EP3194633A1

EP3194633A1 - A steel for a lead cooled reactor

Info

Publication number: EP3194633A1
Application number: EP15840063.0A
Authority: EP
Inventors: Peter Szakalos; Jesper EJENSTAM; Janne Wallenius
Original assignee: Blykalla Reaktorer Stockholm AB
Current assignee: Blykalla AB
Priority date: 2014-09-14
Filing date: 2015-09-15
Publication date: 2017-07-26
Also published as: CA2960670A1; SE1430130A1; EP3194633A4; WO2016039679A1; CA2960670C

Abstract

A steel for structural components used in contact with liquid lead in nuclear reactors consisting of in weight % (wt. %): Cr 8.0-15.0 Ni 10.0-16.0 Al 2.0-4.0 C 0.02-0.2 N <0.06 Mo <3.0 at least one of: Nb 0.1-3.0 Ta 0.1-3.0 Ti 0.01-1.0 ZrO.01-1.0 Hf 0.01-1.0 Y 0.05-1.0 optionally Si <2.0 Mn <4.0 Cu <4 Co <5 V<1 W<3 B <0.1 Bi <0.2 Se <0.3 Ca <0.01 Mg <0.01 O 0.02-0.50 REM <0.3 balance Fe apart from impurities, wherein the content of REM does not include the amount of Y but only the amount of the elements having an atomic numbers 21 and 57-71.

Description

A STEEL FOR A LEAD COOLED REACTOR

TECHNICAL FIELD The invention relates to a steel for structural components used in contact with liquid lead in nuclear reactors.

BACKGROUND OF THE INVENTION

The worldwide interest in next generation's nuclear power plants has increased over the last decade. Enabling a more efficient use of already mined uranium through recycling, and improved safety features makes generation IV reactors of high interest to society. The Lead-cooled fast reactor (LFR) is one of the concepts that are studied worldwide today. Liquid lead has inherent safety features such as slow reaction to water, effective capture of iodine, and decay heat removal through natural circulation, which makes the LFR technology a candidate for future nuclear power. The predominant issue that scientists face when designing LFR systems is the choice of corrosion resistant steels. Liquid lead attacks conventional structural steels, such as 316L and 15-15 Ti at temperatures above 500 °C, thus limits the operation temperature window of the reactor. In recent years, alumina- forming FeCrAl alloys have been proposed as a promising solution, both as bulk steels and through surface alloying. . Despite the excellent corrosion resistance, conventional ferritic steels do not possess the same mechanical properties as austenitic steels do. A well-known possible solution is to introduce nano- sized oxide particles into the ferritic matrix, through a powder production route. Such steels are called oxide dispersion-strengthened (ODS) steels. An alternative solution would be to increase the corrosion resistance of austenitic stainless steels. Recent years, alumina- forming austenitic stainless steels (AFA) have gained a lot of interest following the successful work carried out by Oak Ridge National Laboratory (ORNL) in USA. The AFA alloys by ORNL have shown great creep resistance in temperature interval of 650 °C to 950 °C, as well as superior corrosion resistance in dry and humid air. The creep strength and the corrosion resistance, i.e. ability to form Al₂0₃, have been related to the niobium content of the alloys. Formation of nano-sized niobium carbides throughout the matrix leads to a significant increase in creep resistance, and simultaneously improved corrosion resistance. Both Cr and Al are strong ferrite stabilizing elements, although essential for the protective oxide formation. Therefore, in order to make the alloy fully austenitic, the amount of ferrite stabilizers has to be kept to a minimum. Increasing ferrite stabilizers in the matrix leads to formation of delta ferrite, which ultimately leads to loss of creep resistance.

DISCLOSURE OF THE INVENTION

The main object of the present invention is to provide an aluminium alloyed steel, which is suitable for use in liquid lead and liquid lead bismuth eutectic (LBE) alloys at high temperatures in a lead cooled reactor.

Another object of the present invention is to provide a steel for the use in structural components in structural component in a lead or lead-bismuth alloy cooled nuclear reactor or in a concentrated solar power plant.

