EP1693855B1

EP1693855B1 - Fuel element for a fast neutron reactor (variants) and a cladding for the production thereof

Info

Publication number: EP1693855B1
Application number: EP04793751.1A
Authority: EP
Inventors: Alexandr Grigorievich Ioltukhovsky; Maria Vladimirovna Leontieva-Smirnova; Alexandr Viktorovich Vatulin; Viktor Nikolaeyich Golovanov; Valentin Kuzmich Shamardin; Tatyana Mikhailovna Bulanova; Valentin Vladimirovich Tsevelev; Igor Alexeevich Shkabura; Jury Alexandrovich Ivanov; Vladimir Alexandrovich Forstman; Mikhail Ivanovich Solonin
Original assignee: Federal State Unitarian Enterprise " AA Bochvar All-Russia Research Institute of Inorganic Materials"
Current assignee: Federal State Unitarian Enterprise " AA Bochvar All-Russia Research Institute of Inorganic Materials"
Priority date: 2003-10-06
Filing date: 2004-10-04
Publication date: 2014-12-17
Anticipated expiration: 2024-10-04
Also published as: CN1890758A; WO2005034139A3; EP1693855A2; CN1890758B; WO2005034139A2; EP1693855A4; RU2003129679A; RU2262753C2

Description

Technical Field

The present invention relates to nuclear engineering range, specifically, to materials used in nuclear engineering and might be applied to fabricating fuel elements and components of fast nuclear reactor cores (below - BN reactors).
Steels used to fabricate fuel element claddings and other components of BN reactors have to meet a series of rigid requirements for characteristics of heat resistance, resistance to low temperature irradiation effected embrittlement, resistance to vacancy effected swelling, processability, weldability and others. Recently much importance has been also attached to requirement for prompt decay of induced radioactivity of steels after their discharge from neutron field of BN reactor core, in other words, to designing so-called low activation steels. One of promising materials for fuel claddings is chromium steels that feature favourable properties, particularly, those that are insignificantly subject to irradiation induced swelling,

Background Art

Low activation irradiation resistant steel is known (see RU patent N 2135623 ) that contains carbon, silicon, manganese, chromium, nickel, vanadium, copper, molybdenum, cobalt, tungsten, yttrium, niobium, aluminum and iron at the following ratio between components, percent by weight: carbon - 0.13-0.18; silicon 0.20-0.35; manganese - 0.30-0.60; chromium - 2.0-3.5; tungsten - 1.0-2.0; vanadium - 0.10-0.35; molybdenum 0 0.01-0.05; nickel - 0.01-0.05; cobalt - 0.01-0.05; copper - 0.01-0.10; aluminum - 0.01-0.10; niobium - 0.01-0.05; yttrium - 0.05-0.15; the balance being iron.
Here, the total content of nickel, cobalt, molybdenum, niobium and copper makes up not more than 0.2 percent by weight, while the ratio (V+0.3W)C varies within 3 to 6.
This steel has low induced activity, however, is not heat resistant at a temperature exceeding 500°C (see M.V.Zakharov, A.M.Zakharov. Heat Resistant Alloys, M.: Metallurgy, 1972).
Also known is low activation heat resistant (up to 550°C) steel (according to RU patent N 2033461 ) that contains carbon, silicon, manganese, chromium, tungsten, vanadium, boron, titanium, cerium and iron at the following ratio between components, mass %: carbon - 0.10-0.20; silicon - 0.02-1.00; manganese - 0.50-2.0; chromium - 10.0-13.9; tungsten - 0.8-2.9; vanadium - 0.05-0.45; titanium - 0.01-0.10; boron - 0.0005-0.008; cerium - 0.001-0.100, the balance being iron.
In the described invention the irradiation properties of this steel are not given. However, as it is revealed by investigations (A.G.Ioltukhovsky, M.V.Leont'eva-Smimova, V.S.Ageev and others "Influence of Original Structure State on Propensity of 12% Chromium Steels for Irradiation Effected Embrittlement". Collected papers of 3d Interbranch Conference on Reactor Materials Science, Dimitrovgrad, 1994, v.1, p.51) a steel that has such a composition has to be subject to low temperature irradiation embrittlement (below LTIE) because of 40-50% of δ-ferrite available in its structure. Hence, the major disadvantages of this steel that is most close to the claimed steel are insufficient heat resistance at a temperature higher than 550°C and a lower resistance to LTIE.
A steel that in terms of composition of ingredients and functioning approaches most closely the claimed one is EP823 steel the composition and properties of which are described in the paper by M.I.Solonin., F.G.Reshetnicov, A.G.Ioltukhovsky and others "Novel Structural Materials for Cores of Nuclear Power Installations", in journal "Physics and Chemistry of Material Processing", 2001, N4, pp.17-27.
The steel contains, percent by weight: carbon -0.14-0.18; silicon - 1.1-1.3; manganese -0.5-0.8; chromium - 10.0-12.0; nickel - 0.5-0.8; vanadium - 0.2-0.4; molybdenum - 0.6-0.9; tungsten - 0.5-0.8; niobium - 0.2-0.4; boron - 0.006 (as calculated) cerium - not more than 0.1; nitrogen - not more than 0.05; sulphur - not more than 0.010; phosphorus - not more than 0.015; the balance being iron.
The major disadvantages of this steel are its high activation ability under neutron irradiation via neutron reactions on Ni, Mo, Nb, Cu, Co and other elements to form long-lived radioactive isotopes as well as low heat resistance at a temperature not lower than 650°C and propensity for LTIE within 270-400°C.

