CRITICAL MASS, the minimum mass of fissile-capable material required to start a CHAIN ​​REACTION in an atomic bomb or atomic reactor. In an atomic bomb, the exploding material is divided into parts, each of which is less than critical ... ... Scientific and technical encyclopedic dictionary

See MASS CRITICAL. Raizberg B.A., Lozovsky L.Sh., Starodubtseva E.B. Modern economic dictionary. 2nd ed., rev. M .: INFRA M. 479 s .. 1999 ... Economic dictionary

CRITICAL MASS- the smallest (see) fissile substance (uranium 233 or 235, plutonium 239, etc.), in which a self-sustaining chain reaction of fission of atomic nuclei can occur and proceed. The value of the critical mass depends on the type of fissile material, its ... ... Great Polytechnic Encyclopedia

CRITICAL mass, the minimum mass of fissile material (nuclear fuel) that ensures the flow of a self-sustaining nuclear fission chain reaction. The value of the critical mass (Mcr) depends on the type of nuclear fuel and its geometric ... ... Modern Encyclopedia

The minimum mass of fissile material that ensures the flow of a self-sustaining nuclear fission chain reaction ... Big Encyclopedic Dictionary

Critical mass is the smallest mass of fuel in which a self-sustaining chain reaction of nuclear fission can proceed with a certain design and composition of the core (depends on many factors, for example: fuel composition, moderator, shape ... ... Nuclear power terms

critical mass- The smallest mass of fuel in which a self-sustaining chain reaction of nuclear fission can proceed with a certain design and composition of the core (depends on many factors, for example: fuel composition, moderator, core shape and ... ... Technical Translator's Handbook

Critical mass- CRITICAL MASS, the minimum mass of fissile material (nuclear fuel), which ensures the flow of a self-sustaining nuclear fission chain reaction. The value of the critical mass (Mcr) depends on the type of nuclear fuel and its geometric ... ... Illustrated Encyclopedic Dictionary

The minimum amount of nuclear fuel containing fissile nuclides (233U, 235U, 239Pu, 251Cf), with chromium, a nuclear fission chain reaction is possible (see Nuclear fission. Nuclear reactor, Nuclear explosion). K. m. depends on the size and shape ... ... Physical Encyclopedia

The minimum mass of fissile material that ensures the flow of a self-sustaining nuclear fission chain reaction. * * * CRITICAL MASS CRITICAL MASS, the minimum mass of a fissile material that ensures the flow of a self-sustaining ... encyclopedic Dictionary

Books

• Critical mass, Veselova N., In the book of Natalia Veselova, a member of the Russian Interregional Union of Writers, a full member of the Academy of Russian Literature and Fine Arts. G. R. Derzhavin, the chosen ones entered ... Category: Other publications
• Critical mass, Natalia Veselova, In the book of Natalia Veselova, a member of the Russian Interregional Union of Writers, a full member of the Academy of Russian Literature and Fine Arts. G.R.Derzhavin, included selected stories ... Category:

The site outlines the basics of electroplating technology. The processes of preparation and application of electrochemical and chemical coatings, as well as methods of coating quality control are considered in detail. The main and auxiliary equipment of the electroplating shop is described. Information on the mechanization and automation of galvanic production, as well as sanitation and safety precautions is given.

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Electrolytic chromium plating is an effective way to increase the wear resistance of rubbing parts, protect them from corrosion, as well as a method of protective and decorative finishing. Significant savings are provided by chrome plating when restoring worn parts. The process of chromium plating is widely used in the national economy. A number of research organizations, institutes, universities and machine-building enterprises are working on its improvement. More efficient electrolytes and chromium plating modes are emerging, methods are being developed to improve the mechanical properties of chrome parts, as a result of which the scope of chromium plating is expanding. Knowledge of the basics of modern chromium plating technology contributes to the fulfillment of the instructions of normative and technical documentation and the creative participation of a wide range of practitioners in the further development of chromium plating.

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By chemical deposition of metals, in combination with galvanic metal coatings are created on a wide variety of dielectrics: plastics, ceramics, ferrites, glass-ceramic and other materials. The manufacture of parts from these materials with a metallized surface ensured the introduction of new design and technical solutions, an improvement in the quality of products and a reduction in the cost of production of equipment, machines, and consumer goods.

