- Email: [email protected]
Analysis of dose rates received around the storage pool for irradiated control rods in a BWR nuclear power plant
Applied Radiation and Isotopes 69 (2011) 1104–1107
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Analysis of dose rates received around the storage pool for irradiated control rods in a BWR nuclear power plant J. Ro´denas, A. Abarca, S. Gallardo n Departamento de Ingenierı´a Quı´mica y Nuclear, Universidad Polite´cnica de Valencia, Camı´ de Vera s/n 46022, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Available online 27 October 2010
BWR control rods are activated by neutron reactions in the reactor. The dose produced by this activity can affect workers in the area surrounding the storage pool, where activated rods are stored. Monte Carlo (MC) models for neutron activation and dose assessment around the storage pool have been developed and validated. In this work, the MC models are applied to verify the expected reduction of dose when the irradiated control rod is hanged in an inverted position into the pool. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Monte Carlo method Neutron activation BWR control rods
1. Introduction
2. Material and methods
Control rods are activated by neutron reactions into the reactor. The activation is produced mainly in stainless steel components and impurities. The dose produced by this activity is not important inside the core, but it has to be taken into account when the rod is withdrawn from the reactor and stored into the storage pool for irradiated fuel of the plant at a certain depth under water. Some models were developed with the MCNP5 code (X-5 Monte Carlo Team, 2005), based on MC method, to simulate neutron activation (Ro´denas et al., 2010a) and the pool containing hanger devices with irradiated control rods (Ro´denas et al., 2010b). Thus, doses potentially received by plant workers in the area surrounding the pool edges as well as in a platform moving over the water surface can be calculated. By comparing calculated doses with experimental measurements near an irradiated BWR control rod, it validated the activation model (Ro´denas et al., 2010c). It also validated the model developed to calculate dose rates around the pool (Abarca et al., 2009). Results of the activation model proved that the rod handle is the most irradiated part of the control rod. Activated control rods are stored in the pool with their handle at the uppermost position. If the rods are inverted, the most irradiated part will be the farthest from the free water surface and consequently doses will be reduced. In this work, the developed MC models were applied to verify the expected reduction of dose when the irradiated control rod is hanged in an inverted position into the pool.
2.1. Activation model
n
Corresponding author. Tel.: + 34 96 387 96 31. E-mail addresses: [email protected] (J. Ro´denas), [email protected] (A. Abarca), [email protected] (S. Gallardo). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.10.019
Major activation reactions are produced in stainless steel mostly in some alloy components and impurities. Reactions considered and isotopes produced are listed in Table 1. Nevertheless, only the isotopes emitting gamma rays with half-lives greater than 60 days were of interest from the point of view of dose calculation. Consulting disintegration schemes (Java-based Nuclear Information Software (JANIS) 2005), they remain the following: Mn-54, Sc-46, Co-60, Zn-65, Nb-94, Ag-108 m, Ag-110 m, Eu-152, Eu-154, and Hf-178. Monte Carlo models developed (Ro´denas et al., 2010a, 2010c) to assess the activity generated in control rods of a BWR are based on a detailed geometry that includes an axial division of the rod to consider the different periods and lengths of insertion during its permanency in the reactor core, so that the movement of control rods during reactor operation can be taken into account in the activation assessment. The interaction rate is calculated by MCNP using F4 tally and FM4 (tally multiplier card), which provides data for the reactions included in the calculation. Afterwards, it can be converted into activity taking into account irradiation and cooling times.
