*8.4.1. Uranium-plutonium fuel*

The best closed fuel cycle characteristics are achieved with use of equilibrium contents of fission materials. But the ideal equilibrium uranium-plutonium cycle has very small contents of fission materials so the neutron leakage plays more important role than in CANDU with raw uranium.

Fission materials contents in equilibrium uranium-plutonium fuel equals 0.34 % in condition of resonant neutrons absorption absence in 238U (in CANDU its contents is more than twice larger – 0.712 %).

Solution of the task is possible in two ways – 1) by increasing of initial contents of 235U by adding it, 2) increasing of fission materials contents by increasing resonant absorption in 238U.

The string 19 of Attachment Table 1shows variant with addition of 0.35 % 235U from mass of 238U, that equals equilibrium contents of 239Pu and 241Pu. This fuel can be made by plutonium isotopes, which are extracted from spent fuel, addition to a mix of raw uranium (half of 238U mass) and 238U from spent fuel.

The string 20 of Attachment Table 1 shows campaign characteristics with initial fuel containing equilibrium contents of 239Pu and 241Pu and addition of 235U, which contents is much less than in previous fuel. Resonant neutron absorption in 238U is 1.2 times higher. Production of 233U is lead. Equilibrium contents of 239Pu and 241Pu is also 1.2 times greater.

Burn-up of 3.49 % and raw uranium usage 12.12 % is achieved.

String 18 of Table 1 of Attachment shows campaign with this technology. Strings 21 and 22 show campaign with no 233U production. Maximal burn-up 3.51 % and maximal raw uranium usage 14.28 % is achieved.

## *8.4.2. Uranium-thorium fuel*

200 Nuclear Power – Practical Aspects

**8.3. Open fuel cycle** 

**8.4. Closed fuel cycles.** 

*8.4.1. Uranium-plutonium fuel* 

CANDU with raw uranium.

larger – 0.712 %).

fuel.

fuel cycle.

fuel element cover under the burn-up till 40-59 МW\*day/kg. Such fuel has being used in

Open fuel cycle can be realized by the initial fuel, which consists of 235U and 238U. This fuel can contain natural uranium, as in CANDU reactors, enriched uranium, as in the majority of modern reactors, and, as it presented on the calculation above, from the 235U impoverished. Fission materials in all of these cases are made from natural uranium passing the chains, which is bounded with recycling of spent fuel. Let's assume that technology of 233U generation is not bounded with recycling of spent fuel and might take the part in opened

The final product of open fuel cycle can be the perfect base or composite part of closed fuel

Open cycles, which are presented in strings 1, 9, 10, 12 -15, 17, 23 and 24 of Attachment Table 1, are described above. Maximum portion of natural uranium usage on it is reached under the conduction of superposition mode areas with 233U generation. Quite high part of using is reached in the superposition mode under the decreased neutrons losses in construction materials and for leakage. In these cases fission isotopes of plutonium are in equilibrium at campaign end. Maximal reached part of natural uranium using in presented cycles forms 5.27%. This amount in cycles with generation of 233U can be increased at the expense of generation mode optimization. Optimization is not appeared by paramount task at this work. Thereby, there are different ways for reaching of high natural uranium usage in open fuel

In this work it is considered that all initial fuel products use technology of spent fuel recycling. In closed cycles it is possible to use raw uranium based fuel or raw thorium based

The best closed fuel cycle characteristics are achieved with use of equilibrium contents of fission materials. But the ideal equilibrium uranium-plutonium cycle has very small contents of fission materials so the neutron leakage plays more important role than in

Fission materials contents in equilibrium uranium-plutonium fuel equals 0.34 % in condition of resonant neutrons absorption absence in 238U (in CANDU its contents is more than twice

Solution of the task is possible in two ways – 1) by increasing of initial contents of 235U by adding it, 2) increasing of fission materials contents by increasing resonant absorption in 238U.

calculations of reactor models, which was presented in the table 1.

cycle. Especially, if fission isotopes of plutonium come to equilibrium.

cycles and reception of closed fuel cycles fission components on its.

Equilibrium cycle with uranium-thorium fuel even without resonant neutron absorption in thorium has high fission materials contents, which mean possibility of high burn-up achieving in detailed campaign. Features of uranium-thorium campaign are high neutron absorption in 233Ра and its long half-life, which are used in technology of 233U production by reactivity margin decreasing.

Detailed campaign with neutron flux about 1014 sm-2s-1 becomes very short even in case of low neutron losses which equals 1.7 %. The campaign with twice lower neutron flux is possible at neutron losses of 5.0 %.

Figure 9 shows detailed campaign characteristics with equilibrium uranium-thorium fuel with 233U production at neutron flux 6\*1014 sm-2s-1 and neutron losses 5.0 %.

**Figure 9.** Detailed campaign characteristics with equilibrium uranium-thorium fuel and 233U production. Neutron flux is 6\*1014 sm-2s-1, neutron loss – 5.0 % (string 25 of Table 1 of Attachment).

High stability of reactor power is obtained in compact campaign. Reactivity margin decrease in detailed campaign is observed in first 5000 hours. During all time of detailed campaign reactivity stays positive. Reactor work prolongation over 24000 hours is possible. At the shown campaign burn-up of 4.35% is reached.

Thermal Reactors with High Reproduction of Fission Materials 203

1 – case of fuel assembly, 2 – gaseous gap, 3 – screen, 4 – coolant, 5 – fuel rod, 6 – beryllium insert for a) and gaseous

Neutron absorptions (black columns) neutron fissions (blue columns) in different fuel nuclides are shown on diagrams. Two types are used 235U – natural raw uranium signed as U235N, and formed during transformations of thorium chain (232Th-233Pa-233U-234U-235U),

Fission nuclides have columns for difference between secondary neutrons and total neutron absorptions (red columns). Difference for raw 232Th and 238U between number of secondary neutrons and number of total absorptions with fission are indicated in yellow columns. Columns 1 and 2 indicate neutron absorptions in construction materials and fission

In left part of each column lay data for reactor with liquid metal coolant, in right part – data

Total height of red and yellow columns must be equal to total height of black columns for nuclides 232Th, 238U, 234U, 240Pu and columns for neutron absorptions in construction

If neutron loss in construction materials and leakage are less, than more neutrons can be

For the compared variants fission activity in raw nuclides and ratio of absorptions and fissions in 241Pu are better. In result total neutron absorption in fission products (8 %) for heavy water coolant reactor is less than total neutron absorption in fission products (13 %) for liquid metal coolant reactor at the same neutron losses for construction materials and leakage. So reactor with liquid metal coolant has campaign with burn-up 25.5 MW\*day/kg when heavy water coolant reactor has campaign with burn-up 11,3 MW\*day/kg. Liquid

absorbed in fission products, which number increases during campaign.

metal cooled reactor has higher raw uranium usage in open cycle campaign.

**Figure 10.** Fuel assemblies with water (a) and liquid metal (b) coolant.

cavity for b).

signed as U235S.

products correspondingly.

for reactor with water coolant/

materials and fission products.

The difference of the campaign from uranium-plutonium campaign is possibility of significant amount increase of fission materials to the end of the campaign. Reproduction coefficient of the campaign, which is shown at figure 20, is more than unity by 11 %.
