Steaming ahead

Kyushu Electric Power Corporation, or Tepco’s, Sendai nuclear power plant in Japan is planning to change all of the steam generators at its 890MW Sendai 2 reactor in Japan in 2018, during a scheduled refuelling outage.

Following the Fukushima Daiichi nuclear plant disaster in March 2011 after a large earthquake and subsequent tsunami, only two of 42 operable reactors in Japan are currently running. Shikoku Electric Power Co’s 890MW Ikata 3 is the only other operating reactor in the north Asian nation at present after Sendai 2.

According to Japan’s Nuclear Regulation Authority, or NRA, the Kyushu Electric Power Co has made an application to replace steam generators at the Sendai 2 unit in 2018. The steam generators are scheduled to be replaced during the reactor’s regularly scheduled maintenance and refuelling outage, the NRA said in a press release. Kyushu Electric has not formally said how long the outage will take, but such maintenance halts normally take a minimum of three months.

Steam generators are heat exchangers used to convert water into steam, from heat produced in secondary coolant loops of the reactor unit.

The Sendai nuclear plant is located in the city of Satsumasendai in the country’s Kagoshima Prefecture. The 890MW Sendai 1 reactor, like the vast majority of other nuclear reactors in Japan, did not generate electricity for over four years after the Fukushima disaster. However, the unit was the first of Japan’s reactors to return to service in August 2015. Sendai 2 restarted operations in October of 2015.

As well as the steam generator replacement at Sendai 2, Kansai Electric Power Company’s 826MW Mihama 3 also has plans for extensive steam generator replacement work.

NRA commissioners at one of the regulator’s regular meetings in November unanimously approved Kansai Electric Power Co’s plan for an extended 20-year period of operation beyond the initial 40 years at Mihama 3. Article 43 of Japan’s reactor and fuel law allows only one 20-year life extension to a unit’s assumed 40-year operating life.

Kansai Electric said in a statement it plans to replace many of the unit’s heavy components, including the top head of the reactor pressure vessel, or RPV, as well as all of the steam generators at Mihama 3.

The NRA decision in November allows Kansai Electric formally to operate Mihama 3 until 30th November 2036. However, Naoki Amano, an NRA specialist in PWR ageing countermeasures, told the commissioners at the November meeting that the company plans to restart the reactor around 2020, after implementing all engineering work that the NRA has approved. All of the meetings of the NRA’s commissioners are open to the public and media and are also webcast.

Japan’s system of nuclear regulation was completely revised after the Fukushima disaster, with the intention of both increasing public confidence in regulation by making it more transparent and accessible to the public, as well as trying to break down a culture of deference in Japan that could potentially pose a safety threat in the nuclear sector.

The replacement of the steam generators at Sendai 2 is regarded as highly important as the unit’s steam generators are one of the older of such components among Japan’s operable nuclear reactors and their replacement was not formally required when NRA granted approval for restart of Sendai 2. The steam generator issue was one of a number of safety issues cited by protestors at the Sendai plant at the time of the restart of the plant’s second unit, but the announcement of the timetable for the replacement in 2018 may go some way to alleviate these concerns.

The restart of reactors in Japan is a complicated process that requires multiple stages of approval by the NRA, including submission of detailed engineering work programmes for each reactor requesting restart by the utilities that own the units.

The work programmes are then analysed and commented on by NRA staff and commissioners and subject to multiple revisions and updates. There is also often a period of public consultation on certain aspects of the restart, preoperational onsite physical inspections and testing of the reactor by regulators, as well as local and prefectural, or county, government approval of the restart. 

The length of the approval and actual restart process for Japan’s operable reactors has been painfully slow, and highly controversial. Individual units seeking permission to restart have been subject to legal challenges, sometimes widespread protests and also political pressure against restart.

There are also a number of outstanding legal challenges which have yet to be resolved by Japan’s courts, as well as others that may still be launched or are subject to appeal to a higher court. For instance, while legal challenges by Japanese nuclear activists against both Sendai 1 and Sendai 2 were both ultimately unsuccessful, this was not the case with the Takahama nuclear power station in western Japan, which is owned by Kansai Electric.