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In an accessible and entertaining way, the site introduces physical nature in terms of the features of ionizing radiation and radioactivity, the effect of various doses of radiation on living organisms, methods of protection and prevention of radiation hazard, the possibilities of using radioactive isotopes to recognize and treat human diseases.

For safe operation with nuclear hazardous fissile substances, the parameters of the equipment must be less than critical. As regulatory parameters for nuclear safety, the following are used: the amount, concentration and volume of nuclear hazardous fissile material; diameter of equipment having a cylindrical shape; flat layer thickness for plate-shaped equipment. The normative parameter is set based on the permissible parameter, which is less than the critical one and should not be exceeded during the operation of the equipment. At the same time, it is necessary that the characteristics that affect the critical parameters are within strictly defined limits. The following valid parameters are used: the number of M add , volume V add , diameter D add , layer thickness t add .

Using the dependence of the critical parameters on the concentration of a nuclear hazardous fissile nuclide, the value of the critical parameter is determined, below which, at any concentration, SCRD is impossible. For example, for solutions of plutonium salts and enriched uranium, the critical mass, volume, diameter of an infinite cylinder, thickness of an infinite flat layer have a minimum in the region of optimal deceleration. For mixtures of metallic enriched uranium with water, the critical mass, as for solutions, has a pronounced minimum in the region of optimal deceleration, and the critical volume, the diameter of an infinite cylinder, and the thickness of an infinite flat layer at high enrichment (>35%) have minimum values ​​in the absence of a moderator (r n /r 5 =0); for enrichment below 35%, the critical parameters of the mixture have a minimum at optimal deceleration. It is obvious that the parameters set on the basis of the minimum critical parameters ensure safety over the entire concentration range. These parameters are called safe, they are less than the minimum critical parameters. The following safe parameters are used: quantity, concentration, volume, diameter, layer thickness.

When ensuring the nuclear safety of the system, the concentration of the fissile nuclide (sometimes the amount of moderator) is necessarily limited by the permissible parameter, while at the same time, when using the safe parameter, no restrictions are imposed on the concentration (or on the amount of moderator).

2 CRITICAL MASS

Whether or not a chain reaction will develop depends on the outcome of the competition of four processes:

(1) Ejection of neutrons from uranium,

(2) capture of neutrons by uranium without fission,

(3) capture of neutrons by impurities.

(4) capture of neutrons by uranium with fission.

If the loss of neutrons in the first three processes is less than the number of neutrons released in the fourth, then a chain reaction occurs; otherwise it is impossible. Obviously, if of the first three processes is very likely, then the excess of neutrons released during fission will not be able to ensure the continuation of the reaction. For example, in the case when the probability of process (2) (capture by uranium without fission) is much greater than the probability of capture with fission, a chain reaction is impossible. An additional difficulty is introduced by the isotope of natural uranium: it consists of three isotopes: 234U, 235U, and 238U, whose contributions are 0.006, 0.7, and 99.3%, respectively. It is important that the probabilities of processes (2) and (4) are different for different isotopes and depend differently on the neutron energy.

To assess the competition of various processes from the point of view of the development of a chain process of nuclear fission in a substance, the concept of "critical mass" is introduced.

Critical mass is the minimum mass of fissile material that ensures the flow of a self-sustaining nuclear fission chain reaction. The critical mass is the smaller, the shorter the fission half-life and the higher the enrichment of the working element with a fissile isotope.

Critical mass - the minimum amount of fissile material required to start a self-sustaining fission chain reaction. The neutron multiplication factor in such an amount of matter is equal to unity.

Critical mass is the mass of the fissile material of the reactor, which is in a critical state.

Critical dimensions of a nuclear reactor- the smallest dimensions of the reactor core, at which a self-sustaining nuclear fuel fission reaction can still be carried out. Usually under the critical size take the critical volume of the active zone.

Critical volume of a nuclear reactor- the volume of the reactor core in a critical state.

The relative number of neutrons that are emitted from uranium can be reduced by changing the size and shape. In a sphere, surface effects are proportional to the square, and volume effects are proportional to the cube of the radius. The escape of neutrons from uranium is a surface effect, depending on the size of the surface; capture with fission occurs in the entire volume occupied by the material, and therefore is

volumetric effect. The greater the amount of uranium, the less likely it is that the emission of neutrons from the volume of uranium will prevail over captures with fission and prevent a chain reaction. The loss of neutrons to non-fission captures is a bulk effect, similar to the release of neutrons in fission capture, so increasing size does not change their relative importance.

