COVER STORY • 7

Interview with Dr. George Apostolakis, Director of Nuclear Risk Research Center, Central
Research Institute of Electric Power IndustryBuilding Trust in Nuclear
Power Stations Needs
Knowledge Sharing Process

By Japan SPOTLIGHT
Nuclear power is still an important energy source, as well as a renewable energy at a time when
promoting clean energy to realize a zero-carbon emission economy is becoming crucial. But the mostimportant question is how to ensure the safety of nuclear power stations. One quantitative analysis
method called Probabilistic Risk Assessment (PRA) is highlighted here, as it could help to achieve it. Thiscan be regarded as another example of Evidence-Based Policy Making (EBPM).
Japan SPOTLIGHT held the following interview with Dr. George Apostolakis, a distinguished nuclear
engineering expert and director of the Nuclear Risk Research Center (NRRC) of the Central ResearchInstitute of Electric Power Industry (CRIEPI) in Japan.
(Interviewed on Feb. 8, 2022)

Introduction

damage to the reactor core, is it a release ofradioactivity, is it public health, and so on.
JS: Could you briefly tell us about
And the last question is, how likely are these
your professional and academic
scenarios? So you have thousands ofbackground?
accident sequences, and you assign
frequency to each one, and according to the
Apostolakis: I did my undergraduate degree
frequency you can prioritize. Instead ofin Athens in electrical engineering, then went
looking at thousands of sequences, it turns
to Caltech in Pasadena, California, where I got
out that for public health effects, it’s usually
my Ph.D. in 1973. Then I joined UCLA as an
about 15 sequences that matter, i.e., they are
assistant professor (eventually promoted tomost likely. From thousands to 15, so that’s aprofessor) until 1995, when I moved to MIT
major advance. The other difference between
in Boston. In 2010, I was nominated by
the top-down and the bottom-up approach is
President Barack Obama and confirmed bythat the top-down approach – which looks at
the US Senate to the Nuclear Regulatory
the plant as a system – recognizes that there
Commission in Washington, so I moved there
are human operators who will intervene inand stayed for four years. Then, in 2015, wecase something goes wrong. The possibility
Dr. George Apostolakis
moved back to Los Angeles where we haveof human actions is included in thefamily.
development of accident sequences. This is a very important point
My area of specialization is Probabilistic Risk Assessment (PRA).
because up until the early 1970s the nuclear community did not pay
We started as a community designing nuclear reactors in the 1950s,
much attention to human error. It was the first PRA – which was
1960s and 1970s. The approach to design and safety was what I callcalled the reactor safety study published in 1974 – that showed thatbottom-up. In other words, you look at the various components andhuman performance is important, which of course makes sense.
systems and you have requirements for valves and pumps and so
So that is what PRA is. It looks at the reactor and the plant as one
on, and we design them according to those requirements. Then, we
system, and asks the three questions: What can go wrong? What are
assume that the plant is safe. A different approach is a top-down.
the consequences? And how likely is it? It’s both quantitative and
You look at the plant, as a system consisting of hardware andqualitative analysis.
humans, and you ask yourself what can go wrong? There are threequestions that define PRA.
How PRA Works
What could go wrong – which means developing accident
JS: Could you provide an example of this assessment
scenarios. What are the consequences of each scenario? Is it
https://www.jef.or.jp/journal/

Japan SPOTLIGHT • May / June 2022

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that might enable us to understand exactly how it
works?
Apostolakis: Suppose that the reactor is operating under normal
conditions producing electric power. Then, for some reason, there isa fire in one compartment at the plant. This is an abnormal situation.
The control room is notified and now we start developing possible
accident sequences. The first thing that the plant people will try to dowill be to isolate the fire and stop it from spreading. Maybe the firebrigade will go. So now we start developing possibilities. Is the fire
extinguished within a short amount of time and if it is, what is thedamage? The other possibility is that the fire is not extinguished, andit damages some equipment in that compartment. That now createsa new group of accident sequences because now you have the fire,you know which components are damaged, and what happens next.
If it propagates to another compartment, you have additional
possibilities that the fire will damage additional equipment and so on.
So you see how you generate many accident sequences simply by
the initiator of a fire. That is what we do, and we have a computer tohelp us, and we end up with thousands of sequences.
JS: How do you assume probability for eachconsequence?

Apostolakis: Each accident sequence starts with what we call an
initiating event. The example I gave you is a fire at a particular place.
Another example is an earthquake, a human error, and so on. The
initiating event frequencies typically are produced by statisticalevidence. We have had several thousand reactor years in operationworldwide, so we have an idea how likely these events are. In somecases, we have to do more analytical work because statisticalevidence is not sufficient. You have the frequency of the initiator;
then we develop probabilities for damage. As I said, a fire may
damage a particular component. The question is now, what is theprobability that given this fire, the component will be damaged. And
there is evidence, experimental, statistical, expert opinion – and weput all these together and come up with a probability of damage. We
do this where it is required, and it is a combination of statisticalevidence, engineering knowledge, testing and expert opinion.
JS: To be as objective as possible on this assessment,

do you need lots of big data?

