Monthly Archives: April 2018

An ever present Nth Korean nuclear hazard

https://www.38north.org/2017/12/mkorda121417/

US-KOREA INSTITUTE, JOHN HOPKINS UNIVERSITY

North Korean Nuclear Reactor Safety: The Threat No One is Talking About
BY: MATT KORDA
DECEMBER 14, 2017

The ability of North Korea to safely operate its nuclear reactors, according to many experts, is increasingly being called into question given the North’s isolation and lack of safety culture. Pyongyang’s ability to respond to a nuclear accident in a timely fashion will make the difference between a small-scale event and a catastrophic disaster. And while the actual contamination would be localized, the lack of transparency from North Korea in dealing with the situation is likely to cause political panic in the region in excess of the actual radiological exposure and environmental impact. The opening of nuclear safety talks with the North to help prevent such an accident from occurring would provide a rare opportunity for regional dialogue and could pry open the door for realistic and productive discussions of North Korea’s nuclear program.

A Disaster Waiting to Happen?

A video of Kim Jong Un smoking next to an untested liquid-fueled missile tells you everything you need to know about North Korea’s nuclear safety culture. The remarkable 14-second clip shows the Supreme Leader taking a puff while a Hwasong-14 intercontinental ballistic missile is erected on the launch pad mere feet away—prompting a torrent of snarky Twitter commentary expressing regret that Kim’s lit cigarette had not “solved the problem for us.” Kim’s recklessness is certainly notable, and it hints at an underemphasized and potentially devastating possibility: the threat of a nuclear accident in North Korea.

At the March 2014 Nuclear Security Summit in The Hague, then-South Korean President Park Geun-hye claimed that Yongbyon, North Korea’s primary nuclear research center, “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.” While her damage assessment is likely an exaggeration—researchers from 38 North assess Chernobyl’s power output to have been 3,000 percent greater than Yongbyon—the potential for a nuclear accident is not.

Niko Milonopoulos and Edward D. Blandford noted previously that a sudden fault in North Korea’s outdated power grid could prevent the Yongbyon reactors from being adequately cooled and could potentially trigger a meltdown. Such an event could also be prompted by a natural disaster or abnormal weather patterns. Complementary analysis by Nick Hansen indicates that North Korea’s 5 MWe plutonium production reactor had to be briefly shut down following a flood in July 2013 which destroyed parts of the cooling systems. He noted with concern that “if a major flood cuts off the cooling water supply to the reactors before they can be shut down, a major safety problem could occur.” This is exactly what prompted the series of nuclear meltdowns at Fukushima.

In 2010, a team of Stanford scientists led by Dr. Siegfried Hecker visited North Korea’s 25-30 MWe Experimental Light Water Reactor, which was still under construction at the time and will likely be operational soon. Their subsequent analysis expressed a lack of confidence in North Korea’s ability to operate the site safely upon completion, citing insufficient concrete quality, the lack of an independent nuclear regulator, and the inexperience and isolation of the design team as particular concerns….” end quote. read the rest at the link above.

There is no reason to be optimistic about the radiological state of North Korea. It is likely to be a dangerous mess around and in the nuclear test sites, and other nuclear sites, military or civilian.

The population of the country are slaves at all levels. Over the decades millions must have suffered and died prematurely from all manner of things, including nuclear hazards by now, I think, well embedded into the biosphere of the place. The ones worst off are those closest in and that has always been the case.

There is no data in the public domain which pertains to the radiological state of North Korea, and correcting that global ignorance must surely be a high priority. Given that the man with worst barber in the world says he wants peace. Open disclosure by the North Koreans to the rest of the world is the minimum we should accept.

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Newsweek: NORTH KOREAN SOLDIERS ARE BEING TREATED FOR RADIATION EXPOSURE

http://www.newsweek.com/north-korean-soldiers-being-treated-radiation-exposure-after-nuclear-test-698246

Which would be worse? The US nuclear test sites, or the North Korean ones? How much has the USA spent trying to cleanup its underground tests sites? How much does the routine hydrological monitoring cost the USA ? Who is going to clean up North Korea’s sites? Who is going to pay? Who compensates their nuclear veterans? No one. What are the chances of justice for North Korean nuclear vets? Either none or Buckley’s. This isn’t a furphy Mr Brownowski.

