Corresponding author: Pavel A. Alekseev ( palekseev@ippe.ru ) Academic editor: Yury Korovin
© 2018 Pavel A. Alekseev, Aleksei D. Krotov, Mikhail K. Ovcharenko, Vladimir A. Linnik.
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:
Alekseev PA, Krotov AD, Ovcharenko MK, Linnik VA (2018) Minimize fission power peaking factor in radial direction of watercooled and watermoderated thermionic conversion reactor core. Nuclear Energy and Technology 4(1): 711. https://doi.org/10.3897/nucet.4.29453

The paper investigates the possibility for reducing the radial power peaking factor k_{r} inside the core of a watercooled watermoderated thermionic converter reactor (TCR). Due to a highly nonuniform power density, the TCR generates less electric power and the temperature increases in components of the thermionic fuel elements, leading so to a shorter reactor life.
A TCR with an intermediate neutron spectrum has its thermionic fuel elements (TFE) arranged inside the core in concentric circles, this providing for a nonuniform TFE spacing and reduces k_{r}. The watercooled watermoderated TCR under consideration has a much larger number of TFEs arranged in a hexagonal lattice with a uniform pitch. Power density flattening in a core with a uniformpitch lattice can be achieved, e.g., through using different fuel enrichment in core or using additional incore structures. The former requires different TFE types to be taken into account and developed while the latter may cause degradation of the reactor neutronic parameters; all this will affect the design’s economic efficiency.
It is proposed that the core should be split into sections with each section having its own uniform lattice pitch which increases in the direction from the center to the periphery leading so to the radial power density factor decreasing to 1.06. The number of the sections the core is split into depends on the lattice pitch, the TFE type and size, the reflector thickness, and the reactor design constraints. The best lattice spacing options for each section can be selected using the procedure based on a genetic algorithm technology which allows finding solutions that satisfy to a number of conditions.
This approach does not require the reactor dimensions to be increased, different TFE types to be taken into account and developed, or extra structures to be installed at the core center.
Thermionic converter reactor, thermionic fuel element, power peaking factor, lattice pitch, genetic algorithm, optimization
The use of thermionic converter reactors (TCR) for nuclear power systems (NPS) will make it possible to build a cost effective compact independent heat and electricity source.
Instead of zirconium hydride and sodiumpotassium eutectics (
A watercooled watermoderated converter reactor for a small NPP (AISTMP), capable to serve as a cost effective independent heat and electricity source, is described in (
A cooperation of J SC «SSC RFIPPE named after A.I. Leypunsky», FSUE «SRI SIA «LUCH» and JSC «Afrikantov OKBM» has presented an innovative design of a small NPP based on a thermionic nuclear system for supply of power to remote facilities in the Arctic region (Fig.
Such a large number of TFEs in the core is arranged in a hexagonal lattice, in contrast to the arrangement in concentric circles as in a traditional TCR with an intermediate neutron spectrum. This defines the reactor dimensions and contributes to the natural coolant circulation in the core, and, so, to the NPS cost effectiveness.
To compare with, the TCRs in the TOPAZ and Yenisey nuclear power systems had 79 and 37 TFEs respectively. Heat was removed by the forced coolant circulation in the annulus of each TFE (
With a large number of TFEs arranged in concentric circles, the power density flattening through a nonuniform spacing leads to a greater core diameter.
On the other hand, with TFEs arranged in a uniformpitch lattice, all other conditions being equal, it is not possible to influence the power density distribution.
Fig.
It follows from the figure that the steadystate and continued operation of a TCR with the TFEs arranged in a uniformpitch lattice requires k_{r} to be reduced.
Power density flattening in a core with a uniform lattice pitch can be achieved, e.g., through using different fuel enrichment in core or using additional structures inside the core. The former requires different TFE types to be taken into account and developed, while the latter may lead to worsened reactor neutronic performance; all this will affect the design’s economic efficiency (
The paper investigates the possibility for reducing the radial power peaking factor in a core with a uniform lattice pitch. It is proposed that the core should be split into sections, each with its own uniform pitch which increases in the direction from the center to the periphery.
