Patent Description:
In a nuclear power plant, a spent fuel further includes fission products (FP) such as an alkali metal element (AM), an alkaline earth metal elements (AEM), a platinum element and the like, in addition to transuranium elements (TRU) such as U, Pu and the like. By utilizing an appropriate process to reprocess the spent fuel to separate the U and the Pu from the spent fuel, fissionable materials can be recovered, economic benefits can be improved, and the volume of the spent fuel to be processed subsequently can be greatly reduced.

Through the development of more than half a century, an aqueous reprocessing process represented by PUREX process is becoming increasingly perfect. However, with the expansion of the spent fuel reprocessing scale, significant drawbacks of the aqueous process have become more and more prominent. Since a liquid acid is used to dissolve the spent fuel, a large amount of radioactive aqueous and organic salt solutions are produced, and thus a large amount of high level waste (HLW) to be stored is accumulated. Complex treatments need to be conducted to the high level liquid waste: firstly dehydrating, and then converting into a micro-soluble compound (glass) state, or other solid state that is convenient for storage or disposal. For such a spent nuclear fuel reprocessing process using the aqueous solution and the organic solution, the increase in scale thereof is more and more difficult to meet the industrial safety and nuclear safety requirements in production. On another hand, the hydrochemical process has significant limitations on the spent fuel of a novel reactor: an inability to handle advanced oxide fuels, including an inert matrix fuel, a high burnup fuel, and a fuel with a high Pu content and a short cooling time.

Hitachi proposed a FLUOREX process (<CIT>) combined a fluoride volatility method with the aqueous reprocessing process, which is consisted of "dry" and "wet" steps: firstly, the U in the spent fuel is converted into UF<NUM> and separated from the Pu, the FP and so on, and meanwhile, the Pu is controlled to be non-volatile; the not volatilized U, the Pu, the FP and the like are converted into oxides, and then dissolved with nitric acid, and then a separation process is carried out by using the aqueous PUREX process. The Pu is kept in the non-volatile state in the FLUOREX process, and then recovered by the aqueous process, thereby the problem of low Pu yield in the original fluoride volatility method is avoided. The fluoride volatility step can remove more than <NUM>% of the U in the spent fuel, and the process drastically reduces the burden of the subsequent PUREX process by separating a large amount of the uranium first. However, there is still an aqueous process link in the above process, the production of the high level liquid waste is inevitable, and the economy of the "dry" and "wet" combination process is to be verified.

A dry reprocessing technique in the full sense mainly includes a fluoride volatility method or a metal molten extraction method, and has the following advantages:.

The fluoride volatility method is based on special properties of that the uranium, the plutonium and the neptunium are converted into a volatile hexafluoride form, while most fission products (lanthanides) and transplutonium elements existing in the spent fuel are converted into non-volatile trifluoride. These properties form several processes based on the spent fuel fluorinated by a strong fluorinating agent (such as BrF<NUM>, BrF<NUM>, ClF<NUM>, NF<NUM> and even pure F<NUM>).

In the field of the reprocessing, the fluoride volatility method is considered to be a promising advanced thermochemical reprocessing technology, which can process not only the spent fuel in light water reactors, but also the spent fuel difficult to be processed by the wet process, including: the oxide spent fuel of the fast reactor of the fourth-generation reactor, and advanced oxide fuel types (such as a fuel with an inert matrix, and/or, a fuel with a high burnup, a high plutonium content and an extremely short cooling time), and the spent nuclear fuel types of a nitride, a carbide, an alloy or the like, are pre-oxidized into oxide.

Depending on different types of the medium, the fluoride volatility method can be sorted as a molten salt fluorination method and a gas direct fluorination method. In all known dry pyrochemical reprocessing processes, only the gas direct fluorination method, that is, the gas flame fluorination technology, is the only one method not based on the molten salt, by merely using a simple fluorinating agent and an adsorbent, the required performance and productivity can be achieved, the set quality of the amount of fissile materials recovered can be achieved, and the minimum amount of the radioactive waste is produced, thus the industrial safety and the nuclear safety is improved, and the negative influence on the environment is dramatically reduced. In addition, almost all the fissile materials (UF<NUM>, PuF<NUM>) participate directly in most processes and operation links, with no externally added diluting agent, solvent and the like, so equipments and arrangements thereof used in the process procedure can be very compact.