BY SOFIA LOTTO PERSIO ON 11/1/17 AT 8:20 AM

North Korean soldiers and their families are being treated in a military hospital for radiation exposure after the September hydrogen bomb test at the Punggye-ri nuclear facility.

More than a thousand troops of the North Korean army were deployed to the site to dig tunnels and patrol the surrounding area, Japanese newspaper The Asahi Shimbun reported Wednesday, citing anonymous sources with knowledge of North Korean affairs.

The news comes after reports in the Japanese press indicated that around 200 people died in an accident at the facility due to a tunnel collapse in October.

After reporting a series of small earthquakes and a landslide in the area near where the facility is located, south of the Mantapsan mountain, several experts have warned that the site has become too unstable to host further nuclear experiments. Another bomb test would risk a massive collapse and radioactive leaks, Chinese geologists warned.

end quote.

Pity the poor civilian North Koreans. So malnourished their bones must be some percent Sr89 and Sr90 by now.

No hydrological charts for the site as far as I can see. And ground and surface flows through the test sites are the most important long term vectors for radiologic risk.

North Korea’s 2017 Test and its Nontectonic Aftershock J. Liu et. al March 2018

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2018GL077095

North Korea’s 2017 Test and its Nontectonic Aftershock
J. Liu L. Li J. Zahradník E. Sokos C. Liu X. Tian
First published: 14 March 2018 https://doi.org/10.1002/2018GL077095

Geophysical Research Letters.

Abstract
Seismology illuminates physical processes occurring during underground explosions, not all yet fully understood. The thus‐far strongest North Korean test of 3 September 2017 was followed by a moderate seismic event (mL 4.1) after 8.5 min. Here we provide evidence that this aftershock was a nontectonic event which radiated seismic waves as a buried horizontal closing crack. This vigorous crack closure, occurring shortly after the blast, is studied in the North Korea test site for the first time. The event can be qualitatively explained as rapid destruction of an explosion‐generated cracked rock chimney due to cavity collapse, although other compaction processes cannot be ruled out.

Plain Language Summary
North Korea detonated its strongest underground nuclear test in September 2017. It attracted the public interest worldwide not only due to its significant magnitude (6.3 mb) but also because it was followed 8.5 min later by a weaker event. Was the delayed shock a secondary explosion, an earthquake provoked by the shot, or something else? We answer these questions, thanks to unique data from near‐regional broadband stations. We basically solve a simple problem—fitting observed seismograms by synthetics. The good fit means that we understand why and how the seismic waves are radiated. According to our model, the explosion created a cavity and a damaged “chimney” of rocks above it. The aftershock was neither a secondary explosion nor a triggered tectonic earthquake. It occurred due to a process comparable to a “mirror image” of the explosion, that is, a rock collapse, or compaction, for the first time documented in North Korea’s test site. Interestingly, shear fault motions, typical for natural earthquakes, were extremely small both in the explosion and in the aftershock. Small natural earthquakes also occur at the test site, and geotechnical works might trigger them. Thus, all studies related to rock stability of the site, and prevention of radioactive leakage, are important.

end quote

full text available at link above.

DTIC – “Geologic Site Characterization of the North Korean Nuclear Test Site at Punggye-Ri: A Reconnaissance Mapping Redux”

United States Defence Technical Information Centre.

http://www.dtic.mil/docs/citations/ADA625872

Accession Number : ADA625872

Title : Geologic Site Characterization of the North Korean Nuclear Test Site at Punggye-Ri: A Reconnaissance Mapping Redux

Descriptive Note : Final rept.

