RU2045788C1 - Fuel element of nuclear power reactor - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: fuel element has pelletized fuel rod in sealed zirconium-alloy cladding. End caps are also made of zirconium alloy; one of end caps comes in contact with fuel through getter neck. Neck length is chosen between two and six thicknesses of cladding wall and neck sectional area S_n to head sectional area S_h ratio is within 1/7≅ S_n/S_h≅ 1/2.. Getter head length l_h is found from relationship l_h/l_fp≥ 0,001·(A/3,5-1),, where A is maximum moisture content admitted by manufacturing conditions in fuel pellets amounting to 3,5 ppm≅ A≅ 7,0 ppm;; l_fp is length of fuel pile of fuel element. Neck may be made from stainless steel pipe section to prevent diffusion of hydrogen from head to neck and end cap. EFFECT: improved design. 4 cl, 5 dwg

Description

 The invention relates to nuclear engineering, and more particularly to the design of fuel rods of industrial nuclear reactors, the shells of which are made of zirconium alloys, for example, fuel elements of the RBMK, WWER, AST, and others.

Known designs of fuel rods with claddings of Zr alloys, in which the fuel can be contacted with a lower plug made (the same as the upper plug) of the same zirconium alloy as the cladding of a fuel element, as well as with a spring made of Zr alloy, ( but of a different composition), a latch installed in the compensation volume of a fuel rod [1]
The disadvantage of such fuel rods (standard design of fuel rods of foreign and domestic RBMK and VVER type reactors) is that in the contact between the plug and the retainer with the fuel, as a result of an increase in the temperature of the parts in contact with the latter above the temperature of the fuel cladding pipe, cases of volumetric local ("sunburst"") hydrogenation plugs and retainer. Hydrogenation of the plug and subsequent diffusion of hydrogen into the closely located interface between the plug and the shell leads to hydrogenation and embrittlement of the plug and ends, as a rule, with such a “heavy” type of destruction of a fuel rod as the separation of the plug from the shell along the shell next to the mating zone of these parts.

 Hydrogenation of the retainer spring contributes to embrittlement and breakdown of part of its turns, which is especially harmful for fuel rods of the lower half of a symmetrical two-tier fuel assembly (fuel assembly) of the RBMK-1000 and RBMK-1500 reactors. This leads to lowering of the fuel column in these fuel rods (the latter in the lower half of the fuel assembly relies on the retainer spring) and an increase in the fuel gap in the central part of the reactor core. In turn, an increase in the fuel gap leads to an increase in the energy release surge in the indicated zone and, as a result, to an increase in the overheating of the claddings of the ends of the fuel rods in the central part of the active zone, which, ultimately, activates the processes of hydrogenation of the claddings in this most dressed up reactor zone.

 These design flaws of the fuel rods lead to a decrease in the reliability of their operation in RBMK reactors, worsen the environment, and cause significant economic damage.

An analogue of the proposed solution is a fuel rod described in Pickman [2] in which thermal resistances are introduced in the form of 2 sintered from Al ₂ O ₃ or ZrO ₂ between the fuel column and the lower fuel cap of the fuel rod and between the fuel column and the spring retainer in the upper part of the same fuel rod pills.

These thermal resistances exclude overheating, and consequently, local hydrogenation of the lower plug and retainer and the destruction that accompanies them. (There have never been any cases of tearing off the plug from the compensation volume of the fuel rods during the entire operation of the fuel rods. Apparently, this is due to the lack of contact on this side of the fuel rods with the plug, i.e., the large thermal resistance of the gas gap between the fuel and the plug in this place)
The disadvantages of the analogue are as follows.

 1. The exclusion of hydrogenation of the lower plug in the presence of hydrogen under the cladding of a fuel rod (mainly from moisture adsorbed on fuel pellets, which to a lesser or greater extent almost always occurs) does not exclude hydrogen under the cladding of a fuel rod. Consequently, the risk of hydrogenation and destruction of the fuel cladding pipe is potentially increased in other places (primarily in areas where local overheating will take place, for example, in places where there is a thick oxide film outside the cladding, or in places where the coolant boils).

