Koreans prepare to build the first advanced PWR20 August 2001
If the Koreans manage to stick to their current schedule of getting the first of their next generation nuclear units in commercial operation by 2010, they can truly claim world leadership in advanced PWR technology.
The nuclear industry has seen a good deal of talk about “advanced PWRs” over the years. But, with the slow down in nuclear construction, very few countries can claim to have concrete plans in place for building such advanced reactors. One notable exception is South Korea, which has pressed on doggedly with an ambitious nuclear power programme (see table, p39) and where nuclear energy currently accounts for about 43 per cent of the total electricity generated in the country.
The Koreans have recently announced plans to build the first of what they now call the APR1400, or Korean Advanced Pressurised Water Reactor. This will mark the culmination of a development programme formerly known as the Korean Next Generation Reactor (KNGR)).
The first two APR1400s will be built at Shin-Kori, becoming units 3 and 4 at that site. Construction is slated to start in 2002, with major component manufacture scheduled for 2003-2006. Commercial operation of Shin-Kori 3 and 4 is expected in 2010 and 2011 respectively, while a further two APR1400 units (sites not yet confirmed) are planned for commercial operation in 2013 and 2014 respectively. These dates reflect a stretching of the original KNGR programme by about three years, an effect of the 1998 economic downturn. But if the schedule is met they will the first operating advanced PWRs in the world, albeit with fewer passive features than originally envisaged.
The APR1400, rated at 1400 MWe, is an evolution of the 1000 MWe Korean Standard Nuclear Power Plant (KSNP). Two KSNP units, Ulchin 3 and 4, are now in operation, with four more under construction, while a further four, of an improved KSNP design, called KSNP+, are at the planning stage (with the first pair, Shin-Kori 1 and 2, due to start construction this autumn). Two KSNP units are also in the early stages of construction in North Korea, under the KEDO programme (see panel p 41). But since the change of president in the USA, the future of this project has been looking uncertain.
The APR1400 incorporates a number of features aimed at improving safety and economics relative to KSNP, including simplification of safety systems, improved measures to prevent and mitigate severe accidents, emphasis on human factors engineering and partial introduction of passive safety features (eg fluidic control in the safety injection tank and passive hydrogen control systems). To use the terminology of the US Department of Energy it might be described as Generation III reactor technology, incorporating some passive features but not a full blown passive reactor system, as exemplified by the proposed pebble bed high temperature gas-cooled reactor, which has been dubbed Generation IV.
The economic target of APR1400 is to offer a 20 per cent cost advantage over coal. Measures to achieve this include high power rating (up to 1400 MWe from 1300 MWe, which was the original rating envisaged for the KNGR), improved construction methods, extensive use of 3D CAD and maximum pre-fabrication and modularisation.
Development work proper on APR1400 started in 1992 and has involved a concerted effort by all the major players in the Korean nuclear industry (Korea Electric Power Corp (KEPCO), Korea Institute of Nuclear Safety (KINS), Korea Atomic Energy Research Institute (KAERI), Korea Power Engineering Company (KOPEC), Korea Nuclear Fuel Corporation (KNFC), Center for Advanced Reactor Research (CARR), Hanjung etc), not to mention Combustion Engineering of the United States, formerly owned by ABB and now part of the BNFL nuclear empire. In fact CE’s System 80 and subsequently the System 80+ NSSS designs have been the foundations of the Korean nuclear reactor programme.
The APR1400 development process essentially has consisted of three stages. The first stage, conceptual design, was completed in December 1994. The second stage, basic design, was completed in February 1999, with submittal of the Standard Safety Analysis Report (SSAR) to the Korean regulator for review. The third stage, involving some more detailed design, is now underway, with the goal of achieving certification of the standard design by the end of this year. The third stage has also included plant optimisation based on probabilistic risk analysis and cost, which has led to the dropping of a passive secondary condensing system that had originally envisaged as part of the design.
The design aims to fully reflect utility needs as set out in the Korean Utility Requirements Documents. The hope is to have about 70 per cent of the detailed design completed at the start of construction of the first unit.
The basic approach to be adopted in the APR1400 was established in stage 1 of the development process, which included reviews of advanced light water reactor (ALWR) designs being developed around the world. The safety and economic goals of the various advanced LWRs were compared quantitatively, and, following this, some 42 “top-tier” design requirements were established. The pressurised water reactor was the preferred reactor type, with the following basic design goals shown in the table below:
Since the APR1400 has evolved from the Korean Standard Nuclear Power Plant, it has much in common with that earlier, and well proven, plant design, for example two steam generators with four reactor coolant pumps in a two-hot-legs and four-cold-legs configuration. But, at 4000 MWt, thermal power has been increased by 40 per cent compared with the KSNP’s rated thermal power of 2815 MWt.