Corporate Author : LOS ALAMOS NATIONAL LAB NM

Personal Author(s) : Coblentz, David ; Pabian, Frank

Full Text : http://www.dtic.mil/dtic/tr/fulltext/u2/a625872.pdf

Report Date : 30 Nov 2013

Pagination or Media Count : 68

Abstract : High-resolution information about the geologic setting for denied-access sites is critical for the monitoring and detection of clandestine nuclear test (e.g., the evaluation of seismic wave propagation, the prediction of gas releases, and evaluation of tunnel layouts). An important case-in-point is the lack of precise, large-scale geologic maps for North Korea’s underground nuclear test site near Punggye-ri. As a proof-of-principle, we have developed and applied a geologic assessment methodology at the NKTS which employed a novel geomorphometric analysis technique to produce a high-resolution (5-meter) geologic map of the site. This map helps refine the USGS reconnaissance geology map (which was based on the analysis of ASTER spectral imagery data and extrapolations from nearby 1930’s Japanese ground survey reporting) at the Punggye-ri site. Our assessment provided the means to evaluate a number of geologic factors related to the testing at the Punggye-ri site, including the proximity of carbonate rocks to the test locations, the relationship between fracture rock and containment, and possible motivation for continued tunneling at the South Portal location.

Descriptors : *GEOLOGY , *MAPPING , *NORTH KOREA , *NUCLEAR EXPLOSION TESTING , *RECONNAISSANCE , DETECTION , GEOMORPHOLOGY , HIGH RESOLUTION , IMAGES , MAPS , METHODOLOGY , MONITORING , POSITION(LOCATION) , PREDICTIONS , SEISMIC WAVES , SITES , SPECTRA , TEST FACILITIES , TUNNELING , TUNNELS , WAVE PROPAGATION

HAVE YOU GOT THIS RICHARD BROINOWSKI: ITS NOT A FURPHY DING BAT

Nuclear Leaks At Nth Korean Test Site – real or “a Furphy” according to Broinowski ? Real, says Langley

I have been following the story of radiological dangers posed by the increasing stressed geology of the North Korean nuclear test site for some months. Over the last week the story was again raised by the Australian newspaper. This motivated me to find the closest Chinese authority. The story was, as far as I can gather, published this week in The South China Morning Post on Wednesday 25 April 2018. The article, written by Stephen Chen, is entitled “North Korea’s nuclear test site has collapsed … and that may be why Kim Jong-un suspended tests“. The first paragraph explains further: “The mountain’s collapse after a fifth blast last fall has led to the creation of a massive ‘chimney’ that could leak radioactive fallout into the air, researchers have found….” before I go any further, there are two questions to ask: 1. How credible is Stephen Chen’s reporting and 2. Who are the researchers involved? eg are they retired diplomats only or are they qualified to comment in a scientific manner? If so have their findings been peered reviewed?
(what journalists say is irrelevant to me except when the articles lead me to find the peer reviewed papers published by scientists. Newspapers seem not to put relevant links to source documents up which is a crying shame.)

1. Stephen Chen: his bio on the SCMP site states: “Stephen covers breakthoughs in science and their impact on society, environment, military, geopolitics, business – pretty much all aspects of life. His stories often travel across the globe. Stephen is an alumnus of Shantou University, the Hong Kong University of Science and Technology, and the Semester at Sea programme which he attended with a full scholarship from the Seawise Foundation. In his spare time, Stephen reads and writes novels. He lives in Beijing with a beautiful wife and two lovely kids.”

fair enough, signs look hopeful that the article is not reporting a scientific “furphy”, as Broinowski described the generic story on the Australian ABC TV this morning. But let’s dig a nanometer deeper. Who are the researchers Chen is referencing? Does he name them and are they famous? (fame = mass readership and lots of grant money):

“A research team led by Wen Lianxing, a geologist with the University of Science and Technology of China in Hefei, concluded that the collapse occurred following the detonation last autumn of North Korea’s most powerful thermal nuclear warhead in a tunnel about 700 metres (2,296 feet) below the mountain’s peak.

The test turned the mountain into fragile fragments, the researchers found….” end quote from the SCMP/Chen.

Further, Chen reports: “A research team led by Wen Lianxing, a geologist with the University of Science and Technology of China in Hefei, concluded that the collapse occurred following the detonation last autumn of North Korea’s most powerful thermal nuclear warhead in a tunnel about 700 metres (2,296 feet) below the mountain’s peak.

