Mitsubishi Heavy Industries, Ltd. (MHI) recently completed replacement work on a nuclear power plant in Japan; the event marks the world’s first all-in-one-piece extraction and replacement work of its kind in a pressurized water reactor (PWR).
Junichi Uchiyama, Mitsubishi Heavy Industries, Ltd. Japan
When Shikoku Electric Power Co., Inc contracted MHI to replace the upper and lower reactor internals of the 566 MW Unit 1 at its Ikata nuclear power plant located on Shikoku island, Japan, the high levels of radiation that contractors could be exposed to was of serious concern. To combat this, MHI developed a concept that saw the project become the world’s first use of an all in one extraction technique for replacement work on a pressurized water reactor (PWR).
PWR reactor internals consist of upper and lower core internals that support the core (fuel assembly) and form the flow route of primary coolant. Upper core internals (UCI) guide control rods and support the upper end of the fuel assembly. UCIs are removed from the reactor vessel (RV) and reinstalled into the RV every refueling outage. Lower core internals (LCI) involve and support the fuel assembly, and lead primary coolant from the RV inlet nozzles to the outlet. LCI can be removed from the RV during in-service inspection and reinstalled to the RV.
Figure 1. PWR reactor internals for replacement
All of the reactor internals of the near 30 year old Ikata Unit 1 were replaced at this time to accommodate the increased number of control rods needed for the use of high burn-up fuel and to improve reactor equipment reliability for preventive maintenance, that is, to prevent damage of baffle-former bolts of the kind reported overseas that was caused by stress corrosion cracking (SCC).
Several cases of boiling water reactor (BWR) shroud replacement have been reported, but a case of UCI replacement for a PWR has only been reported at the Prairie Island plant (in 1986). The work at Ikata Unit 1 was the first time anywhere in the world that all reactor internals had been replaced using a new all-in-one extraction method. MHI provided engineering and operation techniques for both the baffle former bolt exchange and the replacement of all internals for preventive maintenance. Some of the utilities in Japan consequently could reach to select a measure of the reactor internals replacement for the objective described above based on their own consideration.
The old reactor internals, with over 20 years operation, had a strong radiation dose rate on the thermal shield outer surface estimated conservatively to be 2X105 mSv/h. Since the replacement operation had to be performed without workers coming into close contact with the high levels of radiation, MHI developed an operation method where both the UCI and LCI were directly removed as one piece in air from the RV, without being split or cut into pieces. This was named the all-in-one extraction method.
Decontamination to reduce strong radiation from the old reactor internals would not have been effective because the high radiation was caused by a radioactive product such as 60Co in the material. The all-in-one extraction method developed was based on three engineering techniques:
- A method through an equipment hatch (EH) of containment vessel (CV) to bring out the old reactor internals and to bring in the new reactor internals
- Shielding and storage cask design
- A method to lift the heavy weight over the capacity of the CV polar crane.
The removed materials were hoisted by a specially installed crane and stored in a special steel container cask. The total weight of the cask after housing the removed reactor internals was over 450 t. The all-in-one extraction method not only shortened the replacement time to 70 days (one-third of the period compared with a cutting method), but it also reduced worker exposure to about one-tenth of what they would have been exposed to if the cutting method had been implemented.
The old reactor internals were stored in the container cask and transported to the steam generator storage facility in the site. The METI Commercial Reactor Rules in Japan were applied to the project for transportation and waste disposal on site. A shielding design adopted steel thickness of about 280 mm. The storage cask corresponding to the all-in-one method reached dimensions of about 12 m in height, 3.8 m in outer diameter, and weighed 450 t including the old reactor internals.
Figure 2. Extraction plan for the old CI
The cask had to be designed and manufactured with high precision due to the all-in-one extraction method. For example, cylindricality within 3 mm of the cask body over whole height was requested because accurate lifting of the old reactor internals from the reactor vessel without a RV guide stud was needed. Before actual operation, setting up of the storage cask and the special crane was finished in the MHI Kobe machinery works, where the verification test and training were also conducted.
New reactor internals were designed and manufactured based on the newest standard design of two loop reactor internals in Japan as follows:
- Standard design adopted: Inverted top hat type of upper support plate; Panel type neutron pad with reduced width.
- Modified baffle structures: L-type baffle plate; Longer baffle former and barrel former bolts; Flow hole on the former plate of bolt location for cooling.
- To increase number of guide tubes.
The old reactor internals had been installed in the reactor vessel at the initial plant construction by means of measuring gaps between the vessel and internals by hand. When the new reactor internals were set in the water-filled reactor vessel, extremely high accuracy was required because of a restriction on bypass flow and to maintain the structural integrity of the LCI during seismic events. MHI therefore developed a new high precision remote controlled measurement and installation procedure for underwater applications. A gap of 0.4 mm between the vessel and the internals operated by the system, equivalent to what was needed during initial construction of the plant, was created.
Figure 3. The cask design
Prior to executing the work on-site, a series of elaborate verifications were performed and rigorous training in the new work method was conducted to ensure full satisfaction of the project’s stringent work requirements.
Shikoku Electric Power is planning to carry out the same reactor internals replacement work during the next inspection of the Ikata Station Unit No. 2, which was scheduled to start in September 2005. MHI aims to use its experience and technology gained from the Ikata power plant project to continue its active pursuit of preventive maintenance for nuclear reactors worldwide.