RU2668546C2 - Method for changing reactivity in pulsed nuclear power plants of periodic action on fast neutrons with threshold-fissionable isotopes - Google Patents

Abstract

FIELD: neural physics.SUBSTANCE: invention relates to the field of neutron physics and the physics of nuclear power plants, namely to the methods for changing reactivity in nuclear power plants. Method of changing reactivity in the pulsed nuclear devices of periodic action on fast neutrons with threshold-dividing isotopes consists in the introduction of a structural element, which contains the neutron moderator, into and out of the active installation zone, while the reactivity is increased by means of withdrawal the structural element and lowered by means of its introduction.EFFECT: technical result is the implementation of the project of a fundamentally new pulsed neutron source on “threshold” isotopes.1 cl, 1 dwg

Description

The invention relates to the field of neutron physics and physics of nuclear installations, and in particular to methods of changing reactivity in nuclear installations. The invention can be used both for a single change in reactivity (as regulatory bodies), and for periodic modulation of reactivity in pulsed nuclear installations.

The following three methods are used to control the fission reaction in nuclear installations:

- Replacement of part of the fissile material of the active zone by non-fissile. This method is effective for fast neutron reactors, both energy and special purpose reactors, experimental, research, transport. Examples: BOR-60 reactor [1], self-extinguishing pulse reactors in the defense research centers of the USA and Russia). This method was used to create periodic pulses of fission power in the world's first pulsed research reactor IDB [2] in Dubna.

- Change in the position of the reflector (or part thereof) of neutrons relative to the core. The reflector is usually present in the design of a nuclear installation to save a critical mass of nuclear fuel and is made of a material with a large neutron scattering cross section. Removing the reflector from the core reduces the number of neutrons returning to the reactor after scattering in the reflector, thereby reducing the reactivity and, accordingly, the speed of the chain reaction. The effect of the reflector on the neutron multiplication coefficient is significant only in small reactors, where fission neutrons from the core are large. Therefore, this method is used only in research and experimental facilities for fast neutrons. So, at the IBR-2 in Dubna, a movable reflector made of nickel alloy in the form of two rotating rotors of a special shape is used to modulate reactivity and generate successive powerful neutron pulses [3].

- Introduction of neutron-absorbing material into the core. This method is most effective and is used in all slow neutron reactors (thermal or intermediate), starting from the first reactors of the 40s of the 20th century. Examples of modern domestic plants: VVER-type energy water-water reactors [4], RBMK graphite power [5], VVR-M research reactors [6], SM-3 [7], PIK [8]. The principle and operating conditions of this method are presented in [9]. With this method, a change in reactivity is carried out by introducing and extracting neutron-absorbing material into the reactor core. Positive reactivity is introduced in this case when removing neutron-absorbing material from the core. The difference from the proposed method is the use of a moderator instead of an absorber.

All three methods for changing reactivity are applicable in nuclear devices using uranium-233, uranium-235, plutonium-239, or other isotopes of transuranium elements, fissile under the influence of neutrons of the entire energy spectrum. The so-called “threshold” isotopes are effectively separated only by the absorption of fast neutrons. For uranium-238, the fission threshold is 0.6 MeV, and the chain reaction in the reactor with this isotope is unattainable. A lower fission threshold of neptunium-237 - 0.4 MeV - allows for the implementation of a chain reaction at fast neutrons. However, due to the threshold nature of the fission reaction, the possibilities of controlling nuclear processes in a non-potassium facility are limited. The use of a method of replacing part of fissile material with non-fissile material for the purpose of rapid modulation of reactivity in a reactor of significant power is technically not feasible. A method of changing the reflection of neutrons due to a small neutron leak from the core will be ineffective. For example, the use of a mobile reflector in a neptunium-based pulsed reactor would give an amplitude change of the neutron multiplication coefficient of no more than 2% instead of 3.5% at IBR-2, where plutonium is used as nuclear fuel (calculations were made in the Monte Carlo N simulation program for the transfer of ionizing radiation Particle Transport Code, abbreviated MCNP [10]). He will be quite effective the last of the three above methods, (absorption of slow neutrons will not affect the reactivity of the installation on the threshold isotope).

An object of the invention is the implementation of a project of a fundamentally new pulsed neutron source on “threshold” isotopes.

The stated technical problem is achieved in that the neutron source contains threshold-fissile isotopes as the main fuel, the introduction of a structural element containing a neutron moderator into the active zone of the installation and withdrawal from it, while the reactivity is increased by removing the structural element and lowering by input.