2.2. Pool model The pool was modeled by a lattice with control rods in appropriate positions and the other cells were filled with water. A VISEDs (Carter and Schwarz, 2005) scheme of the pool MC model can be seen in Fig. 1. The concrete walls, water filling the pool, hangers with control rods, and the air zone above the water, where doses shall be calculated, are shown in this figure. Fuel
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Table 1 Activation reactions produced in stainless steel of control rods. N14 (n, p) C14 Al27 (n, g) Al28 Cl35 (n, p) Cl36 Cl37 (n, 2n) Cl36 Ti46 (n, p) Sc46
Fe54 Fe54 Ni58 Ni58
(n, p) Mn54 (n, g) Fe55 (n, a) Fe55 (n, g) Ni59
Co59 (n, g) Co60 Ni60 (n, p) Co60 Cu63 (n, a) Co60 Ni62 (n, g) Ni63
Zn64 (n, g) Zn65 Mo92 (n, g) Mo93 Nb93 (n, g) Nb94 Ag107 (n, g) Ag108 m
Ag109 (n, g) Ag110 m Eu151 (n, g) Eu152 Eu153 (n, g) Eu154 Hf177 (n, g) Hf178
Fig. 1. VISEDs scheme of the ground plan and elevation of the pool model.
elements are placed about 8 m below the pool surface. Therefore, its contribution to the dose around the pool surface is negligible and they need not to be modeled.
2.3. Variance reduction techniques The main problem in the developed model is that very few photons reach the pool water surface. Therefore, statistics is very poor and large uncertainties are associated with results. To improve statistics, the water volume over the control rods is divided into several slabs with increasing importance values. They can be seen in Fig. 1 on the right hand side. An important reduction in computing time can also be achieved using the SSW/SSR technique. In this technique, a first simulation containing the Surface Source Write (SSW) card is run. The photons emitted from the activated control rod that pass through a defined plane are recorded in a file with all its characteristics, energy, direction, etc. This surface source file is used for subsequent MCNP calculations, where the input deck contains the Surface Source Read (SSR) card. In the second and subsequent simulations, the number of particles can be strongly increased, but the source is simplified as it is just a plane and the recalculation of all interactions before arriving to this plane is avoided. Therefore, statistics can be improved with slowly increase in computer time.
2.4. Dose reduction model The activation model previously validated (Ro´denas et al., 2010c) was applied to a BWR control rod considering 5 axial cells for the rod core and the gain. The activities obtained are listed in Table 2, where it can be seen that the maximum activity for all nuclides considered is located in the handle. It was an expected result because the handle is always into the reactor core, while the other parts of the rod can be more or less introduced into the core. Therefore, rotating the rods by 1801 into the storage pool with handles at a deeper position under water should reduce the dose out of the pool. The same MC model used for dose calculations around the pool (Abarca. et al., 2009) can be applied to verify the predicted dose reduction. It is just necessary to change the cells corresponding to control rods to model the new position. The rest of the model does not change and the same variation reduction techniques can be applied. Again a F4MESH tally has been used to determine dose rate in a mesh in air over the pool, the zone 1 m above the free water surface and 1 m beyond inner surface of the pool walls marked in cyan blue in Fig. 1. 3. Results and discussion Calculated dose rates (mSv/h) are represented in Fig. 2 at the left hand side for control rods in the normal position, while at the right
´denas et al. / Applied Radiation and Isotopes 69 (2011) 1104–1107 J. Ro
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Table 2 Total activity (Bq) in different parts of a control rod.
