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Design features of water-cooled research reactors
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Design features of water-cooled research reactors I.A. Chusov a,∗, A.S. Shelegov a, O.Yu. Kochnov b a Obninsk
Institute for Nuclear Power Engineering, National Research Nuclear University “MEPhI”, 1 Studgorodok, Obninsk 249040, Kaluga region, Russia b Branch of JSC “Karpov Institute of Physical Chemistry”, Kievskoe shosse 109 km, Obninsk 249033, Kaluga region, Russia Available online xxx
Abstract Brief review of the design, specific features of thermal hydraulics of reactor cores and circulation loops of pool-type research reactors is given. Principal distinguishing features of research reactors as compared with industrial power reactor units are outlined. Design of reactor units is examined using the example of two research reactors VVR-M (Gatchina) and VVR-ts (Obninsk). Direction of coolant circulation constitutes the feature of research reactor installations which is of key importance. In contrast to power reactor units, propagation of coolant in research reactors is arranged in downwards direction, i.e. from core top to bottom. In connection with the above, particular design features of reactor support grids are discussed in the present study. A set of data is presented on the values of preset values of alarm and emergency protection triggering thresholds. The issue of modernization of the reactor core implemented by developing the family of fuel assemblies (FAs) of the new type is discussed separately using the example of modernization of the VVR-M reactor. It is demonstrated that by changing the FA design it is possible to significantly increase the neutron flux density and per unit power of reactor units. Tables containing main technical characteristics of different FAs for nuclear reactors of the IRT type are presented. Certain circuit engineering solutions for coolant circulation loops and characteristic design of research loops aimed at the solution of different research tasks are discussed. Copyright © 2016, National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Research reactor; Turbulence; Thermal hydraulics; Heat transfer in nuclear reactor; Fuel assembly; Reactor safety; Pool-type research reactor; VVR-M2; VVR-M3; VVR-M5; VVR-ts fuel assemblies.
Introduction Research nuclear reactors occupy important place in the development of nuclear power generation. Substantiation of safety of commercial NPP operation is impossible without implementation of wide program of fundamental and applied studies conducted on research nuclear reactors (RNR). Studies in the field of nuclear and neutron physics, solid state physics, nuclear and radiation chemistry, material studies, biology, medicine, testing fuel pins of power reactors under design and structural materials for reactor building are conducted in RNR cores [1,2]. ∗
Corresponding author. E-mail addresses: [email protected] (I.A. Chusov), [email protected] (A.S. Shelegov), [email protected] (O.Yu. Kochnov). Peer-review under responsibility of National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2016, n.3, pp. 116-128.
Despite the lower power levels and, correspondingly, smaller amounts of radioactive substances generated as the result of RNT operation, potential hazard of these reactors for public and environment remains nevertheless to be high because of a number of their special features: - High repeatability of transients during operation (reactor start and shutdown operations, variation of power levels); - Frequent core reshuffling and continuous transfers of irradiated items; - High repeatability of load cycles on the main equipment of cores and first cooling loops of reactors; - High neutron flux density in reactor cores; - Presence of high-enrichment fuel; - Smaller, as compared with commercial power reactors, number of physical protective barriers preventing dispersion of fission products; - Location of the majority of RNR in large cities with large population.
http://dx.doi.org/10.1016/j.nucet.2016.11.013 2452-3038/Copyright © 2016, National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: I.A. Chusov et al., Design features of water-cooled research reactors, Nuclear Energy and Technology (2016), http://dx.doi.org/10.1016/j.nucet.2016.11.013
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Fig. 2. (a) Shapes of grids and layout of installation of FA supports in the VVR-M reactor lattice: 1 – FA support; 2, 3 – guiding and support reactor grids. (b) Support grid of the VVR-ts reactor: 1, 2 – segment; 3 – support grid; 4 – shell ring; 5 – FA cell; 6 – CPS moving element cell.
Fig. 1. (a) RNR VVR-M: 1 – tank; 2 – FA; 3 – beryllium reflector; 4 – reactor vessel; 5–7 – baffle, guiding and support grids; 8, 9 – suction and pressure pipelines. (b) RNR VVR-ts: 1 – tank; 2 – FA; 3 – reactor vessel; 4, 5 – baffle and support grids; 6, 7 – suction and pressure pipelines.
Let us note that the present study, although it is of a review nature, does not claim, however, that comprehensive description of pool-type RNR will be provided and does not offer the solution of one of the most important issues outstanding at the present moment, namely, the use in RNRs of FAs loaded with low-enrichment fuel.
Design features of research nuclear reactor installations and the main circulation loop In the most general case pool-type reactor represents a cylindrically shaped tank with height equal to 5.5–8.5 m and with internal diameter equal up to 3.0 m. Wall thickness of the tank varies within the range of 12–20 mm. Spherical or flat bottom with wall thickness equal to 20–35 mm is welded to the lower part of the tank. Upper lid of the tank is flat with wall thickness equal up to 35 mm. Shell ring consisting of two cylindrical parts having larger and smaller diameters is welded in the lower part or the reactor vessel to its bottom (Fig. 1(a) and (b)). Cylindrical parts are connected with each other using conical insert. Separator representing a hollow cylinder to the lower part of which either one grid (support grid) or two grids (guiding and support grids) are welded, is installed in the upper part of the larger diameter cylinder. The main purpose of the guiding and support grids is to ensure rigid fixation of FAs, reactor CPS rods and vertical experimental channels. Guiding and support grids of the RNR VVR-M reactor are shown in Fig. 2(a), and support grid of the RNR VVR-ts reactor is shown in Fig. 2(b).
