RU79210U1 - FAST NEUTRON REACTOR - Google Patents

Abstract

The utility model relates to nuclear technology, namely to nuclear fast reactors (FBR), and is used to increase the service life, efficiency and safety of their operation by significantly (by 30-80%) reducing the neutron flux density (F), " "spectrum of neutrons (decreasing the energy of neutrons) and, accordingly, reducing the damaging dose (D) on the main non-replaceable in-reactor structures and the body. This result is achieved in a fast neutron reactor, which includes a vessel, inside which there is an active zone surrounded by a protective screen. The screen contains a neutron moderator layer, and the effective layer thickness is selected depending on the moderator properties, dimensions, design and composition of the core. As a neutron moderator, a predominantly material containing light atoms was selected from the series: hydrogen, deuterium, isotope B. Zirconium hydride can be used as a neutron moderator. The moderator layer is located at the core boundary. The effective thickness of the retarder layer is selected from the range of 40-150 mm. Additional raw materials for the production of radioisotopes can be placed directly behind the moderator layer. 1 ill.

Description

The utility model relates to nuclear engineering, namely to fast fast neutron reactors (RBN), and is used to increase the service life and efficiency of their work.

Currently, in fast neutron reactors, neutron shielding, which is installed inside the reactor vessel, is used to reduce the neutron flux density (Ф) and the damaging dose (D) to the vessel and other non-replaceable structures.

Known fast neutron reactor [Walter A., Reynolds A. Fast neutron multiplier reactors: Trans. from English - M .: Energoatomizdat, 1986. - p. 333, 335], including a housing, inside of which there is an active zone surrounded by a protective screen. The protective screen is located behind the reproduction zone of the reactor and consists of steel screens, steel bars (stainless steel or nickel-based alloys) and graphite-filled pipes.

This device allows to reduce the effect of radiation (F and D) on the reactor structures and the reactor vessel. Graphite, nickel and, to a lesser extent, iron exhibit the properties of a neutron moderator, acting as a neutron shield. As a result, the radiation decreases behind the protective screen, and, accordingly, on the housing.

The disadvantage of this device is its low efficiency in terms of neutron moderation. As a result, it is not possible to significantly reduce the Φ or it is necessary to use very thick layers of neutron protection (up to several tens of centimeters or 4-5 rows of assemblies), which unproductively increases the size of the reactor. This drawback is due to the low retarding ability of graphite, steel and nickel-based alloys.

The closest analogue in efficiency to the proposed device is a fast neutron reactor [Walter A., Reynolds A. Fast neutron multiplier reactors: Trans. from English - M .: Energoatomizdat, 1986. - p.335], which includes a housing inside which there is an active zone surrounded by a protective screen, which includes an absorber located in separate assemblies.

In new RBN projects, it is proposed to use materials containing absorbers (for example, boron carbide, europium oxide, etc.) as neutron protection, which are placed behind the reproduction zone. The principle of this device is based on the use of nuclides with large neutron absorption cross sections in neutron protection materials.

As a result of neutron absorption, there is a significant decrease in the neutron flux (Ф) and the damaging dose (D) on the in-reactor structures and reactor vessel.

The disadvantage of this device is that its use leads to large energy releases in the absorber. In addition, the neutron absorption reaction on the ¹⁰ V isotope (the main absorber used in the RBN) leads to the formation of helium nuclei. As a result, boron-based neutron protection is not long-lived and quickly loses its strength and neutron absorption properties (burn-out ¹⁰ B). In addition, boron is the “accumulator” of tritium, which is highly migratory and is a global pollutant.

Other absorbers are less effective, expensive to use, and become highly radioactive after irradiation (for example, Eu), which greatly complicates their use in the RBN as an effective neutron shield.

The use of an absorber as a neutron shield, especially in low and medium power RBNs (small RBNs), as well as RBNs without reproduction zones, leads to a significant deterioration in field uniformity

energy release in the reactor core, which negatively affects economic performance and safety.

The claimed technical solution allows to increase the service life of a fast neutron reactor and its safety by significantly (30-80%) reducing the neutron flux density (F), "softening" the neutron spectrum (reducing neutron energy) and, accordingly, reducing the damaging dose (D) by main non-replaceable internal reactor structures and body.

This result is achieved in a fast neutron reactor, including a casing, inside of which there is an active zone surrounded by a protective shield. The screen contains a neutron moderator layer, and the effective layer thickness is selected depending on the moderator properties, dimensions, design and composition of the core.

As a neutron moderator, mainly material containing light atoms was selected from the series: hydrogen, deuterium, ¹¹ B. isotope

Zirconium hydride can be used as a neutron moderator.

The moderator layer is located on the boundary of the active zone in the absence of a reproduction zone. The effective thickness of the moderator layer is selected from the interval of 40-150 mm. Directly behind the moderator layer, raw materials for producing radioisotopes can be additionally placed.

Placing an effective moderator on the path of neutron propagation leads to a "softening" of the neutron spectrum and, consequently, to a decrease in their damaging ability, to a decrease in the probability of neutron leakage from the reactor, to an increase in neutron absorption in the protective shield due to an increase in the capture cross section. As a result, the neutron flux (F) and the damaging dose (D) to the in-reactor structures and body are reduced.

