This is a modified version of an earlier interview conducted by Elisabeth Eaves for the Bulletin of the Atomic Scientists, and contains additional questions from Ruhee Neog for the Institute of Peace and Conflict Studies. Modified and reprinted with permission.

In this interview, Dr Siegfried S Hecker, the former director of the Los Alamos National Laboratory who has visited North Korean nuclear facilities multiple times, weighs in on what North Korea’s sixth nuclear test means, what the country is capable of, and how to get out of the dangerous game of nuclear brinksmanship now embroiling Northeast Asia and the US.

BAS: To the general public, there has been so much nuclear news out of North Korea lately that this one might sound like "just another test." What was different about North Korea’s 3 September nuclear test? How did it differ in magnitude from previous tests, and what does that tell us?

SH: The destructive power of North Korea’s previous five nuclear tests had progressed to about 25 kilotons, roughly the same as the bomb dropped on Nagasaki in 1945. This test was greater than 100 kilotons; that’s a big deal. It indicates they have progressed considerably beyond primitive fission-bomb technologies.

BAS: Was this one really a hydrogen bomb, and how would we know?

SH: The size of the blast was consistent with a hydrogen bomb - that is, a fusion-based bomb. However, it could also have been a large "boosted" fission bomb, in which the hydrogen isotopes deuterium and tritium were used to enhance the fission yield. If any telltale radioactive debris leaked from the underground test site, that could help us differentiate, but so far none has been found. So we can’t be certain.

BAS: What would it mean if it was a hydrogen bomb? Would that be a game changer?

SH: No, I don’t see a hydrogen bomb as a game changer. The North has been steadily enhancing its nuclear weapons in that direction. It was only a matter of time before it got there - although, if this one was a small, modern, two-stage hydrogen bomb, then I am surprised it got there so quickly. For years, I have followed the country’s steady progress on producing plutonium and highly enriched uranium, the fuels for fission bombs. And I concluded some time ago that it also has the ability to produce tritium, which is necessary for a boosted fission bomb or a hydrogen bomb.

BAS: But hydrogen bombs are a thousand times more powerful than fission bombs. Doesn’t that change the military threat?

SH: True, hydrogen bombs can be a thousand times more powerful. In fact, there is no theoretical limit to their destructive power. However, what is much more important is whether any nuclear bomb - fission or a fusion - can be made sufficiently small and light to mount on a missile, as well as robust enough to survive the missile’s launch, flight and atmospheric re-entry. Even a fission bomb of 25 kilotons delivered to Seoul or Los Angeles would cause horrific damage. So sure, a hydrogen bomb with very high destructive power would be worse, and have the advantage of being deliverable on a much-less-accurate missile, but the damage from a fission bomb would already be unacceptable.

BAS: Does the latest test change the political dynamics?

SH: Yes, it does. Washington was already suffering from its preoccupation with keeping North Korea from developing intercontinental ballistic missiles (ICBMs) instead of dealing with the nuclear crisis that already threatened Northeast Asia. President Trump seemed to have made ICBMs his red line, but North Korean leader Kim Jong-un blasted right past that in July and August. If you add the spectre of a hydrogen bomb, that creates an enormous dilemma for the Trump administration in terms of how to assure the American public it will be protected. In Pyongyang, meanwhile, they surely must see being able to field hydrogen bombs as levelling the playing field. A hydrogen bomb would put them in the elite company of the so-called P-5 states, the US, Russia, China, Great Britain, and France. It would increase Pyongyang’s leverage should it ever come back to the negotiating table.

BAS: Has the 3 September nuclear test changed your view that Pyongyang’s ability to reach the continental US with a nuclear-tipped missile is still some years away?

SH: Well, they got closer with this test, as they do with each missile and nuclear test. They may still be a few years away, but they are very competent at climbing a learning curve and making rapid progress. Besides, they are determined. Continued progress with either boosted fission bombs or hydrogen bombs - through more nuclear testing - will make it possible to fit the bombs on an ICBM. However, they still need to do a lot of work to get their weapons to survive the extreme launch, flight, and re-entry conditions.

BAS: Have North Korea’s nuclear and missile tests done any actual physical harm to the US or other countries?

SH: It certainly is confusing for the general public to hear about all these missile tests - flying toward Guam or over Japan. It is important to stress that these are tests of rocket technologies in which the rockets carry surrogates, not explosives or nuclear bombs, so there is no damage.

The nuclear tests, such as the sixth one, are enormously powerful, but the destruction is contained underground in a mountain. We must keep in mind that the US conducted 1,054 nuclear tests between 1945 and 1992, when we stopped. Until 1963, more than 200 of them were detonated in the atmosphere, causing radioactive fallout. The Soviets, by the way, conducted 715 tests over roughly the same time frame, and the Chinese 45. All six North Korean nuclear tests have been underground and well-contained. The possibility of radioactive leakage from these tests, however, is one of China’s greatest concerns since the test site is close to the border.