Organization of coolant circulation in the reactor core (r.c.) constitutes the principal distinguishing feature of pooltype reactor units. In contrast to power reactor units coolant moves in the RNR core in the direction from top to bottom, i.e. the downwards movement takes place, while in commercial power reactor coolant movement is organized in upward direction. RNR type of organization of coolant flow completely excludes buoyancy of FAs under the effects of approach flow, which allows simplification of the reactor core due to the possibility to design the reactor core without the use of FA anchorage in its upper part and makes it easier accessible for installation of diverse experimental devices. Baffle grid intended for leveling the coolant velocity and temperature fields and excluding penetration of unwanted objects on the bottom is installed between the cylindrical shell ring of larger diameter and the reactor tank. Installation of FAs in the reactor core is implemented either manually using a shaft, or using automatic loading mechanism. Extraction of FAs from the reactor core into the cooling pool is achieved only using automatic loading mechanism. Analysis of process circuit flow diagrams for pool-type RNR demonstrates that, despite the diversity of engineering solutions in the choice of configuration and equipment of primary circulation loops (PCL) there exist general principles of construction while the features of hydraulic circuits (application of dedicated equipment) are dictated, in the first place, by the specifics of the RNR intended use and design configuration. Typical conceptual engineering diagram of pool-type RNR is shown in Fig. 3(a). Let us note that this diagram is not exhaustive and many details are not shown in it. Presence of experimental loop installations (EPI) applied for solving different experimental tasks is the typical feature of all RNRs. Conceptual engineering diagram of lowtemperature EPI is shown in Fig. 3(b) as an example.
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Fig. 3. (a) Conceptual circuit diagram of pool-type RNR: 1 – reactor core; 2 – reactor tank; 3 – flow meter; 4 – MCP; 5 – emergency pump; 6 – preliminary filter; 7 – PCL heat exchanger; 8 – ion exchange filter; 9 – circulation pump; 10 – feeding tank; 11 – cooling tower. (b) Conceptual circuit diagram of loop design: 1 – loop channel; 2 – flow meter; 3 – feeding pump; 4 – ion exchange filter; 5 – pressure compensator; 6 – heat exchanger; 7 – circulation pump; 8 – feeding tank.
Fig. 4. Configuration of PCL of VVR-ts reactor unit before (a) and after (b) modernization (modernization was implemented in 2006) 1 – gate valve Ду 300; 2 – gate valve Ду 200; 3 – back flow preventer; 4 – heat exchanger T-2342; 5 – ion exchanger filter; 6 – centrifugal pump; 7 – intake header; 8 – pressure header; 9 – thermocouple; 10 – salt content sensor; 11 – plug; 12 – reactor tank; 13 – flow metering diaphragm; 14 – pressure alarm; 15 – pressure gauge; 16 – FA cladding leak detection.
If several parallel main circulation pumps are installed in the PCL then, as a rule, there is no standby emergency pump. Removal of heat released in the RNR is achieved using shelland-tube heat exchanging equipment items several of which may be included in the circuit, as, for instance, in the case of PCL of VVR-ts reactor before (Fig. 4(a)) and after (Fig. 4(b)) its upgrading. Main cooling pipeline serves for transportation of heated coolant to cooling heat exchangers and its recirculation in the reactor. No special requirements are imposed on the design and tracing the MCP in contrast to power
reactor units and the designers are guided by only the considerations of minimum pipeline length, serviceability and convenience of the whole configuration of the reactor unit. Conceptual circuit diagrams of VVR-ts and RBT10/2 (Dimitrovgrad) reactor units are presented in Figs. 4 and 5. In case of MCP rupture and loss of coolant emergently shutdown of the reactor is triggered by any of the following signals: drawdown of water level in the reactor tank, reduction of pressure in the pressure header, reduction of flowrate in the PCL, etc.
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Fig. 5. Configuration of PCL of the RBT–10/2 reactor 1 – FA basket; 2 – reactor core; 3 – pool pit; 4 – oxygen activity damper; 5 – pressure header; 6 – heat exchanger; 7 – main circulation pump AX-500; 8 – intake header; 9 – support structure.
