**7.2. Fuel is natural uranium**

Let's examine how neutron loss in control elements used for generation of 235U in reactor with initial fuel 235U + 238U influence on campaign characteristics (figures 10, 12 - 14). Characteristics of detailed campaign with 1014 sm-2sec-1 neutron flux, 233U production and 1,7 % neutron loss are shown at the string 15 of Attachment Table 1.

Thermal Reactors with High Reproduction of Fission Materials 197

6. Reactor by the i.5, with fuel assembly shown at the figure 19 (on the right) with

7. Reactor by the i.6, which differs by using of heavy water instead of graphite layer in

№ Parameter\variant 1 2 3 4 5 6 7 1 Leakage 1.7904 1.7789 1.1121 1.1354 0.915 1.041 1.182 2 D 0,163 0.172 0.391 0.221 0.147 0.213 0.254 3 Be - 0,513 - 0,531

5 O 0,443 0,441 0,489 0,456 0,396 0,399 0,410

7 90Zr 0.281 0.283 0.281 0.236 0.240 0.239

9 120Sn 0.0298 0.0287 0.0319 0.023 0.024 0.0254 10 Bi 0.079 0.078 0.0772 11 ∑ 5,061 2,70 2,30 2,481 2,081 1,809 1,660 12 K eff 1,1171 1.1473 1.15489 1.15093 1.1265 1.13233 1.13428 13 RC 0,808 0,804 0,80 0.802 0,871 0,855 0,853 Reflector D2O 21 D2O 62 D2O+C D2O 62 Cell 37 fuel rods 30 fuel rods + Ве Fuel UO2 0.8\*U + 0.2\*Bi

**Table 3.** Neutron loss (string 1-11, %), multiplication factor and reproduction coefficient (string 12, 13,

**Figure 7.** Fuel assembly of CANDU reactor (on the left) and its modification with beryllium insertion (on the right). 1 – fuel assembly casing; 2 – fuel element shell; 3 – fuel rods; 4 – coolant; 5 – beryllium insertion.

relative units) and fuel types and reflector types in variants of CANDU reactor models.

4 C 0.356 0,285 0,327

beryllium insertion.

6 Zr 2,54

8 Sn 0,125

reflector.

Maximum fuel burn-up (5.27%) is reached in this campaign in comparison with previous campaigns; duration of detailed campaign is 34000 hours. Reactivity change during work is close to optimum results (K-1 from 1 % up to 3.2 %). Reactor power is stable. Reactor power change in constant neutron flux is not exceeding 2.5% from the average. 233U generation increase in the campaign with this fuel type is possible at the expense of operating reactivity pike lowering.

Enough good indexes by this technology can be received into the reactor with 5.2% neutron loss. Description of such campaign is presented in string 24 of Attachment Table 1. Indexes of such campaign are high enough.
