Basic Design Requirements for Spent Fuel Reprocessing Process Valves

(1) Radiation resistance of the valve
The material and structure of the valve should be considered in the design to meet the requirements of radiation resistance. The valve body is made from stainless steel which is consistent with the pipe material. Stainless steel meets the requirement for radiation resistance. The material selection of other parts of the valve requires radiation resistance and aging materials. The structure of the valve should minimize radioactive adhesion and effusion, and the roughness of the contact surface between the valve and the medium should be no less than 6.3μm to reduce the adhesion of the radioactive medium and reduce the injury to personnel during maintenance.
 
(2) Corrosion resistance of the valve
The design considers the anti-corrosion of the valve from the perspective of valve materials. Valve material selection requires corrosion resistance to nitric acid and other media. In nuclear power systems, graphite is mostly used as a packing or sealing ring, but nitric acid medium and nitric acid corrodes graphite is often used in spent fuel reprocessing projects. It is recommended to use corrosion-resistant materials such as PTFE or F46.
 
(3) Strength and stiffness of the valve
The design should consider the main parts of the valve under the permanent or instantaneous pressure of various loads, and there should be no obvious elastic-plastic deformation. In addition to conventional strength calculations, methods such as finite element stress analysis and seismic calculation analysis should also be used to ensure the reliability of valves. For 1A valves, anti-seismic experiments must also be done to ensure the reliability of the valve.
 
 (4) Sealing performance of the valve
Since most of the process media are radioactive and corrosive, no leakage is allowed. It must be controlled in the structural design of the valve, the selection of seals (bellows, packing, gasket, etc.), the quality inspection and control of materials and finished products, take strict and effective measures to guarantee. In addition to test verification, effective measures must be taken in structural design to ensure the sealing of chemical safety valves, such as various bellows sealed valve stems.
 
(5) Service life of the valve
The service life of the main pressure-bearing parts such as the valve body and valve bonnet is required to be the same as that of the plant when chemical safety valves or non safety valves are designed. For the wearing parts, the replacement cycle is required to meet the requirements of the operating cycle. Valves should be designed to minimize the number and duration of repairs.
 
3.4 Materials of the valve
All materials used to manufacture valves should comply with current national standards or industry standards, and there should be factory certificates for material manufacturers. The manufacturer conducts re-inspection on the chemical composition and mechanical properties of the purchased materials by the current national standards, and issues a re-inspection certificate. For valves with requirements for radiochemical safety level, the materials can be purchased and re-inspected according to the corresponding standards of RCC-M or ASME.
 
The spent fuel reprocessing process system adopts stainless steel valves, and its valve body, valve core, valve disc and all flow-passing parts are made from stainless steel, and there should be no intergranular corrosion tendency, and intergranular corrosion tests should be carried out. The bolts, nuts, washers, etc. used in the valve shall be made from stainless steel and meet the requirement for the standard.
 
The valve material of radiochemical safety grade must have good corrosion resistance, radiation resistance, impact resistance and intergranular corrosion resistance. For example, low-carbon austenitic stainless steel is selected as the main material. The valve body is generally consistent with the pipe material, which is stainless steel. Select some alloy materials with superior performance such as strength, toughness, pressure resistance, corrosion resistance, erosion resistance, and abrasion resistance for valve stems and sealing surfaces. For example, the sealing surface is surfaced with Stellite alloy, which is wear-resistant and corrosion-resistant. For example, the bellows of the bellows globe valve and the sealing pair of the ball valve use Inconel alloy. For radiochemical safety valves, the main body of the valve and actuator should be made from stainless steel, especially for valves that require butt welding between the valve and the pipeline (the RS-1 valve); low-carbon stainless steel forgings are required. The main body of other valves is made from stainless steel, and the actuators such as handwheels can be made from carbon steel.