Selection of settings for alarm system (AS) and emergency protection system (EPS) thresholds is usually performed in the following way. The limiting condition for normal operation for reactor power level is taken to be equal to 1.1 of the nominal power level; AS setting for pressure at the head of pumps of the primary cooling loop is equal to 0.9 of the rated value; AS setting for power is equal to 1.2 of the rated value; AS setting for coolant flow rate in the primary cooling loop is equal to 0.8 of the rated value; EPS setting for pressure at the head of pumps of the primary cooling loop is equal to 0.8 of the rated value. Emergency core cooling system (ECCS) includes several feeding tanks with volumes from 40 to 90 m3 . Supply of water in the RNR tank is, as a rule, automatically provided in case of triggering any of the above listed settings by natural flow due to the action of gravity forces. Supply of water from fire and household water supply pipelines is envisaged for the case of shortage of water in the feeding tanks. FA design and their main technical characteristics The largest reserve for enhancing safety of reactor operation and RNR performance parameters is associated with optimization of thermal regimes of operation of FAs and of fuel pins included in them. Several types of FAs and their modifications have as of the present moment already passed through the test irradiation in reactor units of VVR type. With rare exclusions (for instance, VVR-SM reactor, Germany) fuel elements of ring and rod types are used in research reactors. In terms of design FAs of pool-type RNR represent a
cylinder, hexahedron or tetrahedron inside which cylindrical, hexagonal or tetrahedral fuel elements are arranged coaxially forming annular channels (Fig. 6). VVR-M FAs consisting of tubular seamless fuel elements loaded with fuel composition in the form of Al + UO2 metal ceramic were entered by the end of the 1960s into operation in standard RNRs of VVR type. VVR-M2 FAs with fuel composition in the form of Al + U alloy started to be used from 1963. A team of experts from the Department of Reactor Physics and Engineering of the Petersburg Nuclear Physics Institute (PNPI) became the originator of thermal hydraulic studies by conducting measurements of hydraulic characteristics of fuel elements, distribution of coolant flow rates in the reactor core elements [3], fuel cladding temperatures in the most thermally stressed locations in the reactor core and limiting permissible heat flux densities [4]. Design characteristics of VVR-M2 FA allowed increasing the specific heat exchange surface by approximately four times relative to the EK-10 FA with fuel rods which was used in pool-type BBR-S reactor. Application of these FAs allowed increasing the RNR power from 2 to 10 MW. By 1973 FAs of VVR-M type had already exhausted their potential. Fuel cladding temperature practically reached saturation temperature and, although the margin before the critical thermal load still remained to sufficiently large (>2.5) [5], further increase of thermal power and, therefore, of neutron flux density, could result in the development of boiling of the surface of fuel elements, which is not acceptable for RNRs. In connection with the above discussion new FAs of VVRM3 type loaded with Al + U fuel were entered into operation from 1973. The key distinguishing features of FAs of this type are the following: (1) increased number of coaxially arranged fuel elements; (2) reduction of thickness of internal and external plating layers; (3) reduction of thickness of fuel layer; (4) use of central rod as the fuel. Heat exchange surface of the new FA was increased by 1.8 times. Based on the meticulously performed and fairly labor consuming experiments [6] PNPI experts demonstrated that specific thermal power of the VVR-M RNR and, consequently, the neutron flux density can be increased by 1.5 times. Preservation of fuel cladding operational temperature regimes constituted the distinguishing feature of the use of the new FAs. The next step in the modernization of FAs was the development of VVR-M5 FA with optimal calculated concentration of 235 U in the reactor core equal to 125 g/l which allowed increasing the fuel load by almost two times, reducing fuel cost component in the balance of operational costs and increasing the reactivity margin [7]. According to the data in Ref. [7] first VVR-M5 fuel assemblies were loaded in the reactor core in October, 1978 and, with reaching the design burnup in fuel assemblies of other types, the reactor load was completely converted to loading with new FAs by the summer of 1980. In the process of conversion to the use of VVR-M5 FAs their comprehensive testing was performed to the extent of their operation at stressful specific heat loads reaching up to 900 kW/l.
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Table 1 Characteristics of pool-type reactor units. Country, location Reactor power start-up, year Reactor power (thermal) (MW) Core volume (l) Core equivalent diameter (m) Core height (m) Maximum specific power (MW/l) Mean specific power (MW/l) Coolant type Moderator type Rated pressure (MPa) Temperature at the core inlet (°C) Temperature at the core outlet (°C) Coolant flow rate (t/h) Reflector Neutron flux density (n/(cm2 s)) FA type FA number Number of fuel elements in the FA Number of CPS elements: - Inherent safety elements - Compensation elements - Emergency protection 1)
VVR-K VVR-ts Kazakhstan, Russia, Alma-Ata Obninsk
VVR-M Russia, Gatchina
VVR-SM RBT-6 RBT10/1-10/23) IRT-T Uzbekistan, Russia, Russia, Russia, Tashkent Dimitrovgrad Dimitrovgrad Tomsk
IVV-2M Russia, Zarechny
IRT-MEPhI Russia, Moscow
1967 6 206 0.64 0.6 0.395 0.066 H2 O H2 O 0.2 <50 <70 1400 H2 O 1.4•1014 VVR-ts 70 3/55)
1964 15 206 0.64 0.6 0.395 0.165 H2 O H2 O 0.35 <50 <60 1500 H2 O 1.86•1014 VVR-ts 70 5
1960 18
1959 10 95
1975 6 132
1983 10 135
1968 6 59.3
1964 15
