North Korea may have more bomb fuel than thought, says US think tank

WASHINGTON, July 14 (Reuters) - Thermal images of North Korea's main nuclear site show Pyongyang may have reprocessed more plutonium than previously thought that can be used to enlarge its nuclear weapons stockpile, a U.S. think tank said on Friday.

The analysis by 38 North, a Washington-based North Korean monitoring project, was based on satellite images of the radiochemical laboratory at the Yongbyon nuclear plant from September until the end of June, amid rising international concerns over North Korea's nuclear and missile programs.

The think tank said images of the uranium enrichment facility at Yongbyon could also indicate operation of centrifuges that could be used to increase North Korea's stock of enriched uranium, its other source of bomb fuel.

RELATED: Satellite images of the Punggye-ri nuclear test site in North Korea

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Satellite images of the Punggye-ri nuclear test site in North Korea
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Satellite images of the Punggye-ri nuclear test site in North Korea
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 1. Activity continues at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 2. Possible new dumping observed at the North Portal spoil pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 3. Probable personnel in formation or equipment in rows at the Main Administrative Area. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 1. No vehicles or trailers remain around the North Portal but well-worn paths are observed. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 2. No new dumping of material on the North Portal spoil pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 3. Small collection of crates or trailers seen in previous imagery has been removed. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 3B. Formations seen in the Main Administrative Area, similar to what was seen in lead up to 2013 nuclear test. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 2. Material dumped at the North Portal tailings pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 4, 2013. Figure 3A. Formations seen in the Main Administrative Area in lead up to 2013 nuclear test. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 1. Continued activity at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 25, 2017. Figure 1. Probable cabling and water drainage seen at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 19th, 2016: Figure 6: Excavation continued underground in the North Portal area suggesting more tests to come in the same tunnel complex directly under Mt. Mantap. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 5th, 2017: Figure 7: The North Portal spoil pile continued to expand into 2017, becoming increasingly broader. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 22nd, 2017: Figure 8: Late January 2017 imagery showing new spoil on top of recent snow. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 19th, 2016: Figure 9. A close-up of the North Portal spoil pile as it appeared in late October 2016. The unstable spoil can sometimes lead to accidents, as in this case of toppled rail cars downslope. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - FEBRUARY 12th, 2017: Figure 10. A close-up of the North Portal spoil pile from February 2017 shows that accumulations had begun move westward with a broadening of the top and bottom west side of the pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 1. DigitalGlobe imagery showing large shipping container or crate seen at the North Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 2. DigitalGlobe imagery showing no changes to pattern and texture of tailings (spoil) pile at the North Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 3. DigitalGlobe imagery showing a small vehicle present at the West Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 4. DigitalGlobe imagery showing a truck present in the southern courtyard of the Main Administrative Area. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 5. DigitalGlobe imagery showing a truck present at the sites Command Center. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 6. DigitalGlobe imagery showing snow cleared at guard barrack and security checkpoint. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 24, 2016: Figure 2. No activity seen at the Sohae launch pad. Date: October 24, 2016. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 24, 2016: Figure 3. Environmental shed remains adjacent to the vertical engine test stand. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 29, 2016: Figure 1C. Increased activity around the North Portal throughout October. Date: October 29, 2016. (Photo DigitalGlobe/38 North via Getty Images)
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There were signs too of at least short-term activity at North Korea's Experimental Light Water Reactor that could be cause for concern, 38 North said.

Reuters received a copy of the report before its scheduled release later on Friday.

The images of the radiochemical laboratory showed there had been at least two reprocessing cycles not previously known aimed at producing "an undetermined amount of plutonium that can further increase North Korea's nuclear weapons stockpile," something that would worry U.S. officials who see Pyongyang as one of the world's top security threats.

It was unclear if the thermal activity detected at the uranium plant was the result of centrifuge operations or maintenance.

It said the thermal patterns at the plant's isotope/tritium production facility suggested it was not operational and was therefore not producing tritium, an essential isotope used in boosted yield and hydrogen weapons.

North Korea manufactures atomic bombs using uranium and plutonium and has tested five nuclear bombs. Officials and experts say it could test a sixth at any time, despite U.S.-led international efforts to curb its program.

Pyongyang said its penultimate test in January 2016 was of a hydrogen bomb, something experts have treated with skepticism.

