RU2164359C2 - Leak detection method - Google Patents

Abstract

FIELD: leak detection in helium-filled fuel elements of light-water reactors; serviceability check of test facility in the course of measurements. SUBSTANCE: method involves testing units by means of helium leak detector and checking serviceability of test facilities in the course of measurement for using heavily loaded hot parts, locating heavy leaks by using pressure tests, selecting optimal test conditions. For testing a part helium is supplied from standard source; amount of helium leakage from part is determined and compared with permissible leakage; test facility charge for each temperature and permissible amount of leakage for given temperature are calculated. Parts are additionally subjected to pressure test with helium, heavy leaks are detected, and rejection characteristic is calculated. EFFECT: improved capacity depending on type of part under test.

Description

 The invention relates to the field of leak detection in products having a free volume, which is filled with helium before sealing. Such products, for example, are fuel elements (fuel elements) of light-water nuclear reactors, for example VVER.

 The search for leaks in them is usually carried out by a mass spectrometric method using a helium leak detector in a vacuum chamber with a loading volume of 1-20 pcs. fuel elements after preliminary pressure testing with helium and heating. Heating serves to improve the detection of small leaks.

 Helium is used as a test gas, the presence of which in the measuring chamber is considered as evidence that it contains an unpressurized fuel element. Before the test, a standard helium source is connected to the measuring chamber, with the help of which the sensitivity of the installation is determined and compared with the permissible value. If it meets the requirements, the installation is considered operational. The tightness of the products is judged by comparing the measured flow of helium leakage into the measuring chamber with the leakage flow (rejection mark) acceptable for this type of fuel elements. The actual amount of leakage flow determines the degree of tightness of the fuel rods.

 The disadvantage of this method is the limited detectability of large leaks and low camera load. If, for example, increase the volume of loading, then the heating time of the products increases. By the time of testing, the amount of helium fed under the cladding of the fuel rods during pressure reduction decreases, the surface leakage flow increases. To reduce the flow of surface leakage, sometimes exposure before testing or flushing the chamber with inert gas is sometimes used, which takes additional time. This makes some large leaks in the fuel rods undetectable.

 Products before crimping already contain helium, and the appearance of large leaks is usually accompanied by the appearance of gross damage or noticeable deviations in the quality of the sealing shell, which are easily detected by their appearance, by ultrasonic or X-ray flaw detection. Therefore, pressure testing with helium is sometimes not used, as, for example, in the method according to US patent N5009835. However, in this case, the products are not heated, the loading volume remains the same. However, there is still no guarantee of detecting large leaks, since defects with a small opening such as cracks are not detected by the indicated methods.

 In such cases, an additional operation specifically designed to control large leaks may be used. So, for example, in US patent N 5625141, G 01 M 3/20, 04.29.97 the presence of a major leak is judged by the change in pressure in the chamber in which the product is located. In US patent N 5307139, G 01 L 1/24, 04/26/94 when vacuum is set, the deformation of the walls of the products is monitored using an interferometer.

 However, these methods are possible only with piece control, they also require sophisticated special equipment and are practically not applicable for the control of fuel elements due to the insufficiently high class of surface treatment and the relatively high stiffness of the cladding.

 There are also known methods for searching for leaks in products containing helium that are closest to the claimed one (US patent N 5009835, G 21 C 17/00, 04.23.89, Europatent EP 0379946 A3, G 01 M 3/22, 11.21.99). In particular, according to US patent N5009835, adopted here as a prototype, helium fed to the fuel elements during their sealing is used as a test gas. Products at normal temperature (one or more) are fed into a vacuum chamber, the chamber closes and is evacuated. At the same time, a known helium stream is fed into the chamber. The leak detector readings taking into account the passport sensitivity of the installation should correspond to this flow. If the leak detector readings show that helium leakage into the chamber is less than the known flow, then the installation is not operable, if they correspond to this flow, then the installation is operable and the products are tight. If the leak detector readings are greater than this value, then there is an untight fuel rod in the chamber. Thus, at the same time there is a check of the operation of the installation and a leak test.

