Temporary cooling system and method for removing decay heat from a nuclear reactor

In combination with a nuclear power generating facility including a composite fuel pool including a reactor cavity and a spent fuel pool fluidly connectable to the reactor cavity, the composite fuel pool at least partially containing a primary fluid, a nuclear reactor vessel positioned in the reactor cavity, a residual heat removal system installed in the facility and fluidly connectable to the reactor vessel, and a spent fuel pool cooling system installed in the facility and fluidly connectable to the spent fuel pool, a temporary cooling system is provided, comprising a primary heat exchange system including a primary heat exchanger for transferring heat from a primary fluid to a secondary cooling fluid. The primary heat exchanger is temporarily locatable in the facility, and is temporarily fluidly connected to the composite fuel pool. A primary pump, also temporarily locatable in the facility, circulates primary fluid through the primary heat exchanger, which cools the primary fluid to a desired point at a faster rate than the spent fuel pool cooling system, allowing fuel to be immediately removed from the reactor rather than waiting for the residual heat removal system to cool the primary fluid to a point at which the spent fuel pool cooling system is able to provide adequate cooling capacity. Particulate filtration and demineralization may also be furnished with the system and method of the invention.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to cooling systems used to cool water in 
the nuclear reactor of nuclear power generating facilities and, more 
particularly, to temporary cooling systems which supplement existing 
cooling systems in such facilities. 
2. Prior Art 
In nuclear power generating facilities, nuclear fuel and water are 
contained in a reactor vessel positioned in what is commonly called a 
refueling cavity or a reactor cavity. During power generation, a primary 
fluid, normally water, is heated by the nuclear fuel, providing steam for 
electric power generation. During shutdowns for refueling and other 
periods when the reactor is not operating, decay heat from the fuel 
continues to heat the water in the reactor vessel. The water must be 
cooled to a desired level before fuel may be removed from the vessel and 
transferred to the spent fuel pool (SFP) of the facility via the reactor 
cavity. The reactor core is cooled of residual decay heat during shutdown 
by a permanently installed residual heat removal (RHR) system. It provides 
heat exchange cooling for decay heat coming from the fuel in the reactor 
core during shutdown. The heat removal capacity of this system is 
necessarily large. During normal shutdown, the RHR system is operated for 
a number of days in order to remove decay heat from the fuel to a point 
where it may be removed from the core. This is due to the fact that the 
SFP, the eventual storage place for the fuel, has a permanently installed 
cooling system, the SFP Cooling System, which does not have sufficient 
cooling capacity to remove the high level of residual heat immediately 
following plant shutdown. 
Thus, in situations requiring removal of the fuel from the reactor core, 
the permanent cooling system configuration in present-day nuclear plants 
requires that the RHR system be operated for a period of days in order to 
cool the fuel such that it may then be removed to the SFP, then allowing 
reactor servicing, such as fuel replacement or the decontamination of 
components such as the reactor recirc system (RRS). The current practice 
prior to the instant invention was simply to wait until cooling by the RHR 
system was complete and then proceed to remove the fuel. This increased 
the facility shutdown period by the number of days required for such 
cooling, thus increasing the cost of the shutdown operation, lost 
revenues, as well as the cost of replacement power purchased during the 
shutdown. The cost of replacement power alone is currently measured in 
hundreds of thousands of dollars per day. However, permanently increasing 
the capacity of the SFP cooling system is inordinately expensive and 
impractical. It is therefore the accepted practice to continue with 
lengthy prior art cooling methods using the existing systems. 
SUMMARY OF THE INVENTION 
Considering the prior art problems discussed above, it is an object of this 
invention to provide a temporary cooling system and method for removing 
decay heat from a nuclear reactor which allow a temporary connection to be 
made to either the spent fuel pool or the reactor cavity of a nuclear 
reactor for supplemental cooling of the primary fluid within the SFP 
and/or the reactor, accomplished with temporarily placed cooling 
equipment. 
It is another object of this invention to provide a temporary cooling 
system and method for removing decay heat from a nuclear reactor wherein 
the primary fluid is both cooled and filtered for particulate matter. 
It is yet another object of this invention to provide a temporary cooling 
system and method for removing decay heat from a nuclear reactor wherein 
the primary fluid is both cooled and demineralized. 
