**7.1. Task formulation**

Previous chapter material analysis shows that campaign characteristics improving, including the reproduction coefficient, with the same fuel type is obtained by decrease of neutron absorption in control elements. Improvement not always can be made by neutron flux change or superposition regime implementation. It is interesting to replace useless neutron absorption in control elements (cadmium, boron, gadolinium etc.) by absorption in raw fuel materials (238U and 232Th). First activation products of raw nuclides must be deleted from the core. This technology should not permit fission materials formation in chain of product transformation in the core. Activation chains of 238U and 232Th are:

$$^{238}\text{U} + \text{n} \rightarrow ^{239}\text{U} \text{ (1412 s)} \rightarrow ^{239}\text{Np} \text{ (2.33 d)} \rightarrow ^{239}\text{Pu} \text{;} \tag{8}$$

$$\text{ }^{222}\text{Th} + \text{n} \rightarrow \text{ }^{233}\text{Th} \text{ (1325 s)} \rightarrow \text{ }^{233}\text{Pa} \text{ (27.4 d)} \rightarrow \text{ }^{233}\text{U};\tag{9}$$

Uranium chain has the minor time reserve, but it is big enough.

Any technology always supplies some part of final fission product into the absorber placing area. It is simple to estimate the negative effect of such penetration. It is defined by purification time outside core from primary activation products (239Np and 233Pa) and degree of fission materials penetration into core loop.

It can be noticed that equal effect from absorption in raw and fission nuclides of uranium chain is obtained at raw to fission nuclides contents ratio 530, and for thorium chain it is equal to 182. Comparison of fission nuclide half-life periods, ratios of raw and fission components with same influence to reactivity, and fission characteristics (233U has larger part of fissions from total neutron absorptions) shows, that thorium is preferable primary candidate for control element development with fission material production. Estimation effectiveness can be made by division of precursor half-time to ratio of raw and fission components with same influence to reactivity. For thorium this parameter is 3.6 hours, and for uranium – 6.33 minutes. Difference is 36 times.

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 zero level in any case of campaign.

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 material amount and energy emitted in core by formula:

$$\mathbf{n} = \mathbf{Q}^\* \mathbf{g}^\* \mathbf{g}^\* \boldsymbol{\xi};\tag{10}$$

Where:

194 Nuclear Power – Practical Aspects

during campaign.

**7.1. Task formulation** 

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

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

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

Previous chapter material analysis shows that campaign characteristics improving, including the reproduction coefficient, with the same fuel type is obtained by decrease of neutron absorption in control elements. Improvement not always can be made by neutron flux change or superposition regime implementation. It is interesting to replace useless neutron absorption in control elements (cadmium, boron, gadolinium etc.) by absorption in raw fuel materials (238U and 232Th). First activation products of raw nuclides must be deleted from the core. This technology should not permit fission materials formation in chain of

Any technology always supplies some part of final fission product into the absorber placing area. It is simple to estimate the negative effect of such penetration. It is defined by purification time outside core from primary activation products (239Np and 233Pa) and degree

It can be noticed that equal effect from absorption in raw and fission nuclides of uranium chain is obtained at raw to fission nuclides contents ratio 530, and for thorium chain it is equal to 182. Comparison of fission nuclide half-life periods, ratios of raw and fission components with same influence to reactivity, and fission characteristics (233U has larger part of fissions from total neutron absorptions) shows, that thorium is preferable primary candidate for control element development with fission material production. Estimation effectiveness can be made by division of precursor half-time to ratio of raw and fission

<sup>238</sup> <sup>239</sup> <sup>239</sup> <sup>239</sup> U n U 1412 s Np 2.33 d Pu; (8)

<sup>232</sup> <sup>233</sup> <sup>233</sup> <sup>233</sup> Th n Th 1325 s Pa 27.4 d U; (9)

are quite close to characteristics of CANDU reactor campaign.

**7. Fission materials production in control elements** 

reactor campaign characteristics matches this calculation in great extent.

product transformation in the core. Activation chains of 238U and 232Th are:

Uranium chain has the minor time reserve, but it is big enough.

of fission materials penetration into core loop.

n – amount of produced fission materials, nuclei;

Q – fission energy emitted in core, J;

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.

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 ~3800 g.

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

$$\mathbf{m1} = \mathbf{m} \, \text{\*} \, \mathbb{Z} \, / \, \text{j} \, \tag{11}$$

Where:

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

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

χ – fuel burn-up during campaign;

j – number of fuel reloads during campaign.

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 SZ, which are shown in the Attachment Table 1.
