A French study on radioactive waste transmutation options 1. Introduction

This study was originally requested by the 2006 Waste Management French Act to obtain an assessment of industrial perspectives on partitioning and transmutation of long lived elements. These studies were carried out in collaboration with EDF, AREVA and the National Radioactive Waste Management Agency (ANDRA, Agence nationale pour la gestion des déchets radioactifs) and in a close link with Generation IV systems development. This legal requirement was detailed in the Decree of 2008 setting the requirements for the National Plan on Management of Radioactive Materials and Waste (PNGMDR), which calls for the CEA to coordinate research conducted on the separation–transmutation of long lived radioelements. This research is based on the analysis of technical and TABLE 3.31. MINOR ACTINIDE TRANSMUTATION EFFECT IN THE

BURNER CORE

Reference core with

5wt% minor actinides C1 C2 C3 C4

Minor actinide loading mass (kg) 632.3 1340.7 942.5 1600.1 1096.0 Minor actinide transmutation

ratio* (%) 8.333 7.605 6.032 6.835 6.517

Specific support ratio by minor actinide transmutation (kg/GW(th)·year)

57.9 207.7 129.7 191.9 125.3

* Minor actinide net consumption over fuel lifetime, including conversion to other actinides by capture reaction, as fraction of the initial content of the minor actinides.

TABLE 3.32. MINOR ACTINIDE INCINERATION EFFECT IN THE BURNER CORE Reference core with

5wt% minor actinides C1 C2 C3 C4

Minor actinide loading mass (kg) 632.3 1340.7 942.5 1600.1 1096.0

Av. fuel burnup (GW·d/t) 73.9 80.2 70.8 78.0 73.0

Minor actinide incineration rate (%) 2.93 2.67 2.16 2.43 2.19

Minor actinide fission fraction (%) 5.5 19.6 12.5 18.3 11.9

Minor actinide specific

economic scenarios taking into account the possibilities of optimization between long lived HLW transmutation processes, their interim storage and their disposal in a geological repository.

In the SYNERGIES framework, the study belongs to Task 3, on the evaluation of options for minor actinide management, providing technical and economical assessment of different option for minor actinide transmutation in France. The complete case study can be found in Annex XXIII on the CD-ROM accompanying this publication.

3.3.4.2. Objective and problem formulation

The objective of the study is a technical and economic evaluation of fuel cycle scenarios along with different options for optimizing the processes between the minor actinide transmutation in fast neutron reactors, their interim storage and geological disposal of ultimate waste. Scenario evaluations take place in the French context which considers the deployment of the first SFR in 2040. By this date, the SFR technology should be mature. Several management options of minor actinides have been studied:

— Plutonium recycling in SFRs (minor actinides are sent to the waste);

— Plutonium recycling and minor actinide (or americium alone) transmutation in SFRs and in the homogeneous mode (minor actinides are mixed with reactor fuel);

— Plutonium recycling and minor actinide (or americium alone) transmutation in SFRs and in the heterogeneous mode (putting minor actinides in radial blankets on a depleted UO₂ matrix);

— Plutonium recycling in SFR and minor actinide transmutation in ADSs.

The key questions to be answered in the study are the following:

— What are the inventories and characteristics of the materials and waste produced by the various scenarios?

— What is the impact of the various scenarios on the features of the installations (reactors, fuel cycle plants and storage facilities) and the transport requirements?

— What is the impact of the various scenarios on geological disposal (geological disposal occupancy and safety, among other things)?

— What is the impact of each scenario on the radiological protection of the public and the workers?

— What is the economic impact of each scenario?

— What is the industrial risk inherent in each scenario?

3.3.4.3. Assumptions, methods, codes and input data used

The assumptions associated with the scenarios under consideration are based on industrial experience transposed as effectively as possible to these new material management options; they do not preclude any future process and technological developments. The scenarios analysed in this first phase have in common the consideration that the current series of reactors would be renewed at constant installed capacity (60 GW(e)) generating 430 TW(e)·h/year. 40 GW(e) of light water EPR type reactors would be deployed between 2020 and 2040, followed by 20 GW(e) of SFRs between 2040 and 2050. The date of 2040 corresponds to a general hypothesis of a possible start of deployment of these SFRs, which would also be consistent with the main renewal dates for power reactors and fuel cycle plants. The introduction of a second series of 40 GW(e) SFR would take place from 2080 to replace the EPR which have reached the end of their service life. Starting in 2100, nuclear power generation capability would consist entirely of fast reactors (see Fig. 3.127).

Among the scenarios considering the deployment of SFRs, several differentiated alternatives have been selected:

(i) Recycling of plutonium only (Scenario F4).

