The Drama at Fukushima By Dr. Richard Lahey

8/7/2019 The Drama at Fukushima By Dr. Richard Lahey

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THE DRAMA AT FUKUSHIMA

The nuclear reactor emergency occurring at the Fukushima power plants in Japan has captured the

attention of the world, but there has been a lot of confusing and contradictory information presented by

the media and other sources. Unfortunately, this may have caused significant fear and anxiety on the

part of the public both here and abroad. The purpose of this white paper is to try to explain whatappears to have occurred at the Fukushima plants to date and what will be the most likely outcome in

terms of the health and safety of the Japanese public and the world.

As can be seen in my Vita (www.rpi.edu/~laheyr/laheyvita.html), prior to going to Rensselaer

Polytechnic Institute (RPI) in 1975 as Chairman of the Department of Nuclear Engineering & Science, I

(Richard T. Lahey, Jr.) was responsible for the boiling water nuclear reactor (BWR) safety-related R&D of

the General Electric Company (GE), the designer of the BWRs at Fukushima . My responsibilities

included core thermal-hydraulic phenomena, emergency core cooling system performance and

performance of the various pressure-suppression containment designs. The particular Fukushima power

plants under consideration are BWR/3 or 4 type nuclear reactors (the first ones having both internal jet

pumps and steam separators) with Mark I pressure-suppression containments (see the attached figure

at the end for details, however please note that Japanese built Mark-I containment systems are

somewhat larger than the GE system which is shown, and Fukushima Daiichi unit-6 has a Mark-II type

containment design). As a consequence of this experience I am very familiar with these types of reactor

systems and how they may respond during accident conditions.

While it has been very difficult to get reliable and self-consistent information on the ongoing nuclear

emergency, I have attempted to make some sense of what is currently known. Since the conditions

associated with the various reactors at Fukushima are different, and even for a particular plant they can

change daily, unless a specific plant is cited, I have chosen to give a generic explanation of what has

occurred. Anyway, we know that a very strong (i.e., 9.0 on the Richter scale) earthquake occurred off

the coast of north eastern Japan and it caused the nuclear reactors which were operating at Fukushima

to SCRAM ( i.e., the control rods were automatically inserted rapidly, thus terminating the fission

process ), which is what these plants were designed to do. Soon after this event, the reactors were put

into a standard shut down cooling mode using the plants closed-loop residual heat removal (RHR)

systems which keep the core covered with water and remove the decay heat generated by the

radioactive decay of the radioactive fission fragments in the nuclear fuel (e.g., subsequent to a SCRAM

the power level of the core drops from full power operation to an initial decay heat power level of about

5% to 3% of full power, and then this power level slowly decays with time after that). This continued for

about an hour until a large tsunami (estimated be 14m high) arrived which knocked out any offsite

electrical power and disabled the plants emergency diesel generators ( which had been brought on right

after the earthquake struck). It should be noted that, in accordance with rules established by the

Japanese regulatory authorities, these plants were designed to take the large loads associated with an

earthquake ( I.e., 8.2 on the Richter scale) and an associated tsunami ( assumed to 5.7 m high), but

obviously not one which had the height/strength of the massive tsunami which actually hit these plants.

Anyway, an alternate battery- powered RHR type system was apparently enabled after the tsunami hit

but this mode of cooling was terminated about eight hours later when the batteries presumably became
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depleted. Subsequently it appears that the water level in the reactor cores began to drop and once the

upper part of the fuel was no longer submerged it began to heatup due to decay heat. When the

zircalloy cladding surrounding the fuel rod pellets gets hot enough and exothermic zirc/steam oxidation

reaction occurs ( Zr + 2H2O = ZrO2 + 2H2 ) which generates a lot of hydrogen gas, and this gas and the

steam generated due to boiling of the residual water in the core, builds up the pressure in the reactor

pressure vessel ( RPV). The pressure in the RPV was subsequently reduced, presumably by discharging

these gases into the water pool of the torodial shaped wet well of the Mark-I ( e.g., Unit-1)pressure

suppression containment ( see the lower part of the attached figure for details ). Hydrogen , being a

non-condensable gas , does not remain in the pressure suppression pool but collects in the upper wet

well and dry well regions of the Mark-I containment ( which was initially inerted with nitrogen gas)

causing it to become pressurized (see attached figure of this containment arrangement).

