Preamble

Research Status and Progress of Mobile Hot Cells
Zhu Haoyuan, Wang Huacai* (China Institute of Atomic Energy, Beijing 102413)

Abstract: The mobile hot cell is a radioactive material handling facility designed for integrated transport and rapid reassembly at target sites. Through modular design, it addresses the flexibility and mobility limitations associated with traditional stationary hot cells. With the development of the nuclear industry, the demand for radioactive material handling and conditioning operations in nuclear facilities continues to grow. While traditional stationary hot cells are technologically mature, their fixed nature and high costs restrict their application scope. As an innovative nuclear facility, mobile hot cells offer advantages such as portability, dismountability, high mobility, and cost efficiency, enabling rapid response to inspection and processing needs in nuclear facilities.

This paper reviews the developmental history and iterative evolution of mobile hot cell facilities with diverse structures worldwide, examining the current research status regarding shielding methods, viewing windows, transport containers, and other components. The article identifies that future development trends for mobile hot cells include sealed interconnection of multiple modules, highly integrated efficient shielding, and intelligent operation and maintenance.

Keywords: hot cell, radiation shielding, mobile hot cell

1 Introduction

A hot cell is a shielded enclosure for conducting experiments and operations with highly radioactive materials, isolated from the surrounding environment to protect personnel from radiation hazards. Hot cell facilities play crucial roles in spent fuel inspection, irradiated material performance verification, and radiochemical experiments. Traditional hot cells are typically constructed within nuclear facility buildings using thick heavy concrete walls for shielding. When handling radioactive materials, operators observe the cell interior through viewing windows and cameras while operating master-slave manipulators to manipulate equipment or samples inside. A series of auxiliary systems ensure operator safety and prevent radioactive material leakage. Hot cell facilities are now widely used globally, with the number of post-irradiation examination hot cells in selected countries shown in Table 1 [TABLE:1].

When no supporting hot cells are available near a nuclear facility, radioactive materials must be packaged and transported to hot cells in other locations. However, some nuclear facilities in remote areas face transportation difficulties, making it challenging to respond quickly to inspection needs. To address this problem, mobile hot cells have emerged. Unlike traditional stationary hot cells, mobile hot cells employ relatively lightweight, detachable walls as their main structure. They can be disassembled into modules, transported to designated locations, and reassembled, offering greater mobility and flexibility. Compared to traditional fixed hot cells, mobile hot cells have lower construction costs, can rapidly respond to inspection and processing demands, and can provide radioactive operation and inspection environments for nuclear facilities lacking hot cell construction conditions, such as remote nuclear reactors, nuclear reactors on offshore drilling platforms, and non-stationary nuclear facilities like marine reactors and nuclear submarines, as illustrated in Figure 1 [FIGURE:1]. A structural schematic of a mobile hot cell is shown in Figure 2 [FIGURE:2].

2 Main Structure

Mobile hot cell design must first clarify functional requirements. Functionally, existing mobile hot cells fall into two main categories: waste radiation source conditioning hot cells and post-irradiation examination hot cells. The former includes the mobile hot cell designed by IAEA in collaboration with South Africa's Nesca [9] and the movable waste radioactive source conditioning device designed by the China Institute for Radiation Protection [5]. These are primarily used for collecting and conditioning discarded industrial or medical radioactive sources, transferring them from original storage containers to new containers suitable for transport and storage, before shipping them to qualified facilities for centralized storage and disposal, thereby reducing risks to public safety and environmental contamination. The latter category includes the mobile shielded inspection hot cell designed by China's Nuclear Power Institute [2], which is mainly used to temporarily construct a hot cell with component inspection capabilities for nuclear power plants and other facilities during emergencies.

The main structure of a mobile hot cell consists of a base, outer walls, shielding modules, viewing windows, master-slave manipulators, and other primary unit modules, together with auxiliary systems including video monitoring, radiation monitoring, transfer systems, and ventilation systems.

