Материал: Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

This section will be focused on description of nuclear technologies and on their analysis from the viewpoint of proliferation resistance. Nonproliferation of nuclear materials may be ensured if they are handled under conditions where their theft or diversion for illegal purposes are so difficult and dangerous and the risk of detection so high that potential transgressors are bound to be discouraged.

Nuclear technologies should be provided with such a system of physical protection, accounting and control that:

a)it would be practically impossible to gain unauthorized access to nuclear materials;

b)theft of small NM quantities by personnel would be quickly discovered, and any further attempts of using this route barred;

c)diversion of NM kept under international safeguards would be easily detected by international inspections.

This chapter will deal mainly with nuclear technologies viewed in the context of non-proliferation.

Nuclear fuel concept

Nuclear fuel is a material containing nuclides which undergo fission when exposed to neutrons. Fissionable nuclides are represented by:

∙natural uranium and thorium isotopes;

∙man-made plutonium isotopes;

∙isotopes of transuranic elements (Np, Am, Cm, Bk, Cf);

∙man-made isotope 233U (resulting from capture of neutrons by 232Th).

Normally, isotopes of uranium, plutonium and thorium with an even mass number (even-numbered isotopes) will undergo fission only under action of fast neutrons (fission reaction threshold approximating 1.5 MeV). On the other hand, uranium and plutonium isotopes with an odd atomic mass number (odd-numbered isotopes) can be split by neutrons of any energy, including thermal neutrons. The fission neutron spectrum is a spectrum of fast neutrons (with the average energy of 2.1 MeV), which are quickly moderated to an energy level below the fission threshold for even-numbered isotopes. It is impossible to have a chain fission reaction on even-numbered isotopes due to a small portion of neutrons with energy above the fission threshold. Neutron moderation is good for sustaining a chain fission reaction on odd-numbered isotopes, considering that fission cross-sections of these isotopes grow as neutron energy decreases.

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It is only natural fissionable isotopes (235U, 238U, 232Th) that are found

in primary nuclear fuel, while secondary nuclear fuel contains man-made fissile nuclides (233U, 239Pu, 241Pu).

The 238U and 232Th isotopes are natural nuclear materials that are of little use as nuclear fuel, since they are only split by fast neutrons. But they can successfully serve for producing fissile nuclides (233U, 239Pu), i.e. for breeding secondary nuclear fuel. These nuclides are often referred to as fertile isotopes.

The present-day nuclear power industry runs on natural uranium, which consists of three isotopes:

·238U with the content of 99.2831 % and half-life Т1/2 = 4.5×109 years;

·235U with the content of 0.7115 % and half-life Т1/2 = 7.1×108 years;

·234U with the content of 0.0054 % and half-life Т1/2 = 2.5×105 years.

The uranium isotope 235U is the only natural nuclear material which is fissionable by neutrons of any energy and gives rise to an excess of fast neutrons. It is owing to these neutrons that a chain fission reaction becomes possible. Most power reactors run on uranium enriched in 235U by 2–5 %. Fast reactors use uranium enriched to 15–25 %. Rese arch reactors operate on uranium of medium or high enrichment (20–90 %).

Enriched uranium contains 235U in amounts exceeding its concentration in natural uranium (0.71 %). It is generally agreed that:

·low enrichment is X5 < 5 %;

·medium enrichment is X5 between 5 and 20 %;

·high enrichment is X5 between 20 and 90 %;

·superhigh enrichment (for weapons-grade U) is X5 > 90 %.

Production of enriched uranium results in depleted uranium, with 235U content below the natural level (normally 0.2–0.3 % ).

The following types of nuclear fuel are in use:

·metals, metal alloys, intermetallic compounds;

·ceramics (oxides, carbides, nitrides);

·cermets (with metal fuel particles dispersed in a ceramic matrix);

·dispersion fuel (coated fuel particles dispersed in an inert, e.g., graphite, matrix).

A fuel element is a basic structural form of nuclear fuel in a reactor, which consists of an active part (fuel core containing fissile and fertile NM) and a cladding. Fuel claddings are usually made of metal, such as stainless steel and zirconium alloys. In the case of spherical elements, fuel particles are coated with layers of silicon carbide and pyrolytic carbon.

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In terms of geometry, fuel elements appear as rods, ring-shaped elements, plates, and spheres. Reactor fuel inventory is distributed among a large number of fuel elements; so a VVER–1000 react or will have 48 000 fuel rods.

Fuel elements are combined into fuel assemblies (FA), with their number varying from several pieces to several hundreds of fuel elements in one FA.

Fuel assemblies are installed to form the reactor core, which is the site of a controlled chain reaction of nuclei fission by neutrons, with nuclear energy converted into heat. This heat is carried off by coolant to be converted into electricity. The reactor core plays the same role as does an ordinary boiler, in which fossil fuel is burned. This analogy allows using customary terminology, such as “nuclear fuel”, “fue l burning”, “fuel burnup”, though nuclear fuel doses not “burn” in th e usual sense of the word.

Nuclear fuel cycle concept

The processes of fuel fabrication, use and reprocessing may be all covered by the notion of nuclear fuel cycle (NFC).

Main stages of the NFC

1.Uranium ore mining and extraction of uranium compounds.

2.Nuclear fuel fabrication.

3.Nuclear fuel use in reactors.

4.Temporary on-site storage of irradiated fuel assemblies (IFA).

Next stages depend on the choice between two options – open NFC or closed NFC.

5a. Burial of irradiated fuel in geological repositories – in an open NFC. 5b. Chemical reprocessing of irradiated fuel – in a closed NFC.

6.Separation of radioactive waste (RW), its treatment and disposal.

7.Recycling of uranium and plutonium, their refabrication into nuclear fuel and subsequent reuse in nuclear reactors.

As regards the expediency of closing the fuel cycle, there two points of view:

1. It is not expedient to close the NFC. Chemical reprocessing of irradiated fuel is associated with technological and political problems, such as:

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a)NM can be stolen for production of nuclear weapons;

b)Fuel reprocessing is complicated and hazardous;

c)RW treatment and disposal are complicated and hazardous.

2. It is expedient to close the NFC. Irradiated fuel contains valuable NM suitable for fabrication of nuclear fuel. A closed NFC affords selfsufficiency in meeting national energy demands.

An NFC can appear in one of several forms: there are one open and two closed options. They are schematically shown in Fig. 2.1.

U3O8

ДобычаU ore miningU-руды

B)

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