Estimation of Radioactivity and Residual Gamma-ray Dose around a Collimator at 3-GeV Proton Synchrotron Ring of J-PARC Facility

Transcription

1 Estimation of Radioactivity and Residual Gamma-ray Dose around a Collimator at 3-GeV Proton Synchrotron Ring of J-PARC Facility Y. Nakane 1, H. Nakano 1, T. Abe 2, H. Nakashima 1 1 Center for Proton Accelerator Facilities, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan. nakane@shield4.tokai.jaeri.go.jp 2 Startcom Co., Ltd., , Hayamiya, Tokyo, Japan Abstract. Estimation of radioactivity and residual dose in accelerator tunnel is very important for the shielding design of a high-intensity proton accelerator facility. In the present work, radioactivity and residual dose rate around the collimator were estimated for 3-GeV proton synchrotron of the J-PARC facility. Exposure dose due to radioactivity in the components of the collimator, local shields and concrete walls of accelerator tunnel around the collimator section were calculated by using PHITS, DCHAIN-SP and QAD-CGGP2. 1. Introduction The high-intensity proton accelerator facility [1,2], J-PARC (Japan Proton Accelerator Research Complex), is now under construction as a joint project between Japan Atomic Energy Research Institute (JAERI) and High Energy Accelerator Research Organization (KEK). The facility comprises accelerator complex of a 400-MeV linear accelerator (Linac), a 3-GeV proton synchrotron (PS) ring of 1MW and a 50-GeV PS ring of 750 kw, and experimental facilities for particle physics, nuclear physics, materials science, life science and nuclear technology. Accelerator components and concrete walls of accelerator tunnel in the facility are activated by primary and secondary hadrons due to a beam loss. At almost all areas of the facility, the beam loss rate is assumed to be in less than 1 W/m, of which condition makes "Hands-on-Maintenance" possible. On the other hand, local beam losses of kw order are assumed at restricted areas such as beam injection, extraction and collimator areas. The access in these areas must be permitted even though the maintenance using remote handling is examined in order to reduce the exposure at the position near apparatus. The estimation of residual gamma-ray dose caused by the activations around the areas is one of the most crucial issues for radiation safety on maintenance. For the 3-GeV PS ring, the estimation of residual dose rate around the collimator is the most important because the largest local beam loss is assumed and much frequency of maintenance is expected. The collimator was placed downstream from the injection area, which will collect 400 MeV and 4 kw proton beam halo. In the present work, radioactivity and the residual dose around the collimator in the 3-GeV PS ring were estimated in the calculations on the bases of the actual design of the facility for the condition after accelerator operation of a year. 2. Calculation 2.1. Condition in the calculation FIG. 1 shows an overview of the collimation system in the 3-GeV PS ring. In order to localize the beam loss, six sets of collimators with shields surrounding them were placed downstream from the injection area of the 3-GeV PS ring. Sum of proton beam loss (halo) at the collimator section will be 4 kw. The distributions of the beam loss in the collimators and the components around them were estimated from the calculation [3] by STRUCT [4]. It was found from the results that the total beam losses at the position of the 2nd collimator and the beam duct around the collimator were estimated to be 1566W, which is the largest beam loss in the six collimators. In the present work, radioactivity and the residual dose around the 2nd collimator with the components around them were estimated in the calculations. Exposure dose due to radioactivity in the components, the shields and the concrete walls of accelerator tunnel around the collimator were calculated for the irradiation condition of 5500 hours, on the assumption of a year of accelerator operation of the 3-GeV ring. 1

2 HO beam dump Proton beam from Linac #1 #2 #3 #4 #5 #6 collimat or Injection sect ion Collimator section FIG. 1. Overview of the collimation system in the 3-GeV PS ring of the facility Calculation geometry FIG. 2 shows the top and the side views of the calculation geometry. Three-dimensional calculation geometry shown in the figure was used in the calculations. Proton beams of 400 MeV impinge in the copper collimators modeled as a hollow cylinder, and stainless steel beam ducts near the collimator shown in the figure. Inner diameter and thickness of the collimator and the beam ducts are listed in Table I. From the calculation results by STRUCT, proton beam losses at the 2nd collimator, and the upstream and the downstream beam ducts near the collimator are assumed to be 910W, 520W and 136W, respectively [3]. For both the bulk shielding and the self-shielding for the activations, the collimator is covered with a collimator shield of iron and concrete complex shown in the figure. For driving the collimator, 200-mm-width x 280-mm-height openings are prepared on the x- and y-axis directions in the collimator shield. For the bulk shielding, the local shields of iron and concrete complex shown in the figure are placed around the beam ducts of the upstream and the downstream X concrete iron copper beam loss collimator wall steel W 910W 136W 2550 unitfmm Proton ƒó 377 beam duct Z Y local shield (upstream side) # 1 # # 3 local shield (downstream side) # Proton wall ƒó 377 beam duct Z (a) Top view (b) Side view FIG. 2. Calculation geometry. 2

