<<

Power Distribution and Control Rod Worth in Heavy-Water-Moderated, Cluster-Type Fuelled Core

- Experiment and Analysis -

Toshio WAKABAYASHI, Nobuo FUKUMURA and Yuuki HACHIYA

l

Heavy Water Critical Experimerit Section 0-arai Engineering Center Power Reactor and Development Corp. 4002 Narita, 0-arai-machi Ibaraki-ken, JAPAN

0702000 1 Abstract

Changes in power distribution due to the movement of control rod and control rod worth have been measured for the purpose of confirming the advantage of load following operation using control rods in a heavy-water-moderated, light-water-cooled, pressure tube-type reactor. A 84 C control rod with the same dimension as that of the FUGEN was inserted into the D20 moderator region of the central plutonium ,fuel lattice. Experimental results were compared with calculations with use of codes ‘WIMS-D and CITATION. It has been clarified that the maximum power change due to the movement of the control rod occurs in the nearest fuel pin at the axial middle point of the displacement of the control rod. In a lattice of 25.0 cm pitch, the maxim,um value of power change due to pulling out of the control rod by 10 cm has been estimated to be about 23 %. Both the axial power distribution in fuel pin and the control rod worth were predicted well by the codes WIMS-D and CITATION. The maximum value of calcu- lational power change and the calculational control rod worth are in good agreement with the experimental results within 4 % and 1 %, respectively.

0702000;: Contents

I. INTRODUCTION ...... 1

II. EXPERIMENT ...... 2 (1) Experimental Facilities (2) Measurement of Control Rod Worth (3) Measurement of Axial Power Distribution

III. CALCULATIONAL MODEL ...... 4

IV. RESULTS AND DISCUSSION ...... 5

V. CONCLUSION ...... 6

REFERENCES ...... 7 I. INTRODUCTION As the nuclear percentage of electric power generation becomes larger, load follow capabilities of nuclear plants become more important. The nuclear plants must be an integral part of the power system and must meet the power system’s operating requirements for load follow capability on daily and hourly bases. In a heavy-water moderated, light-water-cooled, pressure tube-type reactor (HWR), the load following operation is under consideration. Power level and power distribution in the reactor are controlled by control rods and concentration in D20 moderator. Therefore, it is important for the load following operation using the control rods to mitigate the power change of fuel. The objectives of present experiment are to measure changes in power distribution due to the movement of control rod and control rod worths 1) and to establish the calculational anccuracy by the analysis of the experimental results for the purpose of confirming the advantage of the load following operation using control rod in the HWR. A series of critical experiments was performed in the Deuterium Critical Assembly (DCA).

-I-

07020004 II. Experiment (1) Experimental Facilities A cluster used for the present experiments consists of 28 fuel pins as shown in Figure 1. The fuel pins in the cluster were arranged in three concentric layers; counting from center there are 4 pins in the first, 8 pins in the second, and 16 pins in the third layer. These fuel pins were arranged into a 28-pin cluster by aluminum sipacers that were supported by aluminum hanger wires between the upper and lower tie plates. The total length of the cluster was 2223 mm, including the standard fuel meet length of 2000 mm. The cluster was located in a double-walled aluminum tube; the inner one is called the pressure tube and the outer one the calandria tube. This pair of tubes was positioned in a square lattice having a 25.0 cm pitch in a cylindrical core tank by upper and lower grid plates also made of aluminum. The pressure tube is filled with H2O for simulation of coolant void fraction of 0 %. The core tank was 3005 mm in diameter and 3500 mm in height, and was made of lo-mm- thick aluminum. Purity of the heavy water moderator was 99.5 mol%. In the Present experiment, 0.5~vX% PUO2-U02 fuel clusters were arranged in the central part of the core, as shown in Figure 2, in a square 5 x 5 lattice, and the surrounding part was loaded with 1.2-W% enriched UO2 fuel clusters having the same dimensions as those of the PuO2-U02 fuel cluster. The specifications for each fuel are listed in Table I. The cross sectional view of an experimental control rod is shown in Figure 3. The experimental control rod consists of small absorber elements assembled in a double annulus of 82-mm o.d. Each absorber element is a stainless- tube of 3.5% mm i.d. and 4.75 mm o.d., containing B4C of 70 % theoretical density. The control rod was inserted into the D20 moderator region of the central plutonium fuel lattice as shown in Figure 2. The control rod positions from the lower grid plate of the core were 505 mm and 605 mm.