In order to prevent an over-pressurization of the containment structure, these gases were apparently

vented by the reactor operators into the reactor building near the spent fuel pool ( see the upper

portion of the attached figure). When hydrogen gas is vented, a combustible mixture may form. As a

consequence, a hydrogen explosion occurred which blew out the upper blow-out panels of the reactorbuilding (these types of chemical explosions have now happened at several of the plants in various

regions apparently including within the containments). Also during drywell venting some radioactive

gases (including radioactive Iodine 131 and Cesium 137) were released to the atmosphere but it

was found to be a relatively small amount and, to date, this and subsequent ventings have caused no

significant off-site health-related issues , even though some contamination of food stuff and drinking

water has been reported.

Next, in order to reflood the cores to keep the fuel cool the Japanese authorities decided to take an

unprecedented emergency response and injected sea water into the cores (later sea water was

apparently injection into the containment systems to fill region below the RPV ( for the Mark-Icontainments up to the height of the main vent pipes to the pressure suppression pool , about m).

This should promote cooling of any corium which may be release from the RPV, prevent failure of the

steel containment liner , scrub any radioactive aerosols form during concrete ablation by the corium,

and help reduce the potential for a hydrogen explosion within the containment. Nevertheless, the novel

feed-and-bleed method being used for core cooling appears to have been helpful, but it requires that

the steam formed due to boiling in the core be periodically vented and replaced with cold salt water.

Nevertheless, as long as the operators are able to remain onsite, this mode of long-term core cooling

can presumably continue and hopefully keep at least some of the damaged cores from achieving an

uncoolable geometry until either operation of the closed circuit RHR system is restored , the decay heat

level is reduced sufficiently, or the flow passages in the cores/RHR systems become plugged with salt (which, even though the melting temperature of salt is well below that of the core structures and

components, could compromise effective core cooling ). Anyway, as long as there is no breach in both

the plants containment and RPV there should be no significant radiation releases from the cores of

these stricken reactors to the environment ; only the amounts released during the periodic venting of

the RPVs/containments. This is something important that the media has so far failed to realize . Thus,

even if, as may have occurred, one or more of the containments has been penetrated due to hydrogen


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explosions, etc. , the RPV of these plants will contain the radioactive core rubble as long as the RPVs

remain intact. If not, a molten core can melt through the lower head of the RPV and drop down into the

dry well region of the containment, which, as noted previously, are filled with liquid to help freeze these

materials in place and to scrub the radioactive releases due to concrete ablation by the corium melt. In

particular, one way or the other, as long as the cores are kept submerged with water there should not

be gross dispersal of the core debris, and thus there will not be radioactive releases of the type that

occurred at Chernobyl (a Russian nuclear reactor which had no containment structure at all), in which

there was a massive amount of core melting which resulted in the vaporization and release directly to

the environment of previously solid fuel materials. Once released to the relatively cool atmosphere

these highly radioactive vapors formed an extremely dangerous aerosol fog that spread across and

contaminated a lot of Eastern Europe. As I said previously, this type of release will not occur at

Fukushima , indeed only gross contamination of the nuclear park should occur , and even this will be

greatly mitigated as long as the RPVs/containments are intact , long core cooling is properly maintained

and the lower part of the dry wells of the intact containments remains filled with water.

