- Low- and intermediate-level waste typology
- Waste Conditioning
- Specific application for conditioning of resins
Different types of nuclear and conventional waste are produced in a nuclear reactor. Previous articles have focused on the production of waste and its specific treatment. In this article, the objective is the treatment and conditioning of low- and intermediate level waste.
As a summary, it should be remembered that high-level waste (e.g. Uranium-235, U-238 and Pu-239) is found in the zircaloy sheaths in the reactor core; while the fission fragments from the nuclear reaction are confined to the pods themselves.
Once the fuel cycle has finished, all this waste is transported to the storage ponds where it remains until transfer to the final deposit site, either on the surface or as a deep geological deposit. There is another type of waste generated as a consequence of neutron activated nuclear reactions, from the capture of different types of particles (neutrons) and gamma adsorption processes.
A particular example is the liquid coolant that circulates through the primary circuit, which has the double mission of extracting the energy produced by nuclear reactions and cooling the reactor core where nuclear events occur.
This liquid that cools the reactor consists of specific substances such as:
- Boric acid: This adsorbs the neutrons to decrease the number of fissions and moderate the nuclear reaction. Its concentration varies according to the activity of the nucleus, but is in the range of 1000-2000 ppm.
- Lithium hydroxide: This is used to control the coolant pH to prevent corrosion processes.
- Hydrogen peroxide: This regulates the dissolution potential. It helps keep the metals in the solution in the ionic form to prevent deposition in bends or at specific points in the circuit.
- Corrosion products: Different metals from the steel structure of the primary circuit which undergo corrosion and are incorporated into the coolant solution (e.g. Co-60, Mn-54 and Co-58).
- Tritium, From activation of the natural deuterium in the water due to the effect of neutrons. Tritium is a β-emitter and one of the main problems in nuclear waste processing.
There is a whole series of operational wastes that may contain variable concentrations of Cs-137, Sr-90, Co-60 produced by specific valve cleaning, movements of spent fuel to the ponds and sludge cleaning, for example.
The treatment of gases will not be discussed in this article, as it is a very specific topic with problems centered on a group of iodine species, noble gases and tritium, for example. The focus will be on low- and intermediate-level liquid waste effluents.
Radioactive waste can be classified according to different criteria: for example, according to its physical state (gas, liquid or solid); which in turn can be classified into compactable, incinerable and metallic, for example); or according to the type of radiation emitted (alpha, beta or gamma rays, neutrons); the half-life (short or long); and its specific activity (high-, intermediate- and low-level).
The classification of radioactive waste management focuses on the specific activity level and decay period as these two factors determine the type of isolation and conditioning required for its storage.
The International Atomic Energy Agency has proposed a classification with a view to the definitive storage of waste (ref. 11), whose criteria are summarized in Table 1 and schematically represented in Figure 2.
As can be seen, several quantitative limits are considered in the classification:
- A maximum effective dose for members of the public of 10 µSv/year, as the limit for exemption or declassification of waste.
- 30 years half-life as the cut-off value between short- and long-lived waste.
- An average content of 400 Bq/g and maximum of 4,000 Bq/g for long-lived alpha emitters for the waste to be considered long-lived.
- A calorific output greater than 2 kW/m3 for the waste to be considered high-level (the limit must also be exceeded in long-lived alpha emitters).
