**8.5. About neutron balance in the campaign**

In previous materials model of abstract reactor with parameter "neutron losses in constructive materials and leakage" is used. Dependence of reactor characteristics and its campaign is more complex in real life.

More detailed presentation of these relations can be obtained from neutron balance during reactor work. Neutron creation is present not only in fission nuclides but in raw fuel components and such constructive materials as heavy water and beryllium.

Figure 11 show neutron balance in reactors with two types of fuel assemblies with different coolants. Fuel assembly for heavy water coolant is shown at figure 10-a, and fuel assembly for liquid metal – at figure 10- b. Measures for neutron energy loss prevention at fuel location are used in fuel assembly for liquid metal coolant.

Reactor calculations with use of program [19] are conducted, which are the base for reactor campaign calculations with use of program [20].

1 – case of fuel assembly, 2 – gaseous gap, 3 – screen, 4 – coolant, 5 – fuel rod, 6 – beryllium insert for a) and gaseous cavity for b).

**Figure 10.** Fuel assemblies with water (a) and liquid metal (b) coolant.

202 Nuclear Power – Practical Aspects

*8.4.3. Mixed fuel* 

[18].

for it are also less.

shown campaign burn-up of 4.35% is reached.

equilibrium between fission and raw materials.

**8.5. About neutron balance in the campaign** 

campaign is more complex in real life.

High stability of reactor power is obtained in compact campaign. Reactivity margin decrease in detailed campaign is observed in first 5000 hours. During all time of detailed campaign reactivity stays positive. Reactor work prolongation over 24000 hours is possible. At the

The difference of the campaign from uranium-plutonium campaign is possibility of significant amount increase of fission materials to the end of the campaign. Reproduction

Mixed fuel (with uranium-thorium raw materials) is rational to use in closed cycles with

This fuel is less dependent from neutron flux than pure thorium fuel. Together with it the fuel is more sensitive to the neutron losses. At 238U contents 75 % in campaign beginning, fission materials contents 1.16 % neutron losses 2.8% neutron flux less than 8\*1013 sm-2s-1 is

Campaign with mixed fuel can supply simultaneous use of thorium and uranium fuel with burn-up corresponding to its natural contents with high and even full use of raw uranium

Closed fuel cycles with uranium fuel after several cycles can have increased contents of 242Pu, which is feeble burning out and being absorber and filler decreasing of campaign effectiveness. The possible way of solving is plutonium isotope separation. In mixed fuel 242Pu accumulation will be less and its critical contents will be reached much later. The losses

In previous materials model of abstract reactor with parameter "neutron losses in constructive materials and leakage" is used. Dependence of reactor characteristics and its

More detailed presentation of these relations can be obtained from neutron balance during reactor work. Neutron creation is present not only in fission nuclides but in raw fuel

Figure 11 show neutron balance in reactors with two types of fuel assemblies with different coolants. Fuel assembly for heavy water coolant is shown at figure 10-a, and fuel assembly for liquid metal – at figure 10- b. Measures for neutron energy loss prevention at fuel

Reactor calculations with use of program [19] are conducted, which are the base for reactor

components and such constructive materials as heavy water and beryllium.

location are used in fuel assembly for liquid metal coolant.

campaign calculations with use of program [20].

coefficient of the campaign, which is shown at figure 20, is more than unity by 11 %.

possible effective work. The duration of the campaign can be 34000 hours.

Neutron absorptions (black columns) neutron fissions (blue columns) in different fuel nuclides are shown on diagrams. Two types are used 235U – natural raw uranium signed as U235N, and formed during transformations of thorium chain (232Th-233Pa-233U-234U-235U), signed as U235S.

Fission nuclides have columns for difference between secondary neutrons and total neutron absorptions (red columns). Difference for raw 232Th and 238U between number of secondary neutrons and number of total absorptions with fission are indicated in yellow columns. Columns 1 and 2 indicate neutron absorptions in construction materials and fission products correspondingly.

In left part of each column lay data for reactor with liquid metal coolant, in right part – data for reactor with water coolant/

Total height of red and yellow columns must be equal to total height of black columns for nuclides 232Th, 238U, 234U, 240Pu and columns for neutron absorptions in construction materials and fission products.

If neutron loss in construction materials and leakage are less, than more neutrons can be absorbed in fission products, which number increases during campaign.

For the compared variants fission activity in raw nuclides and ratio of absorptions and fissions in 241Pu are better. In result total neutron absorption in fission products (8 %) for heavy water coolant reactor is less than total neutron absorption in fission products (13 %) for liquid metal coolant reactor at the same neutron losses for construction materials and leakage. So reactor with liquid metal coolant has campaign with burn-up 25.5 MW\*day/kg when heavy water coolant reactor has campaign with burn-up 11,3 MW\*day/kg. Liquid metal cooled reactor has higher raw uranium usage in open cycle campaign.

Thermal Reactors with High Reproduction of Fission Materials 205

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

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

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.

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

The first. There is a need to steep increase of coolant temperature, up to 1000 оC. It causes

The second is fuel rod design complication caused by higher coolant temperature. Besides

The third is power increase on the Briton shafting four times higher than Rankin steam

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

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

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

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** 

on acceptable water pressure in a core and its temperature correspondingly.

Pressure of coolant in reactor vessel can be close to atmospheric.

increase of expenses for reactor and energy transformation installation.

expenses increase it causes tendency of fission products release increase.

turbine with the same power output and cost rise, correspondingly.

negative effects. Main effects are listed below.

realization.

*9.1.1. Technology problems* 

transformation were tested.

**Figure 11.** Liquid metal and heavy water cooled reactors neutron balance in campaign with equilibrium by fuel nuclides regime.
