Lessons from Fukushima for Future Nuclear Power Plants

by William Hannum, 22 March 2011

Background
On 11 March 2011, a massive earthquake hit Japan.  The six reactors at Fukushima-Dai-ichi suffered ground accelerations somewhat in excess of design specification.  It appears that all of the critical plant equipment survived the earthquake without serious damage, and safety systems performed as designed.  The following tsunami, however, carried the fuel tanks for the emergency diesels out to sea, and compromised the battery backup systems.  All off-site power was lost, and power sufficient operate the pumps that provide cooling of the reactors and the used-fuel pools remained unavailable for over a week.  Heroic efforts by the TEPCo operators limited the radiological release.  A massive recovery operation will begin as soon as they succeed in restoring the shutdown cooling systems.

It is important to put the consequences of this event in context.  This was not a disaster (the earthquake and tsunami were disasters).  This was not an accident; the plant experienced a natural event (“Act of God” in insurance parlance) far beyond what it was designed for.  Based on the evidence available today, it can be stated with confidence that no one will have suffered any identifiable radiation-related heath effects from this event.  A few of the operators may have received a high enough dose of radiation to have a slight statistical increase in their long term risk of developing cancer, but I would place the number at no more than 10 to 50.  None of the reports suggest that any person will have received a dose approaching one Seivert, which would imply immediate health effects.

Even ignoring the possibility of hormetic effects, this is approaching the trivial when compared with the impacts of the earthquake and tsunami, where deaths will likely come to well over 20,000.  Health impacts from industrial contamination, refinery fires, lack of sanitation, etc., etc. may reasonably be supposed to be in the millions.  Even the "psychological" impacts of the Fukushima problems must be seen to pale in contrast to those from the earthquake and tsunami.  

The radiological impact on workers is also small relative to the non-radiological injuries suffered by them.  One TEPCO crane operator died from injuries sustained during the earthquake. Two TEPCO workers who had been in the turbine building of Unit 4, are missing.  At least eleven TEPCO workers were take to hospital because of earthquake-related physical injuries.

TEPCo has suffered a major loss of capital equipment, the value of which is non-trivial even in the context of the earthquake and tsunami devastation.  They also face a substantial cost for cleanup of the contamination which has been released from the plants. These are financial costs, not human health and well being matters.

The Sequence of Events

Following the tsunami, the operators had no power for the pumps that circulate the primary coolant to the heat exchangers.  The only way to remove the decay heat was to boil the water in the core.  After the normal feed water supplies were exhausted, they activated the system to supply sea water to the core, knowing this would render the plant unfit to return to operation.  In this way, the reactors were maintained in a relatively stable condition, allowing the water to boil, and releasing the resulting steam to the containment building. Since this is a Boiling Water Reactor (BWR), it is good at boiling water.  Operating with the water level 1.7 to 2 meters below the top of the core, they  mimicked power operation; the core normally operates at power with the water level well below the top of the core, the top part being cooled by steam.   Cold water in, steam out, is a crude but effective means of cooling.  

Before using sea water, according to reports, water levels are thought to have dropped far enough to allow the fuel to overheat, damaging some of the fuel cladding.  When overheated, the cladding (Zirconium) reacts, claiming oxygen from the water.  Water, less oxygen, is hydrogen.  When vented to the containment and then to the outer building, the hydrogen built up, and eventually exploded, destroying the enclosing building.  With compromised fuel, the steam being vented contains radioactive fission products.  The design of BWRs is such that this venting goes through a water bath (in the Torus), that filters out all but the most volatile fission products. 

With time, the heat generated in used fuel (both in the core and in the fuel pool) decreases.  From an initial power of about 2% of full power an hour after shutdown (when the coolant pumps lost power) to about 0.2% a week later, the amount of steam venting decreases, and releases can be controlled and planned for favorable weather conditions. 

A second concern arose because of the inability to provide cooling for the used-fuel pool in Unit 4, and later Unit 3.  The Unit 4 pool was of concern because, for maintenance, the entire core had been off-loaded into the pool in November (it is believed that two older core loadings were also in this pool, awaiting transfer to the central storage pool).  With only a few months cooling, the residual heat is sufficient to raise the temperature of the water in the pool to boiling within several days or weeks.  There is also some suggestion that the earthquake may have sloshed some water out of the pool.  In any case, the fuel pools for Units 3 and 4 eventually were thought to be losing enough water such that the fuel would no longer be adequately cooled.  Since the fuel pools are outside the primary containment, leakage from these pools can spread contamination more readily than that from the reactor core.  High-power water hoses have been used to maintain water in the fuel pools.

While many areas within the plant complex itself, and localized areas as far away as 20 Km may require cleanup of the contamination released from the reactors and from the fuel pools, there is no indication that there are any areas that will require long term isolation or exclusion.

Lessons Learned

It is not the purpose of this paper to anticipate the lessons to be learned from this event, but a few items may be noted.  One lesson will dominate all others: 

Prolonged lack of electrical power must be precluded.