The core of a watercooled watermoderated TCR for an NPS is composed of TFEs arranged in a uniformpitch hexagonal lattice. The core also contains reactivity control rods (CR), safety rods (SR) and displacers. The core is surrounded by a beryllium reflector (Fig.
The MNCP code (
This model contains 301 TFEs, and the core diameter is such that the rotarytype CRs or the control drums (CD) (see Fig.
For the power density calculation, the CRs were assumed to be positioned at such height as would lead to the effective neutron multiplication factor equal to unity, that is, to a critical reactor state. With the uniform lattice pitch equal to the TFE diameter plus the gap, the k_{r} value was 1.39, and the maximum power density was as in the central TFEs. With such k_{r}, one can expect a 10% reduction in the output electric power as compared to the design value.
Fig.
It can be seen from the power density distribution that even a minor increase in the lattice pitch leads to a major reduction of k_{r} (to 1.12), that is, the power density on the reactor periphery grows due to a change in the wateruranium ratio in the two final TFE rows.
The core splitting into three sections (Fig.
The CRs at the hexagon apexes forming the central section required the channel cladding to be reshaped (the CR channel cladding is normally a regular steel cylinder). The core splitting near the TFEs adjoining the CRs leads to the formation of excessive water and, therefore, the power density grows. To remove excessive water, the tube cladding was designed as an elliptic cylinder which led to a reduced power density in the considered region (se Fig.
In a general case, the power density distribution will depend not only on the lattice pitch but also on the TFE type and size, the reflector thickness, and the reactor design constraints. So, for the reactor with another TFE design and another material composition, as well as with a larger reflector thickness, the k_{r} reduction to 1.08 became possible with the core split into four sections (Fig.
It is worth noting that the core diameter was 5.7% as small with the minimum uniform lattice pitch, but this required the reactivity margin decrease to be compensated by a larger reflector thickness. The k_{r} value was equal to 1.20. The core diameter did not change with a large lattice pitch and the reactivity margin was larger, but the k_{r} factor was equal to 1.28. With the average pitch, the core diameter was insignificantly smaller, and the reactivity margin coincided with the reactivity margin in the option with the core split into sections, and k_{r} was equal to 1.27.
Therefore, one may talk only about k_{r} being influenced exactly by the core splitting into sections with a uniform lattice pitch in each.
Such approach to power density flattening is useful when one needs to increase the TFE number without changing the core diameter. Thus, adding several tens of TFEs to the core with a specified diameter and the power density flattened (k_{r} equal to 1.08) thanks to a nonuniform spacing leads to the necessity to increase the diameter while k_{r} does not decrease to below 1.15.
And with the TFEs arranged in a hexagonal lattice and the core split into three sections, the k_{r} reduction to 1.10 can be achieved without changing the core diameter; the power density distribution by fuel elements for such case is shown in Fig.
The pitches for each of the sections were found in this case using an optimization procedure based on a genetic algorithm technology (
As a result, the lattice pitches related as 1/1.018/1.055 have been achieved, leading to k_{r} = 1.10. And the smallest relative power density corresponds to the TFEs situated near the SR channels and, for individual TFEs, on the periphery. The power density on the periphery can be reduced by optimizing the shape and the positions of the water displacers.
The power density surge in the central TFEs takes place due to the excessive water formed as the result of the core splitting, that is, such surges can be suppressed through the optimization of the SR tube shape and position.
This paper investigates the possibility for reducing the radial power peaking factor inside a core with a uniform lattice pitch. The core splitting into sections, each having its own uniform pitch with the lattice pitch increasing in the direction from the center to the periphery, leads to the radial power peaking factor reduction to 1.06. This approach does not require the reactor dimensions to be increased greatly, different TFE types to be taken into account and developed, or extra structures to be used at the core center.