The test turned the mountain into fragile fragments, the researchers found. ” source: ibid.

Describing these findings and the dangers posed by the scientific observations as reported by Chen does not smack of “Furphy” or fantasy Richard B. (No I don’t mind who your sister is, you should know better).

Wen Lianxing et al have been tracking North Korean nuclear tests, as far as I can find (hamstrung as I am, because I cannot speak or read Chinese), from at least 2006, and certainly since 2009, when the team became the first in the world to precisely locate the location of a North Korean nuclear test. : “High-precision Location of North Korea’s 2009 Nuclear Test”
Article in Seismological Research Letters 81(1):26-29 · January 2010 authors: Lianxing Wen, University of Science and Technology of China (Hefei, China); Hui Long, Stony Brook University (Stony Brook, United States).

Have a read of it Mr Broinowski. You might find it sensible and not a furphy.

Ok, on with the real matters at hand. Underground nuke tests invariably leak radionuclides into the biosphere. The US underground nuclear test regime has created a legacy of cost and risk, to put it mildly, which continues to this day. Name a US underground shot, and go to DOE Opennet and enter the shot’s code name. Up pops reams of documents detailing the test, the immediate result, and the long term consequences in terms of risk and costs.

There is no reason to suspect that the risks and costs of North Korea’s underground will be any more “furphy” ridden that the US underground tests were. And continue to be.

So without any further ado, even if I have to drag Richard B kicking and screaming into 1954, is some more non furphy from Chen and the SCMP:

“It is necessary to continue monitoring possible leaks of radioactive materials caused by the collapse incident,” Wen’s team said in the statement.

The findings will be published on the website of the peer-reviewed journal, Geophysical Research Letters, likely next month.

North Korea saw the mountain as an ideal location for underground nuclear experiments because of its elevation – it stood more than 2,100 metres (6,888 feet) above sea level – and its terrain of thick, gentle slopes that seemed capable of resisting structural damage…..

“The mountain’s surface had shown no visible damage after four underground nuclear tests before 2017.

But the 100-kilotonne bomb that went off on September 3 vaporised surrounding rocks with unprecedented heat and opened a space that was up to 200 metres (656 feet) in diameter, according to a statement posted on the Wen team’s website on Monday. ….

“As shock waves tore through and loosened more rocks, a large section of the mountain’s ridge, less than half a kilometre (0.3 mile) from the peak, slipped down into the empty pocket created by the blast, leaving a scar visible in satellite images.

Wen concluded that the mountain had collapsed after analysing data collected from nearly 2,000 seismic stations. ….

“Three small earthquakes that hit nearby regions in the wake of the collapse added credence to his conclusion, suggesting the test site had lost its geological stability.

Another research team led by Liu Junqing at the Jilin Earthquake Agency with the China Earthquake Administration in Changchun reached similar conclusions to the Wen team. ….

“The “rock collapse … was for the first time documented in North Korea’s test site,” Liu’s team wrote in a paper published last month in Geophysical Research Letters.

The breakdown not only took off part of the mountain’s summit but also created a “chimney” that could allow fallout to rise from the blast centre into the air, they said. …

“Zhao Lianfeng, a researcher with the Institute of Earth Science at the Chinese Academy of Sciences in Beijing, said the two studies supported a consensus among scientists that “the site was wrecked” beyond repair.

“Their findings are in agreement to our observations,” he said.

“Different teams using different data have come up with similar conclusions,” Zhao said. “The only difference was in some technical details. This is the best guess that can be made by the world outside.” ….

“Speculation grew that North Korea’s site was in trouble when Lee Doh-sik, the top North Korean geologist, visited Zhao’s institute about two weeks after the test and met privately with senior Chinese government geologists.

“Although the purpose of Lee’s visit was not disclosed, two days later Pyongyang announced it would no longer conduct land-based nuclear tests.”