RELATED: North Korea's missiles

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North Korea's Missiles
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North Korea's Missiles
The scene of the intermediate-range ballistic missile Pukguksong-2's launch test in this undated photo released by North Korea's Korean Central News Agency (KCNA) May 22, 2017. KCNA/via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
The long-range strategic ballistic rocket Hwasong-12 (Mars-12) is launched during a test in this undated photo released by North Korea's Korean Central News Agency (KCNA) on May 15, 2017. KCNA via REUTERS REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
Missiles are driven past the stand with North Korean leader Kim Jong Un and other high ranking officials during a military parade marking the 105th birth anniversary of North Korea's founding father, Kim Il Sung, in Pyongyang, April 15, 2017. REUTERS/Sue-Lin Wong
North Korean leader Kim Jong Un inspects the long-range strategic ballistic rocket Hwasong-12 (Mars-12) in this undated photo released by North Korea's Korean Central News Agency (KCNA) on May 15, 2017. KCNA via REUTERS REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT. TPX IMAGES OF THE DAY
People cheer as a missile is driven past the stand with North Korean leader Kim Jong Un and other high ranking officials during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
Military vehicles carry missiles with characters reading 'Pukkuksong' during a military parade marking the 105th birth anniversary of country's founding father, Kim Il Sung in Pyongyang, April 15, 2017. REUTERS/Damir Sagolj TPX IMAGES OF THE DAY
North Korean leader Kim Jong Un supervised a ballistic rocket launching drill of Hwasong artillery units of the Strategic Force of the KPA on the spot in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang March 7, 2017. KCNA/via REUTERSATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. NO THIRD PARTY SALES. NOT FOR USE BY REUTERS THIRD PARTY DISTRIBUTORS. SOUTH KOREA OUT. NO COMMERCIAL OR EDITORIAL SALES IN SOUTH KOREA. TPX IMAGES OF THE DAY
A fire drill of ballistic rockets by Hwasong artillery units of the KPA Strategic Force is pictured in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang September 6, 2016. KCNA/via Reuters ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NO THIRD PARTY SALES. SOUTH KOREA OUT. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS.
FILE PHOTO - An underwater test-firing of a strategic submarine ballistic missile is seen in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on April 24, 2016. KCNA/File Photo via REUTERS. ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. SOUTH KOREA OUT.
A view of a firing contest among multiple launch rocket system (MLRS) batteries selected from large combined units of the KPA, in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on December 21, 2016. KCNA/via Reuters ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NO THIRD PARTY SALES. SOUTH KOREA OUT. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS.
Ballistic rocket is seen launching during a drill by the Hwasong artillery units of the KPA Strategic Force in this undated picture provided by KCNA in Pyongyang on July 21, 2016. KCNA/via ReutersATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. SOUTH KOREA OUT. NO THIRD PARTY SALES. NOT FOR USE BY REUTERS THIRD PARTY DISTRIBUTORS.Ã TPX IMAGES OF THE DAY
A test launch of ground-to-ground medium long-range ballistic rocket Hwasong-10 in this undated photo released by North Korea's Korean Central News Agency (KCNA) on June 23, 2016. REUTERS/KCNA ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. NO THIRD PARTY SALES. NOT FOR USE BY REUTERS THIRD PARTY DISTRIBUTORS. SOUTH KOREA OUT. NO COMMERCIAL OR EDITORIAL SALES IN SOUTH KOREA. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. TPX IMAGES OF THE DAY
A new multiple launch rocket system is test fired in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang March 4, 2016. REUTERS/KCNA ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. NO THIRD PARTY SALES. SOUTH KOREA OUT. NO COMMERCIAL OR EDITORIAL SALES IN SOUTH KOREA
A rocket is launched during a demonstration of a new large-caliber multiple rocket launching system attended by North Korean leader Kim Jong Un (not pictured) at an unknown location, in this undated file photo released by North Korea's Korean Central News Agency (KCNA) on March 22, 2016. REUTERS/KCNA/Files ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. NO THIRD PARTY SALES. SOUTH KOREA OUT. NO COMMERCIAL OR EDITORIAL SALES IN SOUTH KOREA. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. TPX IMAGES OF THE DAY
A rocket is fired during a drill by anti-aircraft units of the Korean People's Army (KPA) in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang November 3, 2015. REUTERS/KCNA ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. NO THIRD PARTY SALES. SOUTH KOREA OUT. NO COMMERCIAL OR EDITORIAL SALES IN SOUTH KOREA.
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North Korea has been working to develop a nuclear-tipped missile capable of hitting the United States and last week tested its first intercontinental ballistic missile, which experts said could hit all of Alaska and parts of the U.S. Pacific Northwest.

Experts at 38 North estimated in April that North Korea could have as many as 20 nuclear bombs and could produce one more each month.

A report by U.S.-based nuclear expert Siegfried Hecker published by 38 North last September estimated North Korea had stockpiles of 32 to 54 kg (70 to 119 pounds) of plutonium, enough for six to eight bombs.

(Reporting by David Brunnstrom and Matt Spetalnick; Editing by Peter Cooney)

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