(ii) Recycling of plutonium and transmutation of all or part of the minor actinides in the homogeneous mode:

— Of all minor actinides (Scenario F2A);

— Of americium alone (Scenario F2B).

FIG. 3.127. Production by reactor technology.

(iii) Recycling of plutonium and of all or part of the minor actinides in the heterogeneous mode in the radial blankets:

— Of all minor actinides (Scenario F1G);

— Of americium alone (Scenario F1J);

(iv) Recycling of plutonium in SFRs and transmutation of minor actinides in a dedicated ADS stratum (Scenario F7).

The methods applied involved grouping under a number of key questions (all the questions of the various stake holders) and then, in a second phase, defining the indicators that serve to answer these questions. The questions are summarized in Annex XXIII on the CD-ROM accompanying this publication.

The results should be analysed through several criteria. Assessment criteria has to take into account the view of all the players (scientists, industry, politicians and public, among others), cannot be redundant and needs to form a coherent whole set that is as robust as possible. The following criteria were used to analyse different scenarios should consider technical and scientific aspects, requirements for potential industrialization and public acceptance:

— Inventories and characterization of materials and waste;

— Impact on the waste repository (footprint and safety impact on the industrial facilities, reactors, fabrication plant, reprocessing plant and interim storage) and transport needs;

— The impact on radiological exposure of the public and workers;

— Economic impact;

— Industrial risks.

All of the scenarios described in this study refer, to varying degrees, to processes or technologies which have not been implemented at present, notably for industrial use and in some cases, which are still concepts. The maturity of the processes and technologies to be implemented in the different scenarios is a major parameter which is required for understanding the technical and economic risks related to the different options. Nevertheless, risks will be limited if the selected options provide sufficient flexibility and if the new technologies are implemented progressively. For instance, processes which are continuously updated through successive addition of new functions involve fewer risks than processes requiring dedicated facilities, which are, in most cases, not easily adaptable to new options.

It is obvious that these risks will be all the higher, as the technologies employed will not be mature. This is the case for the scenario involving the transmutation of minor actinides in ADSs. Although significant progress has been made regarding the feasibility demonstration, ADSs are complex systems, whose development requires the design of components with a high technicality and whose feasibility is not yet guaranteed.

FIG. 3.127. Production by reactor technology.

For all of the scenarios described in this study, it is assumed that the fourth generation SFRs will be implemented from 2040. The feedback from fifteen years expected experience with the ASTRID prototype operation would contribute to the technological maturity for SFR and fuel multi-recycling. The SFR core concept considered was developed by the CEA and French partners [3.70, 3.71]. The ADS model selected is the one designed for the EUROTRANS project (ADS Pb–EFIT) [3.72].

3.3.4.4. Summary presentation and analysis of the results

(a) Inventory

Contrary to a system of water cooled reactors, which inevitably produces increasing amounts of plutonium (reaching 1600 t in 2150 for a 60 GW(e) PWR fleet fed with UOX fuel), a system of plutonium recycling SFRs would stabilize the plutonium inventory at about 900–1000 t. Stabilization implies that SFRs operate on a break-even core (i.e. that they produce as much plutonium as they consume).

The main results at equilibrium and interim period can be summarized. At equilibrium period, in the case of recycling plutonium alone in SFRs without minor actinide transmutation, the minor actinide content in the irradiated fuel is about 0.4% (compared to about 0.1% in a PWR UOX fuel). Transmutation of minor actinides requires considering, at equilibrium, a content of about 1.2% of minor actinides in the fuels in the case of transmutation in homogenous mode, in which all the SFRs are involved; or one row of radial blankets containing 20% of minor actinides in 75% of the reactors of the system; or the deployment of 18 ADSs of 385 MW(th) in case of transmutation in dedicated reactors (an ADS with a higher power could certainly lead to more attractive performances). Transmutation of americium alone requires at equilibrium in all SFRs, a content of about 0.8% in homogeneous mode or one row of radial blankets containing 10% of americium.

The interim period proves to be more restrictive, because the quantity of minor actinides to be transmuted and the number of fast reactors available are not necessarily matched. This is the case in particular between 2040 and 2080. This means: in homogenous mode, having to significantly exceed the limit of 2.5% of minor actinides in the fuel; and in heterogeneous mode, considering two rows of blankets containing 20% of actinides until around 2100.

These results are conditioned by the assumptions made in the scenarios, and in particular the limitations associated with them. Hence, they demand further optimization of the interim phase to try to smooth the peaks encountered, which dimension the fuel cycle installations [3.73].

