RU2284068C1 - Device for generating loading radiation field of objects during radiation resistance tests - Google Patents

Abstract

FIELD: physics of reactor radiation in various materials.

SUBSTANCE: proposed device is made in the form of truncated cone whose smaller base is oriented to object and has set of water-containing neutron moderator plates alternating with converter plates made of material absorbing thermal neutrons in radiation capture reaction. Diameters of cone larger and smaller bases are chosen so as to ensure shading of entire object under test and its middle part from direct rays of reactor, respectively. Cone height is chosen so as to ensure desired ratio of neutron attenuation in middle part of object.

EFFECT: reduced nonuniformity of neutron distribution lengthwise of object under test, enhanced gamma-ray dose contribution in object radiation loading field.

1 cl, 4 dwg

Description

The invention relates to the field of physics of the transformation of reactor radiation in various materials. It can be used when testing objects, especially large ones, for radiation resistance. The tested objects can be robot devices equipped with radio-electronic units and performing special operations in emergency situations (accidents) at research reactors, nuclear power plants or ships with nuclear power plants, as well as aircraft compartments or other equipment.

The need to develop this device is due to large non-uniformities in the spatial distribution of radiation in the reactor room and the need to increase the contribution of gamma radiation to the radiation loading field of the object. Neutron fluences along the length of an object can vary up to two times, and the dose of gamma radiation at the object’s location can be (30 ... 50)% less than the required value, which is unacceptable when conducting radiation resistance tests.

A technical solution is known for the formation of fields of penetrating radiation at a research nuclear reactor BARS using a set of materials from lead, iron and polyethylene for the calibration of neutron detectors [Sevastyanov VD, Kazantsev VV et al., Questions of atomic science and technology. Series: Physics of Radiation Exposure to Radioelectronic Equipment, 1994, issue 3-4, pp. 65-68]. The task was to form a model reference field with certain spectral characteristics of neutrons and a small contribution of the accompanying gamma radiation at a distance of 82 cm from the reactor core (A3).

Technical solution by A.S. No. 1762945 of March 23, 1990, is intended for the formation of gamma-neutron radiation fields at reactors using boron and titanium-containing filters for solving radiobiological problems when it was necessary to isolate the hard component of the neutron spectrum with a small contribution of gamma radiation in the total dose.

Wang J., Wang B., Chen D., Luo Y. [The response of dogs to mixed neutron radiation research, 1991, No. 128, p. 42-46] for the formation of mixed gamma-neutron fields and changing the ratio of components In radiobiological experiments, lead filters of various thicknesses were used.

In the work of Kolesov V.F. and Shtareva S.K. [Questions of atomic science and technology. Series: Pulse reactors and simple critical assemblies, issue 2, M., 1988, p.16] theoretically considered the possibility of amplifying fast neutron and gamma-ray fluxes in reactors using concave reflectors and flat converters made of polyethylene with cadmium oxide.

The prototype of the proposed technical solution is a device for irradiating biological objects (RF Patent No. 2226729 dated 12/07/2002), containing a volume neutron moderator from a hydrogen-containing material with a cavity for irradiating biological objects and a converter from Cd or Cd and W that converts delayed neutrons into gamma quanta and installed in the cavity of the moderator.

However, this technical solution, as well as other given analogues of the proposed one, cannot be applied when testing objects for radiation resistance, since they do not provide an acceptable irregularity of the radiation field along the length of the object and the required ratio of neutron fluence to gamma radiation dose. In addition, in the prototype device, large thicknesses of the moderator lead to the loss of almost all fast neutrons, and a gamma radiation field is mainly formed in the working cavity.

The technical result, which the invention is directed to, is to reduce the uneven distribution of neutrons along the length of the object and increase the contribution of the dose of gamma radiation.

The technical result is achieved by the fact that the proposed device is made in the form of a truncated cone, oriented with a smaller base on the test object, from a set of plates of a hydrogen-containing neutron moderator (polyethylene or organic glass), alternating with converter plates (cadmium) in the moderator-converter sequence. Moreover, the cone parameters are determined by the following relationships:

where h is the height of the cone;

D is the diameter of the larger base of the cone;

d is the diameter of the smaller base of the cone;

L is the length of the test object;

l is the distance from the center of the AZ to the field driver;

R is the distance from the center of the AZ to the object;

λ is the relaxation length of neutrons with energies of more than 0.1 MeV in the moderator;

K is the set ratio of neutron attenuation by the field driver in the middle part of the object;

e is the base of the natural logarithm.

List of figures:

Figure 1. Device for forming a field of radiation loading of test objects

1 - moderator;

2 - converter;

D is the diameter of the larger base of the cone;

d is the diameter of the smaller base of the cone;

h is the height of the cone.