The critical dimensions of a device containing uranium can be defined as the dimensions at which the number of neutrons released during fission is exactly equal to their loss due to emission and captures that are not accompanied by fission. In other words, if the dimensions are less than critical, then, by definition, a chain reaction cannot develop.

Only odd isotopes can form a critical mass. Only 235 U is found in nature, and 239 Pu and 233 U are artificial, they are formed in a nuclear reactor (as a result of neutron capture by 238 U nuclei

and 232 Th followed by two subsequent β-decays).

AT in natural uranium, a fission chain reaction cannot develop with any amount of uranium, however, in isotopes such as 235 U and 239 Pu chain process is achieved relatively easily. In the presence of a neutron moderator, a chain reaction also occurs in natural uranium.

A necessary condition for the implementation of a chain reaction is the presence of a sufficiently large amount of fissile material, since in small samples, most neutrons fly through the sample without hitting any nucleus. A chain reaction of a nuclear explosion occurs when

fissile material of some critical mass.

Let there be a piece of matter capable of fission, for example, 235 U, into which a neutron enters. This neutron will either cause fission, or it will be uselessly absorbed by the substance, or, having diffused, will come out through the outer surface. It is important what will happen at the next stage - will the average number of neutrons decrease or decrease, i.e. weaken or develop a chain reaction, i.e. whether the system will be in a subcritical or supercritical (explosive) state. Since the emission of neutrons is controlled by the size (for a ball, by the radius), the concept of the critical size (and mass) arises. For the explosion to develop, the size must be greater than the critical one.

The critical size of a fissile system can be estimated if the neutron path length in the fissile material is known.

The neutron, flying through the substance, occasionally collides with the nucleus, it seems to see its cross section. The size of the cross section of the core σ=10-24 cm2 (barn). If N is the number of nuclei in a cubic centimeter, then the combination L =1/N σ gives the mean neutron path with respect to the nuclear reaction. The neutron path length is the only dimensional value that can serve as a starting point for evaluating the critical size. In any physical theory, similarity methods are used, which, in turn, are built from dimensionless combinations of dimensional quantities, characteristics of the system and matter. So dimensionless

the number is the ratio of the radius of a piece of fissile material to the length of the path of neutrons in it. If we assume that the dimensionless number is of the order of unity, and the path length at a typical value of N = 1023, L = 10 cm

(for σ = 1) (usually σ is usually much higher than 1, so the critical mass is less than our estimate). The critical mass depends on the cross section of the fission reaction of a particular nuclide. So, to create an atomic bomb, approximately 3 kg of plutonium or 8 kg of 235 U (with an implosive scheme and in the case of pure 235 U) are required. of such a mass is approximately 8.5 cm, which is surprisingly well in line with our estimate

R \u003d L \u003d 10 cm).

Let us now derive a more rigorous formula for calculating the critical size of a piece of fissile material.

As is known, the decay of a uranium nucleus produces several free neutrons. Some of them leave the sample, and some are absorbed by other nuclei, causing their fission. A chain reaction occurs if the number of neutrons in a sample begins to grow like an avalanche. The neutron diffusion equation can be used to determine the critical mass:

	∂C	D C + β C
	∂t

where C is the neutron concentration, β>0 is the neutron multiplication reaction rate constant (similar to the radioactive decay constant has the dimension 1/sec, D is the neutron diffusion coefficient,

Let the sample be spherical with radius R. Then we need to find a solution to equation (1) that satisfies the boundary condition: C (R,t )=0.

Let us make the change C = ν e β t , then

∂C

∂ν

v = D

+ βνe

∂t

∂t

We have obtained the classical equation of heat conduction:

∂ν

Dv

∂t

The solution to this equation is well known

π 2 n 2

ν(r, t)=

sin n re

π 2 n

β −

C(r, t) =

sin n re

r n = 1

The chain reaction will go under the condition (that is,

C(r, t)

t →∞ → ∞ ) that for at least one n the coefficient in

exponent is positive.

