**8.2. Construction of fuel assembly with composite core**

Good neutron-physical characteristics of metallic fuel are well-known [13, 14]. Such fuel is used on reactors by the first stages of atomic energetic progress. Significant swelling under the enough small burn-up is the lack of it. Maximum allowed burn-up of "KC-150" [15] reactor is settled by the level in 15 MW\*day/kg. It is indicated that this burn-up equal to 4 MW\*day/kg the maximal extension of fuel element is laying down for 5-7% under the temperature to 3500C. Rod-shaped fuel element of traditional construction with such characteristics of swelling cannot be perspective for reactors with burn-up of 40 MW\*day/kg and higher.

Situation can be changed, if we will use the fuel element with composite metal core. 2 variants of such fuel element construction are presented at the figure 8. Core of such fuel element is containing fuel elements 2 and 3, liquid-metal filler 4, which are placed under the protection cover.

In variant, which is placed on the left, the core contains 8 leaf fuel cells and one cylindrical fuel element. These elements at swelling occupy the space, which was filled by liquid metal (with fuel working temperature). At the second variant only 2 fuel elements are used. These elements are identical and inserted into each other. Cuttings, which bring down the pressure during swelling in radial direction, are made on elements surfaces.

Bismuth, lead [16] and tin can be filler material. If cover is made from zirconium, then lead, by preliminary estimations, leaves the list of candidates because it actively interacts with zirconium and breaks its initial structure. It is possible, that bismuth in clean type will actively interact with zirconium with such result.

Tin at the time of interaction with zirconium forms the solid compound – zirconium stannide Zr3Sn2 on its surface, which is melted at temperature 1985 0C [17]. Tin is included into the composition of zircaloy, which is used in fuel element covers. Shortage of tin (its enough high absorption cross-section) is possible to delete by using tin enriched by 120Sn isotope. 120Sn has the best properties among tin isotopes with even atomic weight.

It is possible, that alloy of tin with bismuth and lead also will form the tin stannide on the surface of zirconium, which prevents the interaction of zirconium with bismuth and lead. Rigorous research in this direction is necessary.

1 – fuel element cover; 2 and 3 – fuel elements; 4 – filler.

198 Nuclear Power – Practical Aspects

calculation.

protection cover.

Analysis of calculation results allows saying:

1. Initial variant of reactor has the worst characteristics;

of this changing has decrease till 5.06% to 2.70%.

also leads to appreciable lowering of neutrons loss.

which is differ by reflector construction, are minor.

**8.2. Construction of fuel assembly with composite core** 

during swelling in radial direction, are made on elements surfaces.

actively interact with zirconium with such result.

metallic, including the reproduction of fission materials.

2. Major change of properties is reached by the change of natural zirconium and tin to its isotopes 90Zr и 120Sn. These isotopes have the best properties among the rest of isotopes of these elements and have the considerable contents in natural elements. Neutrons loss

3. Decreasing of leakage at the expense of reflector thickness increasing at the i.3 and 4

4. Important improvement of characteristics is obtained by replacement of oxide fuel by

5. The best parameters among the presented variants have the reactor, with fuel assembly made from 30 fuel rods and beryllium insertion. Differences between 6 and 7 variants,

1.7 %, 2.8 % and 5.2 % neutron loss are used in Table 1 as results of CANDU reactor

Good neutron-physical characteristics of metallic fuel are well-known [13, 14]. Such fuel is used on reactors by the first stages of atomic energetic progress. Significant swelling under the enough small burn-up is the lack of it. Maximum allowed burn-up of "KC-150" [15] reactor is settled by the level in 15 MW\*day/kg. It is indicated that this burn-up equal to 4 MW\*day/kg the maximal extension of fuel element is laying down for 5-7% under the temperature to 3500C. Rod-shaped fuel element of traditional construction with such characteristics of

Situation can be changed, if we will use the fuel element with composite metal core. 2 variants of such fuel element construction are presented at the figure 8. Core of such fuel element is containing fuel elements 2 and 3, liquid-metal filler 4, which are placed under the

