Container for irradiated nuclear fuel

A container, termed a multi-element bottle, to receive irradiated nuclear fuel is housed within a flask for transport and both the bottle and flask contain water. Ullage space within the fuel chamber in the bottle and flask allow for thermal expansion under normal conditions. Additional ullage is provided as a safety measure in accident conditions involving fire and resulting rise in temperature with consequent increase in pressure within the bottle and the flask. The additional ullage space is provided by a closed chamber (10) at one end of the bottle, the chamber (10) being closed by a wall (11) which can collapse or rupture when the pressure exterior thereof exceeds a predetermined value. The normal ullage space within the fuel chamber (7) in the bottle can be increased by providing a further chamber (20) within the bottle between the fuel chamber (7) and the closed end chamber (10) having the collapsible wall with means (25) providing communication between the fuel chamber and the further chamber.

BACKGROUND OF THE INVENTION 
The present invention concerns a container for the storage and transport of 
nuclear fuel, in particular irradiated nuclear fuel after it has been 
removed from a nuclear reactor and before the fuel is reprocessed to 
separate reusable nuclear fuel materials from the products of nuclear 
fission. Such a container generally accommodates a plurality of nuclear 
fuel element assemblies and for convenience is termed a multi-element 
bottle (MEB). 
A MEB comprises an elongate cylindrical bottle closed at one end and having 
a removable lid at its opposite end, the lid being fitted with a seal 
which seats on a flange at the end of the bottle and being releasably 
secured to the flange by bolts. The bottle can accommodate a plurality of 
fuel element assemblies which can be located in compartments which allow 
for free circulation of cooling water whilst retaining the fuel in a 
criticality safe array. Typically the bottle can contain seven individual 
compartments but the number and arrangement of the compartments within the 
bottle is a matter of design choice. The bottle is contained within a 
flask for transport and both the bottle and the flask are filled with 
water. Ullage space within the bottle and the flask allow for thermal 
expansion of the water under normal conditions. 
In the event of an accident involving fire and resulting in a rapid rise in 
temperature with consequent rise in pressure it is required to provide 
additional ullage in order to reduce the pressure within the bottle and 
the flask. The invention provides a bottle having additional ullage for 
accident conditions. 
SUMMARY OF THE INVENTION 
According to the present invention a multi-element bottle to receive 
irradiated nuclear fuel comprises an elongate vessel divided internally 
into at least two chambers, a first chamber having a number of 
compartments to receive fuel assemblies and closed by a removable lid at 
one end of the vessel, a second chamber constituting an ullage compartment 
and closed at the opposite end of the vessel by a wall, at least a part of 
the wall being collapsible when the pressure exterior of the second 
chamber exceeds a predetermined value.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, multi-element bottle 
(MEB) comprises an elongate cylindrical vessel 1 having a removable lid 2 
secured to a flange 3 at one end of the vessel by bolts 4. The vessel and 
the lid are conveniently formed from stainless steel. The lid is fitted 
with remotely operable valves 5 which permit water level adjustment and 
flushing operations to be performed during normal fuel handling 
operations. 
A plate 6 is fixed permanently, as by welding, at a position spaced 
inwardly from the opposite end of the vessel. Chamber 7 formed between the 
lid 2 and the plate 6 contains a number of separate compartments 8 to 
receive nuclear fuel element assemblies together with neutron absorbing 
material. Conveniently the compartments 8 can be bounded by sheets 9 of a 
boron containing material and can be arranged in the form of a cruciform. 
The compartments are supported at one end by the flange 3 and by support 
plates intermediate the lid and the plate 6. The nuclear fuel element 
assemblies can be from a pressurised water reactor. The portion of the 
vessel extending beyond the plate 6 constitutes a cylindrical chamber 10 
which is closed by a wall formed by a further plate 11. The chamber 10 
constitutes an ullage compartment at the end of the vessel remote from the 
lid. 
In use to transport irradiated nuclear fuel element assemblies, the MEB is 
submerged upright in a fuel storage pond and the fuel element assemblies 
are loaded, under water, into the compartments 8 within the chamber 7. The 
lid 2 is then secured in position and the MEB is placed within a transport 
flask which is also filled with water to a level to provide ullage within 
the flask. A required volume of ullage (air space) can be obtained witn 
the chamber 7 by means of a dip pipe (not shown). The ullage spaces within 
the bottle and the flask allow for normal expansion of the water resulting 
from the heat emitted by the irradiated nuclear fuel assemblies. During 
transport the flask and the MEB are in a substantially horizontal 
orientation. 