 2. The introduction of heat insulators into standard fuel elements complicates and increases the cost of their production, as it requires the development of new technology and equipment for pressing and sintering ceramic tablets, their introduction into fuel elements, the control of their presence in fuel elements, etc.

 The closest in technical essence to the proposed solution is a fuel rod [3] in which one of the plugs, through the neck, which plays the role of thermal resistance, and the head, which acts as a getter on hydrogen and having a diameter smaller than the inner diameter of the shell, is in contact with the fuel. In this case, the head, neck and body of the plug are made as a whole, the ratio of the cross-sectional areas of the neck and head is recommended as 1: 16, and the plug from the compensation volume of the fuel element is made similar to the plug from the side of its contact with the fuel.

 The disadvantages of the prototype are as follows.

1. The recommended ratio of the cross-sectional areas of the neck and head 1: 16 practically corresponds to the ratio of thermal resistances if a heat insulator, for example from Al ₂ O ₃ , was used instead of the neck, with a diameter equal to the diameter of the getter head.

Such a large value of the thermal resistance of the neck virtually eliminates heat from the head-getter heating causes the latter to temperatures exceeding 750-800 ^{° C,} which causes decomposition of zirconium hydride and recovering therefrom a hydrogen vnutritvelnuyu back into the atmosphere. The latter reduces the efficiency of the getter head in absorbing hydrogen, bringing this design option closer to the analog and its main drawback.

 2. It is not indicated to what extent in order to achieve the main objective of the invention, the neck length of the thermal resistance should be selected.

 3. An essential factor determining the reliability of the getter head is the volume of metal of this head, which should be associated with the maximum possible hydrogen content under the cladding of the fuel rod. Data on determining the height of the getter head, and therefore its volume in the prototype, are also absent.

 4. The plug from the compensation volume of the fuel rod in the prototype is made similar to the plug from the side of its contact with the fuel.

 It is useless to do this, since hydrogen will be absorbed by the head only if it is heated to temperatures approximately twice the temperature of the internal surface of the fuel cladding.

 The purpose of the invention is to eliminate the above disadvantages of the prototype, to increase the reliability of a fuel rod by increasing the efficiency of the getter head, as well as to simplify its design and manufacturing technology.

≅

≅

при этом длина головки-геттера l_г определена из соотношения

≥

-1

,
где А максимально допускаемое по условиям изготовления содержание влаги в таблетках топлива в пределах 3,5ppm ≅ A≅ 7,0 ppm;
l_тс длина топливного столба твэла.The goal is achieved by the fact that in a well-known fuel rod of an energy nuclear reactor containing a zirconium alloy shell filled with ceramic fuel pellets, sealed on both sides with zirconium alloy plugs, one of which through the neck, which acts as thermal resistance and a getter head with a diameter smaller than the internal diameter of the shell in contact with the fuel, the neck length is chosen in the range of from two to six fuel rod cladding wall thickness, and the ratio of cross sectional area of neck S to the cross-sectional area S _d head chosen in the range:

≅

≅

the length of the getter head l _{g is} determined from the ratio

≥

-1

,
where A is the maximum moisture content in the fuel pellets under manufacturing conditions within 3.5ppm ≅ A≅ 7.0 ppm;
l _{tf is the} length of the fuel column.

 To simplify the technology of introducing a resonant neutron absorber into the fuel rod, which is very important for two-tier fuel assemblies (RBMK reactors), since it allows you to suppress the energy release surge in the most stressed part of the reactor core, the place where the fuel breaks in the center of the active zone, there may be a ring is cut on one side and crimped along this neck, made of metal hafnium wire, with an outer diameter not exceeding the diameter of the head disk.

 To ensure the realization of the advantages of the proposed fuel rod on the shells, the diameter of which does not exceed 8 mm, which makes the continuous neck not strong enough, the head with the neck can be made separately from the plug.

 In this case, it is made in the form of a disk with a central hole and slots on the fuel side, with a neck tubular part on the opposite side of the head disk (the hole and slots in the head disk are necessary for communicating the neck cavity with the fuel rod atmosphere).

 Moreover, in the end part of the working disk of the plug from the fuel side, a cylindrical recess is made with a diameter smaller than the outer diameter of the neck (its tubular part) by the interference fit. A getter head is pressed into the recess of the working disk of the plug with a tubular part by the neck.