|Typical characteristics of the various categories of radioactive waste proposed by the IAEA|
|Category of the waste||Typical characteristics||Storage systems|
|1. Exempt or declassified waste (RE initials in Spanish)||Levels of activity whose release does not mean an annual dose to members of the public in excess of 10 µSv||Without radiological restrictions|
|2. Low or medium activity waste (RBMA, initials in Spanish)||Levels of activity whose release may mean an annual dose to members of the public in excess of 10 µSv and which have a thermal potential of under 2 kW/m3|
|2.1 Low or medium activity waste with short life (RBMA-VC)||Limited concentration of long-life nuclides (4000 Bq g of long-life alpha emitters in individual batches, with an average value of 400 Bq/g overall)||Surface storage or geological storage systems|
|2.2 Low or medium activity waste with long life (RBMA-VL)||Concentrations of radionuclides with long life greater than those of short-lived waste.||Geological storage systems|
|3. Highly active waste (RAA, initials in Spanish)||Thermal potential greater than 2 kW/m3 and concentrations of radionuclides with long life greater than those of short-lived waste||Geological storage systems|
The following can be specified about low- and intermediate-level waste, which is dealt with in this article:
- Low- and intermediate-level wastes (RBMA) are those with a radionuclide concentration which produces sufficiently low thermal energy during their disposal. Acceptable values are established at the evacuation site after a safety assessment.
- Short-lived waste (RBMA-VC) contains nuclides with a half-life less than or equal to Cs-137 and Sr-90 (approximately 30 years), with a limited concentration of long-lived alpha radionuclides (limitation of alpha-emitting radionuclides to 4,000 Bq/g in individual waste batches and a general average of 400 Bq/g in the total waste volume).
- Long-lived waste (RBMA-VC) contains radionuclides and long-lived alpha-emitters with a concentration higher than the limits for short-lived waste.
The classification system is intended to be used only for solid waste, although it should be noted that some radioactive waste is liquid and could be treated as transitional waste. This type of waste comes from hospitals and other medical activities.
For Spain, it should be noted that essentially EC recommendations are followed; although, another waste group of very low level activity is being considered within the RBMA category. This contains radionuclides at very low concentrations whose storage does not require isolation systems as complex as for the rest of the RBMA category. Therefore, this subdivision applies exclusively from the point of view of the type of storage required.
In the management of low- and intermediate-level radioactive waste, the concept of “package” is used. This is understood as the unit formed by the radioactive waste, the conditioning agent and the packaging that houses it. Most low- and intermediate-level waste is managed through packages. Normally, the package is a 220 L drum with a complete protective structure and a conditioning system for the waste itself.
The volume and percentage of low- and intermediate-level nuclear waste managed in Spain is indicated in the following table. Practically 92% comes from NPPs and their decommissioning.
|TOTAL AMOUNT OF RBMA MANAGED IN SPAIN|
|Nuclear Power Plant Operation||36.071||20,46%|
|Nuclear Power Plant decommissioning||127.185||72,12%|
|Operation and Decommissioning of Fuel Assembly Plants + PIMIC (Integrated Research Facility Improvement Plan)||1.593||0,90%|
|Radioactive and similar facilities||4.861||2,76%|
|Pollution incidents and others||6.636||3,76%|
To date, the only low- and intermediate-level radioactive waste from decommissioning Spanish nuclear power plants has come from the Level 2 decommissioning of the Vandellós-I nuclear power plant, which has reached about 20% of the total expected from this plant when its decommissioning is complete and finally closed.
The amount of RBMA from the decommissioning already carried out has reached about 3,400 m3, which is barely 3% of the volume expected to be managed in the decommissioning of all nuclear power plants.
Low- and intermediate-level waste typology
There are 7 types of waste that can be established:
- Resins: Suspensions of ion exchange resins which are discharged from the purification system demineralizers, once exhausted. Boron, cesium and cobalt retention exchange systems are among the most important for cleaning the coolant isotopes. However, one of the problems with resins is their swelling capacity. After contact with the solution, they incorporate aqueous solvent in their structure which affects the final volume of the waste. Thus, other options such as specific isotope adsorbents are being studied.
- Evaporator concentrates: Concentrated salt solutions from evaporators for the treatment of boric acid and process effluents.
