Method of controlling nuclear fuel for thermal resistance

FIELD: nuclear fuel technology.

SUBSTANCE: invention relates to production of pelleted fuel and consists in controlling nuclear fuel for thermal resistance involving preparation for selecting pellets from nuclear fuel lot for measuring diameter, which preparation consists in dedusting. Selected pellets are placed in temperature-stabilized box together with measuring instrument. Diameter of each pellet is them measured and measurement data are entered into computer. Thereafter, pellets are charged into heat treatment vessel, wherein pellets are heated in vacuum at residual pressure not exceeding 7·10-2 Pa at heating velocity not higher than 10°C/min to 100-160°C and held at this temperature at most 2 h, whereupon heating is continued under the same conditions to 1470-1530°C and this temperature is maintained for a period of time not exceeding 4 h, after which hydrogen is fed with flow rate 2-6 L/min. Humidity of gas mix is measured in the heat treatment outlet. If humidity of gas mixture in the heat treatment outlet exceeds 800 ppm, hydrogen feeding is stopped and material is subjected to additional vacuum degassing at residual pressure below 7·10-2 Pa and held at 1470-1530°C in vacuum for further 4 h. Hydrogen feeding is the repeated at 2-6 L/min. If humidity of gas mixture in the heat treatment outlet is below 800 ppm, preceding temperature is maintained not longer than 2 h and raised to 1625-1675°C at velocity 40-60°C/h and then to 1700-1750°C at velocity 15-45°C/h. When outlet humidity of mixture is 500-750 ppm, hydrogen feeding is lowered to 1 L/min. Temperature 1700-1750°C is maintained during 24±2 h, after which pellets are cooled to 1470-1530ºC at velocity not higher than 10°C/min. Hydrogen is replaced with argon and cooling is continued to temperature not higher than 40°C, which temperature is further maintained. Outside diameter of each pellet from the selection is measured to find average diameter of pellets before and after heat treatment in order to calculate residual sintering ability. When this parameter equals 0.0-0.4%, total lot of pellets is used in fuel elements and in case of exceeding or negative residual sintering ability the total lot of pellets is rejected.

EFFECT: improved pellet quality control.

2 dwg

 

The invention relates to the nuclear industry and can find application in the manufacturing pelletized fuel from uranium dioxide (UO2for fuel elements (FE), collected in the fuel Assembly (FA)used in nuclear reactors. It is known that during operation of the fuel assemblies in the reactor core of a nuclear reactor, the nuclear fuel tablets of a fuel rod temperatures can reach up to 2000°that can lead to radial and axial redistribution of nuclear fuel (see Development, production and operation of fuel cell power reactors, book 1, page 53. Edited Fgurative, M., Energoatomizdat, 1995).

In this regard, one of the main characteristics of fuel pellets is their stability in geometric size during high-temperature processing, which is quite important to first know how to behave in the fuel column in Fe - “swell”, will decrease in volume or preserve their geometric dimensions. “Swelling” of fuel pellets in a fuel rod is unacceptable, because it is possible that local overheating of the shell and is not excluded depressurization shell of a fuel rod. The reduction in volume pills will violate the tolerance on the outside diameter of the pellets relative to the internal diameter of Talinay shell.

It is not possible bias is tabletki and possible jamming of the shell when the linear thermal elongation of the column of pellets in the shell. Jamming of the pills does not exclude local overheating of the membrane jamming and not excluded depressurization of the fuel element.

The known method of control of nuclear fuel (see patent RU 2158448, Mpk7 G 21 17/06 from 03.06.1999 year), which includes control of thermal stability, but the essence of it is not disclosed.

The closest in technical essence and the achieved effect is a method of control of nuclear fuel thermal stability of tablets, including the heat treatment at a temperature of 1700-1750°C for (24±2) hours in a mixture of argon and hydrogen (volume ratio of 7-8%) humidity (350±50)RRT with a valid value of depechemode 0.3 to 0.5% and monitoring the relative change of the outer diameter after heat treatment (see Development, production and operation of fuel cell power reactors. Book 1, edited by Fgurative - M.: Energoatomizdat, 1995, s).

The method solves an important characteristic of quality tablets their thermal stability, i.e. the preservation of their density and, as a consequence, the sizes in the initial period of reactor operation (several tens of hours) (see ibid., s). However, the prototype method is not fully characterizes the inventive method.

Not reflected preparing and measuring the diameter of the tablets before the heat treatment, and the error in measurement may lead to an error in the end the flax output thermal stability after heat treatment. Does not reflect the number of layers load of pills in the container to heat treatment tablets.

