By Niko Milonopoulos and Edward D. Blandford
03 April 2014
In 2010, when the first Americans were allowed to visit North Korea’s new experimental light water reactor (ELWR), among the signs located at the construction site, one stood out to the group. It was a warning: “Safety first—not one accident can occur!” Early in the construction of the reactor, North Korea appeared to be making a visible effort to ensure safety was a top priority. However, available evidence since then raises doubts as to whether the North’s engineers can achieve the goal proclaimed on that sign.
This concern was echoed as South Korean President Park Geun-hye, in her statement at the 2014 Nuclear Security Summit in The Hague last week, warned, “North Korea’s Yongbyon is home to such a dense concentration of nuclear facilities that a fire in a single building could lead to a disaster potentially worse than Chernobyl.” Yet, this claim, which was first made in a recent study by a South Korean physicist, has not been critically evaluated using available, and known, information about North Korea’s past practices in reactor operation and construction, and the technical capacity of the reactors at the Yongbyon site.
Our analysis differs from these recently prevailing speculations, in terms of the scale and scope of a potential nuclear meltdown at Yongbyon, in part, due to the significantly smaller size of North Korea’s experimental reactor and available radionuclide inventory. However, based on our analysis of the potential safety vulnerabilities of the ELWR’s construction, North Korea’s isolation, and its lack of safety culture, we believe the probability of an accident at the ELWR is high and should be a concern for the international community. In the event of a severe accident, Pyongyang’s ability to respond in a timely fashion will make the difference between a small-scale accident and a catastrophic disaster. And while an accident at Yongbyon would have serious consequences for the local community, the political consequences and public outcry in a region where relationships are already frayed would far exceed the actual radiological exposure and environmental impact.
A New Experimental Light Water Reactor (25-30 MWe) at Yongbyon
Very few details are known about the design of North Korea’s ELWR. What is known was obtained from information gathered by a visiting American delegation in November 2010 and through close examination of satellite imagery of the facility. During the site visit by Dr. Siegfried Hecker and his Stanford University colleagues, the reactor’s chief engineer stated that the ELWR would use a pressurized water reactor (PWR) design—the most common commercial reactor constructed around the globe. The reactor will operate with a full fuel-load of four tons of low-enriched uranium (LEU) at 3.5 percent of the uranium-235 isotope—a level of enrichment consistent with typical PWR fuel—to be produced in the newly constructed uranium enrichment facility at Yongbyon. At the time of the November visit, the concrete foundation of the reactor’s containment vessel was estimated to be 28 meters squared. The chief engineer said the vessel was expected to be 22 meters in diameter, 0.9 meters thick, and 40 meters high. These dimensions have been confirmed by analysis of commercial satellite imagery.
As of January 2014, the external construction of the reactor appeared complete. The exterior of the containment structure has been completed, the turbine generator hall seems finished, a ventilation shaft has been erected, and a transformer park used for electricity generation has been set up on the east side of the reactor. Still, there is probably a significant amount of work to be done inside the reactor before it becomes operational. For example, the control system will need be installed and all the internal components, including the vessels and piping, will need to be designed, installed, and tested. The testing of the reactor’s components may occur in the coming months. However, it is unlikely the ELWR will be fully operational for one to two years at the earliest, assuming the North Koreans do not run into any large-scale problems.
North Korea’s technical specialists have some limited experience with LWR technology. As part of the 1994 Agreed Framework, Pyongyang was promised two 1,000 MWe LWR units in exchange for a commitment to shut down and dismantle its plutonium production facilities. These two units were to be constructed by the Korean Peninsula Energy Development Organization (KEDO) with North Korean involvement. Under the agreement, KEDO’s member states, including South Korea, Japan, the European Union, and the United States, would site and construct the reactors and train the North Koreans to operate them. Construction began in 1997, but was terminated in 2006 amidst political tensions when the project was only about 34.5 percent complete. The reactors under construction were based on the Korean Standard Nuclear Plant (KSNP) model developed by the South Koreans.
