Effects of fuel rod cladding temperature and stressed conditions on hydride reorientation

Размещено на http://www.allbest.ru/

EFFECTS OF FUEL ROD CLADDING TEMPERATURE AND STRESSED CONDITIONS ON HYDRIDE REORIENTATION

Chernyayeva T.P.

Grytsyna, V.M.

Krasnorutskyy, V.S.

Riedkina A.P.

Petelguzov, I.A. Slabospitskaya Ye.A.

`Nuclear Fuel Cycle' Science and Technology Establishment National Science Center “Kharkov Institute of Physics and Technology” Kharkov, Ukraine



Abstract

This paper presents results of experimental research into hydride reorientation and hydrogen embrittlement, which may occur in the SNF FR cladding at conditions simulating normal and some accident modes of SNF handling. We performed simulation experiments with examination of how SNF overheat to 450 °C and stressed condition during loading into a SNF dry storage facility affect hydride reorientation at different hydrogen concentrations in the dummy claddings. Research was done to study changes in the hydride morphology during hydride reorientation tests at conditions simulating the handling operations of SNF loading into and sealing in storage baskets, and accident temperature increase in the SNF dry storage facility during SNF storage. It was established that at hydrogen concentrations in excess of 170 ppm, hydrogen reorientation in the dummy claddings during thermal tests begins at tensile stresses exceeding 55-60 MPa. For hydrogen concentrations in the claddings above 250 ppm, hydride reorientations begin at lower tensile stresses. In the dummy claddings at at 450 °c >70 MPa and hydrogen concentration above 250 ppm, hydrogen reorientation occurs and the hydride reorientation factor increases significantly. Thermal cycling at the stage of SNF holding in the SNF dry storage facility significantly intensifies hydride reorientation. In the claddings at at 450 °c=70 MPa and hydrogen concentration of 400 ppm, thermally cycled three times in the temperature range of 180 °C-450 °C, almost complete hydride reorientation occurs.

Mechanical tests of samples with various hydrogen concentrations before and after the hydride reorientation tests were performed.

The results obtained and the information search conducted allow a prediction on the risk margin due to SNF cladding degradation related to the phenomena induced by presence of hydrogen accumulated in the Zr-1%Nb alloy FR cladding during operation.



Introduction

hydrogen hydride zirconium alloy

The management and long-term storage of spent nuclear fuel is an integral part of the nuclear fuel cycle. The safety of spent nuclear fuel handling operations largely depends on the SNF cladding condition. The SNF dry storage facility commissioned on August 24, 2001 at ZNPP (Zaporozhye NPP) site has considerably raised the demand for research into the phenomena responsible for the FR cladding degradation during SNF handling and long-term storage.

For many years Zirconium-base alloys has been used for the cladding of the fuel rods and structural components of fuel assemblies in water cooled power reactors, like pressurized water reactors (PWRs, VVER), boiling water reactors (BWRs, RBMK) and pressurized heavy reactors (HPWRs) [1] Upon completion of their operation, fuel rods remain internally pressurized [2, 3] and retain a number of degrading factors, such as hydrogen, accumulated during operation [4].

Degradation phenomena induced by presence of hydrogen in zirconium include:

- hydrogen embrittlement (HE), which is a significant decrease in ductility caused by hydrogen uptake [5, 6];

- delayed hydrogen cracking (DHC), which is gradual crack growth caused by discrete cracking along the hydrides formed at its tip [7, 8, 9];

- hydrogen redistribution and hydride blistering (formation of large hydrides in the local hydrogen accumulation areas) [10].

It is commonly believed that spent nuclear fuel (SNF) handling is mostly jeopardized by hydride reorientation and increased hydrogen embrittlement efficiency caused thereby [11].

Traditionally, the cladding tube manufacturing predetermines their proneness to formation of circumferentially (tangentially) oriented hydrides. The potential for hydride embrittlement of cladding fuel rods may increase drastically if the hydride platelets are reoriented from their normal circumferential to the radial direction of the cladding [12, 13, 14]. Hydride reorientation can occur as a result heating during the vacuum drying process in which the cladding temperature can reach up to 450 °C, causing the hydrides to partially or completely dissolve. During subsequent cool-down, the hydrogen in solid solution can re-precipitate as radial hydrides if the hoop stress caused by the end-of-life internal pressure of the tangential stress exceeds the threshold stress level for the formation of radial hydrides [15].

By the end-of-life, the fuel rod internal pressure under the cladding, depending on the initial pressure (2.0...3.45 MPa), in the PWR rods at a room temperature is 4.6 MPa [2, 16]. The dependence of the end- of-line pressure versus burnup (Bu) can be stated as fol- to 80 microns long predominantly are tangentially ori- lows [2]: ented [18]. Due to such low hydrogen concentration, no P_end=2.8781+0.0224Bu (1) where Pend are internal fuel rod pressure at end of life; Bu is burnup in MW-D/kgU, 2.8781 - mean initial pressure under the shell (fitting parameter).