Disclosure of Invention

The technical objective of the invention is to create a BN reactor fuel element with a cladding having the following properties: low induced activity and a higher rate of its decay after exposure to neutrons, higher resistance to embrittlement in the temperature range from 270 to 400°C under neutron irradiation and a high heat resistance at temperatures up to 700°C.
The technical result is to create a fuel element with a cladding having a high resistance to embrittlement in the temperature range from 270 to 400°C as well as a higher level of heat resistance at temperatures up to 700°C and high performance.
The set up objective pertaining to the first version of realizing a BN reactor fuel element is put in practice via fabricating its cladding from ferritic-martensitic class steel having a non-uniform structure along a cladding length that consists of at least two zones; with the steel structure in the top zone of a fuel element providing its higher heat resistance and in the bottom zone enhancing its resistance to low temperature irradiation embrittlement. The invention is given in the claims.

The set up objective pertaining to the second version of realizing a BN reactor fuel element is put in practice via fabricating its cladding from low activation steel containing carbon, silicon, manganese, chromium, vanadium, tungsten, boron, cerium and/or yttrium, titanium, tantalum, zirconium, nitrogen, iron and unavoidable impurities at the following ratio between components, percent by weight :

carbon	- 0.10-0.21;
silicon	- 0.1-0.8;
manganese	- 0.5-2.0;
chromium	- 10.0-13.5;
tungsten	- 0.8-2.5;
vanadium	- 0.05-0.4;
titanium	- 0.03-0.3;
boron	- 0.001-0.008;
cerium and/or yttrium in total	- 0.001-0.10;
zirconium	- 0.05-0.2;
tantalum	- 0.05-0.2;
nitrogen	- 0.02-0.15;
iron	- the balance

at the ratio of the total content of vanadium, titanium, zirconium and tantalum to the total content of carbon and nitrogen being from 2 to 9; the structure of the steel along a cladding length consists of at least two zones; with the steel structure in the top zone of a fuel element providing its higher heat resistance and containing α -ferrite, δ - ferrite, sorbite, chromium carbides M₂₃C₆ and M₆C, carbides and carbonitrides of the steel components (V, Ta, Ti, Zr, W and others), intermetallics of the Fe₂(W) type while the structure in the bottom zone enhancing its resistance to low temperature irradiation embrittlement consists of sorbite, δ-ferrite, α-ferrite, residual austenite, carbides and carbonitrides of steel components (Cr, V, Na, W and others); in this case high-angle boundaries of grains are occupied by carbides M₂₃C₆ and M₆C while grains of both sorbite and δ -ferrite have but individual precipitates of carbides and carbonitrides VC, V(CN), Ti(CN) and Ta(CN) and the other elements (Fe, Mn, Mo, W, Si and others) enter into the compositions of either complex carbides M₂₃C₆ and M₆C or a FeCr solid solution.