In variant, which is placed on the left, the core contains 8 leaf fuel cells and one cylindrical fuel element. These elements at swelling occupy the space, which was filled by liquid metal (with fuel working temperature). At the second variant only 2 fuel elements are used. These elements are identical and inserted into each other. Cuttings, which bring down the pressure

Bismuth, lead [16] and tin can be filler material. If cover is made from zirconium, then lead, by preliminary estimations, leaves the list of candidates because it actively interacts with zirconium and breaks its initial structure. It is possible, that bismuth in clean type will

Tin at the time of interaction with zirconium forms the solid compound – zirconium stannide Zr3Sn2 on its surface, which is melted at temperature 1985 0C [17]. Tin is included into the composition of zircaloy, which is used in fuel element covers. Shortage of tin (its

swelling cannot be perspective for reactors with burn-up of 40 MW\*day/kg and higher.

**Figure 8.** Variants of fuel element construction with composite metallic core.

We should specify requests to whole of possible varieties of core composite elements, which must have high workability of fuel element with maximal level of burn-up.

In this case the statement is follows: space between particles must create stable conditions for separate particles location under fuel element cover when increasing volume in specified limits without appearing of additional effort between particles;

This claim dissects for some independent claims:


It should be marked that important factor in such construction of fuel element, which decrease the metallic fuel elements swelling, is their small thickness. The small thickness increases the migration of fission gaseous splinters over the external surface with decreasing of inner mechanical stresses.

By the estimations, forms changing of metallic fuel rods under the execution of given claims and initial content of liquid-metal filler in 20% by the volume is not leading to trespassing of fuel element cover under the burn-up till 40-59 МW\*day/kg. Such fuel has being used in calculations of reactor models, which was presented in the table 1.

Thermal Reactors with High Reproduction of Fission Materials 201

The string 19 of Attachment Table 1shows variant with addition of 0.35 % 235U from mass of 238U, that equals equilibrium contents of 239Pu and 241Pu. This fuel can be made by plutonium isotopes, which are extracted from spent fuel, addition to a mix of raw uranium (half of 238U

The string 20 of Attachment Table 1 shows campaign characteristics with initial fuel containing equilibrium contents of 239Pu and 241Pu and addition of 235U, which contents is much less than in previous fuel. Resonant neutron absorption in 238U is 1.2 times higher. Production of 233U is lead. Equilibrium contents of 239Pu and 241Pu is also 1.2 times greater.

String 18 of Table 1 of Attachment shows campaign with this technology. Strings 21 and 22 show campaign with no 233U production. Maximal burn-up 3.51 % and maximal raw

Equilibrium cycle with uranium-thorium fuel even without resonant neutron absorption in thorium has high fission materials contents, which mean possibility of high burn-up achieving in detailed campaign. Features of uranium-thorium campaign are high neutron absorption in 233Ра and its long half-life, which are used in technology of 233U production by

Detailed campaign with neutron flux about 1014 sm-2s-1 becomes very short even in case of low neutron losses which equals 1.7 %. The campaign with twice lower neutron flux is

Figure 9 shows detailed campaign characteristics with equilibrium uranium-thorium fuel

with 233U production at neutron flux 6\*1014 sm-2s-1 and neutron losses 5.0 %.

**Figure 9.** Detailed campaign characteristics with equilibrium uranium-thorium fuel and 233U production. Neutron flux is 6\*1014 sm-2s-1, neutron loss – 5.0 % (string 25 of Table 1 of Attachment).

0 5000 10000 15000 20000

**time, h**

U 233 U 2235 W Pa 233 K-1 SZ K-1 unwr K-1 oper

0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,1 0,11

**K-1**

Burn-up of 3.49 % and raw uranium usage 12.12 % is achieved.

mass) and 238U from spent fuel.

uranium usage 14.28 % is achieved.

*8.4.2. Uranium-thorium fuel* 

reactivity margin decreasing.

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

**C FM**

possible at neutron losses of 5.0 %.
