**1. Introduction**

Efficiency of power reactors is determined by the expenses of raw material for its work and by efficiency of heat to mechanical energy transformation.

In general modern nuclear power plants (NPP) use thermal reactors with enriched by isotope 235U uranium (comparing to raw uranium) in the beginning of campaign. This fuel allows having sufficient reactivity margin for obtaining burn-up more than 30-50 MW\*day/kg. This reactor type is developed by itself during short period of nuclear power plants development. Raw uranium cannot supply required burn-up even in reactors with best heavy-water moderator. An effective technology of isotope separation was made for military purposes.

Known shortage of thermal reactors is small usage of raw uranium during its work (0.5 – 1.0 %). Stocks of cheap ore for these reactors are enough for 40-50 years at power level of 4000 GW [1].

The next step in nuclear power plants development is suggested usage of fast neutron reactors. This transition is connected with fuel enrichment increase and it supplies possibility of fission reactions on fast neutrons, which produce more secondary neutrons. By solving technical problems at this direction, nuclear power plants supplied with cheap enough fuel for many centuries can be built. But this development direction has one shortage. It is extremely expensive.

When it`s advocates say that high price is because of modern technical solutions shortages, they are half right. Insuperable high price is because of high raw uranium requirement for its start. For using nuclear power plants with total power of 4000 GW, which are supplied for 3000 years, all cheap uranium stocks must be processed in 40-50 years [2]. It is connected with ecological problems and some complexity in non-proliferation of nuclear fission materials.

© 2012 Kotov, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Kotov, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Thermal Reactors with High Reproduction of Fission Materials 181

sp nat o k I M M M; (1)

Q I / M ; u sp nat (2)

Q I M / C / M u sp Pu dv nat (3)

 (4)

**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

**In closed cycles** after the end of campaign fuel is reprocessed for extraction of fission

**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

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

> 2 n1 D Y \* 1 ; is

precise for condition MPu / Cdv < 0.5 \* Mnat, that characterize modern thermal reactors.

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

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

loading with enriched uranium [3]. Raw uranium usage Isp is calculated as:

Mnat – raw uranium mass, required for initial fuel producing;

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

Where:

Where:

Where:

production;

Mo – mass of initial fuel loading;

Mc – fuel mass at the end of reactor campaign.

material rests. Different situations are possible.

Cdv – fission materials contents in initial fuel.

Y – fuel nuclides burn-up during campaign;

n – campaign number of this fuel cycle.

new fuel. Portion of raw uranium usage in this case is:

There are designs of heavy water reactors, which allow improvement of technical and economical performance of it. In general it is related with use of thorium in fuel.

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 achieved.

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

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 shown.
