ML20236C304
| ML20236C304 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 09/14/1976 |
| From: | Jaffe D Office of Nuclear Reactor Regulation |
| To: | Lear G Office of Nuclear Reactor Regulation |
| References | |
| TAC-46174, NUDOCS 8707290420 | |
| Download: ML20236C304 (18) | |
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UNITED STATES
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. f 'k NUCLEAR REGULATORY COMMISSION i Ph WASHINGTON, D. C. 20555 "K
September 14, 1976
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Docket No. 50-336 George Lear, Chief, Operating Reactors Branch #3, D0R FORTHCOMING MEETING WITH NORTHEAST NUCLEAR ENERGY COMPANY ON NEUTRON SHIELDING FOR MILLSTONE UNIT NO. 2 (2.*) *
.A.M., Tuesday, September 28, 1976 TIME AND DATE:
g%
6 LOCATION:
AMaryland National Bank Building, Maryland PURPOSE:
Northeast Nuclear Energy Company (NNECO) will present several concepts for a pro-posed neutron shield for Millstone Unit No. 2 g
PARTICIPANTS:
NNECO (R. Kasich, et al)
/
NRC (C. Grimes, D. Jaffe and J. Nehemias)
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s-David Jaffe Operating Reactors Branch #3 Division of Operating Reactors
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c x HGol.1 er'3 TJCarter GLear Project Manager _ DJaffe CParrish OELD OI6E (3)
ACRS (16)
NRC Participants OSD (3)
Receptionist BFaulkenberry DEisenhut LShao BGrimes RBaer ASchwencer DZiemann RReid W. Mcdonald D. Thompson J. Reece CGrimes JNehmias 9
D
SUMMARY
OF THE NEUTRON STREAMING PROBLEM AT MILLSTONE UNIT NO. 2 Northeast Utilities became aware of the neutron streaming problem in the fall of 1975.
It was discovered at Calvert Cliffs Unit No. 1 during startup testing that a radiation streaming prob-Icm existed.
A study by the architect-engineer for Calvert Cliffs l'
and Millstone indicated that Millstone Unit No. 2 also would exhibit neutron streaming.
This was confirmed during the Millstone Unit No. 2 startup test program.
In both cases, a vertical attenuated i
beam of radiation emanated from the annulus between the reactor vessel and the primary shield walls and was scattered by the con-tainment dome and other structures.
Both gamma and neutron flux levels were beyond design levels within containment and, because of leakage paths created by certain materials of cons.truction, radiation levels were excessive outside of containment as well.
A major difference exists between the radiation levels at Mill-stone Unit No. 2 and Calvert Cliffs Unit No. 1.
Due to strtctural variations between the plants, radiation levels outside of contain-ment were much lower for Millstone Unit No. 2.
The neutron streaming at Millstone Unit No. 2 has caused high radiation levels within con-tainment.
To date, the streaming problem has severely restricted access to containment during plant operation, and has limited access f
to outside areas immediately adjacent to.the personnel and equipment hatches and certain areas of piping and electrical penetrations.
I NNECO has minimized occupational exposures through the use of prudent administrative controls and radiation practices.
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SUMMARY
OF THE NEUTRON STREAMING PROBLEM AT MILLSTONE UNIT NO. 2 Page 2 Northeast Utilities began searching for a resolution to the neutron streaming problem in the fall of 1975.
Designs for special shielding were formulated by the architect-engineer and proposals for shielding at other plants were studied.
The scope of these in-vestigations is shown in Tables I and II.
The original shield design selected to attenuate the radiation streaming upward around the reactor vessel was a subdivided shield.
l Boron impregnated silicon rubber balls slightly smaller than base-balls comprised the shield.
These balls were to be contained in i
baskets supported directly over the annulus around the reactor vessel.
The baskets were hinged to their supports so that the blowdown from a hypothetical LOCA occurring at a reactor vessel nozzle-to-pipe weld would blow the shielding out of the path of steam escapihg from the reactor vessel cavity.
This design was purused through actual procurement and fabrication of the shield baskets and letting of bids for the silicon rubber balls.
During the design and fabri-cation of this shield NUSCO closely followed developments at other plants.
The moveable subdivided shield offered a reasonable compro-mise of radiation attenuation with minimum impact on plant operation during shield installation and plant refueling.
These plans were interrupted by NRC questions which required large scale prototype testing and additional design calculations prior to NRC acceptance of the subdivided shield.
The time and cost of the required testing and calculations were found to be substantial and a review of all possible shield designs was performed.
Bachtel
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SUMMARY
OF THE NEUTRON STREAMING PROBLEM AT MILLSTONE UNIT NO. 2 Page 3 I
Power Corporation was contracted to develop three alternate conceptual shield designs and to recommend the most. promising design based on L
l cost, schedule.and.licenseability.
I The result of the Bechtel work is a proposed concrete torus
}-
shield.
This shield design-has the following attributes:
,s
.. acceptable radiation attenuation d
d9 fy
.. simple supports
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reasonable cost
.. common materials of construction-L
.. comparatively easy to remove / install
.. no missile gener.ation from credible accidents.
The evolution of the conceptual design into a final item of
. plant hardware will commence shortly.. Schedular impact is'possible from several sources including plant capacity factor and fabrication
-of shield components, but the target date for project completion tentatively is the first refueling' outage.
WGK: caw 9/24/76 Attachments
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SUMMARY
OF THE NEUTRON STREAMING PROBLEM AT MILLSTONE UNIT NO. 2 Page 4 TABLE I NEUTRON SHIELD DESIGNS CONSIDERED FOR MILLSTONE UNIT NO. 2 A.