The foregoing objects, as well as additional advantages are achieved to a significant measure by providing a steel having a composition as set out in the alloy claims. The inventive alloys form protective Al-rich oxides on the surface when exposed to the corrosive conditions of Pb and LBE at high temperatures.

The invention is defined in the claims. DETAILED DESCRIPTION

The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims. Chromium is to be present in a content of at least 8 % in order to provide a good oxidation and corrosion resistance. Cr is a ferrite stabilizing element, which reacts with carbon to form carbides. Cr also favors protective alumina scale formation. If the chromium content is too high, this may lead to the formation of undesired phases at lower temperatures such as 4 - 600 °C. The chromium content is therefore limited to 15 %. The lower limit may be 8.5 %, 9.0 % 9.5 %, 10.0 %, 10.5 %, 11.0 %, 11.5 % or 12.0 %. The upper limit may be 12%, 12.5 %, 13.0 %, 13.5 %, 14 % or 14.5 %. Nickel is an austenite stabilizer and its primary purpose is to stabilize austenite. The amount of Ni necessary depends on the amount of ferrite stabilizing elements and the amount of other austenite stabilizers. Ni is easily dissolved in liquid lead and has therefore an upper limit depending on the alloys ability to form a protective and Ni-free oxide. The lower limit may therefore be 10.0 %, 10.5 %, 11.0 %, 1 1.5 %, 12.0 %, 12.5 % 13.0 % or 13.5 % and the upper limit may be 12.0 %, 12.5 %, 13.0 %, 13.5 %, 14.0 %, 14.5 %, 15.0 %, 15.5 % or 16.0 %.

Aluminum is essential for the formation of the Al-rich oxides and is therefore added in an amount of 2.0 - 4.0 %. However, too much Al may result in the formation of undesired phases. Aluminum is beneficial in case of ferrite precipitation at low temperatures such as 400 - 500 °C, since it suppresses phase separation ( '-formation) and spinodal decomposition. The lower limit may therefore be 2.0 %, 2.25 % or 2.5 % and the upper limit may be 2.5 %, 2.75 %, 3.0 %, 3.25 %, 3.50 %, 3.75 % or 4.0 % Carbon is always present in steels, it forms carbides and stabilizes the austenite. The upper limit for carbon may be set to 0.2 %, 0.15 %, 0.10 %, 0.09 %, or 0.06 %. The lower limit may be 0.02 % or 0.04 %.

Nitrogen may be present in the steel in an amount of < 0.06 % because N reacts with Al. Molybdenum increases the high temperature mechanical properties and is a strong carbide forming element and also a strong ferrite former and may result in the formation of brittle Laves phase. The amount of molybdenum should be restricted to maximum 3 %, preferably to 2 % or less. If the alloy composition is prone to lave phase

precipitation, the higher limit may be 2 %, 1.5 %, 1 %, 0.5 % or 0.1 %.

Niobium forms carbides, nitrides and carbo-nitrides and is beneficial for strength and creep resistance. In addition, Nb tends to improve the oxidation resistance and to form influence on the formation of intermetallic precipitates. Nb is therefore present in an amount of 0.1 - 3 %, preferably 0.6 - 1.2 %.

Tantalum

forms carbides, nitrides and carbo-nitrides and is beneficial for strength and creep resistance. In addition, Ta tends to improve the oxidation resistance and to form influence on the formation of intermetallic precipitates. Ta is therefore present in an amount of 0.1 - 3 %, preferably 0.6 - 1.2 %.

Ti, Zr & Hf

Reactive elements that promote formation of a protective alumina scale. Strong carbide formers and strong oxide particles formers, beneficial for high temperature mechanical properties when alloying with oxygen, so called ODS alloys.

The amount of Ti, Zr & Hf, individually, may be 0.01-1 %. If alloyed with oxygen, the preferred amount is 0.5 - 1 % (ODS). If no oxygen is deliberately added, the amount may be < 0.5 %.

