∙a mockup FA is inserted to check the channel for passability;
∙a fresh FA is loaded into the channel;
∙the channel is pressurized, pressure in the machine drops, and the machine is disconnected from the channel.
The reactors permitting refueling without power reduction and reactor depressurization, such as heavy-water CANDUs and water-graphite RBMKs, are the greatest proliferation hazards.
Discharged fuel assemblies are kept for 3 to 10 years in the on-site water pool to reduce their activity and residual heat.
The irradiated fuel pools are equipped with:
∙a water cooling system;
∙an ion exchange facility to remove radioactive substances and to clean the pool water;
∙a ventilation system to pass air through filters and to vent waste gas to the atmosphere.
Transportation of irradiated nuclear fuel
Irradiated fuel assemblies are carried in special casks by rail, motor and water transport. Such shipping casks weigh about 80–110 t, of which fuel accounts for merely 2–5 %. The rest is contributed by safety features.
A shipping cask looks as follows:
1.It is a large hollow thick-walled cylinder (with diameter of 2 m; height of 4–6 m, and wall thickness of 40 cm). It may be oriented either horizontally or vertically, and its structural material is steel, cast iron, or concrete.
2.Inside, it is lined with stainless steel for better corrosion resistance, which may have interlayers of neutron moderating material.
3.Its outer surface may be ribbed to increase the heat transfer surface area.
4.It contains metal racks for fuel assemblies. During transportation it is filled with coolant which will remove heat by natural convection or forced circulation.
5.Casks are made tight by strongly sealed lids.
6.Casks are fitted with an interior system for monitoring activity, heat release, temperature, and pressure and with an emergency decontamination system.
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Technologies for reprocessing of irradiated nuclear fuel
The purposes of fuel reprocessing are:
1)to separate plutonium and uranium for recycling;
2)to separate fission products (FP) and transuranic elements as waste. Irradiated nuclear fuel is generated in the world at a rate of 7000 t per
year, while the existing facilities can reprocess annually about 5100 t of INF.
INF reprocessing methods
1.Aqueous (“wet”) methods:
∙extraction processes, with uranium and plutonium extracted from solutions by organic compounds;
∙precipitation processes, with poorly soluble uranium and plutonium compounds precipitating from solutions.
2.Non-aqueous (“dry”) methods:
∙pyrochemical processes, e.g., fluoride gas technology, with its principle lying in different volatility and sorption capacity of uranium and plutonium fluorides and FP;
∙pyrometallurgical processes, e.g., electrolytic refining, based on difference in transport of uranium, plutonium and FP in molten metals and salts.
Irradiated nuclear fuel is reprocessed using one of the above processes at a special radiochemical facility. Aqueous extraction processes are the best developed and proven technologies.
Main stages in the aqueous extraction technology (PUREX)
Disassembly and cutting.
1.Fuel assemblies are taken apart by cutting off end-pieces, cutting shrouds by disk saws, disassembling fuel lattices.
2.Fuel rods are cut, e.g., by guillotine shears or lasers in inert environment (nitrogen or argon).
Pre-oxidation (voloxidation).
Pre-oxidation of irradiated fuel takes place in oxygen at an elevated temperature. Uranium dioxide, UO2, transforms into uranium octaoxide,
U3O8:
3 UO2 + O2 → U3O8.
This gives rise to the following effects:
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Fuel decompaction. The density of UO2 is tangibly different from that of U3O8: γ(UO2) = 11 g/cm3, γ(U3O8) = 8.3 g/cm3. Fuel increases its volume
by 30 %, becoming porous and loose; Change of crystalline lattice;
Intensive release of fission and tritium gas.
INF dissolution. Fuel is dissolved in boiling nitric acid, HNO3:
UO2 + HNO3 → UO2 (NO3)2 + NOX + H2O.
Zirconium and steel claddings will not dissolve. They are removed from solution and are treated as solid radioactive waste.
Preparation of INF solution for extraction proceeds as follows.
1.Clarification of solution:
∙filtration through cermet or porous polypropylene;
∙centrifugation with addition of coagulants.
2.Removal of volatile FP and fission gas from solutions:
∙air bubbling to remove iodine present as I– , IO3, which is then captured at silver nitrate (AgNO3) filters:
6 AgNO3 + 3 I2 + 3 H2O → 5 AgI + AgIO3 + 6 HNO3;
∙ozone bubbling to remove ruthenium, Ru4+:
Ru4+ + 2 O3 + 2 H2O → RuO4 + 2 O2 + 2 H2.