 The disadvantages of the prototype method are high requirements for the stability of the installation, which should have a constant sensitivity. Otherwise, if there is an unpressurized fuel rod during the simultaneous measurement of the leakage stream from this fuel rod and the flow from a calibrated helium source, positive test results can be obtained. These high requirements become unattainable when testing the tightness of hot products, as is required in the control of WWER fuel rods, as well as with increased loading volumes.

 The disadvantages of the prototype also include the inability to detect larger leaks through which the source helium manages to escape and which would be detected by crimping the products with helium. However, the use of crimping fuel rods according to the prototype would create another source of instability - the surface flow of helium from compressed fuel rods, which varies from load to load, depending on many factors.

 The aim of the present invention is to propose such a method for detecting leaks using a helium leak detector and verifying the operation of the installation during the measurement process, in which it would be possible to test hot products with a large load, and also to search for large leaks using crimping, but so that this does not affect neither on the operability of the installation, nor on the detectability of large leaks, so that it is possible to select the optimal control modes with maximum performance depending on the type of product and vany them.

 This can be done if in the prototype method two measurements are taken during product testing: one measurement with a calibrated helium source connected, the other with a disconnected one. The difference in readings determine the sensitivity of the installation, the second indication - the degree of tightness.

 In addition, it is necessary to additionally search for large leaks (with a small opening) by the same method, but after preliminary pressure testing of the products with helium. When testing fuel rods for tightness without crimping, the main goal is to warm the entire load of products for a sufficient time for this, no matter how large this load is. When testing fuel rods for large leaks with pressure testing, the main thing is to reduce to a minimum the period of time between pressure testing and testing, no matter what the temperature of the fuel rods and the sensitivity of the installation. The use of two control operations allows you to create the most favorable conditions for detection for each type of leak - small and large.

 A large leak gives a clearly visible leak detector signal that exceeds all background levels. The boundary leak will give a signal that rapidly decreases in time from the upper limit of sensitivity of the leak detector to the lower. Therefore, even an indicator method of determination is suitable for its identification. However, for the most complete overlap of the ranges for detecting large and small leaks, a specific value of the allowable leakage flow should be adopted in the sensitivity range of the leak detector, in which this leakage can be confidently determined above the background levels.

 Determination of the sensitivity of the installation during the measurement makes it possible to eliminate the error associated with fluctuations in the flows of surface leakage from load to load, which can be quite large at elevated product temperatures and large volumes of loads.

 Of great importance for the detection of small leaks is the pressure of helium under the shell. Depending on the design of the fuel rod (pressure under the cladding) and product requirements (test temperature, rejection mark), the same defect in the cladding may produce a different leak. On the other hand, the magnitude of the leakage flow indirectly characterizes the size of the damage to the shell, much depends on the degree of disclosure of the defect. This sometimes makes it necessary, on the one hand, to set the allowable leak for certain (standard) gas flow conditions (for example, normal), on the other hand, in the manufacture of products, some products should strive to control for a higher degree of tightness than the specification requires, and others - at the level of requirements, but with maximum performance. To enable the manufacturer in each case to choose the most optimal control modes is another objective of the present invention.

 The search order for leaks by the proposed method can be described in more detail as follows.

Suppose it is required to control the product temperature T _es with the internal pressure of the helium P _tv, with a permissible leakage q ^m _additional, predetermined temperature T ₀ and pressure P _0.

производят вычисления:

.Preliminarily determine the specific flux of surface leakage from one fuel rod q ^m _detachment . _Pov . For this amount of product N, corresponding to full load of the furnace is heated to the test temperature T _es determine background value by product

make calculations:

.

После этого определяют n и q^м _бр в зависимости от принятого соотношения из выражения:
nq^м _{удел.пов}=q^м _бр
Затем загрузку в количестве n штук изделий нагревают в печи до температуры T_исп и перегружают в измерительную камеру установки контроля герметичности.Then find the value of the rejection sign q ^m _br , which corresponds to q ^m _extra at the test temperature. It can be shown using gas laws that:

After that, determine n and q ^m _br depending on the adopted ratio from the expression:
nq ^{m long} _range = q ^m _br
Then loading in an amount of n pieces of products in an oven heated to a temperature T _es and transferred into a measuring chamber installation leakage control.

