Материал: Апсе ENVIRONMENTAL PROTECTION 2014

self-sustaining chain fission reaction. The chain fission reaction can generate a large enough amount of fission neutrons to produce secondary nuclear fuel through radiative neutron captures by fertile isotopes. Large fraction of fertile uranium isotope 238U in primary fuel of nuclear power reactors (95-98%) can realize a partial reproduction of nuclear fuel.

Fuel element (fuel rod or fuel pin) is a main constructive form of nuclear fuel in the reactor core. Cylindrical fuel rod consists of a central active part (fuel meat) containing fissile and fertile isotopes and hermetical cladding around. Usually, the claddings are made of metals (Zrbased alloys and stainless steels). In spherical fuel elements of hightemperature gas-cooled reactors (HTGR) micro fuel particles are clad by thin layers of silicon carbide and pyrolytic carbon, and then these fuel particles are uniformly dispersed in graphite matrix.

Fuel rods are united into fuel assemblies (FA) containing from several fuel rods up to several hundred fuel rods. Inside FA fuel rods are stiffly fastened by the spacing grids (spacers). Also, certain conditions must be guaranteed to provide a reliable heat removal by coolant from fuel rods and compensate temperature-induced expansion of structural and fuel materials.

Complete set of all fuel assemblies disposed in a nuclear reactor constitutes the reactor core where the controlled chain fission reaction can be initiated to convert nuclear energy into thermal energy and then into electrical energy. It appears the reactor core plays a similar role with a traditional thermal pile, or boiler, where fossil organic fuel (charcoal, oil or natural gas) is burnt to produce heat. This analogy allows us to use such habitual terms as “fuel”, “inci neration” or “burnup” although no any burning or incinerating process es, in their traditional sense, occur in the nuclear reactor core.

Indeed, there are many substantial distinctions between nuclear and organic fuel. Main distinctions are briefly described below.

1. Significantly higher calorie content in nuclear fuel.

Incineration of one carbon atom in chemical reaction with oxygen can produce thermal energy at the level of 4 eV only:

C + O2 → CO2 + 4 eV.

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fraction of nuclear fuel quantity that exceeds its critical mass and provides super-criticality, or reactivity margin needed to make up the negative effects caused by fuel burn-up and build-up of fission products.

Usually, nuclear fuel burn-up is evaluated either by FP quantity per total fuel mass (for example, 10% fuel burn-up means that 10% of fuel mass was burnt-up and converted into FP) or by quantity of produced thermal energy per total fuel mass, MWd/t. It may be shown that 1% of fuel burn-up is approximately equal to specific energy yield of 10 GWd/t.

Typical, currently achievable values of fuel burn-up are as follows:

∙Heavy-water CANDU-type reactors - 10÷12 GWd/t, or 1 ÷1,2% of heavy metals (HM).

∙Light-water reactors (LWR) of VVER, PWR and BWR type - 40÷50 GWd/t (or 4-5% HM).

∙Fast LMFBR-type reactors – up to 100 GWd/t (or ~10% HM).

Upon exhaustion of the reactivity margin, spent fuel assemblies have to be replaced by fresh ones completely or partially. Spent nuclear fuel (SNF) contains large amounts of fertile and fissile nuclides. One ton of

SNF discharged from VVER-440 contains, in addition to 30 kg FP, about 950 kg 238U, 12,5 kg 235U and 6,5 kg of plutonium isotopes (mainly, 239Pu and 240Pu).

3.Possibility to organize repeat usage (recycle) of fertile and fissile nuclides.

The recycle can reduce significantly the demands for natural uranium mining and for its isotope enrichment with 235U.

4.Possibility to organize reproduction of fissile nuclides.

Fissile nuclides can be reproduced in any nuclear reactor which, in addition to fissile nuclides, contains fertile nuclides 238U or 232Th. When capturing neutron, fertile nuclide 238U converts into fissile nuclide 239Pu. Similarly, 232Th converts into fissile nuclide 233U. The reproduction process is conventionally characterized by the breeding ratio (BR), i.e. by ratio of secondary fuel generation rate to primary fuel incineration rate. Depending on the BR value, the following options of nuclear fuel reproduction are marked out: partial reproduction (BR < 1); full reproduction (BR = 1) and extended reproduction (breeding), if BR > 1.

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The reproduced secondary fuel can slow down the reactivity slump, prolong the reactor lifetime and generate some additional amount of thermal energy. In stationary operation mode of typical LWR, when the reactor core contains both fresh and partially incinerated FA, total generation rate of thermal energy includes a considerable contribution (up to 40% and above) from fissions of secondary fissile nuclide 239Pu.

Thanks to some neutron-physical peculiarities, the best conditions for extended reproduction (breeding) of nuclear fuel can be formed in fast breeder reactors loaded with mixed uranium and plutonium dioxides, i.e. with mixed oxide (MOX) fuel. Fast breeder reactors are able to produce such plutonium quantity that is sufficient to meet fuel demands of the reactor-producer and create an initial fuel loading for a new reac- tor-consumer. If large stockpiles of natural uranium are available or if there are no incentives for intense NPP deployment, then fast reactors can operate in fuel self-sustainability regime with the BR value about unity.

Similar situations can be formed for mixed thorium-uranium fuel. Good neutron-multiplying properties of 233U and huge abundance of natural thorium keep a customary interest to nuclear power reactors loaded with thorium-uranium fuel. However, nuclear technologies related with fabrication of fresh and reprocessing of spent (Th-U) fuel assemblies encountered some specific difficulties, and till now these technologies are not developed yet up to an industrial scale.

5. “Incineration” of nuclear fuel does not require oxidizer.

Incineration of conventional organic fuel in traditional thermal power plants requires roughly three-fold mass of oxygen taken from the Earth’s atmosphere. Moreover, the incineration process is followed by release of toxic wastes (smoke, ashes, sulphur and nitrogen oxides).

“Incineration” of nuclear fuel does not require an oxidizer at all. Radioactive fission products and spent nuclear fuel, which may be regarded as nuclear wastes, are retained within fuel rods for a rather long time period and, then, after appropriate treatment, transported into wellprotected geological repositories.

Data on fuel consumption and waste production by two electrical power plants (TPP and NPP) of the same power (1000 MWe) are presented in Table I.1.

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The well-known Kyoto protocol concerning global warming-up of the Earth’s climate proposes to introduce some ecological constraints on release of CO2 into the atmosphere by coal-fired TPP and establish certain economic sanctions for the exceeding of these constraints at the level of 60 US dollars (about 40 euros) per one ton of carbon dioxide. Following from the scale of global coal-fired power system, it is easy to evaluate total scope of these economical sanctions.

Table I.1 Fuel consumption and waste production by electrical power plants

6. Accumulation of radioactive FP. Residual heat generation after reactor shutdown. Induced radioactivity of structural materials and coolant.

Fission reactions of heavy nuclides produce fission products, i.e. relatively lighter nuclides whose mass numbers cover the range from ~70 a.m.u. up to ~160 a.m.u. As a rule, fission reaction is an asymmetrical act, i.e. instead of splitting a heavy nucleus into a couple of fission products with approximately equal masses, fission reaction produces two nuclides with mass ratio about 2:3 (95 a.m.u. and 140 a.m.u., for example). Common form of FP yields dependency on their mass is roughly the same for all fissile nuclides and for all neutron energies. (a symmetrical two-peak curve, as is presented in Fig. I.1). FP accumulation rate in nuclear power reactors is about 1000 kg per one GWe·year.

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