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Safety of nuclear fuel

Objective

Specific Objectives

1. To investigate the restructuring of LWR fuel (UO2 and MOX) at very high burnups, check its thermo physical and thermodynamic stability, and define parametric limits for safe in-pile operation;

2. To determine the phase diagram of the U-FP-O system at very high FP contents;

3. To confirm evolution of Young Modulus vs. burn-up, suggesting progressive decrease of the bond strength (stiffness) with burn-up in the range 0-50
GWd/tM, including structural explanation;

4. To measure fission-gas release and sublimation rate of fission products and actinides from high burn-up UO2 and MOX fuel by means of Knudsen-cell effusion experiments under oxidising conditions;

5. To describe the fuel performance and calculate with mechanistic computer codes the safe operation limits and fuel behaviour during operational and accidental temperature transients;

6. To determinate the annealing temperatures for alpha and neutron damage recovering (relation to fuel restructuring) using X-Ray diffraction (including micro-beam technique) and TEM;

7. To develop the fabrication processes for innovative fuels: thorium-based fuel, nitrides, CERMET. (Note: this could also be placed in the Project objectives);

8. To improve the Pu distribution in MIMAS MOX by using additives or advanced UO2 (Five parameters on each method); the goal is to reach 95% TD thermally stable pellets with high homogeneity);

9. To produce Th-MOX by infiltration of ThO2 micro spheres (one batch) and to measure Thermo physical and thermo chemical properties.
Planned Deliverables

Specific deliverables to DGs:

- Contribution to formulation of EU policies in the area of waste management and fuel cycle safety;

- Publications, reports;

- Participation in advisory and consultants meetings.

As a result of the research:

- Collection of PIE micro-and macro-structural data of fuel rods of UO2 and MOX irradiated up to above 70.000MWd/t;

- Further measurements and analysis of the fission gas release enhancement at high burnups;

- Extension of the TRANSURANUS -VVR version to reactor power transients (in connection with a PECO Project)

- Development of TRANSURANUS for new IM fuels;

- Data base enlargement (for code predictions) regarding elastic and plastic deformation of UO2 vs. irradiation time;

- Production of two fuel pins for the THOMOX irradiation programme in BR2.

Summary of 2000 Deliverables: 31/12/2000

Highlight 2000: Successful start of new European network collaborations. In 2000, a number of new projects (running in the next 3 to 4 years), were started with European partners, in the frame of the DG RTD shared cost actions (e.g. fabrication of MOX with a new microstructure, production of coated particles for the HTR, evaluation of thorium fuel for plutonium incineration, study of nitride inert matrix fuel and evaluation of fuel-cladding interaction). In these projects ITU covers both the manufacturing of new types of nuclear fuel (using innovative methods such as sol-gel) and the analysis of this fuel after irradiation (by using the equipment of the ITU hot cells). It permits to understand and optimise the fuel behaviour and to analyse e.g. the safety in transient conditions in a power reactor.

Systematic micro X-ray diffraction studies were carried out of LWR-fuels in the range of 30 to 110GWd/tM to determine the level of irradiation damage, build-up (and recovery) of strains before and after rim-transformation, effects of solid fission products and Oxygen/Metal ratio. The innovative equipment and measurement technique used for this purpose has been patented.

From the high burn-up studies on UO2 it could be derived that radiation enhanced diffusion can result in a high percentage of fission gas release from the transformed grains of the rim-structure during a single irradiation cycle. Most significantly, radiation enhanced diffusion can account for the observation made that the transformed grains contain a fixed, low concentration of fission gas independent of the burn-up accumulated since the transformation of the microstructure occurred.

The release of the fission products and actinides and the vaporization curve of the UO2 matrix were measured to the temperature of complete vaporization of the sample (>2500K). A model describes the fractional release of the fission gases, during thermal annealing, in term of combined diffusion and trapping processes. Three gas migration steps were defined: venting of pores and grain boundaries, volume diffusion from the body to the grain boundaries and release of the gas trapped in immobile intragranular bubbles. It has been observed that the relative importance of these steps is strongly dependant on the irradiation conditions, burn up and in pile temperature of the samples.

In the TRANSURANUS code the high burn up model (Xe depletion and fission gas release) was further developed, the first tests of the homogeneous MOX model were compared to experimental results and knowledge transfer was implemented in a Transuranus user meeting with 20 participants from 8 countries.

Output Indicators and Impact

- Workshops and Courses;

- Knowledge and computer code transfer to industry, licensing authorities, and Research Centres within Europe, including candidate member states in Eastern Europe.

Global output: 4 publications in refereed journals, 2 ITU internal reports and 6 conference participation
Summary of the project

The objective is to contribute to nuclear safety improvements by studying in detail phenomena that occur in light water reactor fuel rods at extended times of operation and which may endanger the integrity of fuel rods. More specifically, the studies will concentrate on technical factors that limit the achievement of higher burn-up of fuel, such as the fuel cladding mechanical interaction, the outer corrosion of the zircaloy cladding, phenomena related to the special MOX structure, the resulting hydrogen pick-up and the enhanced fission gas release associated with the formation of the high burn-up structure. Furthermore detailed investigations of fuel behaviour under LOCA or reactor accident conditions will be carried out.

The focus of all these investigations will evolve from high burn-up UO2 to high burn-up mixed oxide fuel. The fuel performance code TRANSURANUS will be further developed to handle Gd and MOX fuel.

Advanced fuel fabrication techniques will be developed to overcome problems related to accumulation of dust that results in an increase in personnel doses and in maintenance work that will be particularly difficult during decommissioning. These advanced techniques aim therefore at increased operational safety and efficiency and are important in view of the potential recycling of minor actinides in a partitioning and transmutation scheme.

Rationale

The safety of nuclear installations remains a major public concern even if energy production by nuclear fission is considered a mature technology. Licensing authorities are vigilant and push for safety improvement whereas industry is pressed to increase operational efficiency and constantly integrate new developments. Recent examples are the extension of the lifetime of fuel in the nuclear reactor and the development and testing of new fuels designed to reduce the civil and military stockpiles of plutonium.

Incineration or transmutation of long-term radiotoxic plutonium and other actinides require advanced technology for the fuel fabrication.

The Institute supports these efforts, which are at the core of its mission. The unique facilities, equipment and recognised competence in the area of nuclear fuel properties and behaviour make it the appropriate place and an ideal reference centre for fuel performance data that are of relevance to nuclear safety.

Funding Scheme

JRC - Joint Research Centre research

Coordinator

Institute for Transuranium Elements
Address

Karlsruhe
Germany