However, two problems of the direct fluorination method should be overcome: one is that the temperature is high and the time is long when fluorinating, and the corrosion of the fluorination gas (F<NUM> or HF) to the equipment is serious; the other one that is the balance constant of the chemical balance PuF<NUM> ↔ PuF<NUM> + F<NUM> is large under the fluorination temperature of the plutonium, the stability of the PuF<NUM> itself is poor, thus a decomposition of the PuF<NUM> occurs while fluorinating, a gas channel is blocked by the PuF<NUM>, and the recovery rate of the plutonium is reduced. To solve these problems, the rate of the fluoridation should be increased, and appropriate cooling measures should be taken to prevent the thermal decomposition of the PuF<NUM> in terms of dynamics.

In the process of the flame fluorination of the spent fuel, due to the fission products existing in the spent fuel, and a short processing time, the conversion of the <NUM> components in the spent fuel into the hexafluoride cannot be fully realized, and these components reside in a slag in a form of lower valent fluorides. Basically, the powder of these fluorides is collected in a collection container, but at most <NUM>% of the powder deposits on the walls of the reactor. In order to clean the slag from reactor, the reactor must be dismantled and thus the operation is complex and difficult.

Publications <CIT>, <CIT> and <CIT> are considered to be relevant to the present application.

A technical problem to be solved by the present disclosure is to provide a spent fuel dry reprocessing method based on plasma.

A technical solution adopted by the present disclosure to solve the technical problem is to provide a spent fuel dry reprocessing method based on plasma, including the following steps:.

Preferably, in the step S1, the spent fuel powder is delivered into the plasma reactor through a scroll feeder or a nozzle under an action of an inert carrier gas.

Preferably, in the step S1, the spent fuel powder has a particle diameter of no larger than <NUM> µ m.

Preferably, in the step S2, the plasma containing F atoms is generated by conversion of the fluorination medium via gas discharge and then delivered into the plasma reactor; or, the plasma containing F atoms is generated by conversion of the fluorination medium in the plasma reactor.

Preferably, the plasma containing F atoms is generated by at least one of a high-voltage discharge, a DC-arc discharge, a high-frequency discharge, a microwave discharge and a laser ionization.

Preferably, in the step S2, when the fluorination medium is F<NUM>, chemical reaction formulas of UO<NUM> and U<NUM>O<NUM> in the spent fuel powder reacting in the plasma reactor are represented by the following formulas (<NUM>) and (<NUM>):.

UO<NUM>+3F<NUM>→UF<NUM>+O<NUM>     (<NUM>).

U<NUM>O<NUM>+9F<NUM>→3UF<NUM>+4O<NUM>     (<NUM>).

Preferably, in the step S2, a temperature of a reaction region in the plasma reactor is larger than <NUM>; and a temperature of a wall of the plasma reactor is controlled below <NUM>.

Preferably, the step S5 includes: performing a dry or wet processing to the mixed gas of the UF<NUM> and the PuF<NUM> to obtain a mixed oxide of UO<NUM> and PuO<NUM>, and HF; or,
the step S5 includes: separating the UF<NUM> and the PuF<NUM> of the mixed gas, performing a dry or wet processing to the UF<NUM> to obtain UO<NUM> and HF, and performing a dry or wet processing to the PuF<NUM> to obtain PuO<NUM> and HF.

Preferably, the step S5 includes: performing an electromagnetic separation processing to the mixed gas of the UF<NUM> and the PuF<NUM> to obtain U, Pu, and F<NUM>; or,
the step S5 includes: separating the UF<NUM> and the PuF<NUM> of the mixed gas, performing an electromagnetic separation processing to the UF<NUM> to obtain metal U and F<NUM>, and performing an electromagnetic separation processing to the PuF<NUM> to obtain metal Pu and F<NUM>.

