RU2464124C1 - Method of rotary casting of multilayer billets for shipping packages (sp) for waste nuclear fuel from high-strength iron with ball-shape graphite, and sp case one-piece casting thus produced - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to nuclear machinery building. Proposed method consists in teeming different-composition iron under flux layer, layer-by-layer, in rotary mould to produce SP one-piece case. Teemed outer layer is ferritic high-strength iron with ball-shape graphite that features neutron absorption capacity while inner layer is rustproof austenitic iron with ball-shape graphite.

EFFECT: high reliability of protection and corrosion resistance.

5 cl, 2 dwg

Description

The invention relates to nuclear engineering, in particular to a technology for the manufacture of centrifugal casting of TUK casings for SNF by centrifugal casting of multilayer castings (including mono-, bi- and polymetallic), large-tonnage blanks-cases of TUK for transportation and storage of SNF and other radioactive waste (RAS) from ferritic and special cast irons with spherical graphite.

State of the art

A method for centrifugal casting of hollow products is known from the patent literature, including pouring the melt into a rotating mold, its crystallization and alloying / cm. RF patent No. 2067914, 1991 /.

A known method of centrifugal casting of two-layer rolling rolls with a working layer of high alloy cast iron, comprising sequential pouring of metal into a rotary form / see RF patent No. 2148471, 1998 /.

A known method of centrifugal casting of cast iron bimetallic billets with an axial cavity, comprising applying a heat-insulating coating to the mold and sequential casting of the outer and inner layers of the metal in a mold / cm rotated around a horizontal axis. RF patent No. 2067914, 1991 /. A known machine with a horizontal axis of rotation of the mold for centrifugal casting / cm RF patent 2146182, 1998 /. Known mold for centrifugal casting of rolls / see USSR copyright certificate No. 1779461, 1990 /.

The closest analogues are the method of centrifugal casting of bimetallic billets, including the sequential pouring of metals of different composition, and the pouring of the next metal layer is carried out after cooling the previously cast metal to a temperature below the solidus of 100-350 ° C / cm. USSR author's certificate No. 358074, 1969 /.

At present, in our country and a number of other countries, the problem of providing facilities using nuclear fuel, TUK for transportation and long-term storage of spent nuclear fuel requires an urgent solution, as the accumulation of these materials poses a real threat to the environmental safety of large regions adjacent to the places for temporary storage of spent nuclear fuel, located, as a rule, near facilities using nuclear fuel.

An analysis of the main trends in world practice in solving the problem of the handling of irradiated nuclear fuel and radioactive waste showed that one of the main quality indicators of VChShG (K1s) (fracture toughness) in the temperature range from 20 to -40 ° C is higher than that of carbon steel at the same temperatures.

The fracture toughness of ductile iron and carbon steel Mark Metal The fracture toughness K1C, N / mm ² m _^1/2 material the basis -fifty +20 +100 Hf-40 ferrite> 90% 70 110 130 Hf-40 ferrite> 80% 60 one hundred 110 Carbonist. steel Perlite ferrite fifty 90 110

Extensive studies of the structural state, mechanical and physico-chemical properties of high-frequency alloys on samples cut from the body of large-capacity castings, container bodies have shown that the developed process regulations ensure the stable production of high-quality large-castings in the molded state, in accordance with the requirements of the IAEA rules and regulatory and technical RF documentation:

- container bodies made of ductile iron have significant advantages over other materials;

- the presence of 3.5% carbon in the composition of cast iron, which during crystallization from the melt is released in the form of free graphite (occupying about 15% of the total casting volume), provides better protection against radiation than other alloys;

- casting the hull of the TUK from VChShG significantly reduce costs and manufacturing time in comparison with the cost of manufacturing welded forged steel casings;

- allows for ultrasonic quality control of cases at all stages of manufacture and operation;

- allows for recycling after a 50-year period of operation of the hull from VChShG by simple remelting as a charge.

The large-tonnage container bodies made of VChShG are massive cylindrical products with an outer diameter of up to 2.5 meters with a wall thickness of 300-500 mm and a height of up to 10 meters. Now container bodies made of ductile iron are manufactured by the stationary method, i.e. pouring the VChShG melt into metal molds (chill molds) installed assembled (with rods) on the parade ground or in the caissons of the foundry. The manufacturing cycle of one container body according to this technology is (depending on its weight) from 20 to 30 days, of which approximately 15-20 days, the body is cast in a mold until it reaches a temperature of 300-350 ° C.

With such a long cycle of manufacturing one case casting, the Foundry of CJSC Petrozavodskmash, recognized as the best enterprise in our country for the production of large-tonnage products from VChShG.

The inner surface of the container body and the end surfaces in contact with the gaskets of the inner and outer end caps (not shown in the drawings) have a protective nickel coating. The outer surface of the TUK case is covered with the OS-51-03 organosilicate composition according to TU-84-725-78.

The container for transportation and / or storage of radioactive elements contains a housing with a lid and a bottom (see publ. Application of the Russian Federation No. 2004123577 from 01/27/2006).

