Atomic Energy, Vol. 91, No. 5, 2001
DEVELOPMENT AND TESTING OF FUEL ELEMENTS WITH VIBRATIONALLY COMPACTED OXIDE FUEL FOR FAST REACTORS
A. A. Maershin, V. A. Tsykanov, V. N. Golovanov, O. V. Skiba, G. I. Gadzhiev, A. S. Korol’kov, N. V. Boborova, V. A. Kislyi, and A. A. Teikovtsev
UDC 621.039.542:621.039.548
The main results of a series of scientific-research and technological studies performed at the State Science Center of the Russian Federation – Scientific-Research Institute of Nuclear Reactors to substantiate the use of fuel elements with vibrationally compacted oxide fuel in fast reactors are presented. In the course of this work, the physical-mechanical and technological characteristics of granular UO2 and UPuO2 fuel were studied; radiation tests and materials-engineering investigations of experimental and test fuel elements were performed in BOR-60, BN-350, and -600 reactors. More than 30,000 fuel elements were fabricated. Maximum burnup ~30% heavy atoms was attained in BOR-60 using fuel assemblies with the standard construction and 32.3% heavy atoms was obtained using experimental fuel elements with a collapsible fuel assembly. In testing fuel elements with vibrationally compacted UPuO2 in BN-600, maximum burnup of 9.6% (~10.8% heavy atoms for individual fuel elements) was achieved. Postreactor investigations showed that the service life of the fuel elements is determined only by the choice of the cladding material. In accordance with the concept developed at the Ministry of Atomic Energy of Russia for the utilization of weapons plutonium, the Institute set about to implement in practice a technology for converting the metallic weapons-grade plutonium into mixed uranium–plutonium oxide fuel on the basis of pyroelectrochemistry and vibrational compaction.
Comprehensive investigations to prove that a short closed fuel cycle for nuclear reactors can be implemented have been going on for the last 25 years at the State Science Center of the Russian Federation – Scientific-Research Institute of Nuclear Reactors. In contrast to conventional water methods for processing nuclear fuel, the present work is based on the principles of dry technology: – pyroelectric chemical methods for reprocessing high-level nuclear fuel in fused salt systems with production of granular material directly usable for fabricating fuel elements by vibrational compaction; – vibrational compaction for fabricating fuel elements from granulated fuel; and – automated remote control processes for fabricating granulated fuel, fuel elements, and fuel assemblies. Such technologies make it possible to take account of modern safety and ecological requirements for the process and to expand substantially the possibilities of oxide fuel for improving the technical-economic indicators of the fuel cycle as a whole. The first stage of the work on scientific-experimental substantiation of fuel elements for fast reactors with vibrationally compacted uranium–plutonium oxide fuel is now by and large completed. The construction of a fuel element and an State Science Center of the Russian Federation – Scientific-Research Institute of Nuclear Reactors. Translated from Atomnaya Énergiya, Vol. 91, No. 5, pp. 378–385, November, 2001. 1063-4258/01/9105-0923$25.00 ©2001 Plenum Publishing Corporation
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TABLE 1. Production of Fuel Elements with Vibrationally Compacted Oxide Fuel Form of the fuel
Reactor
Number of fuel assemblies
Number of fuel elements
UPuO2*
BOR-60
426
15,762
UPuO2
**
BOR-60
32
1184
UPuO2**
BN-350
2
254
UO2
***
BN-350
7
889
UO2***
BN-600
4
508
UPuO2
**
BN-600
9
1143
UPuO2*
BN-600
4
508
*, ***
UPuO2
UO2*** + PuO2**** ***
UO2
BFS
8
1016
BOR-60
1
7
BOR-60
235
8695
* **
,
Reactor (high-background) and weapons (low-background) grade plutonium, respectively. Product obtained by reprocessing spent fuel at RT-1. **** Production obtained by reprocessing spent BOR-60 (24%) and BN-350 (4.9%) fuel. ***