Table 1. Main parameters of the cladding tubes used

Material	Chemical composition	Condition	Grain size, microns	Texture	Dimensions, mm
Alloy E110	Zr-1 wt.%Nb	Recrystallized	3-5		Outer diameter - 9.13; Wall thickness - 0.68

According to US NRC requirements, the following criteria shall be met to guarantee fuel rod integrity during SNF handling and long-term storage [16, 17]:

- tangential stress in the fuel rod cladding at 400 °C should not exceed 90 MPa;

- hydrogen concentration in the fuel rod cladding should not exceed 400-500 ppm.

The amount of hydrogen accumulated in Zr-1 %Nb alloy during six years of operation (70 MW-D/kgU) is ~50...60 ppm and does not exceed 80 ppm; hydrides up limit was set for hydrogen uptake previously, however at present a design limit of 400 ppm has been implemented to match U.S. and western design criteria [17, 19].

This paper describes the hydrogen reorientation tests simulating the temperature mode and stress condition of Zr-1%Nb dummy claddings with hydrogen concentration up to 400 ppm and different scenarios of SNF handling and storage.

The cladding used in the present study was nonirradiated Zr-1%Nb cladding tubes in the delivery condition (finish annealing 580 °C, 3 hours); their main parameters are shown in Table 1.

Hydride reorientation tests were performed on internally pressurized dummies with Zr-1%Nb cladding (Fig. 1). The overall dummy length was 112 mm, with the gas-filled cavity length of 66 mm. The gas (helium) pressure under the cladding was from 3 to 5 MPa.

Fig. 1 - Pressurized dummy for the hydride reorientation test

According to the specification, hydrogen concentration in non-hydrogenated Zr-1%Nb (E110) cladding tubes does not exceed 15 ppm (ppm -part per million); its standard concentration varies within 4-7 ppm [20]. The dummy claddings were saturated with hydrogen using a “dry” hydrogenation method at 380 and 420 °C. Titanium hydride powder was used as the source of gaseous hydrogen. The hydrogen concentration was determined by weighing and using metallographic structure images (transverse cross-section). The hydrogen concentration in the cladding of the hydrogenated dummies ranged from 60 to 400 ppm.

The test temperature mode was based on SNF handling safety criteria.

The currently assumed safety criteria system of the Spent Nuclear Fuel Dry Storage at Zaporozhye NPP (ZNPP) meets the following [21]:

- the fuel rod peak cladding temperature operated in the fuel assemblies for the normal long-term dry storage in the helium environment is 350 °C;

- for extreme weather conditions and during SNF handling the temperature may rise to 450 °C for not more than 8 hours during the entire handling and storage period.

Changes in the cladding temperature of SNF fuel rods with 1 kW decay heat for long-term storage in the ZNPP SNF Dry Storage are shown in Fig. 2. The peak fuel rod clad temperature at the beginning is 349°C (Fig. 3) and decreases with time. In 5 years this temperature will equal 275 °C, and in 45-50 years it will reduce to 180 °C.

Fig. 3 - Temperature Mode during Tests Simulating Handling (Mode 1) and Tangential Stresses in the Dummy Cladding during Tests (P₂₉₃ =3, 4 and 5 MPa)

Fig. 2 - Changes in Fuel Rod Clad Temperature during Storage in ZNPP SNF Dry Storage

The temperature mode during tests simulating handling conditions (Mode 1) is shown in Fig. 3.

It should be noted that in these tests the stressed condition of the dummy cladding changes with temperature according to the Charles law, just like on the fuel rod cladding during handling and long-term storage.

Changes in tangential stresses in the dummy claddings under pressure of 3, 4, and 5 MPa during tests in the mode simulating fuel handling and normal operating conditions are shown in Fig. 3.

Changes simulating the handling temperature mode and the limiting design-basis accidents included:

- Fuel rod dummy tests under pressure of 3, 4 and 5 MPa in the mode: heating to 450 °C, exposure at 450 °C for 3 hours, cooling to 300 °C; exposure at 300 °C for 1 hour and subsequent three thermal cycles from 300 °C (temperature after ~2 years of storage) to 450 °С. Exposure in each of the cycles at 300 and 450 °C for 1 hour (Mode 2) (Fig. 4)

Fig. 4 - Test temperature mode with three thermal cycles 300-450 °С. Mode 2

- Fuel rod dummy tests under pressure of 5 MPa in the mode: heating to 450 °C, exposure at 450 °C for 3 hours, cooling to 300 °C; exposure at 300 °C for 1 hour and subsequent five thermal cycles from 300 °C to 450 °С. Exposure in each of the cycles at 300 and 450 °C for 1 hour (Mode 3).

