Ten reactor years of ABWR experience

The advanced boiling water reactor (ABWR) design integrates BWR technology with advanced instrumentation and control (I&C) technology. The ABWR also features reactor components such as fine-motion control rod drive (FMCRD) and reactor internal pump (RIP).

The ABWR represents a significant step in the BWR evolution process. Since improvements have been continually incorporated into the BWR5 series, the performance targets for ABWRs were already met in the later BWR5 units. However, the ABWR design features improvements in the man-machine interface and has safety advantages over later BWR5 units.

Kashiwazaki-Kariwa (KK) 6 and 7 are twin 1356MWe ABWRs, developed jointly by BWR vendors (GE/Toshiba and Hitachi) and Japanese utilities. The first ten reactor years of combined operating experience for Tokyo Electric Power's (Tepco's) KK6&7 ABWR units have demonstrated:

• ABWRs are performing up to their expectations.

• The unplanned shutdowns have been due to conventional problems and do not suggest there are any ABWR-specific problems.

• BWR technology is becoming safer and more economic.

• Compared to earlier BWR technology, ABWRs have lower occupational radiation exposure, increased availability, higher load factors and lower O&M costs.

• ABWRs would operate more efficiently under less severe operating constraints and with improved asset management strategies.

Operating KK

Operations and maintenance (O & M) activities at the seven BWR units of Tepco's KK site are carried out by 950 Tepco employees. Of these, 60 operators and 26 dedicated O&M support team members are employed at the twin ABWRs.

Each ten-man shift team working at the shared control room for KK6&7 works in a rotating shift cycle: shift 1, shift 1+2, shift 2, shift 3, shift 3 (shift 1 = 0830-1520, shift 1+2 = 0830-2125, shift 2 = 1500-2120, and shift 3 = 2100-0855). The shift schedule is designed so that each operator has eight weeks of shift duty out of 12 calendar weeks.

Operational performance

The Figures on page 23 show the operational history of KK6 and 7. The Table, also on page 23, shows that the overall operational performance meets the target set in the late '70s for four performance indicators. As mentioned earlier, these targets have in general already been achieved by the newer units of the BWR5 (1100MWe) series, due to the continuous evolution of the BWR design and operation.

However, the following four points would not have been possible without the ABWR design:

• Reduced occupational radiation exposure. This is attributable to less primary system piping and consequentially less ISI and a better working environment inside the containment; reduced maintenance requirements for reactor components such as FMCRD; and a reduced radiation level of reactor components.

• Increased availability - FMCRD does not require power reduction during rod pattern change.

• Increased load factor, attributable to less maintenance required for NSSS components, and less ISI activities and through testing before use. Currently, the cumulative load factor of the ABWRs exceeds the average of the later 1100MWe units by 3%.

• O&M costs are reduced by 20% due to less maintenance requirements, increased output and less workforce due to the shared control room.

Although the cumulative load factor of 87% is higher than the average in Japan, this would be even higher if the constraints and practices of Tepco's O&M are changed. Such changes could include lifting the 13-month operational cycle limit, as well as the limit on rated electric power and other constraints; risk-informed ISI and online maintenance of safety systems could be introduced; reliability-centered maintenance and condition-monitoring based maintenance practices could be extended to replace traditional time-based maintenance; and 24-hour activity and good pre-planning could be introduced for outage management. These changes would lead to an increase in load factor by about 9%.

Core performance

The initial fuel design was 8x8 fuel with 39.5GWD/t average burnup and is currently being replaced by 9x9 reload fuel with an expected burnup of 45GWD/t (core average).

The use of FMCRD in ABWRs enables rod pattern adjustment under full power operation with almost no change in core flow whereas, with other BWRs, control rod pattern change needs considerable reactor power reduction, especially towards the end of cycles.

Unplanned shutdowns

To date, KK6&7 have had 0.74/year and 0.63/year of unplanned shutdowns, respectively, during nearly five reactor years of operation. This is of a similar order of magnitude to the Japanese average of 0.48/year. All of the incidents are classified as Level 0 on the IAEA Event Scale (INES). Although the figure for KK6 appears to be somewhat high, this is probably due to the fact that it is a first-of-a-kind (FOAK) reactor. In Tepco's 1100MWe fleet, the FOAK plants Fukushima Daiichi 6 and Fukushima Daini 1 had 1.0/year and 0.4/year unplanned shutdowns, respectively, during their initial five reactor years of operation.

There was a total of nine incidents leading to unplanned shutdowns of the ABWRs during a total of 11.5 reactor-years of operation (including pre-operational stage). Although two of these are related to RIP - an ABWR-specific system - their cause was by conventional means, namely failure of motor cable terminal due to vibration and failure of the digital controller board.