extensive series of studies of the radiation resistance of vibrationally compacted fuel and fuel elements based on this fuel and an automated remote-control technology for fabricating fuel elements are incorporated into the concept of a closed nuclear fuel cycle and have high technical-economic and operational indicators. Experience has been accumulated in designing, fabricating, and operating equipment for fabricating granular uranium and uranium–plutonium oxide fuel and fuel elements. A databank and computer codes have been developed. Since 1981 the BOR-60 core has been operating on vibrationally compacted UPuO2 fuel. Fuel assemblies with standard construction and 30% burnup and individual experimental fuel elements with 32.3% have now been achieved. The pyroelectrochemical method has been used to reprocess ~50 kg of weapons-quality plutonium, and this plutonium has been used to fabricate and test 32 fuel assemblies in the BOR-60 reactor and three fuel assemblies in the BN-600 reactor. Successful tests of six fuel assemblies with vibrationally compacted UPuO2 fuel with up to 9.6% burnup (maximum burnup of individual fuel elements ~10.8%) have been conducted. Fabrication of Vibrationally Compacted Fuel Elements. Vibrational compaction has always been considered for fabricating fuel cores. It makes it possible to decrease substantially the cost of producing fuel elements and improve their operating characteristics. The main advantages of vibrational compaction and fuel elements with vibrationally compacted fuel are: – simplicity and reliability of the production process, which are achieved by reducing the number of technological and control operations to a minimum, thereby facilitating automation and remote control of the process, so that this process can be used for fabricating fuel elements from high-level (reprocessed) fuel in protected chambers; – possibility of fabricating fuel cores with easily variable parameters and multicomponent compositions; – possibility of using a granular material of any form with homogeneous composition and in the form of a mechanical mixture; – smaller, compared with pelleted cores, thermomechanical effect of the vibrationally compacted fuel on the cladding; and – easing of requirements on the inner diameter of the fuel-element cladding. As a result of the vibrational compaction, the effective density of a fuel core for fast-reactor fuel elements is 8.9–9.4 g/cm3. For comparison, the effective density of the fuel core of pelted fuel is 8.5–8.6 g/cm3. The distribution of the effective density of a fuel core and plutonium along the axis does not exceed 5% of the nominal level. At the filler preparation stage, a getter in the form of granules of metallic uranium is introduced into the granular fuel in order to adjust the oxygen potential of the fuel and eliminate any influence of technological impurities. The getter particles are nearly spherical and have a maximum diameter of up to 100 µm. When the technology was perfected, the getter dis924
TABLE 2. BOR-60 Fuel Elements Fuel composition
UO2 + PuO2, UO2 + PuO2 + U, (UPu)O2 + U
PuO2 mass fraction, %
20–28
Getter mass fraction, %
3–10
Diameter × thickness of fuel element cladding, mm
6 × 0.3, 6.9 × 0.4
Maximum power per unit length, W/cm
510
Maximum cladding temperature, °C
722
Maximum fuel burnup, % heavy atoms basic-construction fuel assembly
21
experimental fuel assemblies
30
experimental fuel elements
32.3
Number of fuel assemblies: Total
442
Reaching burnup, % heavy atoms: 10–15
279
15–20
11
>20
10
tribution along the fuel core was uniform to within ±5%. Comprehensive investigations, which included prereactor and reactor tests (BOR-60, BN-350, and -600) and postreactor materials engineering investigations, made to possible to optimize the getter content taking account of the thermodynamic characteristics and the need to achieve the required prestoichiometric oxygen coefficient of the fuel. The possibility of fabricating fuel elements by vibrational compaction was demonstrated in two variants: manual – in glove boxes – and remote-controlled – in protected chambers. The commercial prototype remote-contolled fabrication of vibrationally compacted fuel elements and fuel assemblies for BOR-60, BN-350 and -600 under was achieved in the Orel setup (1977–1986, about 300 BOR-60 fuel assemblies were fabricated), the OIK setup (1989–1997, 26 BN-800 fuel assemblies), and the Kolibri model setup (1992–1997, 63 fuel elements). Two series of glove boxes, on which various experimental programs are implemented, and experimental BOR-60 