The manufacture of the container represents a great technological complexity and a significant part of the cost of manufacturing the kit as a whole. This technology provides for the manufacture of chill castings with a massive central core from cold-hardening mixtures (CTS) with a heat removal system from the casting body. The metal is supplied by a siphon through the lower part of the casting, which leads to a significant consumption of liquid metal. To ensure the density of the casting, a significant profitable part is used. In general, for this part, the metal utilization coefficient does not exceed 0.5. When machining, the processing of channels for neutron protection by the method of deep drilling is of particular difficulty. This is a long technologically complex process requiring special equipment and tools. The second serious problem of ensuring long-term trouble-free operation of TUKs is the protection of the inner working surface of the cast-iron housing from mechanical damage and chemical corrosion during operation, today this is solved by nickel plating. The process entails a large consumption of electricity and a complex environmental problem associated with the disinfection of waste.

TUK is intended for:

- long-term storage of VVER spent fuel assemblies for up to 50 years in the conditions of dry container storage of spent nuclear fuel, which is an open area, or the TUK building is installed vertically;

- transportation of WWER spent fuel assemblies across the territory of the nuclear power plant from power units to the on-site storage of spent nuclear fuel, on a trailer or on rail;

- transportation of WWER spent fuel assemblies from the territory of a nuclear power plant from power units or from on-site storage of spent nuclear fuel by road, water or rail to the existing SNF SNF storage facility (KHOT-1) or centralized SNF.

The composition of the TUK includes a container, which is the main element that ensures the placement of SFA, radiation protection of personnel and the public during transportation, the retention of radionuclides from leaving the container, and the dispersion of residual heat of the SFA into the environment.

SUMMARY OF THE INVENTION

A method for the manufacture of large-tonnage transport and packaging enclosures of sets (TUK) for the transportation and storage of spent nuclear fuel (SNF NPP) from high-strength ferritic and special cast iron with spherical graphite, characterized by the sequential pouring into the rotating mold under a flux layer of the same type of composition ductile cast iron with spherical graphite completely forming a casting of the body with a high degree of isotropic properties along the cross section and height or heterogeneous compositions of cast iron, forming a monolithic casting of a TUK case with high strength and special properties, while the outer layer is poured from high-strength ferritic cast iron with spherical graphite, the intermediate layer is from alloyed cast iron with spherical graphite with increased neutron absorption capacity, and the inner layer is made of corrosion-resistant austenitic cast iron with spherical graphite.

The method is characterized in that the rotation speed of the mold for each subsequent pouring is increased by 5-10%.

The method is characterized in that the layers of the melts are poured under a layer of liquid or powder flux in an amount of 0.7-0.8% by weight of the first melt layer being poured, and with each subsequent pouring of the melt, a flux of 10-15% by weight of the initially introduced amount is introduced. flux.

The method is characterized in that they use a flux consisting of the following components in wt.%:

silicate block (nNa ₂ O mSiO ₂ ) - 23-25; fluorspar (CaF ₂ ) - 23-25; borax (Na ₂ , B ₄ O ₇ ) - 15-17; limestone (CaO) - the rest.

A monolithic casting-casing TUK made by the above centrifugal casting method, characterized by a cylindrical shape with removable end caps, the casing being made by sequentially pouring one or heterogeneous compositions of high-strength cast iron with spherical graphite, and the neutron protection of the casing is made by a middle layer of cast iron alloyed with neutron-absorbing elements, The internal corrosion protection of the casing is made of austenitic nodular cast iron.

When pouring subsequent layers of cast iron melt, most of the remaining flux under the action of centrifugal forces is again displaced onto the inner surface of the newly cast layer. The specified composition of the flux is recommended to be used both when pouring layers of one VChShG melt and when pouring bi- and polymetallic container bodies.

The monometallic container bodies made of high-strength alloys are stationary coated from the inside with an anticorrosive cladding layer applied by the electrochemical method, plasma spraying, or by pressing a shell made of X18H10T steel.

In the proposed technology, it is proposed to carry out an anti-corrosion layer with a thickness of 15-20 mm by pouring melt of austenitic cast iron with spherical graphite, i.e. the entire bimetallic casting is formed into a continuous single product - a large-tonnage container body by layer-by-layer casting of ferritic and austenitic cast iron layers by centrifugal casting.

In the manufacture of polymetallic casting of a container body, consisting of various layers of cast irons of a different composition, the formation of high nuclear (neutron) and radiation protection, as well as corrosion resistance in the container body is carried out by successive pouring of the melts of these layers by centrifugal casting.

A polymetallic monolithic casting of a body with high characteristics of strength and special properties consists of a combination of layers of alloyed cast iron with spherical graphite with high parameters of special properties. The difference between the manufacturing process of bi- and polymetallic castings of casings from monometallic castings is in the thickness of the layers and the temperature regime of their pouring. When pouring a multilayer casting of the container body from one structure of the ductile iron mixture, the first melt layer (80-100 mm thick) is poured into a rotating metal mold (mold) at a temperature of 30-50 ° C higher than the liquidus temperature, the subsequent layers are filled with a thickness of 80-100 mm at the same temperature, but upon reaching, on the contact (inner) surface of the first layer of temperature, 100-150 ° C lower than the solidus temperature of this cast iron.

