Heating and radiological CNGS decay pipe - Heating and radiological issues Heinz Vincke and Graham R. Stevenson

OUTLINE 1. General Information 2. Energy deposition 3. Thermal study 4. Induced radioactivity 5. Dose-rates

General information - decay tunnel Decay tunnel characteristics  Excavated by TBM - 992 m long - Internal diameter of 3.50 m - Slope of 5.6% - Supported by shotcrete - Situated at a depth ranging between 55 and 125 m. Internal diameter of 2.45 m, steel pipe t = 18 mm, surrounded by 50 cm concrete  The gain in beam intensity becomes negligible for larger diameters. 998 m long  Compromise between the gain in beam intensity against the additional cost of a longer decay tube and associated infrastructure. Vacuum 1 Torr  To avoid any loss of pions and kaons through interactions with air (28% loss with air and 7% loss with helium). No access once the facility is operational Radioactivity

Energy deposition - general Proton beam energy for CNGS : 400 GeV Proton beam intensity : 8.0 x 1012 pot/s … 13.8 x 1019 pot/y, ultimate number based on dedicated running with 100 % efficiency. Note, that the nominal number is 4.5 x 1019 pot/y. Beam power: 512 kW Calculation with FLUKA - Monte Carlo particle transport code Energy cut-off for particle transport 0.1 MeV for all particles except for neutrons which were followed down to thermal energies.

Energy deposition in the decay tunnel In the steel pipe: 55 GeV  71 kW In the concrete: 37 GeV  48 kW In the rock: 3 GeV  4 kW --------------------------------------------- TOTAL: 95 GeV  123 kW

Thermal study - general A thermal study of the decay tunnel was carried out in order to check whether the temperature increase in each material remains within an allowable range and is not affecting their thermal properties whether the thermal gradient does not cause excessive stress in the material. The experiment schedule was the following: 200 days of operation followed by a stop of 165 days, and looping this cycle again during a period of 10 years. A long term run over 27 years non stop was also simulated. The temperature map around the tunnel was calculated using a 3D finite difference program. The subsequent stresses in steel, concrete and rock were analyzed using a finite element program.

Thermal study - Energy deposition

Thermal study Results: The maximum temperature increase in the steel and concrete is 45°C, and 38°C in the rock. This is well below a temperature of 100ºC in the rock - the limit to avoid problems in water saturated rock zones. The steel, concrete and rock stresses due to the thermal effect will remain in an allowable range. The installation of a temperature measuring system will enable the behavior of the system to be surveyed and also enable adjustments to be made to the theoretical results of the thermal analysis. A long term simulation over 27 years showed that the temperature increase (DT = 71ºC) will be still acceptable both for the materials behavior and the possible occurrence of water saturated rock zones.

Induced Radioactivity and dose rates The production of radioactive isotopes (residual nuclei) was calculated with FLUKA - Monte Carlo particle transport code. Energy cut-off for particle transport 10 MeV for all articles except for neutrons which were followed down to thermal energies. Muons, electrons and photons were discarded. A postprocessing program was used to evolve the residual nuclei distribution obtained from FLUKA, according to given irradiation and cooling times. 200 days of operation per year with a proton intensity of 8 x 1012 pot/s. 10 years of operation. cooling times: 1d, 30d, 180d, 1y, 2y, 5y, 10y, 50y, 100y and 1000y.

Induced Radioactivity in the steel-pipe after 10 y Fe-55 (73%) H-3 (24%) Ti-44 & Sc-44 (0.6%) after 1 y Fe-55 (68%) Mn-54 (20%) V-49 (8%) after 1000 y Al-26 (83%) Ca-41 (5%) Ni-59 (4%) after 100 y Ar-39 (28%) Ti-44 (21%) Sc-44 (21%)

Dose rate in the decay tunnel After 1000 y steel concrete Al-26 (98%) Al-26 (99.9%) Sc-44 (1%) K-40 (0.1%) Ti-44 (0.1%) After 100 y steel concrete Sc-44 (89%) Sc-44 (54%) Ti-44 (10%) Al-26 (21%) K-42 (0.4%) K-42 (19%) After 10 y steel concrete Sc-44 (57%) Na-22 (99%) Na-22 (16%) Sc-44 (0.3%) Co-60 (11%) K-42 (0.2%) After 1 y steel concrete Mn-54 (95%) Na-22 (90%) Sc-46 (2%) Mn-54 (9%) Co-56 (1%) Sc-46 (0.1%)

Summary and Conclusion The maximum temperature increase of the decay tube, the concrete and the rock will be less than 45ºC after 10 years of operation. Note, that for the nominal beam intensity of 2.6 x 1012 pot/s the temperature rise is 2.5 to 3 times lower. No cooling system will be necessary for the decay tunnel but potential dilatation of the tube must be taken into account by a detailed mechanical design study. The steel pipe will have to be finally disposed of as radioactive waste. Dismantling of the steel pipe is maybe possible after 25 years.