” Hu Xingdou, a Beijing-based scholar who follows North Korea’s nuclear programme, said it was highly likely that Pyongyang had received a stark warning from Beijing.

““The test was not only destabilising the site but increasing the risk of eruption of the Changbai Mountain,” a large, active volcano at China-Korean border, said Hu, who asked that his university affiliation not be disclosed for this article because of the topic’s sensitivity.

“The mountain’s collapse has likely dealt a huge blow to North Korea’s nuclear programme, Hu said.

Hit by crippling international economic sanctions over its nuclear ambitions, the country might lack sufficient resources to soon resume testing at a new site, he said.

“But there are other sites suitable for testing,” Hu said. “They must be closely monitored.”

Guo Qiuju, a Peking University professor who has belonged to a panel that has advised the Chinese government on emergency responses to radioactive hazards, said that if fallout escaped through cracks, it could be carried by wind over the Chinese border.

“So far we have not detected an abnormal increase of radioactivity levels,” Guo said. “But we will continue to monitor the surrounding region with a large [amount] of highly sensitive equipment and analyse the data in state-of-the-art laboratories.”

“Zhao Guodong, a government nuclear waste confinement specialist at the University of South China, said that the North Korean government should allow scientists from China and other countries to enter the test site and evaluate the damage.

“We can put a thick layer of soil on top of the collapsed site, fill the cracks with special cement, or remove the pollutants with chemical solution,” he said.

“There are many methods to deal with the problem. All they need [to do] is ask.” end quote . source: ibid.

For the sake of ignorant ex diplomats everywhere, let me list all the qualified scientists Chen gives as sources for his article:

1. Wen Lianxing
2. Liu Junqing
3. Zhao Lianfeng
4. Hu Xingdou
5. Guo Qiuju
6. Zhao Guodong

The above qualified people consider that North Korean nuclear tests have, and do, pose a continuing radiological risk to North Korea and to China. This is due to the geologic damage the test series have caused. As any rational person with knowledge of the US underground test era knows, such risks are extremely well documented in the case of the US tests and appalling documented in the case of North Korea.

Dissenters from my point view and the content of Chen’s reported based upon his 6 expert sources are: 1. Richard Broinowski, retired diplomat. Not a scientist.

blows rasberry at RD. so sue me.

P.S. and another thing Richard B. You won’t close the South Korean nuclear plants down by going on TV and denying the North Korean radiological mess, which is probably an undisclosed actual disaster for the people there. Underground nuke test sites have many ways of leaking radionuclides. Over the years a test site’s hydrology is main vector, but anything can happen at the time, and, in the US experience has happened. The chances of uncontained radionuclides let loose into the biosphere is very high in North Korea and no ideology can successfully hide that fact. Your comments on the ABC TV this morning were damaging to the movement and frankly, in my opinion, bloody ignorant. Would you accept the facts of the matter if the scientists Chen cites were all born in London and were named “Watt”?

This post has been posted on Mr Stephen Chen’s facebook page. with thanks to him and his sources.

Vulnerability of the Nuclear Power Plant in War Conditions

“The Krško Nuclear Power Plant (Slovene: Jedrska elektrarna Krško, JEK, or Nuklearna elektrarna Krško, NEK, Croatian: Nuklearna elektrana Krško) is located in Vrbina in the Municipality of Krško, Slovenia. The plant was connected to the power grid on October 2, 1981 and went into commercial operation on January 15, 1983. It was built as a joint venture by Slovenia and Croatia which were at the time both part of Yugoslavia.

The plant is a 2-loop Westinghouse pressurized water reactor, with a rated thermal capacity of 1,994 thermal megawatts (MWt) and 696 megawatts-electric (MWe). It runs on enriched uranium (up to 5 weight-percent 235U), fuel mass 48.7 t, with 121 fuel elements, demineralized water as the moderator, and 36 bundles of 20 control rods each made of silver, indium and cadmium alloys to regulate power. Its sister power plant is Angra I in Brazil.[1] ” Wikipedia.

