Validation of Fuel Elements for PU Burning
Validation of Technical Feasibility of Burning Weapon's Grade and Civil PU Via New Generation PWR Fuel Elements Designed in ISTC Project N173-95
Tech Area / Field
- FIR-FUE/Reactor Fuels and Fuel Engineering/Fission Reactors
3 Approved without Funding
All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow
- Kurchatov Research Center, Russia, Moscow
- Belgonucléaire, Belgium, Brussels
Project summaryThis project is a logical continuation of the developments implemented under the contract between ISTC and SSC RF A.A.Bochvar VNIINM in the framework of Project N 173-95.
The implementation of the activities on Project № 173-95 resulted in the development of:
- methods of producing porous and pore-free dispersion cores with different types of high density nuclear fuel;
- laboratory technology of manufacturing dummy fuels with the above cores;
- design documentation on dummy fuels to be research reactor tested.
Based on the developed technologies several fuel design options were worked out to resolve specifically the following goals:
- to extend the burn-up 2 or 3 times higher than the existent one (specifically, up to 150,000 MWday/t U for the VVER and PWR reactors);
- to promote the variability of the U content (particularly, to reduce the enrichment);
- to provide the disposition of Pu in a fuel rod (specifically, to burn ex-weapons and civil Pu);
- to improve the fuel element reliability (specifically, to reduce the stored energy, to increase the heat transfer surface etc.);
- to lower the cost of the electricity generation and to improve the ecology.
To check up the reliability and assess the life-time of those fuel element options a joint SSC RF A.A.Bochvar VNIINM — SSC RF RIAR proposal was submitted to ISTC on Project N 721: «In-Pile Tests of New Generation Fuels for VVERs of Different Purpose» the funding of the Project has not yet been initiated.
The goal of the suggested Project is to analyze the feasibility of utilizing the fuel elements designed pursuant to Project N 173-95 to burn recovered civil and weapon’s grade Pu in domestic thermal reactors.
As an example, figs. 1 and 2 schematically illustrate two fuel element options as applied to the VVER-1000 fuel assembly (FA).
Fig. 1 is the option of a cylindrical Zr-alloy clad fuel element having the standard sizes with spacer grids required to space fuel elements within a FA.
Fig. 2 shows the option of a cross - shaped fuel element clad, e.g., in chromium-nickel alloy. This option makes use of self-spacing.
The fuel elements of both the options have Al-alloy matrices. The U-containing phases are UO2 (Figs. 1a and 2a, fuel elements are Pu generators) and PuO2 (Figs. 1b and 2b, fuel elements are Pu burners).
If needed, a minifuel element with a burnable absorber may be located in the centre of the PuO2 fuel elements.
Options are possible of fuel element assemblage (Figs. 1b and 2b) with minifuel elements containing long-lived toxic actinides to be burnt out.
The fuel element disposition within a reactor will be discussed.
The first schema uses a FA fully assembled of PuO2 fuel elements. This schema looks more preferable since after the irradiation Pu and its train from Pu fuel elements are reprocessed separately without U and its train (Figs. 1b, 2b) (the isotope composition of Pu and its train is different in Pu and U fuel elements). Due to the high reliability of the new fuel elements the cycle of various FAs may be varied in a wide range.
The second schema involved assembling a FA of UO2 (Figs 1a, 2a) and PuO2 (Figs. 1b, 2b) fuel elements.
But in this case if irradiated FAs are disassembled without the subpision fuel elements into two groups, spent U and Pu fuel is to be reprocessed jointly (which is the case of MOX fuel).
The third schema involves the use of PuO2 fuel elements (Figs. 1b, 2b) and standard VVER UO2 fuel elements assembled either within a single FA or in different FAs.
Both the suggested options have a high reliability since, first, the fuel elements are featured by pores that offset swelling (either within UO2 or minifuel PuO2 element). Second, due to the high effective thermal conductivity of a fuel element (because of the Al matrix) its operating temperature is low and, hence, gas induced swelling and deposited energy at the off-normal power are minimal. Aside from this, the self-spaced fuel element is devoid of the limitations inherent in spacer grids (deposits, debris etc.) that reduce the fuel element reliability.
Currently (specifically, ISTC Project N 369) the main and only scenario of using Pu as fuel for the VVER type reactor involves the option of partial (1/3 of the core) or complete loading MOX fuel into a reactor.
However, in the MOX fuel option the amount of generated Pu (discharged from the reactor) exceeds the amount of Pu loaded. This indicates that the accumulated and stored large amounts of isolated civil Pu and weapon’s grade one released as a result of the disarmament programme implementation cannot be reduced even to some extent in VVER-type reactors via the current route of the MOX fuel use. In the best case weapon’s grade Pu in its main mass may be converted to civil one and hence the «stock-piles» of civil Pu will be much increased.
The options of the fuel elements that burn and accumulate Pu (in smaller amounts) that were developed in the framework of Project N 173-95 open up the prospects of implementing the programme of actual Pu utilization but not its accumulation in the reactors of the VVER, PWR and BWR types.
It is assumed that after the accumulated Pu is utilized one will be able to load the VVER-type reactor with fuel elements - Pu burners and accumulators in such a way that the Pu that was generated in the previous cycle will be completely used up in the subsequent one. In countries that do not have significant amounts of accumulated civil Pu or in newly commissioned reactors this optimized loading schema could be realized immediately.
One should specify some positive features (aside from those mentioned above) inherent in those fuel elements:
- relatively small amounts of Pu (civil and weapon’s grade) are located separately aside from a huge mass of UO2 fuel, and they are not blended as in case of MOX;
- to fabricate Pu minifuel elements very simple equipment is needed.
The design of the fuel element allows for separate lines to load Pu and U;
- the fabrication technology and design of fuel elements are such that the requirements placed on UO2 and PuO2 powders are minimized; and the method of their preparation becomes unimportant;
- Pu in minifuel elements may be used essentially in the form of any compound (alloys, intermetallics etc.) if they allow granules to be prepared from them.
- design assessments of the possible safe operation of VVER-1000 type reactor fuelled with novel fuel rods;
- determine the possible quantity of Pu that can be loaded and reprocessed per a FA cycle;
- design assessments of temperature conditions and stress-strained fuel rod state;
- laboratory studies into the feasibility of implementing design charges of Pu and U.