If β − π 2 n 2 D > 0,

then β > π 2 n 2 D and the critical radius of the sphere:

R = n

If π

≥ R , then for any n there will be no growing exponent

If π

< R , то хотя бы при одном n мы получим растущую экспоненту.

We restrict ourselves to the first member of the series, n = 1:

R = π

Critical mass:

M = ρ V = ρ

The minimum value of the ball radius at which a chain reaction occurs is called

critical radius , and the mass of the corresponding ball is critical mass.

Substituting the value for R , we get the formula for calculating the critical mass:

M cr = ρπ 4 4 D 2 (9) 3 β

The value of the critical mass depends on the shape of the sample, the neutron multiplication factor and the neutron diffusion coefficient. Their determination is a complex experimental problem, therefore the resulting formula is used to determine the indicated coefficients, and the calculations carried out are proof of the existence of a critical mass.

The role of the sample size is obvious: with decreasing size, the percentage of neutrons emitted through its surface increases, so that at small (below critical!) sample sizes, a chain reaction becomes impossible even with a favorable ratio between the processes of absorption and production of neutrons.

For highly enriched uranium, the critical mass is about 52 kg, for weapons-grade plutonium, 11 kg. The regulatory documents on the protection of nuclear materials from theft indicate critical masses: 5 kg of 235 U or 2 kg of plutonium (for the implosion scheme of the atomic bomb). For the cannon scheme, the critical masses are much larger. On the basis of these values, the intensity of protection of fissile substances from terrorist attacks is built.

Comment. The critical mass of a 93.5% enriched uranium metal system (93.5% 235 U; 6.5% 238 U) is 52 kg without a reflector and 8.9 kg when the system is surrounded by a beryllium oxide neutron reflector. The critical mass of an aqueous solution of uranium is approximately 5 kg.

The value of the critical mass depends on the properties of the substance (such as the fission and radiation capture cross sections), on the density, the amount of impurities, the shape of the product, and also on the environment. For example, the presence of neutron reflectors can greatly reduce the critical mass. For a particular fissile material, the amount of material that constitutes the critical mass can vary over a wide range and depends on the density, characteristics (material type and thickness) of the reflector, and the nature and percentage of any inert diluents (such as oxygen in uranium oxide, 238 U in partially enriched 235 U or chemical impurities).

For comparison purposes, here are the critical masses of balls without a reflector for several types of materials with some standard density.

For comparison, we give the following examples of critical masses: 10 kg 239 Pu, metal in the alpha phase

(density 19.86 g/cm3); 52 kg 94% 235 U (6% 238 U), metal (density 18.72 g/cm3); 110 kg UO2 (94% 235 U)

at a density in crystalline form of 11 g/cm3; 35 kg PuO2 (94% 239 Pu) at density in crystalline

in the form of 11.4 g/cm3. Solutions of salts of pure fissile nuclides in water with a water neutron reflector have the lowest critical mass. For 235 U the critical mass is 0.8 kg, for 239 Pu it is 0.5 kg, for 251 Cf it is

The critical mass M is related to the critical length l: M l x , where x depends on the shape of the sample and ranges from 2 to 3. The shape dependence is related to the leakage of neutrons through the surface: the larger the surface, the greater the critical mass. The sample with the minimum critical mass is spherical. Tab. 5. Main estimated characteristics of pure isotopes capable of nuclear fission

								Neutrons
		Receipt		critical	Density	Temperature	Heat dissipation	spontaneous
			half-life
		(source)			g/cm³	melting point °C
			T 1/2					105 (kg s)
	231Pa
	232U	Reactor on
		neutrons
	233U
	235U	Natural	7.038×108 years
	236U		2.3416×107 years? kg
	237Np		2.14×107 years
	236Pu
	238Pu
	239Pu
	240Pu
	241Pu
	242Pu
	241Am
	242mAm
	243mAm
	243Am
	243cm
	244cm
	245cm
	246cm
	247cm		1.56×107 years
	248cm
	249Cf
	250Cf
	251Cf
	252Cf

Let us dwell in more detail on the critical parameters of the isotopes of some elements. Let's start with uranium.

