**5.2. Combined work of several fuel types**

Reactor campaign characteristics with 0.64 % initial contents of mix 233U, 235U, 239Pu and 241Pu in uranium-thorium fuel with 238U – 75 % and 9\*1013 sm-2s-1 neutron flux are shown at string 7 of application's table 1.

Contents of fission materials is corresponding to its equilibrium contents. Reproduction of fission nuclides is close to unity. Portions of energy release from fission components are:

$$\text{\$^{233}\text{U}\$ - \$39.76 \text{ \%}\$; \$^{235}\text{U} - 4.26 \text{ \%}\$; \$^{239}\text{Pu -- 42.21 \text{\%}\$; \$^{241}\text{Pu -- 13.76 \text{\%}.}$$

That means in fission components from raw 238U occurs ~56 % fissions, and in fission components from raw 232Th ~44 % fissions.

Positive reactivity at the campaign end in this variant is less than with equilibrium fuel on the base of 232Th and 233U. Campaign with the same fuel, but 5\*1013 sm-2s-1 flux is presented at string 8 of application's table 1. Reached burn-ups in the both cases are close to each other and it is says about decreasing of negative role of thorium nuclides component fuel.

## **5.3. Dynamic loading regime use**

Most part of absorptions in fission products is in 135Хе. Absorption in 135Хе in high neutron flux (more than 5\*1013 sm-2s-1) is close to portion of 135I formation from fission products. Portion of 135I formation is ~6 %, and total absorption in all fission products is slightly above 10 %.

Peculiarity of 135Хе is its formation from 135I with half-life 6 hours and decay with 9 hours half-life.

190 Nuclear Power – Practical Aspects

requirement at specified power output. It is especially valuable for the case when during

Campaign characteristics calculations for above mentioned fuel types with neutron flux in diapason from 2.5\*1013 to 2.0\*1014 sm-2s-1 and campaign durability from 5000 to 40000 hour were carried out. At flux from 2.5\*1013 to 1014 sm-2s-1 change in final mass of fission materials, control rods absorption and reactivity at the end of campaign are slight for all fuel kinds.

Final reactivity for thorium containing fuel under the neutron fluxes more than 1014 sm-2sec-1 is fast becoming less than zero [7]. Flux rising influence is could be watched at string 5 and 6 of application's table 1, where the one fuel type is used, but it has the different neutrons fluxes in 9\*1013 sm-2sec-1 and 5\*1013 sm-2sec-1. At larger flux reactivity is less than zero after 16500 hours, and at smaller flux it is near the 0.03 after 39000 hours. By estimation it

These effects are explained by neutron absorption in 233Ра, which has comparatively high half-time period (27.4 days) and large absorption cross-section (66 barn). However, not everything is so simple. The reactivity in campaign with non-equilibrium fuel (figure 4) and

Reactor campaign characteristics with 0.64 % initial contents of mix 233U, 235U, 239Pu and 241Pu in uranium-thorium fuel with 238U – 75 % and 9\*1013 sm-2s-1 neutron flux are shown at string

Contents of fission materials is corresponding to its equilibrium contents. Reproduction of fission nuclides is close to unity. Portions of energy release from fission components are:

<sup>233</sup> <sup>235</sup> <sup>239</sup> <sup>241</sup> U 39.76 %; U – 4.26 %; Pu – 42.21 %; Pu – 13.76 %.

That means in fission components from raw 238U occurs ~56 % fissions, and in fission

Positive reactivity at the campaign end in this variant is less than with equilibrium fuel on the base of 232Th and 233U. Campaign with the same fuel, but 5\*1013 sm-2s-1 flux is presented at string 8 of application's table 1. Reached burn-ups in the both cases are close to each other

Most part of absorptions in fission products is in 135Хе. Absorption in 135Хе in high neutron flux (more than 5\*1013 sm-2s-1) is close to portion of 135I formation from fission products. Portion of 135I formation is ~6 %, and total absorption in all fission products is slightly above

and it is says about decreasing of negative role of thorium nuclides component fuel.

Change of its parameters for fuel with raw 238U at neutron flux 2\*1014 sm-2s-1 is slight.

campaign ratio of neutron flux to power is almost constant.

becomes zero after 50000 hours of work reactor.