Not reflect the parameters and conditions of the initial heating of the pellets, drying the pellets, degassing furnace, evaluation dehumidification, conditions output on basic mode of heat treatment and cooling of tablets as an abrupt cooling of the tablets will lead to their destruction from heat stroke, and other failure modes will reviewing tablets with deviation from the requirements to the quality of pills thermal stability.

The technical challenge is to improve the quality control of tablets on their thermal stability, identification of tablets with mismatched thermal stability and rejecting the entire lot from which the tablets were taken under control, and improve quality made TVEL.

This technical problem is solved in that in the method of control of nuclear fuel for thermal stability of tablets, including the heat treatment at a temperature of 1700-1750°C for 22-26 hours in a mixture of argon and hydrogen with permissible humidity permissible value of depechemode and monitoring the relative change of the outer diameter after heat treatment;

According to the invention of the batch of nuclear fuel, take a sample of tablets, prepare them for the measurement of the diameter of the through cleaning of dust, the temperature in the box for at least 2 hours at a constant temperature of tablets and the measuring instrument, when 21-29°measure the diameter of each pellet sample, at least nine times with a turn angle of from 60 to 120° in each of the three sections at one end, the middle and near the other end tablets, the results of the measurements is introduced into the electronic computer, tablet sample after measuring the load in the container to heat treatment, where the initial heating of the pellets is carried out at a residual pressure of not more than 7×10-2PA at a speed of not more than 10°C/min to a temperature of 100-160°aged at this temperature for 2 hours, then continue to the specified mode of heating in a vacuum to 1470-1530°To make the shutter speed is not more than four hours and served hydrogen 2-6 l/min, measuring the humidity of the gas mixture at the exit of the heat treatment, and, if the humidity of the gas mixture exceeds 800 ppm, the flow of hydrogen stop and spend additional degassing vacuum to a residual pressure of less than 7×10-2PA and in addition is maintained in a vacuum at a temperature of 1470-1530°With no more than four hours, then start hydrogen repeat 2-6 l/min, if the humidity of the gas mixture at the exit of the heat treatment below 800 ppm, after holding not more than 2 hours the temperature was raised to 1625-1675°With the speed of the 40-60° With/hour after that until 1700-1750°at the rate of 15-35°C/HR humidity of the gas mixture at the outlet 500-750 ppm, reduce the supply of hydrogen to 1 l/min, after holding at 1700-1750°C for 24±2 hours tablets cooled to 1470-1530°With a speed of no more than 10°C/min, replacing the hydrogen with argon with a flow of not less than 5 l/min, continue to cool tablet with speed no more than 10°C/min up to 250°With heat off and are cooled to a temperature of not more than 40°With tablets thermostatic for not more than 2 hours and measure the outer diameter of each pellet sample, determine the average diameter of each tablet before heat treatment and after heat treatment by the formulas:

where- the average diameter of each pellet sample before heat treatment;

- the average diameter of each pellet sample after heat treatment;

Doiand Dtithe results of single measurements of tablets before and after heat treatment, respectively, mm,

thermal stability of the geometric dimensions (depechemode) tablets, % abs., determined by the formula:

where ΔD - depechemode %,

if depechemode [0,0-0,4]% of all tablets party pills is directed to the ammunition is their TVEL, and when exceeding or negative result of the whole party pills is discarded and sent for processing.

The proposed method can improve the quality control of tablets on their thermal stability, identify pills with mismatched thermal stability, discard the entire party, from which were taken these pills for checking thermal stability, and improve the quality of the fabricated fuel elements.

To explain the method of control of nuclear fuel for thermal stability are given.:

figure 1 - flow chart;

figure 2 - chart modes.

The method of control of nuclear fuel for thermal stability is as follows.

Party pills dioxide uranium UO2selected sample of ~ 50 tablets, cleaning them of dust, carry out the temperature control in the box together with the measuring instrument at least 2 hours.

