Equivalent hydrogen control of fuel rods
the problem of damage of fuel rods due to hydrogen absorption was well solved in theory and practice as early as the early 1970s, and a large number of operating experience over 20 years has proved that hydrogenation is no longer the main cause of damage of fuel assemblies, but this problem has always attracted the attention of assembly manufacturers and nuclear power plants. From 1989 to 1996, 798 groups of PWR fuel assemblies in the United States were damaged (the component damage rate was 1.9%), of which 5 groups were damaged by hydrogenation such as impact, accounting for 0.62% of the damaged assemblies. The Chinese industry standard ej323 "design criteria for fuel assemblies of pressurized water reactor nuclear power plants", the French rcc-c "rules for fuel assemblies and construction of pressurized water reactor nuclear power plants" and the nuclear safety guideline haf0410 "quality assurance in the procurement, design and manufacture of nuclear fuel assemblies" all put forward strict requirements for equivalent hydrogen control of fuel rods. The accepted criterion is that the equivalent water content per cubic centimeter of free cold space in the fuel rod in the manufacturing state shall be less than 2mg. YFP strictly controlled the equivalent hydrogen content of UO2 pellets and the tubing process during the manufacturing of fuel rods, ensuring that the equivalent hydrogen content of fuel rods meets the design requirements. The safe operation of fuel assemblies in nuclear reactors has also proved that YFP's control over the equivalent hydrogen of fuel rods is strict and effective. Control of equivalent hydrogen of 2300mw fuel rod design control of equivalent hydrogen of fuel rod is divided into two aspects:
(1) measure the equivalent water content of fuel rod with simulated fuel rod (hereinafter referred to as simulated rod) before each refueling production to obtain the total water equivalent content per unit volume, The design requirements are as follows: "after the hydrogen and water contained or adsorbed by all parts in the rod are completely released into the cold space, the equivalent water content shall be less than 2mg/cm3."
(2) the pellets shall be dried before being piped, and the equivalent hydrogen content shall be sampled and analyzed. The design requirements are "total hydrogen equivalent content of fuel pellets when piped 2 μ g/gU。…， In normal production, only the moisture content of pellets can be monitored before pipe installation, but the determination of water content monitoring indicators should be based on the total hydrogen content in pellets. " Analysis of test results
2.2.1 equivalent water content of fuel rods
in 1984, YFP established the CH4 mass spectrometer to measure "the determination method of equivalent water content in simulated fuel rods". This method not only measures the water in the pellet, but also the water in helium. It also measures the adsorbed water on the inner surface of the fuel rod, the spring, the end plug and the heat insulation block, that is, it measures the total water equivalent content of the hydrogen and water contained or adsorbed by all parts in the rod after they are completely released into the cold space, which can meet the detection accuracy requirements proposed by the designFrom 1987 to 1991, YFP detected 13 data in total. The average value of equivalent water content in the simulation rod was 0.33mg/cm3, and the maximum value was 0.61mg/cm3. It is much lower than the 2mg/cm3 required by the design
in order to intuitively understand the equivalent water content of each component in the fuel rod, YFP conducted scientific research and concluded that "the total water content in the fuel rod mainly comes from UO2 pellets, and the water content in UO2 pellets accounts for about 83% of the equivalent water content in the rod"
from 1995 to 1999, YFP conducted 5 times of determination of equivalent water content in the simulation rod. See Table 1 for the data. It can be seen from table 1 that the average equivalent water content in the simulation rod is 0.177mg/cm3, the maximum is 0.23mg/cm3, and the equivalent water content in the rod is about 1/10 of the design criteria (2mg/cm3). Among them, the residual adsorbed equivalent water content of helium, end plug, spring, insulation block and pipe inner wall only accounts for 32% of the total equivalent water content in the rod. This shows that the current fuel rod manufacturing process can fully meet the control requirements of equivalent water content in fuel rods.
2.2.2 pellet moisture content analysis
now the moisture content statistics of the 2nd refueling pellet drying batch of Pakistan Chashma 300MW Nuclear Power Plant are shown in Figure 1. The results show that the moisture content of the dried pellets meets the control limit requirements of technical conditions. 2.3 summary
(1) the average and maximum of the total water equivalent content in the simulation rod showed a downward trend, the average value decreased from 0.33mg/cm3 to 0.18mg/cm3, and the maximum value decreased from 0.61mg/cm3 to 0.23mg/cm3, which proved that the manufacturing quality of YFP pellets and the fuel rod assembly process were further improved
(2) calculated from table 1, the equivalent water content in the pellet accounts for 58% ~ 83% of the total water content in the simulation rod, and the total water equivalent in the fuel rod mainly comes from UO2 pellet. The water content absorbed by the inner surface of the tube, the end plug and the spring is stable, and the contribution of the inner surface of the fuel rod, the end plug, the spring and the insulation block to the total water equivalent content in the fuel rod is only 0.04mg/cm3 at most
(3) in production, the equivalent hydrogen content in fuel rods can be controlled by controlling the equivalent hydrogen content in pellets
control of equivalent hydrogen of 3900MW fuel rod
3.1 design requirements
(1) pellet batch: the average equivalent hydrogen of each moisture batch measured with 95% confidence shall be less than 0.8 μ g/gUO2。 The equivalent hydrogen of 95% pellets in this moisture batch shall be less than 1.2 μ g/gUO2；
(2) pellets: the maximum equivalent hydrogen content of a single pellet is 1.66 μ g/gUO2。
3.2 inspection results and analysis
3.2.1 analysis of equivalent hydrogen content in fa3g pellet identification
during the first AFA3G pellet identification production in 2001, for the purpose of 3D system, the company also released Geomagic freeform 2017, the most comprehensive 3D hybrid design software in the industry to analyze the impact of drying and storage conditions on equivalent hydrogen of pellets. YFP sampled UO2 pellets for equivalent hydrogen analysis in two states of pellet drying and non drying, See Table 2 for the test results of equivalent hydrogen of dried pellets and table 3 for the test results of equivalent hydrogen of non dried pellets. In the table, the coefficient K1 is selected according to the French statistical analysis standard nfx "calculation of average value", and the coefficient K2 is selected according to the French statistical analysis standard nfx "determination of statistical range"
3.2.2 production control
Framatome ensures the equivalent hydrogen content of fuel rods by controlling the equivalent hydrogen content of pellet batches and the installation time (sampling and analysis are required for more than 240h). YFP dries the pellets before pellet installation and controls their equivalent hydrogen content and installation time (drying and sampling are required for more than 96h). See Table 4 for the analysis results of afa2g pellet equivalent hydrogen over the years