 Such a design is convenient for inserting, for example, a getter onto an iodine into the plug part, which is necessary to prevent SCC (corrosion cracking under tensile stresses) in a medium containing iodine, which is a product of radioactive decay of fuel.

 In addition, in order to practically eliminate the possibility of hydrogen diffusion from the head through the neck and the working disk of the plug into the neck shell, it can be made separately from the head from a pipe segment made of a material with which hydrogen does not interact, for example, from stainless steel pressed in by interference fit the corresponding cylindrical recess in the end of the head disk.

 The invention is illustrated in figures 1-5.

 The proposed fuel rod is depicted in figure 1. It consists of a zirconium alloy shell 2 filled with fuel 1 and sealed on both sides with plugs 3 and 4, each of which has thermal resistance between the fuel column and the end face of the plug, with a retainer 5 in the compensation volume. The cap 4 is made of zirconium alloy. It can also be made of alloys based on other metals, which are analogues of zirconium in terms of physicochemical properties, for example, titanium. At the end of the plug 4, in one piece with it, a cylindrical protrusion is made, consisting of a neck 6, which performs the function of thermal resistance and diffusion resistance of hydrogen, and a head 7, which performs the function of a getter for hydrogen absorption during operation of a fuel rod, i.e. after warming up the fuel column.

 Figure 2a shows a variant in which a ring 9 cut from one side and crimped along this neck is made of a neck 9 made of a metal hafnium wire (resonant neutron absorber). The outer diameter of such a ring should not exceed the diameter of the head disk. The ring should be installed with gaps of the order of 0.3-0.4 mm, preventing the possible diffusion of hydrogen.

 Figure 2, b presents a variant of the lower end of the fuel rod with a head 7 and a neck 6, made separately from the plug 4. In this case, the cross section of the neck is selected the same as in the main version (figure 1). But the neck itself is made in the form of a tubular part with an outer diameter corresponding to the fit diameter of the cylindrical recess in the working disk of the plug. In this case, the wall thickness of the tubular part of the neck is selected so that it settles in diameter when pressing the plug 4 into the cylindrical recess of the working disk. In practice, the neck wall thickness in this case is chosen equal to 0.3-1 of the δ shell (a smaller value corresponds to a larger δ shell and greater than the smaller δ shell), i.e. in the range of 0.3-0.4 mm. To communicate the cavity of the tubular part of the neck with the atmosphere of a fuel rod, a central hole is made in the head (with a diameter of 2-2.5 mm), and slotted slots can be made at its end in contact with the fuel and connected to the central hole in the head.

 Such a neck geometry is preferred for fuel rods with claddings whose outer diameter does not exceed 8 mm. However, it can also be used for RBMK-1000 and RBMK-1500 fuel rods, since it allows the neutron absorber 10, for example, metallic Hf or getter to absorb iodine, to be introduced into the fuel gap of a two-stage fuel assembly.

 In FIG. 2, a variant of the lower end of a fuel element is shown, in which the neck is made separately from both the head and the plug, as a piece of pipe 6 made of a material that is practically unreacted with hydrogen and does not pass it, for example, stainless steel.

 This option is proposed to exclude even the fundamental possibility of hydrogen diffusion from the head into the mating zone of the shell and the plug, and, consequently, exceptions in principle of embrittlement and destruction of this zone.

 In this case, the selection of the wall thickness of the pipe segment made of stainless steel is carried out similarly to the selection of the wall thickness of the tubular part of the neck, respectively (Fig. 2, b, and the cylindrical recesses in the ends of the plug and head are made with a diameter smaller than the outer diameter of the pipe segment by the interference fit .

 Such a choice of the wall thickness of the neck pipe and the fit dimensions of the mating parts provides a draft according to the diameter of the ends of the neck pipe when it is pressed into the plug and head and makes it possible to reliably and reliably connect these parts into a single whole.

 Of course, inside the pipe neck 6, if necessary, various absorbers 10, for example, neutrons or iodine, can be placed in the simplest way. The geometry of the head 7 of the plug 4 was selected based on the following.

