JPH0132477B2 - - Google Patents

Description

[Detailed description of the invention]

[Object of the Invention] (Industrial Application Field) The present invention relates to an oxide-based nuclear fuel element that is used in a power reactor by being sealed in a metal cladding tube. (Prior Art) Power reactors are strongly required from an economic standpoint to be capable of load-following operation in which the output can be freely adjusted depending on the amount of electricity demanded. However, it is known that when the reactor output is suddenly changed, mechanical interaction (hereinafter referred to as PCMI) occurs due to the difference in coefficient of thermal expansion between the oxide nuclear fuel element and the metal cladding. Irradiated and brittle cladding may be damaged when PCMI occurs;
Releases highly radioactive fission products accumulated in the cladding. In order to avoid such a situation, the measures currently adopted in light water reactors, etc. are to minimize the occurrence of PCMI by strictly limiting the conditions for fluctuations in the reactor's output. This has successfully reduced fuel damage in power reactors. However, if limits are placed on the operating conditions of a nuclear reactor in this way, it will naturally take time to increase the output to a predetermined level, reducing plant efficiency and causing large economic losses. Become. Furthermore, the load-following operation described above is undesirable, and finding a more fundamental solution has become an urgent task in the field of nuclear fuel. One way to solve this problem is to use additives. This is done by adding certain additives to the sintered pellet type oxide nuclear fuel element to reduce its strength at high temperatures.
This method minimizes damage to the cladding. Particularly effective additives for oxide nuclear fuel elements include Al ₂ O ₃ , BeO, CaO, MgO, SiO ₂ ,
This is a case where at least two types selected from Na ₂ O and P ₂ O ₅ are included. However, in order to improve the damage to the cladding to a negligible extent, it is necessary to add up to 5% by weight. However, since adding foreign substances to nuclear fuel material reduces the density of fissile material, there are restrictions on the amount of the additives. Also, from the viewpoint of neutron economy, it is necessary to suppress neutron absorption by additives as low as possible. Therefore, it is desirable that a predetermined strength reduction can be achieved by adding a small amount. (Problems to be Solved by the Invention) As described above, in conventional oxide nuclear fuel elements, Al ₂ O ₃ , BeO, CaO, MgO, SiO ₂ , Na ₂ O,
Damage to the cladding can be considerably alleviated by adding up to 5% by weight of additives containing at least two types selected from P ₂ O ₅ , but the addition of foreign substances reduces the density of fissile material, so the amount added should be kept small. There was a contradictory problem in that it had to be kept to a minimum. The present invention was made with attention to this point,
_{Al2O3 ,} BeO, CaO, _MgO , _SiO2 , _Na2O , _P2O5
By adding a third composition (auxiliary additive) to a sintered pellet type oxide-based nuclear fuel element containing at least two types of additives selected from, the required amount of the additive is reduced, The purpose of the present invention is to provide an oxide-based nuclear fuel element that suppresses a decrease in the density of fissile material and suppresses neutron absorption. [Structure of the Invention] (Means for Solving the Problems) In the oxide-based nuclear fuel element of the present invention,
Al ₂ O ₃ , BeO, CaO, MgO, SiO ₂ , Na ₂ O ₅ ,
A sintered pellet type nuclear fuel containing at least two selected from P ₂ O ₅ as additives is characterized in that Nb ₂ O ₅ is added as a third composition in an amount of 0.2 weight percent or more and 2.0 weight percent or less. (Function) In such a device, the strength reduction effect of the nuclear fuel element can be obtained by adding a small amount of Nb ₂ O ₅ , so that the density reduction of the fissile material can be suppressed and neutron absorption can be kept low. (Example) The present invention will be explained in detail below. The effect of the additive is to reduce the apparent strength of the nuclear fuel element by intervening in the grain boundaries of the base material of the nuclear fuel material. It is desirable that the additives are separated as completely as possible. Conversely, additives need to be added in amounts sufficient to separate the crystal grains of the base material. Therefore, in order to obtain the desired effect with as little amount as possible, it is advantageous to increase the crystallinity of the base material. This is because if the shape of the crystal grains of sintered pellet nuclear fuel is modeled as spherical, a simple calculation shows that the total grain boundary area per unit volume decreases in inverse proportion to the crystal grain size. This is because it is required. This is also because the total amount of additives present at grain boundaries is considered to be approximately proportional to the total sum of grain boundary areas. On the other hand, in sintered pellet-type oxide nuclear fuels such as uranium dioxide, it is known that when certain compositions are added, the crystal grains of the base material become coarse. The present invention was made by combining the following relationships, and its characteristics are that by adding the third composition, the crystal grains are coarsened; Nevertheless, the goal is to reduce the high temperature strength of the sintered pellet type oxide nuclear fuel element with a smaller amount of additives. The content of the present invention will be explained in further detail below using specific examples. The composition ratio in weight percent to uranium dioxide powder is SiO ₂ : 47, Al ₂ O ₃ : 38, CaO: 14.5,
After mixing 0.2 and 1.5 weight percent of Fe ₂ O ₃ :0.5 additives, the mixture was compressed at a pressure of 2 tons/cm ² to reduce the diameter.
A molded body of 12.5 mm and a height of 12.5 mm to 13.7 mm was obtained. This was sintered at 1700° C. for 2 hours to obtain three types of sintered bodies with densities of 95.3, 94.6, and 91.8% TD. On the other hand, after adding 1.0% by weight of the above additive to the same lot of uranium dioxide powder,
Nb ₂ O ₅ fine powder 0.1, 0.2, 0.5, 1.0, 2.0, respectively
Using 6 types of raw material powder prepared by adding 5.0% by weight and mixing, compressed at a pressure of 2ton/cm ² ,
The molded body had a diameter of 12.5 mm and a height of 12.6 to 13.1 mm.
This was sintered at 1700℃ for 2 hours, and the density was 95.2.
Sintered bodies with TD of 95.8, 96.2, 95.7, 95.2, and 94.4% were obtained. A total of nine types of sintered bodies were cut and polished, and then their hardness was measured in a temperature range from room temperature to 1200°C, and the results shown in Figures 1 and 2 were obtained. As can be seen from FIG. 1, when an additive having the above composition ratio is added to uranium dioxide, the strength of the uranium dioxide sintered body decreases, and when 5 weight percent is added, the strength decreases by about 60% at maximum. However, when the amount added is 1% by weight, a strength reduction of only about 35% is obtained. As can be seen from FIG. 2, the effect of adding Nb ₂ O ₅ as the third composition is clear. That is, in the case of a material containing 0.5% Nb ₂ O ₅ , even if the additive amount was 1% by weight, a strength reduction equivalent to the addition of 5% by weight in the case of not containing Nb ₂ O ₅ was obtained. Furthermore, if we compare the results in Figures 1 and 2, we find that Nb ₂ O ₅
The optimum amount of addition is 0.2% or more and 2.0% or less. Also, Nb ₂ O ₅ = 0.1% or less or Nb ₂ O ₅ = 3
% or more, it can be seen that it has the effect of increasing the strength of the uranium dioxide sintered body. In addition, in all cases with the addition of Nb ₂ O ₅ , the average grain size becomes coarser, and the growth is 3.6 to 4.5 times larger in the case of Nb ₂ O ₅ than in the case of those without these. observed. This observation result shows that reducing the grain boundary area due to coarsening of the crystal grains is effective in bringing about a specified strength reduction with a small amount of additives. As detailed above, Al ₂ O ₃ , BeO, CaO, MgO,
A third composition, for example, a material that coarsens the crystal grain size such as Nb ₂ O ₅ , is added to an oxide-based nuclear fuel element containing at least two types of additives selected from SiO ₂ , Na ₂ O, and P ₂ O ₅ By doing so, even under conditions where PCMI occurs in normal nuclear fuel elements, there is less damage to the cladding tube, the density of fissile material is suppressed, and neutron absorption is reduced. We also conducted the following experiments. Additives having the same composition ratio as those described in the previous example were added to the uranium dioxide powder, i.e., SiO ₂ :47, Al ₂ O ₃ :38,
Table 1 shows Nb ₂ O ₅ fine powder and TiO ₂ fine powder after addition of 1.0 weight percent of additives of CaO: 14.5 and Fe ₂ O ₃ : 0.5.