The set up objective is also put in practice via fabricating a cladding for a fast reactor fuel element from low activation steel containing carbon, silicon, manganese, chromium, vanadium, tungsten, boron, cerium and/or yttrium, titanium, tantalum, zirconium, nitrogen, iron and unavoidable impurities at the following ratio of components, percent by weight:

at the ratio of the total content of vanadium, titanium, zirconium and tantalum to the total content of carbon and nitrogen being from 2 to 9; the steel structure along a cladding length consists of at least two zones: with the steel structure in the top zone of a fuel element providing its higher heat resistance and containing α-ferrite, sorbite, chromium carbides M₂₃C₆ and M₆C, carbides and carbonitrides of steel components (V, Ta, Ti, Zr, W and others), intermetallics of the Fe₂(W) type while the structure in the bottom zone enhancing its resistance to low temperature irradiation embrittlement consists of sorbite, δ -ferrite, α -ferrite, residual austenite, carbides and carbonitrides of steel components (Cr, V, Ta, W and others); in this case high-angle boundaries of grains are occupied by carbides M₂₃C₆ and M₆C while grains of both sorbite and δ -ferrite have but individual precipitates of carbides and carbonitrides VC, V(CN), Ti(CN) and Ta(CN) and the other elements (Fe, Mn, Mo, W, Si and others) enter into the compositions of either complex carbides M₂₃C₆ and M₆C or a FeCr solid solution.

Variants of Particular Embodiments of Invention

In a particular case of the first version of a fuel element realization the structure of a steel along a cladding length consists of three zones; with the middle zone structure having intermediate values of heat resistance and resistance to low temperature irradiation embrittlement in comparison to those in the bottom and top zones.

In one of special cases of the second version of fabricating a fuel cladding the contents of unavoidable impurities are limited by concentrations:

nickel	- not more than 0.1;
niobium	- not more than 0.01;
molybdenum	- not more than 0.01;
copper	- not more than 0.1;
cobalt	- not more than 0.01;
sulphur	- not more than 0.008;
phosphorus	- not more than 0.008;
oxygen	- not more than 0.005.