Preferably, the spent fuel dry reprocessing method further includes the following step:
S6, performing a recovery process to the solid-state product obtained from separation in the step S3 and the step S4.

The spent fuel dry reprocessing method based on plasma of the present disclosure, compared with the traditional operation processes such as hydrochemical process dissolution, deposition, filtration and the like, avoids the production of a large amount of radioactive waste liquid, and thereby solves the problem of storing and processing the large amount of waste liquid; through the reaction of F atom of the plasma with U, Pu and other elements in the spent fuel, compared with flame fluorination process, the plasma has effects of a wider temperature range and a stronger activity of the fluorinating agent, a complete conversion of Pu to PuF<NUM> is realized, the problem of separation of Pu from the fluoride of the fission product is solved, the separation of U and Pu from the fission products is promoted, the recovery rate is improved, and the process is simplified.

The method of the present disclosure has a wide range of applications, is fitted for processing spent fuels of fast breeder reactors, thermal reactors and other types of advanced reactors, and is fitted for various types of spent fuels, including oxide, nitride, carbide and metal and the like. The process is simple, and the secondary waste is less. Compared to other fluorination methods, the reaction rate of the method is higher, and the temperature is easier to control (by controlling the atomic density of reactant by discharge power, to control the reaction rate; controlling according to components, temperatures of the ion, the electron and the neutral atom being controlled respectively), a maximum conversion of Pu is realized, and it is beneficial to generate a closed nuclear fuel cycle structure.

The present disclosure will now be further described with reference to the accompanying drawings and embodiments, and in the drawings:
<FIG> is a schematic flow diagram of a spent fuel dry reprocessing method based on plasma of an embodiment of the present disclosure.

With reference to <FIG>, a spent fuel dry reprocessing method based on plasma of an embodiment of the present disclosure, includes the following steps:
S1, processing a spent fuel after removal of a cladding into a spent fuel powder, and sending the spent fuel powder into a plasma reactor.

Wherein, the processing of the spent fuel after removal of the cladding may utilize, but not limited to, a mechanical processing, the spent fuel after removal of the cladding is grinded into the powder, that is, the spent fuel powder. The spent fuel powder has a particle diameter of no larger than <NUM> µ m, preferably.

The spent fuel powder may be delivered into the plasma reactor through a nozzle under an action of a carrier gas. The carrier gas adopts an inert gas, without chemical reaction with the conveyed material (spent fuel powder). Of course, the spent fuel powder may alternatively be delivered into the plasma reactor by mechanical delivery (such as a scroll feeder).

S2, fully mixing and reacting the spent fuel powder with a plasma containing F atoms in the plasma reactor, to generate a product including volatile fluorides and non-volatile fluorides.

Wherein, the plasma containing F atoms is generated by conversion of a fluorination medium, and may be generated by gas discharge conversion of the fluorination medium in advance and then placed into the plasma reactor; or alternatively, the plasma containing F atoms is generated by conversion of the fluorination medium in the plasma reactor.

Outside the plasma reactor, the plasma containing F atoms may be generated by at least one of a high-voltage discharge, a DC-arc discharge, a high-frequency discharge, and a microwave discharge. In the plasma reactor, the plasma containing F atoms may be generated by at least one of a high-voltage discharge, a DC-arc discharge, a high-frequency discharge, a microwave discharge and a laser ionization. The fluorination medium as the plasma includes one or more gases of CF<NUM>, NF<NUM>, F<NUM>, HF, BrF<NUM>, BrF<NUM>, ClF<NUM>, and SF<NUM>.

A temperature of a reaction region in the plasma reactor is larger than <NUM>. A temperature of a wall of the plasma reactor is controlled below <NUM>, or below <NUM>, to reduce the risk of being corroded.