BAS: Several hours before the test, the North Korean official news agency KCNA posted photos of Kim Jong-un inspecting what it called a two-stage thermonuclear bomb. Do you believe that is what was tested?

SH: The images undoubtedly showed a model rather than the real device, but it had features generally consistent with a two-stage thermonuclear device, that is, a modern hydrogen bomb. The photos showed Kim inspecting the model in front of a schematic of the Hwasong-14 ICBM re-entry vehicle, and next to a mockup of its nose cone. The model appeared to have dimensions that would allow it to be mounted inside the ICBM. Clearly, that’s what the North Koreans would like us to believe, that they have mastered the ability to deliver a thermonuclear-tipped missile to the US mainland. However, we have no way of knowing if the device tested was of this design. The model could quite easily be constructed based on drawings of two-stage thermonuclear bombs available on the Internet. Nevertheless, I have learned not to underestimate the North Korean nuclear specialists.

BAS: Does the time interval between this nuclear test and North Korea’s last nuclear test tell us anything about technological progress they may be making?

SH: North Korea has been very methodical and deliberate about nuclear testing. The fact that it conducted six tests over such an extended period, beginning in October 2006, gave its nuclear scientists a chance to learn a lot between tests. I believe North Korea learned much more from its tests than did India or Pakistan, which conducted almost all of their six respective tests over a short time period with little chance to learn from one to the next. However, there was another reason for the slow, deliberate pace: North Korea lacked sufficient fissile materials, either plutonium or highly enriched uranium, until quite recently. The regime must also have weighed the likelihood of adverse actions from China, but as this last test shows, it was determined to proceed regardless of Chinese and international reaction.

BAS: The news coverage sometimes implies that Kim Jong-un, who took power in 2011 after his father and grandfather before him, is especially impatient and determined to develop a threatening nuclear arsenal. Do you see it that way?

SH: Not necessarily. North Korea has been making deliberate, steady progress on nuclear and missile advances since at least 2009, when all serious dialogue with Pyongyang ended. Progress, particularly on the missile front, has accelerated since Kim Jong-un took the reins at the end of 2011, but the foundations for the nuclear and missile programmes were already built. It does appear that Kim Jong-un has brought a more effective, hands-on management style to move the programmes forward.

BAS: In photos released by the KCNA, one of the men alongside Kim Jong-un appears to be Ri Hong-sop, head of North Korea’s Nuclear Weapons Institute. A Reuters news report, which identifies Ri in an earlier photo, says you met with him during your visits to Yongbyon. Is that so, and what can you tell us about him?

SH: Dr Ri Hong-sop was director of the Yongbyon Nuclear Research Center during my first visit in January 2004. I was impressed with his technical competency as well as his honest and direct answers to my technical questions during the tour, in which he gave our Stanford team remarkable access to the Yongbyon plutonium facilities. In a fascinating exchange about the intricacies of plutonium metallurgy, he even allowed me to hold a sample of recently produced plutonium - in a sealed glass jar - to convince me it really was plutonium.

BAS: Was that the only time you met with Ri?

SH: No, we met during several of my seven visits to North Korea, although by the fourth visit in 2007, he was no longer director of the Yongbyon Nuclear Center. I was told he had moved to Pyongyang to advise the General Department of Atomic Energy. When I asked about him during my last visit in November 2010, my host told me somewhat sarcastically that my government wouldn’t let me meet him because the latest UN sanctions had put him on a blacklist. Much of what we know about the North Korean nuclear complex comes from discussions we had with technical professionals in Yongbyon. So much for the benefits of sanctions: they didn’t slow down the North’s progress on its nuclear programme, but eliminated one of the few windows we had into it.

BAS: An official KCNA statement quoted Kim Jong-un as saying, "all components of the H-bomb were homemade … thus enabling the country to produce powerful nuclear weapons as many as it wants." You have previously said that North Korea has only limited inventories of fissile materials, the fuel required for bomb-making. Do you still consider that to be the case? How many bombs could it make now?

SH: North Korea cannot produce "as many as it wants," although it is making progress on both fusion and fission fuels. It appears to have produced lithium deuteride, which can be used to produce the tritium fuel for hydrogen bombs, but likely has only small inventories of tritium for boosted fission devices. And it still has relatively small inventories of fissile materials for the fission bombs that are required to trigger the fusion device.

