This chapter presents the neutronic analysis of the DFR reactor concept. The DFR simplied model developed for the calculations is introduced and used with SER-PENT, SCALE and other codes.
The analysis of the reactor is carried out at the full core scale. The DFR reactor core contains a large amount of fuel salt tubes, in which ssion occurs. In the analysis the fuel tubes are not distinguished. First, the state of the reactor under stationary conditions is investigated with the calculation of several important parameters, such as the eective neutron multiplication factor, the delayed neutron data, including the delayed neutron fraction and the delay constants, the generation time of the neutrons, the in-hour equation of the system, the neutron spectra, the reaction rate density for various reactions in the fuel salt and other materials, and the power distribution in the core. Dierent codes and versions of codes, as well as dierent nuclear data libraries are used for this purpose, so that a cross-comparison for consistency can be made.
The general information of the single fuel salt tube including reaction rates, power generation and neutron ux are approximately obtained by the average of the whole core results.
5.1 Geometry
According to the basic information of the DFR discussed in previous chapters, a computational model used for the neutronic analysis is developed in this section.
Generally, the DFR model consists of several concentric cylindrical zones, namely the ssion zone, the reector, and the breeder blanket. Figure 5.1 [WSM15] shows this arrangement from the center to the periphery of the core region, for one quarter of the DFR core taking advantage of its symmetry. Based on the result of a sensitivity calculation done with coolant tubes in the breeding blanket, this component is ig-nored in the calculations, because of its irrelevant eect on the stationary calculation of the DFR reactor. This sensitivity calculation can be found later in Sec. 7.
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32 CHAPTER 5. NEUTRONIC MODELLING OF THE DFR
Figure 5.1: Horizontal section
The radius of the ssion zone rf ission is set to 135cm, the thickness of the re-ector dref lector is set to 20cm, and the thickness of the breeder blanket dbreeder is set to 125cm. The breeder blanket's coolant tubes have a radius of 6cm for the outer surface of the tube wall and a radius of 5cm for the inner surface.
In the axial direction, the ssion zone is located in the central region of the DFR core and its lower and upper parts are limited by the inlet and outlet plena respectively. The radial reector sur-rounds the ssion zone and spans be-tween the top of the outlet plenum and the bottom of the inlet plenum. Beyond these two surfaces are the ow channels of the coolant, which act as axial top and bottom reectors. The breeder blanket surrounds radially all of the internal cylindrical structures as the outermost core region.
Figure 5.2: Longitudinal section
The total height of the ssion zone hf ission is equal to 240cm and the in-let and outin-let plena have each a height hinlet/outlet of 20cm. The part of the coolant ow channels located inside the reactor (above and below the ssion zone) each has a height hcoolant of 140cm, and the breeder blanket has a total heighthbreeder (orhcore) of 560cm. The longitudinal view of the reactor in Fig. 5.2 shows also only one quarter of the reactor due to the symmetry along the central symmetry axis overlapped with the axis of the cylinder and the symmetry axis at half height.
Figure 5.3: Hexagonal array and fuel tube
In the center of the model, inside of the ssion zone, the treatment is slightly dif-ferent depends on the dierent codes.
The main idea aim is to generate a hexagonal array of the fuel salt tubes.
The inner radius of the fuel salt tube rtube,in is set as 0.725cm and the outer radius rtube,out 0.95cm. Each fuel tube can be considered in its own hexagonal cell as shown in Fig. 5.3. The pin pitch between the fuel tubes from center to center, which is also the size of the cell, is 2.2cm. Between the outer wall of the fuel
5.2. FUEL SALT COMPOSITION 33
Isotope Mole% in the fuel Atomic density(1024/cm3)
37Cl 75.000 1.820E-02
238U 19.586 4.734E-03
238Pu 0.113 2.749E-05
239Pu 3.154 7.664E-04
240Pu 1.346 3.271E-04
241Pu 0.467 1.629E-04
242Pu 0.334 8.109E-05
Table 5.1: Proportions of U-Pu fuel
U-Pu fuel Mole%
37Cl 75.0
238U 25.0
Table 5.2: Breeder fertile material conguration
tube and the boundary of the lattice the coolant ows. The fuel salt ows inside of the inner wall and the wall material between the inner and the outer wall holds the ow.
The total number of the fuel salt tubes has been calculated by dividing the horizontal cross section of the ssion zone by the horizontal cross section of a hexagonal cell.
A theoretical total number of 13659 fuel tubes for the model is, thus, obtained.
The total number of complete fuel tubes is 13406, obtained considering only the complete fuel tubes (but the hexagonal cell might not be complete).
5.2 Fuel Salt Composition
As mentioned in the previous chapters, the original design of the DFR reactor concept has selected a mixture of UCl3 and PuCl3 as the fuel salt (U-Pu fuel). From the chemical formula37Cl takes 75 mole% and the rest of heavy nuclides share 25 mole%.
The detailed composition of the heavy nuclides mixture in both mole percentage and atomic density (at 1380K) is listed in Tab. 5.1:
5.3 Breeder Blanket Composition
The composition of the breeder blanket is shown below in Tab. 5.2:
34 CHAPTER 5. NEUTRONIC MODELLING OF THE DFR
Code/Version/ SCALE SERPENT
Calculations 6.1.3 6.2b4 1.1.19 2.1.x Single Fuel Tube
-Reaction rates
-Power distribution
-Neutron distribution
Static Calculation
-ke
-Delayed neutron data
-Generation time
-Neutron spectrum
-Reaction rates
-Power distribution
Sensitivity Analysis
-Nuclide importance
-Thermal feedback
-Geometry feedback
Table 5.3: Correspondence of Codes to Calculations
5.4 Calculations Performed
In order to get a more complete basis for comparison, several codes and versions have been employed. Because of the variety of the codes used in this work, not all the codes are used for every calculation. The following Tab. 5.3 claries this approach.
In the following sections, SERPENT 1 is specied for SERPENT 1.1.19 and SER-PENT 2 is for SERSER-PENT 2.1.23. Other versions for the occasional situation will be individual remarked.