- Fuel rod dummy tests under pressure of 5 MPa in the mode: heating to 450 °C, exposure at 450 °C for 3 hours, cooling to 300 °C and subsequent seven thermal cycles from 300 to 450 °С. Exposure in each of the cycles for 1 hour at 300 °C and 45 minutes at 450 °C (Mode 4).

- Fuel rod dummy tests under pressure of 4 and 5 MPa in the mode: heating to 450 °C, exposure at 450 °C for 3 hours, cooling to 180 °C and subsequent three thermal cycles from 180 °C (temperature after ~45-50 years of storage) to 450 °С. Exposure in each of the cycles at 180 and 450 °C for 1 hour (Mode 5).

- Fuel rod dummy tests under pressure of 4 and 5 MPa in the mode: heating to 450 °C, exposure at 450 °C for 3 hours, cooling to 180 °C and subsequent five thermal cycles from 180 °C to 450 °C. Exposure in each of the cycles at 180 and 450 °C for 1 hour (Mode 6).

- Fuel rod dummy tests under pressure of 5 MPa in the mode: heating to 450 °C, exposure at 450 °C for 3 hours, cooling to 180 °C and subsequent seven thermal cycles from 180 °C to 450 °C. Exposure in each of the cycles for 1 hour at 180 °C and 45 minutes at 450 °C (Mode 7).

Metallographic Examination

The metallographic examinations were done using the optical microscope “Axio Observer.Alm”. The metallographic structure images were used to determine the hydrogen concentration and the hydride orientation coefficient.

Tensile tests

Short-terms mechanical tensile test of circumferential samples (outer diameter 9.13 mm, wall thickness 0.68 mm, width 2.7 mm) were performed on semi-circular supports, 6.0 mm in diameter, at temperature 20 and 350 °C.

Results and discussion re-orientation of hydrides

During the tests, depending on the concentration, the hydrides which initially had a technology-induced orientation relative to the main directions in products (for tubes: radially, tangentially, and axially) dissolve completely or partially, whereas the subsequent cooling releases hydrides whose orientation is to a significant degree determined by the active stresses. Hysteresis exists between the temperature of full hydride dissolution (TSSD) during heatup and the temperature of their precipitation during cooling (hydride precipitation requires some subcooling). The hydrogen concentrations at hydride dissociation temperatures ([H]tssd) during heatup and at the beginning of hydride precipitation during cooling ([H]tssp) in Zr-1%Nb alloy (M5) are determined by the equations [24].

where: R - gas constant (8.314 J/(mole-K); T - absolute temperature, K.

The dependence of hydride dissociation temperatures in Zr-1%Nb during heat up (TSSD) and their precipitation during cooling (TSSP) are shown in Fig. 5.

Fig. 5 - Hydride dissociation temperatures (TSSD) during heat up and at the hydride beginning of precipitation temperatures during cooling (TSSP) in Zr-1%Nb versus hydrogen concentration [H]

The hydrogen reorientation tests in the dummy temperature) simulating fuel handling and normal stor claddings under pressure of 3, 4, and 5 MPa (at room age conditions in the SNF dry storage was performed on the dummies with the hydrogen concentration in Zr- 1%Nb cladding of 170, 250, 320, and 400 ppm. Fig. 9 marks the hydride dissolution and precipitation temperatures at these concentrations (I-V), as well as the maximum test temperature (450 °C (723 K)). The hydride the tests is shown in Fig. 6; the data on hydride orientation (hydride orientation coefficients in the dummy claddings) is provided in Table 2.



Fig. 6 - Hydride microstructure in the dummy (Zr-1%Nb) claddings before and after the hydride reorientation tests in the mode simulating fuel handling in the SNF dry storage (on the right)



Table 2/ Hydride reorientation coefficients in the dummy claddings tests in the mode simulating fuel handling in the SNF dry storage (single cycle heating to 450 °C, holding for 8 hours and subsequent cooling to room temperature (according to regime 1)).

