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Analysis of the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 909–912
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Analysis of the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool J. Ro´denas , S. Gallardo, A. Abarca, V. Juan ´cnica de Valencia, Apartado 22012, E-46071 Valencia, Spain Departamento de Ingenierı´a Quı´mica y Nuclear, Universidad Polite
abstract BWR control rods become activated by neutron reactions into the reactor. Therefore, when they are withdrawn from the reactor, they must be stored into the storage pool for irradiated fuel at a certain depth under water. Dose rates on the pool surface and the area surrounding the pool should be lower than limits for workers. The MCNP code based on the Monte Carlo method has been applied to model this situation and to calculate dose rates at points of interest. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction BWR control rods are irradiated into the reactor by the neutron flux and consequently activation reactions are produced in the materials composing the rod. When they are withdrawn from the reactor, they must be stored into the storage pool for irradiated fuel of the plant at a certain depth under water. The storage is disposed to hang up 12 rods in a hanger device specially designed for this goal. Several hangers can be installed at various positions throughout the pool. Doses potentially received by plant workers in the area surrounding the pool edges as well as in a platform moving over the water surface should be calculated to assure the adequate protection. The MCNP5 code (X-5 Monte Carlo Team, 2005) based on the Monte Carlo method has been applied to model hangers with irradiated control rods and to estimate dose rates at points of interest. This paper presents the model developed to simulate several groups of 12 control rods stored in the pool in order to calculate these dose rates. The simulation with Monte Carlo is a realistic method to calculate dose rates at different points produced by activated materials with complicated geometries such as control rods. Very few references have been found in the literature for this subject (see Agosteo et al., 2005).
and gains of the control rod blades) and H2O. This simplification permits us to reduce the number of surfaces in the model so that the maximum allowed by the MCNP code is not exceeded. For each device hanging up irradiated control rods, dose rates are calculated using MCNP at different points into and out of the pool.
2. Monte Carlo model In order to simplify the model just one control rod has been modelled and then duplicated at 12 positions where they are hanged up. As well the composition of the control rod is simplified considering a unique cell containing an equivalent material composed of a mix of B4C, stainless steel (tubes containing B4C Corresponding author.
E-mail address: [email protected] (J. Ro´denas). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.09.060
Fig. 1. Layout of the model for 12 control rods.
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´denas et al. / Applied Radiation and Isotopes 68 (2010) 909–912 J. Ro
They should be then summed for each point to obtain the total dose rate at this point. The source considered for each control rod into the pool is the activity calculated by MCNP taking into
Fig. 2. Positioning of the 12 control rods of a hanger into the pool.
account the neutron activation of the rod into the reactor. Simulations of this activation and results have been also presented at this conference (Ro´denas et al., 2009). The repeated-structures capability in MCNP allows describing only once, cells and surfaces of any structure that appears more than once in the geometry reducing drastically the amount of input data and computer memory. This tool can be appropriate for simulating in detail many control rods. In other words, using repeated-structures and defining a cell ‘filled’ with other cells and materials, one is able to duplicate it in several places. From the technical point of view, it is possible to do, but the characteristic geometry of the control rod introduces an important difficult during the sampling of the source particle generation. MCNP can use three volume distributions for source sampling: Cartesian, spherical, and cylindrical. Furthermore, a defined volume distribution can be used in combination with the CEL variable to sample uniformly throughout the interior of a cell. Due to the special cross-shaped geometry of the control rod, two options appear to be useful for this case: a cylindrical volume using a cell rejection option or different rectangular parallelepipeds. It has been proved that a cylindral-shaped geometry is inappropriate due to the extremely low sampling efficiency during the source generation produced by the relative low area of the control rod section in comparison to the cylindrical section area of the volume surrounding the control rod. The solution finally adopted consists of two rectangular parallelepipeds for covering the cross-shaped of the control rod. In the definition of the source these rectangular parallelepipeds are translated and rotated to simulate all the control rods stored in the pool. A layout of the model for 12 control rods can be seen in Fig. 1. The positioning of the 12 control rods of a hanger into the pool is shown in Fig. 2. Both designs have been obtained using VISED (Carter and Schwarz, 2005). The source spectrum is characterized using results of the activation run and disintegration schemes from JANIS database (2005). The following radionuclides are included in the source spectrum: Mn-54, Sc-46, Co-60, Zn-65, Nb-94, Ag-108 m, Ag110 m, Eu-152, Eu-154, and Hf-178. All of them are gamma
Fig. 3. Ground plan of the fuel storage pool with hangers.