As has been repeatedly mentioned, 235 U (0.72% clarke) is of particular importance, since it is fissioned under the action of thermal neutrons (σ f = 583 barn), while releasing a “thermal energy equivalent” of 2 × 107 kWh / k. Since, in addition to α-decay, 235 U also spontaneously divides (T 1/2 \u003d 3.5 × 1017 years), neutrons are always present in the mass of uranium, which means that it is possible to create conditions for the occurrence of a self-sustaining fission chain reaction. For metallic uranium with an enrichment of 93.5%, the critical mass is: 51 kg without reflector; 8.9 kg with beryllium oxide reflector; 21.8 kg with full water baffle. Critical parameters of homogeneous mixtures of uranium and its compounds are given in

Critical parameters of plutonium isotopes: 239 Pu: M cr = 9.6 kg, 241 Pu: M cr = 6.2 kg, 238 Pu: M cr = from 12 to 7.45 kg. Of greatest interest are mixtures of isotopes: 238 Pu, 239 Pu, 240 Pu, 241 Pu. The high specific energy release of 238 Pu leads to the oxidation of the metal in air; therefore, it is most likely to be used in the form of oxides. Upon receipt of 238 Pu, the accompanying isotope is 239 Pu. The ratio of these isotopes in the mixture determines both the value of the critical parameters and their dependence upon changing the content of the moderator. Various estimates of the critical mass for a bare metal sphere of 238 Pu give values ​​from 12 to 7.45 kg compared to the critical mass for 239 Pu of 9.6 kg. Since the 239 Pu nucleus contains an odd number of neutrons, the critical mass will decrease when water is added to the system. The critical mass of 238 Pu increases with the addition of water. For a mixture of these isotopes, the net effect of adding water depends on the isotope ratio. When the mass content of 239 Pu is 37% or less, the critical mass of the mixture of 239 Pu and 238 Pu isotopes does not decrease when water is added to the system. In this case, the allowable amount of 239 Pu-238 Pu dioxides is 8 kg. With others

ratios of 238 Pu and 239 Pu dioxides, the minimum value of the critical mass varies from 500 g for pure 239 Pu to 24.6 kg for pure 238 Pu.

Tab. Fig. 6. Dependence of the critical mass and critical volume of uranium on 235 U enrichment.

Note. I - homogeneous mixture of metallic uranium and water; II - homogeneous mixture of uranium dioxide and water; III - solution of uranyl fluoride in water; IV - solution of uranyl nitrate in water. * Data obtained using graphical interpolation.

Another isotope with an odd number of neutrons is 241 Pu. The minimum value of the critical mass for 241 Pu is achieved in aqueous solutions at a concentration of 30 g/l and is 232 kg. Upon receipt of 241 Pu from irradiated fuel, it is always accompanied by 240 Pu, which does not exceed it in content. With an equal ratio of nuclides in a mixture of isotopes, the minimum critical mass of 241 Pu exceeds the critical mass of 239 Pu. Therefore, with respect to the minimum critical mass, the 241 Pu isotope at

239 Pu can be substituted for 239 Pu if the mixture of isotopes contains equal amounts

241 Pu and 240 Pu.

Tab. 7. Minimum critical parameters of uranium with 100% enrichment in 233 U.

Let us now consider the critical characteristics of americium isotopes. The presence of 241 Am and 243 Am isotopes in the mixture increases the critical mass of 242 m Am. For aqueous solutions, there is an isotope ratio at which the system is always subcritical. When the mass content of 242 m Am in a mixture of 241 Am and 242 m Am is less than 5%, the system remains subcritical up to the concentration of americium in solutions and mechanical mixtures of dioxide with water equal to 2500 g/L. 243 Am mixed with 242m Am also increases

the critical mass of the mixture, but to a lesser extent, since the thermal neutron capture cross section for 243 Am is an order of magnitude lower than that for 241 Am

Tab. 8. Critical parameters of homogeneous plutonium (239 Pu+240 Pu) spherical assemblies.

Tab. 9. Dependence of the critical mass and volume for plutonium compounds* on the isotopic composition of plutonium

* The main nuclide is 94 239 Pu.

Note. I - homogeneous mixture of metallic plutonium and water; II - homogeneous mixture of plutonium dioxide and water; III homogeneous mixture of plutonium oxalate and water; IV - solution of plutonium nitrate in water.

