**9. About development directions of nuclear reactors and devices for energy transformation**

Achieving of good results of raw uranium usage with small uranium mass used cannot be the sole claim for highly effective reactor. Claim for effective energy transformation is also significant. High value of efficiency allows not only decrease uranium usage for unit of power production but decrease of negative effect to the nature, because small efficiency means more thermal energy release.

Complexity of nuclear reactor technology, effective work with energy transformation installations requests contradictions make us to look for different technical solutions, which can provide desired result. During development of atomic power plants many solutions, different coolant types, construction materials, schemes of thermal to electrical power transformation were tested.

In field of thermal reactors the base niche is occupied by water-water reactors. Between two types of these reactors (pressurized and boiling) there is a competitive contest. Now in heavy water moderated reactors there are both transfer directions of water coolant thermal energy to turbine. Maximal efficiency of this scheme is limited by ~34 %, which is depending on acceptable water pressure in a core and its temperature correspondingly.

High efficiency can be obtained in fast liquid metal cooled reactors with coolant temperature about 500 оC, which is typical for combustible fuel powered electro stations. Feature of this scheme is decoupling of coolant load conditions and Rankin cycle actuating medium. Pressure of coolant in reactor vessel can be close to atmospheric.

Decoupling of coolant load conditions and actuating medium also exists in schemes with gaseous coolant [23]. Energy transfer direction with use of gaseous coolant in 50-60-ties of XX century has transformed by type of energy receiver from Rankin cycle to Briton cycle [24].

Potential some efficiency increase (up to ~48 %) in this scheme is possible, but has some negative effects. Main effects are listed below.

The first. There is a need to steep increase of coolant temperature, up to 1000 оC. It causes increase of expenses for reactor and energy transformation installation.

The second is fuel rod design complication caused by higher coolant temperature. Besides expenses increase it causes tendency of fission products release increase.

The third is power increase on the Briton shafting four times higher than Rankin steam turbine with the same power output and cost rise, correspondingly.

All these factors are negative for NPP economy, time for solution preparation to practical realization.

The paper [25] shows usage possibility in Briton cycle instead of turbines piston machines, which supply high efficiency obtaining possibility at lower coolant temperature. It is obtained because of absence energy transfer chain from high speed gas to blades of turbine (and vice versa in compressor), which makes basic energy loss in Briton cycle. Shortages of this solution are absence of practical schemes of needed piston machines and less power of a unit.

## **9.1. Heavy water reactor with gaseous coolant in Rankin cycle**

### *9.1.1. Technology problems*

204 Nuclear Power – Practical Aspects

equilibrium by fuel nuclides regime.

than 2,5 % of produced energy).

reactor are shown [22].

**energy transformation** 

means more thermal energy release.

produced energy spent for fuel enrichment.

**reactor** 

**Figure 11.** Liquid metal and heavy water cooled reactors neutron balance in campaign with

**8.6. External neutron sources for reproduction coefficient increase in thermal** 

External neutron sources can be included in neutron balance if it is linked somehow with reactor work. There are schemes, where neutrons, which are formed in reactions between accelerated protons and nuclei of heavy metals, are emitted to the reactor's core. This scheme has significant lack of reactor work reliability because of use less reliable source – electric nuclear installation. When autonomic electric nuclear installation is used only for 233U formation by neutron absorptions in thorium, reactor work almost is not depending on work reliability of electric nuclear installation. Work [21] demonstrates that raw uranium usage can be increased up to 100 % with use of comparatively small energy expenses (less

Note, that modern thermal reactors, which use less than 1% or raw uranium, about 1% of

Possible characteristics of joint work of fission reactor and neutron source based on fusion

Achieving of good results of raw uranium usage with small uranium mass used cannot be the sole claim for highly effective reactor. Claim for effective energy transformation is also significant. High value of efficiency allows not only decrease uranium usage for unit of power production but decrease of negative effect to the nature, because small efficiency

**9. About development directions of nuclear reactors and devices for** 

In channel heavy water reactors portion of energy, which is released in moderator, is lost for energy production because the moderator has low temperature. In channel released energy is used for energy transformation. Together with heat flow to moderator from channel thermal energy loss is about 10 % from total energy released in the core. Maybe it is a reason of HTGR design with high temperature graphite, which transfers energy of neutron moderation to fuel assemblies.

Thermal Reactors with High Reproduction of Fission Materials 207

**Figure 12.** Scheme of coolant ducts and steam loop of heavy water channel reactor with gaseous

Calculation of neutron and thermal characteristics of the reactor are conducted with use of programs [16, 27], campaign characteristics with use of program [19], Rankin cycle and

Sketches of reactor design with thermal power 80 MW with gaseous cooled fuel assembly and with water cooled fuel assembly are worked out. Water cooled fuel assemblies are

The reactor has 85 fuel assemblies, which are located in nodes of triangular grid with step 18 sm. Core is surrounded by two-layer reflector. Inner layer is heavy water, outer layer is

Coolant cross-section is limited by a screen made of zirconium alloy thin shell (thickness 1 mm), and a casing made of the same alloy shell (thickness 3 mm) at distance of 2 mm from the screen. The gap between the screen and the casing is gas filled. The gas pressure equals

**9.2. Description of reactor with thermal power 80 MW** 

Each fuel assembly has 59 fuel rods with outer diameter 6 mm.

to average by channel height pressure of actuating medium.

turbine characteristics with use of programs [28, 29].

located in central part of the core.

*9.2.1. Description of fuel assembly* 

coolant.

graphite.

Rankin cycle with two steam overheating, one at maximal pressure in cycle (at the entrance to turbine) and additional – after steam expansion to specified pressure, is used in modern reactors. At small maximal pressure levels steam at last stage and turbine exit is wet with high specific humidity. It leads to exit blades corrosion, necessity of valuable alloys usage. Blades of last stage have maximal size and determine total cost of turbine.