Accordingly, in combination with a nuclear power generating facility 
including a composite fuel pool including a reactor cavity and a spent 
fuel pool fluidly connectable to the reactor cavity, the composite fuel 
pool at least partially containing a primary fluid, a nuclear reactor 
vessel positioned in the reactor cavity, a residual heat removal system 
installed in the facility and fluidly connectable to the reactor vessel, 
and a spent fuel pool cooling system installed in the facility and fluidly 
connectable to the spent fuel pool, a temporary cooling system is 
provided, comprising a primary heat exchange system including a primary 
heat exchanger for transferring heat from a primary fluid to a secondary 
cooling fluid. The primary heat exchanger is temporarily locatable in the 
facility, and is temporarily fluidly connected to the composite fuel pool. 
A primary pump, also temporarily locatable in the facility, circulates 
primary fluid through the primary heat exchanger, which cools the primary 
fluid to a desired point at a faster rate than the spent fuel pool cooling 
system, allowing fuel to be immediately removed from the reactor rather 
than waiting for the residual heat removal system to cool the primary 
fluid to a point at which the spent fuel pool cooling system is able to 
provide adequate cooling capacity. Particulate filtration and 
demineralization may also be furnished with the system and method of the 
invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
As shown in FIG. 1, in a typical nuclear power generating facility 1 (only 
the pertinent parts are shown), a reactor building 23 contains a reactor 
vessel 2, which contains a core 3, which comprises numerous elements of 
nuclear fuel 4, usually in the form of fuel bundles. During power 
generating operations reactor vessel 2 is closed using top 5. Reactor 
vessel 2 is positioned within a reactor cavity 6, which is fluidly 
connected to a spent fuel pool (SFP) 7 during outages. In the facility 
embodiment shown, the SFP 7 is separated from the reactor cavity by a wall 
8, having a closeable opening 9, closeable by a gate (not shown) or other 
means known in the art so as to isolate the SFP 7 from the reactor cavity 
6. Since various embodiments of facilities 1 are possible, the SFP 7 and 
reactor cavity 6 will be jointly referred to as the "composite fuel pool" 
10, which will refer to any point within either the SFP 7 or the reactor 
cavity 6. An example of an alternate embodiment of the composite fuel pool 
10 is one wherein the SFP 7 and reactor cavity 6 are separated by a 
conduit (not shown) rather than a wall 8. The SFP 7 typically contains 
fuel racks 11, which support spent fuel bundles which are stored in the 
SFP 7. 
During power generating operations, top 5 is closed and primary fluid 12 
(normally water) is contained within reactor vessel 2 at an operating 
level 13 above core 3. The core 3 heats primary fluid 12, generating steam 
which is used to generate electric power. The extensive piping and 
additional apparatus used for generating power is not relevant to the 
instant invention and is thus not shown. The reactor recirc system (RRS) 
14 recirculates water within the reactor vessel 2, and is fluidly 
connected to the residual heat removal (RHR) system 15 during shutdown 
periods. In the facility embodiment shown in FIG. 1, the RRS 14 includes 
an "A" loop 16 and a "B" loop 17. Circulation is maintained by recirc 
pumps 18. Valves 19 provide isolation of the RRS 14 from the RHR system 
15. Of course, many different configurations of piping and valves are 
possible, and vary from facility to facility. 
The facility 1 may be shut down for various reasons, including total or 
partial fuel replacement, decontamination of components, or for other 
reasons. Detailed shutdown procedures are required in order to maintain 
system safety. In order to remove fuel bundles 4 from the core 3, top 5 is 
removed and the level of primary fluid 12 is raised to a refueling level 
20 within the composite fuel pool 10. Following this step the closeable 
opening 9 is in an open position, allowing primary fluid 12 to equalize 
refueling level 20 within both the SFP 7 and reactor cavity 6. Once 
refueling level 20 is stable, the fuel bundles 4 may be lifted from the 
core 3 and placed in fuel racks 11. However, the initial decay heat from 
the fuel bundles 4 must be removed during this procedure. 