Handle Core 1 Core 2 Core 3 Core 4 Core 5 Gain 1 Gain 2 Gain 3 Gain 4 Gain 5 Tubes
Mn-54
Co-60
Nb-94
Ag-108 m
Ag-110 m
Eu-152
Eu-154
Hf-178
Sc-46
Zn-65
7.55E + 11 9.78E + 10 1.54E + 11 2.10E+ 11 1.70E+ 11 1.72E + 11 1.57E + 11 2.72E + 11 3.27E + 11 3.47E + 11 3.09E+ 11 1.07E+ 12
7.82E + 13 2.95E + 12 4.85E + 12 6.21E + 12 6.98E + 12 6.58E + 12 5.95E + 12 1.00E + 13 1.29E + 13 1.30E+ 13 1.30E+ 13 2.83E + 13
8.51E + 07 5.28E + 06 7.11E + 06 9.35E + 06 1.03E+ 07 8.37E + 06 8.01E+ 06 1.23E + 07 1.61E + 07 1.69E + 07 1.61E + 07 4.04E+ 07
3.54E +09 1.26E +08 2.27E +08 2.95E +08 3.05E +08 2.91E +08 2.79E +08 4.53E +08 5.75E +08 5.81E +08 5.85E +08 1.31E +09
1.07E+ 11 4.55E+ 09 1.03E+ 10 9.51E+ 09 1.12E+ 10 1.22E+ 10 9.94E+ 09 1.58E+ 10 2.01E+ 10 2.00E + 10 1.88E+ 10 4.62E+ 10
2.97E+ 10 1.08E+ 09 1.77E+ 09 2.30E+ 09 2.51E+ 09 2.42E+ 09 2.20E+ 09 3.72E+ 09 4.77E+ 09 4.82E+ 09 4.84E+ 09 1.01E+ 10
8.05E+ 08 3.22E + 07 5.40E+ 07 6.71E + 07 8.21E + 07 7.18E + 07 6.30E+ 07 1.06E+ 08 1.34E + 08 1.35E + 08 1.35E + 08 3.02E+ 08
6.34E +10 2.80E +09 5.97E +09 7.57E +09 7.38E +09 7.74E +09 5.38E +09 9.37E +09 1.16E +10 1.15E +10 1.11E +10 2.68E +10
3.88E+ 07 4.39E+ 06 7.91E+ 06 1.10E+ 07 8.87E+ 06 1.01E+ 07 7.78E+ 06 1.39E+ 07 1.65E+ 07 1.74E+ 07 1.54E+ 07 1.41E+ 07
3.59E+ 11 1.34E+ 10 2.28E+ 10 3.05E+ 10 3.18E+ 10 3.06E+ 10 2.74E+ 10 4.74E+ 10 6.05E+ 10 6.20E+ 10 6.01E+ 10 1.36E+ 11
1200
1200
1000
1000
800
800
600
600
400
400
200
200
0
0
200 10
20
400 30
0
600 40
50
0
200 0.005
400 0.01
0.015
600 0.02
Fig. 2. Dose rate (mSv/h). Left—normal position; right—reverse position.
Table 3 Dose rates in selected points over the pool. Calculation points
Dose rate (mSv/h), normal position
Dose rate (mSv/h), reverse position
Ratio
Maximum Middle of the pool Right lower corner Right upper corner Left lower corner Left upper corner
50.00 7.58 22.15 21.47 13.61 29.38
0.0161 0.0037 0.0092 0.0089 0.0043 0.0102
3106 2048 2408 2412 3165 2880
hand side they are rotated by 1801. Uncertainties associated with calculated doses are lower than 10%, except around the red lines in the right graph, where it is about 25% due to the fact that few particles reach the water surface from these points. The number of particles reaching the zone, where dose rates are calculated, is again a problem, even worse than for normal position as photons have a longer pathway when they are started from the handle where the activity is the largest. In any case, as it can be seen in the isodose maps represented in Fig. 2 the dose rate over the pool can be reduced more than 2000 times changing the storage position of control rods. A comparison between dose rates (mSv/h) calculated for both control rod
positions is done for 6 points: maximum, meaning the point where the dose rate is maximum for normal position of control rods; middle of the pool; and the four corners of the pool (right lower, right upper, left lower, and left upper). In Table 3 these values are listed together with the ratio between dose rates at normal and inverse position. Dose rate ratios are represented in Fig. 3, where it can be seen that the huge reduction in dose for the area surrounding the pool is obtained when the irradiated control rods are stored in a position rotated at 1801. It is in this area where the presence of operators is more probable.