While the ELWR appears similar to the LWRs that KEDO was supposed to have built in North Korea, it is unclear how the North’s involvement with that project has influenced the reactor design or how much technology transfer occurred. When the KEDO project was terminated, some reactor components, fabricated abroad, were left onsite. It is unknown any of these components have been used for the new reactor or if the North Koreans—who are very good at reverse engineering—may have used the components instead to learn how to fabricate them for the ELWR. Satellite imagery of the KEDO site from 2009 shows activity around many of the areas where the components were stored indicating the North Koreans may be using them in some way for the new reactor.
Safety Concerns Posed by North Korea’s ELWR
The large amount of unknowns about the ELWR is cause for concern. After closely examining the available data, we have identified four potential safety vulnerabilities.
1. Possible flawed reactor safety design and inadequate quality of construction
From our observations of satellite imagery, we believe the new ELWR has features similar to standard Generation II pressurized water reactors. While the specific safety features of the ELWR are unknown, reactors with a similar design generally rely on “active” safety systems, which require operator actions or electronic feedback to ensure safety of the reactor. The specifications of the safety system can make the difference under a disaster scenario between a reactor core melting down and shutting off just in time. Because these key details remain unknown, it exposes a serious safety vulnerability and raises considerable alarm that a natural disaster onsite could precipitate a core meltdown of the reactor.
For an active safety system to be adequate, onsite power needs to be supplied to the reactor constantly in order to pump cooling water through the core to keep the temperature constant and prevent a meltdown. However, North Korea’s antiquated energy grid is unlikely to be reliable enough to guarantee a constant stream of onsite power. In addition, backup diesel generators are usually used to provide power to the cooling system in the event a station blackout occurs. However, no generators appear present at the site. In short, without a reliable source of onsite power or adequate back-up generators, North Korea’s ELWR runs the risk of losing power and not being able to provide adequate cooling to the core.
The ability of the North Koreans to fabricate specialized safety-related equipment and components is another important concern. Everything in the reactor, from the piping to the pressure and containment vessel to the concrete used for the containment building, requires special materials and quality-assured fabrication to prevent any radiological leaks. Pyongyang does possess a large steel industry, including a 10,000-ton forging press at the Chollima Steel Complex, which could be used to forge reactor components. However, these components require precise and accurate fabrication, a process only a few countries have perfected through many years of trial and error. While not impossible, it would be surprising for North Korea to be able to flawlessly fabricate these components without outside assistance.
An additional concern is that basic details of the ELWR design appear to have been decided on an ad hoc basis even as construction had already begun. As of November 2010, North Korea’s reactor engineers admitted to not yet knowing what type of cladding would be used for the fuel: either zirconium alloy or stainless steel. This decision is usually made early in the reactor design phase, well before construction has begun, because the type of cladding can affect the overall design of the reactor’s core and safety features. Moreover, North Korea has no experience fabricating fuel for an ELWR. That might leave the reactor susceptible to fuel failures. Once the reactor becomes operational, the fuel rod’s protective cladding sleeve could degrade and the hot exposed fuel pellets would leak radiation into the containment vessel, possibly contaminating the reactors components.
2. Inexperienced design and safety engineering
While some North Korean engineers were trained in aspects of LWR construction and operation by KEDO, Pyongyang evidently decided not to capitalize on this experience. Rather, the reactor design team is composed of young men in their 40s without any prior experience designing or building these reactors, making them more susceptible to misjudgment and errors in their calculations. Safe reactor operation is a high-consequence endeavor that requires highly specialized repetitive training.
An additional potential problem is what appeared to be inadequate nuclear standard best practices during the early construction of the containment building. For example, with only a single backhoe visible onsite, Dr. Hecker asked if the North Koreans had conducted the proper seismic analysis to ensure the reactor was not sited in a location susceptible to earthquakes. Although the chief engineer assured him they excavated down to the bedrock, there was little visible evidence that such an analysis had occurred. In addition, the method of laying the foundations for the reactor’s containment structure appeared inadequate. Best practices for reactor design requires special reactor-grade concrete poured in large, unbroken units whose drying must be carefully watched through close temperature control. However, only a small mixer was visible at the site and the concrete containment shell was being poured only one meter at a time.
