Neutron fluence surveillance capsule holder modification for boiling water reactor

An apparatus for carrying out a surveillance program to monitor the neutron fluence and its effect on vessel materials at a position in the annular space between the pressure vessel and core shroud of a boiling water reactor for the purpose of monitoring vessel embrittlement. The apparatus includes an offset capsule holder assembly which fits in an existing capsule holder attached to the inner surface of the pressure vessel wall. The offset capsule holder assembly positions a new capsule holder radially closer to the core, by an amount determined by neutron transport calculations. The new capsule holder is geometrically identical to the original, or a "replacement in kind", allowing the original surveillance capsules to be immediately reinstalled. With the water moderator in the downcomer annulus, the fluence rate increases significantly when the surveillance capsule is moved radially inward from the pressure vessel inside surface toward the core.

FIELD OF THE INVENTION 
This invention is directed to devices that measure and monitor the neutron 
fluence inside a light-water nuclear reactor. In particular, the invention 
relates to devices which measure the neutron fluence and its effect on 
vessel materials at various positions in the annular space between the 
pressure vessel and core shroud of a boiling water reactor for the purpose 
of monitoring vessel embrittlement. 
BACKGROUND OF THE INVENTION 
One type of conventional boiling water reactor, the BWR/6, is shown in FIG. 
1. During operation of the reactor, coolant water circulating inside a 
reactor pressure vessel 10 is heated by nuclear fission produced in the 
nuclear fuel core 20. Feedwater is admitted into the reactor pressure 
vessel 10 via a feedwater inlet 12 and a feedwater sparger 14. The 
feedwater flows downwardly through the downcomer annulus 16, which is an 
annular region between reactor pressure vessel 10 and a core shroud 18. 
The core shroud 18 is a stainless steel cylinder which surrounds the 
nuclear fuel core 20. The fuel core is made up of a multiplicity of fuel 
bundle assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 
1). Each array of fuel bundle assemblies is supported at the top by a top 
guide 24 and at the bottom by a core plate 26. The coolant water flows 
downward through the downcomer annulus 16 and into the core lower plenum 
25. The water in the lower plenum in turn flows upward through the fuel 
core 20. In particular, water enters the fuel assemblies 22, wherein a 
boiling boundary layer is established. A mixture of water and steam exits 
the fuel core and enters the core upper plenum under the shroud head 28. 
The steam-water mixture then flows through standpipes 30 on top of the 
shroud head 28 and enters the steam separators 32, which separate water 
from steam. The water is recirculated back to the downcomer annulus and 
the steam flows out of the RPV and to the gas turbines (not shown). 
The BWR/6 also includes a coolant recirculation system which provides the 
forced convection flow through the core necessary to attain the required 
power density. A portion of the water is sucked from the lower end of the 
downcomer annulus 16 via recirculation water outlet 34 and forced by a 
centrifugal recirculation pump (not shown) into a plurality of jet pump 
assemblies 36 (only one of which is shown) via recirculation water inlets 
38. The jet pump assemblies are circumferentially distributed around the 
core shroud 18 and provide the required reactor core flow. A typical BWR/6 
has 16 to 24 inlet mixers. 
During operation of reactors of the foregoing type, various reactor 
parameters are measured and monitored to ensure safe operation. In 
particular, Federal regulations require the institution of a material 
surveillance program for the purpose of monitoring changes in the fracture 
toughness properties of ferritic materials in the reactor vessel beltline 
region of light-water nuclear power reactors resulting from exposure of 
these materials to neutron irradiation and the thermal environment. Under 
the material surveillance program, fracture toughness test data are 
obtained from material specimens exposed in surveillance capsules. In 
accordance with the requirements of Appendix H of 10 CFR, Part 50, the 
surveillance specimen capsules must be located near the inside vessel wall 
in the beltline region so that the specimen irradiation history 
duplicates, to the extent practicable within the physical constraints of 
the system, the neutron spectrum, temperature history and maximum neutron 
fluence experienced by the reactor vessel inner surface. The capsule 
holders can be attached to the vessel wall or to the vessel cladding. The 
design and location of the capsule holders must permit periodic removal of 
the capsules. 
The BWR/6 and Nine Mile Point 2 (NMP-2) plants have a surveillance capsule 
design different from other boiling water reactors. A major difference is 
the compactness of the BWR/6 and NMP-2 capsules. This compactness, in 
turn, created the requirement to reach further down into the vessel to 
remove the capsules. As a result, the designed locations of the 
surveillance capsules were intended to facilitate removal during an outage 
and do not provide the optimum level of irradiation of the specimens. 
