*9.2.1. Description of fuel assembly*

206 Nuclear Power – Practical Aspects

moderation to fuel assemblies.

*9.1.2. Solution description* 

and maximal pressure 20.0 MPa.

Rankin cycle and decrease thickness of duct walls.

condition of automatics becomes better).

ducts.

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

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.

Scheme of joint work of heavy water reactor with gaseous coolant and Rankin cycle steam turbine, which is based on full use of fission energy (including neutron moderation energy)

First feature of the solution is based on heat emission to the steam in Rankin cycle, which is

The second feature allows avoiding presence of steam with high specific humidity on exit stage blades of turbine. Good parameters are achieved at maximal steam temperature 500 °C

Scheme of coolant ducts and steam loop of heavy water channel reactor with gaseous coolant is shown at figure 12. Differences of this scheme from other solutions are usage of neutron moderation energy in Rankin cycle for water of steam loop heating, separation of coolant duct on four ducts, which supply with energy re-heater and water evaporator, separate steam overheaters. Coolant ducts for heating and water evaporation can be water

Neutron moderation energy transfer to water in Rankin cycle is realized with use of vessel of reactor design with greater than atmospheric pressure, but less than maximal pressure in coolant duct. It allows having moderator with temperature greater than water heated in

Separation of overheaters ducts allows decreasing of danger of hermetization loss of steam loop with high pressure. Actions for sequence avoiding of hermetization loss is made to duct with small portion of reactor power (~20 %). Reactor scheme at figure 12 gaseous coolant duct, which is linked to high pressure steam duct, has vessel, which supplies decrease of maximal pressure at accidental steam leakage to coolant (volume of steam is limited) and increase time of pressure growth in coolant duct at such leakage (work

Besides this separation optimizes energy expenses for coolant pumping by use of

temperature differential in corresponding external heat exchangers.

Blades of last stage have maximal size and determine total cost of turbine.

and three stepped steam entering to turbine, is suggested in the paper [26].

made not in a single process, but several with different temperature intervals.

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

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 to average by channel height pressure of actuating medium.

Gaseous coolant in fuel assembly can be hydrogen or helium. Fuel on base of metallic uranium and thorium is chosen for the work variant (figure 13). Each fuel rod has uranium fuel elements alternating by height with thorium fuel elements [30]. Initial contents of 235U in uranium elements is 0.5 %. Initial contents of 239Pu and 241Pu in uranium elements, and initial contents of 233U in thorium elements equal to equilibrium contents of these nuclides during durable campaign.

Thermal Reactors with High Reproduction of Fission Materials 209

Uranium tablets of first campaign are made from raw uranium with adding of 0,357 % 239Pu and 0,12 % 241Pu. It is possible to have the same mass of nuclides with different mixture contents. In thorium tablets of the first campaign is added 2.3 % 235U of high enrichment.

Uranium tablets are divided on two parts at spent fuel reprocessing. Fission products and plutonium is extracted from 70% of spent fuel uranium mass of the first campaign. This fuel

Fission products are separated from the second fuel part. Add mass of raw uranium which

Fission products are separated from the thorium fuel part. Add mass of thorium which

In reactor core there is 2312 kg of fuel. Spent fuel reprocessing requirement is 680 kg per annum. Raw uranium requirement is 510 kg per annum. Required mass of thorium is 14 kg

For comparison, reactor WWER-1000 uranium requirement is 10 times larger per unit power

Thermal characteristics of fuel assemblies with hydrogen coolant are presented in table 4. Characteristics of fuel assemblies with helium coolant are slightly different. Hydrogen is

overheating

Temperature at fuel assembly entrance, 0С **285.9 285.3 376 141**  Temperature at fuel assembly exit, 0С **510 510 510 375**  Fuel assembly number in reactor 10 12 17 46 Coolant flow in fuel assembly, kg/s 0.286 0.285 0.478 0.274

Coolant velocity at fuel assembly exit, m/s 40.1 40 67.35 31.5 Pressure difference in fuel assembly, Pa 3892 3882 9617 2923 Coolant pump power in fuel assembly, kW 5.24 6.25 36.8 17.34

2 overheating

28.8 28.6 55.36 20

65.63/0,2

1 overheating

Water heating

part is replaced with raw uranium, in which extracted plutonium is added.

equals mass of burned 238U in uranium fuel.

equals mass of extracted fission products.

cheaper than helium so it is preferable.

per annum.

m/s

total

Usage of raw uranium in closed cycle is 5%.

and relative amount of dissipated power is more 1.4 times.