(i) Plutonium availability

If the deployment of the first wave of SFRs from 2040 raises no problem, it is important to anticipate the reprocessing of the spent fuels (cooling time reduced to 3.4 years instead of 5 years), or to alter the reactor concept in order to get the plutonium required to deploy the second wave of SFRs (using radial blanket for example). In these conditions, the deployment of a SFR reactor system as described in these scenarios appears feasible.

The deployment of ADS reactors is faced with a plutonium deficit of about 40 t. Relaxing the limitations applied to the scenario would probably help to circumvent this difficulty. Two alternatives can be examined:

reducing the fuel cooling time and/or larger number of fertile assemblies.

(ii) Inventories of minor actinides

The transmutation of minor actinides helps considerably to limit the quantity present in waste, because with the exception of minimal losses, they are no longer automatically sent to waste. If the non-recycling of minor actinides means the continuous increase in their waste inventory (to reach nearly 400 t in 2150), the transmutation of all the minor actinides helps to stabilize this inventory at around 60 t, regardless of the transmutation mode selected (homogeneous, heterogeneous and ADS). Between these two extremes, the transmutation of americium implies a moderate increase in the waste inventory, due to curium and neptunium: the value reached in 2150 is about 150 t.

FIG. 3.128. Inventory of minor actinides in the fuel cycle.

(iii) Inventory in the cycle

The immediate consequence of implementing the minor actinide transmutation option is the increase of the minor actinide quantities in the cycle inventory, as shown in Fig. 3.128. It is worth to recall that the term

‘cycle’ here designates all the installations of the loop followed by the actinides (fuel fabrication plant, reactor, reprocessing plant and spent fuel storage). The not recycled part of minor actinide inventory is considered as waste and the sum up between recycled and waste forms the overall minor actinide inventory.

In the case of ADSs, the high level of inventory in the cycle is explained as follow: although the reactor inventory is comparable with the transmutation in SFR, the short duration of the irradiation cycle due to the low power of ADSs makes the inventory outside of the reactors (7 years: reprocessing + fabrication) proportionately larger. This term is dominant here.

(b) Impact on waste

ANDRA was asked by the CEA to assess the impact of the HLW and ILW generated by the various transmutation technologies on geological repository size. ANDRA considered the repositories similar in structure to those used in the Cigéo project ongoing for current nuclear power plants. The results allow comparing the underground footprint and the excavated volume for three scenarios (F4, F1G and F1J). The impact of the period of interim storage was also assessed. Approaches to optimize the footprint of the repository were put forward. The advantages and disadvantages of the various transmutation options are analysed in Refs [3.74, 3.75].

HLW and ILW–low level waste (LLW) are disposed of in separate underground zones. This arrangement offers independence in terms of: (i) the management of the various types of waste; and (ii) the phenomenological behaviour of each zone, in view of the specific characteristics of the waste contained.

The repository cells are constructed progressively with waste emplacement, according to a modular architecture which provides for a strict separation between mining and nuclear activities. The underground facility includes a common infrastructure built prior to the operational phase of the repository, a disposal zone for ILW and LLW and a disposal zone for HLW.

In Scenario F4, the first study phase concluded that an increase of the interim storage period from 70 to 120 years would provide a gain of 25% on the footprint of the HLW zone and 7% on the total excavated volume.

The presence of americium in the waste restricts the densification of the repository because of a relatively low decay of the thermal power with time due to the long radioactive half-life of ²⁴¹Am. In the case of the transmutation of all minor actinides (Fig. 3.128), the increase of the interim storage period to 120 years allows a larger gain (60%) of the footprint of the HLW zone and 12% of the overall volume excavated.

FIG. 3.128. Inventory of minor actinides in the fuel cycle.

Based on the results of the first study phase, the second study phase consisted of a search for ways of optimizing the design with the objective of a more drastic decrease of the repository footprint. This second phase has considered only an interim storage period of 120 years for HLW. Indeed, this assumption associated with transmutation scenarios provides a significantly higher gain than a 70 year interim storage period. Compared to the multi-recycling of plutonium in an SFR, the transmutation of minor actinides associated with a design optimization of the repository would provide the following:

— A reduction by a factor of up to 7.3 (americium) to 9.8 (minor actinides) of the footprint of the HLW disposal zone after an interim storage period of 120 years;

— A total reduction by a factor of 3 of the repository footprint taking into account ILW–LLW and common infrastructures;

— A total reduction by a factor of 2 of the excavated rock volume.

Along with a reduction of the footprint of the HLW disposal zone, the partitioning and transmutation of actinides also decreased the thermal phase duration. After an interim storage period of 120 years, the thermal phase is reduced to about 200 years, compared with 1000 years without transmutation.

The long term radiological impact of the deep geological repository is not reduced by partitioning and transmutation of actinides. Indeed, this impact is dominated by long lived fission and activation products with a higher mobility in the geosphere (¹²⁹I and ³⁶Cl).