Source Link :http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/38/056/38056717.pdf

Vulnerability of the Nuclear Power Plant in War
Conditions

A. Stritar, B. Mavko
‘Jožef Stefan” Institute
Ljubljana, Jamova 39
Slovenia

Presented at First Meeting of the Nuclear Society of Slovenia, Bovec 11.-12.6.1992

ABSTRACT – In the summer 1991 the Nuclear Power Plant Krško in
Slovenia found itself in the area of military operations. This was probably
the first commercial nuclear power plant, to which it was threatened with
the air jet attack. A number of never before asked questions had to be
answered by the operating staff and supporting organizations. In this paper
some assets of the nuclear power plant safety in war condition are
described: the selection of the best plant operating state before the attack
and the determination of plant system vulnerability. It was concluded, thdt
the best operating mode, into which the plant should he brought before the
attack, is the cold shutdown mode. The problem of Nuclear Power Plant
safety in war conditions should be addressed in more detail in the future.

INTRODUCTION
The Nuclear Power Plant Safety in war conditions is practically a completely new
area, which is seldom addressed in the literature. Although the physical protection of the plant in such a condition is the task of military, police and civil defense forces, it is the responsibility of nuclear professionals to investigate possible scenarios and their consequences from the nuclear safety point of view. Everybody involved with the Nuclear Power Plant operation in Slovenia, found himself in a situation, that was hardly encountered before. Our Nuclear Power Plant was probably the first commercial PWR in the world, that found itself in the middle of the war area. The Iraqi research reactors in a Gulf war 1991 were maybe in the similar situation. Very little guidance about the appropriate preventive measures for such a situation could be found in the literature. Number of preventive steps were performed on the site by the plant personnel, while this paper describes some of related activities of the Reactor Engineering Division of “Jožef Stefan” Institute at that time.

ANALYSIS OF THE EXPOSURE OF PLANT VITAL SYSTEMS
No Nuclear Power Plant anywhere in the world is designed against an air attack by military bombs or missiles! Provisions are only taken against the terrorist attack from the ground by the light weapons. Some nuclear power plants are designed to withstand the crash of an aircraft, but without any explosive materials. It was clear at the beginning of the crisis in Slovenia, that there is no way to completely prevent damage to the equipment in the case of an attack. All our efforts were therefore directed into the minimization of the possible public radiological consequences during such an event.
The critical safety functions in such conditions were checked:
a) Subcriticality,
b) Core cooling,
c) Heat sink,
d) Integrity,
e) Containment,
0 Inventory.
Subcriticality is not a problem, if the plant is shut down and the primary water
properly borated. All other functions are endangered: Containment is directly
exposed to missile attacks and may be considered as the most endangered one. Next is heat sink, since the water intake structures and pump stations at the river are also completely exposed. Integrity of the primary system is endangered in the case of heavier bombing. Inventory may be endangered as a consequence of the loss of integrity, while the core cooling is primarily vulnerable through the loss of off-site power. Fortunately, if the plant is shut down, it is much easier to maintain or replace ail degraded critical safety functions.
In order to get an estimate about the vulnerability of different plant system to the missile attack, die exposure of each system was evaluated and the “rose of
vulnerability” similar to the example in Figure 1 was constructed. The vulnerability of each system after the attack from different sides may be seen from it. This kind of diagram is helpful in preparation of preventive measures and determination of most vulnerable systems.

The rose was constructed with the help of 3D computer plant layout model. The
assumption was, that the system is completely destroyed, if it is located on the side, from which the attack is coming from, that it is damaged, if it is in
the shade of one building and that it is not damaged, if it is located on the opposite side of die site.

Our main concern was further devoted to maintenance of the core cooling in the core and to integrity of the spent fuel pit.

