Treatment of effluents in the decommissioning of a nuclear power plant

SECTIONS

• Decomissioning
• Operation
• Treatment of waste from the primary circuit
• Treatment of other liquid waste
• Treatment of gaseous effluents
• Treatment of radioactive solid waste
• Example of the treatment process in a decommissioning

DECOMMISSIONING

The decommissioning of a nuclear power plant is the stage that brings to a close the useful life cycle of the facility and entails the obligation to leave the facility in safe state and monitor it, especially in terms of radiation.

It is important to define milestones, alternatives, technologies, investments and decommissioning techniques. Currently, due to the sequence of scheduled start dates of nuclear power plants in Spain, we are approaching the end of the useful life of some of these facilities, including extensions to their end dates, and so it is essential to plan for this right now.

The competent bodies in Spain in charge of managing the decommissioning of a nuclear power plant and deal with the waste that this produces are the Consejo de Seguridad Nuclear (Nuclear Safety Council) and ENRESA. World-wide, the decommissioning process has already started. Based on the information supplied by the CSN (initials in Spanish of the Nuclear Safety Council), with regard to low and medium activity waste, we can summarize the processes already under way in the following table:

Source: OECD Nuclear Energy Agency Report “Decomissioning of nuclear facilities: Feasibility, Needs and Costs”, París 1986

This volume of low and medium activity waste calls for processes that enable the volume of the waste to be reduced for reasons of safety and economy. In these processes of concentration, the industrial processes of evaporation and crystallization play a crucial role.

Specifically, in the EU there are some 95 large nuclear facilities of which 39 are nuclear power plants, 25 are reactors that do not generate electricity and 32 are nuclear facilities that are mostly involved in the fuel cycle.

The power plants located in Spain are at the mature production stage and in some cases their productive life has been extended. In any case, we are approaching the point where decommissioning processes will have to begin and these have to be scheduled and studied, as shown in the following graphic:

The termination of the active life of a nuclear power plant may be due to many factors: economic, the owner’s interest, technological (useful life), safety reasons, etc.

The end of a nuclear power plant’s activity does not mean the end of the risk of exposure to ionizing radiations, due to the neutronic activation processes of the materials, which is induced by fragments from fission, and which have affected elements such as the concrete, the steel, the cooling circuit, the steam generators, fuel storage pools, primary chemical and volumetric treatment circuits, etc. It is essential to conduct a radiological planning for the decommissioning process.

Three categories of waste are produced in a nuclear power plant: operation waste (low and medium activity), waste to the spent fuel (highly active), and decommissioning waste (low and medium activity). The management of highly active waste is highly specific and calls for its own methods of treatment.

This article sets out to focus attention on certain decommissioning operations which, due to their characteristics, are within the scope of CONDORCHEM ENVIROTEC’s skill set and previous experience,such as the processes of evaporation, filters and technology associated with the minimizing of waste.

Shutting down a nuclear power plant means withdrawing it from service in a safe manner and the reduction of the activity of waste to levels that allow the production process to be terminated without restrictions of the site and the termination of the licenses from the competent authorities. The IAEA (International Atomic Energy Agency) defines three levels in the process of decommissioning a nuclear power facility:

• Level 1: Supervised closure of the site
• Level 2: Partial and conditional use of the site
• Level 3: Unrestricted use of the site

Furthermore, three processes are undertaken when decommissioning a power plant:

Process 1: Supervised closure of the facility, disposal of the spent fuel and radioactive waste from the operation.

Process 2: Elimination of the radioactive elements outside the containment enclosure, as well as the conventional structures and elements, which are to be stored and sealed in the containment enclosure,the components with the most specific activity. The containment building may or may not be buried. The site is available for use subject to restrictions.

Process 3: Complete dismantling and demolition of the structures, restoring the site to use without restrictions.

OPERATION OF A NUCLEAR REACTOR

A PWR (Pressurized Water Reactor) uses water as the coolant for the reactor core. The operating conditions are between a pressure of 150 bars and temperatures in the region of
370oC. The primary water circuit, which cools the reactor core, is in contact with the zirconium alloy rods which contain inside the UO2, enriched by 4% with U-235. Once the fission process starts, a flow of neutrons in operation is produced of 3.1013 cm-2. s-1.

This flow of neutrons needs to be moderated. The fission processes of the fuel itself and the flow of neutrons generates heat that must be extracted using a coolant (water) contained in a primary circuit.