3 Inner diameter of hollow cylinder (cm) Table I. Calculation conditions. thickness of hollow cylinder (cm) length of hollow cylinder (cm) Collimator Beam duct (upstream) 23.8 (downstream) side of the collimator shield. The concrete walls of the accelerator tunnel around the collimator were considered in the calculations. Residual dose around the collimator #2 were estimated at the four estimation positions shown in the figure. The positions #1 and #2 were placed on the side of the collimator, of which the collimator could watch directly through the opening of the collimator shield. The positions #3 and #4 were placed on the side of the downstream local shield Calculation method Nuclides produced by the reactions of the components with the primary proton beam halo and the secondary particles above 1 MeV and secondary neutrons above 20 MeV were calculated in the present work by using the particle and heavy ion transport code system, PHITS [5]. The PHITS is a new version of the high-energy particle transport code, NMTC/JAM [6] with the extension to heavy ion transport calculation. The energy spectra of secondary neutrons below 20 MeV are also calculated in the code. The induced radioactivity and the energy spectra of gamma-ray in the components were calculated by using the DCHAIN-SP 2001 code [7]. In the calculation, nuclide productions in the components due to neutrons below 20 MeV are obtained from the energy spectra with the activation cross sections library prepared in the code, while the calculated results by PHITS are used for the productions due to charged particles above 1 MeV and neutrons above 20 MeV. Using the results, the residual dose rates at four estimation points shown in the FIG. 2 were calculated by using the point kernel code, QAD-CGGP2 [8] with effective dose conversion coefficients defined in ICRP Pub.74 [9]. 3. Results and discussion 3.1. Time evolution of residual dose rate FIG. 3 shows the calculated results of time evolution of residual dose rate caused by the activations of the components around the collimator for the condition after accelerator operation of a year. The figure shows that the dose rate after a day cooling from the end of operation is about half of that after an hour cooling, and that after a year cooling is about an order smaller than that after a day cooling. For the condition after a month cooling from the end of operation, calculated results of dose rate at the positions #1 and #2 are 11.6 and 2.1 msv/h, respectively. This means that it is difficult to access at the positions for a longer time without additional shield for the condition after a month cooling. On the other hand, the dose rate at the positions #3 and #4 are 23 and 118 µsv/h, respectively, for the condition after a month cooling from the end of operation. This means that it is possible to access for several hours at the position for the condition after a month cooling Contribution of dose rates due to activation in each component Calculated results of dose rates due to activations in each component for the conditions after a day cooling from the end of operation of a year are listed in Table II. The results show that the contributions of dose rates due to activations of the collimator and the collimator shield are dominant at the position #1, #2 and #4 because of the positions could watch them directly. On the other hand, those due to activations of the downstream local shield around the beam ducts are dominant at the position #3 because the position is near the local shield and could not watch the collimator directly. The dose rate at the position #4 is higher than that at the position #3, though the distance from the beam line to the position #4 is longer than that to the position #3. This can be attributed that the dose 3

4 rate due to the activations of the collimator and the collimator shield is higher at the position #4 because the position could watch the collimator directly. It was found from the results that the contribution of dose rate due to activation of concrete wall of accelerator tunnel is small since the collimator and the ducts are covered with the collimator shield and the local shields during operation. Dose rate (µsv h -1 ) Position #1 Position #2 Position #3 Position # Cooling time (days) FIG. 3. Calculated results of time evolution of residual dose rate caused by activations of components around the collimator for the condition after a year of accelerator operation. Table II. Dose rates (µsv/h) due to activations of each components at the condition of a day cooling. Position #1 Position #2 Position #3 Position #4 Collimator Iron shield around collimator Concrete shield around collimator Upstream beam duct Iron shield around upstream duct Concrete shield around upstream duct Downstream beam duct Iron shield around downstream duct Concrete shield around downstream duct Concrete wall of accelerator tunnel Total Additional calculations of dose rates on the assumption of maintenance In the present work, two kinds of additional estimations were performed on the assumption of the maintenance of the collimator and the apparatus around the beam duct. One is the calculation of dose rates with removing the downstream local shields around the beam duct after the end of operation for the maintenance of the collimator and the apparatus, the other is the calculation with an additional lead plug of 5-cm-thick inserted in an opening of the collimator shields after the end of operation for the 4