The technical result achieved by the invention consists in:

- improving the reliability of protection against radiation from spent nuclear fuel, the corrosion resistance of the TUK hull and increasing the productivity of TUK manufacturing;

- optimization of manufacturing technology TUK;

- ensuring stable production of quality large-capacity castings TUK;

- the ability to ensure quality control of TUK at all stages of the manufacture and operation of TUK;

- the possibility of disposal after a 50-year period of operation of the hull from the ductile iron.

The invention is illustrated by drawings, where:

Figure 1 - a cylindrical housing TUK; (1 - container body; 2 - end cover);

Figure 2 is the same with the end caps (for centrifugal casting).

It is known that obtaining high-quality massive castings, such as container bodies with wall thicknesses up to 500 mm with any casting method, is associated with certain technological difficulties associated mainly with the inability to ensure the directional solidification principle during their crystallization, and obtaining high mechanical properties throughout cross section of the casting. In the stationary manufacturing method, in this case, they usually resort to setting up massive feeding profits, but these measures, although they prevent the formation of concentrated defects of shrinkage origin, but do not provide high isotropic properties of cast iron. Physico-mechanical properties and ductility in the metal in massive castings made of high-alloy steel with a wall thickness of 300 mm or more are reduced by about 25% compared with the mechanical properties of castings with a wall thickness of casting up to 50 mm.

As studies have shown in castings with a wall thickness of more than 100 mm made of high-strength alloys by centrifugal casting, it is also almost impossible to obtain a quality defect-free casting with a one-time casting. As a rule, such castings show defects such as anisotropy, bandedness, delamination, etc.

Therefore, a fundamentally new technological process is proposed for the production of large-capacity castings of container bodies by centrifugal casting using the principle of multiple layer-by-layer casting necessary to obtain a solid cast large-capacity casting of the TUK case. It is necessary that, when each layer is poured, the contact surface of the initially poured metal layer should be prior to pouring the next portion of the melt with a flux protective cover, introduced into the rotating mold along with the poured or immediately after pouring it.

When casting large container containers, the centrifugal speed is determined on the basis of the need to reduce to the minimum possible values of the alignment time of the angular velocities of the cast metal layer and the mold to achieve high uniformity of the properties of the cast metal. This condition in large-capacity castings of cases is ensured with a gravitational coefficient equal to 120, according to which the frequency of rotation of the mold, determined by the formula: n = 328 / √R, where R is the external radius of the casting (in m).

The new process allows:

1. To reduce (not less than 3 times) the complexity of manufacturing a TUK case.

2. Create a more technological design of TUK.

3. Significantly reduce the wall thickness and weight of the TUK casing by removing holes in the casing wall for neutron protection and increase payload.

The invention allows to obtain high-quality castings by means of multilayer pouring of heterogeneous compositions of VChShG. To ensure uniform, strong welding between each other of the chemical composition of the layers of cast iron, provides high isotropic properties over the entire cross section of the casting of the TUK body. To form a monolithic large-tonnage container body for transporting and storing SNF from nuclear plants and other industry facilities. To create a high level of strength physical and chemical properties and solve the problem of protection against radioactive radiation and corrosion of the main structural elements.

Claims (5)

1. A method of manufacturing a large-tonnage case of a transport-packaging set (TUK) for transporting and storing spent nuclear fuel, characterized in that cast iron of various compositions is layered layer-by-layer under a flux and a monolithic casting of the TUK body is formed, while the outer layer is filled from ferritic ductile iron with spherical graphite, the intermediate layer is made of alloyed spheroidal graphite iron with neutron absorption capacity, and the inner layer is made of orrosion-resistant austenitic nodular cast iron. 2. The method according to claim 1, characterized in that the rotation speed of the mold with each subsequent filling of the layers is increased by 5-10%. 3. The method according to claim 1 or 2, characterized in that a liquid or powder flux is used, which, when pouring the first layer, is introduced in an amount of 0.7-0.8% by weight of the first melt layer being poured, and each time the melt is poured further flux is introduced in an amount of 10-15% by weight of the initially introduced amount of flux. 4. The method according to claim 3, characterized in that use a flux containing the following components in wt.%:
Silicate block 23-25 Fluorspar 23-25 Borax 15-17 Limestone rest. 5. The case of the transport packaging package (TUK) for transportation and storage of spent nuclear fuel, characterized in that it is made of monolithic layers of ductile iron with spherical graphite of different composition, while the outer layer is made of high-strength ferritic iron with spherical graphite, an intermediate layer - from alloyed cast iron with spherical graphite, which has neutron-absorbing ability, and the inner layer is from corrosion-resistant austenitic cast iron with spherical graphite.

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