1967 2.5 59.3
0.576 0.5 0.9 0.25 H2 O H2 O 0.2 20–62 <62 1720 Be 4,0•1014 VVR-M5 145 61)
0.58
45 <65 1200 Be 2.3•1014 IRT-3M 32 6/84)
0.35 0.256 0.0625 H2 O H2 O 0.165 <60 <70 550–600 H2 O 1.4•1014 TVS-SM2 56 160/1882)
0.35 0.308 0.075 H2 O H2 O 0.18 60 <75 1000 H2 O + Be 1.5•1014 TVS-SM2 78 160/1882)
0.58 – 0.101 H2 O H2 O – 45 51 900 H2 O 1.76•1013 IRT-3M 78 6/84)
<40 <65 1200 Be 5.0•1014 IVV-2M 36–42 5
H2 O + Be 4.8•1013 IRT-3M 6/84) 6/84)
1 6 2
1 5 3
1 6 2
1 8 2
1 6 6×2
1
1 6 2
3 1 6×2
1 6 3
0.4 H2 O H2 O
6×2
0.5 0.5 0.207 0.205 H2 O H2 O + Be
0.58 0.042 H2 O H2 O 45 <65
2)
Data are provided for VVR-M5 FA. FAs of SM-3 reactor of the first (160 rods) and second (188 rods) types. 3) Two IR reactors in one pool. 4) FAs consisting of 6 or 8 pipes are used (including 6 or 8 fuel elements, respectively, and one control rod). 5) FAs consisting of 3 or 5 pipes are used (including 3 or 5 fuel elements, respectively, and one control rod). Table 2 Comparative characteristics of FAs and fuel elements of VVR-M, VVR-ts and IRT-T RNRs. Parameter
VVR-M
VVR-M2
VVR-M3
VVR-M5
VVER-ts
IRT-T
Year of putting into operation Fuel element thickness (mm) Plating layer thickness (mm) Active layer thickness (mm) Active layer length (mm) Hydraulic diameter (mm) Fuel type Cladding material Flat-to-flat size (mm) Lattice spacing (mm) Gap between fuel elements (mm) Number of fuel elements in the FA (items) 235 U enrichment (%) Heat exchange surface (m2 /m3 )
1958 2.3 0.7 0.9 500 6 Al + UO2 САВ-1 32.0 35.0 1.5 3 20 2.54
1963 2.5 0.9 0.7 5001) 6 Al + U САВ-1 32.0 35.0 1.5 3 36 3.62
1973 1.25 0.48 0.29 500 3.1 Al + U САВ-1 33.5 35.0 1.5 6 90 6.6
1978 1.25 0.36 0.53 500 3.1 Al + UO2 САВ-1 33.5 35.0 1.5 6 90 6.6
1964 2.3 0.85 0.6 600 6.14 Al + UO2 САВ-1 65.3 71.0 2.85 5 36 3.54(2.71)
1967 1.4 0.5 0.4 620 3.82 Al + UO2 САВ-1 69.4 71.5 2.5 8(6) 90 1.56(1.37)
1) Active 2) There
layer length in the VVR-S FA modification is equal to 600 mm. exists modification of VVR-M5 FA equipped with fins which is denominated VVR-M7 FA.
Characteristics of some pool-type reactor units are presented in Table 1; some characteristics of FAs of VVR-type reactors including FAs of VVR-ts reactor are shown in Table 2; characteristics of FAs and fuel elements of IRT reactors are provided in Table 3. As the experience of operation showed [8], the value of spacing between the external hexagonal fuel elements and experimental devices installed in the core of the reactor unit varies within the range from 0.75 to 2.5 mm depending on the type of the device and cannot be controlled. Direct contact
of the fuel element and the device can cause local overheating of the fuel element surface and even its melt-down. It is specifically these considerations which served as the basis for subsequent modification of the FAs. In order to exclude direct contact of cladding of fuel elements between each other or with experimental devices it was decided to apply the well proven method, namely equipping the external hexagonal fuel element with fins. Authors of Ref. [8] provided the following considerations in favor of such design solution:
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Fig. 6. (a) FA of the VVR-M reactor unit: A – VVR-M2; B – VV5-M5; C – VVR-M5 with fins (VVR-M7); 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank. (b) FA of the VVR-ts reactor unit with fins: 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank. (c) FA of the VVR-S IRT-3 M reactor unit: 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank; 5 – cartridge; (d) FA od the IVV-2 M reactor unit: 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank; 5 – cartridge. Table 3 Comparative characteristics of FAs and fuel elements of the IRT-M type. Parameter
IRT-M
IRT-2M
Number of fuel elements in FA (items) 235 U content in FA (g) 235 U enrichment (%) Fuel element thickness (mm) Spacing between fuel elements (mm) Active layer thickness (mm) Active layer length (mm) Thickness of plating layer (mm) Fuel type Specific heat exchange surface (m2 /l)
2/3 120/155 36 3.2 5.3 1.2 500 1.0 U + Al 0.2
3/4 147/171 90/80 2.0 4.5 0.4 580 0.8 U + Al 0.265
- Possibility of direct contact of cladding of fuel elements between each other and with experimental devices is excluded; - Possibility of damage of fuel cladding is eliminated; - Change of FA geometry due to the arrangement of fins does not negatively affect the core assembling process. Testing of the FA which was given the name VVR-M7 demonstrated that core assembling process was not impeded and no criticism was made during reshuffling the core loaded with VVR-M7 fuel assemblies. The implemented experiments demonstrated that operability of VVR-M7 fuel assemblies equipped with fins until 40% burnup was reached did not differ from operability of VVRM5 FAs. External appearances of FAs of VVR-M RNR in different modifications and of FAs of VVR-ts RNR are shown in Fig. 6(a) and (b), respectively. Similar or almost similar
IRT-3M 3/4 198/230 36 2.05 0.64 600 0.68 Al + UO2
4/6/8 200/265/300 90/80 1.4 1.85 0.4 580(600) 0.8 U + Al 0.525
IRT-4M 6/8 309/353 36
6/8 265/300 19.7 1.6
0.5 600 0.45 Al + UO2
0.78 600 0.54 Al + UO2 0.513
FAs were applied in other RNRs. General view of FAs of VVR-s and IVV-2 M RNRs is presented in Fig. 6(c) and (d). More detailed description of all phases of modernization of cores of pool-type research reactor units is provided in Refs. [9–17]. Conclusion General characteristic is given of pool-type research nuclear reactor units. Main principles of arrangement and operation and operational safety requirements are formulated as applicable to research nuclear reactors. Brief description of design of reactor cores and coolant circulation loops is given. Example of VVR-M reactor is used to discuss the issue of the possibility to enhance operational parameters of the reactor unit as a whole. Comparative characteristics are given of fuel assemblies of nuclear reactors of different types.