4. Conclusions The activation of control rods due to neutron irradiation during its stay into the reactor core is more important for the rod handle as in BWR it is always into the core while the other parts of the rod can be more or less introduced. Therefore, an inversion of the position of control rods stored into the irradiated fuel pool will produce an important reduction in the dose around the pool. Monte Carlo models developed and validated to assess activity produced in control rods and dose rates in the area surrounding the storage pool were applied to the case of a BWR power plant, considering the usual position of the control rods and the reverse
´denas et al. / Applied Radiation and Isotopes 69 (2011) 1104–1107 J. Ro
1200
1200
1000
1000
800
800
600
600
400
400
200
200
0
1107
0 0
200
400
600
1500 2000 2500 3000 3500 4000 4500 5000
0
100 500
200
300 1000
400 1500
500
600
700
2000
Fig. 3. Dose rate ratio. Left—over the pool; right—around the pool.
position into the storage pool with the handle at a deeper position under water. Results show an important reduction in dose rate by a factor of approximately 2000 in the zones where operators are usually working. Therefore, it is very advisable to introduce the appropriate changes in the hanger devices to achieve this important dose reduction. References Abarca, A., Gallardo, S., Ro´denas, J. Validation of the Monte Carlo model developed to estimate doses around the irradiated fuel pool produced by activated control rods discharged from a BWR. In: Proceedings of the 2009 International Nuclear
Atlantic Conference—INAC 2009, Rio de Janeiro, Brazil, September 27–October 2, 2009. Carter, L.L., Schwarz, R.A. MCNP Visual Editor Computer Code Manual, for Vised Version 19 K, November 2005. Java-based Nuclear Information Software (JANIS). /http://www.nea.fr/janis/S 2005. Ro´denas, J., Gallardo, S., Abarca, A., Juan, V., 2010a. Estimation of the activity generated by neutron activation in control rods of a BWR. Applied Radiation and Isotopes 68 (4,5), 905–908. Ro´denas, J., Gallardo, S., Abarca, A., Juan, V., 2010b. Analysis of the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool. Applied Radiation and Isotopes 68 (4,5), 909–912. Ro´denas, J., Gallardo, S., Abarca, A., Sollet, E., 2010c. Validation of the Monte Carlo model developed to assess the activity generated in control rods of a BWR. Nuclear Instruments and Methods in Physics Research A 619, 258–261. X-5 Monte Carlo Team, MCNP -A General Monte Carlo N-Particle Transport Code, Version 5. Los Alamos National Laboratory, 2003 (revised 10/03/2005).
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Analysis of dose rates received around the storage pool for irradiated control rods in a BWR nuclear power plant J. Ro´denas, A. Abarca, S. Gallardo n Departamento de Ingenierı´a Quı´mica y Nuclear, Universidad Polite´cnica de Valencia, Camı´ de Vera s/n 46022, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Available online 27 October 2010
BWR control rods are activated by neutron reactions in the reactor. The dose produced by this activity can affect workers in the area surrounding the storage pool, where activated rods are stored. Monte Carlo (MC) models for neutron activation and dose assessment around the storage pool have been developed and validated. In this work, the MC models are applied to verify the expected reduction of dose when the irradiated control rod is hanged in an inverted position into the pool. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Monte Carlo method Neutron activation BWR control rods
1. Introduction
2. Material and methods
Control rods are activated by neutron reactions into the reactor. The activation is produced mainly in stainless steel components and impurities. The dose produced by this activity is not important inside the core, but it has to be taken into account when the rod is withdrawn from the reactor and stored into the storage pool for irradiated fuel of the plant at a certain depth under water. Some models were developed with the MCNP5 code (X-5 Monte Carlo Team, 2005), based on MC method, to simulate neutron activation (Ro´denas et al., 2010a) and the pool containing hanger devices with irradiated control rods (Ro´denas et al., 2010b). Thus, doses potentially received by plant workers in the area surrounding the pool edges as well as in a platform moving over the water surface can be calculated. By comparing calculated doses with experimental measurements near an irradiated BWR control rod, it validated the activation model (Ro´denas et al., 2010c). It also validated the model developed to calculate dose rates around the pool (Abarca et al., 2009). Results of the activation model proved that the rod handle is the most irradiated part of the control rod. Activated control rods are stored in the pool with their handle at the uppermost position. If the rods are inverted, the most irradiated part will be the farthest from the free water surface and consequently doses will be reduced. In this work, the developed MC models were applied to verify the expected reduction of dose when the irradiated control rod is hanged in an inverted position into the pool.