- Sludge: Sludge from deposits, precipitates, cleaning of communication routes for the transfer of spent fuel and sludge from filters. All this sludge must be made inert. One of the sources of sludge is the removal of the pre-coating and insoluble matter from reactor water purification systems and irradiated fuel pond filters. Other sludge which is part of this waste stream is from the liquid radioactive waste treatment system, due to sedimentation of the water stored in the additional waste tanks, as well as from condensate backwash. An example of this waste production is the decanters in the sludge treatment from the Santa Maria de Garoña plant. Sludge is currently processed in the settling tanks by fluidization after re-suspension, mixing, filtration, drying and packing in drums. The volume of sludge contained in the settling tanks is 300 m3 and it is expected to produce 580 drums of 135 L of conditioned waste. In the additional waste tanks, there are another 50 m3 of decanted sludge which will also be processed, depending on the operational situation of the liquid radioactive waste treatment system.
- Compactable materials: These refer to specific PPE: e.g. clothing, ventilation filters, rags and plastic utensils.
- Non-compactable solids: e.g. tools, metal parts, rubble and wood.
- Liquid circuit filters: Metallic filters for process systems.
- Non-operational waste or that from specific actions, such as treatment of effluent with Sb-125.
All this waste must be immobilized and placed in duly approved drums. 220 L drums are commonly used. There are two types of packages, classified by their activity levels: 1 and 2.
Level 1 packages: These are individually packed in a 0.22 m3 container and do not exceed the maximum activity value indicated in the annex to the technical specification, 031-ES-IN-0002. The individual package values depend on different configurations: number of packages per conditioning unit, type of package and container properties.
Level 2 packages: Their activity level is higher than Level 1, with a maximum value not exceeding the limits derived from the Level 2 conditioning unit values, with due consideration to the heterogeneity factor in Annex II of specification 031-ES-IN-0011.
The following table shows the different package classification levels:
|TYPICAL PACKAGE CLASSIFICATION|
|HOMOGENEOUS LIQUID WASTE||Level 1||Evaporator concentrates, powdered resins, sludges and mixtures incorporated into the hydraulic binder matrix.|
|SOLID WASTE (HETEROGENEOUS OR HOMOGENEOUS, TAKEN TO DRYNESS)||Level 1||Ball resins incorporated into the hydraulic binder matrix.|
|Level 1||Liquid circuit filters immobilized by a hydraulic binder.|
|Level 1||Non-compactable solids in a container (1.3 m3)|
|Level 1||Compactable and non-compactable solids|
|Level 1||Solid or dry waste immobilized by a hydraulic binder.|
Nuclear waste is conditioned in all nuclear power plants after being treated appropriately. Before evaporation, operating or decontamination resins in the primary circuit undergo a dehydration process, and a hydraulic binder (cement or mortar) is used as a solidifying or immobilizing agent to make a homogeneous, stable matrix. The concentrated effluents, spent resins and sludge are mixed with cement. This cement must meet the requirements of technical reference, A32-ES-CB-0063. For example, according to document JC-LP-29, the conditioning water: cement ratio must not exceed 0.47 and the percentage of dry resin per package must not exceed 4%.
For non-stable solids, especially certain types of filters, immobilization is with mortar, which forms a surrounding matrix that covers the residue and stabilizes it.
As a summary, the main criteria that package conditioning must meet are:
- Specific and global activity limits contained in the tables in annexes I and II.
- The maximum amount of complexing agents is 8%.
- The conditioned waste must contain organic liquids incorporated in the matrix above 3%.
- Conditioned waste must not contain substances which are pyrophoric or susceptible to strongly exothermic reactions.
- The waste is conditioned by incorporation into the matrix or wall of hydraulic binder.
- Voiding will be minimized. The filling rate for packages incorporated in a solid matrix will be 95% ± 5%. The filling rate for packages with a wall will be ≥ 98%.
- Free liquid in conditioned packages must not exceed 0.5% after setting.
- In general, hydraulic binder walls or matrices will provide the package with the minimum mechanical strength established in instruction 031-ES-IN-0011:
8.1 – Level 1 packages immobilized by incorporation into a solid matrix; the matrices must have an average compressive strength ≥ 3 MPa.