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Design features of water-cooled research reactors I.A. Chusov a,∗, A.S. Shelegov a, O.Yu. Kochnov b a Obninsk
Institute for Nuclear Power Engineering, National Research Nuclear University “MEPhI”, 1 Studgorodok, Obninsk 249040, Kaluga region, Russia b Branch of JSC “Karpov Institute of Physical Chemistry”, Kievskoe shosse 109 km, Obninsk 249033, Kaluga region, Russia Available online xxx
Abstract Brief review of the design, specific features of thermal hydraulics of reactor cores and circulation loops of pool-type research reactors is given. Principal distinguishing features of research reactors as compared with industrial power reactor units are outlined. Design of reactor units is examined using the example of two research reactors VVR-M (Gatchina) and VVR-ts (Obninsk). Direction of coolant circulation constitutes the feature of research reactor installations which is of key importance. In contrast to power reactor units, propagation of coolant in research reactors is arranged in downwards direction, i.e. from core top to bottom. In connection with the above, particular design features of reactor support grids are discussed in the present study. A set of data is presented on the values of preset values of alarm and emergency protection triggering thresholds. The issue of modernization of the reactor core implemented by developing the family of fuel assemblies (FAs) of the new type is discussed separately using the example of modernization of the VVR-M reactor. It is demonstrated that by changing the FA design it is possible to significantly increase the neutron flux density and per unit power of reactor units. Tables containing main technical characteristics of different FAs for nuclear reactors of the IRT type are presented. Certain circuit engineering solutions for coolant circulation loops and characteristic design of research loops aimed at the solution of different research tasks are discussed. Copyright © 2016, National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Research reactor; Turbulence; Thermal hydraulics; Heat transfer in nuclear reactor; Fuel assembly; Reactor safety; Pool-type research reactor; VVR-M2; VVR-M3; VVR-M5; VVR-ts fuel assemblies.
Introduction Research nuclear reactors occupy important place in the development of nuclear power generation. Substantiation of safety of commercial NPP operation is impossible without implementation of wide program of fundamental and applied studies conducted on research nuclear reactors (RNR). Studies in the field of nuclear and neutron physics, solid state physics, nuclear and radiation chemistry, material studies, biology, medicine, testing fuel pins of power reactors under design and structural materials for reactor building are conducted in RNR cores [1,2]. ∗
Corresponding author. E-mail addresses: [email protected] (I.A. Chusov), [email protected] (A.S. Shelegov), [email protected] (O.Yu. Kochnov). Peer-review under responsibility of National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2016, n.3, pp. 116-128.
Despite the lower power levels and, correspondingly, smaller amounts of radioactive substances generated as the result of RNT operation, potential hazard of these reactors for public and environment remains nevertheless to be high because of a number of their special features: - High repeatability of transients during operation (reactor start and shutdown operations, variation of power levels); - Frequent core reshuffling and continuous transfers of irradiated items; - High repeatability of load cycles on the main equipment of cores and first cooling loops of reactors; - High neutron flux density in reactor cores; - Presence of high-enrichment fuel; - Smaller, as compared with commercial power reactors, number of physical protective barriers preventing dispersion of fission products; - Location of the majority of RNR in large cities with large population.
http://dx.doi.org/10.1016/j.nucet.2016.11.013 2452-3038/Copyright © 2016, National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Fig. 2. (a) Shapes of grids and layout of installation of FA supports in the VVR-M reactor lattice: 1 – FA support; 2, 3 – guiding and support reactor grids. (b) Support grid of the VVR-ts reactor: 1, 2 – segment; 3 – support grid; 4 – shell ring; 5 – FA cell; 6 – CPS moving element cell.
Fig. 1. (a) RNR VVR-M: 1 – tank; 2 – FA; 3 – beryllium reflector; 4 – reactor vessel; 5–7 – baffle, guiding and support grids; 8, 9 – suction and pressure pipelines. (b) RNR VVR-ts: 1 – tank; 2 – FA; 3 – reactor vessel; 4, 5 – baffle and support grids; 6, 7 – suction and pressure pipelines.
Let us note that the present study, although it is of a review nature, does not claim, however, that comprehensive description of pool-type RNR will be provided and does not offer the solution of one of the most important issues outstanding at the present moment, namely, the use in RNRs of FAs loaded with low-enrichment fuel.
Design features of research nuclear reactor installations and the main circulation loop In the most general case pool-type reactor represents a cylindrically shaped tank with height equal to 5.5–8.5 m and with internal diameter equal up to 3.0 m. Wall thickness of the tank varies within the range of 12–20 mm. Spherical or flat bottom with wall thickness equal to 20–35 mm is welded to the lower part of the tank. Upper lid of the tank is flat with wall thickness equal up to 35 mm. Shell ring consisting of two cylindrical parts having larger and smaller diameters is welded in the lower part or the reactor vessel to its bottom (Fig. 1(a) and (b)). Cylindrical parts are connected with each other using conical insert. Separator representing a hollow cylinder to the lower part of which either one grid (support grid) or two grids (guiding and support grids) are welded, is installed in the upper part of the larger diameter cylinder. The main purpose of the guiding and support grids is to ensure rigid fixation of FAs, reactor CPS rods and vertical experimental channels. Guiding and support grids of the RNR VVR-M reactor are shown in Fig. 2(a), and support grid of the RNR VVR-ts reactor is shown in Fig. 2(b).
