RU2522580C2 - Fire-resistant neutron-protective material - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to materials with neutron-absorbing properties for protection against neutron radiation. Claimed is a fire-resistant neutron-protective material, consisting of a magnesium phosphate binding agent (24-33 wt %) and a powder part (76-67 wt %, with the powder part containing titanium hydride TiH₂ (90.3-95.5 wt %), magnesium oxide MgO (2.7-4.5 wt %) and boron carbide B₄C (1.8-5.2 wt %). Components are mixed to a homogeneous state and poured into a special cavity, and after hardening are subjected to thermal processing.

EFFECT: obtained material possesses long-term mechanical strength, heat resistance to ≈300°C, high heat conductivity, a temperature coefficient of linear expansion, close to the coefficient of construction steels, and high specific density of hydrogen and boron, contained in it, which ensures high coefficients of neutron radiation attenuation.

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Description

Technical field

The invention relates to materials with neutron-absorbing properties for protection against neutron radiation. In particular, the material is intended to protect personnel from neutron radiation when handling radioactive materials.

The material can be used as a protective layer in transport packaging systems (TUKs) during transportation and storage of spent nuclear fuel (SNF) with high burnup and, therefore, energy release, for example from VVER-1000 reactors. Or as a protective layer of high-temperature radiation protective screens at nuclear facilities.

State of the art

Most of the existing neutron-shielding materials (INS) are composite materials, which include hydrogen-containing substances, boron-containing substances and, if necessary, substances containing elements with a high serial number (z), for example, heavy metals. Hydrogen is the most effective neutron moderator, boron is an effective absorber of fading neutrons, and elements with large z increase the attenuation coefficient of the accompanying γ-radiation by protective layers. If boron (or its compound) in the form of a separate component can be introduced into almost any composite material, then the presence of hydrogen in currently known neutron shields is ensured by its presence in the structure of the main substance of the composite material.

A high hydrogen content is characteristic of many high molecular weight compounds of organic origin. Solid hydrogen-containing materials are used: polyethylene, polypropylene, rubbers and other materials and compositions. The highest concentration of hydrogen is possessed by polyethylene, polypropylene; in other polymeric materials, the concentration of hydrogen is lower.

The disadvantage of neutron protection based on polyethylene and polypropylene is that when exposed to elevated temperatures, they change their geometric parameters and undergo partial destruction with the release of gases and an increase in pressure in the volume they occupy to values that are dangerous for the design of the outer shell of the TUK.

In addition, the disadvantage of neutron protection from polypropylene is that this material has a much higher coefficient of thermal linear expansion (CTLR) compared to steel [(10-15) · 10 ^-6 K ^-1 ] equal to 110 · 10 ^{- 6} K ^-1 according to GOST 26996-86.

The paste-like material for protection against radioactive radiation, shown in RF patent No. 2 111 558 dated 06/25/1996., IPC6 G21F 1/10, authors: Lazebnik IM, Andreev VV, Starostin B.S., was selected as an analogue ., which includes a plastic synthetic mass, such as rubber, and a filler, which is used as titanium hydride in a ratio of 60-96 wt.% to the total composition of the paste. The resulting paste allows you to create protective layers of complex geometric shapes, fill cable shafts and other hollow channels in protective structures and thereby prevent the spread of n-, γ-radiation through them. The presence in this material of a significant amount of titanium hydride with a high specific hydrogen content (ρ _n ≈ 0.15 g / cm ³ ) provides it with good neutron-shielding properties. The disadvantage of the paste is that it contains a synthetic component (rubber). In rubber in the presence of oxygen, oxidative reactions with loss of mass due to degradation (depolymerization) occur already at (200 ... 230) ° C and are greatly accelerated at temperatures of 250 ° C and above. Therefore, this material is not suitable for use as a neutron protection material in TUK designs designed for transportation and storage of SNF with high energy release, when the working temperature of the neutron protection material in normal operating conditions can exceed 200 ° C, and in emergency conditions reach 800 ° C .

As a prototype, the stabilizer material (MS) was selected, which is described in RF patent No. 2362225, dated April 16, 2007, IPC G21F 9/00, (authors: Sharygin L.M., Muromsky A.Yu., Kalyagina M.L., Davidenko N.N., Lebedev V.I., Shary O.A.), including magnesium-phosphate binder (56-66 wt.%) And powder part (44-34 wt.%). Magnesium phosphate binder consists of P ₂ O ₅ (30-40 wt.%), MgO (7-8.5 wt.%), H ₂ O - the rest. The composition of the powder part: Al ₂ O ₃ (78-90 wt.%), MgO (22-10 wt.%), Boron-containing component (in terms of boron - (0-1) wt.%). The mixture is used to fix SNF or for other purposes during the “lifetime” of the mixture in the liquid state.