CORE COOLING
Immediately after the shutdown of the reactor, it had to be decided, in what
operational state the plant should be brought to. The following
proposals were considered:

1. Removal of the fuel from the core,
2. Refuelling mode,
3. Normal cold shutdown.
The analysis had to be performed fast, therefore there was no time for any extensive
analytical work using sophisticated computer codes. It was mainly based on
engineering judgment and expert opinion. Following are main results of the analysis:
1. Removal of the fuel from the core would certainly assure, that there would
be no big danger for the exposure of the population in the case of the major
damage of the containment building. But the only possibility for the
immediate removal is into the spent fuel pit on site. Spent fuel pit is, like in
most similar NPPs, located outside the containment building and is even
more exposed, than the structures inside it. In addition, the sole process of
fuel removal is increasing risk and is also requiring considerable time. The
reactor vessel and the core are located so deep in the containment and
shielded by various structures (containment building, steel containment,
concrete structures, steam generators, pressurizer etc.), that very strong or
concentrated repetitive missile deployment would be needed to achieve it’s
destruction. The idea about fuel removal was therefore rejected.

2. Refuelling mode was considered as the only normal operational mode, in
which the primary system may be opened. The reactor vessel is covered with
the few meters high layer of water, which would certainly represent some
protection against falling structures or missiles. On the other hand there is a
high probability, that missile impacts would cause displacements of some
structures in the containment and deformation of the seal ring around the
reactor vessel. The refuelling water would leak into the reactor cavity and be
lost as a protection of the core. If the reactor vessel head happens to be
removed from the top of the core in such a case, there would be an
immediate release of the radioactivity into the containment. This option was
also rejected.
3. Cold shutdown mode was found to be the most appropriate option. The
primary system is at the atmospheric pressure, therefore there is no problem,
even if part of the system gets damaged. The core cooling is provided by the
residual heat removal system, the heat is transported via the essential service
water system into the river. The core is protected by all the structures above
the reactor vessel and by the reactor vessel itself. There are also no
complicated manoeuvres necessary to bring the plant into that state.
After selecting the cold shutdown mode as a most suitable operational state of the plant, further studies of the vulnerability were done. Besides the possibility of the direct damage to structures, that would cause release of the radioactivity, were are two important subjects, that have to be considered:
a. AC and DC power supply,
b. ultimate heat sink.
In the coW shutdown mode the residual heat from the core is removed by the
Residual Heat Removal system via Essential Service Watei> system to the river. RHR pumps are electrically driven. To prevent heat-up of the core the long term power supply and ultimate heat sink have to be assured. But, since the residual power of the core is relatively low, it is possible for the plant to withstand even a certain period without it. In order to get an approximate answer about the time, during which the plant may loose the power and heat sink without a damage, the following scenario was analyzed by a simple hand calculation.

LOSS OF AC POWER AND HEAT SINK IN COLD SHUTDOWN
It was assumed, that prior to the loss of heat removal capabilities (AC power and/or beat sink), the power plant was in cold shutdown for several days, producing about 5 MW of constant heat, which is about 2.6% of full power (in reality the power level is decreasing with time). The heat had been removed via RHR system. There was only about 25% of water in the pressurizer, the rest was filled with the Nitrogen. Both Steam Generators were filled with water. At the beginning of the transient the heat removal from the primary system via RHR is completely lost.

The heat, generated in the core, would cause the primary system to heat up. Since we have assumed, that the Power Operated Relief Valve at the top of the Pressurizer is closed, the primary pressure would rise. The natural circulation would establish in both primary loops, transferring heat from the core to the secondary side of Steam Generators. Secondary side would also heat up, but with some delay. PORVs at the secondary side were assumed to be opened, therefore the highest temperature, that the secondary water could reach, is the boiling temperature at atmospheric pressure, i.e. around 373 K. It may be expected, that because of the increased pressure the subcooling in the primary system would not be lost. Therefore the natural circulation would be sustained and the heat from the core could be transferred to the secondary side.
The equation (1) has been used for calculation of the time needed for the whole
primary and secondary system to reach boiling temperature at atmospheric pressure,
373 K (100 °C). see original text for calculaiton
t, Time to heat up primary and secondary water to 373 K,
mpr Mass of liquid in primary system,
mSG Mass of liquid in secondary side of the Steam Generator,
hs
Liquid saturation specific enthalpy at atmospheric pressure,
hpr Initial specific enthalpy of primary liquid,
Qt
Core power.

