t***h
发帖数: 5601 | 1 http://www.ctbto.org/press-centre/press-releases/2013/ctbto-det
CTBTO detects radioactivity consistent with 12 February announced North
Korean nuclear test
Vienna, 23 April 2013
The CTBTO's radionuclide network has made a significant detection of
radioactive noble gases that could be attributed to the nuclear test
announced by the Democratic People's Republic of Korea (DPRK) on 12 February
2013.
The detection was made at the radionuclide station in Takasaki, Japan,
located at around 1,000 kilometres, or 620 miles, from the DPRK test site.
Lower levels were picked up at another station in Ussuriysk, Russia. Two
radioactive isotopes of the noble gas xenon were identified, xenon-131m and
xenon-133, which provide reliable information on the nuclear nature of the
source.
The ratio of the detected xenon isotopes is consistent with a nuclear
fission event occurring more than 50 days before the detection (nuclear
fission can occur in both nuclear explosions and nuclear energy production).
This coincides very well with announced nuclear test by the DPRK that
occurred on 12 February 2013, 55 days before the measurement.
Using Atmospheric Transport Modelling (ATM), which calculates the three-
dimensional travel path of airborne radioactivity on the basis of weather
data, the DPRK test site was identified as a possible source for the
emission.
CTBTO radionuclide expert Mika Nikkinen said: "We are in the process of
eliminating other possible sources that could explain the observations; the
radionuclides could have come from a nuclear reactor or other nuclear
activity under certain specific conditions, but so far we do not have
information on such a release."
On 12 February, the DPRK event was detected immediately, reliably and
precisely by 94 seismic stations and two infrasound stations of the CTBTO's
International Monitoring System. The first data were made available to CTBTO
Member States in little more than one hour, and before the DPRK announced
that it had conducted a nuclear test.
The event recorded at 02.57.51 (UTC) was found to have a magnitude of 4.9
using the CTBTO International Data Centre's magnitude scale. The location
was in the vicinity of the two previous nuclear tests (Lat.: 41.313 degrees
north; long.: 129.101 degrees east).
The radionuclide technology is the only one of the four monitoring
technologies employed by the CTBTO that can provide clear evidence of the
nuclear nature of an explosion. After an underground nuclear explosion,
radioactive noble gases can seep through layers of rock and sediment until
they escape into the air. Alternatively, the radioactivity may also be
released by man-made activities at the test site.
Exposed to prevailing winds, the noble gases are dispersed in the atmosphere
and may, after a certain period of time, be detected thousands of
kilometres away from the explosion site. The CTBTO currently has 66
radionuclide stations in operation across the globe, of which 30 are able to
detect noble gases.
The CTBTO is the international organization responsible for ensuring that no
nuclear explosion goes undetected. It uses four complementary verification
technologies: seismic, hydroacoustic, infrasound and radionuclide monitoring
. Traces of the radionuclide noble gas xenon 133 were registered after the
2006 DPRK nuclear test, but there were no detections following the 2009 DPRK
test. In total, over 87 percent of the CTBTO's 337 monitoring facilities
are now up and running.
The DPRK's action had been strongly condemned by Tibor Tóth, the Executive
Secretary of the CTBTO, and by Jan Petersen, Chairperson of the CTBTO's
executive body. | t***h
发帖数: 5601 | 2 http://www.ctbto.org/the-treaty/developments-after-1996/2013-dp
Media questions and answers on radionuclide detection
Vienna, 23 April 2013
1. How come the detection was made so late (when you said on 12 March that
it was improbable)?
A: The main detection was made on 8 and 9 April, a smaller one from 12 to 14
April. Detection of radioactive noble gas more than 7 weeks after an event
is indeed unusual, we did not expect this and it did not happen in 2009. We
do not know what has happened at the nuclear test site but if the
radionuclides are from the announced test, the observations are consistent
with an instantaneous release of 1 to 10% of the noble gasses remaining in
the cavity. A large instantaneous release is unlikely the result of normal
seeping through the hard rock this long after the announced test.
Follow-up: Why could this have happened?
A: We don't know why and we don't want to speculate.
Follow-up: But why hadn't the isotopes decayed?
A: Huge quantities of xenon gasses are produced in fission, the nuclear
reaction that is found in both nuclear weapons and nuclear reactors; after
55 days there is still a substantial amount left if the test is well
contained. A release of a large portion of the remaining gasses is still
within the detection capability of the CTBTO International Monitoring System
network.