0,4 мм.The diameter of the disk head D _g selected equal
D _g = d- (0.5-2) δ, where d is the inner diameter of the cladding of a fuel rod; δ wall thickness of the cladding of a fuel rod, based on the fact that, on the one hand, there must be a gap between the head and the cladding, eliminating the possibility of hydrogen diffusion directly from the head into the cladding, on the other hand, this gap should be sufficient so as not to complicate assembly and welding fuel cladding with a plug. And, finally, the last of this gap should not significantly reduce the head volume, otherwise it will require an increase in its height and, therefore, for a fuel assembly like RBMK, an increase in the fuel gap, which is unacceptable. The lower boundary of the limit (0.5–2) δ is chosen mainly for cladding of fuel elements with a wall thickness δ of the order of 1 mm, and the upper limit for claddings in which δ

0.4 mm.

≥

-1

.The length (height) of the head l _{g was} determined using the ratio

≥

-1

.

 This ratio is obtained on the basis of the following considerations.

 The volume of the metal of the head (the minimum should be proportional to the maximum amount of hydrogen that can be contained under the cladding of the fuel rod.

 The maximum amount of hydrogen in a fuel rod is proportional to the mass of the fuel and the maximum moisture content in it, the decomposition of which gives hydrogen.

 The first of these conditions is defined as follows.

 With the same moisture content in all fuel pellets, the amount of hydrogen in the fuel rod is proportional to the length of the fuel column.

This amount of hydrogen must be absorbed either by the volume of zirconium of the head V _g , or, if the plug is made without a neck and head, part of the metal of the plug and part of the metal of the shell. The latter can be seen from Fig. 3, which shows the end of an RBMK fuel rod, which was depressurized due to hydrogenation of the interface between the plug and the shell during operation (for RBMK fuel elements, the maximum moisture content in the fuel is 7ppm, fuel column length l _mf = 3420 mm). Figure 3 shows that the volume of the hydrogenated metal V _Zr is equal to (taking into account the uneven distribution of the hydride distribution of hydride in zirconium 1.25 g and the safety factor for the volume of zirconium 2) the volume of the metal ring with an outer diameter of d + 2δ, wall thickness 2 δ and height 4 δ, i.e.

V _Zr ≥ 10 π d δ ² .

Therefore, such a minimum volume V _Zr should have a head for absorption of the maximum amount of hydrogen under the cladding of a fuel rod, i.e.

V _g ≥ 10 π d δ ² .

, а отношение объема головки V_г к объему топливного столба V_тс будет равно отношению длины головки l_г к длине топливного столба l_тс, т.е.If the head of the plug has a diameter D _g equal to the diameter of the fuel pellet D _t and equal to the inner diameter of the cladding of the fuel element d, then, to a first approximation, we can accept, since these values differ slightly in practice, the length (height) of the head l _g will be equal to
l _g =

and the ratio of the head volume V _g to the volume of the fuel column V _tf will be equal to the ratio of the head length l _g to the length of the fuel column l _tf , i.e.

Учитывая последние соотношения, найдем численное значение

применительно к твэлам РБМК-1000 (d=11,7 мм; D

D_г=11,5 мм; δ1мм; l_тс=3420 мм):

Очевидно, что это соотношение будет справедливо для всех твэлов, имеющих 7ppm влаги в топливе и одинаковую плотность таблеток топлива (при D_г

D_т).

Given the last relations, we find the numerical value

in relation to RBMK-1000 fuel rods (d = 11.7 mm; D

D _g = 11.5 mm; δ1mm; l _tf = 3420 mm):

Obviously, this ratio will be true for all fuel elements having 7ppm of moisture in the fuel and the same density of fuel pellets (at D _g

D _t ).

 Given that, due to cladding hydrogenation, on average 2-3 fuel rods out of 1000 fail, and that the moisture in the fuel ranges from 0 to 7ppm (i.e., fuel pellets with moisture and 0ppm and 7ppm can be present in one fuel rod), practice it was found that up to about 3.5ppm hydrogen does not affect the destruction, or rather, does not give them (997 fuel elements survive the campaign without damage to the cladding, and 2-3 fuel elements out of 1000, in which all, or almost all fuel pellets have 7ppm moisture, are destroyed).