The ullage within the bottle and the flask is sufficient to cater for the 
expansion of the water under normal conditions, that is the expansion 
resulting from the heat emitted by the fuel assemblies. However, in the 
event of an accident resulting in fire the consequent increase in 
temperature will produce a corresponding pressure increase within the 
flask and the bottle. At a certain predetermined pressure a bursting disc 
or discs in the lid will break to allow the water in the bottle to mix 
with the water in the flask. As a further safety precaution and to provide 
additional ullage the plate 11 will rupture or collapse at a predetermined 
pressure within the flask whereby the chamber 10 can accommodate the 
expansion of the water and thereby relieve the pressure. The plate 11, or 
a portion thereof, will be designed so as to rupture or collapse and 
release the additional ullage in the chamber 10 to thereby reduce the 
overall pressure within the flask and the MEB. In FIG. 1, the plate 11 is 
shown formed with a central region of reduced thickness constituting a 
bursting disc 12. The disc 12 can be protected from possible physical 
damage by a cover 13 supported on and spaced from the plate 11 by legs 14. 
The structure of the embodiment in FIG. 2 is similar to that of FIG. 1 and 
where appropriate the same reference numerals are used to denote 
corresponding parts. 
In the embodiment of FIG. 2, a further ullage chamber 20 is provided 
between the chambers 7 and 10. The chambers 20 and 10 are separated by a 
plate 21. Conveniently the plate 6 can be secured to and supported by a 
ring 22 which is fixedly secured, by welding, to the wall of the vessel. 
Similar supports can be provided for the plates 11 and 21. 
Pipes 23 and 24 extend between the lid 2 and the chamber 20. At the lid, 
the ends of the pipes 23 and 24 are secured in apertures in the flange 3 
and communicate with the valves 5 on the lid. The opposite ends of the 
pipes 23 and 24 pass through the plate 6 and open into the chamber 20 with 
the pipe 24 terminating a short distance from the plate 6. A further pipe 
25 provides communication between the chambers 7 and 20, this pipe 25 
passing through the plate 6 at a position adjacent the periphery of the 
plate. The end of the pipe 25 within the chamber 20 terminates at a 
position closer to the plate 21 than the corresponding end of the pipe 24. 
As before, the bottle is loaded under water and in an upright position. 
During loading of fuel into the compartments 8 within the chamber 7 water 
flows through the pipe 25 to fill the chamber 20. After loading the lid 2 
is secured in place and water in the chamber 20 is expelled by air under 
pressure applied to the pipe 23 to force the water out of the chamber 
through pipe 24. The expulsion of water continues until the level within 
the chamber 20 falls below the end of the pipe 24. As the end of the pipe 
25 is below the end of the pipe 24 the former is always submerged and the 
air introduced into the chamber through pipe 23 does not enter the chamber 
7 containing the fuel assemblies. The valves 5 at the ends of the pipes 23 
and 24 are closed and water within the chamber 7 can drain into the 
chamber 20 through pipe 25 until a balance is obtained and the trapped air 
volume within the chamber 20 supports the water in the chamber 7. 
The bottle is lifted into a flask filled with water for transport in a 
horizontal position. The bottle can be lifted by means of a pintle 26 on 
the lid and keyways 27 at the opposite end of the vessel cooperate with 
keys in the flask for location of the bottle within the flask. 
The chamber 20 provides additional ullage space within the bottle. Under 
normal conditions the ullage spaces within the compartments 6 and 20 cater 
for thermal expansion of the water. In any abnormal condition resulting in 
a rapid rise in temperature and consequential increase in pressure within 
the bottle the excess ullage space within the chamber 20 can relieve the 
pressure. As in the case of the FIG. 1 embodiment the lid is provided with 
a bursting disc or discs which will rupture if the pressure within the 
bottle exceeds a certain predetermined value to allow the water in the 
bottle to mix with the water within the flask and to utilise the ullage 
space within the flask. Finally, if the pressure within the flask exceeds 
a predetermined value the bursting disc 12 in the plate 11 will rupture to 
release the ullage space in the chamber 10. 
Although not shown in the drawings further inlet and outlet pipes can 
extend from valves on the lid to terminate within the chamber 7 adjacent 
the plate 6. Such pipes can be used for flushing the chamber 7.