(2) Measurement of Control Rod Worth Control rod worths were obtained by integrating the D20 level coefficient of reactivity between two critical D20 heights with and without control rod in the core. This coefficient in cents per centimetre is defined as follows by the one-group diffusion theory: -=-.dP 1 2Z“MZ . 1 %f Beff km u&S)3

c-c-- (1) (H+s Y

-2- where ,dcff = effective delayed fraction ,Mz = migration area km J infinite neutron multiplication factor a = 2 aZM2/,Q~j~fkm(cent -cm21 H = D2O height (cm) B = effective axial extrapolation distance (cm)

/ The axial distribution of copper activation measured at the center of the plutonium fuel cluster was fitted to a cosine function to determine the effective axial extrapola- tion distance. The value of 8 was 10.8 + 1.4 cm. The D20 level coefficients of reactivity were obtained by the positive period method at various critical D20 heights. Values of (r were obtained from measured reactivity coefficients and critical D20 heights using Eq. (1). The variation in CLvalues with the critical heights was within the experimental uncertainty (24%). Consequently, the a was assumed constant. Control rod worths, in cents, corresponding to the change in critical D2O height from hl (without control rod) to h2 (with control rod) were evaluated by,

(3) Measurement of Axial Power Distribution Axial power distributions in plutonium fuel cluster adjacent to the control rod were measured by scanning T -rays emitted from the FP in the irradiated fuel pins a . For measurements of the axial power distribution, three fuel pins at positions A, B and C, as shown in Figure 4, were selected. The fuel pins were irradiated for 2 hours at a power of 1 kW (-log n/cm2.s). After irradiation, the fuel pins in the central plutonium fuel cluster for measurement of the axial power distribution were cooled for about one week. Beyond the one week after irradiation, the main r -rays in FP activity are 1.60 MeV T -rays from 140La This nuclide has a 40 h half-life, and attains the equilibrium with its parent nuclide - 140Ba (half-life: 12.8d). The detector for the r -ray counting was a solid-type 2 in. dia. x 2 in. thick NaI(T1) scintillation counter, shielded with Pb blocks. Between the fuel pin and the detector, a slit of width 6 mm was used. For the effect of tilted 140La distribution in a pin, the fuel pin was rotated on its axis with an electric motor during r -rays counting.

-3-

0702OOOrll III. CALCULATIONAL METHOD The WIMS-03)y ‘) and the CITATION51 codes were used to calculate the control rod worths and the axial power distributions. The WIMS-D code is general lattice cell program that uses transport theory to calculate flux as a function of energy and position in a cell. The basic cross-section library is in 69 groups with 14 fast, 13 resonance and 42 thermal groups. The transport equation is solved by a collision probability method using up to 69 neutron energy groups. The WIMS-D gives D, x a, S rem, 2 f and K-infinitive for the whole unit cell. The group constants are given in few energy groups suitable for use with other computer codes such as the CITATION. The WIMS-D code can give the group constants of unit cell with control rod by using multi-cell option. In the present calculation, a multi-cell model with a control rod and four fuel clusters was used to obtain fourteen group constants of unit cell with the control rod as shown in Figure 5. The fourteen group constants in each region generated by the multi-cell option were condensed into two groups ones by the one-dimensional diffusion calculation using 100 fine mesh intervals. The one-dimensional model is shown in Figure 6. This group condensation prodedure using detailed spatial distribution made improvements on calculational accuracy of control rod worth compared with the normal condensation method by the WIMS-D. The control rod worths and the axial power distributions for the full core inserted a control rod were calculated in three-deimensional diffusion theory of two groups. The three dimensional (X-Y-2) model of the DCA core was employed for the present calculation using the CITATION. Maps of the CITATION geometry in the (X-Y) plane are shown in Figures 7 through 9. Figure 7 shows the whole core map. Detailed maps of the CITATION geometry surrounding the control rod are shown in Figures 8 and 9. The CITATION geometry in the 2 direction is shown in Figure 10. The CITATION calculations were done for the full core using 35880 (46 x 60 x 13) mesh intervals and two energy groups. l

