Catalin Frujinoiu
"Horia-Hulubei" National Institute of Physics and Nuclear Engineering
P.O Box MG-6; Bucharest-Romania

Generally, the decision factors are less excited in promoting remedial actions in case of relatively low risk chronic exposure situations - i.e.- those in which exposures persist in time. However, this subject tends to become of a greater interest because of the existence of large areas radioactively contaminated, including some urban areas, and on the other hand of our moral responsibility for the future generation’ environment. The "pre-existing" low risk level contaminated sites, affected by "historical wastes", cannot be treated by fixed legislative provisions. Although radon is the most relevant subject from the area of chronic exposures, there are human activities that release substances containing radionuclides in quantities that should be considered. For instance, man-made radioactivity such as: mining industry, coal-fired power plants and fertilizer industry, imply significant emissions of long half-time radionuclides. The low level contaminated sites should be treated as chronic exposure situations and characterized As Realistic As Possible. The final decision should ensure that the exposure to the public will de decreased As Low As Reasonable Achievable. The paper explores, in a new and comprehensive approach, the criteria for identification and characterization of low risk radioactively contaminated sites.

In the sixtieth document the International Commission for Radiological Protection (ICRP), [1], shared the nuclear activities in practices and interventions. While practices are those activities that increase the risks of exposure- e.g., by introduction of a source in the environment- interventions are those activities intended to decrease a priori risk- e.g., by removing an existent source. The basic radiation protection principles are applicable both for practices and for interventions but in a distinct manner. Thus, in case of emergencies the principle of dose limitation is not applicable, we have instead intervention levels or action levels. Following the last international recommendations ,[2], in the case of intervention we could have intervention following an emergency exposure situation -in this case we rely on intervention levels- or after a chronic exposure situation that is associated with action levels.

Interventions are events that require protective action to reduce or avert temporary exposure. These include:

1. accidents and emergencies in which a remedial plan or emergency procedures have been activated; and
2. any other temporary exposure situation defined by the Regulatory Authority or the Intervening Organizations warranting intervention.

Chronic exposure situations require remedial action to reduce or avert a long time exposure. These include:

1. natural exposure, such as exposure to radon in dwellings and workplaces;
2. exposure to radioactive residues from past events, such as: the radioactive contamination caused by accidents, after the situation requiring protective action has been terminated, as well as from the conduct of practices and the use of sources not under the system of notification, and authorization; and
3. any other chronic exposure situation specified by the Regulatory Authority or the Intervening Organization as warranting intervention.

Typically, the environmental restoration of low level contaminated sites could be classified as intervention due to chronic exposure situation. In this respect, the most frequent case is (ii), i.e. exposure to radioactive residues from the past. These situations should be assessed as "pre-existing" ones, [1], in which the limitation system is not applicable and the estimation should be made "As Realistic As Possible" ARAP. Decision to be made is to intervene or not for restoring the environment to reduce the radiation exposure "As Low As Reasonable Achievable" ALARA. The estimation obtained ARAP and the decision obtained ALARA are phases of the same process -usually accepted as ALARA program- that precede the implementation of the restoration itself.

One of the most delicate issue associated to environmental restoration is the intervention for "reestablishing" of low risk level contaminated sites. These areas are characterized by low-level radioactive waste that are mainly recognized as "historical waste", i.e. those wastes originated in "past activities" for which the initial producer can no longer be considered responsible.

These "past activities" were performed under standards that became outdated and most of them replaced. Recently, the radiation protection community has been proposing new basic standards that recommend, among others, new lower limits. Thus, the lowering of the limit for the public from 5mSv annually to 1 mSv annually it is expected to lead to a considerable increase of the number of low risk level contaminated sites.

Generally, the society is less excited in promoting remedial actions in case of chronic exposure situation- an exposure that persists in time. The time range of this exposure depends of radionuclide’s physical half-time, chemical and residence form. Thus, in case of radionuclides with long physical half-time it is quite sure that they will affect generations. As a first step to a right attitude in face of the future we have the responsibility to quantify the long time effects of our present comfort.

