Complex and stable waste streams

As discussed in Section 3, part of the traceable waste streams from NPPs can be classified as complex and stable. Main characteristics of this type of waste are as follows:

— sampling is easy but must be representative (for instance, the total radioactivity in ion exchange resins may not be stable with time)

— fingerprint is known and stable

— matrix and origin of waste is known

— easy to measure using NDA or DA methods

— scaling factor method for difficult to measure radionuclides is applicable

— physical and chemical properties are easily determined by destructive analysis

Examples of this category include evaporated concentrates, ion exchange resins and solid waste.

4.3.2. Reprocessing 4.3.2.1. Subject and scope

The scaling factor methodology is applicable to nuclear facilities where complex radioactive substances) are handled, for instance, a reprocessing plant. It is further applicable if a fraction of the radionuclides present in the mixture can be assayed directly by non-destructive measurement such as gamma spectrometry, dose rate measurement and passive or active neutron counting. A list of radionuclides likely to be found in the initial reprocessing sludge is given in Annex II.

The activity of DTM and ITM radionuclides (called DTM in this Section) is determined from the activity of the directly measurable radionuclides by means of prior correlations established either from destructive analysis or using calculation codes.

4.3.2.2. General application criteria of the scaling factor methodology

From the standpoint of radionuclide behaviour, the physicochemical processes implemented in a reprocessing nuclear facility can be represented schematically as follows:

Process 2 Process 1

Process n

…

Final radionuclide mixtures Initial radionuclide

mixture

Initial mixture Processing

FIG 4.1. Physicochemical processes in a reprocessing facility.

The physicochemical process may:

⎯ modify the isotopic ratios of a single chemical element (e.g. enrichment);

⎯ conserve the isotopic ratios of the chemical elements but modify the chemical element concentrations in the initial solution.

The following values are defined:

⎯ the decontamination factor (DF) of a radionuclide is equal to the ratio of the activity of the radionuclide in the initial mixture to its activity in the final mixture:

⎯

activity Final

activity Initial

DF=

The DF is a mass or volume factor, depending on whether the initial and final activity is expressed in terms of the mass or volume of the initial and final mixtures;

− the reconcentration factor (RF) is equal to the ratio of the activity of the radionuclide in the final mixture to its activity in the initial mixture:

−

activity Initial

activity Final

RF=

It is equal to the inverse of the decontamination factor:

DF RF= 1 .

4.3.2.3. Selection criteria for Key Nuclides (KN)

To establish representative correlations, KNs are selected according to the following criteria:

⎯ the KN must be present in the assay sample in significant quantities;

⎯ the radioactive half-life of the KN must be long compared with the time necessary for sampling, measurement, and physicochemical processing;

⎯ the KN and DTM in the initial mixture should preferably be formed by similar mechanisms. For example, in nuclear power plants, the radionuclides arising from fission reactions in the fuel should be distinguished from radionuclides due to activation of structural materials;

⎯ the chemical behaviour of the KN and DTM in the reprocessing steps should be similar;

it is thus preferable to choose isotopes of the same chemical element.

4.3.2.4. Establishing correlations between KN and DTM in the initial mixture

Technique based on sampling and destructive measurement

A representative sample is taken in the initial mixture. The laboratory analysis procedure implements techniques capable of quantifying the activity of the desired radionuclide(s) in the prepared sample.

In a complex sample the analysis may reveal a non linear relation between the activities of the KN and DTM. In most cases this relation can be reduced to the following expression:

(

)

DTM a Activity Activity = ⋅

The above activities refer to the measured specific activities or volume activities. The terms a and b are defined by fitting the preceding relations to the experimental values using suitable mathematical methods (e.g. least-squares method) to minimize the deviation.

Technique based on calculation codes

The radionuclide activities in the initial mixture can in some cases be determined using specific calculation codes such as the following:

⎯ fuel evolution codes for nuclear facilities in the back end of the fuel cycle;

⎯ activation codes in facilities concerned by material irradiation.

⎯

The correlation is expressed by a ratio:

KN DTM

Activity Activity

.

The above activities refer to the measured specific activities or volume activities.

When the activity of a DTM can be correlated with the activity of several KN, the characteristic correlation ratio can be established from the sum of the KN activities:

=

∑

KN DTM

Activity Activity ratio

n Correlatio

4.3.2.5. Establishing correlations in the final mixture

Technique based on sampling and destructive measurement

The technique discussed in 4.3.2.4. is also applicable when quantifying the radionuclides in the final mixture with the same constraints as for the initial mixture.

Technique based on knowledge of the characteristics of the initial mixture and of the physicochemical treatment applied

The correlations between KN and DTM in the initial mixture are known. The effects of physicochemical processing on the radionuclides concentrations have first to be characterized by destructive analysis (the DF or RF values of the radionuclides are known).

By defining the decontamination or reconcentration factors, the following relations can be established:

If a DTM is correlated with several KN in the initial mixture, only the directly measurable radionuclides with non-zero reconcentration factors (and with a specified decontamination factor) will be used as tracers for the final mixture.

