Reactor pressure vessel vented head

A head for closing a nuclear reactor pressure vessel shell includes an arcuate dome having an integral head flange which includes a mating surface for sealingly mating with the shell upon assembly therewith. The head flange includes an internal passage extending therethrough with a first port being disposed on the head mating surface. A vent line includes a proximal end disposed in flow communication with the head internal passage, and a distal end disposed in flow communication with the inside of the dome for channeling a fluid therethrough. The vent line is fixedly joined to the dome and is carried therewith when the head is assembled to and disassembled from the shell.

The present invention relates generally to boiling water nuclear reactors, 
and, more specifically, to a removable reactor pressure vessel head. 
BACKGROUND OF THE INVENTION 
A boiling water nuclear reactor (BWR) includes a pressure vessel containing 
a nuclear reactor core and water which is boiled thereby for generating 
steam for producing power such as by driving a turbine-generator for 
producing electrical power. The pressure vessel typically includes a 
cylindrical shell having a removable head at the top, and an integral head 
at the bottom thereof. The top head typically includes an arcuate dome 
having an integral mounting flange which mates with a complementary 
supporting flange on the shell. A plurality of circumferentially spaced 
apart bolts extend through the head flange into the shell supporting 
flange for bolting the upper head to the shell to form a pressure vessel 
capable of withstanding the relatively high pressures generated within the 
pressure vessel during operation of the reactor core. 
The upper head typically includes an integral nozzle at the top center 
thereof which is conventionally joined to a vent line, or a spray line, or 
both. The vent line communicates with the head nozzle for releasing or 
venting non-condensable gases such as nitrogen before startup of the 
reactor core, for example. And, the spray line communicates with the 
nozzle for injecting or spraying water therein for cooling purposes, for 
example. In both configurations, the vent and spray lines must be 
disconnected from the upper head prior to removing the upper head from the 
shell during a maintenance operation. This increases the complexity and 
time for accomplishing routine maintenance and increases the duration of 
radiation exposure to maintenance personnel. 
SUMMARY OF THE INVENTION 
A head for closing a nuclear reactor pressure vessel shell includes an 
arcuate dome having an integral head flange which includes a mating 
surface for sealingly mating with the shell upon assembly therewith. The 
head flange includes an internal passage extending therethrough with a 
first port being disposed on the head mating surface. A vent line includes 
a proximal end disposed in flow communication with the head internal 
passage, and a distal end disposed in flow communication with the inside 
of the dome for channeling a fluid therethrough. The vent line is fixedly 
joined to the dome and is carried therewith when the head is assembled to 
and disassembled from the shell.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Illustrated schematically in FIG. 1 is an exemplary boiling water nuclear 
reactor 10 which includes an annular pressure vessel 12 having a 
longitudinal centerline axis 14. Disposed inside the pressure vessel 12 is 
a conventional nuclear reactor core 16 submerged in reactor water 18 and 
effective for generating heat to boil the water 18 for generating steam 
18a. Disposed above the core 16 is a conventional assembly of steam 
separators 20 and in turn a steam dryer assembly 22 to remove moisture 
from the steam 18a. 
The pressure vessel conventionally includes a cylindrical shell 24 and a 
lower head 26 conventionally welded to the bottom of the shell 24, and an 
upper head 28 conventionally bolted to the top of the shell 24 and 
including an integral vent line 30 in accordance with one embodiment of 
the present invention. The shell 24 also includes a main discharge nozzle 
32 to which is conventionally joined a main steam outlet line 34 which 
channels the steam 18a to a conventional steam turbine-generator, for 
example, for generating electrical power. 
The upper portion of the pressure vessel 12 is illustrated in more 
particularity in FIG. 2. The upper head 28 includes an arcuate dome 36 
suitably configured for withstanding the relatively high pressures 
generated in the pressure vessel 12 and conventionally includes an 
integral annular mounting or head flange 38 disposed coaxially with the 
centerline axis 14 and extending around the perimeter of the dome 36. The 
shell 24 includes an integral annular supporting or shell flange 40 also 
disposed coaxially about the centerline axis 14 and extending around the 
top of perimeter of the shell 24 for supporting the upper head 28. 
As shown in more particularity in FIG. 3, the head flange 38 includes an 
annular and flat head mating surface 42 at its bottom which faces 
downwardly for sealingly mating with the shell 24 upon assembly therewith. 