Although they do involve great uncertainty, I believe my previous estimates still hold: By the end of 2016, North Korea had enough bomb fuel - roughly 20 to 40 kilograms of plutonium and 200 to 450 kilograms of highly enriched uranium - to make 20 to 25 nuclear weapons, with an annual production capacity of six to seven bombs' worth. If they continue to test and develop more sophisticated hydrogen bombs that could use less fissile material, we’ll have to revise that upwards. However, I don’t concur with the leaked intelligence estimate that they have up to 60 nuclear weapons now.

BAS: The KCNA statement also touted North Korea’s ability to launch a "super-powerful EMP attack" against the US. EMP is short for electromagnetic pulse. Could you explain what an EMP attack is, and whether this is a credible threat?

SH: The idea of an EMP attack would be to detonate a nuclear weapon tens of miles above Earth’s surface with the goal of knocking out the US power grid and causing other electrical disruptions.

I don’t see this as something the US needs to worry about now. First, North Korea has a lot of work to do to develop the right nuclear device for an intense EMP weapon. Second, how would an EMP attack help Pyongyang achieve its objective of deterring the US? If Pyongyang used such a weapon against the US, Washington would consider that an act of war, which would likely lead to the end of the Kim Jong-un regime.

What the EMP comment does show, however, is how closely the North Koreans follow the US press, which has published reports by some American alarmists wringing their hands about this threat. The North Koreans were even clever enough to have researchers from Pyongyang’s Kim Chaek University of Technology write a short brief about EMP, with the conclusion that it represents an important "strike" method.

BAS: Could the comment by US' UN Ambassador Nikki Haley that North Korea is "begging for war" hold any truth - that is, might Kim Jong-un see some benefit in getting to the point of actual military conflict?

SH: I don’t think so. Kim Jong-un’s only hope of survival is to avoid war. He apparently believes that in order to survive, he has to be able to threaten the US not only with ICBMs, but with ICBMs tipped with hydrogen bombs.

BAS: You’ve previously argued that the Trump administration must talk directly to North Korea as the next step in resolving the nuclear crisis. But both Haley and Trump have said the "time for talking is over." So now what?

SH: I’m afraid the Trump administration is compounding the mistakes of past US administrations with such comments, along with threats of "fire and fury." This rhetoric will make it all the more difficult for Washington to take the necessary steps to avoid a nuclear confrontation with North Korea. We need to face reality - the way we got into this situation is that we haven’t talked seriously since 2009.

BAS: "Talks" can mean different things to different people. Should the US negotiate? Or accept a nuclear-armed North Korea? Does talking constitute "appeasement," as Trump accused South Korean President Moon Jae-in of pursuing?

SH: The US administration should dispatch a small team to talk to Kim Jong-un to establish mechanisms to avoid misunderstandings, miscalculations, or misinterpretations that could quickly send us over the cliff into nuclear war. The talks would not be a reward or a concession to Pyongyang, nor should they be construed as signalling acceptance of a nuclear-armed North Korea. Such talks are not meant to appease Pyongyang as they would not offer any rewards. They could, however, deliver the message that while Washington fully intends to defend itself and its allies from any attack with a devastating retaliatory response, it does not otherwise intend to attack the North or pursue regime change. I realise that talking so soon after North Korea made such a major nuclear weapons advance may make it look like the US administration blinked first. But I consider that much less dangerous than stumbling into a nuclear war, which could happen if we pursue other actions being considered by the administration.

These talks would not be negotiations - not yet. Rather, they are a necessary step toward re-establishing critical lines of communication to avoid a nuclear catastrophe. Negotiations on denuclearisation might follow, but that would require a much longer time frame and coordination with China, Russia, and US allies.

RN: What is the potential fallout of the so-called "war of words" between Donald Trump and Kim Jong-un? Does it support or weaken US deterrence towards North Korea?

SH: The exchange of threats by the heads of the two governments increases the likelihood of stumbling into a war that will inflict unacceptable damage to both sides. It also greatly reduces the chance of talking to reduce tensions – so, it is a most unwelcome development.

RN: What is your analysis of the new round of sanctions on North Korea issued by executive order on 21 September?

SH: These will likely inflict greater pain on North Korea, including its people, but will not, in my opinion, force the regime to denuclearise. Sanctions without concomitant dialogue have been and will continue to be ineffective to that end.

RN: India has recently sought an investigation of North Korea’s proliferation linkages and asked that those involved "be held accountable." This has widely been read as an indirect reference to Pakistan. Could you comment on the type and degree of technological exchanges between North Korea and Pakistan? Has Pakistan’s assistance contributed directly to advancements in North Korea’s nuclear programme?

SH: Based on the words of former President Musharraf, Pakistan assisted North Korea with the start of its uranium enrichment programme. North Korea has been very clever in using a leaky international export control regime throughout much of the industrial world. However, one should not underestimate North Korea’s ability to grow its nuclear programme – they have done much of that indigenously.