∙volatile ruthenium oxide (RuO4) is removed from gas in a reaction with NaOH;
∙removal of inert Kr and Xe by bubbling with gas sorption at zeolite or activated carbon at low temperatures.
Extraction is breakdown of a substance into two immiscible fractions: light organic fraction (TBP + diluent) and heavy aqueous fraction (acid solution of INF). A significant drawback of extraction is radiolysis of organic agents, i.e., decomposition under irradiation.
Separation of plutonium from uranium. Plutonium found in an INF
solution can be 3-, 4- or 6- valent. Separation of uranium and plutonium relies on the fact that U6+, Pu6+ and Pu4+ are easily dissolved both in the aqueous phase and in the organic phase, while Pu3+ is only slightly dissolved in the organic phase. When plutonium is restored to a condition in which it becomes 3-valent, it will completely pass into the aqueous
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solution and will be removed from the process, whereas uranium will remain in the organic phase.
In separation of plutonium from uranium, hexavalent plutonium is first restored to a condition of tetravalency, and then to trivalency. Plutonium is converted from the hexavalent to tetravalent condition in a reaction with potassium nitrite, KNO2:
PuO2(NO3)2 + KNO2 → Pu(NO3)4 + KNO3 ,
i.e. Pu6+ is turned to Pu4+, which is then restored to Pu3+ in a reaction with Fe2+ compounds:
Pu4+ + Fe2+ → Pu3+ + Fe3+.
Iron gives up one valent electron to plutonium in a reaction with U4+:
Pu4+ + U4++ 2 H2O → Pu3+ + UO22+ + 4 H;
through electrochemical reduction of plutonium. An electric current is passed through the solution:
Pu4+ + e– → Pu3+;
UO22+ + 2e– + 4 H+ → U4+ + 2 H2O; and U4+ will act as another reducing agent.
When the organic phase is washed with a reducing solution, Pu3+ will go into the aqueous phase, while U will remain in the organic phase. In reextraction of the organic phase with a weak solution of nitric acid, uranium will pass into the aqueous phase (re-extract).
Stages of one extraction–re-extraction cycle
1.Fuel dissolution in nitric acid.
2.Extraction of uranium and plutonium compounds from solution using TBP. Uranium and plutonium pass into the organic phase.
3.Re-extraction with a reducing solution. Hexaand tetravalent plutonium turns into its trivalent variety and goes into the aqueous phase.
4.Uranium re-extraction from the organic phase with diluted nitric acid. Uranium passes into the aqueous phase.
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Control for non-proliferation of nuclear materials at a reprocessing facility
A reprocessing facility is one of the most sensitive components of the nuclear fuel cycle in terms of proliferation resistance. The main problem here is control over plutonium, which is made even more complicated by a number of factors:
Large plutonium quantities. The existing reprocessing facilities are capable of treating about 1000 t of INF per year. One tonne of irradiated fuel from light water reactors contains 6–7 kg of p lutonium, which means that 6-7 t of plutonium can be put through the facility in one year.
High accuracy requirements. The Significant Quantity of plutonium, SQ(Pu), adopted by the IAEA is 8 kg. Suppose plutonium needs to be monitored at the reprocessing facility with an accuracy of 1 kg. With 7 t of plutonium passing though the reprocessing facility every year, the monitoring accuracy should be in the order of ~ 10–2 %. The realistic accuracy of Pu mass measurements is 0.1–1 %. The pe rmissible imbalance is about 0.1 %, which is close to the limit of measuring capabilities. As a result, inventory has to be taken several times a year, with the facility divided into material balance areas for location of a potential plutonium diversion point.
Plutonium is found in different phase states (solid, liquid, organic); it may be part of compounds with different valences, and is involved in periodic, continuous or semi-continuous processes.
The following factors are considered in assessing the extent to which plutonium compounds may be attractive for theft:
Density factor f1 defines the Pu content in its compounds. Factor f1 is treated as a function of NM volume containing 1 g of Pu. For Pu metal, this factor is assumed to be f1 = 1. The density of Pu metal is 19.8 g/cm3; i.e. its specific volume is I ~ 5×10–5 l/g. The datum point of function f1(Vуд) is 1, given Vsp=5×10–5 l/g. Other Pu–bearing materials have larger specif ic volumes and, hence, smaller values of factor f1.
Time factor f2 accounts for the time it takes for a group of specialists armed with modern equipment to convert a Pu–bearing material into the charge of a nuclear explosive device. It is assumed that plutonium metal can be turned into such a charge in a week’s time, i.e. the time factor for Pu metal is f2 = 1 with t = 7 days. For other Pu–bearing materials, the char ge production time is longer and the f2 value is smaller.
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