Once the core 3 is shut down, decay heat continues to be generated by the 
fuel 4. The RHR system 15 is designed to provide heat exchange to cool the 
primary fluid 12, removing the tremendous initial decay heat generated 
upon system shutdown. As shown by flow arrows 21, the RHR system 15 cools 
primary fluid 12 and recirculates the cooled primary fluid 12 back to the 
reactor vessel 2. As stated above, the prior art method of cooling the 
primary fluid 12 requires operation of the RHR system 15 for a number of 
days until the initially large amount of decay heat is removed from the 
primary fluid 12. The amount of heat removed during RHR system operation 
can be on the order of 50,000,000 BTU/hr. The RHR system 15 was operated 
until the fuel bundles 4 were cooled to a point where they could be 
removed to the SFP 7, where the smaller capacity SFP cooling system 22 
would continue to circulate primary fluid from the SFP 7 (see flow arrows 
26) and remove the decay heat at a much smaller rate, for example 
1,000,000 BTU/hr. The RHR system 15 and SFP cooling system 22 are 
permanently installed in the facility 1. Due to the permanent nature of 
the installation, as well as safety, redundancy, licensing and 
contamination problems, modification of the permanent cooling systems 
would be impractical and overly expensive. 
The temporary cooling system 30 provides immediate increased cooling 
capacity with no additional permanent connections to the facility 1. The 
invention 30 comprises a primary heat exchange system 31, which includes a 
primary heat exchange means 32 for transferring heat from primary fluid 12 
to a secondary cooling fluid, a primary pump 33, a primary pump suction 
line 34, a primary pump discharge line 35 and a primary return line 36. 
Primary fluid 12 is circulated in primary heat exchange system 31, where 
heat is transferred to a secondary cooling fluid from a secondary heat 
exchange system 37. All heat exchange equipment, pumps and other 
components of the invention 30 may be mounted on skids 55 (see FIG. 3) and 
temporarily located within the facility 1. Due to severe space limitations 
within facilities 1, components of the invention 30 may be located in 
various places within a facility 1, as shown in FIG. 2. Due to radioactive 
particles circulating in the primary heat exchange system 31, it is 
preferable to locate the primary heat exchange system 31 within the 
containment of building 23. FIG. 2 shows a building 23 schematically (with 
walls removed for clarity), with the primary heat exchange system 31 
located within the building 23 near a stairwell 24. Stairwell 24 provides 
an opening for primary pump suction line 34 and primary return line 36. 
Secondary heat exchange system 37 may be positioned at a point on the 
exterior of the building 23, such as a roof area 25. In this case, 
building penetrations 38 will need to be provided for secondary cooling 
fluid supply line 39 and secondary cooling fluid return line 40. 
Alternately, secondary heat exchange system 37 may also be positioned at a 
point on the interior of the building 23. 
A more detailed schematic of a preferred embodiment of the invention 30 is 
shown in FIG. 3. As can be seen, the system 30 is provided with some 
redundancy in order to assure adequate heat exchange capacity. Thus, two 
primary pumps 33, two primary heat exchangers 41, two secondary pumps 42 
and two secondary heat exchange means 43 are provided. One or both of each 
of these components (if properly sized for the desired heat transfer rate) 
will adequately function in the system 30. The primary heat exchange means 
32 preferably comprises a primary heat exchanger 41. It has been found 
that a plate-type heat exchanger (such as a Graham Manufacturing Company, 
Inc. Model No. UFX-51 plate heat exchanger) works well for this 
application, although other heat exchange means known in the art, such as 
chillers, may be used. Primary heat exchangers 41 each have a primary 
inlet 45, a primary outlet 46, a secondary inlet 47 and a secondary outlet 
48. Primary inlets 45 are fluidly connected to outlet ends 49 of primary 
pump discharge line 35, and primary outlets 46 are fluidly connected to 
inlet ends 50 of primary return line 36. Secondary inlets 47 are fluidly 
connected to outlet ends 51 of secondary cooling fluid supply line 39, and 
secondary outlets 48 are fluidly connected to inlet ends 52 of secondary 
cooling fluid return line 40. For the purposes of this disclosure, the 
terms "fluidly connected" or "fluidly connectable" refer to the ability 
for fluid to flow from one element to another element. There may be 
numerous components, such as piping, valves, pumps, measuring devices, 
etc. interposed between such elements, which are not necessarily claimed 
as part of the invention 30 and which are simply part of the fluid 
connection or potential fluid connection. 