Although the radon is by far the most important radionuclide for a life-span other radionuclides are important for the committed effective collective dose. There are examples of man-made radioactivity (e.g., production of power in coal fired plants, residues from mining, and fertilizer production) in which there are released in the environment substances containing Ra-226, U-238, Th-232, K-40, Pb-210, Po-210 and others. In spite of this, the international community failed to agree on setting action levels in case of chronic exposure situations,[2], that include those that could be subject for environmental restoration of low level contaminated sites. Below, there are offered three examples that may meet similarities in other areas of the world. All these could be considered for their classification as being long time exposures from low level contaminated sites and they have in common man-made radioactivity -or better radioactivity modified technologically by man. Coal, as well as all natural materials contain traces of natural radionuclides. The process of coal-firing assumes an intrinsic enhancing of radioactive product and once emitted in atmosphere a possible chronic exposure situation. Measurements performed in Romania, [3], at several thermo power plants under a research contract devoted to radiological impact of coal-fired power plants reveal exceeding of the values taken as reference from other areas. Also, the measurements (50mBq/m³ for Ra-226, 10 mBq/m³ for Th-232 and 100 mBq/m³ for U-238) show the persistence of the radioactivity in volatile ash. The collective effective dose exceeds the world average, [4]. Moreover, for some old coal-fired power plants it was find that one GWe produced annually corresponds to an effective dose around 20 mSv and a committed collective dose of 2 manxSv. The problem is more serious because these values exceed even the exemption levels from regulatory control in nuclear field (10 mSv and 1 manxSv) agreed internationally in basic safety standards, [2]. Should we regulate even these radioactive sources?

Environmental problems are raised from appreciable quantities of residues following exploration and exploitation of the ores both with nuclear and non-nuclear interest. A recent study performed in Romania on the radioactive mining exploitation and exploration [5], states that after more than 40 year of exploitation and exploration of raw material ore, piles with radioactive ores cover more than 1.8x10⁴ m² and piles with low level radioactivity together to pile of sterile cover more than 1.4x10⁶ m². A much larger value is assumed for non-radioactive mining industry.

After [5], in Romania the phosphate fertilizer industry, located in eight large industrial plants, processes almost 3 million tons of phosphates annually. About 85% of these phosphates are of sedimentary origin therefore are radioactive, containing 0.008 - 0.015% uranium, usually in secular equilibrium with its decay products. An eventual elimination, and recovery, of uranium could lead to 200 tons of uranium yearly. Depending on the phosphate rock used, the radioactivity for phosphates of sedimentary origin is 400-1 000 Bq/kg. A considerable quantity of waste is deposited around the plants. Over 20 million tones of waste are spread on more than 100 hectares in Romania.

Although the radiological criteria are the major factors during the identification and characterization of low level radioactive contaminated sites there are numerous other contributors such as: the deep, the area and the volume of the contaminated site as well as geology, hydrology, population, future utilization, costs, public concern etc.

Moreover the radiological and non-radiological risks, e.g. chemical hazards, are cumulative. This approach is a priority for radionuclides of uranium and thorium because the toxicity exceeds the radiation effects.

In case of low level contaminated sites, it is necessary to develop ALARA programs, or ARAP-ALARA programs, that are supposed to offer to human subject proper alternatives for the site classification:

• prohibited to human access;
• allowed for restricted release;
• allowed for unrestricted release.

Generally, the decision is taken under large uncertainties due to the multitude of aspects involved, the lack of information and the considerable dispersion of the sources of data. At the first phase it is advisable to consider an ARAP judgment in which you should rely on the information already available and the readily accessible data in order to reach an opinion selected for future in deep considerations. The goal of this approach is to detect all the suspicious areas. Any new data and information would cost -e.g., complex screening measurements- and will influence the balance of final ALARA program.

Thus it is opportune to propose a methodology to support a preliminary decision - "realistic but sufficient conservative", in case of low level radioactive contaminated sites that is taken in conditions of uncertainties of the initial conditions. Due to the strong influence of the subjective factor in this process we should consider a special model that is characterized by a higher degree of abstraction, capable to describe the judgment under uncertainty. By this model a preliminary decision is expected. Subsequently, more specific criteria have to be adopted. These encompass screening measurements (locally gamma dose rate and concentrations in soil samples) as well as other relevant information such as: public concern, land use forecast, demographic data and projections, agricultural land forecast, hydrology, other interests that should be protected, costs. However, for a final decision the remedial actions should be set up ALARA for each site by considering in deep all the relevant factors. It is not possible to set up fixed standards for environmental restoration of low level contaminated sites.

