Large-Scale Purified Inert Atmosphere Cell for Pyrochemical Treatment of Spent Nuclear Fuel

The Siberian Chemical Combine has been building an experimental base for testing prototype pilot production equipment for the spent fuel pyrochemical treatment and verifying design concepts of inert atmosphere cells and support systems.

In 2018–2022, the Sosny R&D Company developed and manufactured the cell, equipment, and operation support systems for it.

The basic element is a double-wall inert atmosphere cell with a volume of 72 m3, an inert gas purification system ensuring oxygen and water contaminants less than 5 ppm, and a cooling system.

It also incorporates a power manipulator, a glove box, an airlock chamber for transferring large-size equipment, an airlock for transferring materials and small-size equipment to the cell and the box, and other ancillary equipment and engineering system.

Regulations and rules require that shielded containments have negative pressure to prevent a gas flow from "contaminated" room into "clean" ones in case of leaks. Implementing this requirement along with the internal high-purity inert atmosphere requirement entails labor-intensive efforts to ensure air-tightness of the process equipment during the manufacture and operation. So, we took a decision to design a double-wall cell. The wall-to-wall gap and the internal volume of the cell are filled with inert gas, argon. The gap is filled up with the gas only once, during the commissioning of the cell. Whatever the operation mode, the gap is always under excessive pressure. A dedicated valve releases the surplus (redundant) gas into the atmosphere.

The cell has a rigid frame of channels that ensures its physical endurance and steadiness. It has a rigid frame of channels that ensures the physical endurance and steadiness of the hot cell. The external and internal surfaces of the frame are coated with corrosion-resistant steel plates. For technological purposes, the containment has feed-through ports and viewing systems.

The feed-through port serves as an inlet for power cables, signal electric lines, and gas supply. The feed-though ports are designed as caissons. Similar to the double-wall containment, the feed-through ports are pressurized with inert gas.

 

General view of the inert atmosphere cell: 1 – airlock chamber; 2 – inert atmosphere cell; 3 – feed-through ports; 4 – windows; 5 – airlock; 6 – glove box

 

The inert atmosphere cell during installation

 

The major purpose of the airlock chamber is loading and unloading large-size equipment with the quality of the internal atmosphere maintained. For the loading operations, the airlock chamber has gas-tight doors, an equipment transfer system, a vacuum system to maintain the residual pressure below 10 Pa, and an infrared heating system.

The glove box is a non-process unit that can operate with air or high-purity inert gas. It is intended for experimental and analytical work.

Materials and tools are transferred between the cell and the glove box through the airlock.

 

Glove box

 

The argon recirculation system maintains the required purity of the inert gas in the cell and the glove box. It is a closed system.

The basic elements of the argon recirculation system are three argon recycling units that purify argon from oxygen and water. The inlet and outlet pipelines of these recycling units allow simultaneous, sequential, or parallel operation of one, two, or all of them. In parallel, we can purify the internal gas atmosphere and purge the cell and the glove box.

The argon recycler operates in the purge or regeneration modes. In the purge mode, argon with oxygen and water flows to the argon recycling unit at a rate of 90 m3/h.

Purification of argon from oxygen takes place in the reactor with copper-based chemisorbent R3-11G. The chemisorbent absorbs oxygen because of copper oxidation. The reactor has a heater to maintain the optimal temperature of the chemisorbent. The oxygen concentration in the purified argon is below 1 ppm.

A reactor with NaA-type artificial zeolite removes water from argon. The capacity of zeolite is over 1 kg of water, with up to 2 ppm of water in the purified argon.

Maintaining the inert atmosphere of the required quality with argon recycling units was verified experimentally.

 

Argon recycling unit

 

The in-cell gas cooling system is a dual-circuit closed system.  The primary coolant is an aqueous ethylene glycol solution in the primary heat exchangers installed in the cell. The ethylene glycol heated inside the cell flows to the outside secondary heat exchanger. Freon R404a is the secondary coolant that gives off heat to the environment through the radiator outside the room.

The cooling system removes heat of up to 30 kW from the inert atmosphere cell.

Bench tests verified the proper performance of the system.

The main purpose of the medium change and pressurization system is changing the internal gas composition when transiting from one operation mode to another and maintaining the preset pressure.

Several operation modes are set for the cell:

Mode 1. The cell is filled up with atmospheric air. The cell stays in this condition after the manufacture and during scheduled maintenance operations.

Mode 2. Substituting the atmospheric air by the inert gas.

Mode 3. The hot cell is filled up with the inert gas. The basic operation mode.

Mode 4. Substituting the inert gas by the atmospheric air. The mode enables transiting from the basic operation to the maintenance mode.

In modes 1, 2, and 4, the system has a permanent connection with the active ventilation system and supports a continual gas feed or discharge. The air and argon flow to the cell through the pressure regulators connected to corresponding high-pressure pipelines.

Mode 3 allows refilling and evacuation of surplus gas in relatively small amounts.

At the initial stage of operation, the cell will be used for the technology adjustment without involving nuclear materials or radioactive substances, including:

  • changing the composition and maintaining the specified parameters of the in-cell atmosphere under steady-state and transient conditions of operation;
  • loading the equipment in the cell with testing the vacuum and thermal degassing of different materials in the airlock chamber;
  • installation of the pyrochemical equipment on the in-cell bench with a power manipulator, the remote partial assembling/disassembling of the process equipment;
  • remote connection/disconnection of electric and gas lines in the feed-through ports and process equipment;
  • remote replacement of components for the ancillary equipment and systems (TV cameras, lights, filters, thermocouples, etc.).

The performance-based findings will provide a framework for the development of typical solutions for the pyrochemical technology involving the closed nuclear fuel cycle in the new-generation nuclear industry, and for the industries requiring large-size high-purity inert atmosphere cells.

 

 

 

 

 

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