At different azimuthal locations in the vessel, there are peaks and valleys 
in the neutron fluence. Ideally, the surveillance capsules should be 
located at a peak, leading the accumulation of fluence on the vessel (thus 
the term "lead factor"). In the subject group of plants, the capsules are 
located in or near fluence valleys, so that the capsules lag the vessel in 
fluence accumulation. 
Currently, NRC regulation 10 CFR, Part 50, Appendix H, references ASTM 
Standard E185, which specifies that the lead factor should be between 1 
and 3. This was not specified at the time of BWR/6 design, and in the 
cases of BWR/6 and NMP-2 reactors, the lead factor relative to the vessel 
inside surface is about 0.3-0.7. 
Thus, there is a need to design a modification to the surveillance capsules 
for the BWR/6 and NMP-2 nuclear power plants to increase the lead factor 
to a value to be determined on a plant-by-plant basis. The modification 
should achieve the desired lead factor (1-3) at minimal cost with minimal 
outage time. 
SUMMARY OF THE INVENTION 
The present invention is a device for carrying out a surveillance program 
to monitor radiation embrittlement in the reactor pressure vessel. The 
proposed idea is to develop a design modification for the BWR/6 and NMP-2 
surveillance programs to accelerate the accumulation of future fluence 
(i.e., increase the lead factor). The similarity of the surveillance 
capsules, and their brackets on the vessel wall, will allow one 
modification concept to be applicable to NMP-2 and all BWR/6's, with 
possible minor subtleties addressed in plant-specific variations. 
The current capsule holder configuration for the NMP-2 and BWR/6 nuclear 
power plants is against the inside surface of the reactor vessel, near the 
midcore height. The capsule holder is configured to receive and hold a 
remotely insertable surveillance capsule in a generally upright position. 
The capsule is installed on the vessel wall and left in place for a 
predetermined period of reactor operation time for the purpose of 
monitoring the neutron fluence. After the surveillance period has 
terminated, the capsule can be removed from the reactor. Then the 
specimens are removed from the capsule and examined in a laboratory to 
determine the neutron fluence and its effect on vessel materials. 
The surveillance capsule lead factor could be increased by changing the 
azimuthal location, but this would be costly and technically difficult. 
The same goal of increasing fluence, and thus, increasing the lead factor, 
can be accomplished in accordance with the preferred embodiment of the 
invention by moving the surveillance capsule radially closer to the core. 
With the water moderator in the downcomer annulus, the fluence rate 
increases significantly when moving radially inward from the pressure 
vessel inside surface toward the core. This increase in fluence rate is 
shown schematically in FIG. 2. 
The preferred embodiment of the present invention is an offset capsule 
holder assembly which fits in an existing capsule holder attached to the 
inner surface of the pressure vessel wall. The offset capsule holder 
assembly positions a new capsule holder radially closer to the core, by an 
amount determined by neutron transport calculations. The new capsule 
holder is geometrically identical to the original, or a "replacement in 
kind", allowing the original surveillance capsules to be immediately 
reinstalled. 
The surveillance capsule holder modification of the present invention 
applies to the BWR/6, NMP-2 type of capsule holder design. It requires no 
special tools to install or remove either the capsule holder or the 
capsule itself. The invention provides a quick and relatively low cost way 
to proactively improve the surveillance capsule program at the BWR/6 and 
NMP-2 nuclear power plants. Plant operators will be able to customize the 
capsule position to meet their plant specific needs, which may include 
license renewal. The "replacement in kind" approach allows the plant 
operators to use existing surveillance capsules, thus simplifying the 
option of future capsule reinstallations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 3, the preferred embodiment of the capsule holder 
modification in accordance with the invention comprises an offset capsule 
holder assembly 40 which is inserted in and supported by a capsule holder 
42 affixed to the inner surface of the pressure vessel 10. The offset 
capsule holder assembly 40 comprises an offset capsule holder 44 
preferably having the same configuration as that of the capsule holder 42. 
However, it will be apparent that the offset capsule holder 44 may differ 
somewhat from the capsule holder 42 as long as the offset capsule holder 
44 is capable of performing its function of holding the same type of 
surveillance capsule 46 which the capsule holder 42 was designed to hold. 
The offset capsule holder assembly 40 further comprises a positioning arm 
assembly 48 having a generally U-shaped profile. As best seen in FIG. 5, 
the positioning arm assembly 48 comprises a holding lug 50 welded on one 
side to an end of a left arm 52 and welded on the other side to an end of 
a right arm 54. Preferably, the left and right arms are mutually parallel, 
so that the offset capsule holder assembly will hold the offset capsule 
holder in a position which is displaced relative to, but not rotated 
relative to, the position of the capsule holder 42. 