Organization of coolant circulation in the reactor core (r.c.) constitutes the principal distinguishing feature of pooltype reactor units. In contrast to power reactor units coolant moves in the RNR core in the direction from top to bottom, i.e. the downwards movement takes place, while in commercial power reactor coolant movement is organized in upward direction. RNR type of organization of coolant flow completely excludes buoyancy of FAs under the effects of approach flow, which allows simplification of the reactor core due to the possibility to design the reactor core without the use of FA anchorage in its upper part and makes it easier accessible for installation of diverse experimental devices. Baffle grid intended for leveling the coolant velocity and temperature fields and excluding penetration of unwanted objects on the bottom is installed between the cylindrical shell ring of larger diameter and the reactor tank. Installation of FAs in the reactor core is implemented either manually using a shaft, or using automatic loading mechanism. Extraction of FAs from the reactor core into the cooling pool is achieved only using automatic loading mechanism. Analysis of process circuit flow diagrams for pool-type RNR demonstrates that, despite the diversity of engineering solutions in the choice of configuration and equipment of primary circulation loops (PCL) there exist general principles of construction while the features of hydraulic circuits (application of dedicated equipment) are dictated, in the first place, by the specifics of the RNR intended use and design configuration. Typical conceptual engineering diagram of pool-type RNR is shown in Fig. 3(a). Let us note that this diagram is not exhaustive and many details are not shown in it. Presence of experimental loop installations (EPI) applied for solving different experimental tasks is the typical feature of all RNRs. Conceptual engineering diagram of lowtemperature EPI is shown in Fig. 3(b) as an example.
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Fig. 3. (a) Conceptual circuit diagram of pool-type RNR: 1 – reactor core; 2 – reactor tank; 3 – flow meter; 4 – MCP; 5 – emergency pump; 6 – preliminary filter; 7 – PCL heat exchanger; 8 – ion exchange filter; 9 – circulation pump; 10 – feeding tank; 11 – cooling tower. (b) Conceptual circuit diagram of loop design: 1 – loop channel; 2 – flow meter; 3 – feeding pump; 4 – ion exchange filter; 5 – pressure compensator; 6 – heat exchanger; 7 – circulation pump; 8 – feeding tank.
Fig. 4. Configuration of PCL of VVR-ts reactor unit before (a) and after (b) modernization (modernization was implemented in 2006) 1 – gate valve Ду 300; 2 – gate valve Ду 200; 3 – back flow preventer; 4 – heat exchanger T-2342; 5 – ion exchanger filter; 6 – centrifugal pump; 7 – intake header; 8 – pressure header; 9 – thermocouple; 10 – salt content sensor; 11 – plug; 12 – reactor tank; 13 – flow metering diaphragm; 14 – pressure alarm; 15 – pressure gauge; 16 – FA cladding leak detection.
If several parallel main circulation pumps are installed in the PCL then, as a rule, there is no standby emergency pump. Removal of heat released in the RNR is achieved using shelland-tube heat exchanging equipment items several of which may be included in the circuit, as, for instance, in the case of PCL of VVR-ts reactor before (Fig. 4(a)) and after (Fig. 4(b)) its upgrading. Main cooling pipeline serves for transportation of heated coolant to cooling heat exchangers and its recirculation in the reactor. No special requirements are imposed on the design and tracing the MCP in contrast to power
reactor units and the designers are guided by only the considerations of minimum pipeline length, serviceability and convenience of the whole configuration of the reactor unit. Conceptual circuit diagrams of VVR-ts and RBT10/2 (Dimitrovgrad) reactor units are presented in Figs. 4 and 5. In case of MCP rupture and loss of coolant emergently shutdown of the reactor is triggered by any of the following signals: drawdown of water level in the reactor tank, reduction of pressure in the pressure header, reduction of flowrate in the PCL, etc.
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Fig. 5. Configuration of PCL of the RBT–10/2 reactor 1 – FA basket; 2 – reactor core; 3 – pool pit; 4 – oxygen activity damper; 5 – pressure header; 6 – heat exchanger; 7 – main circulation pump AX-500; 8 – intake header; 9 – support structure.