Even the siting of the ELWR raises serious questions. Located adjacent to the Kuryong River, the reactor uses the river as its ultimate heat sink—the source of water to cool the core of the reactor. A constant, reliable supply is needed to cool the reactor system to prevent a meltdown. However, based on satellite imagery, rather than constant and reliable, the supply seems variable depending on rainfall. The river tends to flood during the late summer monsoon period and dries up and freezes during the winter. This unpredictable water level complicates the reactor design’s need for active cooling.
3. Lack of a strong safety culture and an independent nuclear regulator
The absence of a strong, transparent nuclear regulatory framework in North Korea is of significant concern. One of the key objectives during the KEDO project was to establish such a framework through working with the DPRK’s State Nuclear Safety Regulatory Commission (SNSRC), the authority that is responsible for licensing and overseeing the ELWR’s construction. That was to be accomplished by training operators and teaching its specialists best practices in nuclear safety.
The SNSRC’s ability to effectively implement its mission is in doubt for three reasons. First, the employees of the SNSRC, while university graduates, probably only have practical experience working at Pyongyang’s gas-graphite reactor and have never licensed an LWR. This lack of experience is a significant problem. The preliminary safety analysis report for the ELWR was accepted by the SNSRC and a construction permit issued despite the fact that many details of the reactor were not decided until after construction began. As a result, it is likely this analysis was done in a piecemeal fashion without a broad, comprehensive safety analysis of the plant’s entire system until late in the construction phase. Experienced nuclear engineers would have conducted this analysis before concrete was ever poured. Not doing so early in the design phase could lead to significant gaps in the reactor’s safety system.
Second, the SNSRC is unlikely to qualify as an independent and strong body operating outside the political influence of the regime, an essential requirement for ensuring nuclear safety. The need for such an authority is a lesson many countries unfortunately do not learn until it is too late. For example, Japan established such an authority in the aftermath of Fukushima, which, unlike its predecessor, is solely regulatory with no responsibility for technology promotion. In the case of North Korea, there are no government agencies that operate outside of the control of the regime and the importance of Pyongyang’s nuclear program ensures an even greater degree of oversight.
It should be noted that North Korea does have experience constructing and operating reactors in a safe manner. Since 1986, North Korea has been operating its 5 MWe gas-cooled, graphite-moderated reactor off and on, which was constructed indigenously, without any major accidents or safety issues. However, because of fundamental design differences between the gas-graphite reactors and LWRs, especially related to reactor safety, this experience does not directly translate or inform the ability of North Korea’s engineers to ensure LWR safety. In fact, the gas-graphite design has more inherent safety features than LWRs and is less susceptible to a severe accident.
How Disastrous Would an Accident Be?
With little information emanating from North Korea about its ELWR, tremendous uncertainty exists about the probability of an accident occurring at the site and how bad it would be. This uncertainty has left some experts to speculate that an accident at the 5 MWe reactor at Yongbyon would be worse than Chernobyl. While we believe there is a high probability that some sort of accident will occur at the ELWR, based on the known specifications of the reactor, that accident would probably not reach the scale of some of the worst nuclear disasters. However, a number of devastating localized effects would occur. The nuclear facilities at Yongbyon are located adjacent to large fields used to grow crops and many of the buildings at the nuclear research center, are interspersed with others associated with processing agricultural products. If an accident were to occur, there would be a high risk of contaminating the surrounding agricultural areas and ground soil. In addition, if the accident is not successfully managed, this contamination could be made worse, for instance, it might also spread to the adjacent Kuryong River.