для твэлов с содержанием влаги в топливе отличном от 7ppm, но превышающим 3,5ppm, должен быть добавлен коэффициент К, равный
K

1.Assuming that the dependence of the number of fractures on the moisture content in the fuel, as a first approximation, sufficient for practice, is linear, in relation

for fuel elements with a moisture content in the fuel other than 7ppm, but exceeding 3.5ppm, a K coefficient equal to
K

1.

≥

1

, где А допустимое содержание влаги в топливе, превышающее 3,5ppm.Thus, we finally obtain

≥

1

where A is the permissible moisture content in the fuel in excess of 3.5ppm.

 The geometry of the neck 6 of the plug 4 was determined as follows.

Thermal resistance R _w neck should be great enough to impede heating disposed below the working drive stub (welded to shell) to a temperature greater than the temperature of the inner surface of the shell by more than ^about 30-50 C than hydrogen uptake virtually eliminated working disc plugs from free volume of a fuel rod, greater absorption of hydrogen by the cladding.

On the other hand, R _w should not be too great, otherwise, if the contact of the head with the fuel and, moreover, at the very head temperature exceeds 700 ^C, the hydrogen absorbed in the head to form a final hydrides, will be partially released from hydrides back to the intra-atmosphere, i.e. the head as a getter will work poorly.

, из которого определяется диаметр шейки Dш и соответственно R_ш, была определена расчетом (на ЭВМ по специально разработанной программе, методом конечных элементов, применительно к условиям эксплуатации твэлов РБМК) с условием, что температура в контакте топлива с головкой не должна превышать 700^оС.Therefore, the lower boundary of the ratio of the cross-sectional area of the neck S _w to the cross-sectional area of the head S _g i.e.

Of which is determined by the diameter of the neck DmN and respectively R _w, was determined by calculation (with a computer on a specially designed program, the method of finite elements, in relation to operating conditions fuel RBMK) with the proviso that the temperature in the contact of the fuel with the head must not exceed 700 ^{° C.} .

≥

При этом учитывалось, что при эксплуатации, вследствие различных температур по оси (до 1500^о) и на периферии (до 650^оС) таблеток топлива, и следовательно, различных термических расширений этих зон, а также вследствие возможных деформаций размягченной сердцевины таблеток топлива при термомеханическом взаимодействии топлива с оболочкой (храповой эффект) возможна выборка зазора в лунке таблетки топлива. Т.е. расчет вели, считая, что головка имеет контакта с таблеткой по всей поверхности ее торца, а не только по наружному кольцу.The calculations (in relation to RBMK fuel rods) showed (with the head diameter D _g = 10 mm and the neck length l _w = l _g = 4.0 mm) that this condition is satisfied by the inequality
D _w ≥ 3.8 mm, i.e.

≥

When taken into account that during operation, due to the different temperatures along the axis (1500 ^a) and the periphery (650 ° ^C) fuel pellets, and therefore different thermal expansions of these zones, and also due to possible deformations of the softened core fuel pellets thermomechanical interaction of the fuel with the shell (ratchet effect), it is possible to sample the gap in the well of the fuel tablet. Those. the calculation was carried out, assuming that the head has contact with the tablet over the entire surface of its end face, and not only along the outer ring.

(If it is assumed that the head is in contact with the pellet fuel only on the ring outer diameter of the latter, i.e. in an area where even in the absence of contact with the head surface temperature on the cylindrical fuel pellets due to radial heat sink to the coolant does not rise above 650 ^C. , then, as calculations show, R _W can be taken equal to R _{and the} thermal resistance of the insulator of ZrO ₂ .

·

R_и=

λ_Zr= 0,2;

= 0,012.In this case
R _W =

·

R _and =

λ _Zr = 0.2;

= 0.012.

