Plate of 150 mm thickness is sufficient to take the load of concrete blocks with maximum deflection is 0.82 mm at center maximum von-mises stress of 20.5 MPa which is well within the permissible limits

Design of Movable Shield for the Vertical Test Stand Cryostat

Introduction VTS has internal shielding consists of 200 mm lead, 100 mm Steel and 100 mm borated polyethylene enclosed inside a vacuum vessel. Radiation dose outside the external shield, comprising of 150 mm steel and 480 mm of concrete, due to bremsstrahlung x-rays and photo-neutrons are simulated using the Monte-Carlo code, FLUKA [1]. The absorbed dose rate data for both with and without external shield are given Table-1.

Hemant Kumar Patel, Ashish Kumar Shukla, S. K. Joshi, P.K. KushRaja Ramanna Center for Advanced Technology, Indore, India

hemantpatel@rrcat.gov.in

Vacuum jacketed transfer line and Pumping line from VTS to vacuum pumping station have been routed through trench Liquid helium is transferred to VTS from 10,000 liters deewar connected to liquefaction system Vacuum jacketed line with LN2

shielding has been made to transfer LHe Evaporative cooling to bring down temperature for 4.2 K to 2 K (minimum temp achieved is 1.65 K)

System for cryogen filling

Table-1: Photon dose rate outside the pit with and without external shielding

Location

Dose rate (µSv/h)Without external shield

With external shield

A- Top of external shield in beam

direction6.67 x 103

< 1.38 x 10-2

B- Top of external shield along the edge

of the pit4.94 x 103

C- Adjacent to external shield outside

the pit2.48 x 101

D- Inside adjacent pit<9.50 x 10-3

(less than Background)

Size Optimization of movable shield

Analysis for plate thickness

150 mm thick bottom plate 100 mm thick bottom plate

Max deflection – 0.82 mm Max deflection – 2.7 mm

Drive mechanism designed to move the shield of weight ~ 62 tonnes with stringent alignment requirement Requirement of 10 mm gap between the floor and shield is achieved Interference checking and small parts detailing was done using UG NX 7.5 Helical bevel geared motor drive with quintuple reduction and skid rollers of 15 tonnes capacity each (10 Nos) were selected Special type of spacer plates of different thickness were designed for maintaining the gap between floor and the shielding. Actual deflection measurements after erection of shield is 0.75 mm Tolerance of 2 mm and 4 mm achieved on length 5.3 meter and 11.5 meter respectively.

Erection and commissioning

Thickness of 150 mm steel with 480 mm concrete thickness is sufficient for VTS radiation shielding. Radiation level after shielding is of the order of 1.38 x 10-2 µSv/h The size of the shielding lid is optimized based on changes made in the pit design and operational requirements. Fabrication and erection has been completed successfully Actual deflection measurements after erection of shield is within 10 % of calculated value

Conclusion

150 mm of steel with 480 mm of concrete thickness is sufficient Length (5.3m) and width (4.85m) of the lid is optimized to provide minimum thickness required Total weight of the shield is around 62 tonne A concrete thickness correction factor - 20 mm spatial radiation has been prevented using staggered arrangement of the plates and concrete blocks

ABSTRACT Vertical Test Stand Cryostat (VTS) is a facility for testing super conducting radio frequency cavity at a temperature of 2K (size - Dia 1.3 m x Length 5.4 m). About 3000 liters of liquid helium is transferred at a temperature of 4.2 K. Temperature is further reduced to 2K by reducing the pressure over helium bath. During testing of cavities, due to impurities and geometrical imperfections etc. there is a chance of radiation coming out of the VTS pit. A movable radiation shield to cover the cryostate is deployed to prevent accidental radiation in the vertical upward direction. The 4.2 K helium transfer line and 2K pumping line puts constraints on the designing of radiation shielding arrangement. Highlights of the design also includes the selection of the roller, Chain, Sprocket and geared motor which can move the shield (Approximate weight 62 tonnes) To and Fro over VTS pit 1 and 2. This paper is focusing on the Mechanical design aspects of the movable shield with size and cost optimization of the system based on design conditions and actual site scenario. It also discusses planning and layout of cryogenic pipe lines in conjugation with movable shielding.

4850

630

5300

Fig. 1 VTS vertical sectional view

Fig. 2 VTS building layout

Fig. 3 Radiation shielding model

Fig. 3 Radiation shielding model vs Actual site picture

ICEC26

9-P2-116