Corresponding author: Vladimir V. Knyshev ( vvk28@tpu.ru ) Academic editor: Yury Kazansky
© 2021 Vladimir V. Knyshev, Aleksandr G. Karengin, Igor V. Shamanin.
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:
Knyshev VV, Karengin AG, Shamanin IV (2021) Subcriticality control elements in a reactor system with an extended plasma source of neutrons with regard for temperature. Nuclear Energy and Technology 7(2): 97-101. https://doi.org/10.3897/nucet.7.68949
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Materials have been selected for the shim rods and burnable absorbers to compensate for the excessive reactivity of the facility’s blanket part and to provide for the possibility of reactivity control in conjunction with a plasma source of neutrons.
Burnable absorber is a layer of zirconium diboride (ZrB2) with a thickness of 100 μm applied to the surface of fuel compacts. Boron carbide (B4C) rods installed in the helium flow channels and used to bring the entire system into a state with keff = 0.95 have been selected as the shim rod material. Throughout its operating cycle, the facility is subcritical and is controlled using the neutron flux from the plasma source.
Verified codes, WIMS-D5B (ENDF/B-VII.0) and MCU5TPU (MCUDВ50), as well as a modern system of constants were used for the calculations.
The facility’s neutronic performance was simulated with regard for the changes in the inner structure and temperature of the microencapsulated fuel and fuel compact materials caused by long-term irradiation and by the migration of fission fragments and gaseous chemical compounds.
Fusion-fission reactor system, plasma neutron source, criticality, burnable absorber, control and protection system
High-temperature gas-cooled reactors (HTGR), due to the peculiarities of the core design composition and layout, feature advantageous characteristics in terms of nuclear safety and reliability (
Conventional reactor systems operate in a critical mode with the fission chain reaction controlled by influencing the balance of neutrons in the core’s breeding environment. Operation of the facility under consideration, which consists of a blanket (the facility’s energy generating part) and an external plasma source of fusion neutrons, differs from the traditional operating modes of the reactor, still the principle of control is basically the same and consists in influencing the balance of neutrons in the facility’s active blanket part. The effective multiplication factor for the loaded blanket of the facility under consideration is made up for to the values 0.95 to 0.98 and is maintained at the preset level using criticality control elements and systems. The plasma source generates extra neutrons and ensures, in conjunction with the control system, safe operation of the entire facility (
The efficiency of compensating systems depends on a number of parameters investigated by the authors in (
Fig.
Structurally, the blanket consists of hexagonal graphite blocks having channels for the accommodation of fuel and injection (flow) of helium surrounded by two rows of graphite blocks which perform the reflector function. At the top and at the bottom, the blanket part is shielded with graphite blocks installed in one row. The fuel block has 76 small-diameter channels for fuel and seven large-diameter channels for helium. The width across flats is 0.207 m, and the height is 0.8 m. The graphite blocks that shield the blanket part at the top and at the bottom have a width across flats of 0.207 and a height is 0.3 m. The microencapsulated (coated particle) fuel for the graphite fuel compacts represents spherical kernels of (Th,Pu)O2 with a diameter of 0.350×10–3 m covered with PyC and Ti3SiC2 layers of the thickness 0.90×10–4 m and 0.35×10–4 m respectively. The coated particle fuel is dispersed into the graphite matrix of the fuel compact to the surface of which an outer force coat of SiC is additionally applied. The fuel compact diameter is 10.17×10–3 m, the height is 20.10–3 m, and the thickness of the outer SiC layer is 0.3×10–3 m.
The BOL maximum fuel temperature was estimated based on a condition that there was bulk heat generation, qv (z,r), and surface heat generation, qs (z,r), in the fuel block. With the rated power of the facility being 60 MW, the maximum values qvmax(z,r) and qsmax(z,r) are respectively equal to 8.41×104 and 2.01×104 kW/m3.
The computational studies presented in (
A code, MCU5TPU (MCUDВ50), is used to calculate the evolution of the fuel’s nuclide composition and to estimate the radiation dose and the neutron fluence. The MCU-5 geometrical module makes it possible to simulate 3D systems with a geometry of any complexity when using a combined approach based on describing complex systems by combinations of elementary bodies and surfaces (MCU Project). The MCU-5 nuclide library includes an extensive list of isotopes and allows calculating the evolution of the nuclide composition and the criticality.