fuel elements in fuel assemblies are fabricated, are now in operation. The technological equipment used in the experimental-research complex was found to be highly reliable. The yearly production yield exceeded 98%. In all (according to the situation in May 2001), 728 fuel assemblies and 29,966 fuel elements have been fabricated (Table 1). BOR-60 has been operating on vibrationally compacted uranium–plutonium oxide fuel since 1981. In the last few years, reprocessed uranium dioxide has also been introduced into the reactor fuel cycle. About 500 fuel assemblies with mixed fuel have been and are being tested in the reactor (Table 2). Comprehensive investigations to optimize the fuel-element construction showed that the introduction of a getter in the form of metallic-uranium powder into the fuel core is the most radical method for improving the working capacity of fuel elements. Adding getter in quantities up to 5 mass% has made it possible to achieve burnup up to 11.5% heavy atoms (maximum burnup was 15–16% heavy atoms). Increasing the getter content from 5 to 10% and increasing the density of the fuel core have made it possible to obtain just as deep burnup but with higher heat loads and cladding temperature. Reactor Tests and Materials-Engineering Investigations. Postreactor materials-engineering investigations of fuel assemblies have confirmed the behavior previously obtained under irradiation for experimental fuel elements with mixed vibrationally compacted fuel: gas release, change in the outer diameter of the cladding, formation of the structure of the fuel core, distribution of fission products (Zr, Ce, Ru), and others. The mechanical properties and swelling of fuel-element claddings were close to the values for experimental fuel elements with homogenized fuel (UPu)O2 and the mechanical mixture UO2 + PuO2, as well as fuel elements with fuel consisting of uranium dioxide pellets. It was shown that in fuel elements 925
16
12
∆V/V, %
1
4
8 2 4
3
5
0 10
6
20 30 F, 1022 cm–2 (E > 0.1 MeV)
Fig. 1. Swelling of stainless steel 316 (1), the alloy ÉI-847 (2), ChS-68 (3), modified steel 316 (4), EM12 (5), and MT-9 (6).
Fig. 2. Micro- and macrostructure of the transverse section of BOR-60 fuel element with 32% heavy atoms burnup.
with getter mass fraction >3% there is no physical-chemical interaction of the fuel with the cladding, even with maximum burnup, specific heat load, and temperature of the cladding. The only limiting factor, which prevents reaching burnup greater than 20% heavy atoms for BOR-60 fuel elements with given structure, is greater swelling of austenitic cladding materials under damaging doses exceeding 80 displacements/atom (Fig. 1). Reactor tests and materials-engineering investigations of basic-construction fuel elements with the fuel containing a getter consisting of metallic-uranium powder and possessing cladding made of the promising ferrite-martensite steel confirmed a high working capacity up to burnup ~30% heavy atoms. Individual experimental fuel elements reach burnup >32% heavy atoms. No thermomechanical and physicochemical interactions of the fuel with the cladding were observed in postreactor investigations in any of the tranverse sections of the fuel elements investigated. The typical micro- and macrostructure of vibrationally compacted (UPu)O2 + U fuel after tests in BOR-60 to burnup 32% heavy atoms are presented in Fig. 2. Analysis of the radiation characteristics of the working capacity of fuel elements with vibrationally compacted oxide fuel showed the following: 926
TABLE 3. Characteristics of BOR-60 Fuel Assemblies Rearranged During Operation
Type of rearrangement
Fuel type
Within a single row
Pelleted
Number of fuel Burnup, assemblies % heavy atoms
4
Number of unsealed fuel assemblies
10–12
2
Vibrationally compacted
16
8–24
2 (2 runs after rearrangement)
Pelleted
18
10–12
–
Vibrationally compacted
57
7–22
–
Pelleted
4
12–17
3
Vibrationally compacted
85
8–25
1 (5 runs after rearrangement)
From center to periphery From periphery to center
TABLE 4. Tests of Fuel Assemblies with Vibrationally Compacted Fuel BN-350
BN-600
Fuel assembly P1–P7
Ts–585
Ts–586
NF–0187
NF–0287
Fabrication year
1982
1984
1984
1987
1987
Fuel composition
UO2
(UPu)O2
(UPu)O2
UO2
(UPu)O2
(UPu)O2
5
5
10
