Corresponding author: Sergey B. Vygovsky ( vigovskii@mail.ru ) Academic editor: Georgy Tikhomirov
© 2019 Sergey B. Vygovsky, Fedor V. Gruzdov, Rashdan T. Al Malkawi.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Vygovskiy SB, Gruzdov FV, Al Malkawi RT (2019) A study into the dependence of the claddingfuel pellet gap conductance on burnup and the effects on the reactor core neutronic performance. Nuclear Energy and Technology 5(2): 97102. https://doi.org/10.3897/nucet.5.35579

This paper presents the results of the research to study the dependence of the VVER1000 (1200) cores neutronic characteristics on the cladding – fuel pellet gap conductance coefficient in the process of the fuel burnup. The purpose of the study was to determine more accurately the dependence of the cladding – fuel pellet gap conductance coefficient on the fuel burnup as shown in the Final Safety Report for the Bushehr NPP and to determine the extent of the effects this dependence had on the spatial distribution of the neutron field, on the xenon accumulation rate, and on the kinetic and dynamic behavior of the reactor facility. The paper presents the results of calculating the parameters using which the heat engineering safety of the reactor core is monitored in the process of the fuel burn up (for a generalized fuel load of a VVER1000) during the transition to an 18month nuclear fuel cycle. This paper also includes the results of a numerical research to determine the cladding – fuel gap conductance coefficient depending on the fuel burnup. These results have shown that, in reality, the gap conductance coefficient dependence on the burnup does not affect greatly the steadystate characteristics. At the same time, it affects to rather a great extent the xenon accumulation rate, specifically in the event of an extended fuel life. In conditions of maneuvering (load following) modes accompanied by the xenon processes in the reactor core. These facts should be into consideration in design of engineering codes, that used to support the operation of the VVER1000 (1200) and fullscale simulators.
VVER1000, gap conductance coefficient, burnup, xenon oscillations, reactivity, Doppler effect
The paper presents the results of the studies into the conductance of the gas gap between the cladding and the fuel pellet (the gap hereinafter for brevity) in the VVER fuel as a function of fuel burnup and the effects this dependence has on the neutronic performance of the reactor core. The timeliness of the studies is explained by the need for the gap conductance parameters and their influence on the magnitude of the power reactivity effect to be taken into account with greater accuracy in fullscale codes. This is especially important for the more accurate simulation of incore xenon transients during the reactor plant modes with transition from one power value to another. Such modes involve incore xenon transients leading potentially to local power fluctuations across the reactor core.
The purpose of the study was to investigate the dependence of the VVER1000 (1200) cores neutronic performance on the gap conductance behavior in a fuel element in the process of burnup. The study deals with a problem of determining the extent of influence the socalled small effects in reactor physics have on the neutronic performance of thermal neutron reactors. A small effect has nothing to do with manufacturing tolerances or design errors. It is the authors’ opinion that the effect in question is the result of the failure to take into account the thermal resistance of the gap between the fuel cladding and the fuel matrix (Thermal Contact Resistance or the TCR hereinafter) in the VVER1000 as a function of fuel burnup in standard codes for the computational support of the NPP operation which is of a vital importance for some of the operating modes. These phenomena manifest themselves largely in maneuvering modes taking place during the reactor plant transition from one power to another and involving incore xenon processes, as well as in fuel burnup modes in conditions of extended fuel cycles (up to 18 months). While, earlier, the VVER1000 maneuvering modes were oneoff in the event of primary and secondary frequency regulation in the energy system, VVER1200 NPPs nowadays have scheduled testing of daily modes with power maneuvers in a broad range of their values at an arbitrary reactor life point (ATOMEXPO2010). In daily modes with a change of power, Doppler reactivity effect, which accounts for the most part of the power effect, is the key stabilization factor for the xenon local power oscillations across the core. As the gap conductance affects considerably the temperature distribution in the fuel and, hence, the magnitude of the power reactivity effect, greater emphasis needs to be placed on a more accurate calculation of this reactivity effect in integrated models used in engineering codes, that used to support the operation of the VVER1000 (1200) and fullscale simulators.
With only the dependence of the gap conductance relied on for fresh fuel with no burnup effects taken into account, the magnitude of the temperature reactivity effect at different moments of the fuel cycle can be misjudged severely. The physics of the phenomena taking place in fuel during burnup is as follows (
The fuel matrix swells and the fuel cracks in the radial direction at the initial burnup point as gaseous fission products are formed. This leads to a reduction of the fuel element gap between the fuel and the cladding and to an increase in the gap conductance.
The intensity of these processes depends to a large extent on the fuel pellet diameter, the availability of a central hole, and the specific heat load. The larger the diameter of the pellet and the smaller the pellet’s central hole are, and the greater the fuel’s specific heat load is, the more pronounced the process described is. For example, there is a more marked TCR increase of a stable and steadystate nature observed at high burnup values for the KONVOIdesign fuel matrix used in the fuel elements of the reactor core at the Gösgen NPP, Switzerland, as shown by the calculated TCR dependences in the design documentation. The data on the PWR reactor fuel TCR dependences on power and fuel burnup has also been provided by the Gösgen experts.