The effects of the fuel component’s nuclide composition and the blanket part’s thermophysical properties and temperature on the neutronic performance and efficiency of the compensating system have been calculated for an equivalent 2D cylindrical cell based on models in (
The blanket part of the facility is a modified HTGR core with a neutron spectrum close to the epithermal spectrum. Compounds employed traditionally in high-temperature reactors (B4С, B4С-SiC, Dy2O3TiO2, CrB2+Al, Gd2O3, ZrB2, AgInCd, Mo+Eu2O3, Hf-Zr, and others) were therefore used to select the effective materials for the shim rods (SR) and the burnable absorbers (BA) (
Passive reactivity should be achieved with the use of SRs and BAs in conjunction with a plasma neutron generator which, together with the SR and BA system, is required to compensate for the effects caused by the nuclear fuel burn-up, and by the blanket slagging and poisoning in the process of the startup and during long-term operation. This was achieved in a 69-group diffusion approximation by the joint use of the WIMS-D5B (ENDF/B-VII.0) code (
After identifying the neutron absorbing material, which is effective in the operating neutron spectrum, the SRs and BAs are calculated. The best possible SR arrangement in the facility’s blanket is also generated.
The calculation, the results of which are presented in Figs
The accumulation of fission fragments and gaseous compounds and the migration of these in conditions of long-term irradiation lead to a temperature growth in the kernel (
The results of investigating the coated particle fuel and fuel compact materials have shown that the conditions of operation do not exceed the permitted working maximums (
An analysis of the findings has shown that the best reactivity compensation options are those with a ZrB2 coat of the thickness 0.1·10–3 m applied to the surface of the fuel compacts and with B4C (Table
Material | keff, initial | keff, with rod | ρ of system | dρ (weight of one rod) |
---|---|---|---|---|
Rod diameter 0.01 m | ||||
B4C | 1.207558 | 1.207012 | 0.171508 | 0.000375 |
Eu2O3 | 1.207558 | 1.207119 | 0.171581 | 0.000301 |
Gd2O3 | 1.207558 | 1.207235 | 0.171661 | 0.000222 |
Rod diameter 0.03 m | ||||
B4C | 1.207558 | 1.205919 | 0.170757 | 0.001126 |
Eu2O3 | 1.207558 | 1.206108 | 0.170887 | 0.000996 |
Gd2O3 | 1.207558 | 1.206296 | 0.171016 | 0.000866 |
Rod diameter 0.05 m | ||||
B4C | 1.207558 | 1.205016 | 0.1701355 | 0.001747 |
Eu2O3 | 1.207558 | 1.20537 | 0.1703792 | 0.001503 |
Gd2O3 | 1.207558 | 1.20542 | 0.1704136 | 0.001469 |
Using ZrB2 makes it possible to reduce the effective multiplication factor to a value of keff = 1.069833. Further blanket subcriticality of up to keff = 0.95 was achieved using SRs of 0.01 m in diameter accommodated in the helium channels. The required number of rods is 138. The SR arrangement in the fuel block is presented in Fig.
The SR channel material is steel of the KhN55MVTs-VI(ID) (ChS57-VI(ID)) grade (Heat-resistant nickel-based alloy) which has proved itself to perform well in conditions of long-term in-pile irradiation.
Simulation of various options for the SR arrangement in blocks and for the block positions in the blanket made it possible to obtain the optimum SR arrangement presented in Fig.
The arrangement of fuel blocks with SRs shown in Fig.
We shall note that the use of a plasma neutron source improves the facility’s nuclear safety since switching off the injection of neutral atoms causes the generation of neutrons to drop by approximately a half for the initial 2.5 ms and by a factor of 20 more for the further 5 ms (
The most advantageous method for reactivity compensation is to apply a ZrB2 coat of the thickness 0.1·10–3 m to the surface of the fuel compact and to use В4С SRs of 0.01 m in diameter installed in the coolant channels. Coat application and sintered В4С manufacturing technologies are successfully developed by research teams at the Tomsk Polytechnic University.
The burnable absorber and shim rod materials have been chosen with regard for the changes in the inner structure of the microencapsulated fuel and the fuel compact caused by long-term irradiation and by the migration of fission fragments and gaseous chemical compounds affecting the thermophysical properties and the temperature of the compact’s fuel part.
The reported study was funded by RFBR, project number 19-38-90132 and number 19-29-02005.