2.1. Activation model
n
Corresponding author. Tel.: + 34 96 387 96 31. E-mail addresses: [email protected] (J. Ro´denas), [email protected] (A. Abarca), [email protected] (S. Gallardo). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.10.019
Major activation reactions are produced in stainless steel mostly in some alloy components and impurities. Reactions considered and isotopes produced are listed in Table 1. Nevertheless, only the isotopes emitting gamma rays with half-lives greater than 60 days were of interest from the point of view of dose calculation. Consulting disintegration schemes (Java-based Nuclear Information Software (JANIS) 2005), they remain the following: Mn-54, Sc-46, Co-60, Zn-65, Nb-94, Ag-108 m, Ag-110 m, Eu-152, Eu-154, and Hf-178. Monte Carlo models developed (Ro´denas et al., 2010a, 2010c) to assess the activity generated in control rods of a BWR are based on a detailed geometry that includes an axial division of the rod to consider the different periods and lengths of insertion during its permanency in the reactor core, so that the movement of control rods during reactor operation can be taken into account in the activation assessment. The interaction rate is calculated by MCNP using F4 tally and FM4 (tally multiplier card), which provides data for the reactions included in the calculation. Afterwards, it can be converted into activity taking into account irradiation and cooling times.
2.2. Pool model The pool was modeled by a lattice with control rods in appropriate positions and the other cells were filled with water. A VISEDs (Carter and Schwarz, 2005) scheme of the pool MC model can be seen in Fig. 1. The concrete walls, water filling the pool, hangers with control rods, and the air zone above the water, where doses shall be calculated, are shown in this figure. Fuel
´denas et al. / Applied Radiation and Isotopes 69 (2011) 1104–1107 J. Ro
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Table 1 Activation reactions produced in stainless steel of control rods. N14 (n, p) C14 Al27 (n, g) Al28 Cl35 (n, p) Cl36 Cl37 (n, 2n) Cl36 Ti46 (n, p) Sc46
Fe54 Fe54 Ni58 Ni58
(n, p) Mn54 (n, g) Fe55 (n, a) Fe55 (n, g) Ni59
Co59 (n, g) Co60 Ni60 (n, p) Co60 Cu63 (n, a) Co60 Ni62 (n, g) Ni63
Zn64 (n, g) Zn65 Mo92 (n, g) Mo93 Nb93 (n, g) Nb94 Ag107 (n, g) Ag108 m
Ag109 (n, g) Ag110 m Eu151 (n, g) Eu152 Eu153 (n, g) Eu154 Hf177 (n, g) Hf178
Fig. 1. VISEDs scheme of the ground plan and elevation of the pool model.
elements are placed about 8 m below the pool surface. Therefore, its contribution to the dose around the pool surface is negligible and they need not to be modeled.
2.3. Variance reduction techniques The main problem in the developed model is that very few photons reach the pool water surface. Therefore, statistics is very poor and large uncertainties are associated with results. To improve statistics, the water volume over the control rods is divided into several slabs with increasing importance values. They can be seen in Fig. 1 on the right hand side. An important reduction in computing time can also be achieved using the SSW/SSR technique. In this technique, a first simulation containing the Surface Source Write (SSW) card is run. The photons emitted from the activated control rod that pass through a defined plane are recorded in a file with all its characteristics, energy, direction, etc. This surface source file is used for subsequent MCNP calculations, where the input deck contains the Surface Source Read (SSR) card. In the second and subsequent simulations, the number of particles can be strongly increased, but the source is simplified as it is just a plane and the recalculation of all interactions before arriving to this plane is avoided. Therefore, statistics can be improved with slowly increase in computer time.