2. When did you make the detection (and why have you not informed about it
earlier)?
A: The initial detection was made on 9 April, but needed to be analysed and
Atmospheric Transport Modelling needed to be done in order to narrow down
the source region. We are still in the process of excluding other possible
sources and this is a rather laborious task. We still cannot exclude
completely that the emission could have come from another source. Our Member
States have been informed about our technical findings and analyses
continuously as they always are, the first time on April 10.
Atmospheric Transport Modelling, short ATM, is the calculation of the travel
path of airborne radionuclides, using meteorological data. This calculation
can be performed in two ways: 1. as backtracking ATM, which identifies the
area from which a radionuclide may have been released, calculated from the
location where it was observed. 2. as forward ATM, which predicts where
radionuclides may travel from their known point of release.
3. How can you be sure it's not from Fukushima again?
A: The ratios of the two xenon isotopes detected clearly indicate a nuclear
fission that is recent, less than 60 days old. This kind of isotope ratio is
not possible from Fukushima. Also, the atmospheric transport modelling is
indicating that the source is not from Fukushima.
4. What does this say about the DPRK's nuclear weapons programme (HEU or Pu)?
A: This detection does not give an indication of the type of fissile
material used. To be able to distinguish between uranium and plutonium, it
helps if a detection is made early (before the decay of isotopes) and the
amount of registered radioactivity is large.
5. Why has nobody else made this detection, for example the national systems
deployed along the Korean border?
A: Our Atmospheric Transport Modelling shows that the winds blow the
emissions initially away from the Korean border and over the Sea of Japan.
This explains why our radionuclide station at Takasaki detected them
initially. Some small traces of the radionuclides were detected later in
Ussuriysk, Russia.
6. Are you sure that this comes from the DPRK test site and not somewhere
else?
A: The ratio of the detected xenon isotopes is consistent with a nuclear
fission event (fission can occur in both nuclear explosions and nuclear
energy production) occurring more than 50 days before the detection. This
coincides very well with the announced nuclear test by DPRK that occurred 55
days before the measurement. We are in the process of eliminating other
possible sources that could explain the observations; the radionuclides
could have come from a nuclear reactor or other nuclear activity under
certain specific conditions, but so far we do not have information on such a
release. We have also conducted Atmospheric Transport Modelling to narrow
down the possible source region and the model coincides very well with a
possible release in DPRK.
Follow-up: How sure are you (in %)?
A: We do not want to give any percentage. At this time, based on our
analyses and the sources of information that we have, we are confident that
the DPRK test site is among the possible source regions. But we cannot
exclude that the emission could have emanated from other sources.
7. Could it have been faked, i.e. the noble gas simply bought and released?
A: Theoretically, samples of both xenon isotopes could be released to
simulate an emission. However, as experts have said, creating an explosion
of 4.9 magnitude using conventional explosives, including its clandestine
preparation, would be technically very challenging.
8. How could an on-site inspection help?
A: On-site inspection happens close to the possible source of radioactivity,
so the measurements of radioactivity have better sensitivity. On-site
inspections will be possible after the CTBT has entered into force. An on-
site inspection would involve a team of up to 40 inspectors searching the
ground using a range of geophysical and other methods, including sampling at
the purported test site for radioactive traces.
9. Will this detection influence the ratification process in the holdout
countries?
A: Verifiability is a crucial consideration for any arms control treaty. The
latest detection shows again that the CTBT's verification regime is indeed
ready to provide confidence that no nuclear explosion will escape detection.
10. Why was there no detection after the 2009 DPRK nuclear test?
A: Underground nuclear tests may or may not release radioactivity into the
atmosphere. This depends on many factors, some of which are hard to
influence. Statistics from the Cold War have shown that a significant
percentage of all underground nuclear tests vented radioactivity. It should
be remembered, though, that the 2009 DPRK nuclear test was detected by 61
CTBTO seismic stations confidently and reliably. This alone would have
largely sufficed for an on-site inspection request. The seismic magnitude
also in this case was rather high - 4.52 - and this and the signals made it
possible for experts to say with reasonable confidence that this was a
nuclear explosion.