By inserting data for the NPP Krško into the equation (1) gives approximately 4
hours before the entire water in the primary and the secondary side reaches boiling temperature.
The equation (2) could be used to estimate the time rlb before the whole water on the secondary side of both steam generators is boiled dry.

see original text for calculation

where h, is saturated steam specific enthalpy. Specific plant data gave about 27
hours.
If we also consider boiling water in the primary system (in which case the PORV on the pressurizer should be opened), we get some 47 hours available, before the whole primary system completely dries out.
In order to remove the residual heat by steaming secondary water while maintaining the water level in Steam Generators, the following amount of water has to be added
from the river:
4b K-K
(3)
where hr is the river water specific enthalpy. For the conservatively assumed core residual power of 5 MW the water flow rate q amounts to less than 2 kg/s.

There is one further concern connected with the heat removal by natural circulation
from the core to the secondary side of the steam generator. If the primary water
starts to boil, the natural circulation flow in primary loops may be interrupted
because of the formation of steam in upper parts of steam generator U tubes. If the
PORV on the pressurizer is left closed, the pressurization of the primary system may
be expected. That would prevent primary coolant from boiling. The following
calculation checks the amount of the primary pressure increase.
With the next equation we can calculate the level change in the pressurizer after the
heat-up of the whole primary system from 323 to 373 K (50 to 100 °C).
see original text for calculation
where
Apr pressurizer cross section,
p50 primary coolant specific density at 323 K,
pm primary coolant specific density at 373 K.

For the amount of primary coolant in our NPP and the cross section of it’s
pressurizer, we got an estimate of the level increase to be 1.3 m.
In the cold shutdown there is a nitrogen in the upper part of the pressurizer. If weassume it to be an ideal gas, we can get an estimate of the pressure increase after above calculated level change in the pressurizer. The compressibility of the primary coolant is of course neglected.

see original text for calculation

wherePso> T$o» V50 pressure, temperature and nitrogen volume at 323 K (50 °C),
Pia» T](X» VJOO pressure, temperature and nitrogen volume at 373 K.
The piessure in the plant would rise from atmospheric pressure to 1.57 bar. That
would prevent primary coolant from boiling, provided the heat is transferred to thesecondary water at 373 K.
Results are summarized in the table 1. These results give the operator ample time for mitigating actions. More then a whole day is available to provide required amount of water for heat removal. This amount is raljier small (less than 2 kg/s) and can be easily supplied even by a mobile pump ie. fire engine. This supply of the water even doesn’t have to be constant and continuous. The level in the secondary side of steam generators may be recovered in batches every several hours if necessary.
The described scenario has assumed a closed primary system. But even if it is
opened or damaged, one can expect, that reflux boiling in steam generator U tubes would provide enough heat transfer for successful cooling of the core. This phenomenon, however, couldn’t be easily calculated by hand. More thorough system analysis code calculations would be
needed.

All the calculations in equations (1) to (4) have been done under the assumption, that there is no heat loss from the primary and secondary system into the containment and environment. But since this is not true and heat
losses at that power level are expected to be relatively high, even
longer time before the complete dry-out and even less water for the successful heat removal would be needed. In that respect our assumption is conservative.

CONCLUSIONS
Quick analysis during the crisis has shown, that consequences of a military attack to the plant by jet fighters could be serious, but with the proper preventive measures and preparedness the environmental consequences could be minimized. The plant in the cold shutdown condition can sustain loss of all off-site power and cooling for a period long enough for establishing a variety of possible emergency solutions.
There should be more attention addressed to this subject by national and international bodies responsible for nuclear safety all over the world. In addition to purely political measures by international community, that would minimize the probability of an military attack to the nuclear power plants, detailed studies covering all different aspects of nuclear safety in war conditions should be initiated and included in emergency plans for each nuclear power plant. The help in a form of expert advice should be available and provided to any reactor operator, whose power plant would be found in an area of military activity.
It appears, that in the future the efforts of the international research community should nevertheless be devoted to gaining better understanding of true vulnerabilities and to enhancing measures for protecting nuclear power plants in the event of military attack.