The magnitude of an accident would depend on North Korea’s ability to adequately respond in a timely fashion. The failure of the reactor operators to quickly address the accident at Fukushima led to meltdown of the reactor cores and the release of significant quantities of radiation. It is unclear whether the North Korean reactor operators are sufficiently trained to handle such contingencies, particularly given their lack of experience in operating an LWR. Nor is it clear whether the North has in place the capabilities to respond to such an emergency on a national level. Being able to mitigate the accident in a timely manner could make the difference between a limited disaster and a catastrophe.
Pyongyang’s lack of transparency should an accident occur, is likely to pose even greater risks to its neighbors and the international community. Like the Soviet Union during Chernobyl, North Korea is unlikely to be forthcoming about an accident or its scale, forcing the international community to rely on its own means, such as detecting released radionuclide with the Comprehensive Test Ban Treaty Organization’s International Monitoring System. The lack of information could induce panic, particularly in South Korea and China where the public may be uncertain as to the direct impact of an accident and the possible adverse effects of radiation exposure. This occurred after Fukushima despite the transparency of Japanese government and regulators. Moreover, it may be that an accident could aggravate tensions on the peninsula and in the region if Pyongyang fails to be forthcoming about its details and public concern in these countries puts enormous pressure on their governments to respond.
This issue of ELWR safety should be of concern to the international community. However, little can be done to address these safety vulnerabilities if tensions between North Korea and the United States continue and there are no talks about Pyongyang’s nuclear weapons program. If those talks resume, nuclear safety should be addressed. But in the meantime, national governments, if they have not done so already, should prepare for the day when Pyongyang’s ELWR becomes operational and the possibility that an accident will occur.
 Siegfried S. Hecker, A Return Trip to North Korea’s Yongbyon Nuclear Complex, Center for International Security and Cooperation, Stanford University; November 20, 2010, p. 3.
 Sebastien Falletti, “Expert warn of ‘Chernobyl’ risk at Yonbyon nuclear plant,” CBRN Assessment, IHS Jane’s Defense Weekly, January 26, 2014.
 For comparison, Cherynobl’s Unit 4, which melted down, had a power output of 1,000 Megawatts versus the DPRK’s 5 Megawatt plutonium production reactor and its planned 25 Megawatt light water reactor. The Cherynobl unit has a power output 3000% greater than the DPRK’s units combined.
 Siegfried S. Hecker, A Return Trip to North Korea’s Yongbyon Nuclear Complex, p. 3.
 Jack Mulligan, “Preserving the Donghae Nuclear Station,” Presentation to the LIANS, December 15, 2004, Slide 7.
 KEDO Annual Report 2005, p. 6-8, 14.
 The most recent imagery of the Kumho site is from October 19, 2009 and September 24, 2009.
 Siegfried S. Hecker, Chaim Braun, and Robert L. Carlin, “North Korea’s Light-Water Reactor Ambitions,” Journal of Nuclear Materials Management, Vol. 39, No. 3, Spring 2011, p. 22.
 Siegfried S. Hecker, A Return Trip to North Korea’s Yongbyon Nuclear Complex, p. 3.
 Ibid, p. 3
 Ibid, p. 3
 Siegfried S. Hecker, Chaim Braun, and Robert L. Carlin, “North Korea’s Light-Water Reactor Ambitions,” p. 23.
 This report, like all safety analyses done by the SNSRC have never been made public.
 This gas-graphite reactor is similar to the UK’s Magnox reactor in Calder Hall, UK. This reactor is named for the Magnesium alloy used for fuel cladding. At the time North Korea constructed the reactor, over five years in the 1980s, North Korea used the plethora of information about the reactor design available in the public domain to design and build the reactor. This reactor was ideal for North Korea since it operates using natural uranium fuel, of which, North Korea has a large amount.
 They have also been operating a small research reactor, an IRT-2000, built for them by the Soviet Union, since 1967.
 S.E. Jensen and E. Nonbel, Description of the Magnox Type of Gas Cooled Reactor (MAGNOX) (Roskilde, Denmark: Riso National Laboratory, November 1998): p. 15.