Stable temperatures 21-29°measure the diameter of each pellet sample, at least nine times with a turn angle of from 60 to 120° in each of the three sections at one end, the middle and near the other end tablets, the results of the measurements is introduced into the electronic computer, tablet sample is loaded into the container to heat treatment, where the initial heating of the pellets is carried out in vacuum at a residual having the attachment is not more than 7· 10-2PA (5·10-4mm Hg) at a speed of not more than 10°C/min to a temperature of 100-160°aged at this temperature for 2 hours, then continue to the specified mode of heating in a vacuum to 1470-1530°To make the exposure time longer than 4 hours and served hydrogen 2-6 l/min, measuring the humidity of the gas mixture at the exit of the heat treatment, and if the humidity of the gas mixture exceeds 800 ppm, the flow of hydrogen stop and spend additional degassing vacuum to a residual pressure of less than 7·10-2PA (5 to 10-4mm Hg) and in addition is maintained at 1470-1530°in a vacuum of not more than 4 hours, then start hydrogen repeat 2-6 l/min, if the humidity of the gas mixture at the exit of the heat treatment below 800 ppm, after holding not more than 2 hours the temperature was raised to 1625-1675°With speeds of 40-60°C/h, followed that up 1700-1750°at the rate of 15-35°C/HR humidity of the gas mixture at the outlet 500-750 ppm, reduce the supply of hydrogen up to 1 l/min, after 24-hour exposure at 1700-1750°With cool tablet to 1470-1530°With a maximum speed of 10°C/min, replacing the hydrogen with argon with a flow of not less than 5 l/min, continue to cool tablet with a maximum speed of 10°C/min up to 250°With heat off and are cooled to a temperature of not more than 40°With tablets thermostatic for not more than 2 hours and edit the accelerate outer diameter of each tablet sample determine the average diameter of each tablet before heat treatment and after heat treatment by the formulas:

where- the average diameter of each pellet sample before heat treatment ;

- the average diameter of each pellet sample after heat treatment;

D0iand Dtithe results of single measurements of tablets before and after heat treatment, respectively, mm,

thermal stability of the geometric dimensions (depechemode) tablets, % abs., determined by the formula:

where ΔD - depechemode %,

if depechemode [0,0-0,4]% of all tablets party pills is directed to equipment TVEL, and when exceeding or negative result of the whole party pills is discarded and sent for processing.

In the way that all parameters are optimal and any deviation in more or less side will not solve the technical problem.

The method is tested in a production environment and there was a positive effect.

The method of control of nuclear fuel for thermal stability, comprising the heat treatment of the pellets of nuclear fuel at a temperature of 1700-1750°C for 22-26 h in hydrogen atmosphere with humidity is, with depechemode and monitoring changes in their external diameter after heat treatment, characterized in that batch of nuclear fuel, take a sample of tablets, prepare their diameter measured by cleaning from dust, carry out the temperature control in the box together with the measuring instrument for at least 2 h, at a constant temperature 21-29°With, measure the diameter of each pellet sample, at least nine times with a turn angle of 60-120°With each of the three sections at one end, the middle and near the other end tablets, the results of the measurements are injected into the electron-computer, tablet sample after measuring the load in the container to heat treatment, where the initial heating of the pellets is carried out in vacuum at a residual pressure of not more than 7·10-2PA at a speed of not more than 10°C/min to a temperature of 100-160°aged at this temperature no more than 2 h, and then continue to the specified mode of heating in vacuum to a temperature 1470-1530°To make the shutter speed is not more than 4 h and serves hydrogen 2-6 l/min, measuring the humidity of the gas mixture at the exit of the heat treatment, and if the humidity of the gas mixture exceeds 800 ppm, the flow of hydrogen stop and spend additional degassing vacuum to a residual pressure of less than 7·10-2PA and additionally incubated at 1470-1530°in HAC the mind no more than 4 hours, then start hydrogen repeat 2-6 l/min, if the humidity of the gas mixture at the exit of the heat treatment below 800 ppm, after holding not more than 2 hours the temperature was raised to 1625-1675°With speeds of 40-60°/h after that to 1700-1750°at the rate of 15-35°/h humidity of the gas mixture at the outlet 500-750 ppm, reduce the supply of hydrogen to 1 l/min, at the end (24±2)-hour exposure at 1700-1750°With cool tablet to 1470-1530°With a maximum speed of 10°C/min, replacing the hydrogen with argon with a flow of not less than 5 l/min, continue to cool tablet with a maximum speed of 10°C/min up to 250°With heat off and are cooled to a temperature of not more than 40°With tablets thermostatic within no more than 2 hours and measure the outer diameter of each pellet sample, determine the average diameter of each tablet to heat treatment and after heat treatment by formulas

where- the average diameter of each pellet sample before heat treatment;- the average diameter of each pellet sample after heat treatment; D0iand Dtithe results of single measurements of tablets before and after heat treatment, respectively, mm,

thermal stability of the geometrical sizes of tablets, % abs., determined by the formula

where ΔD - depechemode %,

if depechemode 0,0-0,4% of all tablets party pills is directed to equipment fuel elements, and when exceeding or negative result of the whole party pills is discarded.



 

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