This power plant is presently shut due to a court injunction handed down by a court in Fukui prefecture which was obtained by local anti-nuclear citizen activists on safety grounds and which legally prevents the operation of any nuclear reactor at the power station. The injunction is presently the subject of a legal appeal to a higher court in Japan which could well be successful based on the outcome of similar appeals against nuclear bans to Japan’s higher courts. However, the process is one that takes a large number of months, is very costly to the utility and local government, and is subject to a high degree of uncertainty.

The unexpectedly slow restart of Japan’s operable nuclear reactors has also represented a blow to the Japanese Government’s target of maintaining the share of nuclear power generation in the country’s total energy mix at around 20%. This figure currently stands at around five percent, a stark contrast to official targets to have nuclear represent over 20% of the total power generation mix in Japan in 2030 and beyond.

On the other hand, the large number of shut reactors has been financially painful for a number of the country’s utilities which have had to import either liquefied natural gas (LNG) or find other fuel sources such as natural gas or fuel oil to replace power generated by the shut nuclear units. These fuel replacement activities have imposed unexpected costs on many of the nuclear- owning utilities in Japan, which have few other power sources available to them.

In addition, enhanced safety costs related to new regulatory requirements and, in the case of Tepco, the owner of the Fukushima Daiichi plant, prohibitively expensive decommissioning and clean up costs.

Looking at the science behind testing steam generators can, however, explain to an extent the slow and expensive process of restarting Japan’s nuclear plant fleet, as well as maintaining power stations anywhere in the world using similar PWR technology.

IAEA steam generator standards

The IAEA, the Vienna-based international regulatory and safety body for the nuclear industry, says in a document on steam generator safety on its website that “managing the safety aspects of nuclear power plant (NPP) ageing requires implementation of effective programmes for the timely detection and mitigation of ageing degradation of plant systems, structures and components (SSCs) important to safety, so as to ensure their integrity and functional capability throughout plant service life.” The document further notes that “simultaneous rupture of several steam generator tubes can lead to a plant transient [change in reactor coolant system’s pressure], which is difficult to control and radioactive levels released to the environment which may exceed site limits.”

The IAEA adds that with respect to steam generator safety that “sudden rupture of several steam generator tubes also results in a rapid depressurisation of the primary coolant system and possibly may uncover the [reactor] core and cause core melting.”

A review of safety at 20 pressurised water reactors, or PWRs, nuclear power plants in the US showed that risk associated with steam generator tube ruptures can be as high as 75% of the total plant risk to such nuclear generating stations.

The IAEA’s website adds that “the major safety function of the steam generator is to act as a barrier between the radioactive primary side [of a nuclear power plant] and the non-radioactive secondary side.”

In the US, California’s San Onofre 3 was closed January 2012 after a tube leak was found in one of two replacement steam generators installed at the unit in 2011. Extensive tube damage was found in one of the two steam generators during inspection after the closure of the reactor. In June 2013, the plant’s owner announced it would permanently shut San Onofre 2 and 3, which had not operated for roughly 18 months, citing the high cost of regulatory uncertainty related to the closure of the two reactors.

Primary conditions for future generators

A further analysis of safety conditions for steam generators includes a number of factors that should be looked at to provide the best solutions for “primary side conditions for PWR steam generators”, as defined by the IAEA. As the “primary coolant in the reactor coolant system (RCS) serves as a moderator and is a medium for transporting heat from the core to the steam generators, it must not endanger plant operation by the corrosion of materials and consequences thereof,” according to IAEA.

Beside the function as a moderator, the task of water chemistry can be divided into metal release rates of the structural materials, which should be minimal, and the occurrence of localised forms of corrosion, which should be counteracted. The IAEA says “the transport and deposition of corrosion products must be influenced in such a manner that contamination of the primary coolant system is kept low”. It also recommends “the deposition of corrosion products on heat transfer surfaces, particularly on fuel assemblies, which should be prevented as far as possible, while the radiolytic formation of oxygen should be suppressed”.

Along this scientific investigation, the IAEA has found that the contribution of nickel is much more important to the primary side corrosion product inventory than the iron.