[H], ppm	P293, MPa	сте at 450 °C, MPa	TSSD, °C	TSSP,°C	сте at TSSP, MPa	Fn
60	before the test	-	-	-	-	0.04
	3	46	293.72	231.40	32.09	0.05
	4	61.3	293.72	231.40	42.78	0.06
	5	76.7	293.72	231.40	53.48	0.06
170	before the test	-		-	-	0.04
	3	46	388.70	332.53	38.52	0.07
	4	61.3	388.70	332.53	51.36	0.1
	5	76.7	388.70	332.53	64.20	0.12
250	before the test	-	-	-	-	0.05
	3	46	432.48	381.10	41.61	0.14
	4	61.3	432.48	381.10	55.48	0.15
	5	76.7	432.48	381.10	69.35	0.32
320	before the test.	-	-	-	-	0.06
	3	46	463.68	416.49	43.86	0.16
	4	61.3	463.68	416.49	58.48	0.18
	5	76.7	463.68	416.49	73.10	0.48
400	before the test	-	-	-	-	0.07-0.08
	3	46	494.36	451.96	46.12	0.18
	4	61.3	494.36	451.96	61.49	0.23
	5	76.7	494.36	451.96	76.87	0.58

The results obtained (Fig. 6, Table 2) evidence that the dummy tests under the pressure of 3, 4, and 5 MPa in the mode simulating fuel handling and normal storage conditions do not lead to hydride reorientation in the claddings with low hydrogen concentration (60 and 170 ppm (Fig. 5)), whose hydride dissociation and precipitation temperatures are well below 450 °C. This may be related to low stresses in the dummy claddings at the temperature of precipitation onset of low-hydro- gen concentration hydrides. At hydrogen concentrations of 250...400 ppm, with a pressure increase from 0 to 4 MPa, the hydride orientation coefficient increases only insignificantly, but rises dramatically with a pressure increase from 4 to 5 MPa (Fig. 6, Table 2). Zr- 1%Nb cladding propensity towards radial hydride formation under internal pressure increases with the hydrogen concentration growing from 250 to 400 ppm, which is in good qualitative agreement with the dependence of tangential stress in the dummy claddings at the hydride precipitation temperature (TSSP) (See Fig. 5).

For illustration, Fig. 7 shows the dependence of the hydride orientation coefficient versus tangential stress at 450 °C for the dummy claddings (Zr-1%Nb) with a hydrogen concentration of 400 ppm (TSSP=451.96 °C), tested in the mode simulating fuel handling and normal storage conditions in the SNF Dry Storage, as well as in the modes with three, five, and seven thermal cycles 300^450 °C.

Fig. 7 - Dependence of the hydride orientation coefficient versus tangential stress at 450 °C in the dummy cladding (Zr-1%) of 400 ppm hydrogen concentration tested in the following modes: Mode 1 simulating fuel handling and normal storage conditions; and Modes 2-4 with three, five, and seven thermal cycles 300-450°C

According to the data obtained, the dummy claddings with a hydrogen concentration of 400 ppm tested in the mode: heating to 450°C, exposure at 450°C for 8 hours and cooling at a rate of ~2 deg./min., the threshold hydride reorientation stress is «55-60 MPa (breakpoint on the function FnCTe (Fig. 7)).

Hydride distribution in the dummy claddings before and after the tests in the modes: heating to 450°C, exposure for 3 hours, cooling to 300 °C and subsequent 3, 5, and 7 thermal cycles of 300^450 °C is shown in Fig. 8; data on hydride orientation (hydride orientation coefficients in the dummy claddings) is provided in Table 3.

Fig. 8 - Hydride microstructure in the dummy claddings (Zr-1%Nb) before and after the hydride reorientation test in the mode: heating to 450 °C, exposure for 3 hours, and cooling to 300 °С and subsequent 3 (a,b), 5 (c), and 7 (d) thermal cycles 300-450 °C

Table 3. Hydride reorientation coefficient in the dummy claddings tested in the mode: heating to 450 °C, exposure for 3 hours, and cooling to 300 °С and subsequent 3, 5, and 7 thermal cycles 3000450 °C

[H], ppm	P293, MPa	сте at 450 °C, MPa	TSSD, °С	TSSP,°С	сте at TSSP, MPa	Fn
3 thermal cycles 300 0450 °C
80	before the test	-	-	-	-	0.04
	3	46	317.12	255. 80	33.64	0.06
	4	61.3	317.12	255.80	44.85	0.07
	5	76.7	317.12	255.80	56.07	0.06
150	before the test	-	-	-	-	0.04
	3	46	375.64	318.28	37.62	0.092
	4	61.3	375.64	318.28	50.15	0.32
	5	76.7	375.64	318.28	62.69	0.38
220	before the test					0.05
	3	46	417.34	364.16	40.53	0.14
	4	61.3	417.34	364.16	54.05	0.38
	5	76.7	417.34	364.16	67.56	0.48
330	before the test	-	-	-	-	0.06
	3	46	467.77	421.18	44.16	0.16
	4	61.3	467.77	421.18	58.88	0.53
	5	76.7	467.77	421.18	73.60	0.73
400	before the test	--	-	-	-	0.07-0.08
	3	46	494.36	451.96	46,12	-
	4	61.3	494.36	451.96	61.49	0.68
	5	76.7	494.36	451.96	76.87	0.82
5 thermal cycles 300 0450 °C
60	before the test	-	-	-	-	0.04
	5	76.7	293.72	231.40	53.48	0.07
170	before the test	-	-	-	-	0.04
	5	76.7	388.70	332.53	64.20	0.4
250	before the test	-	-	-	-	0.05
	5	76.7	432.48	381.10	69.35	0.52
320	before the test	-	-	-	-	0.06
	5	76.7	463.28	416.49	73.10	0.79
400	before the test	-	-	-	-	0.08
	5	76.7	494.36	451.96	76.87	0.89
7 thermal cycles 300 0450 °C
60	before the test	-	-	-	-	0.04
	5	76.7	293.72	231.40	53.48	0.13
170	before the test	-	-		-	0.04
	5	76.7	388.70	332.53	64.20	0.58
250	before the test	-	-	-	-	0.05
	5	76.7	432.48	381.1	69.35	0.76
320	before the test	-	-	-	-	0.06
	5	76.7	463.68	416.49	73.10	0.88
400	before the test	-	-	-	-	0.08
	5	76.7	494.36	451.96	76.87	0.98

