**2. About usage problems of raw uranium and thorium**

In fission reactors can be used uranium and thorium. But only in raw uranium there is fission isotope - 235U. Fission material in thorium is absent, but can be obtained by neutron irradiation (233U). The basic raw uranium isotope - 238U at neutron irradiation becomes fission nuclide - 239Pu.

One of reactor characteristics is raw uranium usage, obtained in its fuel cycle. If core has no conditions for producing 239Pu from 238U, then raw uranium usage is less than isotope 235U portion in it.

**In open cycles** fuel is once used. A fuel with initial contents of fission and raw isotopes is loaded to a core. At the end of campaign the fission materials contents is decreased and the fuel not used any more. If enriched by 235U uranium is used as an initial fuel, then for raw uranium usage calculations is necessary to calculate raw uranium mass, required for a core loading with enriched uranium [3]. Raw uranium usage Isp is calculated as:

$$\mathbf{I}\_{\rm sp} = \mathbf{M}\_{\rm nat} - \mathbf{M}\_{\rm o} + \mathbf{M}\_{\rm k} \mathbf{\!} \tag{1}$$

Where:

180 Nuclear Power – Practical Aspects

achieved.

shown.

fission nuclide - 239Pu.

portion in it.

The best thermal reactors related to high reproduction of fission materials are heavy water moderated reactors. Today there are several types of such reactors. But its potential still is not fully discovered. In CANDU reactors and like-CANDU reactors in the best cases are used fuel on the base of natural uranium as advantage. But achieved burn-up in these reactors is significantly lower than in light water reactors. Besides it, neutron moderation energy and heat leak from channels energy is lost in heavy water. These factors and compactness of light water vessel reactors have caused its leadership in modern nuclear

power plants. Now this is shadowing potential performance of heavy water reactors.

economical performance of it. In general it is related with use of thorium in fuel.

moderator and Briton cycle. Using gas cooled heavy water reactor is not in favor.

**2. About usage problems of raw uranium and thorium** 

There are designs of heavy water reactors, which allow improvement of technical and

In fifties gaseous coolant in heavy water reactors have been tested, which allowed to use different values of pressure in reactor and maximal pressure in Rankin cycle. With use of fuel rods, which are much the same design as used in majority of modern reactors, coolant temperature up to 500 оС (EL-4, France) and even a little more (KKN, Germany) was

Efficiency at temperature, which is similar to achieved, at thermal power plants is more 40%. In the mentioned reactors efficiency is close to 30 % only. Possibly, that this experience served as a reason of transition to high temperature gas cooled reactors with graphite

If heavy water channels reactors allow better characteristics than existing WWER, PWR, BWR, then it is necessary to know technical solutions, which are needed for this transition.

The purpose of the work is demonstration of thermal reactors development possibility in direction of fission materials reproduction increase, which is sufficient for obtaining burnup comparable with burn-up of the best modern reactors. This development direction shows that these reactors have high raw uranium usage and can supply high durability of nuclear power plants work at high power with modest requirements in uranium mining. Small amount of fission materials in spent fuel reprocessing is significant advantage. At the same control level it allows less possibility of fission material proliferation. The possibility of reaching the high efficiency coefficient of nuclear plants with the proposed reactors is

In fission reactors can be used uranium and thorium. But only in raw uranium there is fission isotope - 235U. Fission material in thorium is absent, but can be obtained by neutron irradiation (233U). The basic raw uranium isotope - 238U at neutron irradiation becomes

One of reactor characteristics is raw uranium usage, obtained in its fuel cycle. If core has no conditions for producing 239Pu from 238U, then raw uranium usage is less than isotope 235U Mnat – raw uranium mass, required for initial fuel producing;

Mo – mass of initial fuel loading;

Mc – fuel mass at the end of reactor campaign.

Portion of raw uranium usage Qu, as relative quantity is calculated as:

$$\mathbf{Q}\_{\mathbf{u}} = \mathbf{I}\_{\mathbf{sp}} / \,\,\mathbf{M}\_{\mathbf{nat}} \,\,\mathbf{:}\,\tag{2}$$

**In closed cycles** after the end of campaign fuel is reprocessed for extraction of fission material rests. Different situations are possible.