Typically, elements contained in the spent fuel powder include uranium (U), plutonium (Pu), lanthanide (Ln), and minor actinides (for example, neptunium Np, americium Am, curium Cm), and so on. After reacting with the plasma, wherein U is converted into UF<NUM> (non-volatile fluoride), Pu eventually is converted into PuF<NUM> (non-volatile fluoride), most of the fission products (such as Sr, Ba, Y, Cs and lanthanides) are converted into non-volatile fluoride, and a fraction of the fission products (such as Nb, Ru, Te, Mo, I and minor actinides) are converted into volatile fluoride.

Taking F<NUM> as the fluorination medium as an example, chemical reactions occurring in the plasma reactor are described in detali herebelow, and the reaction mechanism is similar when the fluorination medium takes other fluorine source gas.

When the fluorination medium is F<NUM>, the chemical reactions of UO<NUM> and U<NUM>O<NUM> in the spent fuel powder reacting in the plasma reactor are represented by the following formulas (<NUM>) and (<NUM>):.

UO<NUM> (s) +3F<NUM> (g) →UF<NUM> (g) +O<NUM> (g)     (<NUM>).

U<NUM>O<NUM> (s) +9F<NUM> (g) →3UF<NUM> (g) +4O<NUM> (g)     (<NUM>).

In practice, the fluorination medium reacts with UO<NUM> and U<NUM>O<NUM> in a state of atomic F in the plasma reactor, and the reaction rate is faster, and therefore, the above formulas (<NUM>) and (<NUM>) are respectively represented by the following formulas (<NUM>) and (<NUM>):.

UO<NUM> (s) +6F (p) →UF<NUM> (g) +O<NUM> (g)     (<NUM>).

U<NUM>O<NUM> (s) +18F (p) →3UF<NUM> (g) +4O<NUM> (g)     (<NUM>).

The chemical reactions of PuO<NUM> in the spent fuel powder reacting in the plasma reactor are represented by the following formulas (<NUM>) to (<NUM>):.

PuO<NUM> (s) +2F<NUM> (g) →PuF<NUM> (s) +O<NUM> (g)     (<NUM>).

PuO<NUM> (s) +3F<NUM> (g) →PuF<NUM> (g) +O<NUM> (g)     (<NUM>).

PuF<NUM> (s) +F<NUM> (g) ↔PuF<NUM> (g)     (<NUM>).

Wherein, the PuF<NUM> generated according to the formula (<NUM>) then reacts with a sufficient F<NUM> to promote the reaction of the formula (<NUM>) to the right end, so that the PuF<NUM> is completely converted into the PuF<NUM>.

In practice, the fluorination medium reacts with the PuO<NUM> in the state of atomic F in the plasma reactor, and the reaction rate is faster, and therefore, the above formulas (<NUM>) to (<NUM>) are represented by the following formulas (<NUM>) to (<NUM>), respectively:.

PuO<NUM> (s) +4F (p) →PuF<NUM> (s) +O<NUM> (g)     (<NUM>).

PuO<NUM> (s) +6F (p) →PuF<NUM> (g) +O<NUM> (g)     (<NUM>).

PuF<NUM> (s) +2F (p) ↔PuF<NUM> (g)     (<NUM>).

The chemical reaction of the lanthanide in the spent fuel powder reacting in the plasma reactor is represented by the following chemical formula:.

2Ln<NUM>O<NUM> (s) +6F<NUM> (g) -4LnF<NUM> (s) +3O<NUM> (g)     (<NUM>).

Corresponding to the state of atomic F, the formula (<NUM>) is represented by the following formula (<NUM>) in practice:.

2Ln<NUM>O<NUM> (s) +12F (p) -4LnF<NUM> (s) +3O<NUM> (g)     (<NUM>).

The chemical reactions of the minor actinides in the spent fuel powder reacting in the plasma reactor are represented by the following chemical formulas:.

NpO<NUM> (s) +3F<NUM> (g) →NpF<NUM>+O<NUM> (g)     (<NUM>).

NpO<NUM> (s) +2F<NUM> (g) →NpF<NUM> (s) +O<NUM> (g)     (<NUM>).