**Coolant Н<sup>2</sup>**

**Gas in the gap of fuel assembly СО<sup>2</sup>**

**9.2.3. Thermal characteristics of fuel assemblies** 

Function of fuel assembly 3

Coolant velocity at fuel assembly entrance,

Total coolant pump power, kW / % from

This design supplies constant energy release distribution by height of fuel rod independent from fuel type under the shell – uranium or thorium.

1 – rod shell, 2 – uranium fuel element, 3 – thorium fuel element.

**Figure 13.** Disposition of uranium and thorium fuel elements by fuel rod height.

Quality of fuel rod height energy release distribution becomes better with increase of portion of equilibrium nuclides and, correspondingly, raw uranium usage portion increase.

Difference of fuel elements form with uranium and thorium has significant role at reprocessing of spent fuel, when there is need to separate uranium and thorium fuel.

With use of equivalent fuel elements with mix of uranium and thorium, where also can be obtained uniform energy release, we will have problem of separation fission 233U and 235U from raw 238U. Without this separation raw uranium usage portion has steep decrease.

### *9.2.2. Campaign characteristics*

Change of reactivity margin during two variants of the campaign without fuel replacement and with one fuel replacement.

At constant neutron flux in core ratio of reactor power at campaign end to power in campaign beginning in variant without fuel replacement equals 37,6 %, and with one replacement – 17,6%. Fuel burn-up is 5,1 % or 48,7 MW\*day/kg.

Uranium tablets of first campaign are made from raw uranium with adding of 0,357 % 239Pu and 0,12 % 241Pu. It is possible to have the same mass of nuclides with different mixture contents. In thorium tablets of the first campaign is added 2.3 % 235U of high enrichment.

Uranium tablets are divided on two parts at spent fuel reprocessing. Fission products and plutonium is extracted from 70% of spent fuel uranium mass of the first campaign. This fuel part is replaced with raw uranium, in which extracted plutonium is added.

Fission products are separated from the second fuel part. Add mass of raw uranium which equals mass of burned 238U in uranium fuel.

Fission products are separated from the thorium fuel part. Add mass of thorium which equals mass of extracted fission products.

Usage of raw uranium in closed cycle is 5%.

208 Nuclear Power – Practical Aspects

durable campaign.

from fuel type under the shell – uranium or thorium.

1 – rod shell, 2 – uranium fuel element, 3 – thorium fuel element.

*9.2.2. Campaign characteristics* 

and with one fuel replacement.

**Figure 13.** Disposition of uranium and thorium fuel elements by fuel rod height.

replacement – 17,6%. Fuel burn-up is 5,1 % or 48,7 MW\*day/kg.

Quality of fuel rod height energy release distribution becomes better with increase of portion of equilibrium nuclides and, correspondingly, raw uranium usage portion increase. Difference of fuel elements form with uranium and thorium has significant role at

With use of equivalent fuel elements with mix of uranium and thorium, where also can be obtained uniform energy release, we will have problem of separation fission 233U and 235U from raw 238U. Without this separation raw uranium usage portion has steep decrease.

Change of reactivity margin during two variants of the campaign without fuel replacement

At constant neutron flux in core ratio of reactor power at campaign end to power in campaign beginning in variant without fuel replacement equals 37,6 %, and with one

reprocessing of spent fuel, when there is need to separate uranium and thorium fuel.

Gaseous coolant in fuel assembly can be hydrogen or helium. Fuel on base of metallic uranium and thorium is chosen for the work variant (figure 13). Each fuel rod has uranium fuel elements alternating by height with thorium fuel elements [30]. Initial contents of 235U in uranium elements is 0.5 %. Initial contents of 239Pu and 241Pu in uranium elements, and initial contents of 233U in thorium elements equal to equilibrium contents of these nuclides during

This design supplies constant energy release distribution by height of fuel rod independent

In reactor core there is 2312 kg of fuel. Spent fuel reprocessing requirement is 680 kg per annum. Raw uranium requirement is 510 kg per annum. Required mass of thorium is 14 kg per annum.

For comparison, reactor WWER-1000 uranium requirement is 10 times larger per unit power and relative amount of dissipated power is more 1.4 times.