**In the first**, the most undesirable situation, not all fission nuclides can be separated from raw nuclides. For example, 235U remains in raw 238U in small amounts so this mix cannot be loaded to a core. Produced 239Pu and 241Pu can be extracted from this spent fuel by chemical methods. Extracted fission materials must be diluted in portion of remained 238U to produce new fuel. Portion of raw uranium usage in this case is:

$$\mathbf{Q}\_{\rm u} = \left(\mathbf{I}\_{\rm sp} + \mathbf{M}\_{\rm Pu} / \, \mathbf{C}\_{\rm dv}\right) \, / \, \mathbf{M}\_{\rm rat} \tag{3}$$

Where:

MPu – mass of isotopes 239Pu and 241Pu, extracted from spent fuel and used in new fuel production;

Cdv – fission materials contents in initial fuel.

This formula is not taking into account difference between properties of initial and final fission materials and following history of fuel usage. It is estimation. This formula is more precise for condition MPu / Cdv < 0.5 \* Mnat, that characterize modern thermal reactors.

Account of following fuel usage history can be conducted by formula:

$$\mathbf{D}\_{\rm is} = \mathbf{Y} \begin{array}{c} \mathbf{Y} \ \mathbf{\*} \\ \end{array} \left( \mathbf{1} + \ \boldsymbol{\upmu} \ \mathbf{+} \\ \right. \quad \left. \ \boldsymbol{\upmu} \ \mathbf{Y} \ \mathbf{+} \\ \end{array} + \begin{array}{c} \boldsymbol{\upmu} \\ \end{array} \right) ; \tag{4}$$

Where:

Y – fuel nuclides burn-up during campaign;

ψ – ratio of extracted fuel material mass at the end of campaign to its initial mass;

n – campaign number of this fuel cycle.

The shortage, connected with impossibility of cheap extraction of fission isotope 235U from raw isotope 238U during spent fuel reprocessing, can be overcome by two ways. The first way is in increasing of initial 235U burn-up, so at the end of campaign its contents is negligibly small. Another way is in using fuel with different nucleus charge – using fission isotope of uranium in thorium, fission isotopes of plutonium in raw 238U. Usage of both ways simultaneously is possible.

Thermal Reactors with High Reproduction of Fission Materials 183

**Figure 1.** Dependence of multiplication factor and fission nuclides reproduction coefficient for reactors

**С FM**

Ряд1 Ряд2 Ряд3 Ряд4 Ряд5 Ряд6 Ряд7 Ряд8 Ряд9 Ряд10

0,003 0,005 0,007 0,009 0,011 0,013 0,015

Data for cross-sections and number of secondary neutrons, used in calculations, are taken

 8a 5f *КВ* 1 n \* / n \* ; 

From data, which shown at figure 1 for hypothetical reactor, it can be seen that there is diapason of uranium enrichment (from 0.46 up to 0.66 %) in which reproduction coefficient and multiplication factor are more than unity simultaneously. For each of shown variants at

To estimate possibility of real reactor work at this diapason of enrichment is necessary to take into account two factors – presence of additional neutron losses in construction materials and neutron leakage, and production influence of secondary fission materials,

Estimation of these factors in point model of reactor is possible by introduction of neutron loss in construction materials and leakage term in system of equations, describing accumulation and neutron absorption in initial fuel nuclides and additional nuclides

Sufficiently precise estimation of reactor campaign characteristics with different fuel types

Fission materials reproduction coefficient in initial fuel is calculated:

reproduction coefficient equal unity multiplication factor is close to 1.1.

actinides and fission products, which are sufficient neutron absorbers.

with different fuel types and no neutron loss.

Ряд11 Ряд12

produced during reactor work.


can be made taking into account following nuclides:




from [5].

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

**К**

(6)

Because of fission isotopes absence in raw thorium there is no problem with raw thorium usage.

Known stocks of thorium are bigger than known stocks of uranium. It will be ideal if nuclear power industry use both elements in its work. Basic raw material in the present time is uranium. Spending cheap stocks of uranium will lead to necessity of depleted uranium reprocessing from dumps of enrichment plants and spent fuel and thorium usage.

One of the problems with thorium fuel is 232U production, which is source of high energy gamma-ray quanta [4]. One way of this problem solution is usage of automatics at thorium spent fuel reprocessing.