Для твэлов РБМК с оболочками размером 13,6х11,7 мм, полагая D_и=D_т=11,5 мм и учитывая, что

находим
D_ш=2,8 мм.Assuming: R _W = R _and ; l _W = l _and and D _and = D _t , where l _and ; D _and ; S _{and the} length, diameter, cross-sectional area (respectively) of the insulator tablet;
l _w ; D _W ; S _w length, diameter, cross-sectional area of the neck of the stub;
D _{t is the} diameter of the fuel tablet;
λ the thermal conductivity of Zr and ZrO ₂ , we find

For RBMK fuel rods with claddings 13.6 x 11.7 mm in size, setting D _and = D _t = 11.5 mm and considering that

we find
D _W = 2.8 mm

If such R _w taken into account that the contact with a head of fuel available around the head end, it will cause an increase in contact temperature, calculations show an average of ^about 800 C, which as mentioned is unacceptable because deteriorates head job of the getter. In this case, a recess can be made at the end of the head, eliminating the contact of fuel across the entire end of the head.

In addition, D _W = 2.8 mm (at D _g = 11.5 mm or D _W = 2.5 mm at D _g = 10 mm) is insufficient to counteract axial bending and flattening under the influence of possible axial compression forces of the fuel column.

(или нижний предел R_ш), учитывая сложность и неточность его определения рассчетным путем, обусловленную, главным образом, необходимостью определения границ гидрирования, имеющих лучеобразный характер, различный в разных плоскостях сечений, определялся в реакторном эксперименте на шлифах концов твэлов полномерной опытной сборки РБМК-1000. Один из этих твэлов разгерметизировался (продольная трещина в оболочке), по-видимому вследствие распухания топлива и увеличения диаметра оболочки (до 14,5 мм).Upper limit of relationship

(or lower limit R _W ), taking into account the complexity and inaccuracy of its determination by calculation, mainly due to the need to determine hydrogenation boundaries having a beam-like character, different in different plane sections, was determined in a reactor experiment on the thin sections of the ends of the fuel rods of the full-scale experimental assembly RBMK- 1000. One of these fuel rods was depressurized (a longitudinal crack in the cladding), apparently due to fuel swelling and an increase in cladding diameter (up to 14.5 mm).

 A photograph of a longitudinal section of the lower sealing joint of an untight fuel rod is shown in Fig. 4, b.

 In FIG. 4a, a longitudinal section is shown of a stub in the form of a “glass” with a central “rod” and a “leg” used to seal these fuel elements (a number of other issues were simultaneously solved during their testing).

 From FIG. 4b it can be seen that only the walls of the glass and the central core in it underwent hydrogenation and destruction, while hydrogen almost only started to reach the glass's legs. The inner burr of the weld and the interface between the shell and the plug from hydrogen are free. Thus, in this case, the head in the form of a “shot glass” played the role of a getter and protected the inner fillet of the weld and the interface between the cladding and the plug from hydrogen, despite the fact that the amount of hydrogen in this depressurized fuel element, most likely from the heat transfer medium, was big enough.

 There is no doubt that, if the fuel was in contact with the internal burr (Fig. 3), then the hydrogenation of the latter and the interface zone with the shell would occur, and then the test would end by detaching the plug through the shell.

 Moreover, for a sufficiently large time from depressurization, i.e. when hydrogen appeared, before the test was stopped, hydrogen did not reach the “proper” plug, which suggests that this will not happen for the entire campaign, although in principle, at significantly longer campaign times, hydrogen can still reach the plug and shell due to diffusion.

 To prevent destruction at significantly longer durations of the operation campaign, a solution should be used that almost completely eliminates the diffusion of hydrogen and the head into the neck, plug and shell.

Это соответствует

что и принято за верхнюю границу этого отношения.From figure 4, b (photo taken at magnification x 7) using large-scale measurements found that the ratio

It corresponds

which is accepted as the upper limit of this relationship.

 It should be noted that the identity of the heat conduction and diffusion equations allows us to consider the neck not only as thermal resistance, but also as diffusion resistance, i.e., as a barrier to diffusion, in this case, hydrogen from the head to the working disk of the plug welded to the shell, which contributes to the concentration of hydrogen in the head and prevents the hydrogenation and embrittlement of the shell in the area adjacent to the plug.

должно находиться в пределах

≅

≅

Если

<

то R_ш будет достаточно велико для исключения перегрева и гидрирования рабочего диска заглушки и примыкающей к ней части оболочки, но в этом случае возможен перегрев головки и выход вследствие этого водорода из гидрида головки, что недопустимо.Thus, to fulfill the task, the ratio

should be within

≅

≅

If

<

then R _W will be large enough to exclude overheating and hydrogenation of the working disk of the plug and the adjacent part of the shell, but in this case the head may overheat and, as a result, hydrogen may escape from the head hydride, which is unacceptable.