2.1 Shielding Walls

Shielding design must first identify the types, quantities, sizes, and shapes of radionuclides to be handled, as well as the anticipated activity types and radiation levels of the radioactive materials. Primary radionuclides considered include actinides such as U, Pu, and Np that emit α particles and neutrons; 90Sr, 90Y, 137Cs, and 60Co that emit β radiation; and 137Ba and 60Co that emit γ radiation [16]. Post-irradiation examination hot cells mainly handle inspection of irradiated fuel, cladding, and structural materials, while waste radioactive source conditioning hot cells primarily process radioactive isotopes. Shielding structures and transfer channels should be designed according to specific handling requirements. Based on the type of radiation shielded, hot cells can be divided into two categories: β-γ hot cells and α-β-γ hot cells. Both types can shield β, γ, or neutron radiation, but β-γ hot cells are relatively sealed, while α-β-γ hot cells require complete sealing with higher sealing standards. Mobile hot cell design sealing standards should refer to EJ/T 1096-1999 "Sealed Enclosure Classification and Test Methods" and EJ/T 1108-2001 "Design Principles for Sealed Enclosures."

Traditional shielding materials include water, lead, concrete, and stainless steel. Water is an easily obtainable, low-cost material with high hydrogen content that effectively shields neutrons, but its low density provides poor attenuation of γ rays. When applied to mobile hot cells, it is best used in combination with other high-density shielding materials. Lead, with a density of 11.3 g/cm³, is a high-density shielding material that effectively shields γ rays but provides poor neutron shielding. Its heavy weight may create difficulties in mobile hot cell assembly and transport. Iron and stainless steel, characterized by high density and good mechanical properties, are widely used in hot cell and reactor design. The mobile shielded inspection hot cell designed by China's Nuclear Power Research Institute uses cast iron shielding modules [3]. Sand is primarily used as a shielding material in radioactive source conditioning mobile hot cells, offering advantages of low cost, easy availability, and convenient installation and recycling. The shielding effectiveness varies with sand type and particle size. Shen Fu et al. [4] compared the shielding performance of sands with different densities, finding that higher-density sand provides better shielding, and that sand with saturated water content offers stronger shielding capability than dry sand.

Composite materials made by mixing multiple components also find wide application in radiation shielding. Boron steel, often used for spent fuel storage, provides combined neutron and γ-ray shielding, though boron has extremely low solubility in steel, and excessive boron content adversely affects mechanical properties such as ductility. Lead-boron polyethylene composites offer low density and strong comprehensive shielding capability, effectively reducing shielding weight, but suffer from insufficient structural strength and poor corrosion and radiation resistance. Epoxy resin is a widely used polymer radiation shielding material across various fields and represents a hot research topic both domestically and internationally. By incorporating boron, lead, glass fibers, and other components, the shielding capability and mechanical strength of epoxy resin can be further enhanced. Other materials such as nanomaterials and rare earth elements represent important future research directions [17].

Hot cell viewing windows are typically classified as solid windows, liquid windows, or solid-liquid hybrid windows. Solid viewing windows usually consist of multiple lead glass panels coated with anti-reflective materials and are widely used in stationary hot cells due to their reliability. Liquid viewing windows fill the space between hot and cold face glasses with radiation-resistant liquid, typically zinc bromide solution. Their advantages of easy replacement, recycling, and transport make them the most commonly used type for mobile hot cells. Muhammad Hannan Bahrin et al. [6] characterized zinc bromide as a liquid radiation shielding material for viewing windows, obtaining linear attenuation coefficients for ZnBr₂ solutions of different densities (Table 2 [TABLE:2]). Using a 1000 Ci ¹³⁷Cs source, they measured dose rates on the cold side of mobile hot cells with different solution viewing windows, with results shown in Table 3 [TABLE:3].

The results demonstrate that water as a shielding material significantly reduces dose rates compared to no shielding (air), while ZnBr₂ solution with a density of 1.6 g/cm³ further reduces the dose rate by 357 times compared to water, confirming ZnBr₂ solution as an excellent viewing window shielding material. It should be noted that ZnBr₂ solution viewing window tanks should avoid ferrous materials, as contact causes the solution to become turbid, affecting visibility.