Selection of settings for alarm system (AS) and emergency protection system (EPS) thresholds is usually performed in the following way. The limiting condition for normal operation for reactor power level is taken to be equal to 1.1 of the nominal power level; AS setting for pressure at the head of pumps of the primary cooling loop is equal to 0.9 of the rated value; AS setting for power is equal to 1.2 of the rated value; AS setting for coolant flow rate in the primary cooling loop is equal to 0.8 of the rated value; EPS setting for pressure at the head of pumps of the primary cooling loop is equal to 0.8 of the rated value. Emergency core cooling system (ECCS) includes several feeding tanks with volumes from 40 to 90 m3 . Supply of water in the RNR tank is, as a rule, automatically provided in case of triggering any of the above listed settings by natural flow due to the action of gravity forces. Supply of water from fire and household water supply pipelines is envisaged for the case of shortage of water in the feeding tanks. FA design and their main technical characteristics The largest reserve for enhancing safety of reactor operation and RNR performance parameters is associated with optimization of thermal regimes of operation of FAs and of fuel pins included in them. Several types of FAs and their modifications have as of the present moment already passed through the test irradiation in reactor units of VVR type. With rare exclusions (for instance, VVR-SM reactor, Germany) fuel elements of ring and rod types are used in research reactors. In terms of design FAs of pool-type RNR represent a
cylinder, hexahedron or tetrahedron inside which cylindrical, hexagonal or tetrahedral fuel elements are arranged coaxially forming annular channels (Fig. 6). VVR-M FAs consisting of tubular seamless fuel elements loaded with fuel composition in the form of Al + UO2 metal ceramic were entered by the end of the 1960s into operation in standard RNRs of VVR type. VVR-M2 FAs with fuel composition in the form of Al + U alloy started to be used from 1963. A team of experts from the Department of Reactor Physics and Engineering of the Petersburg Nuclear Physics Institute (PNPI) became the originator of thermal hydraulic studies by conducting measurements of hydraulic characteristics of fuel elements, distribution of coolant flow rates in the reactor core elements [3], fuel cladding temperatures in the most thermally stressed locations in the reactor core and limiting permissible heat flux densities [4]. Design characteristics of VVR-M2 FA allowed increasing the specific heat exchange surface by approximately four times relative to the EK-10 FA with fuel rods which was used in pool-type BBR-S reactor. Application of these FAs allowed increasing the RNR power from 2 to 10 MW. By 1973 FAs of VVR-M type had already exhausted their potential. Fuel cladding temperature practically reached saturation temperature and, although the margin before the critical thermal load still remained to sufficiently large (>2.5) [5], further increase of thermal power and, therefore, of neutron flux density, could result in the development of boiling of the surface of fuel elements, which is not acceptable for RNRs. In connection with the above discussion new FAs of VVRM3 type loaded with Al + U fuel were entered into operation from 1973. The key distinguishing features of FAs of this type are the following: (1) increased number of coaxially arranged fuel elements; (2) reduction of thickness of internal and external plating layers; (3) reduction of thickness of fuel layer; (4) use of central rod as the fuel. Heat exchange surface of the new FA was increased by 1.8 times. Based on the meticulously performed and fairly labor consuming experiments [6] PNPI experts demonstrated that specific thermal power of the VVR-M RNR and, consequently, the neutron flux density can be increased by 1.5 times. Preservation of fuel cladding operational temperature regimes constituted the distinguishing feature of the use of the new FAs. The next step in the modernization of FAs was the development of VVR-M5 FA with optimal calculated concentration of 235 U in the reactor core equal to 125 g/l which allowed increasing the fuel load by almost two times, reducing fuel cost component in the balance of operational costs and increasing the reactivity margin [7]. According to the data in Ref. [7] first VVR-M5 fuel assemblies were loaded in the reactor core in October, 1978 and, with reaching the design burnup in fuel assemblies of other types, the reactor load was completely converted to loading with new FAs by the summer of 1980. In the process of conversion to the use of VVR-M5 FAs their comprehensive testing was performed to the extent of their operation at stressful specific heat loads reaching up to 900 kW/l.
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Table 1 Characteristics of pool-type reactor units. Country, location Reactor power start-up, year Reactor power (thermal) (MW) Core volume (l) Core equivalent diameter (m) Core height (m) Maximum specific power (MW/l) Mean specific power (MW/l) Coolant type Moderator type Rated pressure (MPa) Temperature at the core inlet (°C) Temperature at the core outlet (°C) Coolant flow rate (t/h) Reflector Neutron flux density (n/(cm2 s)) FA type FA number Number of fuel elements in the FA Number of CPS elements: - Inherent safety elements - Compensation elements - Emergency protection 1)
VVR-K VVR-ts Kazakhstan, Russia, Alma-Ata Obninsk
VVR-M Russia, Gatchina
VVR-SM RBT-6 RBT10/1-10/23) IRT-T Uzbekistan, Russia, Russia, Russia, Tashkent Dimitrovgrad Dimitrovgrad Tomsk
IVV-2M Russia, Zarechny
IRT-MEPhI Russia, Moscow
1967 6 206 0.64 0.6 0.395 0.066 H2 O H2 O 0.2 <50 <70 1400 H2 O 1.4•1014 VVR-ts 70 3/55)
1964 15 206 0.64 0.6 0.395 0.165 H2 O H2 O 0.35 <50 <60 1500 H2 O 1.86•1014 VVR-ts 70 5
1960 18
1959 10 95
1975 6 132
1983 10 135
1968 6 59.3
1964 15
1967 2.5 59.3
