Spent Fuel Pools; How They Impact Fukushima Daiichi

The following is an in depth look at spent fuel pools and how these technical factors impact the disaster at Fukushima Daiichi.
By SimplyInfo member Dean Curtis

SPENT FUEL POOLS AT FUKUSHIMA

The spent fuel pools (SFP) at the Fukushima Daiichi units 1-4 have been uniquely designed for
each facility and located adjacent to the reactor to allow transfer of fuel from the reactor core to
the spent fuel pool all underwater. The SFPs each have a gate between the pool and the upper
part of the reactor drywell which can be removed to allow fuel transfers during reactor shutdowns
or installed during reactor operation. The general theory for the SFPs is to provide a temperature
and chemistry controlled water environment with specially designed fuel racks to house the
fuel removed from the reactor. The fuel elements which are just removed during a shutdown is
segregated to allow cooling to occur over a reactor operating cycle, typically 18-24 months. Some
of the elements may be used again, the others are stored permanently for 10-20 years and
then sent off for processing or to an interim central SFP or to dry storage in a cask.

SFP water must be maintained to the same water chemistry as the reactor which for BWR’s is
normally demineralized water with specific Ph, conductivity, particulate and biologic controls.
Other water chemistry controls are maintained to prevent unwanted corrosion of the fuel cladding
or biological effects such as algae growth. The water is routinely sampled to ensure all of the
chemistry requirements are met as well as integrity of the cladding material to prevent unwanted
degradation and prevention of unwanted hydrogen concentration buildup.

SFPs are also designed to store the fuel elements under water which acts as a shielding from
harmful radiation. Storage racks hold the fuel elements and maintain physical separation for
thermal controls as well as being raised off the bottom of the SFP with legs to allow convective
cooling in each fuel element. The racks also maintain sufficient separation and contain a reactor
poison material to prevent criticality. The fuel elements are moved from the reactor to the SFP
with aid of a crane as the fuel elements are heavy and would be difficult to handle manually. The
SFP is typically 40 feet deep and sized length and width for the specific reactor. There are usually
small drain lines between the stainless steel and concrete walls and floor which allow leakage
from the pool to be detected (called tell tale lines).

The SFP is lined with stainless steel which is surrounded on the sides and floor by 3-5 feet of
concrete. The SFPs do not have flow piping out the bottom which eliminates draining accidents.
Piping for the recirculation of water is performed with the use of skimmers on or below the
surface of the water line. Water is drawn from the SFP and circulated through a heat exchanger to
control temperature below 50C (120 F) and then typically through filters and special chemistry and
biologic control units to kill bacteria, then returned to the SFP. Radiolysis, the dissociation of
molecules by radiation, is of main concern and is controlled with the circulating water system.

The cooling water flow is usually about 900 gpm via an electric pump and the water is taken from
near the surface of the pool through a cleanup system then entering a heat exchange at about
100F. The heat exchanger then removes 15 degrees of heat and returns the water at about 85F to
the pool.

FUKUSHIMA POST ACCIDENT SPENT FUEL POOL STATUS

The most critical emphasis at a SFP is to keep water on the fuel at any costs. Evidence has
shown that the spent fuel pools at Fukushima have undergone some form of boil off, loss of
water, water displaced to heavy objects being dropped in (perhaps as big as a refueling crane)
etc. Early reports indicated that there could have been an explosion and some fuel melting in the
No 4 reactor and SFP debris being ejected from the No 3 SFP. As the work continues recently
more and more will be found out as workers uncover the main floor of the reactor and cameras
are able to get a good view of the SFP’s.

Chemistry control is another critical parameter that must be maintained at SFP to protect
unwanted corrosion of the fuel cladding material which, if corroded would release the inside
fuel contents to the SFP and/ or environment. To this writer’s knowledge, nothing has been
done to address the chemistry control of the SFP water to date. The main thrust is to get cooling
established and then inject boric acid (boron being seen as a poison or neutron absorber) by

the fuel. This cooling system has been quickly set up with external air heat exchangers, supply
sources such as tanks and well water, pumps and piping consisting of a combination of plastic or
metal pipe.

Nothing has been done to determine the effects of the poor quality of water on the cladding of the
fuel elements or establish sound corrosion rates. It is doubtful the cladding will survive the length
of time currently suggested to be able to remove the fuel from the SFP and reactor vessels.

In the interim period the SFPs for units 1-4 and reactor fuel contained in those SFPs are at the
moment the single biggest threat from an accident due to the same threats we protect nuclear
reactors from. The problem is they are teetering every day on the fence between guarded stability
or disastrous accidents.

Significant efforts need to be focused immediately to increase the robustness of temporary
cooling systems and efforts need to be made to begin to gain some chemistry control and sample
water used to cool the reactors to develop the best estimate of corrosion. This should be done in
part by lowering metal coupons into the canal with as close to the fuel cladding known at the time
of the accident to develop corrosion rates. Other monitoring mechanisms need to be developed
and implements to ensure every science is being used to help get the most life out of the cladding
within the fuel. Efforts must be made to know if fuel melting in the SFPs has indeed happened,
videos need to be made to perhaps use as some measure of the fuel condition inside the reactor
vessels.

All of this takes time, a huge investment in people , technology and engineering to accelerate the
process to more fully understand and plan a correct course of action.

This article would not be possible without the extensive efforts of the SimplyInfo research team
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