Based on the calculated Gösgen TCR dependences, the authors suggest that, with high burnup values and a high intensity of the gaseous fission product accumulation (at a high specific power), gases pass through the radial cracks reaching the fuel pellet periphery and the swelling process slows down. This process is illustrated in Fig.
Data from the Bushehr NPP final safety analysis report was analyzed (
The conductance processes in a fuel element were simulated using the procedures to calculate Thermal conductivity coefficient for uranium fuel (UO_{2}) without taking into account the dependence on fuel burnup and temperature (see Fig.
The gap TCRs were calculated as a function of burnup at different specific power values based on the developed approximation and compared against the TCRs for the PWR1000 KONVOIseries fuel dependence (Western dependence hereinafter) (
With regard for the dependences of the TCR and Thermal conductivity coefficient on temperature and burnup, computational studies were performed, using the PROSTOR code, to determine the extent to which these dependences affected the reactor core neutronic performance. In the first place, a study was conducted to investigate the influence the gap conductance dependence on burnup has on the VVER1000 core steadystate characteristics during burnup or, more specifically, on the duration of the reactor fuel cycle and the spatial neutron field distribution across the core. The fuel burnup was calculated for the stationary fuel load in one of Russian NPPs. This load is a generalized example of fuel loads in VVER1000 during the transition to an 18month fuel cycle. The calculations used the gap conductance dependences on fuel temperature, based on data from chief engineer, without taking into account the dependence on burnup (option 1) Gap Conductance Coefficient taking into account the dependence on burnup (option 2). The calculations were performed for the base power of 3120 MW and fuel cycle extension due to the power reactivity effect at a power reduction to 75% of the nominal value.
Fig.
Fig.
The maximum difference between the calculated and the maximum allowable linear power on the fuel element of the VVER1000 at the end of the cycle for stationary fuel loading.: 1 – Gap Conductance Coefficient without taking into account the dependence on burnup; 2 – Gap Conductance Coefficient taking into account the dependence on burnup
Dependence of boric acid concentration on the operating time of stationary fuel loading of the VVER1000 at the end of the cycle: 1 – Gap Conductance Coefficient without taking into account the dependence on burnup; 2 – Gap Conductance Coefficient taking into account the dependence on burnup
It can be concluded from the calculation results that there is a small increase in the duration of the reactor cycle for stationary fuel load, with regard for the dependence of conductance on fuel burnup, which is explained, on the one hand, by a decrease in the effective fuel temperature which improves the neutron multiplying properties of the VVER fuel lattice (
Despite of a small decrease in the linear power margin, it needs to be noted that this leads to somewhat worsened conditions of the reactor core safe operation. Though the extent of the variations obtained is small, the effects the gap conductance dependence on burnup has on the local power axial distribution across the reactor core shall not be neglected.
The effects the dependence of the gap conductance on burnup has both on the characteristics and the dynamics of incore xenon processes has been investigated. Calculations were performed for stationary fuel load with a base power of 3120 MW for an 18month fuel cycle in different moments of the cycle. The configuration of the incore axial neutron field was changed by the 20% insertion of the group of regulating control rods and retained in the resultant position for three hours. The reactor power was set as equal to and kept at 75 % of the nominal value by changing the critical concentration of boric acid. Calculations were performed for the moment of cycle: 150, 350 and 485 effective days. The dependences of axial offset on time for 485 effective days are presented in Fig.
The need for taking into account the dependence of the gap conductance on fuel burnup is explained by the fact that a high burnup leads to the greatest possible change in the gap conductance values with, accordingly, the maximum influence on the radial temperature distribution in the fuel pellet. The effect of the thermal contact resistance reduction is comparable with that from the decrease in the fuel conductance. Therefore, it is critical to take into account the dependence of the gap conductance on fuel burnup leading to a change in the radial temperature distribution in fuel such that the average temperature and, accordingly, the effective fuel temperature decrease as compared to the calculation of the temperature fields in fuel without this effect taken into account. The Doppler effect improves to a certain extent the reactor core’s neutron multiplying properties and leads to increase in the duration of the reactor cycle and a local power growth in the core’s upper and lower parts. The presence of this effect reduces the reactor stability to the local power xenon oscillations in the core at the end of cycle.
Calculations to justify safe operation of the NPP equipment involve the socalled principle of conservatism that can be briefly described as follows. When employed, any approximations and simplifications in the calculation procedures for the nuclear plant safety justification can reduce the nuclear safety level. A valid conclusion on the nuclear safety during calculation of a phenomenon can be made if the calculations really show a safe operation level. And where they provide the results leading indirectly to an increased level of the nuclear safety, approximations and simplifications are better not to be used. It follows from this that there is a need for an increased calculation accuracy of the gap conductance with its dependence on fuel burnup. These facts should be into consideration in design of engineering codes, that used to support the operation of the VVER1000 (1200) and fullscale simulators.