Tab. Fig. 10. Dependence of the minimum critical mass of 242 m Am on its content in a mixture of 242 m Am and 241 Am (the critical mass was calculated for AmO2 + H2 O in spherical geometry with a water reflector):

	Critical mass 242 m Am, g

With a small mass fraction of 245 Cm, ​​it should be taken into account that 244 Cm also has a finite critical mass in systems without moderators. Other curium isotopes with an odd number of neutrons have a minimum critical mass several times greater than 245 Cm. In a mixture of CmO2 + H2O, the 243 Cm isotope has a minimum critical mass of about 108 g, and 247 Cm - about 1170 g. With respect to

critical mass, we can assume that 1 g of 245 Cm is equivalent to 3 g of 243 Cm or 30 g of 247 Cm. Minimum critical mass 245 Cm, ​​g, depending on the content of 245 Cm in a mixture of 244 Cm and 245 Cm isotopes for СmО2 +

H2O is described quite well by the formula

	M cr = 35.5 +
		ξ + 0.003

where ξ is the mass fraction of 245 Cm in a mixture of curium isotopes.

The critical mass depends on the cross section of the fission reaction. When creating weapons, all sorts of tricks can reduce the critical mass required for an explosion. So, to create an atomic bomb, 8 kg of uranium-235 is needed (with an implosion scheme and in the case of pure uranium-235; when using 90% uranium-235 and with a stem scheme of an atomic bomb, at least 45 kg of weapon-grade uranium is required). The critical mass can be significantly reduced by surrounding the sample of fissile material with a layer of material that reflects neutrons, such as beryllium or natural uranium. The reflector returns a significant part of the neutrons emitted through the surface of the sample. For example, if you use a reflector 5 cm thick, made of materials such as uranium, iron, graphite, the critical mass will be half the critical mass of the "bare ball". Thicker reflectors reduce the critical mass. Beryllium is especially effective, providing a critical mass of 1/3 of the standard critical mass. The thermal neutron system has the largest critical volume and the smallest critical mass.

An important role is played by the degree of enrichment in the fissile nuclide. Natural uranium containing 0.7% 235 U cannot be used for the manufacture of atomic weapons, since the rest of the uranium (238 U) intensively absorbs neutrons, preventing the chain process from developing. Therefore, uranium isotopes must be separated, which is a complex and time-consuming task. Separation has to be carried out to degrees of enrichment in 235 U above 95%. Along the way, it is necessary to get rid of impurities of elements with a high neutron capture cross section.

Comment. When preparing weapons-grade uranium, not only do they get rid of unnecessary impurities, but replace them with other impurities that contribute to the chain process, for example, they introduce elements - neutron breeders.

The level of uranium enrichment has a significant effect on the value of the critical mass. For example, the critical mass of uranium enriched with 235U 50% is 160 kg (3 times the mass of 94% uranium), and the critical mass of 20% uranium is 800 kg (that is, ~15 times greater than the critical mass 94% uranium). Similar coefficients of dependence on the level of enrichment are applicable to uranium oxide.

The critical mass is inversely proportional to the square of the density of the material, M to ~1/ρ 2 , . Thus, the critical mass of metallic plutonium in the delta phase (density 15.6 g/cm3) is 16 kg. This circumstance is taken into account when designing a compact atomic bomb. Since the probability of neutron capture is proportional to the concentration of nuclei, an increase in the sample density, for example, as a result of its compression, can lead to the appearance of a critical state in the sample. In nuclear explosive devices, a mass of fissile material that is in a safe subcritical state is transferred to an explosive supercritical state using a directed explosion that subjects the charge to a high degree of compression.

Allowance for citizens "Caution! Radiation"

atomic fission

The fission of the nuclei of atoms is a spontaneous, or under the action of neutrons, splitting the nucleus of an atom into 2 approximately equal parts, into two "fragments".

Fragments are two radioactive isotopes of elements in the central part of D. I. Mendeleev's table, approximately from copper to the middle of the lanthanide elements (samarium, europium).

During fission, 2-3 extra neutrons are emitted and an excess of energy is released in the form of gamma quanta, much more than during radioactive decay. If one act of radioactive decay usually accounts for one gamma-quantum, then for 1 act of fission there are 8-10 gamma-quanta! In addition, flying fragments have a large kinetic energy (velocity), which turns into heat.

The emitted neutrons can cause the fission of two or three similar nuclei if they are nearby and if the neutrons hit them.