As well as the passive features already mentioned, the APR1400 includes the following advanced design features: four-train safety injection; direct vessel injection; in-containment refuelling water storage tank (IRWST); shut down cooling system with pressure of 900 psi; interchangeable shutdown cooling system and containment spray system; robust double containment with large volume; and digital instrumentation and control.
The general plant layout features a wrap-around and quadrant-type auxiliary building, with a common basemat shared by the auxiliary and containment building. The outer containment is a reinforced concrete cylinder, while the inner containment is a post-tensioned concrete cylinder with hemispherical dome. The emergency diesels occupy their own building, which is separated from the auxiliary building. The design allows for replacement of steam generators (should that prove necessary) in one piece. The units are envisaged as being built in pairs with a common radwaste building, in what the Koreans call a “twin-unit slide along arrangement.”
The main control room features redundant compact workstations for operators, a seismically qualified large display panel for overall process monitoring (visible to all control room staff), multi-function soft controls, fully computerised operating procedures with context sensitive operational guidance, and a safety console with conventional switches and buttons capable of carrying out essential safety functions in emergencies. The instrumentation and control system includes a fibre optic network and a redundant and diverse information processing system.
To help reduce planned outage times, the APR1400 uses an integrated head assembly, with reactor vessel closure head lifting systems, HVAC equipment for cooling of control rod drive mechanisms, head area cable supports and missile shield structures. One-piece removal of the head assembly saves about three days of critical path time in a refuelling outage.
In the APR1400, the emergency core cooling system is designed so that ECCS water injects directly into the reactor vessel. This four-train system eliminates the need for conventional low pressure safety injection.
Conventional spring-loaded safety valves mounted on top of the pressuriser are replaced by pilot operated safety relief valves (POSRVs). The functions of reactor coolant system overpressure protection and safety depressurisation in case of severe accidents are carried out by the POSRVs, with a significant gain in reliability.
As already noted, the refuelling water storage tank is located inside containment. In the event of a leak from the reactor coolant system (RCS), the discharged RCS inventory is directed to this IRWST and quenched there, meaning that the containment remains uncontaminated in the event of RCS coolant loss.
Located at the discharge of the safety injection tank is a fluidic valve, which provides a passive system for regulating the injection of borated water into the reactor coolant system. This arrangement improves the ability of plant to handle loss of coolant accidents, by extending the water injection period.
Severe accident mitigation
Hydrogen explosions are considered to pose the biggest threat to the integrity of a PWR containment in the event of core damage. In the APR1400, any hydrogen build-up is controlled through passive auto recombiners and glow type igniters. The volume of the containment is sufficiently large to keep the hydrogen concentration below 13 per cent (by volume) even in the event of oxidation of 75 per cent of the active fuel cladding.
To prevent direct containment heating and maintain core debris in the reactor cavity in the unlikely event that reactor vessel failure occurs, the cavity under the reactor vessel is large, the possible paths that core debris could take to the upper part of the containment are highly convoluted so that such transport is unlikely, and a cavity flooding system is provided for long term cooling of core debris. The water to flood the cavity comes from the IRWST, needing only gravity – once motor operated valves (battery powered) have been opened.
In addition to these measures for ex-vessel retention of core debris, there are also provisions for cooling the external surface of the reactor vessel in the event of a core melt, to keep molten core material inside the vessel.
As result of these kinds of measure, the estimated containment failure frequency for APR1400 is dramatically lower than for current generation reactors. It is also ten times lower than for KSNP. The probability of containment failure for APR1400 is estimated to be 10-7/reactor-year.
Perhaps the key reason why the Koreans have managed to retain a long term nuclear development programme is that nuclear self-reliance is seen as a key plank of national policy. April 2001 saw the opening of the Korea Power Exchange and the setting up of six generating companies, one responsible for nuclear and hydro generation (Korea Hydro and Nuclear Power Co (KHNP)). All the companies will compete with each other and be looking to reduce costs but KHNP will remain in public ownership. There is expected to be full retail competition by will around 2009.
Keeping nuclear capacity under state ownership is seen as the best way of maintaining nuclear safety, retaining the capability to develop and build new plants and dealing with the KEDO PWR project in North Korea.
The new nuclear company presumably now also has also taken on the responsibility for dealing with nuclear waste. The current plan is to have a repository for low and intermediate level waste in operation by 2008, with a centralised interim storage facility for spent fuel operating in the same area by 2016.
|North Korean project faces uncertainty|
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