The offset capsule holder 44 is mounted to the left arm 52 of the 
positioning arm assembly 48. As seen in FIGS. 5 and 7, the left arm 52 
takes the form of a bar. The left arm 52 has a generally straight portion, 
to which the offset capsule holder 44 is attached, and a curved portion, 
the distal end of which is welded to the holding lug 50. Similarly, the 
right arm 54 has a generally straight portion and a curved portion, the 
distal end of which is welded to the holding lug 50. The generally 
straight portion of right arm 54 has a width which varies along the arm 
axis, i.e., an upper increased-width portion 58 and a lower 
increased-width portion 60. The width of portions 58 and 60 is slightly 
less than the width of the interior space of the capsule holder 42, so 
that portions 58 and 60 act as bearing surfaces which bear against the 
opposing surfaces inside the capsule holder 42 and hold the offset capsule 
holder assembly 40 upright. 
The capsule holder 42 comprises a front plate 62, a channel 64 and a bottom 
plate 66 welded together to form a holder in the shape of a parallelepiped 
which is open at the top. The channel 62 has a pair of opposing sides 
connected by a base, the space between the opposing sides being closed by 
the front plate, as shown in FIG. 9. The top of each side of the channel 
has a respective extension 68 which is inclined at an acute angle relative 
to the plane of the channel side. As seen in FIGS. 6 and 8, the extensions 
68 extend away from each other. Similarly, the front plate 62 has a pair 
of extensions 70, a distal portion of each extension being inclined at an 
acute angle relative to the plane of the front plate. A rod 80 is attached 
to the extensions 70 at the bend line and bridging the gap therebetween. 
The circular handle 81 on the capsule (see FIG. 3) is a spring. The spring 
arm has a bend which engages rod 80, providing a locking device (or active 
retention) of the capsule in the holder. As seen in FIG. 3, the extensions 
70 extend away from the reactor pressure vessel wall 10. 
In the conventional application of the capsule holder 42, during the 
remotely controlled lowering of a surveillance capsule into the capsule 
holder 42, the inclined extensions 68 and 70 act as slide surfaces which 
guide the surveillance capsule into the interior volume of the capsule 
holder 42. In the application of the present invention, the capsule holder 
42 serves to hold the offset capsule holder assembly 40 instead of a 
surveillance capsule. 
As best seen in FIG. 6, the holding lug 50 of the offset capsule holder 
assembly 40 comprises an eyelet 56. The eyelet 56 is adapted to receive 
the coupling element of a grapple or other lifting tool (not shown), which 
can be used to install the offset capsule holder assembly 40 by lowering 
it into position or to remove the offset capsule holder assembly 40 by 
raising it up. As the offset capsule holder assembly 40 is lowered with 
the right arm 54 of the positioning arm assembly 48 overlying the capsule 
holder 42, the right arm 54 will be guided into the capsule holder 42 by 
the inclined extensions 68 and 70. 
As seen in FIG. 4, the capsule holder 42 is attached to a pair of pressure 
vessel bracket pads 72 via a corresponding pair of brackets 74 which 
extend generally transverse to the longitudinal axis of the capsule holder 
42. A U-shaped spring 76 has one leg attached to the bottom plate 66 of 
capsule holder 42. The distal end of the other leg of spring 76 bears 
against a pressure vessel spring pad 78. The pads 72 and 78 are preferably 
welded to the interior surface of the vessel wall. Likewise, brackets 74 
are welded to pads 72 and spring 76 is welded to spring pad 78. 
As seen in FIGS. 3 and 5, the offset capsule holder 44 is attached to the 
left arm 52 of the positioning arm assembly 48. The offset capsule holder 
44 preferably has a structure identical to that of the capsule holder 42, 
except that the brackets 74 and the spring 76 are not needed. The offset 
capsule holder receives a capsule 46, and then the capsule holder is 
lowered into position using a remotely operated positioning tool. As seen 
in FIG. 3, the offset capsule holder assembly allows the surveillance 
capsule 46 to be held at a radial position which is closer to shroud 18 by 
a distance D, which will be specific to a given plant and to a particular 
azimuthal position and removal period. For example, in one plant the value 
D varied from 5.2 to 7.4 inches for different azimuthal positions and 
removal periods. This displacement of the surveillance capsule closer to 
the nuclear fuel core increased the lead factor to a value in the desired 
range of 1-3. 
The preferred embodiment of the offset capsule holder assembly has been 
disclosed for the purpose of illustration. Variations and modifications of 
the disclosed structure which do not depart from the concept of this 
invention will be readily apparent to engineers skilled in the design of 
tooling. All such variations and modifications are intended to be 
encompassed by the claims set forth hereinafter.