RU2373587C1 - Neutron shield composition - Google Patents

Abstract

FIELD: nuclear physics.

SUBSTANCE: invention can be used as a protective layer in transportation-package structures for transporting or storing spent nuclear fuel. A neutron shield composition containing the following, in pts. wt, is proposed: polydimethyl iloxane with terminal hydroxyl groups with molecular weight ranging from 70000 to 100000 - 100, boron-containing compound - 4 to 30, low-molecular polydiethyl iloxane - 8 to 12, ethylsilicate - 0.1 to 0.2, organo-tin catalyst - 0.5 to 1.0. The positive effect is that the proposed composition has good thermal stability, can work for prolonged periods at temperature of 200°C, is fluid at normal conditions and can be used as a protective layer in transportation-package structures for spent nuclear fuel.

EFFECT: neutron shield composition is designed, with good thermal stability and is fluid at normal conditions.

Description

The invention relates to the field of development of materials with neutron-absorbing properties, and can be used, for example, as a protective layer in the manufacture of transport packaging structures (TUK) for transportation or storage of spent nuclear fuel, as well as for biological protection against other cases of neutron radiation.

Known glass-like composition having neutron-absorbing properties, consisting of 50-80% wt. silicon dioxide, 1-30% wt. lithium oxide and 1-25% wt. aluminum oxide. The composition may also include lead oxide and components containing elements selected from the group: Mg, Be, B, F, P or Bi. To obtain patentable material, the mixture of the above components is heated at a temperature of 1250-1400 ° C (US Pat. US 5221646, prior. 06/22/93, C03C 4/08). The glassy composition described in the patent has good chemical and thermal resistance. However, it is characterized by significant disadvantages. The materials obtained on its basis are quite fragile and can be destroyed mechanically from shock and shock. Their strength is not guaranteed under sharp fluctuations in temperature. Very high temperatures are required. This makes them practically unsuitable for use as a neutron-protective layer in the design of TUK.

A known composition used to obtain neutron-absorbing articles containing boron carbide, a powdered phenolic resin and a small amount of a liquid medium with a boiling point less than 200 ° C, preferably water. As the resin using phenol-formaldehyde resin containing a hardener - hexamethylene-tetraamine. The molecular weight of the resin is 1200-10000, preferably 6500. The mass ratio of boron carbide: resin: water is 60-80: 20-40: 2-8. The curing of the composition is carried out at elevated temperatures (US Pat. US 4213883, prior. 12/30/77, CB 35/68).

The composition is easily molded under pressure, does not require the use of very high temperatures to obtain neutron-absorbing materials, but it has a low heat resistance (below 150 ° C). In addition, the resulting materials do not melt and cannot be poured into the cavity of the TCA, which greatly limits the scope of their use.

The closest analogue is a neutron-absorbing composition, consisting of a polymer base - polyethylene with a molecular weight of 2.5 · 10 ⁶ -1 · 10 ⁷ g / mol and boron or boron-containing compounds (boric acid, boron nitride, boron carbide or their mixtures). It is most preferable to use boron carbide with a particle size of 20-80 mm, which prevents the separation of components during processing.

The content of boron carbide in the mixture with polyethylene is 5-50% by weight, most preferably 20-30% by weight. The polyethylene used must not contain substantially impurities. To increase its stability to light, heat and oxidation, a stabilizer is introduced in an amount of 0.1-0.2% by weight. As a stabilizer, for example, 4,4 ^1- thio bis (3-methyl-6-tert-butyl-1-phenol), dilauryl-thiopropionate, etc. can be used.

Neutron-absorbing material is obtained by homogeneous mixing of the starting components in an appropriate mixer, followed by heating the mixture at a temperature of 180-250 ° C under a pressure of 10-15 MPa and subsequent cooling also under a pressure of 3-5 MPa.

Due to the fact that when heated, such a composition is able to acquire fluidity, it can be technologically acceptable methods placed in the cavity of the TUK as a neutron protective layer.

The disadvantage of the composition is the low heat resistance and relatively low thermo-oxidative stability, not allowing its use for a long time at a temperature of 150-200 ° C even in the presence of the stabilizers proposed in the patent. According to well-known data, products based on polyethylene in contact with metal are not operable at temperatures above 70 ° C (Handbook "Electrical Cables, Wires and Cords". Belorusov NI, Saakyan A.E., Yakovleva A.I. Energoatomizdat, 1988 g.), while the walls of the TUK are always made of metal.

The objective of the proposed technical solution is to develop a composition for neutron protection, with fluidity under normal conditions, good thermal stability and can be used as a protective layer in TUK.

The goal is achieved in that the composition for a neutron protection based on a polymer, including a boron-containing compound, uses polydimethylsiloxane with terminal hydroxyl groups and a molecular weight of 70,000-100,000 and additionally contains a low molecular weight polydimethylsiloxane, ethyl silicate and an organotin catalyst in the following ratio of components, wt. hours:

polymer  one hundred polydiethylsiloxane  8-12 boron compound  4-30 ethyl silicate  0.1-0.2 catalyst  0.5-1.0

As boron compounds, for example, boric acid, boron nitride (TU 2036-1045-88) can be used.