8.2 – Level 1 packages immobilized by a hydraulic binder wall. The wall must have a nominal thickness of 5 cm for filters and dispersible waste and an average mechanical strength of 7.5 MPa.
8.3 – Level 2 packages immobilized by incorporation into a solid matrix; the hydraulic binder matrices must have an average compressive strength ≥ 10 MPa.
8.4 – Level 2 packages immobilized by a hydraulic binder wall. The wall must have a nominal thickness of 5 cm for filters and dispersible waste and an average mechanical strength of 25 MPa.
- The contact dose rate should not exceed 100 mSv/h at the time of withdrawal.
- The surface contamination measurement on the outside of the packages must be less than 4 Bq/cm2 in β-emitters and 0.4 Bq/cm2 in α-emitters.
Below is a summary of the tests a conditioned package must be subjected to:
|HOMOGENEOUS AND HETEROGENEOUS SOLID WASTE IMMOBILIZED BY A HYDRAULIC BINDER|
|TESTS (Properties to be measured)||Level 1||Level 2|
|Absence of free liquid||Yes||Yes|
|Hydraulic binder uniaxial compressive strength||Yes||Yes|
|Hydraulic binder indirect tensile strength||No||Yes|
|Radionuclide diffusion through innovative material||No||Yes|
|Wall thermal cycling tests||No||Yes|
|Tritium diffusion (for concentration > 7.4 MBq/kg.)||No||Yes|
|ADR tests (on package)||Yes||Yes|
Specific application for conditioning of resins
The resin conditioning begins by reducing the amount of water incorporated into its structure. They are granular resins associated with the impurities of the retention process. These resins are normally a mixture of cationic and anionic resins with a ratio of 2:1 anionic to cationic parts.
Physically it is a divided solid with a moisture content around 50%. The type of cement used to condition these resins is of the type and category III/B 32.5 N/SR.
The process is carried out using a hydraulic binder. The drums used have a volume of 220 L and a maximum weight of 410 kg.
The fill rate is 95% ± 5%. The 220 L drums are designed and built according to the purchase specification for steel drums for radioactive waste and standards UNE 36563, UNE 36051, UNE 36086 and DIN 933/125.
The maximum external dimensions are 602 mm diameter and 870 mm in height without a lid. The construction material is carbon steel sheet with nominal thicknesses of 1.25 mm for the body and 1.5 mm for the lid.
They have an internal epoxy polyamide coating or a 20 µm phosphochromatizable primer and a 20 µm epoxy enamel finish.
The matrix properties are summarized below:
|Matrix property||Level 1||Level 1 and 2|
|Apparent resin volume made up with water||120 liters||90 liters|
|Resin||81.6 liters||61.2 liters|
|Free water||69.7 liters||81.2 liters|
|Cement||164 kg||191 kg|
|Free water/cement mass ratio||0.40-0.50||0.40-0.45|
The radiological properties of the level 2 matrix are specified with a surface dose rate always lower than 100 mSv/h. The rate must not exceed 10 mSv/h at 1 m from the package. For Level 1 matrices, the radiation rate is expected to be less than 6 mSv/h on contact and 0.5 mSv/h at 1 m.
Finally, a radiological homogenization test is conducted for compliance with the primary package acceptance specifications.
1.-Criterios aceptación de bultos primarios. ENRESA
2.-“Curso sobre gestión de residuos radiactivos 2009” CIEMAT. Ministerio de Ciencia e Innovación. ISBN: 978-84-7834-603-5
3.-CSN .Sede electrónica.
4.- V Jornadas de investigación y desarrollo tecnológico en gestión de residuos radiactivos. ISSN: 1134-380X. D.L.: M-34149-2004 Julio de 2004.
5.-“Control del proceso de solidificación de residuos radiactivos de baja y media actividad” Guia de seguridad nº 9.1 CSN.Madrid Julio 1991.