Technologies and Equipment for Reprocessing the VVER Spent Fuel at the Experimental Demonstration Center of the Mining and Chemical Combine

The Mining and Chemical Combine in Zheleznogorsk has been constructing the Experimental Demonstration Center (EDC) for technological advancement in the VVER-1000 spent fuel reprocessing. It incorporates:

  • a complex of hot cells to test innovative technologies for spent fuels reprocessing with a capacity of up to 5 t/year, 
  • a VVER-1000 SNF reprocessing plant with a capacity of up to 400 t/year.

Since 2015, the Sosny R&D Company has actively participated in development of the in-cell process and auxiliary equipment for the EDC test complex and the reprocessing line.

In 2015, Sosny developed a fully equipped SFA segmentation cell to incorporate it in the complex of the hot cells.

In 2017-2020, the Sosny R&D Company designed and manufactured process equipment for the following five divisions of the EDC reprocessing plant:

  • Fuel Assembly Chopping and Spent Fuel Voloxidation Division,
  • Spent Fuel and Radioactive Waste Preparation Division,
  • The 1st Extraction Cycle Division,
  • Clarification Division,
  • LHRW Vitrification Division.

 

EQUIPMENT FOR FUEL ASSEMBLY CHOPPING AND SPENT FUEL VOLOXIDATION DIVISION

The Division performs incoming visual inspections of the fuel assemblies and burnup and isotopics assays. The SFA top/bottom nozzles are cut off, and the fuel rods undergo the pressured cutting process. Then, the mechanical mixture of the structural elements and the spent fuel comes to the voloxidizer for oxidative recrystallization (voloxidation).

In the voloxidizer, the SNF oxidation separates the fuel from the claddings with simultaneous trapping of volatile fission products in the gas purification system. Then, the fuel powder mixed with fragments of the fuel hulls, spacer grids, and guide channels is fed to the separator drum. The voloxidized fuel powder separated from the structural elements in the separator drum is subject to cooling and dissolving (leaching) at the SNF Dissolution Division, while the structural elements undergo re-extraction of fissile materials in the cleanup tank at the SNF/RW Preparation Division.

The above processes required designing and manufacturing of the following equipment:

  • equipment for the incoming visual inspection, burnup and isotopic assay of the spent fuel,
  • an SFA chopping facility (GIT-1000),
  • a voloxidation facility.

The equipment for the SFA Cutting and SNF Voloxidation Division was designed and fabricated with a comprehensive safety analysis made, including a nuclear and radiation safety analysis. Following the review results, the IPPE Nuclear Safety Department issued a positive expert assessment report.

The SFA chopping facility serves to cut off the SFA top/bottom nozzles, chop the fuel rod bundle together with the guide channels and spacer grids at a given stroke (up to 40 mm), crush the fragments, load them into a storage hopper, and clean up the cutting equipment.

The facility incorporates:

The unique cutting blades designed to replace the chopping ones reduce the facility’s weight and size. Adjusting the cutting stroke extends the blades' service life, and remote replacing of the blade assembly significantly reduces the downtime.

 

  

Fuel Assembly Cutting and Fragmentation Facility

 

The voloxidation facility serves to recrystallize the spent nuclear fuel with a gas reactant at a temperature below 550 °C, remove volatile fission products, separate the SNF powder from the fuel claddings, filter light fractions of the voloxidized SNF powder to prevent a carryover to the gas purification system, cool down and load the fuel into a dissolver, and transfer the structural elements to the cleanup tank.

The Sosny R&D Company designed and fabricated the voloxidation facility internally. The facility incorporates:

  • a voloxidizer,
  • a fuel-clad separator drum,
  • a voloxidized fuel powder cooler with a rotary gate,
  • an I&C system,
  • mounting equipment and tools.

The power manipulator and ancillary equipment designed internally can handle the main components of the voloxidation facility. A sieve of the fuel-clad separator drum has increased longevity and can be replaced remotely with the power manipulator.

