**6.1. Ideal reactor with zero loss of neutrons**

Modern power thermal neutron reactors widely use campaigns with multiple fuel reloading [11, 12]. During each reloading fuel with maximal burn-up is moved out core, fuel with different burn-up is rearranged and fresh fuel is loaded. Necessity of reloading is caused by fission materials concentration decrease during irradiation.

It is shown in presented above data that high concentration of fission materials is stabilized in high fission materials reproduction reactors during campaign burn-up. Burn-up increase in campaign of such reactors is purpose of multiple fuel replacements with entering regime of negative reactivity in some part of core with maximal burn-up.

Terms of detailed campaign and compact campaign are introduced here for better understanding. Detailed campaign is theoretically calculated campaign with continuous reactor work with use of negative reactivity. Compact campaign contains the portions of various fuels with different burn-up level. Durability of compact campaign is equal durability of detailed campaign divided on number of fuel zones with different campaign burn-up. For work regime with fuel replacements is used term "zone superposition".

It can be noted that work in compact regime is possible only if neutron absorption in control elements in detailed campaign is more than zero.

Fuel with nuclides 235U and 238U in zone superposition regime is considered. Figure 6 shows detailed campaign characteristics of initial fuel 235U (0.47 %) + 238U in neutron flux 9\*1013 sm-2s-1 and multiplication factor of compact campaign with four fuel replacements. In superposition regime fuel works in reactor for 25000 hours. Raw uranium usage in campaign increases up to 3.81%. Multiplication coefficient at the end of detailed campaign becomes lower than unity by 0.04.

Characteristics of campaign with the same fuel type and different initial contents of fission materials in the beginning of campaign are presented at the string 10 of Attachment Table 1. Here it is corresponds to natural uranium fission materials contents. Larger burn-up and raw uranium usage in open fuel cycle is obtained in this variant.

**Figure 6.** Detailed campaign characteristics with initial fuel 235U + 238U and multiplication factor of compact campaign with four fuel replacements (string 9 of Attachment Table 1).

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

192 Nuclear Power – Practical Aspects

actinides.

assemblies and replacement mechanism.

**6.1. Ideal reactor with zero loss of neutrons** 

elements in detailed campaign is more than zero.

becomes lower than unity by 0.04.

fission materials concentration decrease during irradiation.

of negative reactivity in some part of core with maximal burn-up.

raw uranium usage in open fuel cycle is obtained in this variant.

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

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

**6. Joint work of zones with different campaign moments in the same core** 

Modern power thermal neutron reactors widely use campaigns with multiple fuel reloading [11, 12]. During each reloading fuel with maximal burn-up is moved out core, fuel with different burn-up is rearranged and fresh fuel is loaded. Necessity of reloading is caused by

It is shown in presented above data that high concentration of fission materials is stabilized in high fission materials reproduction reactors during campaign burn-up. Burn-up increase in campaign of such reactors is purpose of multiple fuel replacements with entering regime

Terms of detailed campaign and compact campaign are introduced here for better understanding. Detailed campaign is theoretically calculated campaign with continuous reactor work with use of negative reactivity. Compact campaign contains the portions of various fuels with different burn-up level. Durability of compact campaign is equal durability of detailed campaign divided on number of fuel zones with different campaign

It can be noted that work in compact regime is possible only if neutron absorption in control

Fuel with nuclides 235U and 238U in zone superposition regime is considered. Figure 6 shows detailed campaign characteristics of initial fuel 235U (0.47 %) + 238U in neutron flux 9\*1013 sm-2s-1 and multiplication factor of compact campaign with four fuel replacements. In superposition regime fuel works in reactor for 25000 hours. Raw uranium usage in campaign increases up to 3.81%. Multiplication coefficient at the end of detailed campaign

Characteristics of campaign with the same fuel type and different initial contents of fission materials in the beginning of campaign are presented at the string 10 of Attachment Table 1. Here it is corresponds to natural uranium fission materials contents. Larger burn-up and

burn-up. For work regime with fuel replacements is used term "zone superposition".

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

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


**Table 2.** Comparison of campaign characteristics with natural uranium and different neutron loss in zone superposition regime.

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– campaign with the same fuel and neutron loss 1.7%.

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 reached.

For comparison, characteristics of reactor campaign with the same fuel and 5.2 % neutron loss but without zone superposition using are presented at the string 23 of Attachment Table 1. Such campaign has the worst characteristics among others by campaign duration, fuel burn-up and raw uranium usage, and Rsz value – average neutron loss in control elements during campaign.

Thermal Reactors with High Reproduction of Fission Materials 195

(10)

(11)

components with same influence to reactivity. For thorium this parameter is 3.6 hours, and

This suggestion realization does not mean that all control elements must produce fission components. There is no need to have it in safety system with total neutron absorption at

At initial development stage it is possible to divide two functions of control system: traditional, for fast regulation of reactor power with efficiency of 1 β and slow regulation system with fission materials production and efficiency of maximal reactivity in campaign.

Part of neutrons involved in fission materials production is linked with produced fission

n Q \* g \* ;

With core power 1000 MW and 1 % of involved in fission materials production neutrons amount of produced fission material is equal to 1.2\*10-4 g per second. For year mass is

It is possible to use value of fuel burn-up and number of fuel reloading during the campaign. Insertion of produced fission materials is made with each new fuel portion. In this case the

> n1 m \* / j;

Calculation of campaign characteristics with fission materials production by use of redundant reactivity compensation is made by iterative method. Adsorption of redundant neutrons at this process leads to additional reactivity insertion. At the good realization of campaign this additional reactivity is added to areas of detailed campaign with negative reactivity, which supplies prolongation of initial fuel work duration. Iterations in carried out calculations are not always optimal. Measures of optimal iteration are values of Rmin and R

n1 – amount of nuclei of produced fission materials, which are inserted with each fuel

g – link coefficient between emitted power and fissions number (3.1\*1010), J-1; ξ – part of neutrons, which are involved into fission materials production.

number of inserted nuclei of produced fission materials is described by formula:

m – amount of fuel nuclei, including raw and fission components;

for uranium – 6.33 minutes. Difference is 36 times.

material amount and energy emitted in core by formula:

n – amount of produced fission materials, nuclei;

Q – fission energy emitted in core, J;

χ – fuel burn-up during campaign;

j – number of fuel reloads during campaign.

SZ, which are shown in the Attachment Table 1.

zero level in any case of campaign.

Where:

~3800 g.

Where:

reload;

In reality campaign at string 23 of Attachment Table 1should be finished at least at 1000 hours earlier on condition of sufficient positive reactivity. Characteristics of this campaign are quite close to characteristics of CANDU reactor campaign.

It should be noted, that calculations shown in the present work are made for point model of reactor. In cases, when fission materials contents change at campaigns is minimal, real reactor campaign characteristics matches this calculation in great extent.
