Ⅳ. OPERATION RECORDS
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Operation of BigRIPS cryogenic plant
K. Kusaka,∗1 M. Ohtake,∗1 K. Yoshida,∗1 M. Ohshima,∗2 A. Mikami,∗2 H. Hazama,∗2 H. Shiraki,∗2 H. Hirai,∗2K. Kimura,∗2 M. Nagano,∗2 H. Shiba,∗2 and S. Okada∗2Following the accident that occurred on December
3, 2021,1) the main motor of the BigRIPS compressor
unit was shipped to the manufacturer’s factory and disassembled. Damages were found in the bearing units
on both the coupling and anticoupling sides. In particular the bearing unit on the coupling side was heavily
damaged such that the ribbon retainer was destroyed
and the surfaces of the bearing balls and rings were ruined (Fig. 1). We also found that the size of the housing
for the damaged bearing unit was 20 µm larger than its
standard dimension (+0 to 25 µm). After cleaning therotor and stator, the motor unit was reassembled using new bearing units and the re-manufactured bracket
housing. In addition to mechanical maintenance, theinsulation was also examined, and little deteriorationof the stator insulation was noticed. We decided toreplace the motor unit in the summer maintenance period.
Fig. 2. Compressor unit with the new motor.

Fig. 1. Damaged bearing unit, disassembled from the coupling side of the motor unit.

We performed two continuous operations of the BigRIPS cryogenic plant in 2022. The ﬁrst operation
period was from February 22 to June 13 and the second was from November 16 to December 28. Between
these dates, we replaced the motor unit with a new one(Fig. 2). Figure 3 depicts the vibration accelerationsin the vertical and horizontal directions as functions ofthe total operation time. Three rapid increases in thevibration acceleration at the operation times of 59,000,71,000, and 78,541 indicate that the damage in thebearing unit occurred in December 2016, June 2019,∗1∗2

RIKEN Nishina Center
Nippon Air Conditioning Services Co., Ltd.

- 209 -Fig. 3. Vibration acceleration of the compressor unit.

and December 2022.1) The red arrow at the operation
time of 81,235 hours indicates the replacement of themotor unit. Although the vibration acceleration does
not change signiﬁcantly, the typical operation currentof the motor unit decreased from 30.0 to 26.6 A.
In 2022, another incident occurred: the damage ofthe ﬁrst expansion turbine T1 in the refrigerator. We
visually inspected T1 and T2 turbines during every
summer maintenance. It was found that the bladesof the expansion wheel of the T1 turbine were broken (Fig. 4). The T1 cartridge was then sent to its
manufacturer, Linde, and disassembled. Linde reportsthat the fractured surface indicates the typical characteristics of a low-cycle fatigue (LCF). The expan-Ⅳ. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 56 (2023)

sion and compressor turbine wheels are replaced withnew ones and re-assembled with new axial bearings.
Before remounting the T1 cartridge on the refrigerator, we investigated the inside of the refrigerator using
a ﬁberscope. No fragments but tiny metal dust wasfound in the downstream piping of the T1 expansion
turbine. After the dust was removed using a vacuum
cleaner, the T1 turbine was mounted on the refrigerator. We switched the refrigerator on for pre-coolingon November 16, and the STQ1-5 cryostats were ﬁlledwith liquid helium on December 3. Although the average liquefaction rate of 26 m3 /h is the same as before,
the rotation speed of 4450 rps of the T1 turbine during
the steady-state operation was lower than its design
value of 4600 rps. The origin of the LFC and the lower
rotation speed of the T1 turbine are currently underinvestigation.

Fig. 4. T1 expansion turbine wheel with broken blades.

Reference
1) K. Kusaka et al., RIKEN Accel. Prog. Rep. 55, 182(2022).

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