Yttrium

The amount of Y may be 0.05 - 1 %. If alloyed with oxygen, the preferred amount is 0.5 - 1 % (ODS). If no oxygen is deliberately added, the amount may be < 0.5 %. Silicon is beneficial for high temperature oxidation properties but is a strong ferrite former and should therefore be limited. The upper limit may be 2.0 %, 0.6 %, 0.55 %, 0.5 %, 0.45%, 0.4 % or 0.35 %. Manganese

Strong austenite stabilizer and may to some extent replace Ni. Mn also improves the mechanical properties to some extent. Mn is included in carbides as well as oxides. Mn tends to promote secondary phases, such as sigma phase, which may cause

embrittlement. The Mn content should be limited to < 4 % for some alloy compositions, but preferably < 3 % for alloy compositions sensitive to sigma phase. The upper limit may be 3 %, 2.5 %, 2.0 %, 1.5 %, 1 % or 0.5 %.

Copper is an optional element, which has austenite stabilizing effects but it may form brittle phases, especially under irradiation. It is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally limited to 3 %, preferably < 0.3 %. Most preferably, Cu is not deliberately added.

Cobalt

The Co-content should be as low as possible in nuclear applications but for other application it is beneficial in stabilizing an austenitic structure and improves the strength at al temperatures. In compositions aimed for nuclear applications, the amount is preferably < 0.1 %. In compositions where Co is deliberately added, the amount may be < 5 %.

Vanadium forms carbides and carbonitrides of the type M(C,N) in the matrix of the steel. However, if stronger carbide formers are present, than the V amount should be < 0.3 %. In other cases, the V amount may be < 1 %. Tungsten

Increases the high temperature mechanical properties and is a strong carbide forming element and also a strong ferrite former and may result in the formation of brittle sigma phase. The amount of molybdenum should be restricted to maximum 3 %, preferably to 2 % or less. If the alloy composition is prone to lave phase precipitation, the higher limit may be 2 %, 1.5 %, 1 %, 0.5 % or 0.1 %. Sulphur

Sulphur should not deliberately be added, lowers the oxidation properties. Boron

Boron may act as a substitution to carbon, but is also a strong neutron absorber. Boron suppresses the nucleation of ferrite on austenitic grain boundaries. The amount of B may be < 0.1 %, but preferably < 0.007 %.

Bi, Se, Ca, Mg

These elements may be added to the steel in the claimed amounts in order to further improve the machinability, hot workability and/or weldability.

Oxygen

In combination with oxygen active elements such as Y and REM in general, form small oxide particles, beneficial for high temperature mechanical properties, so called ODS- alloys. In the case of ODS alloying, the O amount may be < 0.5 %, but preferably 0.05 - 0.15 %. In non-ODS alloys, O should not be deliberately added.

REM Improves the oxide scale properties and are beneficial for high temperature mechanical properties in combination with oxygen, so called ODS-alloys. (Rare Earth Metals) as used in this application embraces the elements with atomic numbers 21 and 57-71 because Yttrium is defined separately. The amount of REM may be < 0.3 %.

EXAMPLE

In the present example three austenitic stainless steels are compared with the inventive steel. The two steels 316 L and 15-15 Ti are commercial steels. The AFA alloy and the inventive steel were casted in a vacuum furnace, approximately 1 kg per batch. The alloys were subsequently rolled into 8 x 1 mm strips in a total of 8 steps, with 5 min heat treatment at 1 100 °C after each rolling step. Full compositional data for all alloys is presentenced in table 1.

Table 1

Alloy Fe Cr Ni Al Mn Mo Si Nb C Cu P Ti

316L (4404) Bal. 17 10 - 1 2 0.5 - 0.02 0.4 0.03 -

15-15 Ti (12R72) Bal. 15 15 - 1.8 1.2 0.5 - 0.09 - 0.01 0.5

AFA 20Ni Bal. 14 20 2.5 2 2.5 0.15 0.9 0.08 - - -

Inventive steel Bal. 14 14 2.5 2 2.5 0.15 0.9 0.08 - - -

All alloys were cut into samples measuring 30 x 8 mm, with varying thicknesses deepening on initial shape. All samples were polished to near mirror like surfaces using Struers abrasive SiC paper (final step #1200) and finally ultrasonically cleaned in ethanol for 10 minutes.