In another special case of fabricating a cladding the total content of impurities of high activation metals, viz., molybdenum, niobium, nickel, copper and cobalt, does not exceed 0.1 %mass. This serves to reduce neutron irradiation induced activation and to accelerate the decay rate of induced activation of steel.
In another special case of fabricating a cladding the total content of impurities of low-melting metals, viz., lead, bithmuth, tin, antimony and arsenic, dots not exceed 0.05 %mass. This serves to enhance the resistance of steel to low temperature irradiation embrittlement under neutron irradiation.
The essence of the invention consists in the fact that the steel structure along a cladding length is made non-uniform and is brought in conformity with the actual temperature drop along the length of the BN reactor core which makes up several hundred degrees. That is why, the claimed invention puts in practice a novel principle of placing different requirements for the properties and structure of a fuel cladding along its length.
The steel structure in the bottom (low temperature) zone of a fuel cladding includes sorbite, α -ferrite, residual austenite, carbides of likely strong carbide forming components of the steel (Cr, V, Ti, Ta, W and others), nitrides and carbonitrides of the above mentioned elements; boundaries of large-angle grains are basically free from precipitates of carbides M₂₃C₆ and M₆C and have an intragranular coagulated carbide phase. As a result of formation of such a structure a solid solution is depleted in chromium and is not capable of irradiation effected precipitation of embrittling phases (υ- phase, α- phase and others) which increases the resistance of a cladding to LTIE processes in its low temperature bottom zone.
To create such a structure the low temperature zone of a fuel element is subjected to cycling (up to 10 cycles) which comprises heating to the temperature A_C1 + 20°C and cooling to room temperature; in this case the rate of product cooling after tempering and in the process of cycling and after it is finished has not to be below 50°C/min upon cooling in air.
As it follows from the above said to ensure the optimal structure of a fuel cladding in high - and low -temperature zones and provide it with low activation as well as heat and irradiation resistances the steel is subjected to complex alloying with elements having a prompt decay of irradiation induced activity to attain a specified ratio between γ-stabilizing elements (C, N, Mn) and α -stabilizing ones (Cr, W, V, Ta, Ti, Zr, Mo, Nb and others).
In a high temperature zone of a fuel element where the operating temperature of a cladding is not lower than 600°C the high level of heat resistance is reached through forming a stable martensitic - ferritic structure of a cladding that contains solid solution strengthening interstitial elements (C,N,B) and substitutional elements (W, V, Cr), strengthening carbide (MC, M₂C, M₂₃C₆ and others), nitride (MN, M₂N) and carbonitride (MCN) phases as well as Laves phases of the Fe(W) type.
This structure in the high temperature zone of a fuel cladding is formed via quenching at a temperature of 1050-1150°C for 40 min followed by tempering at 680-760°C.
An increase in the content of tungsten that is introduced in place of molybdenum at an approximately equivalent ratio provides a lower irradiation induced activation of a cladding and its prompt decay with time after neutron exposure due to a lower section of neutron interaction with tungsten nuclei and a lower half-life of irradiation produced isotopes of tungsten, respectively. An increase in the content of tungsten also promotes preservation of a high level of long-term and short-term strength of steel.
Through introducing zirconium, tantalum and nitrogen the short-term and long-term strength of steel remains at an adequately high level.
Through introducing nitrogen and limiting within 2 to 9 the ratio of the total contents of titanium, tantalum, zirconium and vanadium to the total contents of carbon and nitrogen the resistance of steel to low temperature irradiation effected embrittlement under neutron irradiation increases.
Cerium and/or yttrium introduced in the quantity of 0.001-0.10 %mass promotes the refining and atomizing of steel grains. In this case cerium and yttrium being low activation elements do not increase the induced activity of the steel claimed.
The low levels of cerium and/or yttrium content correspond to the minimal concentration at which its favourable influence on steel refining is noted. The value of the upper limit of cerium and/or yttrium content promotes preservation of the adequate processability by steel at a hot stage.
The low limit of zirconium content is determined by the need of binding a part of nitrogen into finely dispersed and thermodynamically stable particles of zirconium nitride.
The upper limit of zirconium content is determined by the feasible formation of low melting zirconium-iron eutectic which might reduce the processability of steel.
The low limit of titanium content is determined by the need of binding a part of carbon into thermodynamically stable finely dispersed carbides of titanium.
The upper level of titanium content is determined by feasible redistribution of nitrogen between zirconium and titanium which is not desired because of a possible decrease in long-term strength of steel.
The low level of tantalum content is determined by the need of binding a part of carbon into thermodynamically stable tantalum carbides and ensuring its content in solid solution at the level of ultimate solubility.
The upper limit of tantalum content is determined by feasible formation of globular carbide inclusions that reduce the processability of steel.
The low limit of nitrogen content is determined by the need of binding zirconium into finely dispersed particles of zirconium nitride. The restriction of the nitrogen upper limit is required to provide the processability of steel during welding: zirconium and tantalum being low activation elements do not increase the induced activity of the claimed steel.
Nitrogen as an isotope ¹⁴N (99% content) becomes activated under neutron irradiation to form a long-lived isotope ¹⁴C that upon decaying (half-life of 5.7x10³ years) produces α-particle (stable isotope ⁶He) without resulting in γ-radiation, i.e., the availability of nitrogen does not influence the decay of steel radiation activity determined by γ-radiation.
The content of silicon ranges from 0.1 to 0.8 to ensure deoxidation of steel.
To provide for the processability of steel and to lower down the quantity of δ-ferrite the content of manganese is kept at the level of 0.5-2.0 percent by weight.
To provide for heat and irradiation resistance the chromium content of the claimed steel is kept at the level of 10-13.5 percent by weight.
The carbon content of the claimed steel ranges within 0.10-0.21 percent by weight to provide for the high level of structural stability and heat resistance via the process of martensitic transformation.
In a vacuum induction furnace two steel ingots 25 kg each as well as two ingots 500 kg each were produced for use as claddings of the claimed fuel element. Ingots 25 kg in mass were forged into billets 35 mm in the diameter that were then forged into plates 10mm thick and a bar 12mm in the diameter. Ingots 500kg in mass were forged into billets 90mm in the diameter that were then rolled to produce a sheet 6mm thick and a bar 12mm in the diameter. The bar, sheet and plates were heat treated under the standard conditions, namely, normalizing and tempering. The heat treated metal was used to fabricate cylindrical samples with the effective part size Ø5x25 mm to be tested for long-term strength and creep in compliance with GOST(ΓOCT) 10145-81 and GOST(ΓOCT) 3248-81. Tensile properties also after irradiation were determined using standard samples in tensile tests according to GOST (ΓOCT) 10446-80. The indicated samples had to simulate the condition of the metal in the high temperature part of a fuel cladding at the operating temperature not lower than 600°C.
Simultaneously similar small-size g agarin samples were prepared that were to simulate the condition of the metal in the low temperature part of a fuel cladding operating at a temperature not higher than 400°C, i.e., they were used to test their propensity for LTIE.
To create in the metal of those samples a structure having a higher resistance to LTIE the samples were subjected to cycling, i.e., heating to the temperature A_C1+20°C, 10 min holding and cooling to room temperature at a rate of not less than 50°C/min within 600 to 20°C. Altogether 10 cycles were carried out. After the cyclic treatment the samples were subjected to tempering at 720°C for 2 h followed by accelerated cooling (not less than 50°C/min) to room temperature.
The claimed steel was neutron irradiated in research fast neutron reactor BOR-60 at a temperature of 345-365°C to the fluence of (1.14-2.0) 10²² n/cm² (E>0.1MeV) at the damage dose of 5.8-8.0 dpa. Tensile tests were conducted using remotely operated breaking machine 1794-Y5 in air at a strain rate of ~1 mm/min. Under the indicated conditions samples were irradiated that had been heat treated under the standard conditions as well as samples that had been subjected to the cyclic heat treatment.
The chemical compositions of the steels for use as claddings of the claimed fuel element and the known steel are tabulated in table 1, the results of calculating the decay kinetics of the induced activity of those steels are summarized in table 2 and the results of tensile testing are presented in tables 3 and 4.
The data on the calculated kinetics of the induced activity decay (i.e., the γ-irradiation dose rate) of steels after the assumed irradiation in fast neutron reactors BN-600 for 560 h and subsequent holding up to 500 years evidence the advantages of the claimed steel, that are particularly noticeable after holding for more than 10 years (see table 2). After holding for 50 years the claimed steel might be managed without special protection and might be remelted for re-use.
Similar calculations carried out for the neutron spectrum of the fusion reactor DEMO demonstrate that the prompt decay of induced activity makes it safe after 50 years holding.