Initially most PWRs were operated with coordinated chemistry treatment. The key component of primary coolant water chemistry is given by restrictions on the fuel elements and dose rate reduction issues, and not the steam generators, except for Alloy 600MA SG tubes. Once any checks to the actual tubes have to be done, the issue of accessibility becomes paramount.

Generation inspections

Steam generator tube inspection requirements in the US are discussed first by the IAEA because a number of countries with PWR and CANDU units have used those requirements as a starting point for their own requirements. In addition, as the US has one of the most developed civilian nuclear generation markets and longest established regulatory traditions, it is often seen as a leader and regulatory benchmark for other jurisdictions.

The Nuclear Regulatory Commission (NRC) is also widely regarded as both one of the toughest and also most transparent regulators in the world. For this reason, nuclear regulatory standards adopted in the US by the NRC are often copied almost word for word in a number of other countries. For example, when Japan completely overhauled the nuclear regulatory system in the country following the Fukushima Daiichi disaster and a widespread loss of confidence, both domestically and internationally, in the country’s nuclear regulatory regime, the new regulatory environment was based on that of the US to quite a large degree. Thus, the new Japanese nuclear regulator that emerged from this environment, the Nuclear Regulation Authority, took the NRC to a large degree as its reference regulator.

The requirements for steam generator tubing inspections at US plants are included in the plant technical specifications,
which are prepared by the plant operator and approved by the US NRC. Originally, those requirements generally followed
the guidelines presented in the US NRC’s Regulatory Guide 1.83, which has been consequently modified.

Tubing inspection practices in Canada, China, the Czech Republic, France, Germany, Japan, the Republic of South Korea, Russia, Slovakia, Slovenia, Spain, Sweden and Switzerland for which there is a good record, however, differ from country to country.

The different approach is based on the variety of steam generator designs, materials and site-specific conditions. In addition, steam generators are susceptible to different types of ageing degradation. Some types of degradation are easier to detect or give rise to less severe safety consequences than other types of degradation. As well, different tube wall thickness leads to different safety consequences and different probability of leaks or ruptures.

Regulatory Guide 1.83 recommends that at least three percent of the tubes in each steam generator be tested over their entire length during the first inspection, which should be performed after six effective full power months but before 24 calendar months. According to the original US Regulatory Guide, “subsequent inspections should not be less than 12 or more than 24 calendar months apart and may be limited to one steam generator encompassing three percent of the total tubes at the plant”. All non-plugged tubes with previous indications (20%) should be inspected.

Regulatory Guide 1.83 was used as the basis for the steam generator inspection requirements in the technical specifications for only a few years. By the early 1980s, the US utilities (e.g. Southern California Edison, Northern States Power, Georgia Power and Commonwealth Edison) were following the steam generator tube sample selection guidance, i.e. focusing more on particular sections of the tube rather than looking primarily at the duration of operation as an assessment criteria. In this case the tubes selected for each in-service inspection included at least three percent of the total number of tubes in all the steam generators at a unit and are selected randomly except in some special cases.

The Japanese NRA requires that 30% of tubes be inspected every other year when a steam generator has had no leakage and no tube degradation. If any primary to secondary SG leakage or any tube defects are detected, 100% of the tubes have to be inspected each year over their full length, according to the IAEA.

Before each inspection, the steam generator tubes are subjected to a differential pressure test to open tight cracks and make them more detectable. Bobbin coil eddy current equipment is used above the tube sheet region. Eight by one eddy current probes are used in the hot leg tube sheet region in most steam generators in order to detect circumferential degradation. Rotating pancake coil eddy current equipment is used in the tube sheet region of one Japanese plant in order to detect pitting.

However, what slows the process of inspection is that “an appropriate level of steam generator and plant safety can only
be maintained by a suitable combination of inspection and acceptance (fitness for service) requirements,” according to IAEA analysis. Some countries have chosen to apply somewhat more conservative fitness for service criteria and so require their regulators to undertake fewer inspections at existing nuclear power plants. Other countries have chosen less conservative fitness for service criteria, thereby saving money on repairs and more inspection.