The experiment was conducted in a COSTA (COrrosion Test Stand for liquid metal Alloys) setup, constructed by Karlsruhe Institute of Technology (KIT). Samples were fitted into alumina crucibles using alumina holders and 1 mm molybdenum wire as support. Lead shots (2 mm) (Alfa Aesar, 99.95 % metal base) were poured in the crucibles until the samples were completely covered. All crucibles were subsequently placed on nickel trays and placed inside the sealed quartz tubes of the furnace. More information on the COSTA setup is presented in J. Nucl. Mater. 278(2000) 85-95.

The oxygen concentration in the liquid lead was controlled by means a gas mixture containing Ar, H₂ and H₂0. The H₂/H₂0 ratio was set to 1.3, which corresponds to 10^" wt. %. A Zirox SGM5 oxygen analyzer was used to monitor the oxygen partial pressure at the systems gas outlet. Two corrosion tests, lasting 3,000 h and 8,700 h (1 year) respectively, were carried out at 550 °C.

Prior to evaluation, the samples were cleaned with ethanol, and dried with pressurized air. Cross sections were prepared by mounting the samples in acrylic resin with Fe filler, followed by fine polishing.

The two chromia forming steels, 316L and 15-15 Ti, were both attacked by the liquid lead although to various degrees. While a continuous dissolution front was seen for the 316L samples, only localized dissolution attacks were seen in 15-15 Ti. No obvious differences, with respect to dissolution depth, were seen for the exposure times for the two chromia formers. However, for 15-15 Ti the frequency of localized dissolution attacks increase with exposure time, i.e. from 3000 h to 1 year. The lead penetration depth was roughly 100 - 300 μηι for 316L whereas the localized dissolution attacks on 15-15 Ti measured up to roughly 50 μιη. Selective dissolution of Ni, caused by exposure to liquid lead, is clearly seen when the attacked samples are examined by SEM-EDS. In addition, dissolution of an austenite stabilizing element, such as Ni, cause phase transformation from austenite (FCC) to ferrite (BCC).

The 20 Ni AFA alloy and the inventive steel are both alumina forming steels.

For the 20 Ni AFA alloy, localized dissolution attacks were seen. Similar to the 15-15 Ti sample, the depth of the attacks measured up to about 20 μπι for both exposures times. The frequency of attacks was again higher after 1 year exposure. In addition, localized internal oxidation was noted. The oxide nodules, rich in Al, Fe and Cr, measured up to about 10 μπι in size and were unevenly spread out in the metal/oxide interface. No significant difference in size or frequency of oxide nodules was noted with increased exposure time.

The inventive steel was the only alloy in the test that did not suffer from any dissolution attack, neither after 3,000 h nor after 1 year. However, nodular internal oxidation was found. The size of the oxide nodules were up to 10 μπι after 3,000 h, whereas the largest ones measured about 25 μιη after 1 years exposure. As for the 20Ni AFA, the oxide nodules were unevenly spread out in the metal/oxide interface. A thin protective oxide layer, measuring 10 to 100 nm, was covering the sample surface.

In addition to differences in the alloys oxidation properties, differences in

microstructure were found. A part from the NbC present throughout the matrix, darker and lighter areas was detected in the inventive steel. EBSD mapping revealed that the alloy was not purely FCC phased, but rather two-phased (FCC and BCC). The fractions of FCC and BCC in the alloy bulk were calculated to 83% and 17% respectively. The 20Ni AFA was however essentially single phased (FCC). The results are summarized in table 2. Table 2. Summary of corrosion and oxidation results in liquid lead.

3000 h 1 year

Alloy P.O D.A. I.O. P.O. D.A. I.O.

316L 100-300 100-300

No μπι No No μιτι No

15-15 Ti 20-50 20-50

Partially μηι No Partially μτη No

20Ni AFA 10-20 Yes (<10 10-20 Yes (<20

Partially μιη μιη) Partially μm ΐΏ)

Inventive Yes (<100 Yes (<10 Yes (<20 steel nm) No μιτι) Yes No μηι)

P. O.- Protective oxide. D.A.- Dissolution attack. I. O.- Internal Oxidation.

It is evident from table 2 is the only alloy that formed a protective oxide cover and that did not suffer from any dissolution attack at any time. Accordingly, the claimed alloy is considered to have very attractive properties for use as structural components in Pb or LBE cooled reactors or in concentrated solar plants.