All electric drives are deployable outside the radiation hazard zone to enable their maintenance. An additional pin gear maintains the facility operable in case of an accident-caused failure of the primary rotary actuator.

The key advantages of the voloxidation facility are:

  • simultaneous downloading of all fragments of a fuel assembly,
  • nuclear safe design,
  • removal of the voloxidized spent fuel from the hulls by mechanical separation,
  • a unique long-lasting seal on the rotary shaft.

 

 

 

Voloxidation facility. Factory trials

 

EQUIPMENT FOR SPENT FUEL AND RADIOACTIVE WASTE PREPARATION DIVISION

The Sosny R&D Company designed and fabricated all the equipment for the Spent Fuel and Radioactive Waste Preparation Division in-house and performed safety analyses (including nuclear and radiation safety) that received a positive expert assessment.

The Division implements the entire technological cycle of the solid radioactive waste treatment and preparation at the Experimental Demonstration Center.

The cleanup equipment incorporating a set of tanks, pumps, and auxiliary equipment serves for additional extraction by leaching the remaining fuel from the hulls and structural materials after separation in the separator drum. The equipment serves to:

  • receive the chopped contaminated hulls, spacer grids, and guide channels of one fuel assembly from the separator drum,
  • dissolve the remaining spent fuel in a nitric acid solution at a temperature of 115 °C with the agitation by partial decantation in the interim tank,
  • load clean structural materials into SRW barrels,
  • transfer the fuel solution to the primary dissolution line of the Dissolution Division.

The design of the cleanup equipment solves the issue of cooling the contaminated structural materials down to a temperature that excludes intensive evaporation induced by the acid flow. It also ensures heating and maintaining the operating temperatures of the nitric acid and leaching liquids.

The SRW drying equipment removes water and aerosols from the container with clean chopped fuel rod claddings, spacer grids, guide channels of the fuel assembly, and remaining insoluble structural materials from the Clarification Division. The drying operation prevents hydrogen accumulation and ensures explosion safety during the long-term storage of radioactive waste.

The Spent Fuel and Radioactive Waste Preparation Division also houses the following equipment:

  • hoisting and transfer equipment for loading spent cartridges of the gas purification system into SRW barrels,
  • welding equipment for barrel lid sealing and helium leak testing of the welds,
  • SRW barrel decontamination equipment to ensure safe preparation for the transfer to the HLW vault,
  • a transfer system consisting of handling equipment, rail-guided carriages, and transfer hubs. The system continuously supports the entire SRW reprocessing cycle up to 300 days a year,
  • an I&C system for the Spent Fuel and Radioactive Waste Preparation Division.

 

Solid waste drying machine

Solid waste tank welding machine

Transport system elements

 

 

EQUIPMENT FOR THE 1ST EXTRACTION CYCLE DIVISION

The 1st Extraction Cycle Division is the core of the Experimental Demonstration Center intended to extract fuel components from the clarified SNF solution, size the target products, and purify them from fission products and impurities.

It includes the following equipment:

  • an annular pulsed reciprocating plate continuous extraction column,
  • clusters of centrifugal extractors,
  • clusters of mixer-settlers,
  • separators for separation of extracts from aqueous microemulsion,
  • uranium catalytic reduction columns with a platinum catalyst,
  • tanks and vessels,
  • heat exchangers,
  • metering pumps and force pumps,
  • auxiliary equipment,
  • mounting equipment and tools;
  • an I&C system for process control, monitoring, and ensuring the safe operation of the facility.

An innovative solution is the clusters of centrifugal extractors and an annular pulsed reciprocating plate continuous extraction column. The centrifugal extractors increase the productivity of the Division. At the same time, the radiation exposure for the organic extracting agent is drastically reduced, thus decreasing its degradation. The use of the annular pulsed reciprocating plate continuous extraction column avoids electromechanical drives that require regular maintenance and replacement; it significantly decreases operating costs. The extractors can operate both in simultaneous and serial modes.