9\*1013 sm-2sec-1 flux remains high during 39000 hours.

**5.2. Combined work of several fuel types** 

components from raw 232Th ~44 % fissions.

**5.3. Dynamic loading regime use** 

10 %.

7 of application's table 1.

In dynamic loading regime [7] fuel works in reactor during time close to 135I half-life, formation of 135Хе and its neutron absorption is minimal. During fuel exposition out of core <sup>135</sup>Хе is decaying.

Dependence of neutron absorption portion in 135Хе, which formed during fuel work on work duration and neutron flux and portion of remaining 135Хе after fuel exposition out of core is shown on figure 5.

It can be seen that even in maximal neutron flux (1014 sm-2s-1) neutron absorption in 135Хе is 30 % at work time about 8-10 hours.

We can note that portion of remaining 135Хе after fuel exposition practically does not depend on loading characteristics of fuel in reactor.

**Figure 5.** Dependence of ratio neutron absorption in 135Хе to 135I formation on fuel work time and neutron flux.

Data from charts on figure 5 can be used for estimation of 135Хе neutron absorption at dynamic loading regime. For example, in neutron flux 5\*1013sm-2s-1 with 5 work hours and exposition 45 hours neutron absorption in 135Хе is less than 30% of fission product 135I formation and is about ~1.9 %. Saved 4 % of neutrons can be used for construction material absorption and leakage.

For effective usage of dynamic loading duration of working regime can be in the region of 5 to 10 hours and exposition time – 35-50 hours. Increasing exposition time more than 60 hours is unreasonable. So fuel mass can be 3.5 – 5.0 times larger.

Dynamic loading regime for traditional fuel types with rigid placed fuel assemblies in a core is sufficiently complicated. In general it can be made in reactors, which have fuel assemblies' replacements without reactor shut down, such as CANDU or RBMK-1000. But large amount of replacements, needed for this regime realization, decreases durability of these fuel assemblies and replacement mechanism.

Thermal Reactors with High Reproduction of Fission Materials 193



0

0,02

0,04

**K-1**

0,06

0,08

0,1

**Figure 6.** Detailed campaign characteristics with initial fuel 235U + 238U and multiplication factor of

0 5000 10000 15000 20000 25000 **time, h**

U235 Pu239 Pu241 W K-1 SZ K-1 unwrap K-1 oper

The string 11 of Attachment Table 1 shows characteristics of detailed campaign with fuel 239Pu + 239Pu + 238U and multiplication factor of compact campaign with eight fuel replacements. Neutron loss in construction materials and leakage is 1.7% in this campaign.

This campaign is identical to the campaign, which is presented at the string 2 of Attachment Table 1 and figure 3, by the fuel type and composition. In spite of neutron loss presence

Comparison of campaign characteristics with natural uranium and different neutrons loss in the construction materials and leakage is presented at the strings 10, 12, 13, and 14 of

Characteristics of reactor campaign fueled with natural uranium and neutron loss 5.2% is presented at the string 14 of Attachment Table 1, and at the string 12 of Attachment Table 1–

241Pu production in reactor campaign with 5.2 % neutron loss is only reaching stationary level. Stationary level of 241Pu production in the campaign with 1.7 % neutron loss is

1 Neutron loss, % 0 1.7 2.8 5.2 2 The duration of the campaign, hour 34 000 29 000 24 000 19 000 3 Burn-up, % 3,92 3,38 2,84 2,41 **Table 2.** Comparison of campaign characteristics with natural uranium and different neutron loss in

compact campaign with four fuel replacements (string 9 of Attachment Table 1).

larger campaign duration and burn-up is obtained in the campaign.

Attachment Table 1. Selected data from this table is presented in Table 2.

**6.2. Reactors with non-zero loss of neutrons** 

campaign with the same fuel and neutron loss 1.7%.

zone superposition regime.

0

0,2

0,4

0,6

0,8

**C FM**

1

1,2

1,4

reached.

Dynamic loading regime is possible in molted salt reactors [8] and in reactors with spherical fuel circulation in heat-exchange loop [9]. Work [10] shows that this regime can significantly simplify a molted salt reactor technology of fuel purification from fission products and actinides.