10
10
7
–
20
22–28
–
~30
20
27
–
–
–
–
–
8.9–9.1
8.8–9.2
8.8–9.2
8.9–9.1
Getter (U metal) content, % Plutonium content, % 235
U enrichment, %
Effective density, g/cm
3
8.8
Fuel-element cladding material
8.3–8.8 OKh16N15M3B
Fuel-assembly jacket material
Vu1–Vu4 NF03–NF06
É1–É3
1989–1990 1990–1991
1999
OKh16N15M3BR
Kh16N11M3T
ChS-68
05Kh12N2M
05Kh12N2M
Heat load, kW/m
48
51
48
45
45
42
Cladding temperature, °C
710
740
690
670
670
Damaging dose, displacements/atom
45
30
42
53
77
Burnup, % heavy atoms
6.8
9.6
42–44
In reactor
680
680
Same
27
76–77
3.6
9.6 (10.8)
Same *
Same
7.2
4.7
6.8
with respect to gas
1**
0
2**
0
0
0
0
Same
with respect to fuel
0
1
0
0
0
2
0
Same
Presence of unsealed fuel elements:
* **
Individual fuel elements. During shipment operations.
– using fuel with the composition (UPu)O2 + U and a getter makes it possible to eliminate completely the corrosion processes due to the presence of cesium and halogens and also possible technological impurities; this eliminates the burnup limit due to the physicochemical interaction of the fuel and cladding; – as a result of the presence of a peripheral layer in the initial structure of the fuel, the stresses in the cladding during transient processes are many times lower and the stresses relax much more rapidly than in fuel elements with pelleted fuel; in consequence, the arrangements of fuel assemblies in the core with a power increase does not influence the working capacity of the fuel elements (Table 3); the high effective density of the fuel core (>9 g/cm3) gives an adequate margin with respect to the alloying temperature; – the total swelling of vibrationally compacted (UPu)O2 + U fuel was 0.6 ± 0.1%/% burnup; 927
Fig. 3. Mass transfer versus effective density 8.6 (a), 8.5 (b), 8.4 g/cm3 (c) in BN-600 fuel element.
– no effect of the technology used to obtain (UPu)O2 and UO2 + PuO2 granulated fuel on the working capacity of the fuel elements was observed. In the period from 1985 to 1986, two experimental fuel assemblies with mixed vibrationally compacted oxide fuel and seven fuel assemblies with uranium vibrationally compacted oxide fuel were irradiated in BN-350 (Table 4). In the course of operation, on the basis of data provided by the system for monitoring the seal of fuel-element cladding in the reactor, one of the assemblies with mixed vibrationally compacted oxide fuel (fuel assembly Ts-585) was found to be unsealed down to 928
TABLE 5. Basic Parameters of Fast-Reactor Fuel Elements Parameter
BOR-60
BN-50
BN-600
fuel element
1082
1790
2400, 2440
gas cavity
273
300
600
bottom end breeding zone
150
400
350
fuel core
450
1060
950
Length, mm:
100
–
350
Cladding diameter, mm
top end breeding zone
6, 6.9
6.9
6.6
Cladding thickness, mm
0.3, 0.4
0.4
0.4
Size of spacing wire, mm: circular
1.05
1.05
1.15
elliptic
0 × 1.3
0.6 × 1.3
0.6 × 1.3
Effective fuel-core density, g/cm3
8.3–9.5
8.4–8.8
8.8–9.2
Average PuO2 mass fraction in the granular material, %
15–40
20
30
235
45–90
10
–
U enrichment of the granular material, %
Nonuniformity of the distribution of along the core, %: effective density
±5
±5
±5
plutonium
±5
±5
±5
contact of the fuel with the coolant. After indications of a leak appeared on the 45th day of operation (burnup 1.2% heavy atoms), the assembly was irradiated for another 110 effective days. Disassembly revealed one defective fuel element. All other fuel elements did not show any external damage. The destruction of the integrity of the cladding of the defective fuel element occurred in the form of a transverse crack along the center of the core. Fuel was present at the location of the damage to the cladding. No indications of growth of the defect, washing out of fuel, and interaction of the fuel with the coolant were observed. γ-scanning methods revealed fuel elements with axial mass-transfer of fuel and a correlation was found between this phenomenon and the initial effective density of the fuel core (Fig. 3). Investigations established that the axial mass transfer of the fuel and the consequent appearance of a through defect in the cladding in one fuel element were due to the fact that the values of the test parameters exceeded the design parameters in combination with the lowest effective fuel density (~8.3 g/cm3 in some fuel