0.576 0.5 0.9 0.25 H2 O H2 O 0.2 20–62 <62 1720 Be 4,0•1014 VVR-M5 145 61)
0.58
45 <65 1200 Be 2.3•1014 IRT-3M 32 6/84)
0.35 0.256 0.0625 H2 O H2 O 0.165 <60 <70 550–600 H2 O 1.4•1014 TVS-SM2 56 160/1882)
0.35 0.308 0.075 H2 O H2 O 0.18 60 <75 1000 H2 O + Be 1.5•1014 TVS-SM2 78 160/1882)
0.58 – 0.101 H2 O H2 O – 45 51 900 H2 O 1.76•1013 IRT-3M 78 6/84)
<40 <65 1200 Be 5.0•1014 IVV-2M 36–42 5
H2 O + Be 4.8•1013 IRT-3M 6/84) 6/84)
1 6 2
1 5 3
1 6 2
1 8 2
1 6 6×2
1
1 6 2
3 1 6×2
1 6 3
0.4 H2 O H2 O
6×2
0.5 0.5 0.207 0.205 H2 O H2 O + Be
0.58 0.042 H2 O H2 O 45 <65
2)
Data are provided for VVR-M5 FA. FAs of SM-3 reactor of the first (160 rods) and second (188 rods) types. 3) Two IR reactors in one pool. 4) FAs consisting of 6 or 8 pipes are used (including 6 or 8 fuel elements, respectively, and one control rod). 5) FAs consisting of 3 or 5 pipes are used (including 3 or 5 fuel elements, respectively, and one control rod). Table 2 Comparative characteristics of FAs and fuel elements of VVR-M, VVR-ts and IRT-T RNRs. Parameter
VVR-M
VVR-M2
VVR-M3
VVR-M5
VVER-ts
IRT-T
Year of putting into operation Fuel element thickness (mm) Plating layer thickness (mm) Active layer thickness (mm) Active layer length (mm) Hydraulic diameter (mm) Fuel type Cladding material Flat-to-flat size (mm) Lattice spacing (mm) Gap between fuel elements (mm) Number of fuel elements in the FA (items) 235 U enrichment (%) Heat exchange surface (m2 /m3 )
1958 2.3 0.7 0.9 500 6 Al + UO2 САВ-1 32.0 35.0 1.5 3 20 2.54
1963 2.5 0.9 0.7 5001) 6 Al + U САВ-1 32.0 35.0 1.5 3 36 3.62
1973 1.25 0.48 0.29 500 3.1 Al + U САВ-1 33.5 35.0 1.5 6 90 6.6
1978 1.25 0.36 0.53 500 3.1 Al + UO2 САВ-1 33.5 35.0 1.5 6 90 6.6
1964 2.3 0.85 0.6 600 6.14 Al + UO2 САВ-1 65.3 71.0 2.85 5 36 3.54(2.71)
1967 1.4 0.5 0.4 620 3.82 Al + UO2 САВ-1 69.4 71.5 2.5 8(6) 90 1.56(1.37)
1) Active 2) There
layer length in the VVR-S FA modification is equal to 600 mm. exists modification of VVR-M5 FA equipped with fins which is denominated VVR-M7 FA.
Characteristics of some pool-type reactor units are presented in Table 1; some characteristics of FAs of VVR-type reactors including FAs of VVR-ts reactor are shown in Table 2; characteristics of FAs and fuel elements of IRT reactors are provided in Table 3. As the experience of operation showed [8], the value of spacing between the external hexagonal fuel elements and experimental devices installed in the core of the reactor unit varies within the range from 0.75 to 2.5 mm depending on the type of the device and cannot be controlled. Direct contact
of the fuel element and the device can cause local overheating of the fuel element surface and even its melt-down. It is specifically these considerations which served as the basis for subsequent modification of the FAs. In order to exclude direct contact of cladding of fuel elements between each other or with experimental devices it was decided to apply the well proven method, namely equipping the external hexagonal fuel element with fins. Authors of Ref. [8] provided the following considerations in favor of such design solution:
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Fig. 6. (a) FA of the VVR-M reactor unit: A – VVR-M2; B – VV5-M5; C – VVR-M5 with fins (VVR-M7); 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank. (b) FA of the VVR-ts reactor unit with fins: 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank. (c) FA of the VVR-S IRT-3 M reactor unit: 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank; 5 – cartridge; (d) FA od the IVV-2 M reactor unit: 1 – FA head; 2 – crown; 3 – fuel elements; 4 – shank; 5 – cartridge. Table 3 Comparative characteristics of FAs and fuel elements of the IRT-M type. Parameter
IRT-M
IRT-2M
Number of fuel elements in FA (items) 235 U content in FA (g) 235 U enrichment (%) Fuel element thickness (mm) Spacing between fuel elements (mm) Active layer thickness (mm) Active layer length (mm) Thickness of plating layer (mm) Fuel type Specific heat exchange surface (m2 /l)
2/3 120/155 36 3.2 5.3 1.2 500 1.0 U + Al 0.2
3/4 147/171 90/80 2.0 4.5 0.4 580 0.8 U + Al 0.265
- Possibility of direct contact of cladding of fuel elements between each other and with experimental devices is excluded; - Possibility of damage of fuel cladding is eliminated; - Change of FA geometry due to the arrangement of fins does not negatively affect the core assembling process. Testing of the FA which was given the name VVR-M7 demonstrated that core assembling process was not impeded and no criticism was made during reshuffling the core loaded with VVR-M7 fuel assemblies. The implemented experiments demonstrated that operability of VVR-M7 fuel assemblies equipped with fins until 40% burnup was reached did not differ from operability of VVRM5 FAs. External appearances of FAs of VVR-M RNR in different modifications and of FAs of VVR-ts RNR are shown in Fig. 6(a) and (b), respectively. Similar or almost similar
IRT-3M 3/4 198/230 36 2.05 0.64 600 0.68 Al + UO2
4/6/8 200/265/300 90/80 1.4 1.85 0.4 580(600) 0.8 U + Al 0.525
IRT-4M 6/8 309/353 36
6/8 265/300 19.7 1.6
0.5 600 0.45 Al + UO2
0.78 600 0.54 Al + UO2 0.513
FAs were applied in other RNRs. General view of FAs of VVR-s and IVV-2 M RNRs is presented in Fig. 6(c) and (d). More detailed description of all phases of modernization of cores of pool-type research reactor units is provided in Refs. [9–17]. Conclusion General characteristic is given of pool-type research nuclear reactor units. Main principles of arrangement and operation and operational safety requirements are formulated as applicable to research nuclear reactors. Brief description of design of reactor cores and coolant circulation loops is given. Example of VVR-M reactor is used to discuss the issue of the possibility to enhance operational parameters of the reactor unit as a whole. Comparative characteristics are given of fuel assemblies of nuclear reactors of different types.
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