-4- IV. RESULTS AND DISCUSSION (1) Control Rod Worth Experimental and calculated results of reactivity worth of control rod are shown in Table II. The experimental error in these results was estimated to be ~4%. The results of the WIMS-D and the CITATION calculation agree with the experimental results within 1%.

(2) Axial Power Distribution Figures 11 though 13 show normalized axial power distributions at positions A, B and C in Figure 4. The experimental error in the results was estimated to be 24%. It can be seen in these figures that the maximum power change due to movement of control rod occurs in the nearest fuel pin at the axial middle point of the displacement of control rod. A comparison of the axial power distribution between experiment and calculation is shown in Figure 13. The calculational result is in good agreement with the experimental one. Table III shows a comparison of the power change in each fuel pin between experiment and calculation. The power change increases as the position of fuel pin in the cluster approaches the control rod. The maximum value of power change due to pulling out of the control rod by 10 cm is estimated to be about 23 % at the position of fuel pin A in Figure 4. The maximum values of power change in each fuel pin are in good agreement with the calculational ones and are within 4%.

-5-

07020008 v. CONCLUSION Changes in power distribution due to the movement of control rod and control rod worths have been measured for the purpose of confirming the advantage of load following operation using control rod in a. heavy-water-moderated, light-water-cooled, pressure tube-type reactor. The maximum power change due to pulling out of a B4C control rod by 10 cm is estimated to be about 23 % at the nearest fuel pin to the control rod in the fuel cluster. Calculational results for the power changes and the control rod woths are in good agreement with the experimental results within 4% and l%, respectively.

-6- REFERENCES I) M. UEDA, M. MATSUMOTO and T. HAGA, “Reactivity Worths of Annular Control Rods in a Pressure-Tube-Type Heavy Water Lattice”, Nucl. Sci. & Eng., 62, 559 (1977). 2) N. FUKUMURA, “Measurement of Local Power Packing Factors in Heavy-Water Moderated Plutonium Lattices”, J. Nucl. Sci. Tech., 18 (4), 285 (1981). 3) J.R. ASKEW, “A Coarse Mesh Correction for Collision Probabilities”, AEEW-MS89 (1969). 4) M.J. ROTH, IThe Estimation of Collision Probabilities in Complicated Geometries”, AEEW-M857 (1969). 5) T.B. FOWLER, D.R. VONDY and G.W. CUNNINGAM, “ Core Analysis Code: CITATION”, ORNL-TM-2496 Rev. 2, USAEC July (1971).

-7- 0702OO’lG 0.54 wt% 1.2 wt% Fuel Type PuO,-uo, uo,

Fuel Pellet Density, g/cm 3 10.17 10.36 Diameter, mm 14.69 14.80 l PUO, Enrichment, wt% 1.203 (*W) o.542 i PuO,+UO,

Composition, wt% 235U 0.6214 1.057 238U 86.782 86.793 ==apu 0.000102 z30Pu 0.4304 -Pu 0.04115 24’ Pu 0.004359 -Pu 0.000303 l 0 12.12 12.1,5

