.
Sensitivity: radioisotopes are measurable with high sensitivity and
consequently detected in very low concentrations. Measurement “in situ”: in
most cases radiotracers can be detected in the field or in an industrial
plant with no physic contact with the process.
Residence time distributions Radiotracers are the best tool
whenever the knowledge of residence time distribution (RTD) in an
industrial process is needed. As a matter of fact the system response at
an instantaneous tracer injection is precisely the RTD function of this
system. The system response is the tracer concentration against time
curve in its outlet. In the simplest case the knowledge
of the mean residence time and its standard deviation is enough
information to evaluate the process but in
more complex systems it is preferred to analyse their transfer function in
detail, to evaluate all the statistic parameters and to apply mathematical
models. These models allow the engineers to decompose the process in a set
of interconnected simple systems. Then the model can be "tuned" modifying some of
its parameters aiming to the optimisation of the process and the further
increase in the production efficiency. Ideal systems can be classified in
two types: plug flow and total perfect mixers. In the former all the
elements making part of the flow moves through the system with the same
regime and without mixing, being the output concentration curve identical
in shape to the input but delayed. In the case of perfect mixers, the
material is mixed instantaneously and uniformly within the volume of the
system and the output concentration is identical to the concentration
within the reactor. In a real system, the response
to a tracer impulse is its residence time distribution and can take any
arbitrary shape. The first moment of the response curve is the system mean
residence time and can be calculated solving the equation shown in the
picture above numerically. Additionally, real systems are usually more o less
complex but they can be decomposed in small parts connected in series or
in parallel and even including feedback loops to make easier their study.
Specialised software is then used to obtain the mathematical model that
better approaches the dynamic behaviour of the system. Tracers techniques allow the engineers to find dead
volumes, multiple transport paths, internal feedbacks and many other
problems that usually affect industrial processes. Tracer response analysis also contributes to a better
understanding of the system under study and to optimise its working
conditions in order to get higher economic benefits. Leak detection and leak location are
the most widespread radiotracer applications in industrial plants. Any
undesirable interconnection between isolated parts of a same system or
between two different systems may be a leak. A leak is suspected if there
is any abnormal behaviour of a system such as loss of pressure,
contamination of product or loss of process efficiency. Detection of a leak, if any, is
unambiguously achieved by injecting a radiotracer into the suspected part
and measuring for the tracer in the part that should be isolated.
Radiotracers are especially useful when the leak is located in underground
pipelines since in such cases there is no needs of performing excavations. Mass balance is another radiotracer
technique successfully applied for inventory of materials and for
evaluation of process efficiency in many branches of industry. The extend to which a tracer is
diluted in a tagged material constitutes a measure of the volume or the
inventory of the material. The dilution principle is the basis of methods
for material inventory in batch systems or for the determination of
separation efficiency in a continuous system or for the estimation of the
material fraction which recirculates also in a continuous system. One of the main applications of
dilution analysis is mercury inventory in electrolytic cells where
radioactive 203Hg isotope is used as tracer. Since mercury is
used to label mercury, the tracer can be considered as "ideal".
NOLDOR S.R.L.
provides the following duties related with radiotracer techniques: Determination of residence
time distributions. Leak detection and
location. Flow rate measurements. Dilution analysis. H
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