【table】

〔Effect of the invention〕

_Al2O3 , BeO, CaO, _MgO , _SiO2 , _Na2O ,
By adding the third composition Nb ₂ O ₅ to a sintered pellet type oxide nuclear fuel element containing at least two types of additives selected from P ₂ O ₅ , the required amount of the additives can be adjusted. It is possible to obtain an oxide-based nuclear fuel element in which the density of fissile material is reduced, the density of the fissile material is suppressed, and neutron absorption is suppressed to a low level.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the hardness versus temperature of uranium dioxide sintered bodies containing additives at various concentrations, and Figure ₂ is a graph showing the hardness versus temperature of uranium dioxide sintered bodies containing additives at various concentrations. Figure ₃ is a characteristic diagram showing the hardness versus temperature of the sintered body for uranium dioxide containing 1% by weight of additives and the addition of uranium dioxide at the content rate shown in Table 1.
FIG. 2 is a characteristic diagram showing the hardness versus temperature of a sintered body in which Nb ₂ O ₅ and TiO ₂ are added at the same time.

Claims (1)

[Claims] 1 Al ₂ O ₃ , BeO, CaO, MgO, SiO ₂ , Na ₂ O ₅ ,
In a sintered pellet type nuclear fuel containing at least two types selected from P ₂ O ₅ as additives, Nb ₂ O ₅ as the third composition is 0.2% by weight or more 2.0
An oxide-based nuclear fuel element characterized in that an oxide-based nuclear fuel element is added at a weight percent or less.