Industrial Applicability

The results of testing tensile properties (see table 3) corroborate that the steel for the claimed fuel cladding has a substantially higher margin of the LTIE resistance after cycling treatment. So, the values of the per cent elongation of the claimed steel samples as BN-600 reactor irradiated at temperatures of 345-365°C at which LTIE is revealed have lower values at both 20°C (2.6-6.1%) and at the irradiation temperature (1.3-1.7%) while after cycling treatment these values are 1.5-2 times higher.
The results of long-term strength and creep tests carried out according to GOST (ΓOCT)10145-81 and GOST (ΓOCT) 3248-81 demonstrated (see table4) that the steel for claddings of the claimed fuel element after heat treatments under the chosen conditions is heat resistant at temperatures of 650-750°C and even in its modifications having a lower nitrogen content. For instance, the creep rate of the claimed steel at 650°C at a stress of 8kgf/mm² makes up (0.9-7)x10^-4 %/h. Similar results are also observable at stresses of 10 and 12 kgf/mm².

Thus the claimed fuel element cladding is usable in cores of fast neutron reactors. The use of a fuel element with a cladding having the claimed properties shall provide for high cost-effectiveness due to a more prompt decay of induced activity and have high properties of heat resistance and resistance to low temperature irradiation embrittlement. This cost-effectiveness effect will manifest in lower contamination of environment effected by operating nuclear power facilities of novel generation and in feasibility of re-using structural materials.

Table 2. Kinetics of Decay of Absorbed γ-Radiation Dose Rate (Sv/h) in Claimed Steel as Fusion Reactor DEMO Irradiated (neutron burden on first wall is 12,5 MW·year/m², irradiation time - 10 years, E=14 MeV)

Nominal N Of ingot	γ - irradiation dose after end of irradiation (Sv/h)
	1 hour	1 month	1 year	10 years	50 years	100 years	500 years
1	5·10²	2.9·10²	16	1	1·1O^-2**	1·1O^-3	5·1O^-4
2	4·10²	1.5·10²	20	1	1·1O^-2**	1·1O^-3	5·1O^-4
3	6·10²	2.5·10²	17	1	1·1O^-2**	1·1O^-3	5·1O^-4
4	1·10²	4·10²	30	1	1·1O^-2**	1·1O^-3	5·1O^-4

Notes: * Transmutation of elements and induced activity were computed using program FISPACT-30 (R.A. Forrest, J.-CH. Sublet. "FISPACT-3 User Manual", report AEA/FUS/227, 1993). γ - radiation dose rate that attends radioactive decay of
** 1·1O^-2 Sv/h - is safe level of γ - radiation at which according to IAEA rules material might be re-used.

