Nuclear reactor safety in the post-Fukushima world

Since the Fukushima-Daiichi nuclear power plant disaster in 2011, there have been calls for increasing safety levels in nuclear plants worldwide. The incident in Japan was the fourth significant accident in the 55-year history of nuclear reactor operation.

The first occurred in 1957 at the Windscale reactor in the UK. Two decades later, in 1979, a reactor at Three Mile Island in the USA was severely damaged, though radioactive material releases were slight. The third incident is well-known: the 1986 disaster at Chernobyl, Ukraine, where the destruction of a reactor by steam explosion, fire and core disruption had significant health and environmental consequences, mainly due to fission product release and dispersion, as well as a human death toll at the site itself.

But throughout the nuclear age, countries and international bodies have, of course, been developing a variety of approaches and systems to promote safety and minimize the risk of accidents. In the UK and US, for example, nuclear reactor safety has, since the latter part of the twentieth century, been based on a comprehensive risk assessment approach in which experts, at the design stage, identify what could go wrong—based on a detailed knowledge of the components, materials, energy flows and core neutronics.

The experts then identify the various paths that an accident might follow, taking into account the plant design and its safety features such as emergency shutdown control rods, emergency core cooling and pressure vessel strength. They also calculate probabilities for each scenario (how often in ten thousand years a given accident might happen). For each hypothetical accident, they calculate the likely release of radioactive materials and model what the consequences would be for human life, health and the environment. Once all the above is complete, the so-called ‘envelope of risks’ for the planned reactor can be examined. If any part of this represents a risk above that which is judged to be tolerable, improvements can be made.

A landmark event in the UK was the Public Enquiry into the construction of a pressurised water reactor at the existing nuclear site at Sizewell. This enquiry revealed that neither the operators nor the Nuclear Installations Inspectorate had a numerical basis for comparing the tolerability of risks from nuclear reactors to such things as deaths from earthquakes, aircraft crashes and lightning strikes. The Sizewell enquiry was adjourned until these things were developed and approved. The result was two seminal documents: ‘The Tolerability of Risk from Nuclear Power Stations HSE, London 1988 (revised 1992)’; and ‘Safety Assessment Principles for Nuclear Facilities (revised as 2006 Edition, Revision 1, HSE, Bootle, UK).

The US has also engaged in studying these issues. The latest document is the US Nuclear Regulatory Commission’s State-of-the-Art Reactor Consequence Analyses, which analyzed the potential consequences of severe accidents at the Surry Power Station, Virginia, and the Peach Bottom Atomic Power Station in Pennsylvania as real examples of nuclear reactors. The project combined up-to-date information about the plants’ layout and operations with local population data and emergency preparedness plans. This information was then analyzed using computer codes that incorporate decades of research into reactor accidents. More

 

Have Past Accidents Helped Make Today's Nuclear Plants Safer?

American Academy of Arts and Sciences Paper Assesses How Industry, Regulators Have Applied Lessons Learned

Following each of the world's worst nuclear accidents-Three Mile Island, Chernobyl, and Fukushima-governments and plant operators adopted safety and security measures to help prevent future disasters. Most recently, the U.S. Nuclear Regulatory Commission has designed a new set of rules to harden American reactors against earthquake-triggered failures like those that crippled Japan's Fukushima-Daiichi plant last year.

(Photo: http://photos.prnewswire.com/prnh/20120611/DC22177 )

But has the response from industry and nuclear regulators always been adequate? Lessons Learned from "Lessons Learned": The Evolution of Nuclear Power Safety after Accidents and Near-Accidents, a new paper from the American Academy of Arts and Sciences, examines the changes in safety procedures and protocols that were or were not implemented after major nuclear disasters. The authors evaluate several less catastrophic accidents and near-mishaps as well, noting that those less serious incidents also offer critical lessons.

The paper provides recommendations for increasing plant safety and security as commercial nuclear power spreads globally. Authors, Michael M. May and Edward D. Blandford stress the need for better communication among nuclear states. "Mechanisms to facilitate and, where needed, enforce mutual learning have not always been adequate," they write. "Information-sharing, import/export agreements based on safety standards, agreements to facilitate cooperation among regulatory authorities, and the participation of financial interests such as investors and insurers all have a role to play in improving mutual learning among different states."

This paper, published as part of the American Academy's Global Nuclear Future (GNF) Initiative, is available online athttp://www.amacad.org/projects/globalnuclearbooks.aspx . More

 

Spent fuel rods drive growing fear over plant in Japan

TOKYO — What passes for normal at the Fukushima Daiichi plant today would have caused shudders among even the most sanguine of experts before an earthquake and tsunami set off the world’s second most serious nuclear crisis after Chernobyl.