SUMMARY OF THE INVENTION

The invention allows to implement a project of a fundamentally new pulsed research fast reactor on the “threshold” isotopes. This reactor (a fast neutron nuclear installation) will make it possible to obtain a neutron flux in the research channel at least an order of magnitude higher than the IBR-2 (currently the most powerful research reactor). Important characteristics of a pulsed reactor are pulse duration and minimum background reactor power between pulses. To achieve an acceptable (less than 500 μs) pulse duration and reduce the power level between pulses, a change in reactivity of not less than IBR-2, that is, at least 3.5%, is necessary. The problem is solved as follows. The reactivity of a fast neutron nuclear plant with threshold fissile isotopes is changed by introducing a structural element into the reactor core and out of it. The structural element contains a neutron moderator. The introduction of negative reactivity is achieved by introducing this element into the active zone, and the introduction of a positive one by removing it from the active zone. A similar concept of a nuclear research facility using neptunium and reactivity modulation by input-output of a neutron-slowing substance was not previously proposed. Such a facility is the most effective currently operating, as it allows to obtain the highest neutron flux per unit of heat output.

The essential features of the invention are:

- The structural element contains a neutron moderator, which allows you to change the average neutron energy in the core;

- The method is applicable in installations based on threshold fissile elements (for example Np-237) that are not fission by slow neutrons. When introduced into the core, the moderator reduces the average neutron energy. With a decrease in energy, neutrons go beyond the threshold of fission of isotopes of the core, thereby reducing the probability of fission. Accordingly, the smaller the probability of fission, the lower the effective neutron multiplication coefficient. The opposite effect when removing a structural element with a moderator from the core is the introduction of positive reactivity. (Fig. 1).

In FIG. 1 A plot of the fission cross section of Np-237 versus neutron energy is presented.

The abscissa axis represents the neutron energy, and the ordinate axis the microscopic fission cross section. The top line is Pu-239 and the bottom line is Np-237. The graph shows that the deceleration of neutrons (a decrease in their energy) negatively affects the rate of the fission reaction Np-237 compared to Pu-239, that is, Np-237 is practically not fissioned by neutrons with energies below 0.4 MeV.

One possible implementation of this method is a reactivity modulator in a pulsed neptunium reactor. A series of calculations was performed using the MCNP program [10]. In the calculations, we used a cylindrical model of a reactor with neptunium-237 nitride as fuel and a nickel reflector. The efficiency of a moderator based on a moderator was 4.4%, while when using a movable reflector in the same non-potassium reactor, its efficiency was no more than 2%. The use of boron carbide, as in BN-600, gives an effect of 3%. A neutron moderator-based modulator reduces the duration of a neutron pulse by 30% and reduces the harmful background power of the reactor between pulses by more than 2 times.

Literature

1. Fast experimental reactor: [Electronic resource] // State Scientific Center - Research Institute of Atomic Reactors., URL: http://www.niiar.ru/node/101 (Date of access: 08/29/2016).

2. Research fast reactor: [Electronic resource] // IM Neutron Physics Laboratory Franka., URL: http://flnp.jinr.ru/494/ (Date of access: 08.29.2016).

3. Start-up and study of the main characteristics of the IBR-2 reactor with a new heterogeneous type reactivity modulator. V.D. Ananyev N.P. Antsupov A.V. Vinogradov L.V. Edunov V.G. Ermilov A.F. Zatsepin V.L. Lomidze Yu.N. Pepelyshev A.D. Rogov S.V. Rudenko E.P. Shabalin. Dubna, 2004.

4. Water-water energy reactor: [Electronic resource] // Wikipedia., URL: http://ru.wikipedia.org/wiki/Bodo-water energy_reactor (Date accessed: 08/29/2016).

5. High-power reactor channel: [Electronic resource] // Wikipedia., URL: http://en.wikipedia.org/wiki/Peactop_large_power_channel (Date of access: 08.29.2016).

6. Research beam-water reactor: [Electronic resource] // Modern Research Infrastructure of the Russian Federation., URL: http://ckp-rf.ru/usu/73591 / (Date of access: 08/29/2016).

7. High-flow research reactor SM-3: [Electronic resource] // State Scientific Center - Research Institute of Atomic Reactors., URL: http://www.niiar.ru/node/3340/ (Date of access: 08.29.2016 )

8. High-flow research reactor PIK: [Electronic resource] // National Research Center Kurchatov Institute., URL: http://www.nrcki.ru/ pages / main / 6015/7147 / index.shtml (Date of access: 29.08 .2016).

9. Nuclear power plants with fast neutron reactors with sodium coolant. In 2 parts. Tutorial. Authors: Beltyukov A.I., Karpenko A.I., Poluyaktov S.A., Tashlykov O.L., Titov G.P., Tuchkov A.M., Scheklein S.E. Under the general editorship of S. Scheklein, Olga Tashlykova. Yekaterinburg: UrFU, 2013. - Ministry of Education and Science of the Russian Federation. Ural Federal University named after B.N. Yeltsin.

10. MCNP: [Electronic resource] // Wikipedia., URL: http://ru.wikipedia.org/wiki/MCNP (Date of access: 08.29.2016).

Claims (1)

The method for changing the reactivity in pulsed nuclear batch plants with fast neutrons with threshold fissile isotopes, which consists in introducing a structural element containing a neutron moderator into the reactor core and out of it, while the reactivity is increased by removing the structural element and lowering by input.

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