5 maintenance of the collimator. Comparison results of the dose rates at the positions of #3 and #4 for the cases with and without the downstream local shield are listed in Table III for the conditions after 1 and 30 days cooling from the end of operation of a year. It was found from the results, the dose rates for the condition without the local shield at the position of #3 and #4 are several tens times and several times higher than those for the condition with the local shield, respectively. For the case without the local shield, the dose rate at the position #3 is higher than that at the position #4, while that for the case with the local shield at the position #3 is lower than that at the position #4. This can be attributed that the dose due to activity in the beam duct is mainly contributed to the total dose at the positions of #3 and #4 for the case without the local shields. Comparison results of the dose rates at the positions of #1 and #2 for the cases with and without the lead plug are listed in Table IV. With using the lead plug, the dose rate could be reduced about 1~2 order smaller than that without the plug at the positions of #1 and #2 for the conditions after 1 and 30 days cooling from the end of operation of a year. As the results, it is possible to access for a longer time at the position #2 with using the plug for the condition after 30 days cooling. Table III. Calculated results of dose rate (µsv/h) for the cases with and without the local shields. Position #3 Position #4 without shield with shield ratio without shield with shield ratio 1 day cooling days cooling Table IV. Calculated results of dose rate (µsv/h) for the cases with and without the lead plug. Position #1 Position #2 with plug without plug ratio with plug without plug ratio 1 day cooling days cooling Conclusions Radioactivity and residual dose rate around the collimator in the 3-GeV PS ring were estimated in the calculations. Assuming to kw order beam losses in the collimator, the dose rate near the collimator reached more than 10 msv/h for the condition after 30 days cooling from the end of operation of a year. Especially, the dose rate is very high at the positions could watch the collimator directly through an opening of the collimator shield. In order to enable to access for a longer time, inserting an additional shield is required for reduction of the dose due to the activity in the collimator and the collimator shield. Calculated results show that it is permitted to access near the collimator with inserting the lead plug after the end of operation. From the calculation results on the assumption with removing the local shields after the end of operation for the maintenance of the collimator and the apparatus, the dose rate for the case without the local shields is 1~2 order higher than that with the shields because the activity in the beam duct is still high after a month cooling from the end of operation. For more accurate estimations of exposure dose, the examination of maintenance scenario with considerations of the results in the present work will be required. Acknowledgement The authors would like to thank Drs. N. Nakao of KEK, Y. Sakamoto and N. Sasamoto of JAERI for their useful suggestions and discussions about the estimations of radioactivity and residual dose. Authors wish to thank Dr. K. Yamamoto of JAERI for his technical help about the calculations by using STRUCT. 5

6 References 1. The Joint Project Team of JAERI and KEK, The Joint Project for High-Intensity Proton Accelerators, JAERI-Tech , Japan Atomic Energy Research Institute (JAERI) (1999). 2. High-intensity Proton Accelerator Project Team, Accelerator Technical Design Report for High-intensity Proton Accelerator Facility Project, J-PARC, JAERI-Tech , Japan Atomic Energy Research Institute (JAERI) (2003). 3. Yamamoto, K., Kinsho, M., Noda, F., Irie, Y., Shirakata, M., Beam Collimator Design for the 3GeV Synchrotron of the JAERI-KEK Joint Project, in Proceedings of the 2001 Particle Accelerator Conference, Chicago, 2001, p Drozhdim, A., Mokhov, N., The STRUCT Program User s Reference Manual, FNAL (2001). 5. Iwase, H., Niita, K., Nakamura, T., Development of General-Purpose Particle and Heavy Ion Transport Monte Carlo Code. J. Nucl. Sci. Technol., 39: , (2002). 6. Niita, K., Takada, H., Meigo, S., Ikeda, Y., High-energy Particle Transport Code NMTC/JAM. Nucl. Instrum. Methods Phys. Res., B184: , (2001). 7. Kai, T., Maekawa, F., Kosako, K., Kasugai, Y., Takada, H., Ikeda, Y., DCHAIN-SP 2001: High Energy Particle Induced Radioactivity Calculation Code, JAERI-Tech , Japan Atomic Energy Research Institute (JAERI) (2001) (in Japanese). 8. Sakamoto, Y., Tanaka, S., QAD-CGGP2 and G33-GP2: Revised Version of QAD-CGGP and G33-GP, JAERI-M , Japan Atomic Energy Research Institute (JAERI) (1990). 9. International Commission on Radiological Protection, Conversion Coefficients for Use in Radiological Protection Against External Radiation, ICRP Publication 74, Ann. ICRP 26(3/4), Oxford: Elsevier Science, (1996). 6