GB2528268A

GB2528268A - Improved spent fuel handling in a molten salt nuclear reactor

Info

Publication number: GB2528268A
Application number: GB1412529.8A
Authority: GB
Inventors: Ian Richard Scott
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-07-15
Filing date: 2014-07-15
Publication date: 2016-01-20
Also published as: GB201412529D0

Abstract

A method is disclosed to move spent fuel tubes containing molten salt fuel in a molten salt nuclear reactor without lifting the part of the fuel tube containing fuel salt out from the coolant pool. The method can either involve using tubes of asymmetric diameter, i.e. narrower at the top and wider at the bottom, are raised to a level where the part of the fuel tube containing molten fuel salt remains submerged within the coolant but is high enough to move laterally between the upper narrow regions of the fuel tube array or where fuel tubes, or assemblies thereof, are removed from the periphery of the fuel tube array with the resulting gap in the array being migrated in such a way as to allow fresh fuel tubes to be added to the periphery of the array while ensuring an acceptably uniform distribution of fission rate is maintained across the array of fuel tubes.

Description

IMPROVED SPENT FUEL HANDLING IN A MOLTEN SALT NUCLEAR REACTOR

BACKGROUND

A novel design for a molten salt based nuclear reactor was disclosed in UK patent application number 1402908.6 entitled "A practical molten salt fission reactor". The basis for the design was to place the molten salt fissllc material in static tubes from which heat was transferred to a coolant liquid by a combination of conduction and convection.

One option to maintain the required fissile isotope concentration to achieve criticality in the array of fuel tubes as fissfle isotopes are consumed is to remove spent fuel tubes from the array and replace them with fresh fuel tubes. A challenge in doing this is that spent fuel tubes would typically be lifted out of the coolant liquid. Decay heat in spent fuel tubes can be substmtial and such a process can be rendered difficult and dangerous due to the high temperature that the fuel tube will reach when removed from the coolant. and the high flux of radiation that will be emitted by the fuel tube which was formerly substantially absorbed by the coolant.

A way to avoid this problem is described in this application whereby spent fuel tubes can remain immersed in the coolant but away from the neutron flux until the decay process has reached the point where the tubes can be handled safely and conveniently.

DESCRIPTION OF THE INVENTION

In the most efficiently designed reactor cores as described in 1402908.6 there is not sufficient space between fuel tubes for spent fuel tubes to be moved from the central region of the fuel tube array to the periphery without displacing other tubes. There are three basic ways of overcoming this problem which do not involve physically lifting fuel tubes out of the coolant.

In all methods described below, fuel tubes may be single tubes or assemblies of multiple tubes.

First is to use fuel tubes which, as described in 1402908.6, have a wider diameter lower region and a narrower upper region with the fuel salt being restricted to the wide thwer region. Provided there is sufficient depth of coolant, the spent fud tube can he raised so that the wider lower region can pass through the array of tubes at the level of the narrow upper region where there are wider gaps between the narrow regions of the tubes without the region of the fuel tube that contains the fuel salt emerging from the coolant.

Second is to migrate fuel tubes horizontally, removing one or more fuel tubes from the periphery and moving other fuel tubes outwards towards the periphery until the gap in the fuel tube array is close to the centre of the array. The direction of movement of the gap in the array is then reversed so that it moves back towards the periphery until it reaches the periphery and is filled with a fresh fuel tube. By suitable optimisation of the path of migration of the gap in the fuel tube array, a relatively uniform movement of fresh fuel tubes towards the centre of the array can he achieved eoupkd with either a uniform movement of spent fuel tubes out towards the periphery of the array or a more rapid movement of spent fuel tubes out approximately directly along a radius so that several narrow "streams" of spent fuel tubes leave the centre of the array. This latter method can be advantageous in maintaining a more uniform fission rate across the array of fuel tubes.

Third is particularly relevant to a rectangular or similar array of fuel tubes. If the anay has a long axis X and a short axis Y and a centre line parallel to the long axis of C then fuel tubes can be displaced horizontally from either end of the centre line C. The resulting gap is filled by migrating tubes along the centre line until a gap exists along line C which is then filled by migration of tubes parallel to the short axis Y leaving a gap ultimately at the edge of the array which is then filled with a fresh fuel tube. In this way a uniform movement of fuel tubes along axis Y can he achieved with the spent fuel tubes heing relatively quick'y removed from the array by migration along the centre line C. This last method has the substantial advantage of mechanical simplicity due to tubes moving in only one direction until they reach the centrc line then in a single direction to exit the array.

Once the fuel tube according to any of these methods has reached the periphery of the array it can be moved freely away from the array until it is no longer exposed to a significant neutron flux. It can he left in that position in the coolant tank until the rate of decay of fission products has declined to a suitalie level for the fuel tube to he Ii lied out of the coo'ant icr disposal or reprocessing of the fuel salt.

A central neutron reflector, and in particular a partially moderating neutron reflector, may be desired in order to compensate for fissile isotope consumption as fuel tubes approach the centre of the array so that a more uniform fission rate across the core is achieved. For a rectangular fuel tube array this reflector would be located along the centre line C. In this case, the rows of fuel tubes parallel to the long axis of the rectangle on either side of the central reflector would be used to transfer the spent fuel tubes out of the array in the same way as the single row along the centre line C described above.

Fuel tubes migrating to the centre line C close to the centre of the long axis would tend to experience longer times in the reactor core than those towards the end of the long axis as they spend more time migrating parallel to the centre line C. This can be compensated for by adding fresh fuel tubes more frequently at points in the array closer to the centre of the long axis.

Claims

CLAIMS1) A method to move spent fuel tubes containing molten salt fuel in a molten salt nuclear reactor without lifting the part of the fuel tube containing fuel salt out from the coolant pool whereby tubes of asymmetric diameter are raised to a level where the part of the fuel tube containing molten fuel sail remains within the coolant but is high enough to move laterally between the upper narrow regions of the fuel tube array or where fuel tubes, or assemblies thereof, are removed from the periphery of the fuel tube array with the resulting gap in the array being migrated in such a way as to allow fresh fuel tubes to be added to the periphery of the array while ensuring an acceptably uniform distribution of fission rate is maintained across the array of fuel tubes.
2) The method of claim I where the fuel tube array is approximately cylindrical and spent fuel tubes are removed from the penphery. the resulting gap in the array is migrated towards the centre of the array and then back towards the per phery where it is filled with a fresh fuel tube, the migration path being such as to result in movement of fresh fuel tubes towards the centre of the array and of spent fuel tubes towards the periphery of the aray.
3) The method of claim 2 where spent luel tubes move from the centre ol the array to the periphery in an approximately linear radial direction taking a short route to the periphery while fresh fuel tubes move in a less linear path so as to maintain an approximately uniform radial distribution of fuel tube reactivity 4) The method of claim 1 where the array of fuel tubes is approximately rectangular and spent fuel tubes are removed from (he ends of the row or rows of fuel tubes at or close to the centre line of the long axis of the array, the resulting gap in the fuel tube array is migrated some distance towards the centre of the centre line of the thng axis and is then filled by migration of fuel tubes in the column parallel to the short axis of the array resulting in the gap moving to the peripheral row of fuel tubes parallel to the long axis where the gap is then filled with a fresh fuel tube.