WO2016122374A1

WO2016122374A1 - Fuel for water-cooled nuclear reactors

Info

Publication number: WO2016122374A1
Application number: PCT/SE2016/000004
Authority: WO
Inventors: Mikael Jolkkonen; Kyle Johnson; Janne Wallenius
Original assignee: Blykalla Reaktorer Stockholm Ab
Priority date: 2015-01-30
Filing date: 2016-01-29
Publication date: 2016-08-04
Also published as: CA3010876C; SE1500058A1; CA3010876A1

Abstract

The invention refers to a nuclear fuel for use in water cooled nuclear reactors. The fuel, which consists of at least 80 % uranium nitride, has an actinide density higher than 9.7 g/cm³ and an open porosity of less than 0.1 %. Consequently, the fuel is resistant to attacks from water and stream and can be used at temperatures above 250 degrees Centigrade.

Description

Fuel for water-cooled nuclear reactors

Description of the invention

The nuclear fuel is intended to permit longer residence time, alternatively a higher power, or a combination thereof for water-cooled nuclear reactor fuels. The actinide density of the fuel exceeds 9.7 g/cm³ and its open porosity is sufficiently low to significantly improve the corrosion resistance of the fuel. The fuel pellet is manufactured using conventional sintering, hot pressing, field assisted hot pressing, microwave assisted hot pressing or spark plasma sintering (SPS). In a preferred embodiment, the fuel consists of more than 80 volume percent uranium nitride (UN), with an open porosity below 0.1 %.

In another preferred embodiment, the fuel consists of more than 80 volume percent uranium nitride (UN), with an open porosity below 0.1 %. Up to 20 volume percent consists of a secondary phase with a lower melting point than UN, which thanks to higher plasticity acts as pore filler during sintering.

Summary of the invention

The purpose of the present invention is to provide a nuclear fuel combining an actinide density higher than 9.7 g/cm³ with an improved tolerance to water and steam at temperatures exceeding 250°C. A specific purpose of the present invention is to provide a uranium nitride fuel with improved tolerance to water and steam at temperatures exceeding 250°C.

This and other objectives are achieved by the invention as defend in the independent claims.

Yet other advantageous embodiments of the invention are specified in the dependent claims. The invention offers a solution to problems of corrosion of nuclear fuels with an actinide density higher than 9.7 g/cm³ that have been observed during exposure to water and steam at temperatures exceeding 250°C, and therefore facilitates the use of this fuel type in commercial water-cooled nuclear power plants. Especially nitride fuels have historically been difficult to manufacture with high density, resulting in a considerable fraction of open porosity in the pellets produced. Therefore, steam and pressurised hot water may penetrate and deeply oxidise the pellet. The larger specific volume of the forming uranium dioxide causes an inner expansion which rapidly fragmentises and eventually powderises the pellet. By manufacturing uranium nitride fuels, uranium silicide fuels and mixtures of these fuels with a sufficiently low fraction of open porosity, the oxidation process is limited to surface corrosion, which considerably delays the decomposition of the fuel.

Experiments on pellets with different porosity confirm that the rate of attack is more dependent on the porosity of the pellet than on reaction temperature, supporting the conclusion that improved protection against corrosion in steam and pressurised hot water is most easily achieved by eliminating the open porosity.

Uranium nitride fuels with very low porosity may be manufactured using spark plasma sintering (SPS) at a temperature exceeding 1600°C [Malkki 2014]. Another opportunity for eliminating open porosity in a uranium nitride fuel is achieved by blending a certain fraction of uranium silicide, having a melting temperature below 1600°C, into uranium nitride. During pellet manufacture at a temperature above 1600°C the uranium silicide will, thanks to its plasticity, constitute a pore filling second phase, reducing the porosity of the pellet. Industrial applicability

The invention is especially useful in light water and heavy water cooled nuclear power reactors. Example 1)

In this example, laboratory tests were conducted on uranium nitride pellets consisting of more than 80% UN, with a varying degree of porosity and content of UO2, U2N3 and uranium silicide. During exposure of UN to steam, uranium dioxide, ammonia and hydrogen gas form according to the reaction formula:

UN + 2H₂O -> UO₂ + NH₃ + 0.5 H₂

Parameters of the test:

Steam pressure: 0.5 bar Steam temperature: 400 - 425°C

The lab tests showed that the rate of corrosion attack on the uranium nitride pellet is strongly dependent of its porosity.

Figure 1 shows the measured rate of production for hydrogen and total hydrogen production rate as function of time for uranium nitride pellets with 2.3%, 13.0% and 22.4% porosity.

Example2)

In this example, uranium nitride pellets with varying degree of open porosity were manufactured using the SPS-method at different temperatures and pressures. The open porosity was measured using Archimedes' method with chloroform as medium. Figure 2 displays the open porosity as function of total porosity. The measurements show that when the total porosity of the uranium nitride pellet is less than 2.5%, open porosity is less than 0.1 %.

Claims

1 . A nuclear fuel for power production in water-cooled nuclear reactors, wherein the actinide density exceeds 9.7 g/cm³, wherein the fuel consists of at least 80 volume percent UN and wherein the open porosity is less than 0.1 %.

2. A nuclear fuel according to claim 1 , wherein up to 20 volume percent consists of a uranium silicide compound.