The disadvantages of the prototype, leading to the loss of neutron-shielding properties after solidification of the MS and thermal effects on it, can be considered:

- the minimum content of boron-containing component (the amount of boron from 0 to 1 wt.%);

- a significant decrease in the hydrogen content in the MC structure with moderate heat exposure: even if chemically bound crystallization water (T≤150 ° C) is retained, the remaining fraction of hydrogen in it will be ≈1.8 wt.% (ρ _n <0.04 g / cm ³ );

- after prolonged and more intense heating, hydrogen in the MC structure remains only in the chemical compound MgHPO ₄ , which is only about 0.4% of its mass.

Disclosure of the invention.

The objective of the invention is to provide a heat-resistant material with enhanced neutron-shielding properties, taking and maintaining the desired shape.

The technical result consists in the consistency of components, increasing strength, hardness, heat resistance, thermal conductivity, high content, uniform distribution and conservation of bound hydrogen at high temperatures.

The specified technical result is achieved by the fact that in the heat-resistant CCM for the radiation protective layer from neutron radiation, including the magnesium phosphate binder and the powder part, according to the invention, the magnesium phosphate binder is 24-33 wt.%, And the powder part 76-67 wt.% Of the total mass MNZ. Titanium hydride (TiH ₂ ) is added to the powder portion. The powder part contains titanium hydride 90.3-95.5 wt.%, Magnesium oxide (MgO) 2.7-4.5 wt.%, Boron carbide (B ₄ C) 1.8-5.2 wt. % of the total mass of the powder part of MNZ.

MNZ can be used in protective layers in TUKs designed for transportation and storage of spent nuclear fuel with great burnup and energy release; as well as in the protective layers of high-temperature radiation protective shields of nuclear physics facilities.

The uniform distribution of bound hydrogen in the caster is explained by the following. Titanium hydride contains bound hydrogen TiH ₂ . In powder form, it is evenly distributed within the magnesium phosphate binder when mixed. The resulting mixture in an uncured state is evenly distributed in the volume intended for it. A feature of the MNZ is its ability to be evenly distributed in the TUK body volume intended for it to reduce SNF neutron radiation with large burnup. In addition, the caster can be placed on the surface or inside high-temperature radiation-shielding screens.

MNZ is obtained by mixing a magnesium phosphate binder and a filler. The binder is a magnesium phosphate ceramic. Immediately after mixing, it has the ability to take the form in which it is placed. After water (including crystallization) is evaporated from it, the MNE acquires hardness, high heat resistance, up to t ≈900 ° С. The binder plays the role of a matrix in the caster holding the filler, consisting of specially selected components.

The filler to create a promising oil refinery must meet the following basic requirements:

- high hydrogen content in the filler substance (ρ _n ≈0.1 g / cm ³ );

- long-term heat resistance (the ability to maintain their physical properties and not lose hydrogen) at temperatures t ≈200 ° C;

- the possibility of creating solid-state materials with a significant proportion of hydrogen-containing substances (> 50% of the volume) in their composition.

A sufficiently heat-resistant titanium hydride TiH ₂ (ρ _n ≈0.15 g / cm ³ ), which meets all the above requirements, was selected that was sufficiently heat-resistant during prolonged heating up to t ≈300 ° C. The heat resistance of the obtained MNZ is explained by the heat resistance of its constituent components.

An increase in the heat resistance of MNZ is aimed at increasing its thermal conductivity due to the use of titanium hydride as a filler. Since the removal of heat from SNF stabilizes its constant storage temperature, it does not allow the container in which SNF is stored to self-heat and to collapse from this.

After hardening of magnesium-phosphate ceramics, which plays the role of a matrix in the resulting material, and deep drying, the MNZ acquires long-term mechanical strength, and its neutron-shielding properties are provided by heat-resistant titanium hydride with a high hydrogen content and boron carbide in the matrix. The CTLR of the obtained CCM is slightly different from the corresponding values of structural steels. It also increases the strength and hardness of the MNE.

Embodiments of the invention.

The studies conducted by ZAO Termidoksidov Scientific and Production Company, showed that, from the point of view of technological preparation of billet refineries and products from it, the most preferred and functionally suitable are dried materials based on heat-resistant (up to 900 ° C) magnesium phosphate ceramics with chemically inert under operating conditions, fillers in the form of powders of titanium hydride and boron carbide.