Figure 2. Scheme of radiation loading of objects

1 - reactor core;

2 - a device for forming a field of radiation loading of objects;

3 - test object;

L is the length of the test object;

R is the distance from the center of the AZ to the object;

l is the distance from the center of the AZ to the field driver;

h is the height of the cone.

Figure 3. The distribution of the dose of gamma rays along the length of the object

1 - with a device-driver field (polyethylene + cadmium);

2 - with a device-driver field (without cadmium plates).

Figure 4. The distribution of fast neutron fluence along the length of an object

1 - without a field driver;

2 - with a field driver

Device description

The device of figure 1 consists of a set of plates of the moderator 1 and converter 2. It is installed at the reactor core so that the larger base of the cone obscures the entire length of the test object from direct radiation, and the smaller base only the central part of the object (approximately 20% of the length), and was oriented by the smaller base of the cone to the test object. Scheme of radiation loading of the test object is shown in figure 2.

The mechanism of action of the device

From the physics of the interaction of neutrons and gamma rays with matter, it is known that hydrogen-containing materials are effective neutron moderators as a result of their elastic scattering on hydrogen nuclei. Slowing down to thermal energies (0.025 eV), neutrons are absorbed in the converter (for example, in cadmium) with the formation of gamma rays as a result of the radiation capture reaction. At moderator thicknesses <10 cm, the radiation capture process is characteristic mainly for neutrons with energies <0.1 MeV, because the length of the deceleration of these neutrons to thermal energies in hydrogen-containing materials is small and does not exceed 4 cm. Fast neutrons with energies> 0.1 MeV at these thicknesses of the moderator practically do not participate in the radiation capture process, since they have a length of deceleration to thermal energies significantly greater than the thickness of the moderator . However, fast neutrons are effectively attenuated by hydrogen-containing materials. The attenuating properties of materials are usually characterized by the neutron attenuation ratio (K), and the thickness of the material can be conveniently represented in neutron relaxation lengths (λ). This is the length during which the neutron flux density decreases by a factor of e. For neutrons from research reactors (according to the data of A. I. Kiryushkin and E. A. Shlokin, “Fundamentals of Designing the Protection of Reactor Plants”), the neutron relaxation length in polyethylene is 7.2 cm. These physical properties formed the basis for the development of the proposed device.

The results of studies of the topography of neutron fields at the BARS reactor showed that to reduce the uneven distribution of neutrons along the length of the object (at L = 5 ... 6 m) up to 30%, the attenuation rate of fast neutrons by a set of moderator and converter plates should be at least 3. Then the height the cone or thickness of the set of plates of the moderator and the converter should be equal to λ × K / e = 7.2 × 3 / 2.72 = 7.9 cm. For objects with L <3 m, the condition for reducing unevenness to 30% is fulfilled at K = 1,5. In this case, h = 7.2 × 1.5 / 2.72 = 4 cm. Another constructive element to reduce the uneven distribution of neutrons along the length of the object is to determine the diameters of the base of the cone according to relations (2), (3) from the shading conditions of the entire test object and its middle part from direct radiation of the reactor, respectively.

The increase in the contribution of gamma rays to the dose on the irradiated object is ensured by the alternation of the plates of the moderator and converter, which allows you to create a volumetric source of hard secondary gamma radiation that feeds gamma radiation coming from the AZ reactor. In addition, the alternation of the plates of the moderator and converter in the proposed device also allows more efficient use for the generation of gamma rays, mainly slow neutrons with energies less than 0.1 MeV, which do not participate in the formation of the dose load when testing the object.

The proposed device (h = 8 cm, D = 50 cm, d = 13.3 cm) at 4 mm thickness of the moderator plate and 0.5 mm thickness of the converter plate was tested at the BARS reactor when testing an object of 5 m length installed at a distance of 3 m from the center of the AZ reactor. The research results shown in figures 3 and 4 show that the device can reduce the uneven distribution of neutron fluence along the length of the object up to 25% and increase the contribution of the dose of gamma radiation up to 30%.

Claims (1)

A device for generating a radiation loading field of objects when tested for radiation resistance, comprising a neutron moderator from a hydrogen-containing material and a neutron radiation converter from a material that absorbs thermal neutrons in a radiation capture reaction, characterized in that the moderator and converter are made in the form of a truncated cone oriented less based on the object, from a set of plates alternating in the moderator-converter sequence, and the cone parameters are determined by the following relations:

where h is the height of the cone; D is the diameter of the larger base of the cone; d is the diameter of the smaller base of the cone; L is the length of the test object; l is the distance from the center of the reactor core to the field shaper device; R is the distance from the center of the reactor core to the facility; λ is the relaxation length of neutrons with energies of more than 0.1 MeV in the moderator; K is the set probability of neutron attenuation by the device - field shaper in the middle part of the object; e is the base of the natural logarithm.

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