После набора вакуума снимают показание течеискателя от камеры с изделиями -

3атем подключают гелит с потоком гелия g и снова снимают показание течеискателя -

Определяют чувствительность системы по формуле:

Определяют работоспособность установки контроля герметичности, которая наступает при соблюдении условия:
k=k_доп,
где k_доп - допустимая чувствительность, при которой на данной установке можно достоверно замерить поток, равный допустимой утечке q^м _доп
Определяют утечку гелия от изделий Q_и по формуле:

где

фон камеры.Pre-determine camera background

After vacuum is set, the leak detector is taken from the camera with the products -

Then connect the gel with a helium flow g and again take the leak detector -

The sensitivity of the system is determined by the formula:

The operability of the tightness control installation, which occurs when the conditions are met, is determined:
k = k _extra
where k _add - allowable sensitivity at which at this installation it is possible to reliably measure the flow equal to the allowable leak q ^m _add
Determine the leakage of helium from products Q _and by the formula:

Where

camera background.

Compare the obtained value with a valid one. If the inequality holds
Q _and ≅q ^m _br
then all the fuel rods in the chamber are tight. If it is not fulfilled, sequential beam division and search for the leaky product are performed.

Это соотношение позволяет:
- при заданной допустимой утечке выбрать максимальную загрузку,
- при малой загрузке повысить определяемый уровень герметичности изделий,
- менять параметры контроля, не снижая его качества.Thus, the condition of tightness, including the properties of the products, the requirements for tightness and technological parameters, will be the expression:

This ratio allows you to:
- for a given allowable leak, select the maximum load,
- at low load to increase the determined level of tightness of products,
- change the control parameters without reducing its quality.

The same mass spectrometric method determines large leaks with preliminary pressure testing with helium. In this case, an additional rejection sign is used - the value of the allowable leak for large leaks q ^to _dop , which also must satisfy certain requirements. On the one hand, it cannot be less than the total surface leakage flow from products:
nq = q ^to _udel.pov ^to _additional
On the other hand, it should provide overlapping ranges for detecting leaks at both stages of control. In practice, for conditions when the load volume does not change during both operations of leak testing, it occurs when the ratio is observed:
q ^to _extra = (100-1000) q ^m _additional
Reducing the coefficient value to less than 100 does not allow the level of surface leakage from the products, increasing it to more than 1000 is impractical due to a decrease in the degree of overlap of the leak detection ranges in both control operations.

Thus, the condition for the tightness of products in large leaks is the fulfillment of the ratio:
Q _and = (100-1000) q ^m _add
It allows, using the same control methods as for small leaks, to expand the range of leak detection into the region of large leak flows, which, together with high flows of surface leakage from compressed products, would interfere with the search for small leaks if you search in one operation.

The use of the proposed method for the search for leaks can be shown by example. The VVER-1000 fuel rod is a tubular shell sealed at the ends with welded plugs and having a fuel column inside. The free volume of a fuel rod is filled during sealing with helium under pressure P _TV = 17.5 atm. The permissible leakage of helium from the fuel element is q ^m ₀ = 0.185 · 10 μm RT. Art. · L / s under normal conditions (T ₀ = 20 ^o C, P ₀ = 1 ATM). The test temperature T _ut = 300 ^o C. Determined the allowable leak at this temperature for testing for small leaks, q ^m _extra = 5 · 10 ^-5 μm RT. Art. l / s. Having previously determined experimentally the specific flux of surface leakage q ^m _plot.pow = 4.4 · 10 ^-8 μm RT. Art. · l / s from relation (1), found the maximum volume of a single load n = 1136.