To provide protection against neutrons emitted by SNF, titanium hydride (TiH ₂ ) powder was selected. However, the powder itself cannot be evenly distributed and held in the cavity of the TUK casing intended for neutron protection. A necessary substance that helps to evenly distribute titanium hydride is a binder - a stabilizing material. After hardening, the stabilizer material contains hydrogen in the TiH ₂ compound, which gives it neutron-shielding properties. In addition, the TiH ₂ binder, as well as the metal from which the TUK body with SNF is made, have coordinated linear expansion temperature coefficients (TEC). This allows the obtained stabilizer material to maintain protective properties and not collapse.

Heat resistant caster get as follows.

A powder part is added to 25 cm ^{3 of the} magnesium phosphate binder: 70 g of titanium hydride and 5 g of boron carbide. After mixing the mixture in a paddle mixer for 20-30 minutes, a new portion of titanium hydride is added. This operation is repeated until the mixture in consistency looks like a thick dough and it becomes difficult to mix it. Then add 2 g of hardener (MgO). After the addition of MgO, a chemical reaction occurs with the release of heat, under the influence of which the mixture is heated, and after a few minutes it solidifies. For the time before the onset of solidification, the mixture is poured into special cavities in the TUK housing, or into cavities forming protective layers of high-temperature radiation protective shields used in nuclear physics facilities. In addition, casting and pressing can produce neutron protection parts of almost any desired shape.

After hardening, the mixture placed in the cavity is subjected to heat treatment. Deep dehydration consists of two periods. The initial drying period is carried out at room temperature. Further drying is carried out for several days with a gradual increase in ambient temperature from room temperature to maximum. With deep drying, almost complete removal of both sorbed and crystallized water occurs.

In the studies of the physicomechanical and thermophysical properties of the MNZ samples, the following characteristics were obtained:

- the average CTLR values of the MNZ samples are equal to = 9.9 · 10 ^-6 K ^-1 , which is comparable with the CTLR of steel elements of the TUK design;

- the thermal conductivity of the MNZ samples under normal conditions is -1.12 W / (m · K), and at 250 ° C it is 1.6 W / (m · K), which is favorable for using the MNZ in the design of TUK;

- the compressive strength of the MNZ samples is depending on the temperature in the range of 20.7 ... 28.50 MPa.

At normal operating temperatures of about 250 ° C, there is virtually no hydrogen output from titanium hydride. Tests were conducted on the conditions of fire (30 min at a temperature of 800 ° C). In the presence of the gaseous medium volume occupied by the neutron shielding material, a hydrogen partial pressure of about 1 kgf / cm ² TiH compound ₂ (66 at.% Hydrogen) in the material to be dehydrogenated to a compound of TiH _0.72, containing 42 at.% Hydrogen. That is, even during a fire, a material based on magnesium phosphate ceramics with titanium hydride TiH ₂ and boron carbide B ₄ C, with full or partial tightness of the volume with neutron protection, will retain not only its geometric shape, but also due to the remaining uniformly distributed in the material of hydrogen and boron will retain its neutron-protective properties.

The optimal consistency of the components, expressed as their percentage in the CCM, was determined experimentally. As a result of the experiments, it was revealed that the magnesium phosphate binder should be present in the composition of MNZ in the amount of 24-33%, and the powder part, respectively, 76-67%. If the binder is less than 24%, and the powder part is more than 76%, then the mixing of the binder is difficult, it becomes thick, its heterogeneity increases. If the binder is more than 33%, and the powder part is less than 67%, then the neutron-protective properties of the MNZ are deteriorating, which should not be less than the permissible value according to the Rules NP-053-04 of Rosenergoatom, containing radiation safety criteria.

In the powder part, the optimal ratio of the components was experimentally revealed. Titanium hydride should be 90.3% -95.5%. If TiH _{2 is} less than 90.3%, then the neutron-shielding properties of the MNZ are reduced below those allowed by the Rules NP-053-04 of Rosenergoatom, which contain radiation safety criteria.

If TiH _{2 is} greater than 95.5%, then the content of MgO and B ₄ C is reduced. This entails a deterioration in the properties of MNZ. If MgO is less than 2.7%, then the solidification rate of the caster decreases, its strength and hardness decrease. If B ₄ C is less than 1.8%, then the neutron absorption threshold is reduced, which is unacceptable under the Rules NP-053-04 of Rosenergoatom, which contains radiation safety criteria.

If MgO is greater than 4.5%, then the rate of solidification of the mixture increases. The inhomogeneity of the MNE is deteriorating, the uniformity of filling the required cavities is deteriorating, since the time spent by the MNE in the liquid phase is reduced.