Thus, it becomes possible to implement a branching, accelerating chain reaction of fission of atomic nuclei with the release of a huge amount of energy.

If the chain reaction is kept under control, its development is controlled, it is not allowed to accelerate and the released energy (heat) is constantly removed, then this energy ("atomic energy") can be used either for heating or for generating electricity. This is carried out in nuclear reactors, at nuclear power plants.

If the chain reaction is allowed to develop uncontrollably, then an atomic (nuclear) explosion will occur. It's already a nuclear weapon.

In nature, there is only one chemical element - uranium, which has only one fissile isotope - uranium-235. This is weapons-grade uranium. And this isotope in natural uranium is 0.7%, that is, only 7 kg per ton! The remaining 99.3% (993 kg per ton) is a non-fissile isotope - uranium-238. There is, however, another isotope - uranium-234, but it is only 0.006% (60 grams per ton).

But in an ordinary uranium nuclear reactor, from non-fissile ("non-weapon-grade") uranium-238, under the action of neutrons (neutron activation!) the naturally occurring element plutonium. In this case, a fissile isotope of plutonium is immediately formed - plutonium-239. This is weapons-grade plutonium.

The fission of atomic nuclei is the essence, the basis of atomic weapons and atomic energy.

Critical mass is the amount of a weapon isotope at which neutrons released during spontaneous fission of nuclei do not fly out, but fall into neighboring nuclei and cause their artificial fission.

The critical mass of metallic uranium-235 is 52 kg. This is a ball with a diameter of 18 cm.

The critical mass of metallic plutonium-239 is 11 kg (and according to some publications - 9 or even 6 kg). This is a ball with a diameter of about 9-10 cm.

Thus, now humanity has two fissile, weapons-grade isotopes: uranium-235 and plutonium-239. The only difference between them is that, firstly, uranium is more suitable for use in nuclear energy: it allows you to control its chain reaction, and secondly, it is less effective for an uncontrolled chain reaction - an atomic explosion: it has a lower speed spontaneous nuclear fission and more critical mass. And weapons-grade plutonium, on the contrary, is more suitable for nuclear weapons: it has a high rate of spontaneous nuclear fission and a much lower critical mass. Plutonium-239 does not allow reliable control of its chain reaction and therefore has not yet found wide application in nuclear power engineering, in nuclear reactors.

That is why all the problems with weapons-grade uranium were solved in a matter of years, and attempts to use plutonium in nuclear power continue to this day - for more than 60 years.

So, two years after the discovery of uranium fission, the world's first uranium nuclear reactor was launched (December 1942, Enrico Fermi, USA), and two and a half years later (in 1945), the Americans detonated the first uranium bomb.

And with plutonium... The first plutonium bomb was detonated in 1945, that is, approximately four years after its discovery as a chemical element and the discovery of its fission. Moreover, for this it was necessary to first build a uranium nuclear reactor, produce plutonium in this reactor from uranium-238, then separate it from irradiated uranium, study its properties well, and make a bomb. Developed, isolated, manufactured. But talk about the possibility of using plutonium as a nuclear fuel in plutonium nuclear reactors has remained talk, and has remained so for more than 60 years.

The fission process can be characterized by a "half-period".

For the first time, half-life periods were estimated by K. A. Petrzhak and G. I. Flerov in 1940.

For both uranium and plutonium, they are extremely large. So, according to various estimates, uranium-235 has a half-life of approximately 10 ^ 17 (or 10 ^ 18 years (Physical Encyclopedic Dictionary); according to other sources - 1.8 10 ^ 17 years. And for plutonium-239 (according to same dictionary) is significantly less - approximately 10 ^ 15.5 years; according to other sources - 4 10 ^ 15 years.

For comparison, recall the half-lives (T 1/2). So for U-235 it is "only" 7.038 10 ^ 8 years, and for Pu-239 it is even less - 2.4 10 ^ 4 years

In general, the nuclei of many heavy atoms can divide, starting with uranium. But we are talking about two main ones, which have been of great practical importance for more than 60 years. Others are more of a purely scientific interest.

Where do radionuclides come from

Radionuclides are obtained from three sources (three ways).

The first source is nature. This is natural radionuclides, which have survived, survived to our time from the moment of their formation (perhaps, from the time of the formation of the solar system or the Universe), since they have long half-lives, which means that their lifetime is long. Naturally, there are much fewer of them than it was at the beginning. They are extracted from natural raw materials.