Primary pumps 33 each have an inlet 53 and an outlet 54. Primary pump 
suction line 34 has an inlet end 56 removably and fluidly connected to 
composite fuel pool 10. Both primary pump suction line 34 and primary 
return line 36 are shown connected to the SFP 7, but may also be connected 
anywhere in the composite fuel pool 10, depending upon the desired flow 
dynamics for the particular facility 1 in which the invention 30 is 
installed. For example primary pump suction line 34 could be connected to 
the reactor cavity 6 and primary return line 36 could be connected to the 
SFP 7. Outlet ends 57 of primary pump suction line 34 are fluidly 
connected to primary pump inlets 53. Inlet ends 58 of primary pump 
discharge line 35 are fluidly connected to primary pump outlets 54. Thus, 
primary pumps 33 draw primary fluid 12 from composite fuel pool 10, 
circulate it through primary heat exchanger 41 and return it through 
outlet end 59 of primary return line 36, which is removably and fluidly 
connected to composite fuel pool 10. 
The invention 30 may include an anti-siphon means 60 for preventing 
siphoning of primary fluid 12 from composite fuel pool 10 when primary 
fluid 12 falls below an undesirable level 62. Anti-siphon means 60 is 
connected to a portion of primary pump suction line 34 which is 
submersible in composite fuel pool 10, as shown in FIG. 6. Anti-siphon 
means 60 may take the form of one or more anti-siphon holes 61 in primary 
pump suction line 34, as shown in FIG. 6. It is also desirable to reduce 
turbulence from primary fluid 12 reentering composite fuel pool 10 through 
primary return line 36. This may be accomplished by providing a flow 
distribution means 63, connected to a portion of primary return line 36 
which is submersible in composite fuel pool 10, for distributing return 
flow of primary fluid 12 to composite fuel pool 10. Flow distribution 
means 63 may comprise a plurality of flow distribution holes 64 provided 
in primary return line 36, as shown in FIG. 5. It has been found that one 
or more vertical rows of holes 64, facing away from wall 65 of composite 
fuel pool 10 will function adequately. Force from the exiting primary 
fluid 12 will maintain pipe support 68 in an abutted position against wall 
65 for added stability. Of course flow through the end opening 66 of 
primary return line 36 should be restricted by an orifice plate 67 or 
other means known in the art in order to force return flow through flow 
distribution holes 64. The orifice in orifice plate 67 should be 
approximately the same size as holes 64. 
Primary fluid 12 normally contains particulate matter which accumulates in 
the core 3, composite fuel pool 10 and system piping. Since the 
particulate matter is exposed to the fuel 4, it becomes radioactive and 
will contaminate primary heat exchange means 32. Therefore, it is 
preferable that one or more particulate filters 69 be fluidly connected 
between composite fuel pool 10 and primary heat exchange means 32 (either 
in primary pump suction line 34 or primary pump discharge line 35). As 
shown in FIG. 3, it is preferable that particulate filters 69 be located 
in primary pump discharge line 35 between primary pumps 33 and primary 
heat exchange means 32. Particulate filters 69 may take any form known in 
the art, and preferably comprise remotely or semi-remotely removable 
filter cartridges, such as Filterite pleated polyethylene or cloth wound 
filter cartridges. An additional advantage to utilizing particulate 
filters 69 is the simultaneous filtering and resulting decontamination of 
SFP 7 and/or reactor cavity 6 during operation of the temporary cooling 
system 30, saving further time and resulting facility outage normally 
associated with separate and independent filtration operations. 
Similarly, it may also be preferable to conduct simultaneous cooling, 
filtration and demineralization of primary fluid 12. A demineralization 
means 70 is thus provided for removing undesirable minerals from primary 
fluid 12. Demineralization means 70 is fluidly connected between composite 
fuel pool 10 and primary heat exchange means 32 (either in primary pump 
suction line 34 or primary pump discharge line 35), preferably between 
particulate filters 69 and primary heat exchange means 32, as shown in 
FIG. 3. Demineralization means 70 may take any form known in the art, such 
as the ion exchange vessel 71 shown in FIG. 4. The vessel 71 is provided 
with a resin fill inlet 72, a process inlet pipe 73, a process outlet pipe 
74, a sluice outlet pipe 75, inlet screen 76 and outlet screens 77. The 
vessel 71 is filled with ion exchange resin 78, such as Purolite ion 
exchange bead resin. Inlet pipe 73 and outlet pipe 74 are fluidly 
connected to the temporary cooling system 30 such that primary fluid 12 
flows through inlet pipe 73 and inlet screen 76, then downward through 
resin 78 where it is demineralized, then out through outlet screens 77 and 
outlet pipe 74 and back to the temporary cooling system 30. Sluice outlet 
pipe 75 is used to remove spent resin 78. 