NpF<NUM> (s) +F<NUM> (g) ↔ NpF<NUM> (g)     (<NUM>).

2Am<NUM>O<NUM> (s) +6F<NUM> (g) →4AmF<NUM> (s) +3O<NUM> (g)     (<NUM>).

<NUM><NUM>O<NUM> (s) +6F<NUM> (g) →4CmF<NUM> (s) +3O<NUM> (g)     (<NUM>).

Wherein, the NpF<NUM> generated according to the formula (<NUM>) then reacts with a sufficient F<NUM> to promote the reaction of the formula (<NUM>) to the right end, so that the NpF<NUM> is completely converted into the NpF<NUM>.

Corresponding to the state of atomic F, the formulas (<NUM>) to (<NUM>) are respectively represented by the following formulas (<NUM>) to (<NUM>), in practice:.

NpO<NUM> (s) +6F (p) →NpF<NUM>+O<NUM> (g)     (<NUM>).

NpO<NUM> (s) +4F (p) →NpF<NUM> (s) +O<NUM> (g)     (<NUM>).

NpF<NUM> (s) +2F (p) ↔ NpF<NUM> (g)     (<NUM>).

2Am<NUM>O<NUM> (s) +12F (p) →4AmF<NUM> (s) +3O<NUM> (g)     (<NUM>).

<NUM><NUM>O<NUM> (s) +12F (p) →4CmF<NUM> (s) +3O<NUM> (g)     (<NUM>).

In the above formulas, "(s)" and "(g)" respectively represent a solid state and a gas state, and "(p)" represents a plasma state; the solid-state fluorination product (such as LnF<NUM>, AmF<NUM>, or CmF<NUM>) is a non-volatile fluoride, and other gas-state fluorination product is a volatile fluoride.

S3, rapidly cooling the product at a cooling rate of <NUM><NUM> K/s to <NUM><NUM> K/s, to generate a gas-solid two-phase flow and a solid-state product.

Wherein, the product obtained in the step S2 may pass through a rapid cooling area, and the product may be cooled by being rapidly mixed with a cold air flow or by a gas dynamics nozzle in the rapid cooling area, with the cooling rate of <NUM><NUM> K/s to <NUM><NUM> K/s, and meanwhile, along with a flow velocity change, to realize a preliminarily separation of the gas phase from the solid phase, so as to obtain the gas-solid two-phase flow and the solid-state products.

The solid-state products mainly include the fluorination product of the minor actinides such as LnF<NUM>, AmF<NUM>, CmF<NUM> and so on. In the gas-solid two-phase flow, the gas phase products may include the gaseous fluorination products in the above formulas (<NUM>) to (<NUM>), such as UF<NUM> and PuF<NUM>; and the mixed solid-state products may include the fluorination products of minor actinides such as LnF<NUM>, AmF<NUM>, CmF<NUM> and so on.

S4, filtering the gas-solid two-phase flow to remove the solid-state products therein, followed by condensation, adsorption, desorption and distillation, to obtain a mixed gas containing UF<NUM> and PuF<NUM>.

In the step S4, the solid-state products (such as LnF<NUM>, AmF<NUM>, CmF<NUM> and so on) in the gas-solid two-phase flow are separated from the gas phase products by filtration. The filtration may adopt a fine-pore filter equipment, for example a sintered metal filter or a combination with other filter, etc..

The condensation, adsorption, desorption and distillation are conducted to the gas phase products. Wherein, the purpose of the condensation may be to realize a separation of the fluorination products of U, Pu, and Np from the other fluorination products, with a condensation temperature below <NUM>. The adsorption may adopt a fluoride adsorbent, such as NaF or MgF<NUM>, with an adsorption temperature of <NUM> to <NUM>. After the adsorption, the desorption may be realized when the temperature is up to <NUM> to <NUM>. In the above adsorption and desorption, the NpF<NUM> contained in the gas phase products may be adsorbed and removed, thus the mixed gas containing the UF<NUM> and the PuF<NUM> is obtained.