elements). Fuel elements with effective fuel density >8.6 g/cm3, even with high values of the operating parameters, retained their working capacity, and no anomalies were observed on the side of the fuel and the cladding. The Ts-586 assembly, which reached burnup 36% higher than the design value, was sealed according to data from the system monitoring the seal of fuel element claddings in the reactor, but gas activity was detected after the off-loading transport operations. Materials-engineering investigations showed that two fuel elements had gas leaks. The leaks were due to extreme ovalization of ÉI-847 steel cladding as a result of its greater swelling and embrittlement combined with a high level of stress due to mechanical interaction of the bundle of swelled fuel elements with one another and with the casing pipe. The unanticipated test parameters of the fuel assemblies in BN-350, combined with variation of the initial effective density of the fuel core, confirmed experimentally the computational estimates of the limiting operational parameters of the fuel elements. The results of the reactor tests and materials-engineering investigations were used as a basis for making changes in the construction of and the fabrication technology for fuel elements in order to increase the reliablity and service life. Specifically: – the minimum effective density of the fuel core was increased to 8.6 g/cm3 and the getter mass fraction to 10%; – more radiation-resistant structural materials were used. All these improvements in the construction were implemented in the fabrication of experimental BN-600 fuel assemblies (Table 5). 929
Tests of six fuel assemblies with vibrationally compacted uranium–plutonium oxide fuel, as well as four fuel assemblies with uranium oxide fuel were performed in BN-600. Reactor tests of six fuel assemblies with mixed fuel with design operational parameters were completed successfully. The maximum burnup was 9.6% heavy atoms (10.8% for individual fuel elements (see Table 4). The system for monitoring the seal of fuel-element cladding showed no open fuel elements in the assemblies. The results of the comprehensive materials-engineering investigations of the fuel elements in the fuel assemblies with 6.8% heavy atom burnup established the following: – the general state of all components of fuel assemblies and fuel elements is satisfactory; – the maximum changes in the diameter of the cladding and the “turnkey” size of the encased pipe did not exceed 2%, which according to the criterion for admissable shape change attests to an inexhaustible working capacity of the fuel assembly; – no unsealed fuel elements, breaks in the fuel core, or axial mass transfer of fuel was observed. Metallographic investigations did not show any anomalies, indications of overheating, or corrosion damage to the cladding on the fuel or coolant sides. Comparing the structural changes of the fuel core on the basis of the results of metallographic analysis with the results of a thermophysical calculation showed good agreement, which made it possible to verify the computational code VIKOND for fuel elements. In summary, the positive results of the tests performed on six experimental fuel assemblies (two fuel assemblies were fabricated in glove boxes and four fuel assemblies were fabricated by remote control on an automatic line) with vibrationally compacted uranium–plutonium oxide fuel in BN-600 and materials-engineering investigations of the fuel elements confirmed that the technological solutions are effective, but in order to obtain a complete substantiation and statistical data the tests must be continued in order to obtain at least 30 irradiated fuel assemblies. Mass tests of fuel elements with vibrationally compacted uranium–plutonium oxide fuel in BOR-60, together with successful tests of fuel elements in BN-600, and the reliable operation of the systems in the experimental-research complex show that there is a real possibility of using fuel elements with vibrationally compacted oxide fuel to produce safe, economically favorable uranium–plutonium fuel cycle on the basis of a dry technology as well as to use both power and weapons grade plutonium in nuclear reactors.
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