Пучок твэлов n штук нагрели до температуры 300^oC, загрузили в установку контроля мелких течей, набрали вакуум, сняли показания течеискателя,

Затем подключили гелит, определили

определили превышение показания течеискателя за счет гелита

вычиcлили чувствительность установки

0,33·10^-5 мкм рт. ст. ·л/с·мв, убедились в ее работоспособности. Вычислили натекание от изделий

мкм рт. ст.·л/с, сравнили его с допустимым значением при данной температуре контроля, убедились в герметичности всех испытуемых изделий:
3,31·10^-5=5·10^-5 (мкм рт. ст.·л/с).It was decided to limit the actual load to n _real = 450 pcs., Therefore, the level of control can be increased - the rejection sign can be reduced to the value:
^m q = q _{br.umen shenoe br} ^m / n _{is real} / n = 2 · 10 ^-5 mm Hg. Art. l / s
For small n, one can increase the level of control by reducing the allowable leak to a value

A bunch of fuel rods n pieces were heated to a temperature of 300 ^o C, loaded into a small leakage control unit, scored a vacuum, read the leak detector,

Then they connected the gel, identified

determined the excess of the leak detector due to gelite

calculated installation sensitivity

0.33 · 10 ^-5 μm RT. Art. · L / s · mv, were convinced of its working capacity. The leakage from the products was calculated.

μm RT. Art. · l / s, compared it with a permissible value at a given control temperature, made sure all the tested products were tight:
3.31 · 10 ^-5 = 5 · 10 ^-5 (μm mercury column. L / s).

0,33·10^-5 мкм рт. ст.·л/с·мв ·(1200-500)мв=2,3·10^-3 (мкм рт. ст.·л/с)
Сравнили его с браковочным признаком для крупных течей, взятых здесь по формуле
q^к _доп=100q^м _доп=5,0·10^-3мкм рт. ст.·л/с,
убедились в отсутствии крупных течей:
2,3·10^-3<5,0·10^-3 (мкм рт. ст.·л/с).Then the same bundle of products was pressed with helium at a pressure of 5 atm for 10 minutes, loaded into the control chamber, the leakage flow from the products was determined using the same formula

0.33 · 10 ^-5 μm RT. st. l / s · mv · (1200-500) mv = 2.3 · 10 ^-3 (μm mercury st. l / s)
Compare it with the rejection sign for large leaks, taken here according to the formula
q ^to _extra = 100q ^m _additional = 5.0 · 10 ^-3 μm RT. Art. l / s,
convinced of the absence of major leaks:
2.3 · 10 ^-3 <5.0 · 10 ^-3 (μm mercury column. L / s).

 The use of this invention will allow, when organizing the tightness control of products, to solve the problems of choosing technological test modes, the level and quality of control, the cost of its implementation and the issues of hardware design of material and technical means.

Claims (1)

A method for searching for leaks using a helium leak detector in products containing free volume filled with helium, having an allowable leak rate at specified pressure and temperature, and having a surface leakage flow, including verifying the operability of the installation by filling helium from a standard source during product testing, determining the flow helium leakage from the product and its comparison with the allowable leakage value, characterized in that for each test temperature the installation volume is determined and the control value and the permissible leakage at a given temperature from the relationship:

where P _TV - pressure filling products with helium;
q _add - allowable leak specified at pressure P ₀ and T ₀ ;
q $\genfrac{}{}{0ex}{}{\text{m}}{\text{at}}$ _del.pov - surface leakage flow from one product caused by gas separation from its outer surface during leak tests;
q $\genfrac{}{}{0ex}{}{\text{m}}{\text{b}}$ _p is the rejection sign, the maximum allowable leakage flow from the product at the test temperature T _isp. ;
n is the load volume,
and additionally search for large leaks in the volume products, having pre-crimped them with helium, while the value of the reject sign is determined from the ratio
q $\genfrac{}{}{0ex}{}{\text{to}}{\text{b}}$ _p = (100-1000) q $\genfrac{}{}{0ex}{}{\text{m}}{\text{b}}$ _R
q $\genfrac{}{}{0ex}{}{\text{to}}{\text{b}}$ _p - rejection sign for large leaks.