When the thickness of the moderator layer is less than 40 mm, its retarding ability is not effective enough, and an increase in the thickness of the moderator layer of more than 150 mm is impractical.

New significant features are the use of highly effective moderators in the protective shield of the RBN to reduce F and D. It is proposed to use moderators that are well established in the RBN and have shown high reliability during prolonged exposure. In addition, the use of a moderator as a neutron shield will allow:

- to reduce the effect of radiation on biological protection and the reactor shaft, and, accordingly, thermal loads and activation of cooling air;

- to increase the safety of the reactor by reducing the most critical for RBN sodium void reactivity effect;

- reduce thermal loads and neutron fluxes in the in-reactor storage of spent fuel assemblies

- to improve the uniformity of energy release in the reactor core due to its increase in the last rows of fuel assemblies;

- produce mass production of radioisotopes in the “softened” neutron spectrum (for example, ⁶⁰ Co, ¹⁵³ Gd, ¹⁵² Eu, ¹⁵⁴ Eu, etc.).

New essential features, in combination with known features, exhibit new properties not described in the patent and technical literature. Namely, the use of a highly effective moderator in the protective shield of the RBF allows to reduce the neutron flux density and the damaging dose to the reactor structures and the reactor vessel, thereby increasing the service life of the fast neutron reactor and its safety.

The presented figure shows a schematic diagram of a fast neutron reactor, where 1 is the reactor vessel, 2 is the active zone, 3 is the reproduction zone, 4 is the protective screen, 5 is the moderator layer.

The moderator layer (5) in the protective shield (4) is made in the form of standard assemblies of RBN, in which the fuel is replaced with a moderator, or of individual structural layers located directly behind the active zone (2) or behind the reproduction zone (3), if any. The device in the form of assemblies is most optimal for arrangement in a single row behind the reproduction zone (3) on the sides of the active zone (2), and above and below the active zone (2) the moderator is made in the form of a layer (5).

As a result of the selection of the type of moderator, its effective density and location, it is possible to optimize neutron protection for each specific RBN.

Over the past 10 years, the BOR-60 reactor has not had a lateral reproduction zone; therefore, the lateral protective screen consists of steel assemblies and acts as a neutron shield. Replacing the last row (9th row) of steel assemblies of the BOR-60 reactor side protective screen with assemblies with a moderator (zirconium hydride or boron carbide with 3% ¹¹ B) leads to a decrease in F and D by the reactor shell and biological protection by 30 ÷ 70 % In this case, the radial uniformity of the distribution of energy release over the core is improved, the Doppler coefficient (by 15 ÷ 20%) and a number of other reactivity coefficients increase in absolute value, and conditions are created for the mass production of radioisotopes ( ⁶⁰ Co, ¹⁵³ Gd) in cells with “softened” "neutron spectrum. Assemblies with a moderator are similar in design to the reproducing assemblies of the side screen, where ZrH ₂ is loaded into fuel elements (fuel elements) instead of depleted uranium.

In the BN-600 reactor, assemblies with a moderator can be located instead of one row of reproducing assemblies of the side protective shield or steel shield. Installing assemblies with a moderator (ZrH ₂ , ¹¹ V) leads to

reduction of F and D on the reactor structures and reactor vessel by 40 ÷ 80% depending on the effective density of the moderator. At the same time, the sodium void effect of reactivity decreases (by 5-15% depending on the location of the moderator), the energy release and neutron fluxes in the in-reactor storage of spent fuel assemblies are significantly reduced, and conditions are created for the mass production of radioisotopes in the formed cells with a “softened” neutron spectrum .

It is proposed to use assemblies with a moderator in design similar to the replicating assemblies of the side shield, where instead of depleted uranium, load ZrH ₂ into fuel rods. (loading of dump boron is also possible - 97% ¹¹ V).

In the newly designed RBNs (BN-800, BN-1600, BREST, MBIR, SVBR, etc.), the moderator, its placement and design can be performed at the design stage of the reactor, which will reduce the neutron flux density and the damaging dose to the internal reactor design and reactor vessel by optimizing the location of the moderator, its density, composition, etc., and, therefore, to increase the service life of fast neutron reactors and their safety.

Claims (6)

1. A fast neutron reactor, including a housing, inside which an active zone is placed, surrounded by a protective shield, characterized in that the shield contains a neutron moderator layer, and the effective layer thickness is selected depending on the moderator properties, dimensions, design and composition of the active zone. 2. The reactor according to claim 1, characterized in that as the neutron moderator, a material containing light atoms is mainly selected from the series: hydrogen, deuterium, ¹¹ V. 3. The reactor according to claim 1, characterized in that zirconium hydride is selected as a neutron moderator. 4. The reactor according to claim 1, characterized in that the effective thickness of the moderator layer is selected from an interval of 40-150 mm 5. The reactor according to claim 1, characterized in that the moderator layer is located on the boundary of the active zone. 6. The reactor according to claim 1, characterized in that immediately beyond the moderator layer additionally placed raw materials for the production of radioisotopes.