S5, performing a recovery process to the mixed gas to generate a corresponding oxide or metal.

As an option, the mixed gas may be recovered and processed to generate the oxide, with a method as following: the mixed gas of the UF<NUM> and the PuF<NUM> may be processed by dry or wet processing to obtain a mixed oxide of the UO<NUM> and the PuO<NUM>, and the HF.

Or alternatively, the UF<NUM> and the PuF<NUM> of the mixed gas may be separated; the UF<NUM> may be processed by dry or wet processing to obtain the UO<NUM> and the HF; and the PuF<NUM> may be processed by dry or wet processing to obtain the PuO<NUM> and the HF. The separation of the UF<NUM> and the PuF<NUM> can be conducted in a heating up manner, and the PuF<NUM> may volatilize so as to be separated from the UF<NUM>.

In the above options, the dry processing is preferred to be adopted: the UF<NUM> and the PuF<NUM> respectively react with vapor to directly generate UO<NUM>, HF and PuO<NUM>, HF respectively. Taking the UF<NUM> as an example, the reaction formula is as following:.

UF<NUM>(g)+H<NUM>(g)+H<NUM>O(g)→UO<NUM>+6HF(g).

As another option, the mixed gas may be recovered and processed to generate the metal, with a method as following: the mixed gas of the UF<NUM> and the PuF<NUM> may be processed by electromagnetic separation to obtain alloys of U and Pu, and F<NUM>.

Or alternatively, the UF<NUM> and the PuF<NUM> of the mixed gas may be separated; the UF<NUM> may be processed by electromagnetic separation to obtain the metal U and F<NUM>; and the PuF<NUM> may be processed by electromagnetic separation to obtain the metal Pu and F<NUM>.

The electromagnetic separation processing may be implemented using the prior art, and may refer to <CIT>, etc..

The oxide and the HF, the metal and the F<NUM> obtained by recovery may be respectively sent back to relevant sections for reuse. For example, the mixed oxide of the UO<NUM> and the PuO<NUM> recovered as described above, after supplied with the uranium (usually depleted uranium oxide), may pass through a nuclear fuel processing section to generate a mixed oxide fuel (MOX fuel). The HF and the F<NUM> can be used as a fluorine source to be returned to the plasma reactor.

Furthermore, the spent fuel dry reprocessing method of the present disclosure further may include the following steps:
S6, performing a recovery process to the solid-state products obtained from separation in the step S3 and the step S4.

The solid-state products obtained from separation in the step S3 and the step S4 mainly includes fluorination products of minor actinides such as LnF<NUM>, AmF<NUM>, CmF<NUM> and so on.

The recovery process may include a stabilization process or a consumption process to the solid-state products, products convenient for storage or disposal may be generated after the stabilization process is conducted; the consumption process is to apply the solid-state products to a fast neutron reactor and the like.

Claim 1:
A spent fuel dry reprocessing method based on plasma, wherein the method comprises steps of:
S1, processing a spent fuel after removal of a cladding thereof into a spent fuel powder, and sending the spent fuel powder into a plasma reactor;
S2, fully mixing and reacting the spent fuel powder with a plasma containing F atoms in the plasma reactor, to generate a product including volatile fluorides and non-volatile fluorides;
wherein in the step S2, the plasma containing F atoms is generated by conversion of a fluorination medium; wherein the fluorination medium comprises one or more gases of CF<NUM>, NF<NUM>, F<NUM>, HF, BrF<NUM>, BrF<NUM>, ClF<NUM>, and SF<NUM>;
S3, rapidly cooling the product at a cooling rate of <NUM><NUM> K/s to <NUM><NUM> K/s, to generate a gas-solid two-phase flow and a solid-state product;
S4, filtering the gas-solid two-phase flow to remove a solid-state product therein, then carrying out condensation, adsorption, desorption and distillation, to obtain a mixed gas containing UF<NUM> and PuF<NUM>;
S5, performing a recovery process to the mixed gas to generate a corresponding oxide or metal.