JPH0820535B2 - (Ca, U) O ▲ Lower 2 ▼ Manufacturing method for fuel pellets - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing nuclear fuel pellets, particularly (Ca, U) O ₂ -based fuel pellets.

[Background of the Invention]

In recent years, in nuclear power generation, extension of burnup of a fuel body has been studied for effective use of fuel.

In the combustion of fuel bodies, the release of fission products (FP) in the fuel increases rapidly as the burnup increases, and the decrease in thermal conductivity due to this increases the fuel temperature and internal pressure. Inconvenience occurs.

Therefore, there is an idea to increase the burnup by containing FP, especially FP gas such as Kr or Xe, in the fuel pellet, and one of them is CaO-added pellet. That is, the purpose is to reduce the diffusivity of U ions by the addition of CaO, and eventually to reduce the diffusivity of FP gas to prolong the residence time in the pellet.

By the way, generally, UO ₂ pellets are produced by compressing and sintering UO ₂ powder. In this case, the sintering is, in theory, solid-phase sintering, and is caused by diffusion of U ions or O ions. Sintering proceeds. Where UO ₂ powder to C
When aO is added, the diffusivity of U ions decreases, so the sintering does not proceed smoothly at the time of sintering, high temperature and long time are required for sintering, and the crystal grain size of the sintered product also becomes small. Will occur. For this reason, in the past, after sintering in a slightly oxidizing atmosphere mainly composed of CO ₂ , heating was performed in a reducing atmosphere at the same temperature, and the oxygen ratio to all metals was additionally set to O / M = 2.0. Making adjustments.

[Problems to be solved by the invention]

An object of the present invention is to promote sintering without lowering the diffusivity of U ions even at a low temperature and particularly in an oxidizing atmosphere, and CaO-containing pellets having a larger crystal grain size, particularly (Ca, U). It is to provide a manufacturing method of O ₂ fuel pellets.

[Means for solving problems]

The method of the first invention of the present application for achieving the above-mentioned problems is
CaO is added to UO ₂ powder for compression molding and sintering (Ca,
U) In the method for producing an O ₂ -based fuel pellet, a mixing step of adding Ca to the UO ₂ powder in the form of Ca _x U _1-x O _y (0 <x <1, 3 ≦ y ≦ 4) before molding. And a sintering step of heating the compression-molded body of the powder raw material obtained in the mixing step in a reducing atmosphere under a temperature condition of 1400 to 1800 ° C. for 2 hours or more.

The method of the second invention of the present application for achieving the above-mentioned object is a method for producing (Ca, U) O ₂ -based fuel pellets, in which CaO is added to UO ₂ powder and compression molding and sintering are performed. A mixing step of adding Ca to the UO ₂ powder in the form of Ca _x U _1-x O _y (0 <x <1, 3 ≦ y ≦ 4), and a compression molding of the powder raw material obtained in the mixing step. CO
_A sintering step of heating for 30 minutes or more under a temperature condition of 1100 to 1400 ° C. in a slightly oxidizing atmosphere containing _2, and after the sintering step, the molded body is placed in a reducing atmosphere containing H _2. O / U which heats up under the temperature condition of 1100-1400 ℃
The ratio adjustment step is included.

[Action]

When CaO is added in a trace amount, it can form a (Ca, U) O ₂ structure without deforming and destroying the crystal structure of UO ₂ . Because of this, UO ₂
There is an advantage that the object can be achieved without largely changing the characteristics of.

A solid solution similar to this occurs in an oxidizing atmosphere, and (Ca,
U) Can form O ₄ .

On the other hand, (Ca, U) O _4, it is decomposed properties CaO + UO ₃ becomes a high temperature, UO ₃ further comprising a UO ₂ +1/2 · O _2.

The present invention has been made in view of this point, UO ₂
By adding (Ca, U) O ₄ and heating and sintering,
In particular, even if not in an oxidizing atmosphere, an oxygen source is created inside, and the presence of this source in the vicinity of CaO, which is a sintering inhibitor, reduces the sintering inhibiting ability of CaO,
Therefore, the diffusivity of U ions becomes sufficient without heating to unnecessarily high temperature, the sintering proceeds smoothly, and (Ca, U) O ₂ -based fuel pellets with larger crystal grains are produced. Is.