When the activity of the power plant comes to an end, the burnt fuel remains inside the zirconium alloy rods. This fuel will be removed and stored in the fuel pools so that it can be cooled; subcriticality is controlled with borated water and the process of decay of some of the radionuclides present is likewise controlled. In addition, the transfer of the spent fuel to a pool entails temporary storage under the appropriate conditions of safety due to the water barrier.

Various chemical substances can be identified in the chemical composition of the waste that circulates in a primary circuit as coolant in a WPR reactor.

a) Chemical substances from the fission fragments. This waste, which is generated in fission reactions, may escape through cracks in the fuel rods and reach the coolant. The isotopes Cs-137, Sr-90 and I-129 are among the most important of these. They are highly reactive waste products.

b) This group of waste products also contain impurities that come from the zirconium alloy construction, which may have been activated by the flow of neutrons., Waste products are generated by nuclear capture and feedback reactions.

c) Chemical substances from transuranium elements. Waste of this type comes from the radioactive progeny of Uranium-238 and Uranium-235. Within this group, Pu-239 is important
for its long life: 29,400 years, and its toxicity. They are also highly reactive waste products. This type of waste, normally held inside the rods, can also escape through cracks and reach the primary circuit.

d) Products of radioactive structural elements. The primary circuit contains some 10,000m2 of interchange surface area made from Inconel steel (75% Ni, with the main impurity being Co-59). These structural elements become radioactive with the flow of neutrons and produce low and medium radioactive isotopes that are shed from the steel structure by corrosion (the corrosion rate is 2 mg/dm2 month). The presence of H2O2, leads to the oxidized form, which is then captured by the exchange resins. C-14 is formed due to activation of the air(CO2) dissolved in the coolant liquid. H-3 emerges as a result of activation of the hydrogen present in the water, in the LiOH (which regulates the pH) and the boric acid protons.

The following substances can also be detected as low and medium activity waste, most of them generated by neutron capture or activation of the structure:

We can highlight the following as highly radioactive substances that will need to undergo treatment:

The presence of B-10 in the coolant must be taken into account, its purpose is to moderate the flow of neutrons. This element is transformed into Li-7, which is stable, by neutron capture. The larger amount of low and medium activity waste is produced in the primary circuit. These are evacuated by a system of cation interchange resins. LiOH is used to regulate the pH and the neutronic activations of the oxygen and hydrogen from the water.

Treatment of waste from the primary circuit

Apart from this, the primary circuit of a PWR reactor has a purification system so that about 17,000Kg of the 175,000 Kg of coolant in circulation is extracted for treatment and sent to a purification subsystem, with the objective of treating the activation elements (Co-60, Mn-54), some of the fission products diffused through the rods (137Cs, Sr-90) and specifically recover and modulate the boron concentration. This entire process is conducted by means of the chemical and volumetric system.

Treatment of other liquid waste

The main liquid effluents that are treated are:

• Drainage of equipment.
• Drainage of soil.
• Spaces controlled from the primary circuit to de-gas and purify the circuit.
• Purging the steam generator.
• Decontamination and washing processes and laboratories.

The low and medium activity liquid effluents are collected and stored in two tanks.

a) The laundry, showers and decontamination washing water.

b) The tank for floor run-off water, highly active liquid waste, elutions from recoveries of boron and the purging of steam generators.

The liquid effluents are sent to tanks under volumetric control having passed through filters and based on their conductivity characteristics they are treated with ionic exchange resins. This process begins with a decrease in temperature of the effluent, and it is then sent to a mixed bed resins system, to retain the various ions present in the coolant. The specific activity of the coolant in the primary circuit must be less than 37 MBq/Kg ( 1μCi/g) in an equivalent dose of I-131. The cationic bed in the form of Li-7, which is highly acidic, will retain Co, Cs, Sr, Ni, Mn.

The anionic bed in the ionic form OH – will retain iodine. Subsequently, the effluent is sent to a boron recovery system, with the aim of retaining the species H2BO32-, and moderating the concentration of boron in solution. Finally, the effluent is sent to volumetric control tank where it is re-inserted in the primary circuit.

Another system of resins will process the purges from the steam generators that belong to the secondary circuit. The resins and elution water are treated with pump injection convection evaporators to reduce the volume and concentrate the solids.