The second and third sources are artificial.

Artificial radionuclides are formed in two ways.

First - fragmentation radionuclides, which are formed as a result of the fission of the nuclei of atoms. These are "fragments of fission". Naturally, most of them are formed in nuclear reactors for various purposes, in which a controlled chain reaction is carried out, as well as in the testing of nuclear weapons (uncontrolled chain reaction). They are found in irradiated uranium extracted from military reactors (from "industrial reactors"), and in huge quantities in spent nuclear fuel (SNF) extracted from power reactors of nuclear power plants.

Previously, they got into the natural environment during nuclear tests and processing of irradiated uranium. Now they continue to get during the processing (regeneration) of spent nuclear fuel, as well as during accidents at nuclear power plants, at reactors. If necessary, they were extracted from irradiated uranium, and now from spent nuclear fuel.

The second ones are radionuclides of activation origin. They are formed from ordinary stable isotopes as a result of activation, that is, when a subatomic particle enters the nucleus of a stable atom, as a result of which the stable atom becomes radioactive. In the vast majority of cases, such a projectile particle is a neutron. Therefore, to obtain artificial radionuclides, the neutron activation method is usually used. It consists in the fact that a stable isotope of any chemical element in any form (metal, salt, chemical compound) is placed in the reactor core for a certain time. And since a huge number of neutrons are produced in the reactor core every second, therefore, all chemical elements that are in the core or near it gradually become radioactive. Those elements that are dissolved in the reactor-cooling water are also activated.

The method of bombarding a stable isotope in elementary particle accelerators with protons, electrons, etc. is less commonly used.

Radionuclides are natural - of natural origin and artificial - of fragmentation and activation origin. An insignificant amount of radionuclides of fragmentation origin has always existed in the natural environment, because they are formed as a result of spontaneous fission of uranium-235 nuclei. But there are so few of them that it is not possible to detect them with modern means of analysis.

The number of neutrons in the core of various types of reactors is such that about 10^14 neutrons fly through any section of 1cm^2 at any point in the core in 1 second.

Measurement of ionizing radiation. Definitions

It is not always convenient and expedient to characterize only the sources of ionizing radiation (SIR) and only their activity (the number of decay events). And the point is not only that activity can be measured, as a rule, only under stationary conditions at very complex installations. The main thing is that in a single act of decay of different isotopes, particles of different nature can be formed, several particles and gamma quanta can be formed simultaneously. In this case, the energy, and consequently, the ionizing ability of different particles will be different. Therefore, the main indicator for characterizing IRS is the assessment of their ionizing ability, that is (in the end) the energy that they lose when passing through a substance (medium) and which is absorbed by this substance.

When measuring ionizing radiation, the concept of dose is used, and when assessing their effect on biological objects, correction factors are used. Let's name them, give a number of definitions.

Dose, absorbed dose (from Greek - share, portion) - the energy of ionizing radiation (II) absorbed by the irradiated substance and often calculated per unit of its mass (see "rad", "Gray"). That is, the dose is measured in units of energy that is released in the substance (absorbed by the substance) when ionizing radiation passes through it.

There are several types of doses.

Exposure dose(for x-ray and gamma radiation) - determined by air ionization. The unit of measurement in the SI system is "coulomb per kg" (C/kg), which corresponds to the formation of such a number of ions in 1 kg of air, the total charge of which is 1 C (of each sign). The non-systemic unit of measurement is "roentgen" (see "C/kg" and "roentgen").

To assess the impact of AI on humans, we use correction factors.

Until recently, when calculating the "equivalent dose" were used "radiation quality factors "(K) - correction factors that take into account the different effects on biological objects (different ability to damage body tissues) of different radiations at the same absorbed dose. They are used when calculating the "equivalent dose." Now these coefficients are in the Radiation Safety Standards (NRB-99 ) was called very "scientifically" - "Weighing factors for individual types of radiation when calculating the equivalent dose (W Rradiation risk coefficient

Dose rate- dose received per unit of time (sec., hour).

Background- the exposure dose rate of ionizing radiation in a given place.

natural background- exposure dose rate of ionizing radiation, created by all natural sources of IR (see "Radiation background").