Any source of secondary cooling fluid 81 (such as water or freon) may be 
supplied to primary heat exchange means 32. As shown in FIG. 3, such a 
source may comprise a secondary heat exchange system 37. Secondary heat 
exchange system 37 includes secondary heat exchange means 43 for cooling 
secondary cooling fluid 81, which may comprise any suitable secondary heat 
exchangers 44, such as cooling towers. It was found that Baltimore Aircoil 
Company Series V cooling towers, Model VT1-N346-Q worked well in a test 
application. Circulation in secondary heat exchange system may be provided 
by secondary pumps 42, or other means, such as gravity. Secondary heat 
exchangers 44 are provided with secondary cooling fluid inlets 79, fluidly 
connected to outlet ends 82 of secondary cooling fluid return line 40, and 
secondary cooling fluid outlets 80, fluidly connected to inlet ends 83 of 
secondary cooling fluid supply line 39. Secondary pumps 42 may be fluidly 
connected in secondary cooling fluid supply line 39, as shown, or 
elsewhere in the circuit as necessary, depending upon the location and 
type of secondary heat exchange means 32 employed. 
As shown in FIG. 3, the substantial portion of secondary heat exchange 
system 37 may be located outside of containment walls 84. Therefore, it is 
desirable that potentially radioactive primary fluid 12 be prohibited from 
entering the secondary heat exchange system 37. A regulator means 85 is 
thus provided for maintaining an operating pressure of secondary cooling 
fluid 81 higher than the operating pressure of primary fluid 12. One 
embodiment of regulator means 85 is a backpressure valve 86, fluidly 
connected in secondary fluid return line 40 between primary heat exchange 
means 32 and secondary heat exchange means 43. Backpressure valve 86 may 
be set to maintain an upstream pressure greater than that of the primary 
heat exchange system 31 such that, if a leak occurs in primary heat 
exchanger 41, secondary fluid 81 will flow into primary heat exchange 
system 31, maintaining primary fluid 12 within the reactor building 23. 
Backpressure valve 86 may be of the type manufactured by the Ames Company, 
Model A820. Regulator means 85 may also include a system shutdown feature 
such as differential pressure transmitter 89 which will shut down and 
isolate both the primary and secondary heat exchange systems if the 
pressure in secondary heat exchange system 37 is not greater than the 
pressure in primary heat exchange system 31. 
Operation of the system 30 may be observed in FIGS. 1 and 3. Primary fluid 
circulation is shown by flow arrows 87, and secondary cooling fluid 
circulation is shown by flow arrows 88. Initially, top 5 of reactor vessel 
is removed and the level of primary fluid 12 is raised from operating 
level 13 to refueling level 20, at least partially filling reactor cavity 
6 and/or SFP 7 with primary fluid 12. The temporary cooling system 30, 
which has been temporarily positioned in the facility 1 on skids 55, is 
then operated as a partial or full off-load of fuel bundles 4 takes place. 
Primary fluid 12 is circulated within primary heat exchange system 31, 
transferring heat from primary fluid 12 at a faster rate than that 
attainable by the SFP cooling system 22. Secondary cooling fluid 81 is 
circulated in secondary heat exchange system 37, removing the transferred 
heat from the secondary cooling fluid 81. During the cooling of primary 
fluid 12, particulate matter is filtered by particulate filters 69, and 
demineralization is accomplished by demineralization means 70. Circulation 
is maintained in order to maintain a desired temperature of primary fluid 
12 during the outage. 
In a test application, primary fluid 12 was circulated at approximately 
3,000 gallons per minute in primary heat exchange system 31, with primary 
heat exchangers 41 maintaining primary fluid 12 at a safe temperature 
during a full core off-load. The operation of the temporary cooling system 
30 eliminated seventeen days of pre-cooling by the RHR system 15, saving 
millions of dollars in replacement power costs without the expense or 
complication of additional permanently installed equipment. Other 
embodiments of the invention will occur to those skilled in the art, and 
are intended to be included within the scope and spirit of the following 
claims.