Claims

1. A steel for structural components used in contact with liquid lead in nuclear reactors consisting of in weight % (wt. %):

Cr 8.0-15.0

Ni 10.0-16.0

Al 2.0-4.0

C 0.02-0.2

N <0.06

Mo <3.0 at least one of:

Nb 0.1-3.0

Ta 0.1-3.0

Ti 0.01-1.0

Zr 0.01-1.0

Hf 0.01-1.0

Y 0.05-1.0 optionally

Si <2.0

Mn <4.0

Cu <4

Co <5

V < 1

w <3

B <0.1

Bi <0.2

Se <0.3

Ca < 0.01

Mg < 0.01

O 0.02-0.50

REM <0.3 balance Fe apart from impurities, wherein the content of REM does not include the amount of Y but only the amount of the elements having an atomic numbers 21 and 57-71.

2. A steel according to claim 1 containing in weight % (wt. %):

Cr 9.5-14.5

Ni 10.0-15.0

Al 2.5-3.5 C 0.01-0.15

Nb 0.6-1.5

A steel according to claim 1 fulfilling at least one of the following requirements (in wt. %):

Cr 9.0-12.0

Ni 10.0-14.5

Al 2.

3-3.7

C 0.02-0.1

N < 0.04

Si 0.1 - 1.0

Mn 2.0-4.0

Mo 0.5-2.8

4. A steel according to claim 1 fulfilling at least one of the following requirements (in wt. %):

Cr 9.0-11.0

Ni 10.0-14.5

Al 2.5-3.

5

N < 0.03

Nb 0.

6-1.2

Si 0.1-0.5

Mn 0.3-1.0

Mo 0.5- 1.5

A steel according to any of any of the preceding claims fulfilling at least one of the following requirements (in wt. %):

Ti < 0.3

V < 0.3

Nb 0.8-1.0

A steel according to any of claims 1 or 2 fulfilling at least one of the following requirements (in wt. %)

Cr 9.5-13.0

Ni 10.0-14.0

Al 2.5-3.2

N < 0.03

C 0.02 - 0.09

Nb 0.7-1.1

Si 0.1-0.5

Mn 0.4-2.5

Mo 1.5-2.7

7. A steel according to any of the preceding claims fulfilling at least one of the following requirements (in wt. %):

Ti < 0.1

V < 0.1

Cu < 1

Co < 3

W < 1

B < 0.01

Bi < 0.02

Se < 0.03

Mg < 0.001

REM < 0.12

8. A steel according to any of the preceding claims fulfilling at least one of the

following requirements (in wt. %):

Cu < 0.3

Ti < 0.005

P < 0.025

S < 0.005

9. A steel according to any of the preceding claims fulfilling at least one of the

following requirements (in wt. %):

Ti < 0.05

V < 0.05

10. A steel according to any of the preceding claims fulfilling at least one of the

following requirements (in wt. %):

Ti 0.01 - 1.0

Zr 0.01 - 1.0

Hf 0.01 - 1.0

Y 0.05 - 1.0 and wherein the steel fulfil the requirement (in wt. %):

O 0.02-0.50

11. A steel according to any of claims 1-9, wherein the steel contains Nb but has no deliberate addition of any of the elements Ta, Ti, Zr, Hf and Y.

12. A steel according to any of the preceding claims, wherein the ratio Ni/Cr+Al is < 0.85, preferably larger than 0.95.

13. Use of a steel as defined in any of claims 1-12 for a structural component in a lead or lead-bismuth alloy cooled nuclear reactor or in a concentrated solar power plant.

14. Use of a steel as defined in claim 13, wherein the molten lead or lead-bismuth alloy has a temperature of < 600 °C and/or an oxygen content of at least 10^" wt. %.

15. Use of a steel as defined in claim 13 or 14, wherein the relative velocity between the molten lead or lead-bismuth alloy and the structural surface is less than 5 m/s.