Three of the other seven shutdowns were due to fuel failures (single rod failure thus no technical specification implications), and the rest due to failure of turbine pressure instrument pipe, failure of generator excitation system, failure of 500kV power line protection relay, and component cooling water leakage inside the containment vessel.

RIP-related incidents

During operation of KK7, there was an unplanned shutdown due to the failure of a RIP power supply cable terminal due to vibration. Although continued operation was possible since ABWRs can operate with one of their ten pumps out of service (and can operate at reduced power with up to three pumps out of service), the reactor was shutdown to investigate the root cause, and the design was modified immediately. Another example of RIP-related failure, in this case of the RIP power supply controller, happened during pre-operation of KK6. Again, continued operation was possible by re-starting the tripped RIP and utilising the redundant controller. In this case, the plant was manually shut down for 19 days to investigate the root cause.

Defective fuel

There have been five instances of defective fuel, one of which was subject to PIE and the cause of failure was found to be debris-induced fretting corrosion. Iodine levels in the reactor coolant had allowed continued operation within the limits of the operational technical specifications. Continued operation by power suppression techniques was preferred in some cases. Although only one of the defective fuels was subjected to PIE, debris failure is a suspected cause for many cases since the fuel in the core does not differ in design and manufacturing from those in other reactors, and operational limits were kept well within the preset boundaries.

Since the population of defective fuel (3 x 10-5 for 8x8 fuel) is higher than for other Tepco reactors, an analysis has been done to see if there is any design-specific cause inducing debris failure. Drain-forward system design in the balance of plant, no annular region in the RPV and relatively flat-shaped bottom shell of the RPV were suspected as potential factors leading to debris failure susceptibility. However, the analyses were not able to confidently support these premises. A debris filter installed for reload fuel is expected to eliminate this concern.

Maintenance

In accordance with time-based maintenance practices, a large number of components have been subject to inspection and parts replaced where necessary. This has led to extended periods of refuelling and maintenance outages, typical for Japanese plants. This practice will be subject to change in the future for more efficient asset management, based on operating experience and degradation analysis. The use of digital control in safety-grade systems has never caused serious incidents. However, due to the increased number of digital control applications (safety or non-safety grade), Tepco has approximately 190 printed boards on screening criteria, based upon failure data and required times for procurement of replacement parts.

Personnel opinion

Generally speaking, Tepco's employees involved in O&M have a high opinion of the ABWR plant:

• Operations. "Excellent" rating for the large display; more sophistication in automation for control rod withdrawal needed (criticality access); benefits in ganged motion; complexity in tag control during outage (touch operation versus hard switch).

• Maintenance. "Excellent" rating for recirculation system component performances; "good" workspace inside the containment; "good" design consideration for maintenance; outages will be shortened with experience.

• Engineering. Initially difficulties in calibration of core flow measurement; fuel leaks; some problems will be fixed in the subsequent design, since they require only minor modifications.

Future of the ABWR

Due to a harsh winter, the construction of KK6 and 7 took 37 and 39 months, respectively, from first concrete pouring to fuel loading, which is still a reduction of 13-15 months compared to the first unit at the KK site. Construction of subsequent units will be possible in the 33-36 month range and, if steel plate reinforced concrete is used, 21.5 months schedule can reasonably be expected. This is due to the reduced on-site work volume that would be necessitated for rebar works in the conventional building structure. Given the fact that the escalation of the wages of rebar workers exceeds that of steel plates, reduced on-site rebar works would also improve the construction economics. Besides soaring labour costs, the decline in the number of qualified workers and shortage of young labour supports a shift in favour of the new design. Increase in seismic load carrying capability is also possible with this new design.

Because of standardisation, the ABWR fleet is expected to reduce its capital cost as happened for Tepco's 1100MWe plants. It is interesting to note that the FOAK ABWR plant, even if the $600 million T&D cost is to be born by the initial four plants, was built at $23/kW lower than the last unit of Tepco's 1100MWe series, primarily due to reduced buildings/components volume and the benefit of plant scale.

ABWR deployment programmes are taking place in Taiwan (Lungmen) as well as in Japan. In total, Japan has ten ABWR units under various stages of deployment, including two units under construction by Chubu Electric and Hokuriku Electric and also one full-MOX ABWR plant by EPDC (Electric Power Development Corporation). This full-MOX version of the ABWR design exploits the advantages of BWR technology - the core design can relatively easily alter the U/H ratio inside the fuel.

Beyond the ABWR design, Tepco is looking at the ABWR-II design for better safety, economics and operability, and expects incremental improvements in economics throughout the series.
Tables

Tepco’s BWR fleet