More specifically, the shell flange 40 includes an annular and flat shell 
mating surface 44 at its top which faces upwardly for sealingly mating 
with the head mating surface 42. At least one and preferably two annular 
conventional head seals 46 are conventionally disposed between the head 
and shell mating surfaces 42, 44 for providing an effective seal to 
prevent leakage of the high pressure steam from within the pressure vessel 
12 from escaping therefrom between the head and shell flanges 38, 40. In 
the exemplary embodiment illustrated in FIG. 3, the head seals 46 are 
disposed in complementary recesses in the shell mating surface 44 and are 
suitably compressed by the head mating surface 42 when the upper head 28 
is installed on the shell flange 40. As shown in FIGS. 2 and 4, a 
plurality of circumferentially spaced apart, conventional head bolts 48 
extend downwardly through the head flange 38 and are threaded into the 
shell flange 48 for removably fixedly mounting the upper head 28 to the 
shell flange 40 at the top of the shell 24. 
In a conventional design, the upper head 28 is provided with an 
non-integral vent line which must be suitably disconnected from the upper 
head 28 prior to removing the upper head 28 from the shell 24. However, in 
accordance with the present invention, the vent line 30 as illustrated in 
FIG. 2, for example, is integrally joined to the upper head 28 and may be 
carried therewith when the head 28 is assembled to and disassembled from 
the shell 24. This may be accomplished in accordance with the present 
invention by suitably joining the vent line 30 in flow communication 
through the shell flange 40 to a stationary, primary vent line 50 
conventionally fixedly joined to the shell flange 40. In this way, the 
primary vent line 50 remains joined to the shell flange 40, whereas the 
vent line 30 disposed integrally with the upper head 28 is removable 
therewith. As shown in more particularity in FIG. 3, the head flange 38 
includes an internal head flow passage or conduit 52 extending through the 
head flange 38, with a first opening or port 52a being disposed on the 
head mating surface 42, and a second, opposite port 52b disposed on a top 
surface 54 of the head flange 38 spaced above the head mounting surface 42 
and parallel therewith. The head flange top surface 54 is disposed outside 
the dome 36 and also provides the mounting surface against which the 
several bolts 48 are mounted. 
Similarly, the shell flange 40 includes an internal shell flow passage or 
conduit 56 extending through the shell flange 40 with a shell first port 
56a being disposed on the shell mating surface 44 and aligned and disposed 
in flow communication with the head first port 52a. The shell internal 
passage 56 also includes a shell second port 56b disposed on a shell outer 
surface 58, which itself is disposed vertically and substantially 
perpendicularly to the shell mating surface 44. 
The vent line 30 includes a proximal end 30a disposed in flow communication 
with the head internal passage 52, and a distal end 30b disposed in flow 
communication with the inside of the dome 36 as shown in FIG. 2 for 
channeling a fluid 60 therebetween. In the exemplary embodiment 
illustrated in FIG. 2, the dome 36 includes an integral vent nozzle 62 at 
its top center which extends therethrough from a convex outer surface 36a 
of the dome 36 to a concave inner surface 36b of the dome 36. Also in the 
exemplary embodiment illustrated in FIG. 2, the vent line 30 is disposed 
externally of the dome 36 and extends along the dome outer surface 36a. 
More specifically, the vent line 30 extends between the top surface 54 of 
the head flange 38 as illustrated in FIG. 3 along the curvature of the 
dome 36 to the vent nozzle 62 as shown in FIG. 2. The vent line proximal 
end 30a is conventionally fixedly joined to the head flange 38, by welding 
for example, in flow communication with the second port 52b at the top 
surface 54 as shown in FIG. 3, and the distal end 30b as shown in FIG. 2 
is disposed in flow communication with the dome nozzle 62 by being 
conventionally fixedly joined thereto by welding for example. In this way, 
the fluid 60, which for example may be non-condensable nitrogen gas, may 
be vented from inside the dome 36 by being channeled through the dome 
nozzle 62, through the vent line 30 and in turn through the head flange 38 
and shell flange 40 through the respective internal passages 52 and 56 
therein. The fluid 60 is discharged through the shell flange 40 through 
the primary vent line 50 joined thereto. 
Accordingly, the upper head 28 may be vented through the integral vent line 
30 and in turn through the head and shell flanges 38, 40. Since the vent 
line 30 is fixedly joined to the dome 36 itself, it may be carried 
therewith when the upper head 28 is assembled to and disassembled from the 
shell 24. When the bolts 48 are removed from the head flange 38 the entire 
upper head 28 including the integral vent line 30 may be conventionally 
lifted away together, with the head first port 52a being automatically 
separated from the shell first port 56a breaking the connection 
therebetween. On reassembly of the upper head 28 to the shell flange 40, 
the head internal passage 52 is suitably realinged with the shell internal 
passage 56, with the respective first ports 52a, 56a thereof being 
realigned for reestablishing the flow connection therebetween. 