ARTICLE IN PRESS ´denas et al. / Applied Radiation and Isotopes 68 (2010) 909–912 J. Ro
911
emitters and their intensities have been obtained from activities calculated by neutron activation (Ro´denas et al., 2009). The model for control rods is developed as a unique cell for the cross shape rod, but values of activity are taken from the detailed model of the rod in order to obtain more realistic results (Ro´denas et al., 2009).
Fig. 5. Model (only one hanger) with increasing importance.
Calculations have been done for a group of four hangers situated close to a wall of the pool. A ground plan of the fuel storage pool with hangers is shown in Fig. 3 and an elevation section of the fuel storage pool with hangers, control rods, and irradiated fuel can be seen in Fig. 4.
3. Results and discussion
Fig. 4. Elevation section of the fuel storage pool with hangers, control rods, and fuel.
The main problem with the Monte Carlo model is that very few photons reach the pool water surface. Therefore, statistics is very poor and high uncertainties are associated with results. To improve statistics, the water volume over the control rods is divided into several slabs with increasing importance as it can be seen with different colors (or gray scale) in Fig. 5. The space under the rods receives zero importance to avoid photons going towards the bottom of the pool.
ARTICLE IN PRESS 912
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A F4MESH tally, which is an especial F4 tally for measuring particle flux in a defined mesh has been obtained during simulation. The mesh was defined to cover a cell of 0.5 m thickness above the pool water surface extended 1 m out of the pool edges. The code allows us to manage the information provided by F4MESH tally, in order to obtain isodose curves at the pool surface level. For this purpose, Matlab code (1999) was used to plot isodose curves as shown in Fig. 6. The highest values for dose rate can be found at the points on the vertical of each hanger. The maximum is 15.52 72.25 mSv/h corresponding to a point above hanger 3. In the surroundings of the pool (1 m out of edges) dose rates take a maximum value of 170.52 mSv/h. This uncertainty is so elevated because few photons are reaching the zone out of the pool. Uncertainties associated to dose rates calculated are represented in Fig. 7 also using Matlab code (1999). It can be seen that uncertainties are very high at the corners where it arrives few photons. It also happens so in points far of the vertical of the hangers, while on points over this vertical uncertainties are lower. Fig. 6. Isodose curves (mSv/h) at the pool surface level.
4. Conclusions The simulation with Monte Carlo is a realistic method to calculate dose rates at different points on the pool water surface and surrounding the pool produced by activated materials stored into it with complicated geometries such as BWR control rods. Nevertheless, this complicated geometry causes an increase of the computer calculation time to obtain acceptable uncertainties. A planned future development is to obtain the history of a control rod on irradiation time and positioning into the reactor. Then, the neutron activation run can be done with a space distribution of neutron flux. Therefore, the photon source for the second run calculating dose rates also will depend on axial position in the rod and results can be compared with measurements into the pool so that models can be validated. References
Fig. 7. Uncertainties for isodose curves.
Anyway, giving the maximum value possible to importance in these slabs, statistics are not good. Consequently, a huge number of photons should be started at the source and the computer time is increased.