The condensate is sent to control tanks and the gases are treated in retention tanks to reduce the hydrogen and diminish the radioactive isotopes with short lives. In the evaporators, radioactivity concentration values of between 10 and 50 are reached, with decontamination factors of 104 and 105.

These concentrated solids are treated with cement and agglomerant and are placed in 220 -liter drums for further management. In every nuclear power plant there are the STS (Standard Technical Specifications), subject to inspection by the regulator, which set out the operational restrictions of the effective equivalent dose due to the total effluents, which is 100μSV / year, and which must be distributed among the liquid and liquid and gaseous effluents.

In the event of discharges into the environment, according to the specifications of 10CFR20, the discharge must comply with:

Treatment of gaseous effluents

The gaseous waste products in a PWR come from:

– Purges of steam from the primary circuit
– Degasification of the primary circuit
– Expansion of coolant when it heats up
– Drainage and leaks from the reactor building
– Ventilation of potentially contaminated buildings

The treatment of the gaseous effluents is carried out using different techniques:

a) Active carbon adsorbents

These are the most satisfactory devices for retaining gaseous fission products in nuclear reactors. They consist of tightly packed beds of carbon granules. The most usual application is the retention of all radioactive material both in the elemental and organic forms; they are also effective for the removal of noble gases. They are positioned adjacent to highly efficient filters and immediately after them sequentially.

b.-Retardant storage

When there are short-lived radionuclides, it is advisable to delay the emission into the atmosphere of radioactive gaseous effluent, so that their activity decreases, resulting in minimal discharges of noble gases. With 35-40 day retentions, all the isotopes of Kr and Xe are eliminated, except for some Xe-133 and Kr-85 that are not altered. Active carbon beds and HEPA filters are retardant zones. Pipes or tanks are also used to delay the emission. Retarding storage.

c.-Filtration

Gaseous effluents contain particles in suspension that are too fine to be retained in normal filters, using HEPA (High Efficiency Particulate Air) filters with an efficiency of 99.97% for particles smaller than 0.3 microns. To prolong its life, pre-filters (normal filters) and humidity separators are installed. The HEPA filter is the most important one in the extraction system, and is very reliable. They are positioned where the concentration of particles is highest. The filtration and adsorption elements are placed in drums and managed as radioactive waste.

Treatment of radioactive solid waste

Solid waste may come from the treatment of liquid effluent or gaseous products (resins, filters, concentrates, sludge) or from processes that have resulted in superficial contamination (cladding, solids, etc.) or the activation of components in zones of high radiation. The treatment consists in the immobilizing and confinement of the waste to facilitate its transportation and prevent migration or dispersion of radionuclides by natural processes.

Drums for low and medium active waste must meet a series of requirements to be accepted in a definitive storage facility. These requirements establish two levels of waste: Level 1 (low activity) and Level 2 (medium activity):

Example of the treatment process in a decommissioning

One of the elements that is addressed in the decommissioning process of a nuclear power plant is boric acid. Boric acid is used as a moderator to regulate the reactivity of the reactor.

This moderation of the nuclear reaction in the primary circuit is carried out using a system of basic resins. This is not the only use of boric acid, however. In order to prevent an event in which the reactor core is without coolant, all plants have auxiliary tanks with borated water, which, in a situation where the plant is compromised by the lack of coolant, the auxiliary water can regulate the reactor’s activity.

There is a third scenario in which borated water is used. This is the cooling of highly active waste in temporary storage pools. In total, a PWR has some five tonnes of boric acid to treat. These solutions have to be concentrated and crystallized before they can be removed in a decommissioning.

Conclusions

In a relatively short period from now, Spain’s nuclear power plants will have to undertake decommissioning processes to make the sites safe. This decommissioning, which will be conducted at three different levels, will call for industrial technologies to facilitate a reduction in the volume of the waste and its subsequent environmental management.

The innovative processes of evaporation and crystallization enable the optimally effective concentration of waste. Primary and secondary effluents,purges from the steam generators, boric acid deposits, auxiliary deposits…there is an extensive field of work for waste management technologies. A particular field of work has to do with the effluent that comes from the treatment of boric acid.

Bibliography

– Curso sobre Gestión de residuos Radiactivos. Serie Ponencias. ISBN: 978-84-7834-603-5
– Minutes of Inspection published by the CSN.
– References published by the CSN. REF..- CSN/PDT/CNVA2/VA2/1005/241 SUPPLEMENT 2