Solid-liquid hybrid viewing windows consist mainly of high-density glass with smaller thicknesses than solid glass windows, with oil filling the gaps between glass panels. They are suitable for shielding mixed neutron and gamma radiation sources.

2.2 Manipulators

Manipulators typically consist of a master arm (active arm) and a slave arm (passive arm) connected through a wall penetration assembly. The master arm is installed in the cold area of the operator zone, while the slave arm is installed in the hot area inside the hot cell. This design ensures operators can manipulate objects inside the hot cell from a safe area. Depending on operational requirements, manipulators should have sufficient degrees of freedom to complete various tasks. For example, a manipulator may have 7 degrees of freedom to achieve complex spatial movements. The end-effector directly contacts the manipulated object and should be designed according to specific tasks. For grasping and handling tasks, the end-effector should have appropriate shape, size, and gripping force. Relevant standards for manipulators include EJ/T 566-91 "General Technical Conditions for Master-Slave Manipulators." A schematic diagram of the manipulator structure is shown in Figure 3 [FIGURE:3].

2.3 Transfer Containers

The transfer channel of a mobile hot cell is used to receive radioactive materials into the hot cell interior. Its characteristics and dimensions should be designed according to experimental requirements, adapting to the shape and size of samples to be transferred. The transfer container and channel should be designed as a matched set to ensure sealing and safety during docking. Fixed rails and supports can be used to carry transfer containers and guarantee docking precision, while ensuring the container itself possesses adequate risk resistance. To withstand impacts and drops without damage or radioactive leakage, containers typically adopt a multi-layer structure. Both the container and hot cell feature end caps or valve designs to prevent direct contact between the hot and cold sides.

A schematic diagram of container-hot cell docking is shown in Figure 4 [FIGURE:4]. Containers for waste radioactive source conditioning hot cells are designed primarily for long-term storage, collecting individually stored sources for centralized storage and management at specific locations. Therefore, containers must be capable of long-distance transport and materials must withstand prolonged radiation exposure. The Long-Term Storage Shield (LTSS) from UK's RWE NUKEM [9], for example, features four compartments for source storage with a maximum designed capacity of 10 kCi [9]. Transfer containers for post-irradiation examination mobile hot cells generally do not require long-distance transport, but because nuclear fuel is α-radioactive, containers must meet higher sealing standards. The modular fuel rod transfer device designed by China's Nuclear Power Institute for mobile shielded inspection hot cells uses lead as the main shielding material for γ-ray attenuation. The shielding body adopts a through-wall design with end caps at both ends and an interlock device to connect with the hot cell transfer channel, ensuring the container end cap can only be opened after tight docking. The container opening features a z-shaped shielding structure, and since fuel rods are extracted from water pools into the container, a drainage channel is designed to remove water from the cylinder.

3 Future Prospects

As a specialized radioactive protection facility, mobile hot cells have broad development prospects. With increasing global demand for clean energy, nuclear power is receiving greater attention as an efficient, low-carbon energy source. Many countries are actively promoting nuclear power plant construction and development, which will drive growth in related industries, including mobile hot cell research, production, and application.

3.1 Lightweighting

The continuous emergence of novel radiation protection materials will provide strong support for mobile hot cell performance improvement. For example, high-performance composite materials with advantages of light weight, high strength, and effective radiation shielding can be used to manufacture hot cell walls and shielding doors, reducing weight and improving mobility and flexibility. Li Honghui et al. [14] developed a vehicle-mounted low-activity waste radioactive source conditioning device using a container-type sealed structure, making the entire device more lightweight and improving rapid transport capability. The device is equipped with conditioning equipment and auxiliary systems to ensure safe and controllable radioactive material operations.