In the normal long term evolution safety scenario, the study shows that the densification of the repository as allowed by partitioning and transmutation does not significantly change the radiological impact of fission and activation products, despite concentrations in the near field increase with densification.

In an altered evolution scenario such as one involving intrusive drilling, the impact of fission and activation products may increase to some extent because of the densification of the repository. Nevertheless, this impact remains acceptable with regard to the dose limit (0.25 mSv/a) provided by the basic safety guide issued by the French Nuclear Safety Authority (ASN, Autorité de sûreté nucléaire).

(c) Impact on the cycle facilities (i) Impact on manufacturing

Powder metallurgy processes comparable to that employed in the MELOX plant can be considered for the fuel manufacturing. Such processes include successive steps involving powder preparation, the manufacture of pellets via lamination and sintering, the manufacture of rods and the installation of the subassemblies. Nevertheless, the presence of minor actinides requires reinforced shielding systems (neutron emission and gamma radiation).

Furthermore, the heat released by the radioactive materials involves making specific provisions for controlling temperatures during the manufacture of minor actinide bearing subassemblies. However, as americium and the mixture of minor actinides are less penalizing than plutonium, controlling the criticality risk, which is possible by means of the customary control procedures, is not problematical. Table 3.33 compares the thermal power and neutron emissions of the oxide powders used for manufacturing the fuels. These values are standardized with respect to the SFR fuel without minor actinides.

It is clear that the scenarios involving the transmutation of all minor actinides are extremely impeded by the presence of curium, and particularly of the ²⁴⁴Cm isotope. The manufacture of the fuels would obviously require the construction of a shielded enclosure with remote controlled devices. With high curium content, whole new technology development will be required (more pronounced for the ADS option). The scenarios with transmutation of americium are less restrictive for manufacturing operations [3.76].

(ii) Impact on the processing

Minor actinide bearing subassemblies are processed in facilities designed to receive the different types of spent fuel to be recycled (UOX, MOX and SFR). Processing operations are assumed to be based on the hydrometallurgy processes, such as at La Hague, including for ADS fuel. ADS fuel can also be processed by pyrometallurgy. The processing facility is broken down into workshops. Most of the workshops are dedicated to separate the nuclear

TABLE 3.33. THERMAL POWER AND NEUTRON EMISSION OF POWDERS DURING MANUFACTURING (AT EQUILIBRIUM)

MA transmutation Am transmutation

Scenario F4 F2A F1G F7 F2B F1J

Fuel Fr–core Fr–core MABB ADS Fr–core AmBB

Thermal power

(W/kg) 1.5 9 55 160 3 7

Neutron emission

(relative value) (1) (120) (1600) (3500) (1.4) (3)

Note: ADS — accelerator driven system; BB — bearing blankets; Fr — fast reactor; MA — minor actinide.

material from metal structures and to place it into solution, to separate via extraction cycles with solvents the chosen elements, to convert the separated elements into oxides and to condition structural waste and fission products.

The scenarios including the transmutation of minor actinides obviously involve the implementation of a process for separating chosen elements. According to the scenarios, this process may consist in a sequential separation process (DIAMEX-SANEX), in which the actinides are recovered separately, in a combined separation process (GANEX), in which plutonium and minor actinides are extracted together, or in an individual separation process (EXAM), in which americium can be recovered selectively. All of these processes are still being developed.

Their feasibility has been demonstrated at the laboratory scale; however, their industrial implementation still requires a long R&D process.

Studies on reprocessing facilities for core fuels recycling (minor actinides or americium) or bearing blankets recycling (minor actinides or americium) do not show important difficulties. However, scientific and technical feasibility has to be investigated for ADS spent fuels.

Criticality constraints have been analyses in preliminary studies: additional analyses are required for specific functions as conversion of product containing curium. Thermal and radiation constraints have also been considered: the controlling systems and biological shielding are to be reinforced, particularly during the minor actinide conversion step [3.76].

(iii) Impact on transport

The heat released by new or spent actinide bearing subassemblies can be a problem for transport operations between the reactors and the fuel cycle plants (see Table 3.34). Impacts of transmutation scenarios on fresh and spent fuels annual transport have been evaluated. Thermal, radiation and criticality constraints have been taken into account to propose cask concepts for normal conditions. No difficulties appear for americium transmutation scenarios (homogeneous or heterogeneous). When fuels contain curium, transport uncertainties increase because of important heat release requiring dividing fresh fuels and technological innovations development (MABB and ADS).

The number of canister to be transported and the number of transport journeys required are significant factors

The number of canister to be transported and the number of transport journeys required are significant factors

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