Fuel Pin Cladding Material Zircaloy-2 Al Cladding I.D., mm 15.06 15.0:3 Cladding O.D., mm 16.68 16.7:3 Gap Material , Air TableD Comparison of Reactivity Worth of Control Rod between Experiment and Calculation

Experiment Calculation Withdrawal Height Error* (%) l of Control Rod (mm) ($) ($)

605 -(l.O6t-0.07) -1.05 -0.9

505 -(1.65+,0.07) -1.64 -0.6

* Error = Ca’$xp-

07020012 Table III Comparison of Maximum Power Change between Experiment and Calculation

Pin Position Experiment 1 Calculation Error *(%) Fuel Number Channel

Center

* Error =Cal.--Em Exp. Moderator Calandria Tube (Ae) Air Gap Pressure Tube (Ad) Cladding (Zircaloy-2) Fuel - Coolant -2.0 -

(Dimension : mm)

Figure 1 Cross Sectional View of 28 Fuel Pins Cluster 000000000

0000000

3005 c

Total Number of Fuel Clusters : 97

0 1.2 w/o UO, (72 channels)

0.54 w/o PuO,-UO, (25 channels)

@ Control Rod

Figure 2 Configuration of DCA Lattice Ha.ving 25cm Pitch 0 D,O--- Air - Supporting Rod - (SUS 304) _ Air Absorber Tube Absorber Element (36) (SUS 304) Absorber Element (48) (Dimension : mm)

Absorber Element Absorber B,C 34.8f0.5g Effective Length 1975 mm Density 70% TD Cladding (SUS 304) Diameter 3.58mm I.D. Thickness 0.59mm Length 1983mm

Figure 3 Cross Sectional View of Control Rod

.07020016 Channel

(Dimension mm)

Figure 4 Configuration of Fuel Pins and B,C Control Rod Fuel Cell

Control Rod Cell

-25- (Dimension : mm) a Figure 5 WIMS Multi-Cell Model for Control Rod and Surrounding 4 Plutonium Cells

0702001~ (Dimension : mm)

Figure 6 Geometrical Model of One-Dimensional Diffusbn Calculation for Condensation of Multi-Group Constants.

07020011:' (Dimension : mm)

Plane Mesh Points : 2 Mesh Points/Unit Cell (= 125mm)

Figure 7 X-Y Plane Model of Three-Dimensional CITATION Analysis (Full Core)

0702002!! Figure 8 X-Y Plane Model of Three-Dimensional CITATION Analysis (Plane with Control Rod)

0702002"i 0.0

11.521

21.465

31.616 35.34 45.1 a7

35.915 C/T, Air, , Hz0 66.148

87.0

92124

C/R,

125.0 \ Equivalent Control Radius

(Dimension : mm)

Figure 9 Fine Mesh Structure of Fuel Channel

‘07020022 of Mesh Intervals Mesh Posit 951.5 940.5 7 1 94S(Hc:/R=505mm)83.5(Hc/R=605mm) 857.0

1 120.0 737.0 i

649.0 f 1: 605.0 T 33.33

505.0 -I- ? 44.0 - 417.0

I- 1,2 88.0

-- - 241 .O Hc~w=505mm Hcln=60Smm

Lower Grid Plate ) o..

(Dimension : mm)

Figure 10 Z-Direction Mode! of Three- Dimensional CITATION Analysis -; s 8 0 HwR=505mm $ 1.0 a 1 0

I 1 I d.O- 5bo 750 1000 Fuel Height (mm) Figure:1 1 AxA;n:wer Distribution of Center Pin at Central 0” 88 @~H~/n=605mm O~t+4=505mm 0 I.5 - e e 0

.”

I I I 1 1 1.0 - 0 250 500 750 1000 Fuel Height (mm) a Figure 12 Axial Power Distribution of Back Pin at Central . Channel --- 0 Hc,n =505mm

Fuel Height (mm) Figure1 3 Comparison of Axial Power Distributions of Front Pin at Central Channel

0702uO2b