Agosteo, S., Cammi, A., Garlati, L., Lombardi, C., Padovani, E., 2005. Gamma dose from activation of internal shields in IRIS reactor. Radiat. Prot. Dos. 115 (1–4), 86–91. Carter, L.L., Schwarz, R.A., 2005. MCNP Visual Editor Computer Code Manual, for Vised Version 19K. Java-based Nuclear Information Software (JANIS), 2005. /http://www.nea.fr/ janis/S. MATLAB, 1999. User’s guide version 5.3, The MathWorks, Inc. Ro´denas, J., Gallardo, S., Abarca, A., Juan, V., 2009. Estimation of the activity generated by neutron activation in control rods of a BWR. Appl. Radiat. Isotopes, in press, doi:10.1016/j.apradiso.2009.09.059. X-5 Monte Carlo Team, 2005. 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 the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool J. Ro´denas , S. Gallardo, A. Abarca, V. Juan ´cnica de Valencia, Apartado 22012, E-46071 Valencia, Spain Departamento de Ingenierı´a Quı´mica y Nuclear, Universidad Polite
abstract BWR control rods become activated by neutron reactions into the reactor. Therefore, when they are withdrawn from the reactor, they must be stored into the storage pool for irradiated fuel at a certain depth under water. Dose rates on the pool surface and the area surrounding the pool should be lower than limits for workers. The MCNP code based on the Monte Carlo method has been applied to model this situation and to calculate dose rates at points of interest. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction BWR control rods are irradiated into the reactor by the neutron flux and consequently activation reactions are produced in the materials composing the rod. When they are withdrawn from the reactor, they must be stored into the storage pool for irradiated fuel of the plant at a certain depth under water. The storage is disposed to hang up 12 rods in a hanger device specially designed for this goal. Several hangers can be installed at various positions throughout the pool. Doses potentially received by plant workers in the area surrounding the pool edges as well as in a platform moving over the water surface should be calculated to assure the adequate protection. The MCNP5 code (X-5 Monte Carlo Team, 2005) based on the Monte Carlo method has been applied to model hangers with irradiated control rods and to estimate dose rates at points of interest. This paper presents the model developed to simulate several groups of 12 control rods stored in the pool in order to calculate these dose rates. The simulation with Monte Carlo is a realistic method to calculate dose rates at different points produced by activated materials with complicated geometries such as control rods. Very few references have been found in the literature for this subject (see Agosteo et al., 2005).
and gains of the control rod blades) and H2O. This simplification permits us to reduce the number of surfaces in the model so that the maximum allowed by the MCNP code is not exceeded. For each device hanging up irradiated control rods, dose rates are calculated using MCNP at different points into and out of the pool.
2. Monte Carlo model In order to simplify the model just one control rod has been modelled and then duplicated at 12 positions where they are hanged up. As well the composition of the control rod is simplified considering a unique cell containing an equivalent material composed of a mix of B4C, stainless steel (tubes containing B4C Corresponding author.
E-mail address: [email protected] (J. Ro´denas). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.09.060
Fig. 1. Layout of the model for 12 control rods.
ARTICLE IN PRESS 910
´denas et al. / Applied Radiation and Isotopes 68 (2010) 909–912 J. Ro
They should be then summed for each point to obtain the total dose rate at this point. The source considered for each control rod into the pool is the activity calculated by MCNP taking into
Fig. 2. Positioning of the 12 control rods of a hanger into the pool.
account the neutron activation of the rod into the reactor. Simulations of this activation and results have been also presented at this conference (Ro´denas et al., 2009). The repeated-structures capability in MCNP allows describing only once, cells and surfaces of any structure that appears more than once in the geometry reducing drastically the amount of input data and computer memory. This tool can be appropriate for simulating in detail many control rods. In other words, using repeated-structures and defining a cell ‘filled’ with other cells and materials, one is able to duplicate it in several places. From the technical point of view, it is possible to do, but the characteristic geometry of the control rod introduces an important difficult during the sampling of the source particle generation. MCNP can use three volume distributions for source sampling: Cartesian, spherical, and cylindrical. Furthermore, a defined volume distribution can be used in combination with the CEL variable to sample uniformly throughout the interior of a cell. Due to the special cross-shaped geometry of the control rod, two options appear to be useful for this case: a cylindrical volume using a cell rejection option or different rectangular parallelepipeds. It has been proved that a cylindral-shaped geometry is inappropriate due to the extremely low sampling efficiency during the source generation produced by the relative low area of the control rod section in comparison to the cylindrical section area of the volume surrounding the control rod. The solution finally adopted consists of two rectangular parallelepipeds for covering the cross-shaped of the control rod. In the definition of the source these rectangular parallelepipeds are translated and rotated to simulate all the control rods stored in the pool. A layout of the model for 12 control rods can be seen in Fig. 1. The positioning of the 12 control rods of a hanger into the pool is shown in Fig. 2. Both designs have been obtained using VISED (Carter and Schwarz, 2005). The source spectrum is characterized using results of the activation run and disintegration schemes from JANIS database (2005). The following radionuclides are included in the source spectrum: Mn-54, Sc-46, Co-60, Zn-65, Nb-94, Ag-108 m, Ag110 m, Eu-152, Eu-154, and Hf-178. All of them are gamma
Fig. 3. Ground plan of the fuel storage pool with hangers.
