**5. About possibility of practical campaign realization with high reproduction**

The basic difference of real reactors is presence of neutron absorption in construction materials and neutron leakage from reactor. These factors can be researched in the described models without reference to the reactor design by additional term insertion for these neutron losses.

Neutron flux values used in previous calculations are not always applicable. Possibility of neutron flux change and its influence on campaign characteristics must be examined.

Arrangements for reactor campaign improvements and its effects should be also considered. Without reference to a reactor design following arrangements can be considered:


## **5.1. Neutron flux influence on campaign characteristics**

Campaign characteristics calculations, which were presented above, use ~9\*1013 sm-2sec-1 neutron flux. Flux increase allows improving economical issues – decrease of fuel requirement at specified power output. It is especially valuable for the case when during campaign ratio of neutron flux to power is almost constant.

Thermal Reactors with High Reproduction of Fission Materials 191

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

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

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

It can be seen that even in maximal neutron flux (1014 sm-2s-1) neutron absorption in 135Хе is

We can note that portion of remaining 135Хе after fuel exposition practically does not depend

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

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

0 10 20 30 40 50 60 70 80 90 100 **time, h**

Xe in stop Xe burn-up 10^12 Xe burn-up 10^13 Xe burn-up 10^14

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.

half-life.

<sup>135</sup>Хе is decaying.

shown on figure 5.

neutron flux.

absorption and leakage.

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

**Quantity .**

30 % at work time about 8-10 hours.

on loading characteristics of fuel in reactor.

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

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 becomes zero after 50000 hours of work reactor.

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