Three thermal cycles of 3000450 °C significantly increase the hydride reorientation level, with significant reorientation taking place in the dummy claddings with a hydrogen concentration of 150 ppm, whereas in the dummy claddings with a hydrogen concentration of 250...400 ppm significant reorientation also takes place during pressurized tests at Р29з=4 MPa.

It should be noted that the hydride orientation coefficient is significantly increased by three thermal cycles of 300 0450 °C, whereas further increase in the number of cycles to 5 and 7 (P293= 5 MPa) causes the hydride orientation coefficient to grow only insignificantly (see Table 2 and 3; Fig. 7).

Hydride distribution in the dummy claddings before and after the tests in the modes: heating to 450 °C, exposure for 3 hours, cooling to 180 °C and subsequent 3, 5, and 7 thermal cycles of 1800450 °C is shown in Fig. 9; the data on hydride orientation (hydride orientation coefficients in the dummy claddings) is provided in Table 4.

As in the case of three 300^450 °C thermal cycles, three 180^450 °C thermal cycles substantially increase the hydride reorientation level, with significant reorientation taking place in the dummy claddings with a hydrogen concentration of 170 ppm, whereas in the dummy claddings with a hydrogen concentration of 250.400 ppm significant reorientation also takes place during pressurized tests at Р29з=4 MPa. The hydride orientation coefficient is significantly increased by three 180^450 °C thermal cycles, whereas the effects of further increase in the number of cycles to 5 and 7 (P293=5 MPa) is insignificant and ambiguous (see Table 4).



Fig. 9 - Hydride microstructure in the dummy claddings (Zr-1%Nb) before and after the hydride reorientation test in the mode: heating to 450 °C, exposure for 3 hours, and cooling to 180 °С and subsequent 3 (a,b), 5 (c), and 7 (d) thermal cycles 180-450 °C

hydrogen hydride zirconium alloy

Table 4. Hydride reorientation coefficient in the dummy claddings tested in the mode: heating to 450 °C, exposure for 3 hours, cooling to 180°С, and subsequent 3, 5, and, 7 thermal cycles 1800450 °C

[H], ppm	P293, MPa	сте at 450 °C, MPa	TSSD, °C	TSSP, °C	сте at TSSP, MPa	Fn
3 thermal cycles 1800450 °C
60	before the test	-	-	-	-	0.04
	3	46	293.72	231.40	32.09	0.09
	4	61.3	293.72	231.40	42.78	0.08
	5	76.7	293.72	231.40	53.48	0.08
140	before the test	-	-	-	-	0.07
	3	46	368.65	310.71	37.13	0.09
	4	61.3	368.65	310.71	49.51	0.1
	5	76.7	368.65	310.71	61.89	0.1
270	before the test	-	-	-	-	0.05
	3	46	441.9	391.7	42.28	0.14
	4	61.3	441.9	397.7	56.38	0.09
	5	76.7	441.9	391.7	70.48	0.59
320	before the test	-	-	-	-	0.06
	3	46	463.28	416.49	43.86	0.15
	4	61.3	463.28	416.49	58.48	0.73
	5	76.7	463.28	416.49	73.10	0.68
400	before the test					0.07
	3	46	494.36	451.96	46.12	0.18
	4	61.3	494.36	451.96	61.49	0.78
	5	76.7	494.36	451.96	76.87	0.89
5 thermal cycles 180^450 °C
60	before the test	-	-	-		0.04
	5	76.7	293.72	231.4	53.48	0.12
170	before the test	-	-	-	-	0.04
	5	76.7	388.70	332.53	64.20	0.38
250	before the test	-	-	-	-	0.05
	5	76.7	432.48	381.1	69.35	0.42
320	before the test	-	-	-	-	0.06
	5	76.7	463.28	416.49	73.10	0.58
400	before the test	-	-	-	-	0.08
	5	76.7	494.36	451.96	76.87	0.82
7 thermal cycles 180^450 °C
60	before the test	-	-	-	-	0.04
	5	76.7	293.7	231.4	53.48	0.11
170	before the test	-	-	-	-	0.04
	5	76.7	388.7	332.53	64.20	0.38
250	before the test	-	-	-	-	0.05
	5	76.7	432.48	381.1	69.35	0.48
320	before the test	-	-	-	-	0.06
	5	76.7	463.28	416.49	73.10	0.59
400	before the test	-	-	-	-	0.08
	5	76.7	494.36	451.96	76.87	0.92