Apostolakis: Not really. The issue of how objective a PRA is, is avery important one. We have approached this in several ways. Theinternational community has developed standards that whensomeone in Japan or in Sweden wants to do a PRA, they follow
these standards. But even if you follow the standards, there is a lot of
30 Japan SPOTLIGHT • May / June 2022

utilization of expert opinion which is subjective. So, the second
method for ensuring objectivity is to have peer review. You put
together a group of experienced PRA experts and engineers who
have nothing to do with the original PRA, and they spend serious
amounts of time reviewing the PRA at hand. They make numerous
comments that the original team must respond to. These are the twomain ways that we are using to ensure objectivity in the PRA.
JS: You have clearly outlined how PRA works forsafety in nuclear power stations. Is this PRA method
applied in other areas?

Apostolakis: It has been applied in space shuttles, and I know thatthe coast guard in the United States has also used it, as well as somechemical facilities. The extent to which they are using it varies fromindustry to industry; the nuclear power industry is more rigorous
because of the regulations. Other industries are not as heavily
regulated as nuclear power.
JS: Would you say that PRA is limited to certain areasin which the risk is considered very important?

Apostolakis: If you don’t have significant potential risk, there is no
reason to do a PRA. You can do some PRA that is a little shallow, notas detailed as that for nuclear reactors.

What PRA Has Achieved So Far
JS: How is the information obtained from PRA to be

used for specific risk management in a nuclear power
station? Do you think so far that a nuclear power
station’s safety has been enhanced by PRA?

Apostolakis: Yes, there have been several studies that I am aware ofin the US. One of the metrics that we use is frequency of coredamage. This is a very important metric – the frequency per year ofdamage in the core. If you look at the plot of the core damage
frequency as a function of years going back to 1970 up until today,
there is a very clear picture of the curve going down. The coredamage frequency is improving over the years. This is to a large
extent due to the utilization of PRA.
JS: Does the IAEA consider PRA as an importantanalytical tool for ensuring the safety of nuclear
power stations?

Apostolakis: Yes, they have issued safety guides and otherdocuments where they promote the utilization of PRA.

JS: The IAEA set up its own safety standards fornuclear power stations. PRA is a very importantcomponent to encourage people to observe thesestandards.

Apostolakis: The standards in each country more or less comply
with those of the IAEA.
JS: Do all nuclear power stations have to apply PRA

today?

Apostolakis: Yes, it complements the traditional safety principles.
We have tools that look at the plant as a whole. We are noteliminating the traditional requirements on systems andcomponents; by and large they are still in force. It is just a different
way of looking at the plant.
JS: Is it costly for companies to apply PRA?
Apostolakis: Yes, it is costly. But it is a very ambitious undertaking
because it models the plant no matter what happens – a fire, anearthquake and so on. So it is really an ambitious undertaking andrequires many different types of expertise which means it becomes
expensive. You need systems people who understand how the plant
works; you may need seismic experts, fire experts, humanperformance experts. In fact, this is one of the strengths of PRA – itbrings together such diverse disciplines. I remember when we
started paying serious attention to human error, we had meetings ofengineers and psychologists. Psychologists were never part ofdesigning nuclear power plants, and here we had them advising us
on human performance. This is one of the great strengths. The otherthing that makes it expensive is the peer review that I mentioned.
This can also be extensive and expensive because you have to pay
the experts, the reviewers, and these are usually experienced people
who are expensive.
JS: Related to your explanation on how PRA works fornuclear plants, in terms of thinking about possible
events initiating accidents, how do you define therisk and how do you allocate probability to each risk?

Apostolakis: As I mentioned, the list that is widely used now – there
is one for pressurized water reactors and one for boiling water
reactors – comes from experience, international experience. You
have reactors operating in the US, Japan, Sweden, and Taiwan andthere are international organizations that keep track of the evidence.
So, for example, in the early days, people thought that the most
severe possible accident is the guillotine break of the largest pipewith water in the plant, a so-called large loss of coolant accident. We
have never seen it. The statistical evidence says that this is anextremely unlikely event. All over the world we have never seen amajor coolant accident. So you have the statistical evidence and theexpert opinion, and we try to combine the two. We cover all the risks
including all the unexpected ones.
JS: Could you please give us your overview on someOECD nations’ application of PRA such as the US,
the United Kingdom or France? Do you think PRA
has been widely used in the world?

