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Thermal Damage of the Fissile Material Container Protection Layers

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Fissile Material Container Behavior Modeling in the Event of Fire-Induced Thermal Damage of the Heat Protection Layers

Tech Area / Field

  • ENV-MRA/Modelling and Risk Assessment/Environment
  • CHE-THE/Physical and Theoretical Chemistry/Chemistry
  • MAT-OTH/Other/Materials

Status
8 Project completed

Registration date
19.09.2001

Completion date
02.03.2004

Senior Project Manager
Tyurin I A

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Collaborators

  • Los Alamos National Laboratory / D-10 Nuclear Systems Design & Analysis, MS K575, USA, NM, Los-Alamos

Project summary

The project goal is to develop computer simulation tools for thermal processes in a container with fissile materials considering thermal decomposition of the container protection layers.

The project deals with the containers intended for the transportation and storage of excessive fissile materials (plutonium, uranium and their alloys). The excessive fissile materials (further referred to as fissile materials) mean materials obtained from dismantling of nuclear warheads and transferred to the civil structures. AT-400R container serves as an example of such container. This container was developed for fissile material storage and transportation at Russian Fissile Material Storage Facility, built at Mayak site in Russia.

Several field experiments with the AT-400R containers were carried out to study the effects of the fire that is more severe than that stated by the IAEA requirements (fire temperature is 800 °C, fire duration is 0.5 hour). These experiments demonstrate the occurrence and intensive progress of physical and chemical phenomena and processes (melting, vaporization, sublimation, carbonization and even burn-out of the heat protection layers of the container) changing significantly the safety characteristics of the container.

This necessitates the measures to increase the safety and to reduce the risk of container handling in such situation. For the development of these measures, the computer simulation tools are of great importance (computational and experimental models, computer codes).

At this point the results of computer simulation of the container thermal state under severe fire have the evaluation nature for the following reasons:

– heat transfer processes are modeled only in chemically stable materials;


– no thermophysical models are available to describe the thermal decomposition in thermal protection layers;
– no characteristics are available for the decomposition of protection layer materials under high temperatures.

Our project would fill this gap. The main project goal is to develop the methods and codes and to obtain experimental data allowing to model the thermal processes and thermal destruction effects in container protection layers.

The project would result in an efficient tool for predicting the container thermal mode under intensive and long-duration fires. The project would allow a higher accuracy prediction of the container behavior under fire which in turn will contribute to the minimization of the number of field experiments. The results of container modeling under fire are especially important for obtaining the estimates and for the development of recommendations for increasing the safety and for the reduction of the risk of container handling. Currently this issue is urgent because of building of fissile materials storage facility at the Mayak plant.

The project efforts will integrate the high skill experts in thermal physics, heat and mass transfer, fissile material handling, computer simulation and thermal analysis (experimental study of thermal destruction and thermophysical material characteristics). Particularly, they have a long experience in modeling thermal modes of fissile material containers and in studies of materials that can bear the thermal destruction.

The expected results of the project would be the development of the following elements:

– Heat transfer models for chemically and thermodynamically unstable materials (in terms of chemophase transitions) and their implementation in computer codes.

– Kinetic and thermophysical characteristics of thermally decomposing polymer materials used in the protection layers of fissile material containers. The characteristics of volatile decomposition products including the toxic properties.

– Computational method for thermal modes of fissile material containers under fire to include phase transitions, thermal decomposition and burn-out of protection layers, formation and motion of volatile decomposition products, radiant heat exchange in occurring container cavities.

– Computer model for the AT-400R container under intensive and long duration fire. The modeling results for АТ-400R and safety evaluation in terms of thermal modes.

The above mentioned project results could be used for:

– the analysis of existing and development of new FM containers;


– the safety analysis and acquisition of risk estimates for the container handling;
– thermophysical calculations of various processes involving thermally decomposing materials.

It should be noted that the analysis of AT-400R container behavior under fire is interesting both for Russia and USA since USA use the containers with similar material characteristics.

The work is essentially of applied nature. The scientific significance of the results is that they are relatively general and therefore can be used to study a variety of processes followed by the thermal decomposition of materials.

The project proposed would contribute to the following ISTC goals and objectives:

– allow the weapons scientists and engineers to redirect their efforts to peaceful activity;


– promote the integration of CIS scientists to the international community;
– support the basic and applied research and development of technologies for peaceful purposes particularly in the environment protection, energy production and nuclear safety;
– contribute to the resolution of national and international engineering problems;
– support the transition to the market economy meeting the civil needs.

The project implementation will require to accomplish the following tasks:

– Develop and implement the thermal decomposition model for polymer materials.


– Obtain the kinetic and thermophysical characteristics of thermally decomposing polymer materials used in FM containers.
– Develop the computational method for the thermal modes of FM containers under fire to include phase transitions, thermal decomposition and burn-out of protection layers, formation and motion of volatile decomposition products, radiant heat exchange in occurring container cavities
– Develop a computer model for AT-400R container under intensive and long duration fire, carry out the calculations and compare the theory with the experiment. Evaluate the effect of the variation of the container characteristics on the safety parameters.

The project submission to the ISTC is supported by the foreign collaborator from LANL. The collaboration with LANL would be in the following forms:

– Consultations, recommendations, supervision by LANL.


– Data exchange in the course of the project efforts.
– Comments to the final technical report submitted to the ISTC.
– Aid in publication of results, in participation of project participants in international workshops.
– Cross verification of the project results.
– Joint workshop.
– Joint evaluation of the impact of the project results on the safety characteristics of AT-400R container.

The development of the thermal decomposition model would use the existing theoretical and experimental data in this field.

To obtain the characteristics of thermally decomposing materials, we would use the experimental thermal analysis methods such as thermal gravimetry, calorimetry, detection of released gases and thermophysical method. The following variations of the polymer property study conditions would be provided:

– materials: foamed polyurethane, polypropylene, polyethylene;


– medium: inert, air;
– medium pressure: 1-2.5 at;
– heating rate: 0-100 K/min;
– varying density of foamed polyurethane.

The computational method for FM container thermal modes will rely upon the following modifications of the heat conduction problem:

– thermophysical material properties as a function of the decomposition temperature and kinetics;


– introduction of convection terms in the energy equation to describe the filtering of gaseous decomposition products and front nature of thermal destruction of the protection layer.

The method would be implemented as customizing of ANSYS code based on FEM. The radiant heat exchange would be accounted by the ANSYS tools. A detailed analysis of fluxes inside the container would be carried out by Flow-3D code.

The models and methods would be tested and verified by comparing the prediction and experiment data (both existing and obtained) demonstrating the effect of fire on the polymer samples and on the FM containers.


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