However, there are other important safety concerns that must be addressed. In particular, on3/14/2011 there was apparently a fire in the spent fuel pool (i.e., large open pools which resemble

swimming pools) of Fukushima Daiichi Unit 4, a reactor which was shut down for refueling prior to the

earthquake. This fire apparently was put out by the Japanese but it resulted in a relatively large release

of radioactivity to the local environment (i.e., 40 REM/hr onsite readings). Also, since these spent fuel

pools are not located within containment buildings, rather above the containment in the reactor

buildings, this radiation readily escapes to the environment. The most likely cause of this fire is that the

earthquake and/or subsequent related events caused the pool cooling and/or the normal water level to

be lost in the spent fuel pool thereby uncovering the previously submerged spent fuel. If this fuel still

has enough decay heat in it (since unit-4 was being refueled its core had been put into the pool last

November and is thus still quite radioactive) the zircalloy clad fuel rods can heat up sufficiently to causethe zircalloy to undergo a violent exothermic chemical reaction (with the steam and/or air which is

present) in which the heat of combustion can exceed the decay heat, creating a run-away oxidation

reaction; it will literally burn. Unfortunately, a propagating Zirc fire may liberate and disperse many of

the fission products stored in the spent fuel (which are often much larger than those in the cores),

thereby causing the observed spike in the onsite radiation level. It should be noted that the Japanese

use mixed oxide type fuel ( i.e., Plutonium as well as Uranium is used for fuel ), however it appears that

no fuel of this type is currently in the spent fuel pools at Fukushima ( i.e., just in the core of Unit-3).

Anyway, once the spent fuel pool fire is extinguished, the fix is to keep the spent fuel pool properly

cooled and full of water, which can be done in several ways , including with a fire hose. Unfortunately it

appears that this may not have been done after the initial fire was extinguished (perhaps because it was

felt that the fire may have been an oil fire instead). In any event a new fuel pool fire was reported

(3/16/11) and it has apparently been difficult to fight this fire because of the associated high radiation

levels and possibly the high upward velocity of the hydrogen/steam produced in the pool fire retarding

the flow of water into the pool. Also, there are six other spent fuel pools onsite that need to be

inspected and monitored to make sure that they are filled with water and adequately cooled.

Nevertheless, I would expect that the Japanese will eventually manage to quench this fire and hopefully


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will finally understand what caused it and how to prevent future fires. If so, this should not be a

significant source of radiation releases in the future. If not, significant radiation could be released to the

environment.

In my opinion, the expected end-state of this accident scenario should be one that is somewhat similar

to the accident at TMI: the affected power plants have highly damaged cores and the plants areessentially destroyed ( e.g., if by nothing else , by salt water injection into the RPV). However, the bulk

of the incore radioactivity should be contained within the plant boundaries ( even if corium is released

to the flooded containments). Moreover, I would expect that the spent fuel pools will be filled with

water, cooled and put into a stable state. However, it is important that those in charge of managing

this accident in Japan clearly understand what is going on and take the proper steps to mitigate things.

In this regard, the release of more accurate and timely information would allow them to get helpful

input from the scientific community, and to reduce the level of fear and anxiety that currently exists in

the public at large. In the future SWAT teams of nuclear reactor safety experts should be organized

within each country that have nuclear power plants to give the authorities timely advice during

emergencies and a similar international group should be organized and coordinated by the IAEA.

There have been some innovative and even heroic actions on the part of the utility (TEPCO) that owns

and operates the Fukushima reactors. They are to be congratulated on their continuing efforts to

contain this potentially serious nuclear accident and it is hoped that the government authorities in Japan

will let them continue to do their job. In addition, even though some workers and soldiers have been

injured by the earthquake/tsunami and subsequent hydrogen explosions, etc. (including one that

unfortunately died as a result of injuries sustained by the earthquake), and some onsite workers have

apparently had excess radiation exposure, to date, there has been no physical harm done to the general

public due to the nuclear emergency at Fukushima, but many have been badly frightened by the way

this nuclear emergency has been report to them by the media . Anyway, this is in stark contrast to themassive destruction and loss of thousands of lives attributed to the earthquake and tsunami. My

prayers go out to the Japanese people and I hope that my input will be helpful in mitigating the possible

consequences of the unfolding drama at Fukushima and in preventing future emergencies of this type

around the world.

Dr. Richard T. Lahey, Jr. NAE March 16, 2011 (Revised 4/28/11)

The Edward E. Hood Professor Emeritus of Engineering

Rensselaer Polytechnic Institute / Troy, NY USA

Domicile : St. Augustine, Florida / (904) 814-8071


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The Drama at Fukushima By Dr. Richard Lahey

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