Fourteen months after the accident, a pool brimming with used fuel rods and filled with vast quantities of radioactive cesium still sits on the top floor of a heavily damaged reactor building, covered only with plastic.

The public’s fears about the pool have grown in recent months as some scientists have warned that it has the most potential for setting off a new catastrophe, now that the three nuclear reactors that suffered meltdowns are in a more stable state, and as frequent quakes continue to rattle the region.

The worries picked up new traction in recent days after the operator of the plant, Tokyo Electric Power Company, or Tepco, said it had found a slight bulge in one of the walls of the reactor building, stoking fears over the building’s safety.

To try to quell such worries, the government sent the environment and nuclear minister to the plant on Saturday, where he climbed a makeshift staircase in protective garb to look at the structure supporting the pool, which he said appeared sound. The minister, Goshi Hosono, added that although the government accepted Tepco’s assurances that reinforcement work had shored up the building, it ordered the company to conduct further studies because of the bulge.

Some outside experts have also worked to allay fears, saying that the fuel in the pool is now so old that it cannot generate enough heat to start the kind of accident that would allow radioactive material to escape.

But many Japanese scoff at those assurances and point out that even if the building is strong enough, which they question, the jury-rigged cooling system for the pool has already malfunctioned several times, including a 24-hour failure in April. Had the outages continued, they would have left the rods at risk of dangerous overheating. Government critics are especially concerned, since Tepco has said the soonest it could begin emptying the pool is late 2013, dashing hopes for earlier action.

“The No. 4 reactor is visibly damaged and in a fragile state, down to the floor that holds the spent fuel pool,” said Hiroaki Koide, an assistant professor at Kyoto University’s Research Reactor Institute and one of the experts raising concerns. “Any radioactive release could be huge and go directly into the environment.” More

 

Fukushima Reactor #4 and its spent fuel pool

Akio Matsumura talks about nuclear power plants, spent fuel pools, and the trouble with Reactor 4 at Fukushima.

Dear Akio –

This photo is quite sobering. As you know the pool at Unit No. 4 contains 1,538 fuel assemblies, including a full core that was freshly discharged prior to the accident.

The Fourth Reactor at Fukushima on February 20, 2012. The yellow area is the containment vessel. --The Asahi Shimbum Digital

As I might have mentioned, based on data from the U.S. Department of Energy, a spent fuel assembly from a typical boiling water reactor contains about 30,181 curies ( ~1.1E+15 becquerels) of long-lived radioactivity. So the Unit No. 4 pool contains roughly 49 million curies (~1.8E+18 Bq), of which about 40 percent if Cs-137. (Source: U.S. Department of Energy, Final Environmental Impact Statement, for a Geologic Repository for the Disposal of Spent Nuclear Fuel and High-Level Radioactive Waste at Yucca Mountain, Nye County, Nevada, 2002, Appendix A, Tables A-7, A-8, A-9, A-10, BWR/ Burn up = 36,600 MWd/MTHM, enrichment = 3.03 percent, decay time = 23 years.)

As you know, the risk of yet another highly destructive earthquake occurring even closer to the Fukushima reactors has increased, according to the European Geosciences Union. This is particularly worrisome for Daiichi’s structurally damaged spent fuel pool at Reactor No. 4 sitting 100 feet above ground, exposed to the elements. Drainage of water from this pool, resulting from another quake could trigger a catastrophic radiological fire involving about eight times more radioactive cesium than released at Chernobyl.

Best Regards,

Bob Alvarez

Dear Bob:

I thank you very much for your comments on the photo of the Fukushima reactor unit 4.

You and Dr. Gordon Edwards of Canada used the same word, “sobering,” when you saw the photo.

My great concern is what Dr. Hans-Peter Durr, former Director of Astrophysics at the Max Planck Institute in Germany told me ten months ago. Japan had not yet admitted the core had melted down, but Hans-Peter knew that Fukushima was close to bringing us beyond the limits of our scientific knowledge and strongly suggested that Japan’s government should create an independent assessment team to bring the best minds of the nuclear scientists and structural engineers to seek the best solution. In such circumstances, Dr. Edwards accurately said, “It is important to seek the advice of experts who are genuinely independent having no conflict of interest and no need to save face. National pride makes it understandably difficult to seek help from outside, but sometimes it is the best thing to do.”

I have to admit that my country’s strong national pride makes it unrealistic to see an independent assessment team form soon enough, and so I now turn to rely on U.S. leadership to act out of the need for common, global security before a strong earthquake comes in the near future. If the reactor unit 4 building collapses, we will face a global environmental and human catastrophe larger than ever before.

I ask you and your colleagues to help bring this to the attention of U.S. senators and representatives so we may overcome the political hurdles and find a constructive solution.

Yours truly,

Akio

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