ARTICLE IN PRESS ´denas et al. / Applied Radiation and Isotopes 68 (2010) 909–912 J. Ro
911
emitters and their intensities have been obtained from activities calculated by neutron activation (Ro´denas et al., 2009). The model for control rods is developed as a unique cell for the cross shape rod, but values of activity are taken from the detailed model of the rod in order to obtain more realistic results (Ro´denas et al., 2009).
Fig. 5. Model (only one hanger) with increasing importance.
Calculations have been done for a group of four hangers situated close to a wall of the pool. A ground plan of the fuel storage pool with hangers is shown in Fig. 3 and an elevation section of the fuel storage pool with hangers, control rods, and irradiated fuel can be seen in Fig. 4.
3. Results and discussion
Fig. 4. Elevation section of the fuel storage pool with hangers, control rods, and fuel.
The main problem with the Monte Carlo model is that very few photons reach the pool water surface. Therefore, statistics is very poor and high uncertainties are associated with results. To improve statistics, the water volume over the control rods is divided into several slabs with increasing importance as it can be seen with different colors (or gray scale) in Fig. 5. The space under the rods receives zero importance to avoid photons going towards the bottom of the pool.
ARTICLE IN PRESS 912
´denas et al. / Applied Radiation and Isotopes 68 (2010) 909–912 J. Ro
A F4MESH tally, which is an especial F4 tally for measuring particle flux in a defined mesh has been obtained during simulation. The mesh was defined to cover a cell of 0.5 m thickness above the pool water surface extended 1 m out of the pool edges. The code allows us to manage the information provided by F4MESH tally, in order to obtain isodose curves at the pool surface level. For this purpose, Matlab code (1999) was used to plot isodose curves as shown in Fig. 6. The highest values for dose rate can be found at the points on the vertical of each hanger. The maximum is 15.52 72.25 mSv/h corresponding to a point above hanger 3. In the surroundings of the pool (1 m out of edges) dose rates take a maximum value of 170.52 mSv/h. This uncertainty is so elevated because few photons are reaching the zone out of the pool. Uncertainties associated to dose rates calculated are represented in Fig. 7 also using Matlab code (1999). It can be seen that uncertainties are very high at the corners where it arrives few photons. It also happens so in points far of the vertical of the hangers, while on points over this vertical uncertainties are lower. Fig. 6. Isodose curves (mSv/h) at the pool surface level.
4. Conclusions The simulation with Monte Carlo is a realistic method to calculate dose rates at different points on the pool water surface and surrounding the pool produced by activated materials stored into it with complicated geometries such as BWR control rods. Nevertheless, this complicated geometry causes an increase of the computer calculation time to obtain acceptable uncertainties. A planned future development is to obtain the history of a control rod on irradiation time and positioning into the reactor. Then, the neutron activation run can be done with a space distribution of neutron flux. Therefore, the photon source for the second run calculating dose rates also will depend on axial position in the rod and results can be compared with measurements into the pool so that models can be validated. References
Fig. 7. Uncertainties for isodose curves.
Anyway, giving the maximum value possible to importance in these slabs, statistics are not good. Consequently, a huge number of photons should be started at the source and the computer time is increased.
Agosteo, S., Cammi, A., Garlati, L., Lombardi, C., Padovani, E., 2005. Gamma dose from activation of internal shields in IRIS reactor. Radiat. Prot. Dos. 115 (1–4), 86–91. Carter, L.L., Schwarz, R.A., 2005. MCNP Visual Editor Computer Code Manual, for Vised Version 19K. Java-based Nuclear Information Software (JANIS), 2005. /http://www.nea.fr/ janis/S. MATLAB, 1999. User’s guide version 5.3, The MathWorks, Inc. Ro´denas, J., Gallardo, S., Abarca, A., Juan, V., 2009. Estimation of the activity generated by neutron activation in control rods of a BWR. Appl. Radiat. Isotopes, in press, doi:10.1016/j.apradiso.2009.09.059. X-5 Monte Carlo Team, 2005. MCNP—A general Monte Carlo N-Particle transport code, version 5, Los Alamos National Laboratory, 2003 (revised 10/03/2005).