11. What is the difference between the detections in 2006 and now?
A: In 2006, we had only one detection, of xenon-133, at a lower level and at
a single station that was 7,000 kilometres away from the source, in
Yellowknife, Canada. Now, we have two isotopes, xenon-131m and xenon-133,
detected at two stations and at significantly higher levels, at least for
the Takasaki station. Also, the station where the main detection was made is
seven times closer, which makes the process of excluding other potential
sources easier.
One of the reasons for this better detection is the progress in build-up of
our monitoring stations. The number of noble gas-capable stations has
increased from 11 in 2006 to 30 (40 in final configuration) to now include
stations in Japan, China and Russia. | t***h
发帖数: 5601 | 3 http://www.reuters.com/article/us-korea-north-nuclear-idUSBRE93
Gases "consistent with" North Korea's February test found
World | Tue Apr 23, 2013 5:22am EDT
VIENNA | By Fredrik Dahl
Radioactive gases that could have come from North Korea's nuclear test in
February have unexpectedly been detected, a global monitoring body said on
Tuesday, possibly providing the first "smoking gun" evidence of the
explosion.
But the April 9 measurement - almost two months after Pyongyang said it had
carried out the underground detonation - gave no indication of whether
plutonium or highly enriched uranium was used, it said.
The time that had passed before the so-called noble gases were picked up
made it "very difficult" to distinguish between the two fissile materials,
said spokeswoman Annika Thunborg of the Comprehensive Nuclear-Test-Ban
Treaty Organisation (CTBTO).
The isolated east Asian state is believed to have tested plutonium bombs in
its previous two such blasts, in 2006 and 2009. Any switch to uranium would
increase international alarm as it could enable Pyongyang to greatly expand
its arsenal.
North Korea threatened nuclear attacks on the United States, South Korea and
Japan after new U.N. sanctions were imposed in response to its latest
atomic test. But USA officials have cast serious doubt on whether it could
launch a nuclear missile.
Pyongyang's third nuclear test was registered virtually instantaneously via
seismic signals around the world. But no radioactive traces that would have
constituted conclusive proof were found in the immediate weeks afterwards.
The Vienna-based CTBTO, which has a worldwide network of monitoring stations
, said in mid-March that it was highly unlikely any such radioactivity would
be detected.
But Tuesday's statement said it made a significant detection of radioactive
noble gases two weeks ago in Takasaki, Japan, about 1,000 km (620 miles)
from the test site. Lower levels were picked up at another station in
Ussuriysk, Russia.
"Two radioactive isotopes of the noble gas xenon were identified, xenon-131m
and xenon-133, which provide reliable information on the nuclear nature of
the source," it said.
"Detection of radioactive noble gas more than seven weeks after an event is
indeed unusual. We did not expect this and it did not happen in 2009," the
CTBTO added, referring to the reclusive country's previous nuclear test.
BOTH BOMB PATHS
Large amounts of xenon gases are produced in fission, a nuclear reaction
occurring both in nuclear arms and reactors.
"We are confident that the (North Korean) test site is among the possible
source regions," the CTBTO said. But it could still not "exclude completely"
that the radioactive traces came from somewhere else.
North Korea's February test yielded a stronger blast than its previous
explosion four years ago, and Pyongyang said it had made progress in
miniaturizing an atomic weapon, essential to fitting it into the cone of a
missile.
Tension on the Korean peninsula soared after the test.
While estimates of the explosive power of the latest test vary, most
officials and experts assessed it was at least five kilotons - still smaller
than the power of the atomic bomb the United States dropped on Hiroshima in
World War Two.
North Korea abandoned plutonium production six years ago in response to
international pressure, but later acknowledged that it had built facilities
to produce enriched uranium, which can also be used in bombs if refined to a
high degree.
Experts say plutonium, a byproduct of nuclear reactors, can be difficult to
use as bomb material because specifications have to be precise. It could be
easy for North Korea to make large quantities of highly enriched uranium.
"It would not surprise me if they have been pursuing both paths to the bomb,
" Siegfried Hecker, a prominent USA scientist who has often visited North
Korea, said in an interview published this month on a Stanford University
website.
The test-ban treaty was negotiated in the 1990s but has not taken effect
because some holders of nuclear technology have not yet ratified it,
including the USA and China.
But the CTBTO already monitors possible breaches, deploying about 290
stations around the world to look out for signs of atomic tests, including
seismic waves and radioactive traces. |
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