2.4. Dose reduction model The activation model previously validated (Ro´denas et al., 2010c) was applied to a BWR control rod considering 5 axial cells for the rod core and the gain. The activities obtained are listed in Table 2, where it can be seen that the maximum activity for all nuclides considered is located in the handle. It was an expected result because the handle is always into the reactor core, while the other parts of the rod can be more or less introduced into the core. Therefore, rotating the rods by 1801 into the storage pool with handles at a deeper position under water should reduce the dose out of the pool. The same MC model used for dose calculations around the pool (Abarca. et al., 2009) can be applied to verify the predicted dose reduction. It is just necessary to change the cells corresponding to control rods to model the new position. The rest of the model does not change and the same variation reduction techniques can be applied. Again a F4MESH tally has been used to determine dose rate in a mesh in air over the pool, the zone 1 m above the free water surface and 1 m beyond inner surface of the pool walls marked in cyan blue in Fig. 1. 3. Results and discussion Calculated dose rates (mSv/h) are represented in Fig. 2 at the left hand side for control rods in the normal position, while at the right
´denas et al. / Applied Radiation and Isotopes 69 (2011) 1104–1107 J. Ro
1106
Table 2 Total activity (Bq) in different parts of a control rod.
Handle Core 1 Core 2 Core 3 Core 4 Core 5 Gain 1 Gain 2 Gain 3 Gain 4 Gain 5 Tubes
Mn-54
Co-60
Nb-94
Ag-108 m
Ag-110 m
Eu-152
Eu-154
Hf-178
Sc-46
Zn-65
7.55E + 11 9.78E + 10 1.54E + 11 2.10E+ 11 1.70E+ 11 1.72E + 11 1.57E + 11 2.72E + 11 3.27E + 11 3.47E + 11 3.09E+ 11 1.07E+ 12
7.82E + 13 2.95E + 12 4.85E + 12 6.21E + 12 6.98E + 12 6.58E + 12 5.95E + 12 1.00E + 13 1.29E + 13 1.30E+ 13 1.30E+ 13 2.83E + 13
8.51E + 07 5.28E + 06 7.11E + 06 9.35E + 06 1.03E+ 07 8.37E + 06 8.01E+ 06 1.23E + 07 1.61E + 07 1.69E + 07 1.61E + 07 4.04E+ 07
3.54E +09 1.26E +08 2.27E +08 2.95E +08 3.05E +08 2.91E +08 2.79E +08 4.53E +08 5.75E +08 5.81E +08 5.85E +08 1.31E +09
1.07E+ 11 4.55E+ 09 1.03E+ 10 9.51E+ 09 1.12E+ 10 1.22E+ 10 9.94E+ 09 1.58E+ 10 2.01E+ 10 2.00E + 10 1.88E+ 10 4.62E+ 10
2.97E+ 10 1.08E+ 09 1.77E+ 09 2.30E+ 09 2.51E+ 09 2.42E+ 09 2.20E+ 09 3.72E+ 09 4.77E+ 09 4.82E+ 09 4.84E+ 09 1.01E+ 10
8.05E+ 08 3.22E + 07 5.40E+ 07 6.71E + 07 8.21E + 07 7.18E + 07 6.30E+ 07 1.06E+ 08 1.34E + 08 1.35E + 08 1.35E + 08 3.02E+ 08
6.34E +10 2.80E +09 5.97E +09 7.57E +09 7.38E +09 7.74E +09 5.38E +09 9.37E +09 1.16E +10 1.15E +10 1.11E +10 2.68E +10
3.88E+ 07 4.39E+ 06 7.91E+ 06 1.10E+ 07 8.87E+ 06 1.01E+ 07 7.78E+ 06 1.39E+ 07 1.65E+ 07 1.74E+ 07 1.54E+ 07 1.41E+ 07
3.59E+ 11 1.34E+ 10 2.28E+ 10 3.05E+ 10 3.18E+ 10 3.06E+ 10 2.74E+ 10 4.74E+ 10 6.05E+ 10 6.20E+ 10 6.01E+ 10 1.36E+ 11
1200
1200
1000
1000
800
800
600
600
400
400
200
200
0
0
200 10
20
400 30
0
600 40
50
0
200 0.005
400 0.01
0.015
600 0.02
Fig. 2. Dose rate (mSv/h). Left—normal position; right—reverse position.