>

, то температура головки будет ниже 700^оС и разложение гидрида головки будет исключено, но R_ш будет недостаточно, температура рабочего диска заглушки возрастет выше температуры оболочки и поглощение водорода этой зоной может пойти параллельно с поглощением водорода головкой, что также недопустимо.If

>

, The head temperature is below 700 ^{° C} and decomposition head hydride will be omitted, but R _w is not enough, the working disk temperature rise above the plug and the shell temperature of the hydrogen absorption area can go along with the hydrogen absorption head, which is also unacceptable.

For the neck l _w accepted equal
l _w = (2-b) δ,
based on the fact that the head disk must not be guaranteed to come into contact with the inner weld bead, and the neck should have sufficient thermal and "diffusion" resistance. At the same time, the neck should be sufficiently strong and resistant to axial bending under the influence of possible axial forces of the fuel column and its height should not lead to a significant increase in the fuel gap in the RBMK fuel assemblies.

 Moreover, a smaller value of the selected limit (2-6) refers mainly to shells with a larger δ, and a larger one to shells with a smaller δ.

 The proposed fuel rod works as follows.

After filling the core with fuel rods and starting up the reactor (or reloading the rods “on the fly” in RBMK reactors), the fuel pellets are heated, and the contact of the fuel with the head of the lower plug reaches a temperature of 450-600 ^о С, while the inner surface of the shell along its entire length, including the interface zone with its plugs, it is heated only to 330-350 ^о С.

 As a result, all the hydrogen present under the cladding of the fuel rod and released during operation, for example, during the decomposition of moisture in the fuel, cracking of the tablets, etc. absorbed by the cap head and protects the shell from hydrogenation and destruction. At the same time, the thermal resistance of the neck, excluding the overheating of the plug relative to the shell, eliminates the possibility of hydrogen absorption by the plug and the adjacent part of the shell from the intrafuel atmosphere, and the "diffusion" resistance of the neck, being a barrier to diffusion of hydrogen from the getter head, protects the working disk of the plug from penetrating hydrogen from the head due to diffusion.

 In addition, the Hf ring, reducing the neutron burst in the fuel gap zone, reduces overheating in this zone, increasing the safety and controllability of the reactor.

 If the getter on iodine is used, the possibility of the development of stress corrosion cracking (SCC) in a medium containing this element is sharply reduced, which protects the fuel cladding from damage during operation.

The proposed technical solution allows, due to the use of getter properties of Zr alloys with respect to hydrogen and the location of the head disk acting as a getter, in the zone where its temperature is (100-200) ^о С higher than the temperature of the fuel cladding, together with the separation of the getter head from the plug with thermal resistance in the form of a neck with a cross section (compared with the head disk), it is essential to reduce the content of free hydrogen in the free volume of a fuel rod and thereby protect not only the area of collisions from embrittlement and destruction the tension of the shell and the plug, as provided by the prototype, but also the shell along its entire length, as well as a spring clip.

 In addition, the proposed technical solution simplifies the design and manufacturing technology of fuel rods and allows practically without increasing the length of the fuel break zone in RBMK reactors to introduce a resonant neutron absorber into this most energetic zone of the reactor, which, in addition to lowering the temperature in it and correspondingly reducing the possibility of local processes hydrogenation of the cladding of fuel rods in this zone increases the controllability of the reactor and its safety.

 The specified technical solution does not require the organization of new industries, for example, heat insulators and getters, as well as changes in the manufacturing technology of fuel elements, including their sealing. For its implementation, you only need to change the program for cutting machining of the automatic lathes on which plugs are made, and slightly change the overpass for supplying plugs to the welding zone on automatic welding machines in the lines of automated production of fuel elements.