Actually, (Ca, U) O ₄ is Ca _x U _1-x O _y (0 <x <1,3 ≦ y ≦
4), and in this case, x and y can be changed depending on the addition ratio of Ca or the oxidation conditions during production.

On the other hand, UO ₃ and U ₃ O ₈ produced from Ca _x U _1-x O _y have been conventionally added to UO ₂ for density adjustment and pore adjustment, and therefore (Ca , U) O ₂ even when the CaO concentration is constant, there is also an advantage that density adjustment and pore adjustment can be similarly performed by changing the addition amount by changing _{x of} Ca _x U _1-x O _y. .

Sintering does not necessarily have to be performed in a non-oxidizing atmosphere. In the first invention of the present application, the reducing atmosphere is used, but in the second invention, the sintering is performed in a slightly oxidizing atmosphere and then the reduction is performed. The O / U ratio is adjusted by heating in a neutral atmosphere.

To further clarify the features and advantages of the present invention, non-limiting examples of the present invention are as follows.

= Example = Instead of CaO, in order to obtain a finer product, CaCO ₃
The powder was mixed with UO ₂ powder in a molar ratio of 10:90 and formed into pellets, which was heated in an air stream at 900 ° C for 3 hours and then re-formed and the same heating was repeated twice. 700 ° C
Ca _x U by heating in air flow for 3 hours
_1-x O _y was obtained.

The mixing ratio of this Ca _x U _1-x O _y powder to UO ₂ powder is Ca: U
= 0.005: 1, and then compression molding was performed to obtain a pellet-shaped molded body having a molding density of 5.90 g / cm ³ .

The compact was subjected to a sintering treatment at 1700 ° C. for 4 hours in a hydrogen / nitrogen mixed stream containing a small amount of water vapor.

The obtained sintered pellet was a good sintered body having a density of 94.5% TD and an average crystal grain size of 27 μm.

= Example 2 = A molded product similar to the pellet-shaped molded product in Example 1 was subjected to a sintering treatment at 1200 ° C for 3 hours in a CO ₂ / O ₂ stream, and then an H ₂ / N ₂ stream. O by heating at 1200 ℃ for 1 hour
/ U adjustment was done.

The obtained sintered pellet was a good sintered body having a density of 93.8% TD, an average crystal grain size of 20 μm, and O / U = 2.0.

〔The invention's effect〕

As described above, according to the present invention, CaO is added to Ca _x U _1-x
By adding to the UO ₂ powder in the form of O _y , even in a relatively low temperature and in a non-oxidizing atmosphere, or in an extremely low temperature of around 1200 ° C. in a slightly oxidizing atmosphere, Although an O / M adjustment step is required in the subsequent step, the sintering proceeds smoothly without impeding the U ion diffusing ability,
(Ca, U) O ₂ -based fuel pellets having larger crystal grains can be produced, and the density of the pellets can be adjusted.

Claims (2)

[Claims] 1. A method for producing (Ca, U) O ₂ -based fuel pellets, wherein CaO is added to UO ₂ powder for compression molding and sintering, and Ca _x U _1-x O _y (0 <x < UO in the form of 1,3 ≦ y ≦ 4)
₂ a mixing step of adding Ca to the powder, and a sintering step of heating the compression molded body of the powder raw material obtained in the mixing step under a temperature condition of 1400 to 1800 ° C. for 2 hours or more in a reducing atmosphere, A method for producing (Ca, U) O ₂ -based fuel pellets, which comprises: 2. A method for producing a (Ca, U) O ₂ -based fuel pellet, in which CaO is added to UO ₂ powder for compression molding and sintering, Ca _x U _1-x O _y (0 <x < UO in the form of 1,3 ≦ y ≦ 4)
_{2 The} mixing step of adding Ca to the powder, the compression molding of the powder raw material obtained in the mixing step, CO ₂
In a slightly oxidizing atmosphere containing 1100 ~ 1400 ° C. heating step for 30 minutes or more under temperature conditions, after the sintering step, the molded body, 1100 ~ in a reducing atmosphere containing H ₂ ~ A method for producing (Ca, U) O ₂ -based fuel pellets, comprising: an O / U ratio adjusting step of heating under a temperature condition of 1400 ° C.