In the preferred embodiment of the invention illustrated in FIG. 3, the 
pressure vessel 12 further includes an annular vent seal 64 disposed 
between the head and shell mating surfaces 42, 44 around the respective 
first ports 52a, 56a thereof for restricting leakage of the fluid 60 from 
the vent line 30 and between the head and shell flanges 38, 40. In the 
exemplary embodiment illustrated, the vent seal 64 is suitably disposed in 
a complementary recess formed into the shell mating surface 44 which is 
effective for retaining the vent seal 64 therein during assembly and 
disassembly of the upper head 28, and which is suitably compressed upon 
assembly of the upper head 28 to the shell 24. 
Accordingly, the improved upper head 28 having the integral vent line 30 
therein provides an effective vent for the pressure vessel 12 and has a 
continuous flow passage to carry the fluid 60 to the stationary primary 
vent line 50, while allowing disassembly of the upper head 28 with the 
vent line 30 being carried therewith. The vent line 30, itself, therefore 
does not require separate disconnection as in prior art designs, but is 
automatically disconnected from the primary vent line 50 upon disassembly 
of the cooperating head and shell flanges 38, 40. 
Illustrated in FIGS. 5 and 6 is an alternate embodiment of the present 
invention wherein the vent line 30 is disposed internally of the dome 36 
and extends along the dome inner surface 36b. In this embodiment, the head 
flange 38 further includes an inside surface 66 facing radially inwardly 
and disposed above the head mating surface 42 and forms a portion of the 
dome inner surface 36b. And, the second port 52b of the head internal 
passage 52 is disposed on the inside surface 66 of the head flange 38. The 
vent line 30 extends from the flange inside surface 66 along the dome 
inner surface 36b preferably to the top center thereof as illustrated in 
FIG. 5, with the proximal end 30a being disposed in flow communication 
with the second port 52b at the flange inside surface 66 as shown in FIG. 
6, and the distal end 30b being open inside the dome 36 in flow 
communication therewith as illustrated in FIG. 5. In this way, the fluid 
60 is vented directly into the vent line distal end 30b and is carried 
through the vent line 30 and into the head internal passage 52 and in turn 
through the shell internal passage 56 for discharge through the primary 
vent line 50. 
In the exemplary embodiment illustrated in FIG. 6, the conventional head 
seals 46 are preferably spaced radially outwardly from the vent seals 64 
for providing an additional barrier against leakage of the fluid 60 
between the head and shell flanges 38, 40. The head seals 46 provide an 
effective barrier against leakage of the high pressure steam from inside 
the pressure vessel 12 through the joint between the head and shell 
flanges 38 and 40. By positioning the respective first ports 52a, 56a and 
the cooperating vent seals 64 radially inwardly of the head seals 46, any 
leakage therefrom will be further sealed by the head seals 46 themselves. 
In the embodiment of the invention illustrated in solid line in FIG. 3, the 
respective first ports 52a, 56a and vent seal 64 are disposed radially 
outwardly of the head seals 46. However, in an alternate embodiment shown 
in phantom, the head and shell internal passages 52, 56 may be suitably 
angled radially inwardly for positioning the respective first ports 52a, 
56a radially inwardly of the head seals 46 as in the embodiment 
illustrated in FIG. 6 to provide improved sealing of the vent line 30 if 
desired. In the embodiments disclosed above, the internal or external vent 
lines 30 are fixedly joined to the upper head 28 and, therefore, are 
removable therewith during assembly and disassembly of the upper head 28 
and the shell 24 with automatic disconnection between the head and shell 
internal passages 52, 56. This will decrease the amount of time required 
for removing and reinstalling the upper head 28 and therefore also reduces 
the time which maintenance personnel are subject to radiation exposure. 
Although the vent line 30 is preferably used for venting the fluid 60 from 
within the dome 36, it could also be used in alternate embodiments for 
providing a passage for channeling a fluid such as water for example into 
the pressure vessel 12 if desired. 
While there have been described herein what are considered to be preferred 
and exemplary embodiments of the present invention, other modifications of 
the invention shall be apparent to those skilled in the art from the 
teachings herein, and it is, therefore, desired to be secured in the 
appended claims all such modifications as fall within the true spirit and 
scope of the invention. 
Accordingly, what is desired to be secured by Letters Patent of the United 
States is the invention as defined and differentiated in the following 
claims: