Материал: Апсе ENVIRONMENTAL PROTECTION 2014

Heavy-water CANDU-type reactors

The CANDU-type reactors are able to use even natural uranium containing only 0,72% 235U as fuel material. Initial fuel loading of CANDU-600 is equal to about 100 t UO2. Secondary fuel is produced with a specific rate ~350 kg Pu/GWe·year. Isotope composition of reac- tor-grade plutonium extracted from SNF of the CANDU-type reactors is very close to reactor-grade plutonium extracted from SNF of the VVER-type reactors in respect of fissile isotopes content and in respect

of 240Pu content. Typical plutonium extracted from spent fuel of CANDU-type reactors contains about 66% 239Pu, 27% 240Pu, 5% 241Pu and 2% 242Pu, i.e. the same 71% of fissile plutonium isotopes and almost the same content of 240Pu (25% in VVER via 27% in CANDU).

The CANDU-type reactors, quite like the RBMK-type reactors, can represent a potential threat to nuclear non-proliferation regime because their operation modes with continuous refuelings can be easily re-tuned (by proper selection of fuel irradiation time, for instance) to form the best conditions for wide-scale production of weapon-grade plutonium. Besides, the operation mode with continuous refueling can require a permanent presence of the IAEA inspectors to control proper utilization of primary fuel and accumulation of secondary fuel, potentially dangerous material for non-proliferation of nuclear weapons.

By the way, plutonium for the first atomic bombs exploded in July 1945 in the USA and in August 1945 over Japan was produced by heavy-water reactors for about half a year.

5. Liquid-metal fast breeder reactors (LMFBR)

Currently, the LMFBR-type reactors are still loaded with uranium oxide (UOX) fuel, not mixed uranium-plutonium oxide (MOX) fuel as it was anticipated earlier. The UOX fuel is based on middle-enriched uranium (15-25% 235U). Initial fuel loading of LMFBR-1000 is equal to about 10-15 t UO2. Secondary fuel is produced with a specific rate ~1500 kg Pu/GWe·year in the once-through NFC option. If the NFC becomes closed, then large fraction (up to 80%) of the produced plutonium is recycled to provide fuel self-sustainability of the LMFBRproducer, and net rate of plutonium production for other purposes is equal to ~250 kg Pu/GWe·year.

There are no intense neutron absorbers among fission products within high-energy range of the LMFBR-type reactors. That is why

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typical values of fuel burn-up in the LMFBR-type reactors can reach ~100 GWd/t, or 10% HM, i.e. roughly twice higher than acceptable values of fuel burn-up in LWR. Thanks to the higher values of fuel burn-up and, as a consequence, longer fuel lifetimes, plutonium produced by the LMFBR-type reactors is characterized by such isotopic composition which is low suitable for manufacturing of nuclear explosive devices.

Some data on consumption of fresh primary fuel and production of secondary fuel are gathered in Table 3.2 for various reactor types.

3.3. Advanced proliferation-resistant SNF reprocessing technologies

3.3.1. Aqueous SAFAR reprocessing technology

Main idea of nuclear non-proliferation ensuring within the frames of the SAFAR (Safeguarded Fabrication and Reprocessing) technology consists in incomplete separation of uranium, plutonium and fission products. Consequently, at any stage of the SAFAR-technology, pluto-

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nium can not be extracted in the form suitable for its diversion and manufacturing of nuclear explosive devices.

The following specific features can distinguish the SAFARtechnology from traditional PUREX-technology:

1. Incomplete separation of plutonium from uranium and fission products. Plutonium and uranium are recovered jointly (co-extraction) by using only two cycles of the extraction - re-extraction process. As a result, plutonium is deliberately contaminated with uranium and radioactive fission products ( 1% of their initial content). The decontamination factors are about 100 instead of 106-107 in traditional PUREXtechnology.

2.Pure uranium and plutonium dioxides are not produced. Final products of the SAFAR-technology are spherical micro-granules of mixed oxide uranium-plutonium (MOX) fuel. These micro-granules are formed by using the sol-gel process that is described below.

3.The re-fabricated MOX-fuel is characterized by the elevated radioactivity due to the relatively large content of fission products. The elevated radioactivity of the MOX-fuel can be estimated as a certain positive factor from nuclear non-proliferation point of view: the radiation barrier against the MOX-fuel diversion for manufacturing of nuclear explosive devices; unattractiveness for thefts; easy control (high detectability) of any MOX-fuel movements. However, some additional countermeasures must be undertaken to enhance radiation protection of the staff involved.

The sol-gel process, as a key stage of the SAFAR-technology, should be described in more details. Sol is a suspension-like substance, gel is a colloid, or a jelly-like substance. So, the “sol-ge l” term means a gradual densification of the SNF solution through consecutive transformations from the liquid SNF solution into the SNF suspension, then into the SNF colloid and, ultimately, into the solid MOX-fuel granules followed by the fuel pelletization. Main mission of the sol-gel process is to avoid technological operations with finely dispersed powders of uranium and plutonium dioxides and to work with sufficiently large MOX-fuel granules.

The sol-gel process uses the acidic SNF solution after two cycles of the extraction – re-extraction process for partial removal of fission products as an initial feed material. So, the SAFAR-technology can be

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regarded as an advanced version of the solvent-extraction PUREXtechnology. Then, the following operations are performed:

1.Addition of the chemical reagents which are able to upgrade alkaline properties of the SNF solution (urea (NH2)2CO, for example).

2.Infusion of the SNF solution into a water-absorbing organic material (ethyl-benzoate, for example). This infusion converts the SNF solution into the colloid-like substance (U,Pu)O2(OH)0,4(NO3)1,6.

3.Injection of the colloid-like substance into an ammonia-based organic material for further gradual dehydration. This injection converts the colloid-like substance into the jelly-like spherical granules (U,Pu)O2(OH)2 0,5 NH3 0,5 H2O with typical sizes within the range of 40-100 microns, i.e. they are large enough for further pelletization (cold pressing and sintering).

4.Thermal treatment of the jelly-like granules with gradual elevation of temperature. Residual ammonia-based organics is removed at 950С. Ultimate dehydration occurs at 125-2000С with the formation of (U,Pu)O2(OH)4. All residual organic substances are completely evapo-

rated at 300-4000С. Ultimately, solid MOX-fuel granules are calcined at 400-5000С.

5. Fabrication of fresh MOX-fuel rods and fuel assemblies.

The SAFAR-technology can be estimated as a well proliferationprotected spent fuel reprocessing technology because of the following main reasons:

1.Uranium and plutonium dioxides are extracted from the acidic SNF solution jointly. Plutonium dioxide is never separated from uranium dioxide.

2.The MOX-fuel granules are characterized by the enhanced radioactivity due to residual content of radioactive fission products after only two cycles of the solvent-extraction process.

Evolutionary progress of aqueous solvent-extraction technologies is being achieved now in following main directions:

1.Application of new, more effective and radiation-resistant organic extractants for selective removal of minor actinides from the raffinate produced at the extraction stage.

2.Implementation of some new stages intended to extract the most harmful fission products (extraction of radioiodine by volatilization, and technetium by the raffinate radiolysis).

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3. Co-extraction of uranium, plutonium, neptunium and some fission products in order to enhance the barriers against unauthorized NM diversion to any undeclared applications.

According to these directions, some new advanced processes have been developed in France (COEX), Japan (NEXT), USA (family of UREX+ technologies) and so on. These technologies can apply new, more effective extractants, use new organizational schemes for material flows and the reprocessing operations. For example, all SNF reprocessing technologies from UREX+ family apply the following sequence of actions: at first, uranium is extracted from acidic SNF solution and, then, the residuals are treated to co-extract plutonium and minor actinides for further recycling in nuclear reactors.

For the last time the US specialists have developed a whole series of SNF reprocessing technologies under the common name UREX+ (Uranium Recovery by Extraction) family. This name underlines uranium extraction at the very initial stage of all UREX+ technologies. One else common property of the UREX+ technologies is the absence of pure plutonium among all the SNF reprocessing products.

Таблица 3.3 Products at various stages of the UREX+ technologies

Non-aqueous pyrochemical and pyrometallurgical technologies are currently considered as advanced and very promising options of SNF reprocessing but they are not ready yet for wide industrial usage.

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