3.2 Intelligentization

With continuous development of artificial intelligence and Internet of Things technologies, mobile hot cells will achieve higher levels of intelligence. Establishing digital twin models for hot cells and mobile hot cells is currently a hot research direction [24]. Digital twin intelligent control systems can enable real-time monitoring and regulation of hot cell environments, including precise control of temperature, humidity, radiation dose, and other parameters. Future mobile hot cells will be more widely equipped with advanced remote monitoring systems and operation interfaces, allowing operators to monitor various parameters and exercise precise control from safer distances using terminals such as mobile phones and computers. This will not only improve operational convenience but also minimize operator exposure time in radiation environments, safeguarding personnel health.

Zheng Tianyu [26] proposed building a hot cell digital twin system to construct a virtual reality simulation model in virtual space, enabling operators to observe hot cell information from multiple angles. Based on 6-DoF pose information of objects, target detection algorithms such as Yolo and RCNN and path planning algorithms are introduced to achieve collision detection and motion path planning for robotic arms, providing operators with precise operation guidance for more efficient and accurate manipulator activities. Guo Peng [27] designed a virtual reality (VR) simulation system based on Unity3D software, which realized real-time reconstruction of important hot cell environment elements in the VR simulation environment. As technology advances, the integration of virtual and real elements in digital twin technology will become more complete, representing an important pathway for future intelligentization and visualization of mobile hot cells and providing new solutions for current issues such as low operation precision and observation difficulties.

3.3 Modularization

Future mobile hot cells will place greater emphasis on modular design and integrated technology applications. By standardizing the design and manufacturing of various functional modules, hot cells can be rapidly assembled and disassembled according to different user requirements, improving versatility and scalability. Integrated technology can consolidate multiple devices and systems onto a single platform, reducing volume and weight while improving overall performance and reliability. To enhance mobility and flexibility, structural design will increasingly trend toward compactness and lightweighting. By optimizing internal layouts, adopting advanced manufacturing processes, and using lightweight yet high-strength materials, hot cell volume and weight can be reduced, making transport, installation, and relocation easier and expanding application scope.

Modular design enables mobile hot cells to address different working condition requirements, including both component-level and overall modular design. Traditional hot cells are often zoned by function, with each cell performing only one task such as non-destructive testing or metallographic sample preparation. Due to space limitations, mobile hot cells must be designed with different modules for different inspection tasks, allowing equipment modules to be replaced according to specific needs. For scenarios requiring multiple hot cells, mobile hot cells should be capable of interconnecting two or more units, necessitating standardized interface design and dedicated connection channels for series connection.

3.4 Standardization

As mobile hot cell applications increase, the establishment and improvement of industry standards will become inevitable. Unified industry standards will help regulate interface design, improve product quality and safety, and promote healthy development of the mobile hot cell industry. Currently, some international standards and specifications exist for mobile hot cell design, manufacturing, use, and maintenance, but further refinement and detailed development are needed. As technology advances and application requirements evolve, industry standards will continue to be updated and upgraded.

4 Summary

In recent years, continuous global research progress has advanced mobile hot cell technologies. Structural designs vary according to function, and balancing shielding capability, lightweighting, and material lifespan represents an important future research direction. Targeted modifications to critical components such as viewing windows, manipulators, and transfer containers will drive mobile hot cells toward lightweight, modular, standardized, and intelligent development.

This paper has analyzed the research and development status of existing mobile hot cell facilities worldwide. As a high-end nuclear technology equipment involving multiple cutting-edge disciplines, mobile hot cells are receiving increasing attention from research institutions globally. Countries are successively developing and manufacturing mobile hot cells, with research advancing toward modularization, intelligentization, and enhanced safety. China has made significant progress in modular nuclear technology and digital applications, while international experience focuses on advanced processing technologies and standardized collaboration. Future breakthroughs are needed in key technologies such as lightweight shielding design and multi-functional processing unit integration, along with promoting standard formulation to expand application scope.

In summary, mobile hot cells are irreplaceable in the nuclear field, providing essential guarantees for safe radioactive material operations, nuclear fuel cycle applications, and research support. With continuous technological innovation and application expansion, mobile hot cells will play an increasingly important role in the future nuclear industry.

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