Apostolakis: Yes, it has. It depends on the culture and the politicalsystem of the country. In the US, we tend to be very legalistic, so wedevelop rules, the authorities get involved, and the documents spellout clearly what you are supposed to do and so on. In othercountries, my experience is that they are not so formal. The French,
the British, the Swedes, the Finns, Taiwanese, South Koreans have allused PRA. But the utilization is not as public as it is in the US. That
doesn’t mean they are not doing a good job; it is just that they don’t
put it out in the public for everyone to see. In Japan, with the NRRC
and especially after the Fukushima accident, we are trying to be as
open as we can. We are publishing everything we do on our website;
we have our own advisory committee that reviews our work. They
write letters to us and sometimes are critical. We publish thoseletters too, as well as our answers. So we are trying to bring more
openness in the system, at least when it comes to PRA because wemust gain the trust of the public. If the public suspects that you are
not completely open with them, then trust disappears.
JS: In Japan, you mentioned that PRA has been

applied to nuclear plants. Could you explain a little
about the history of PRA in Japan? Was it introduced
a long time ago or more recently applied?

Apostolakis: It was used to some degree before Fukushima. But I
must say, the results of PRA were not really taken so seriously, andthe system was still deterministic – a bottom-up approach. This
meant that there was a tendency to produce very low probabilities
for accidents. Peer review was almost non-existent. Now that we aretrying to build up trust, we have asked the PRAs of two model units– Kashiwazaki Kariwa and Ikata – to be reviewed by international
experts. So we implemented the PRA requirement of theinternational community. These reviews worked very well. The
experts made many substantial comments, and the utilities
responded by modifying their PRAs. The understanding was that theother utilities would follow what was happening at Kashiwazaki
Kariwa and Ikata, and upgrade their own PRA. So this was one of theJapan SPOTLIGHT • May / June 2022

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most important elements in this area in Japan.
At the same time, as Japan is vulnerable to severe natural events,
we are developing probabilistic methods for severe earthquakes andtsunamis and the combinations of both, because in other countries
some of these natural events have been analyzed in detail, such as
earthquakes, while others such as tsunamis are unique to Japan. So,
we are developing the tools that the utilities can use to quantify thesequences for earthquakes, tsunamis, tornadoes, volcanoes –hazards that are really important for Japan.

Restoring Trust in Nuclear Power Stations
by PRA
JS: Building up trust is very important in the case of

the utilization of nuclear power. In your assessment
so far, has PRA worked well in building up trust inJapan?

Apostolakis: No. First of all, when you talk about trust, you have toconsider the general public and also the expert community. I think
PRA has made progress among the technical community. There is
still some reluctance on the part of the regulators to use PRA in
regulating but we are making progress. The Nuclear Regulation
Authority (NRA) is now using the reactor oversight process thatutilizes risk insights. This is the oversight process that we use in the
US. When it comes to the general public though, this is perhaps toomuch of a technical issue. I don’t think people are impressed, and so
I am not sure we are making significant progress in that regard. As
many people have said, trust is easy to lose but gaining it back isreally very difficult. But I think the openness that I mentioned early is
one element. We publish everything on our website and people cansee what we are doing right and what we are doing wrong.
JS: I think nuclear power would be an importantenergy source for Japan in achieving zero carbon
emissions. To take full advantage of nuclear power toachieve a green economy, PRA would be important
evidence encouraging safe utilization of nuclear
power. You mentioned that openness is very
important in achieving progress, but how about otheraspects? For example, communication skills toconvince ordinary people of the safety of nuclear
power plants. How can we improve communication
with the general public on PRA?

Apostolakis: This is the million-dollar question that nobody knows
the answer to. We have a risk communication group in the center,and they are doing their fair share, but in the US I don’t think that the
32 Japan SPOTLIGHT • May / June 2022

average person really cares about the quantitative analysis thatsomething is safe. So, I don’t think anybody knows the answer; ittakes a long time to build trust, and when something happens as anindustry, we have to be very open and not to cover up even minor
incidents. It’s really a very slow process. I don’t think that trying to
educate the general public on PRA is particularly meaningful. So let’s
start with the experts, the regulators, and the industry.
JS: Scientists seem to play an important role inexplaining very complicated events to the generalpublic.

Apostolakis: If it works, fine with me. I don’t know of anybody who
can do that. I am sure you are aware, this was a really big accident.
Fukushima was not just a minor thing, and it really shook up the
public’s trust in the industry and the government, and the regulatory
agency. So it will take a long time. Already I believe that 10 units
have restarted. If you look at it after such a major incident, we have
10 reactors that have restarted 10 to 12 years later; that is probably a
good sign.
JS: Not limited to the case of nuclear power, should

people become more accustomed to quantitative
analysis?

Apostolakis: Yes, absolutely. The attitude of the public or membersof the public is a mystery sometimes. Look at the reaction against
vaccinations all over the world. I can’t believe that people go down
and demonstrate against the vaccines which quantitatively have
proven they can help you against Covid-19, yet you havedemonstrations in Paris, Washington, Brussels and so on. Who arethese people? Communicating with the public on technical matters isreally an unsolved problem in my opinion.
JS: We need a long-term solution and improving

science education could be a solution in the long run
to help people understand what quantitative analysis
is. Today, science, life and business are closely
related to each other.

Apostolakis: Absolutely.

Written with the cooperation of Joel Challender who is a translator, interpreter,
researcher and writer specializing in Japanese disaster preparedness.