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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10.Plutonium disposition in special storage facilities.

11.Final disposal of RW in geological repositories.

C. Closed NFC with use of regenerated uranium and plutonium

1.Uranium ore mining.

2.Production of U3O8.

3.Conversion of U3O8 to UF6.

4.Enrichment of UF6.

5.Fabrication of nuclear fuel (fuel elements and assemblies).

6.Use of nuclear fuel in reactors.

7.On-site INF storage.

8.INF reprocessing with separation of uranium, plutonium and RW.

9.Return of regenerated uranium to conversion and enrichment stages.

10.Fabrication of mixed uranium–plutonium oxide fu el (MOX fuel) based on regenerated uranium and plutonium.

11.Final disposal of RW in geological repositories.

INF reprocessing capabilities are available in seven countries: USA, UK, France, Russia, China (nuclear-weapons states), Japan and India. One of the main objectives pursued in development and use of nuclear technologies is control over NM at all NFC stages to prevent their diversion for military purposes.

There are three possible ways of diverting NM from energy uses to military applications:

1.Forcible theft following an external terrorist attack at a nuclear facility or a vehicle carrying NM.

To prevent such events, physical protection systems are created.

2.Non-forcible, covert theft by site personnel.

To prevent it, NM accounting and control systems are set up. 3. Covert diversion authorized by the national government.

This is prevented by a system of international safeguards and agreements on peaceful use of NM:

a)Treaty on the Non-Proliferation of Nuclear Weapons (NPT);

b)regional agreements on non-proliferation of nuclear weapons;

c)Zangger Committee (Nuclear Suppliers Group) for control over export of nuclear materials, technologies and components.

Periodic inspections of nuclear sites by IAEA experts are the main mechanism of control at the governmental level.

Let us consider the main factors making NM theft attractive at various NFC stages.

1. NM quantity and quality required for producing nuclear explosives:

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∙critical mass of 100 % 235U – 50 kg;

∙critical mass of 100 % 239Pu – 15 kg;

∙critical mass of 100 % 233U – 17 kg.

The critical mass values are given for a metal sphere without a reflector (which almost halves the critical mass). The IAEA introduced the unit of Significant Quantity (SQ). Detection of stolen 1 SQ calls for a special investigation and for notification of the UN Security Council. The SQ value is roughly half that of the critical mass (metal sphere minus reflector):

1 SQ (239Pu, 233U) = 8 kg; 1 SQ (235U) = 25 kg.

2.Simplicity of stealing NM, with low probability of detection.

3.Simplicity of turning NM into a nuclear charge.

The attractiveness of NM diversion at various NFC Stages is qualitatively assessed in Fig. 2.2.

Fig. 2.2. NFC stages in terms of their attractiveness for NM diversion.

The number of dots corresponds to the attractiveness of a stage. The most attractive stages are those of INF reprocessing, plutonium separation, fabrication of mixed uranium-plutonium fuel and its recycling in reactors.

The main NFC stages will be now regarded from the viewpoint of proliferation resistance (Table 2.1).

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*H – L (IAEA): High to low, depending on monitorin g by the IAEA.

1.Uranium ore mining and primary processing

Vulnerability to theft (VT): low. For 25 kg of weapons-grade plutonium to be produced, it is necessary to have about 5000 kg of natural uranium or 5000 t of uranium ore. It is difficult to steal such a quantity of uranium ore without being noticed.

Vulnerability to diversion by personnel (VDP): high. Uranium mines and primary processing facilities are not covered by the IAEA safeguards.

Proliferation risk (PR): low. Natural uranium is not suitable for making nuclear explosive devices.

2.Conversion to nuclear fuel (unenriched uranium for CANDU reactors) or to uranium hexafluoride for enrichment.

VT: low, as in uranium mining.

VDP: dependent on whether the processes are covered by the IAEA safeguards.

PR: low. Natural uranium is unusable in nuclear explosives.

3.Uranium enrichment in 235U.

VT: high. A nuclear explosive device would only take 25 kg of weapons-grade uranium. Such a weight can be carried by one person.

VDP: dependent on whether the process is covered by the IAEA safeguards.

PR: high. The Nuclear Suppliers’ Group placed an unofficial embargo on export of enrichment technologies.

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4. Fabrication of nuclear fuel (fuel elements and assemblies).

VT: low. One FA weighs 300 to 500 kg and would take special transport for conveyance.

VDP: dependent on whether the processes are covered by the IAEA safeguards.

PR: between low and high depending on the fuel enrichment.

5. Use of nuclear fuel in reactors.

VT: low, due to FA weight, radioactivity and location inside the reactor vessel.

VDP: dependent on whether the reactor is covered by the IAEA safeguards.

PR: between low and high depending on fuel enrichment and on availability of reprocessing facilities.

6. INF storage.

VT: low, due to FA weight, radioactivity and residual heat.

VDP: dependent on whether the storage facility is covered by the IAEA safeguards.

PR: between low and high depending on the availability reprocessing facilities.

7. INF reprocessing.

VT: high. INF reprocessing relies on remotely controlled equipment standing between personnel and NM. Though there are areas where Pucontaining materials are accessible for theft.

VDP: dependent on whether the reprocessing facility is covered by the IAEA safeguards.

PR: high. Reprocessing facilities generate plutonium, which can be used in nuclear explosives. The Nuclear Suppliers’ Group placed an unofficial embargo on export of reprocessing technologies.

8. Radioactive waste disposal.

VT: low, due to high radioactivity and heat release, with low content of fissile materials.

VDP: low, due to small percentage of fissile materials.

PR: low, due to high radioactivity and heat release, with low content of fissile materials.

Reactors of different types differ in the attractiveness for NM theft. There are two characteristics determining such attractiveness (Table 2.2):

1)quantity and quality of nuclear fuel loaded;

2)quantity and quality of nuclear fuel produced.

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1. Research reactors

Some research reactors still run on highly enriched uranium fuel. But their thermal power being low (several megawatts), the 235U inventory is no greater than 5–10 kg.

Following the IAEA resolution, research reactors are being converted to fuel of medium enrichment, with 235U content making no more than 20 %. The critical mass of such uranium is 830 kg.

Secondary fuel is not produced in research reactors as their fertile material quantities are small and neutron fluxes are low.

2. High-temperature gas-cooled reactors

These reactors use highly enriched dispersion fuel (93 % 235U) and thorium as fertile material. Fuel particles clad in pyrolytic carbon and silicon carbide are dispersed in a graphite matrix to be subsequently

fabricated into spherical or rod-type fuel elements.

The starting charge of a 770 MWe HTGR contains 8.1 t of 232Th in the form of ThO2 and 0.7 t of 235U as UC. By the end of in-pile irradiation, the inventory will include: 7.5 t of 232Th, 40 kg of 235U and about 180 kg of 233U,

which means that HTGRs generate ~ 200 kg of 233U/GWe×year.

3. Light water reactors

Vessel-type reactors (PWR, BWR, VVER). Such reactors using water as moderator and coolant run on low-enrichment (3–5 % 235U) uranium

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