If the following relations occur:

⎯ ADTM init and ADTM final are the specific and volume activities of the DTM in the initial and final mixtures;

⎯ AKN init(i) and AKN final(i) are the specific and volume activities (subscripted i) of the n KN in the initial and final mixtures with non zero reconcentration factors;

⎯ DFDTM and RFDTM are the mass or volume decontamination and reconcentration factors for the DTM; and

⎯ DFKN (i) and RFKN (i) are the mass or volume decontamination and reconcentration factors (subscripted i) of the n KN.

then the correlation between the sum of the activities of the key radionuclides and the activity of the DTM in the final mixture will be as follows:

∑ ∑

The expression with the reconcentration factor RF is determined from the above expression by substituting 1/RF for DF. The KN subscripted ( j) in the preceding expression is generally the KN with the most significant final specific activity or volume activity and the most easily measurable by the method used.

4.3.3. Waste form 4.3.3.1. Introduction

Conditioning is defined as those operations that produce a waste package suitable for handling, transport, storage or disposal. Conditioning may include converting the waste to a solid waste form, enclosure of the waste in containers, and, if necessary, providing an overpack [17]. In some cases, the waste may be immobilized into a typical fixation matrix (for example, bitumen, cement or glass). This category of waste is well characterized, with a stable and known fingerprint, knowledge of chemical and physical parameters, radiation stability, and long term behaviour. Classification of this waste is complex and stable.

Main characteristics:

− stabilized waste form (physical and chemical properties are stable and known)

− stable and known fingerprint

− scaling factor method for activity determination is applicable

− radiochemical sampling is recommended but not strictly required (process and final product control should be sufficient along with possible NDA)

− physical and chemical property measurement will use DA (leaching, compressive strength, penetration test, etc.).

Examples: cemented, bituminized, vitrified products.

4.3.3.2. Vitrified waste

For most Member States, the reference waste form for high level waste generated from reprocessing of spent fuels is borosilicate glass [19]. Reprocessing wastes are often compositionally complex due to the variety of spent fuels, complicated chemical processing schemes, and storage schemes used. Due to waste composition complexity, glass composition formulation must be optimized to meet the often competing property and composition constraints, such as waste loading and cost, processing rate, viscosity and electrical conductivity of the melts, melter corrosion, phase stability, chemical durability, and regulatory compliance. However, a well-designed waste conditioning process produces homogeneous glass within a composition range in which the property distributions do not vary significantly, allowing for relatively easy representative sampling if required.

With enough initial non-radioactive and radioactive characterization work, key waste glass properties can be predicted as functions of composition (e.g. for a qualified glass composition region within a narrow composition range, most glass properties can be accurately modelled as a linear function of composition, i.e., stable). In this situation, waste characterization data generated during the conditioning process can be used for process control and compliance activities [see references in Annex IV (Section 4) for glass composition versus property modeling]. If the spent fuel parameters, such as initial fuel mean

composition, neutron flux and burnup and conditioning process, are sufficiently known, it has been demonstrated that fission yield scaling factors can be determined to predict the concentration of fission products and actinide radionuclides for repository acceptance of the vitrified waste [20]. This determination of the scaling factors is based on DA results on HLW samples.

Additionally, the use of fission yield scaling factors may be beneficial in controlling or understanding the conditioning process via mass balance closure. For example, one could measure the radionuclides contained in the glass and recognize that certain volatile components were not in the correct ratio and know that they need to be accounted for within the off gas system [21]. This facilitates a scheme of minimal to no waste glass sampling and measurement, given that destructive chemical analysis of glass is costly and hazardous due to the high concentration of radionuclides.

4.3.3.3. Cemented and bituminized waste

Cement solidification for complex and stable waste streams is typically used for NPP operational waste, such as ion exchange resins, evaporator concentrates, and precoat filter sludges. Cement is also used to solidify intermediate level reprocessing sludges. The cement matrix may be the most common ordinary Portland cement (OPC), more durable cements (blast-furnace slag cement, others), or include admixtures. Important considerations in the cementation process are (1) that the cement formulation and the waste must be compatible, (2) the cement-waste mixture must be sufficiently homogenized during the cementation, and (3) that the compressive strength of the cemented waste is sufficient, requiring a minimum curing period [22, 23].

Bituminization is applicable to roughly the same types of NPP operational waste as those that are currently being cemented. Types of waste suitable for immobilization in bitumen are sludges and slurries, ion exchange materials, liquid concentrates, and incineration ashes. Intermediate level reprocessing precipitates are also known to be bituminized. The two types of bitumen mainly used are distilled or blown bitumen, depending on their production process. Blown bitumen is harder than distilled bitumen. Important considerations in the bituminization process are (1) that the bitumen matrix is compatible with the waste, and (2) measures should be taken to prevent flammability during conditioning or afterwards [23-25].

Similar recommendations as applied for vitrified waste can be used to obtain a detailed characterization of the radionuclide inventory of cemented or bitumenized waste. Of course for NPP operational waste, calculations based on the fuel are no longer relevant. Cemented and bituminized waste are known to be less homogeneous at a macro-scale (within the container), e.g. cemented ion-exchange resins. The choice of the location and amount of samples to be taken from a drum, therefore, are more difficult than for vitrified waste.

4.4. SIMPLE AND VARIABLE WASTE STREAMS — EXAMPLES OF NUCLEAR