Media questions / answers on radionuclide detection

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.

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?

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 to the States 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.