Table 3 Dose rates in selected points over the pool. Calculation points
Dose rate (mSv/h), normal position
Dose rate (mSv/h), reverse position
Ratio
Maximum Middle of the pool Right lower corner Right upper corner Left lower corner Left upper corner
50.00 7.58 22.15 21.47 13.61 29.38
0.0161 0.0037 0.0092 0.0089 0.0043 0.0102
3106 2048 2408 2412 3165 2880
hand side they are rotated by 1801. Uncertainties associated with calculated doses are lower than 10%, except around the red lines in the right graph, where it is about 25% due to the fact that few particles reach the water surface from these points. The number of particles reaching the zone, where dose rates are calculated, is again a problem, even worse than for normal position as photons have a longer pathway when they are started from the handle where the activity is the largest. In any case, as it can be seen in the isodose maps represented in Fig. 2 the dose rate over the pool can be reduced more than 2000 times changing the storage position of control rods. A comparison between dose rates (mSv/h) calculated for both control rod
positions is done for 6 points: maximum, meaning the point where the dose rate is maximum for normal position of control rods; middle of the pool; and the four corners of the pool (right lower, right upper, left lower, and left upper). In Table 3 these values are listed together with the ratio between dose rates at normal and inverse position. Dose rate ratios are represented in Fig. 3, where it can be seen that the huge reduction in dose for the area surrounding the pool is obtained when the irradiated control rods are stored in a position rotated at 1801. It is in this area where the presence of operators is more probable.
4. Conclusions The activation of control rods due to neutron irradiation during its stay into the reactor core is more important for the rod handle as in BWR it is always into the core while the other parts of the rod can be more or less introduced. Therefore, an inversion of the position of control rods stored into the irradiated fuel pool will produce an important reduction in the dose around the pool. Monte Carlo models developed and validated to assess activity produced in control rods and dose rates in the area surrounding the storage pool were applied to the case of a BWR power plant, considering the usual position of the control rods and the reverse
´denas et al. / Applied Radiation and Isotopes 69 (2011) 1104–1107 J. Ro
1200
1200
1000
1000
800
800
600
600
400
400
200
200
0
1107
0 0
200
400
600
1500 2000 2500 3000 3500 4000 4500 5000
0
100 500
200
300 1000
400 1500
500
600
700
2000
Fig. 3. Dose rate ratio. Left—over the pool; right—around the pool.
position into the storage pool with the handle at a deeper position under water. Results show an important reduction in dose rate by a factor of approximately 2000 in the zones where operators are usually working. Therefore, it is very advisable to introduce the appropriate changes in the hanger devices to achieve this important dose reduction. References Abarca, A., Gallardo, S., Ro´denas, J. Validation of the Monte Carlo model developed to estimate doses around the irradiated fuel pool produced by activated control rods discharged from a BWR. In: Proceedings of the 2009 International Nuclear
Atlantic Conference—INAC 2009, Rio de Janeiro, Brazil, September 27–October 2, 2009. Carter, L.L., Schwarz, R.A. MCNP Visual Editor Computer Code Manual, for Vised Version 19 K, November 2005. Java-based Nuclear Information Software (JANIS). /http://www.nea.fr/janis/S 2005. Ro´denas, J., Gallardo, S., Abarca, A., Juan, V., 2010a. Estimation of the activity generated by neutron activation in control rods of a BWR. Applied Radiation and Isotopes 68 (4,5), 905–908. Ro´denas, J., Gallardo, S., Abarca, A., Juan, V., 2010b. Analysis of the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool. Applied Radiation and Isotopes 68 (4,5), 909–912. Ro´denas, J., Gallardo, S., Abarca, A., Sollet, E., 2010c. Validation of the Monte Carlo model developed to assess the activity generated in control rods of a BWR. Nuclear Instruments and Methods in Physics Research A 619, 258–261. X-5 Monte Carlo Team, MCNP -A General Monte Carlo N-Particle Transport Code, Version 5. Los Alamos National Laboratory, 2003 (revised 10/03/2005).