Mechanical Properties

Mechanical tests were performed at temperatures of 20 and 350 °C on circumferential samples 2.7 mm wide, cut out from non-hydrogenated dummy claddings, hydrogenated tube cladding sections 75 mm long, and dummy claddings with different hydrogen concentration tested in all the modes described in the “Treatment for Hydride Re-orientation” section.

Fig. 10 a, b shows dependence of the strength limit, yield strength, and relative elongation of the samples cut out from cladding tubes and hydrogenated tube sections at 20 and 350 °C.

At a temperature of 20 °C, non-hydrogenated cladding tubes have the following parameters: CTu=40.75 kg/mm², Ст0,2= 37.5 kg/mm² and 8=39.6%. With the hydrogen concentration increase to 170..400 ppm:

- the strength limit increases to 45.15.48.3 kg/mm²

- the yield strength increases to 42.2.45.3 kg/mm²;

- the relative elongation decreases, but does not go below 27%.

At a temperature of 350 °C, non-hydrogenated cladding tubes have the following parameters: CTu=17.75 kg/mm², cto,2=16.00 kg/mm² and 8=38%. With the hydrogen concentration increase to 170.400 ppm:

- the strength limit increases to 19.1.21.3 kg/mm²;

- the yield strength increases to 16.8.19.5 kg/mm²;

- the relative elongation increases to 40.8.44.2%

Fig. 10 - Dependence of the strength limit, yield strength, and relative elongation of Zr-1% Nb (E110) cladding

Tubes versus hydrogen concentration. Test temperatures: 20 and 350 °C.

No temperature mode used in the work led to any additional hydrogen embrittlement during the hydride reorientation tests. For illustration, Fig. 11a and 11b provide dependencies of the strength limit, yield strength, and relative elongation of the samples cut out from dummy claddings under pressure (P293=5 MPa) after 7 thermal cycles 300^450 °C, which caused virtually complete hydride reorientation (Fn=0.98), versus hydrogen concentration. After testing in this mode at room temperature, the strength of all the hydrogenated dummy claddings is slightly higher than of the non-hy- drogenated ones, and their plasticity virtually does not decrease below 30%. At 350 °C the strength of the dummy claddings virtually does not depend on the hydrogen concentration and their plasticity is slightly higher on the hydrogenated dummy claddings.

Fig. 11 - Dependence of the strength limit, yield strength, and relative elongation of the Zr-1% Nb (E110) dummy claddings tested with 7 thermal cycles 300^450 °C; Test temperatures: 20 and 350 °C



Conclusion

1. Upon completion of their operation, fuel rods claddings retain a number of hereditary degrading factors, such as hydrogen, accumulated during operation. Handling operations and long-term storage of SNF fuel rod claddings created conditions facilitating hydride reorientation, which, based on common understanding, may increase the efficiency of hydrogen embrittlement.

2. Pressurized dummy (3...5 MPa (at room temperature)) with hydrogenated Zr-1%Nb cladding (up to 400 ppm), initially containing tangentially oriented hydrides (Fn=0.04...0.08), were tested for hydride reorientation simulating different scenarios of SNF handling and storage:

- tests simulating handling operations and subsequent normal storage in the SNF dry storage: 450 °C, exposure for 8 hours, and subsequent cooling at ~2 deg./min;

- tests simulating handling operations and limiting design-basis accidents with 3, 5, and 7 thermal cycles 300^450 °C and 180^450 °C.

3. It was established that at internal pressure of 3, 4, and 5 MPa (at room temperature) tests in the modes: 450 °C, exposure for 8 house and subsequent cooling at ~2 deg./min. do not lead to noticeable hydride reorientation in the dummy claddings with a low hydrogen concentration (60 and 170 ppm). At hydrogen concentration 250...400 ppm in the dummy claddings, with a pressure increase from 0 to 4 MPa, the hydride orientation coefficient increases only insignificantly, but rises dramatically with a pressure increase from 4 to 5 MPa (up to Fn=0.58 at 400 ppm).