If B ₄ C is more than 5.2%, then the content of TiH ₂ in the CCM is reduced, which entails a decrease in neutron protection, or the MgO content is reduced, which degrades the strength and hardness of the obtained CCM.

The neutron-shielding characteristics of the proposed MNZ were determined by neutron-physical calculations carried out for a model design of a TUK with 18 spent fuel assemblies (SFA) with spent nuclear fuel from VVER-1000 reactors when a 24-cm thick caster layer was placed on a 22 cm-thick TUK steel side wall. The calculations were carried out using Monte Carlo method described in the article "Justification of the nuclear and radiation safety of the container for spent fuel assemblies of AMB reactors at Beloyarsk NPP", Atomic Energy journal, t. 100, No. 6, p. 423, 2006, av Ora Gorin NV, Ph.D. YA.Z, Chernukhin YI

In the calculations, the attenuation coefficient (K _n ) was calculated with the CCM layer of the dose D _n caused by neutron radiation from the SFA with SNF. The calculations were performed for two experimentally obtained MNE compositions after their deep drying (until almost complete loss of both sorbed and crystallization water), differing mainly in their content of titanium hydride (C ₀ , wt.%) And density (ρ, g / cm ³ ).

Table 1 Compositions and densities of MNZ after deep drying Element Content, wt.% Composition 1 Composition 2 H 3.17 3.32 ¹⁰ B 0.94 0.65 ¹¹ B 3.85 2.66 C 1.33 0.92 O 11.25 8.12 Mg 3.77 2.84 P 6.19 4.32 Ti 69.50 77.17 ρ, g / cm ³ 1.8 2,35

The compositions (wt.%) And densities (ρ, g / cm ³ ) shown in the table correspond to experimentally obtained samples of MNZ with a mass fraction of titanium hydride (TiH2) in the powder part of the initial mixture for their preparation C ₀ ≈90.3 wt.% ( composition 1) and C ₀ ≈95.5 wt.% (composition 2). The main element determining the attenuation coefficient in compositions 1 and 2 is titanium hydride. The remaining elements in the composition are present to maintain the desired state of aggregation of the substance, for example, its fluidity. The attenuation coefficients K _n found in the calculations for these CCM compositions were:

Composition 1: ρ = 1.8 g / cm ³ ; K _n1 ≈820;

Composition 2: ρ = 2.35 g / cm ³ ; K _n2 ≈2880.

It is noticeable that even a relatively small increase in the content of titanium hydride (C ₀ , wt.%) In the proposed MNZ (with deep drying) leads to a significant improvement in its neutron-shielding properties. However, it is difficult to obtain material with C ₀ > 95.5 wt.% (In the powder part) for technological reasons. Therefore, the value of K _n2 ≈2880 is close to the limit for the configuration of the protective layers adopted in the calculations of the TAC model.

Thus, the proposed CCM of magnesium phosphate ceramics with titanium hydride and boron carbide, taking into account the availability of components, acceptable neutron-protective, physico-mechanical and thermophysical properties, can be used as a solid-state heat-resistant CCR. This protection is effective in conditions of elevated temperatures (250 ° C and more) and is able to partially maintain neutron-protective properties in emergency fire conditions (800 ° C).

In the available sources of information, no technical solutions were found that together contain features similar to the distinguishing features of the inventive heat-resistant oil refinery. Therefore, the invention meets the criterion of "novelty."

In our available sources of information there is no information about the effect of the distinctive features available in the claimed invention in aggregate on the achievement of the claimed technical result. Based on this, it was concluded that the invention meets the criterion of "inventive step".

Industrial applicability.

The implementation of the proposed method is quite possible, because To date, the industry has mastered all the main components of which the claimed substance consists, and the technological operations of its preparation. MNZ can be used in TUK designs for the transportation and storage of spent nuclear fuel with a high degree of burnout, as a protective layer of high-temperature radiation protective shields, as well as other heat-stressed equipment of enterprises and facilities of the nuclear complex. This confirms the industrial applicability of MNZ.

Claims (1)

Heat-resistant neutron-shielding material for a radiation-protective layer from neutron radiation, including a magnesium phosphate binder and a powder part, characterized in that the magnesium phosphate binder is 24-33 wt.%, And the powder part is 76-67 wt.% Of the total neutron-shielding material, in the powder part is added titanium hydride (TiH ₂ ), while the powder part contains titanium hydride 90.3-95.5 wt.%, magnesium oxide (MgO) 2.7-4.5 wt.%, boron carbide (B ₄ C ) 1.8-5.2 wt.% Of the total mass of the powder part of the neutron-shielding material.