The clusters of centrifugal extractors resolve the vibration rigidity problem and implement a feature of remote retrieval and transfer of the rotor components to the maintenance area for replacement. All actuators are deployed outside the radiation hazardous area. 

In designing, the issues related to the selection of corrosion-resistant structural materials, tightness of junctions, maintainability, and interchangeability of the standard modules placed in canyons and hot cells were addressed.

 

Centrifugal extractors. Factory trials

Local control system

 

 

EQUIPMENT FOR CLARIFICATION DIVISION

Clarification is a critical operation in the SNF reprocessing that directly affects the steady operation of the Extraction Division.

The SNF nitrite solution coming from the dissolution phase contains both target and non-target products, i.e. undissolved fuel rod fragments, corrosion products,soluble suspended solids, platinum-group intermetallic sediments, molybdenum, zirconium, barium, and silica compounds. The input concentration of these products makes it virtually impossible to implement the "nitric acid-extragent-solvent" extraction of uranium and plutonium.

The removal of the insoluble sediments and impurities from high-concentration SNF solutions rests on the principle of holding the solution at a pre-set temperature with a flocculant added during preparation for centrifugation and double variable-speed centrifugal clarification. The 1st cycle centrifugation removes the sediments with a density higher than that of the input solution, while the 2nd cycle removes the sediments with a density equal to or less than that of the input solution.

The clarification equipment comprises the following equipment prototypes:

  • a vertical decanter centrifuge with scroll discharge of the solids from the rotor to separate them from the SNF solution,
  • a decanter centrifuge with the discharge of the solids by gravity to separate fine suspended solids and clarify the SNF solution.

The clarification process employs the following equipment developed and fabricated for the EDC reprocessing plant:

  • vessels and tanks, including nuclear safe ones, to receive and collect centrates, leaching liquids, suspensions, and sediments;
  • pumps to feed the SNF solution into the centrifuges at a preset flow rate;
  • tanks to feed leaching liquids and cooling solutions into the centrifuges;
  • mounting equipment and tools;
  • an I&C system.

The equipment and piping are furnished with instrumentation and control means for such process variables as temperature, pressure, flow rate, activity and neutron radiation, electric conductivity, solution levels in the tanks.

The equipment is classified according to the safety requirements of NP-016-05 regulations.

 

Clarification vessels

 

 

EQUIPMENT FOR LHLW VITRIFICATION DIVISION

Vitrification is a way to immobilize the liquid high-level waste (LHLW) resulting from SNF reprocessing by converting radionuclides and hazardous chemicals from solutions into a solid form, i. e. glass with high thermal, mechanical, radiation, and chemical resistance.

A cold crucible induction furnace (CCIF) serves as a melter. The CCIF's operational principal is a single-stage process comprising a sequence of the following chemical reactions: dehydration of liquid radioactive waste, nitric acid cleavage, denitration and calcination of salts, sintering of HLW oxides with fluxing additives, production of borosilicate glass as the final product.

The input products are pre-evaporated high-level liquid waste that comes into the buffer tanks as evaporated raffinate from the first extraction cycle, technetium re-extract, and evaporated zirconium-containing solution.

The final product is high-level borosilicate glass placed in air-tight cans for long-term storage.

An LHLW immobilization facility was designed and fabricated to incorporate the vitrification in the core technology at the EDC. It includes:

  • vessels and tanks,
  • a vitrification CCIF furnace,
  • furnace cooling equipment,
  • gas purification equipment,
  • vitrification cell equipment,
  • pumps and metering equipment,
  • mounting equipment and tools,
  • remote maintenance equipment,
  • transport and hoisting equipment, including manipulators,
  • I&C system for control, monitoring, lighting, and video surveillance.

The equipment is classified according to the safety requirements of the NP-016-05 regulations.

 

Vitrification CCIF Furnace

Glass Frit Metering Unit

 

Turntable

 

 

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