As the basis of the composition, dimethylsiloxane rubbers of the SKTN type are used (GOST 13835-73).

As organotin catalysts, for example, dibutyl-tin-caprylate, dibutyl-tin-laurinate, octoate-tin, etc. can be used. Octoate-tin is most preferred because of its low toxicity.

As low molecular weight polyethylsiloxane, for example, PES-5 polydiethylsiloxane liquid is used.

The composition may also contain a small amount (1-2 parts by weight) of silicon oxide (aerosil).

The proposed composition is prepared as follows.

In a device equipped with a stirrer, first dimethylsiloxane polymer and low molecular weight polydiethylsiloxane are mixed for 0.5-2 hours, and then a boron-containing compound, ethyl silicate and an organotin catalyst are introduced. Next, the mixture is transferred to a rotary pulsation apparatus and after a double pass with the help of a pump is fed into the TUK “shirt”.

The following are examples illustrating the proposed technical solution.

Example 1

The required amount of low molecular weight polydimethylsiloxane with a molecular weight of 90 thousand and low molecular weight polydiethylsiloxane (PES-5, GOST 13004-77) with a kinetic viscosity of 200 cSt in an amount of 10 parts by weight are set in the apparatus equipped with a stirrer. per 100 parts by weight rubber and stirred for half an hour. Then, boron nitride (TU 2-036-1045-88) is added to the mixture in an amount of 4 parts by weight per 100 parts by weight rubber, ethyl silicate (GOST 5.1174-71) at a dosage of 0.2 wt.h. per 100 parts by weight rubber and tin octoate in a dosage of 1.0 wt.h. per 100 parts by weight rubber. The mixture is transferred to a homogenizer (RPA - rotary pulsation apparatus) and after a double pass with the help of a pump it is transferred to the TUK jacket.

The possibility of long-term use of the composition when exposed to temperatures up to 200 ° C is determined using the method of predicting changes in properties during thermal aging in accordance with GOST 9.713-86. Since, during operation, the composition in the cured state is inside the sealed TUK shell, a change in the index of hardness conditions is selected as a characteristic indicator of aging depending on the duration of exposure of the composition at various temperatures. The obtained results confirm the permissible change in hardness (7 units) when holding for more than 20 days at 200 ° C. The resulting composition is characterized by a density of 0.98 g / cm ³ , a heat capacity of 0.35 kcal / h ° C and a volume expansion coefficient of 7.10 ^-4 ° C ^-1 .

The viability of the composition is 16 hours. Dynamic viscosity - 18 Pa · s

Example 2

Get the composition under the conditions described in example 1, but containing 30 parts by weight boron nitride per 100 parts by weight polydimethylsiloxane with a molecular weight of 100,000.

The viability of the composition is 11 hours.

Dynamic viscosity - 3.6 Pa · s.

Example 3

Get the composition under the conditions described in example 1, but containing 5 wt.h. boric acid per 100 parts by weight polydimethylsiloxane with a molecular weight of 70,000.

Example 4

Get the composition under the conditions described in example 1, but containing 8 parts by weight of diethylsiloxane liquid, ethyl silicate and 0.8 parts by weight tin octoate per 100 parts by weight polydimethylsiloxane.

The viability of the composition is 14 hours.

Dynamic viscosity - 20 Pa · s.

Example 5

Get the composition under the conditions described in example 1, but containing 12 parts by weight diethylsiloxane liquid, 0.1 parts by weight, ethyl silicate and 0.5 parts by weight, tin octoate per 100 parts by weight polydimethysiloxane.

The viability of the composition is 18 hours.

Dynamic viscosity - 16 Pa · s.

Example 6

Get the composition under the conditions described in example 1, but containing 1 wt.h. dibutyltinocaprylate per 100 parts by weight polydimethylsiloxane.

The viability of the composition is 12 hours.

Dynamic viscosity - 18 Pa · s.

When the above compositions were irradiated with a neutron flux to a fluence of 4 · 10 ¹⁴ neutrons / cm ² and gamma radiation up to an absorbed dose of 1 MGy, no change in material properties was detected. Thus, the proposed composition has fluidity under normal conditions and can easily be poured into the cavity of the TUK. The composition has good thermal stability, is operable for a long time at a temperature of 200 ° C and is able to be used as a protective layer in TUKs from neutron radiation.

Claims (1)

A polymer-based neutron protection composition comprising a boron-containing compound, consisting in the fact that it uses polydimethylsiloxane with terminal hydroxyl groups and a molecular weight of 70,000-100,000 and additionally contains a low molecular weight polydiethylsiloxane, ethyl silicate and an organotin catalyst in the following ratio of components, wt. hours:
polydimethylsiloxane one hundred boron compound 4 ÷ 30 polydiethylsiloxane 8 ÷ 12 ethyl silicate 0.1 ÷ 0.2 organotin catalyst 0.5 ÷ 1.0