 The claimed ratios of the sizes of the disk of the head and neck at the end of the lower plug provide the amount of thermal resistance between the fuel column and the plug needed to heat the getter head in the required temperature range, which makes it possible to efficiently use the getter properties of the zirconium head in contact with the fuel, and thereby achieve the main purpose of the invention is to reduce the hydrogen content in the free volume of a fuel rod during operation, and therefore, to prevent the hydrogenation of cladding ki over its entire length. This allows us to conclude that the claimed features of the invention are interconnected by a single concept. These differences from the prototype allow us to establish compliance with their criterion of "novelty."

 When studying other well-known technical solutions in this technical field, signs that distinguish the claimed solutions from the prototype were not identified. Therefore, they provide the proposed technical solution according to the criterion of "significant differences".

The effectiveness of the proposed solution is confirmed by the following:
1. In the detected cases of destruction of the mating zone between the cladding and the plug on RBMK type fuel rods due to hydrogenation and embrittlement of these zones (Fig. 3), it was established that the causes of the damage are the presence of hydrogen under the cladding of individual fuel rods and overheating of the plug in contact with the fuel. In other words, in the presence of hydrogen under the shell, the contact point of the plug, the place of overheating of the plug relative to the shell is a getter that absorbs hydrogen in the first place.

 2. The fact that the contact site of the plug with the fuel absorbs hydrogen in the first place has been proved experimentally in a laboratory setup for modeling the behavior of the end of a fuel rod during operation during the hydrogenation of whole-hole specimens imitating a welded assembly. The results of these experiments, in which operational temperature gradients and the amount of hydrogen corresponding to the decomposition of the moisture of the fuel 7ppm in each tablet were simulated, are illustrated in figa, b.

3. An increase in the absorption of hydrogen with increasing temperature of the zirconium sample and the partial pressure of hydrogen was noted. The paper shows the corresponding graph which shows that compared with the temperature of 350 ^{° C} (cladding temperature during operation) at a temperature of about 450-500 ^{° C} (temperature end plugs during operation) zirconium hydrogen absorption capacity increases by about an order of magnitude.

 4. Direct confirmation of the effectiveness of the proposed solution was obtained in a reactor experiment in an experimental reactor during tests of an experimental assembly of fuel assemblies of the RBMK type. One of the fuel rods of this assembly was depressurized (a longitudinal crack in the cladding), apparently due to fuel swelling and an increase in cladding diameter (up to 14.5 mm).

 A photograph of a longitudinal section of the lower sealing joint of an untight fuel rod is shown in Fig. 4, b.

 There is no doubt that if the fuel was in contact with the internal burr, then the hydrogenation of the latter and the interface zone with the shell would occur, and then the test would end with the separation of the plug through the shell (similar to figure 3).

 The cost-effectiveness of the proposal is obvious, since the failure of only one RBMK cassette to half of the electricity campaign brings a big loss.

Claims (4)

1. A fuel rod of an energy nuclear reactor containing a zirconium alloy shell filled with ceramic fuel pellets, sealed on both sides with plugs made of zirconium alloy, one of which is in contact with the fuel through a neck and getter head with a diameter smaller than the inner diameter of the shell characterized in that, in order to increase the reliability of the fuel rod by increasing the efficiency of the getter head, the neck length is selected in the range from two to six thicknesses of the wall of the fuel sheath, and the ratio the cross-sectional area of the neck S _W to the cross-sectional area of the head S _g selected within

the length of the getter head l _{g is} determined from the ratio

where A is the maximum permissible moisture content in the fuel pellets under the manufacturing conditions within
3,5rrt ≅ A ≅ 7,0rrt;
l _{t with the} length of the fuel column of the fuel rod.  2. A fuel rod according to claim 1, characterized in that, in order to simplify the introduction of a resonant neutron absorber into it in two-tier fuel assemblies, a ring cut from one side and crimped along this neck, made of a metal hafnium wire, is placed on the neck of the fuel rod plug.  3. A fuel rod according to claim 1, characterized in that the head with a neck is made with a central hole and separately from the plug, and in the end of the latter on the fuel side there is a recess with a diameter smaller than the outer diameter of the neck by the interference fit, while the neck is pressed into recess of a stub.  4. A fuel rod according to claim 3, characterized in that, in order to exclude diffusion of hydrogen from the head through the neck and the plug into the shell, the neck is made separately from the head from a piece of stainless steel pipe, pressed into fit with an interference fit in the corresponding cylindrical recess of the head.