Cutback sensitivity test for boron-free small modular PWR

A soluble boron-free small modular pressurized water reactor (SMPWR) uses burnable absorbers (BA) instead of soluble boron to reduce excess reactivity. As a consequence, the fuel cycle length can be shortened by the residual penalty of BA. This paper performs cutback sensitivity tests to extend the cycle length. The influence of the height of the cutback, of the 235U enrichment rate, and of the BA material on the power peaking factor (Fq), the axial offset (AO) and the fuel cycle length is analyzed with the reactor core design system, CASMO-4E/SIMULATE-3 code system.


Introduction
Soluble boron-free operation of small modular pressurized water reactor (SMPWR) has some advantages: reduction of the plant size owing to the removal of the chemical volume control system (CVCS), reduction of liquid radioactive waste, no corrosion issue due to boric acid [1]. However, large amounts of burnable absorber (BA) must be loaded in the core so as to reduce excess reactivity. As a consequence, the fuel cycle length can be shortened by the residual penalty of BA. This paper performs cutback sensitivity tests at the top and bottom regions of the core to extend the cycle length.
The influence of the height of the cutback, of the 235 U enrichment rate and of the BA material on the power peaking factor (Fq), the axial offset (AO) and the fuel cycle length has been analyzed. The Studsvik's reactor core design CASMO-4E/SIMULATE-3 code system is used for these simulations [2][3][4].

Reference SMPWR Design Parameters
Core design parameters of reference SMPWR are summarized in Table 1. The thermal power is 200 MW, and the power density is 58.4 kW/L. The base model of a fuel assembly (FA) is 17 pins by 17 pins, Westinghouse type. The fuel material is uranium oxide. The BA design uses new type of BA, R-BA, which is a thin Zr-167 Er layer coated inside the cladding, as shown in Figure 1 [5]. The target reactivity swing is below 1,000 pcm to secure enough shutdown margin and to secure stable control of excessive reactivity by control rods.  The two-batch loading pattern with 37 FAs is shown in Figure 2.  Figure 3. The reactivity swing of this core is 548 pcm and the cycle length reaches 798 effective full power days (EFPD, 26.6 months). The fuel cycle length of the reference core is shorter than the target value.

Description of Cutback Sensitivity Test
The R-BA is loaded from the top to the bottom in the reference core as shown in Figure 4. The cutback sensitivity test is performed at the top and bottom regions. The relative power is low at the top and bottom regions; thus those regions may not need to load large amount of BA. The test cases are summarized in Table 2. There is no cutback (0 cm cutback) in the reference core. The height of cutback was then changed from 2 cm to 10 cm by 2 cm step from both the top and the bottom, as shown in Figure 4. For each non-zero cutback, two sensitivity tests were carried out: First In the 2 nd sensitivity test, 2 different BA materials were tested, one after the other. The first BA material tested in the cutback regions is Zr-B instead of Zr-167 Er. The B content in Zr-B was changed from 5 w/o to 10 w/o by 5 w/o step. The second BA material tested is Zr-Gd instead of Zr-167 Er. The Gd content in Zr-Gd was changed from 5 w/o to 20 w/o by 5 w/o step. The content range of BA material in Zr-BA is determined by their phase diagrams [7][8][9].

First Study: Fuel Enrichment variation without BA
The fuel cycle length and the reactivity swing as a function of cutback height and fuel enrichment in the cutback regions are presented in Figure 5. The higher the cutback height is, the longer the fuel cycle length is, due to the absence of BA in the cutback regions.

Second Study: BA Material
The fuel cycle length and the reactivity swing variations, depending on the BA content in Zr-B or Zr-Gd cutback regions, are presented in Figure 6. Using Zr-Gd is better than using Zr-B. Indeed, the higher the B content in Zr-B is, the shorter the fuel cycle length is. On the other hand, the higher the height of Zr-Gd is, the longer the fuel cycle length is.  Figure 6. Fuel cycle length and reactivity swing change according to BA content. Three cases of interest are presented in Table 3. Indeed, for those cases, the cycle length is increased by more than 140 EFPD compared to the reference cycle length. The cutback height for the three cases equals 10 cm and this makes the fuel cycle length longer. The cutback with 4.9 w/o 235 U without BA gives the longest cycle length, but the initial excess reactivity is higher than for the cases with Zr-Gd in cutback regions, as shown in Figure 7. The cutback with Zr-Gd makes the initial excess reactivity low, thus it is good for securing shutdown margin at the beginning of cycle (BOC). In the three cases, the axial offset and the power peaking factor are within target values (-0.4<AO<+0.4 and Fq<4.4 respectively), as shown in Figure 8 and 9. The cutback with Gd 10w/o in Zr-Gd is the best test case, because the reactivity swing of the two other cases is higher than the 1000-pcm reactivity swing target value.  Figure 7. k eff change behavior as burnup

Conclusions
This paper conducted cutback sensitivity tests in SMPWR to extend the cycle length. The cutback region may not need to load considerable amount of BA due to low relative power. At the bottom and top of the core pins, different heights of cutback were tested, along with different fuel enrichment rates and different BA materials (Zr-B or Zr-Gd). The reference core has no cutback region and uses only 167 Er-Zr as BA material, the cycle length of the reference core being 798 EFPD. An optimal cutback region of 10 cm height with natural Gd 10% in Zr-Gd was found, for which the cycle length increases to 942 EFPD. The core with this optimal cutback is satisfied with the design limits, which are the axial offset and the 3D pin peaking factor, over the operation time and the maximum excess reactivity is within 1000 pcm of target value. This is a 4.8-month increase of the reactor cycle length compared with the reference core.