4. On the dummy claddings with a hydrogen concentration of 400 ppm tested in the mode: heating to 450 °C, exposure at 450 °C for 8 hours, and cooling at a rate of ~2 deg./min., the threshold hydride reorientation stress is сте at 450 °c -55-60 MPa (break-point on the function 10>СТе at 450 °C).

5. During spent VVER-1000 fuel assembly handling after 3 years of operation (pressure under cladding ~3.3 MPa), according to the results obtained, the fuel rod claddings are not expected to have significant hydride reorientation and, thus, increased hydrogen embrittlement efficiency.

6. Three thermal cycles of 300^450 °C and 180^450 °C significantly increase the hydride reorientation level, with significant reorientation taking place in the dummy claddings with a hydrogen concentration of 170 ppm, whereas in the dummy claddings with a hydrogen concentration of 250.400 ppm significant reorientation also takes place during pressure tests at Р293=4 MPa.

7. It should be noted that the hydride reorientation coefficient is significantly increased by three 300^450 °C thermal cycles. An increase in the number of 300^450 °C cycles from 3 to 5 and then to 7 causes an additional increase in the degree of hydride reorientation (up to 0.98 in the case of thermal cycles on the claddings with a hydrogen concentration of 400 ppm). Three 180^450 °C thermal cycles increase the degree of hydride reorientation up to 0.89 (on the claddings with a hydrogen concentration of 400 ppm); an increase in the number of 180^450 °С cycles from 3 to 7 does not additionally increase the degree of hydride reorientation.

8. At a temperature of 20 °C, non-hydrogenated cladding tubes have the following parameters: CTu=40.75 kg/mm², cto.2= 37.5 kg/mm² and 8=39.6%. As the hydrogen concentration goes up to 170.400 ppm, the strength limit increases to 45.15.48.3 kg/mm²; the yield strength increases to 42.2.45.3 kg/mm²; and the relative elongation decreases, but does not go below 27%.

9. At a temperature of 350 °C, non-hydrogenated cladding tubes have the following parameters: CTu=17.75 kg/mm², cto.2=16.00 kg/mm² and 8=38%. As the hydrogen concentration goes up to 170.400 ppm, the strength limit increases to 19.1.21.3 kg/mm²; the yield strength increases to 16.8.19.5 kg/mm²; and the relative elongation increases to 40.8...44.2%.

10. No temperature mode used in the work led to any additional hydrogen embrittlement during the hydride reorientation tests.

A logical continuation of this work is to research the effects of conditions in which radial hydrides are formed on the efficiency of hydrogen embrittlement of the alloy Zr-1%Nb.



Reference

[1] Займовский, А. С., Никулина, А. В., and Решетников, Н. Г., '“Циркониевые сплавы в ядерной энергетике,” Москва: Энергоатомиздат, 1994.

[2] Machiels, A., “End-of-Life Rod Internal Pressures in Spent Pressurized Water Reactor Fuel,” EPRI Product ID: 3002001949 EPRI, California, Palo Alto: Electric Power Research Institute, 2013.

[3] Cappelaere, C., Limon, R., Gilbon, D., Bredel, T., Rabouille, O., Bouffioux, P., and Mardon, J. P., “Impact of Irradiation Defects Annealing on Long Term Thermal Creep of Irradiated Zircaloy-4 Cladding Tube,” Zirconium in the Nuclear Industry: Thirteenth International Symposium, ASTM STP 1423, G. D. Moan and P. Rudling, Eds., ASTM International, West Conshohocken, PA, pp. 720-739, 2002.

[4] Couet, A., Motta, A. T., Comstock, R. J., “Hydrogen pickup measurements in zirconium alloys: Relation to oxidation kinetics” Journal of Nuclear Materials, Vol. 451, № 1-3, pp. 1-13, 2014.

[5] Coleman, C. E., and Hardie, D., “The Hydrogen Embrittlement of a-Zirconium - A Review,” Journals of the Less Common Metals, Vol. 11, № 3, pp. 168185, 1966.

[6] Bertolino, G., Meyer, G., Perez Ipina, J. “Degradation of the mechanical properties of Zircaloy-4 due to hydrogen embrittlement,” Journal of Alloys and Compounds, Vol. 330-332, pp. 408-413, 2002.

[7] Nuttall, K, and Rogowski, A. J., “Some Fractographic aspects of Hydrogen-Induced Delay Cracking in Zr-2.5%Nb alloys,” Journals of Nuclear Materials, Vol. 80, № 2, pp. 279-290, 1979.

[8] IAEA-TECDOC-1649, “Delayed Hydride Cracking of Zirconium Alloy Fuel Cladding,” Vienna: IAEA, 2010, ISSN 1011-4289.