Claims

A fuel element for fast neutron reactor comprising a tubular cladding sealed with top and bottom plugs at its ends, pelletized nuclear fuel sited within cladding as well as needed structural elements, wherein its cladding is fabricated from a steel containing carbon, silicon, manganese, chromium, vanadium, tungsten, boron, cerium and/or yttrium, titanium, tantalum, zirconium, nitrogen, iron and unavoidable impurities at the following ratio between components, percent by weight: carbon - 0.10-0.21; silicon - 0.1-0.8; manganese - 0.5-2.0; chromium - 10.0-13.5; tungsten - 0.8-2.5; vanadium - 0.05-0.4; titanium - 0.03-0.3; boron - 0.001-0.008; cerium and/or yttrium in total - 0.001-0.10; zirconium - 0.05-0.2; tantalum - 0.05-0.2; nitrogen - 0.02-0.15; iron - the balance

at the ratio of the total content of vanadium, titanium, zirconium and tantalum to the total content of carbon and nitrogen equal to 2 to 9, with the steel structure along the cladding length having at least two zones; the steel structure in the top zone of a fuel cladding provides higher heat resistance and contains α-ferrite, δ-ferrite, sorbite, chromium carbides M₂₃C₆ and M₆C, carbides and carbonitrides of steel components (V, Ta, Ti, Zr, W and others), intermetallics of Fe₂(W) type while the structure in the bottom zone provides higher resistance to low temperature irradiation embattlement and consists of sorbite, δ-ferrite, α-ferrite, residual austenite, carbides and carbonitrides of steel components (Cr, V, Ta, W and others); with large angle boundaries of grains being occupied by carbides M₂₃C₆ and M₆C while grains of both sorbite and δ-ferrite having only individual precipitates of carbides and carbonitrides VC, V(CN), Ti(CN) and Ta(CN) while the other elements (Fe, Mn, Mo, W, Si and others) entering into the compositions of either complex carbides M₂₃C₆ and M₆C or solid solution FeCr.
A Fuel cladding for fast neutron reactor that is fabricated from a steel containing carbon, silicon, manganese, chromium, vanadium, tungsten, boron, cerium and/or yttrium, titanium, tantalum, zirconium, nitrogen, iron and unavoidable impurities at the following ratio between components, percent by weight: carbon - 0.10-0.21; silicon - 0.1-0.8; manganese - 0.5-2.0; chromium - 10.0-13.5; tungsten - 0.8-2.5; vanadium - 0.05-0.4; titanium - 0.03-0.3; boron - 0.001-0.008; cerium and/or yttrium in total - 0.001-0.10; zirconium - 0.05-0.2; tantalum - 0.05-0.2; nitrogen - 0.02-0.15; iron - the balance

at the ratio of the total content of vanadium, titanium, zirconium and tantalum to the total content of carbon and nitrogen equal to 2 up to 9, with the steel structure along the cladding length having at least two zones; the steel structure in the top zone of a fuel element provides its higher heat resistance and contains α-ferrite, δ-ferrite, sorbite, chromium carbides M₂₃C₆ and M₆C, carbides and carbonitrides of steel components (V, Ta, Ti, Zr, W and others), intermetallics of Fe₂(W) type while the structure in the bottom zone provides higher resistance to low temperature irradiation embrittlement and consists of sorbite, α-ferrite, δ-ferrite, residual austenite, carbides and carbonitrides of steel components (Cr, V, Ta, W and others); with large angle grain boundaries being occupied by carbides M₂₃C₆ and M₆C while grains of both sorbite and δ-ferrite having only individual precipitates of carbides and carbonitredes VC, V(CN), Ti(CN), Ta(CN) and the other elements entering into the compositions of either complex carbides M₂₃C₆ and M₆C or solid solution FeCr.
A fuel cladding as claimed in claim 2, CHARACTERIZED in that the content of unavoidable impurities in steel is restricted by the concentrations: nickel - not more than 0.1; niobium - not more than 0.01; molybdenum - not more than 0.01; copper - not more than 0.1; cobalt - not more than 0.01; sulphur - not more than 0.008; phosphorus - not more than 0.008; oxygen - not more than 0.005.

at the total content of impurities of high activation metals, namely, molybdenum, niobium nickel, copper and cobalt not exceeding 0.10 percent by weight.
A fuel cladding as claimed in claim 2, CHARACTERIZED in that the total content of impurities of low melting metals, namely, lead, bithmuth, tin, antimony and arsenic, does not exceed 0.05 percent by weight.