Dioxide Plutonium Storage
Experimental Investigation and Mathematical Modeling of Radiation-Chemical Processes in the PuO2 – sorbed Water System
Tech Area / Field
- CHE-RAD/Photo and Radiation Chemistry/Chemistry
- ENV-RWT/Radioactive Waste Treatment/Environment
8 Project completed
Senior Project Manager
Genisaretskaya S V
All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow
- CEA / Direction de l'Energie Nucleaire (DEN) / Department de Radiochimie et Procedes, France, Bagnoles sur Sèze\nBritish Nuclear Fuels Ltd (BNFL) / Waste Disposal Research Group Sellafield R&D, UK, Cumbria, Seascale\nLos Alamos National Laboratory, USA, NM, Los-Alamos
Project summaryCurrently the US and Russia are on the way to dispose of excess weapons-grade plutonium withdrawn from defense programs and use it as MOX fuel in commercial nuclear reactors. The plutonium disposition endeavor includes the following main stages: purification of plutonium metal, its transformation to PuO2, homogenization of clean feed materials, manufacture of MOX fuel pellets, placing fuel pellets in fuel rods and then transfer them in the form of fuel assemblies to a specific NPP for burning. At interim stages plutonium dioxide shall be stored for some time.
The pathway of long-term storage of PuO2 must be also under consideration.
In the US, PuO2 is processed, packed and stored according to the DOE 3013-2000 standard. The DOE 3013 standard specifies that PuO2 must be calcined at 950 °C using a temperature profile and verification of moisture content prior to packaging in a sealed container. Preliminarily, PuO2 fed to MOX fuel fabrication facility is to be stabilized at temperatures of up to 800 °C. Since PuO2 stabilization temperature governs the specific surface area (SSA) of stored PuO2 powder at the indicated temperatures SSA will be within 5-30 m2/g PuO2.
Plutonium dioxide is a hygroscopic material. Having been calcined at a temperature of 700 °C and stored within several days in moistened air the material may contain 2 or more wt % water. One may consider that water absorbed from air will be equally distributed onto the powder bulk surface area. In practice there could be phenomena, when water is absorbed on the dioxide from a liquid phase. Furthermore, it forms a film on the powder outer surface. We suggest to categorize these two instances as follows: water absorbed on the powder total surface - H2O (total S.) and water absorbed on the powder outer surface - H2O (outer S.).
Thus, under storage PuO2 powder may have different SSA and contents of water sorbed from air or liquid phase.
As a result of plutonium alpha-emission absorbed water will be subject to decomposition resulting in the formation of hydrogen and oxygen. These phenomena may give rise to hazards associated with the increase of pressure inside the container.
One of the main practical tasks for today is to design and construct a container for safe storage and transportation of plutonium dioxide. For that purpose, reliable experimental data on kinetics of H2 and O2 buildup are needed. A mathematical model would be also obtained to calculate the amount of H2 and O2 generated, as well as their pressure volumes in the container.
In the USA and Russia investigations are under way to determine Н2 and О2 buildup in the PuO2–absorbed water system. The formation of those products was studied experimentally and theoretically in greater detail in the PuO2 – H2O (outer S.) system (М.V.Vladimirova, I.А.Kulikov”, Н2 and О2 formation due to radiolysis of water absorbed onto PuO2”. Radiochemistry. 2002. Vol.44. P. 83); М.V.Vladimirova,”Mathematical model of radiolysis of water absorbed onto PuO2”. Radiochemistry. 2002. Vol. 44. P. 455). It has been shown that the Н2 and О2 formation rates correlate with increase in initial water content (from 0.3 to 3%). If weapons-grade Pu is stored for sufficiently long time periods (600 days) water completely decomposes and the concentration of hydrogen becomes steady-to-state. The mathematical model correctly describes kinetics of hydrogen and oxygen formation for various initial data.
The PuO2 - H2O (total S.) system is described briefly in the scientific literature. In the Los Alamos report LA-13781-MS 2000 experimental data are described, which point out that the amount of hydrogen generated in the system with Н2О (total S.) is much less than in the system with Н2О (outer S.). In the work of M. Duffey and R.R. Livingston presented at the 5th Conference on Spent Nuclear Fuel and Nuclear Materials Storage (Charleston, 2002) it has been demonstrated that the rate of hydrogen generation increases with PuO2 calcining temperature for a constant water content. These data, of substantial importance, have to be completely explained and appropriately reconciled.
Therefore, an important and essential question comes to mind – why is the radiolysis rate of water absorbed onto plutonium dioxide from the gaseous phase is much less than that of water absorbed from the liquid phase.
To correctly understand this issue in this Project systematic research will be carried out to determine the hydrogen and oxygen formation kinetics in the PuO2 – H2O (total S.) system. In addition, radiation-chemical processes of product generation and chemical processes of decomposition will be mathematically modeled. In this aspect, a model developed for the PuO2 – H2O (outer S.) system at earlier stages of investigation will be applied.
The experimental work will be predominately performed at room temperature, although a few tests will be conducted at sample and system temperatures of 50 – 100 °С. Furthermore, a few experiments will be carried out under plutonium dioxide powder gamma-irradiation.
Plutonium dioxide samples will be prepared by oxalate precipitation technology.
The quantities of hydrogen, oxygen and the initial rates of their formation (time 10 – 20 days) depending on SSA (8 – 30 m2/ g PuO2) at the given water content and on the water content (1 – 4%) at the given SSA in vacua and in air shall be acquired experimentally.
The acquired values of initial rates of hydrogen and oxygen formation in presence and absence of air will make it possible to find the rates and rate constants of their formation in radiolysis as well as rates and constants of rates of their recombination to form water.
Using the resultant values of reaction rate constants the quantities of H2 and O2 shall be calculated in wide range of holding time of specimens that shall be compared with the experimental data. Comparison between the calculated and experimental data shall allow a more exact determination of quantitative characteristics of processes under way and mathematical model as well as the basic criteria that influence the rate of hydrogen and oxygen formation in PuO2 – H2O (total S.) system. The mathematical model will make it feasible to calculate the radiolytic gas pressure and kinetics of pressure growth within a container.
The final objectives of the Project are to acquire experimental and calculated data needed to design plutonium dioxide storage container and identify mechanisms of radiation-chemical processes in the PuO2 – sorbed water system.