The complex situations of subject-object interaction are affected by important uncertainties during the decision process and cannot be treated through an ordinary bivalent logic -Boole logic- with values: true and false. This logic is too simple to describe processes that are conditioned by numerous factors, many of them being unknown or partially known. Usually, in the decision process the subject factor is dealing with suppositions and projections that are described by more complex models like the modal logic formalism. By this, a conditioned process is described by logic values such as: True (T), False (F), Impossible (I) and Possible (P). It is accepted that these logic values cannot be projected in real numbers. A comprehensive description of subject-object correlation in the decision process is offered by complex numbers formalism. This is usually presented in geometrical representations, [6]. Thus, the above mentioned T, F, I, and P can be represented by the values of: |b |eⁱf (b is the radius and f the angle); b =0 and |b | =1 being a good measure of Impossible (I) and, respectively Possible (P) values and f that of True (T) and False (F). A simplified general representation of these values for a conditioned process "q" can be as below:

where:

A critical level (CL) is obtained when "q <-->T" is equally to "q <-->F", i.e. the subject factor has equally chances to be right and to be wrong.

The modal representations, like the above, are useful in conditioned processes affected by large uncertainties. In order not to draw right conclusions from wrong hypotheses it is useful to keep the "image" of these uncertainties during the whole judgment.

In order to decide "if a contaminated site should or should not be catalogued as being subject for restoration" the subjective factor uses a logic of suppositions that is different from the classic bivalent logic too simple to describe the associated reality. Through a modal approach we can derive decision diagrams that are intended to support a resolution concerning low risk level contaminated sites. A generic example of a decision diagram based on modal logic is presented below:

where:

By examining diverse positions of RL with respect to OQ and OQcosf one should discover both classic situations: RL > OQ, or RL < OQ, and moreover new aspects such as: OQcosf >CL (the subject factor can separate true of false) or OQcosf <CL (the human factor cannot separate true of false), as well as: OQcosf <RL<OQ, that is proper to object-subject interaction.

The decision diagrams can be combined in flowcharts that may conduct to an ARAP evaluation of low risk level contaminated sites. The primary criterion for the subject’ judgment is considered the radiological significance. Thus the gamma dose rate [m Sv/h], concentrations [Bq/g], associated dump’s dimensions -volume [m³], area [m²]- are considered the necessary operational quantities for an ARAP judgment. An one-criterion ARAP evaluation could be as below:

If we consider all the significant operational quantities we will obtain multi-criterion ARAP flowcharts as below:

where:

Among many particularities, it should be emphasize that the feed-back reaction for uncertainty reduction is a specific feature of the applied modal logic.

1. The "pre-existing" low risk level contaminated sites, affected by "historical wastes", cannot be treated by fixed legislative provisions;
2. Even if the new movement in the radiation protection is quite comprehensive and wholly appreciated there are left some controversy aspects; among them is the enhancing in the number of areas classified as being low risk level contaminated sites due to the lowering of the limits for the public.
3. The low risk level contaminated sites should be treated as chronic exposure situations and may be subject of interventions; a decision to intervene or not should be made As Realistic As Possible (ARAP) based on the readily obtainable data.
4. The intervention for restoring of low risk level contaminated sites should be ARAP and the options should be treated by specific ALARA analysis.
5. For an ARAP estimation of a typical subject’s judgment under uncertainties, bivalent logic is too simple, more complex models, like the modal logic, may be adopted.
6. The methodology developed reveals new situations that can be quantified in decision diagrams and combined in flowcharts that can be taken as useful decision tools.
7. The preliminary ARAP estimations should be followed by specific ALARA analysis.

1. Annals of the ICRP, ICRP PUBLICATION 60; "1990 Recommendations of the International Commission for Radiological Protection", Pergamon Press, Vol. 21, No. 1-3, (1990).
2. International Atomic Energy Agency, SAFETY SERIES NO. 115-I; "International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of Radiation Sources", IAEA- Vienna (1994).
3. Romanian Society for Radiological Protection, RSRP, "Natural Radioactivity in Romania"; in English and Romanian; Bucharest, RSRP edition, (1993).
4. UNSCEAR, REPORT TO THE GENERAL ASSEMBLY WITH SCIENTIFIC ANNEXES; "Sources and Effects of Ionizing Radiation", United Nations publication, sales no. e.82.ix.8, (1993).
5. Proceedings of a Workshop held within the Technical Co-operation Project on Environmental Restoration in Central and Eastern Europe, IAEA-TECDOC-865, "Planning for Environmental Restoration of Radioactively Contaminated Sites in Central and Eastern Europe", volume 1 and 2; International Atomic Energy Agency, (1996).
6. I. PURICA, "Modal Thought in Experimental Sciences", Romanian Academy Edition, in Romanian, (1980).
7. ICRU PUBLICATION 50, "Radiation quantities and units", International Commission for Radiological Quantities and Units, (1990).

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