While the designers believed their design included sufficient redundancies (diesels, batteries, redundant connections to the electrical grid), the simultaneous extended loss of all sources of power left the operators dependant on creative responses.  This lesson is applicable both to the reactor and to fuel pools.

All nuclear installations will probably be required to do a complete review of the security of their access to electrical power.  It may be noted that this lesson is applicable to many more activities than just nuclear power.  Extended loss of electrical power in any major metropolitan area would generate a monstrous crisis.  The loss of power was irrelevant to other activities in the region near the Fukushima plant because they were destroyed by the tsunami.  

Other lessons that will be learned that may be expected to impact existing plants include:

Better means of control of hydrogen buildup in the case of fuel damage may be required.

In addition, detailed examinations of the Fukushimi plants will provide evidence of the margins available in seismic protection.  Detailed reconstruction of the event will give very helpful insights into the manner that fission product can release from damaged fuel, and their transport. 

Applicability of Fukushima Information to MOX-fueled Reactors:

The core of Unit 3 was fueled with plutonium recycled from earlier used reactor fuel.  Preliminary information suggests that the release of hazardous radioactive material, for this type of event, is not significantly different than that non-recycle fuel.  More detailed examinations after the damaged cores are recovered, and models developed to reconstruct the events, will be necessary to verify and quantify this conclusion.

Applicability of Fukushima Information to Gen-III Reactors:

In the period since the Fukushima plants were designed, advanced designs for BWRs (and other reactor types) have been developed to further enhance passive safety (systems feedback characteristics that compensate for abnormal events, without reliance on operator actions or on engineered safety systems), simplify designs, and reduce costs.  The results of these design efforts (referred to as Gen-III) are the ones now under construction in Japan, China and elsewhere, and proposed for construction in the U.S.  

One of the most evident features of the Gen-III systems is that they are equipped with large gravity-feed water reservoirs that would flood the core in case of major disruption.  This will buy additional time in the event of a Fukushima type situation, but the plants will ultimately rely of restoration of power at some point in time.  

The applicability of the other lessons (hydrogen control, fuel pool) will need to be evaluated, but there are no immediately evident lessons beyond these that will affect these designs in a major way.

Applicability of Fukushima Information to Recycling Reactors:

As noted above, Unit-III was fueled with recycled plutonium, and there are no preliminary indications that this had any bearing on the performance of this plant during this event.

Advanced recycling, where essentially all of the recyclable material is recovered and used (as opposed to recovery and recycle of plutonium) presents a different picture.  Full recycling is effective only with a fast reactor.  A metal fuel, clad in stainless steel, allows a design of a sodium-cooled fast reactor with astonishing passive safety characteristics.  Because the sodium operates far from its boiling point in an essentially unpressurized system, catastrophic events caused by leakage or pipe failures cannot occur.  The metal fuel gives the system very favorable feedback characteristics, so that even the most extreme disruptions are passively accommodated.  A complete loss of cooling, such as at Fukushima, leads to only a modest temperature rise.  Even if the control system were rendered inoperable, and the system lost cooling but remained at full power (this is a far more serious scenario than Fukushima, where the automatic shutdown system operated as designed) the system would self-stabilize at low power, and be cooled by natural convection to the atmosphere.  Should the metal fuel fail for any reason, internal fission product gases would cause the fuel to foam and disperse, providing the most powerful of all shutdown mechanisms.

The only situation that could generate energy to disperse material from the reactor is the possibility of s sodium-water reaction.  By using an intermediate sodium system (reactor sodium passes its energy to a non-radioactive sodium system, which then passes its energy to water to generate steam to turn the electrical generator), the possibility of a sodium-water reaction spreading radioactive materials is precluded.

These reactors must accommodate seismic challenges, just as any other reactor type.  While there are many such design features in common with other reactor designs, the problem is simpler for the fast reactor because of the low pressure, and the fact that this type of reactor does not need elaborate water injection systems.

In light of the Fukushima event, one must consider the potential consequences of a massive tsunami accompanying a major challenge to the reactor.  Since it may be difficult to ensure that the sodium systems remain intact under the worst imaginable circumstances, it may be prudent to conclude that a tsunami-prone location may not be the best place to build a sodium facility (whether a nuclear power plant or something else).

Conclusions:

The major lesson to be learned is that for any water-cooled reactor there must be an absolutely secure supply of power sufficient to operate cooling pumps.  Many other lessons are likely to be learned.  At this early point, it appears that design criteria for fuel storage pools may need to be revised, and hydrogen control assessed. 

Given the severity of the challenge faced by the operators at Fukushima, and their ability to manage the situation in such a way as to preclude any significant radiation related health consequences for workers or the public, this event should be a reassurance that properly designed and regulated nuclear power does not pose a catastrophic risk to the public--that, overall, nuclear power remains a safe and clean energy sources.

Given the financial impact this event will have on the utility (loss of four major power plants, massive cleanup responsibilities), it will be worthwhile for the designers, constructors, operators, and licensing authorities to support a thorough analysis of what actually transpired during this event.