[9] Coleman, C., Grigoriev, V., Inozemtsev, V., Markelov, V., Roth, M., Makarevicius, V., Kim, Y. S., Liagat Ali, K., Chakravartty, J.K., Mizrahi, R., and Lal- gudi, R, “Delayed Hydride Cracking in Zircaloy Fuel Cladding - An IAEA Coordinated Research Programme,” Nuclear Engineering and Technology, Vol. 41, № 2, pp. 171-178, March 2009.

[10] Raynaud, P. A., Koss, D. A., and Motta, A. T., "Crack growth in the through-thickness direction of hydrided thin-wall Zircaloy sheet," Journal of Nuclear Materials, Vol. 420, № 1-3, pp. 69-82, 2012.

[11] “Review Of Used Nuclear Fuel Storage and Transportation. Technical Gap Analyses,” FCRD- USED-2012-000215 Draft, PNNL-21596, Prepared for the U.S. Department of Energy Used Fuel Disposition Campaign, Washington, D.C., July 31 2012.

[12] Louthan, M. R., Jr., and Marshall, R. P., “Control of Hydride Orientation of Zircaloy,” Journal of Nuclear Materials, Vol. 9, № 2, pp. 170-184, 1963.

[13] Hardie, D., and Shanaham, A. W., “Stress Reorientation of Hydrides in Zirconium-2.5%Nb,” Journal of Nuclear Materials, Vol. 55, № 1, pp. 1-13, 1975.

[14] Billone, M. C., Burtseva, T. A., and Einziger, R. E., “Ductile-to-Brittle Transition Temperature for High-Burnup Cladding Alloys Exposed to Simulated Drying-Storage Conditions,” Journal of Nuclear Materials, Vol. 433, № 1-3, pp. 431-448, 2013.

[15] Cinbiz, M. N., Koss, D. A., Motta, A.T. ”The Effect of Stress Biaxiality on Hydride Reorientation Threshold Stress,” Proceedings of LWR Fuel Performance Meeting, TopFuel 2015, 13 - 17 September,

2015, Zurich, Switzerland; Belgium, Brussels, Part II, pp. 94-103, 2015.

[16] NUREG 1536, “Standard Review Plan for Spent Fuel Dry Storage Systems at a General License Facility”, Division of Spent Fuel Storage and Transportation, Office of Nuclear Material Safety and Safeguards, 2009.

[17] Новоселов, А. Е., Павлов, С. В., Поленок, В. С., Марков, Д. В., Жителев, В. А., Кобылянский, Г. П., Костюченко, А. Н., и Волкова, И. Н., “Состояние оболочек ТВЭЛов ВВЭР после шести лет эксплуатации,” Физика и химия обработки материалов, № 2, pp. 24-32, 2009.

[18] IAEA-TECDOC-1381, “Analysis of differences in fuel safety criteria for WWER and western PWR nuclear power plants,” IAEA, Vienna, 2003.

[19] Shebaldov, P. V., Peregud, M. M., Nikulina, A. V., Bibilashvili, Y. K., Lositski, A. F., Kuz'menko, N. V., Belov, V. I., and Novoselov, A. E., “E110 alloy cladding tube properties and their interrelation with alloy structure-phase condition and impurity content,” Zirconium in the Nuclear Industry, Twelfth International Symposium, ASTMSTP1354, G. P. Sabol and G. D. Moan, Eds., American Society for Testing and Materials, West Conshohocken, PA, pp. 545-559, 2000.

[20] Лучна, А. Є., Лавренчук, A. І., Сєднєв, В. А., Васильченко, В. М., Двоєглазов, О. М., Медведєв, В. І. и Печера, Ю. М., “Сухе сховище відпрацьаного ядерного палива запорізької АЕС. Забезпечення безпеки” International Conference Current Problems in Nuclear Physics and Atomic Energy, May 29 - June 3, Kyiv, Ukraine, 2006.

[21] Daum, R. S., Chu, Y. S., and Motta, A. T., “Identification and quantification of hydride phases in Zircaloy-4 cladding using synchrotron X-ray diffraction,” Journal of Nuclear Materials, Vol. 392, № 3, pp. 453-464, 2009.

[22] Liu, Y., Peng, Q., Zhao, W., and Jiang, H., “Hydride precipitation by cathodic hydrogen charging method in zirconium alloys,” Materials Chemistry and Physics, Vol. 110, № 1, pp. 56-60, 2008.

[23] Tang, R., and Yang, X., “Dissolution and precipitation behaviors of hydrides in N18, Zry-4 and M5 alloys,” International Journal of Hydrogen Energy, Vol. 34, № 17, pp. 7269-7274, 2009.

Размещено на Allbest.ru

...