Subdivided SL elds 1.
Rotating Basket Supports #
2.
Fixed Grating Supports B.
' Monolithic Shields i
1.
Rectangular Concrete Slab 2.
Concrete Ring TABLE II
' NEUTRON SHIELDS FROM OTHER PLANTS CONSIDERED FOR MILLSTONE UNIT NO. 2
)
A.
Bagged Boric Acid Supported on Grating B.
Rubber Water Bags Supported on Grating C.-
Steel e icketec. ' Subdivided Mineral
)
1 D.
Laminated Wood E.
Monolithic Concrete Slabs 4
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104 5.2.4' Radiation Surveys;
. Discussion The objective of the shielding effectiveness test was to ensure that the design of-the plant provides adequate i
radiation exposure protection to personnel in accordance with 10'CFR 20 and that measured exposure level's are con-7
}.
sistent with the design objectives presented in section 11'2 of the FSAR.
Radiation levels were measured using 9
portable gamma and neutron dose rate meters and measure-ments were'taken at waist level at 0, 20, 50, 80 and u.f 100% reactor power levels.
Film badges, sensitive to Y
j beta, gamma and neutron radiations, were also placed at
- various locations to be used to check meter readings.
y flow'ever the neutron film proved to be inadequate because the proton recoil tracks, in the hydrogen rich emulsion, faded before processing due to the-delay time in transit
[
and the high humidity of the environment.
Therefore, a
with one exception, no detectable levels of neutron 3
3 exposure were measured by film badge.
The one detectable neutron exposure was apparently a false reading since it could not be attributed to any ne'utron radiation.
p7 Results 9k'-
The shielding effectiveness test shows that during normal 4
operation the radiation dose to operating personnel and to the general public will be within the radiation exposure limits set forth in 10 CFR 20.
In addition, all radiation
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s areas are marked and all high radiation area accesses are p-
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marked, locked and alarmed in accordance with,the
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.l requirements of 10 CFR 20. There were seven a.eas where r
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the expected design radiation levels were exceeded.
1 i
These areas are listed in table 4.3.2-1 along with
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i, the probable cause and any necessary corrective action.
These higher than expected levels will add to the doses f
received by operating personnel but will not cause a
these doses to exceed the limits of 10 CFR 20.
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106 TABLE 5.2.4 SHIELDING-EFFECTIVENESS TEST HIGHER THAN EXPECTED DOSE LEVELS'
' AREA AND CAUSE CORRECTIVE ACTION
'l.- Areas of neutron streaming.
This area.has been properly-posted as' a.
- a. Streaming around the edge of the radiation area.
F containment. equipment 1 access door
-has resulted in'approximately 5 mr/hr
'in an expected imr/hr area. See
~
note 1.
j
- b. The area around the containment The access door is locked ~and alarmed and L
personnel access door.and the the area is posted as.'a high radiation associated penetration room is area.
higher than expected due to streaming. The area was expected to be 1 mr/hr but around the con-r tainment access door the level is approximately 15 mr/hr and in the area where the access tunnel and-containment meet the level is approximately 140 mr/hr.
See note 1.
c.-Streaming around the containment The area has been marked as a radiation purge piping in the east penetration area and one particular hot spot of 87'mr room at the 38' 6" elevation has has been roped off and marked.
j raised the area' dose level above the
.j expected 15 mr/hr but it is less than
]
100 mr/hr.
See note l'.
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m 107-AREA AND CAUSE-CORRECTIVE ACTION
- 2. Areas effected by primary flow l-through piping and components..
- a. The development of radiation hot-Both the A'and B shutdown heat
-spots in'the "A" shutdown heat-exchanger rooms are marked as exchanger has caused the le/els in, radiation areas.
the area of the. heat exchanger to exceed the expected 15 mr/hr by approximately 10 mr/hr.
- b. Primary coolant flow through a let--
The area has been marked as a
. down pipe in the west' piping penetra-radiation area.
Since only the. area tion room at the -5' elevation has in the immediate vicinity of the caused the dose levels in the area pipe exceeds 100 mr/hr and since of the pipe to exceed the expected
-this area is not easily accessible the 100 mr/hr.
Contact readings with the penetration room is not considered pipe showed 3500 mr/hr.
The hot spot exceeding 100 mr/hr is roped off and marked.
- c. ;The' primary coolant flow through a The area has been marked as a
' letdown pipe in the auxiliary radiation area.
building at the -5' elevation has caused the dose levels to exceed the expected 1 mr/hr by approximately 2 mr/hr.
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108 f
' Note 1.
tin: a Millstone Nuclear Power Station report sent to the NRC in April,1976,
.f l-the neutron streaming in items.la and 1b of this table were predicted.
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The report, entitled the " Radiation Survey Results in and Around Millstone i
- Unit II Containment Building" can also be used to explain.the streaming 1
.in item ic although this specific area was not mentioned.
From this
' report it can be concluded that the streaming, mentioned in items la,1b
,e h
and ic of this table, results from a lack of neutron shielding above the v
annular region surrounding the reactor vessel.- The levels are higher than-
,?
' design expectations. in the areas where streaming occurs.hecause.when the L
L design dose levels'were initally computed,'it kas anticipated that l
~ shielding would be placed above the annular region. However. when this shielding was not installed, the resultant streaming was expected 'and.an.
h b
evaluation of the situation showed that full power operation could be conducted without exceeding the personnel limits of 10 CFR 20. The necessity and method of install.ing annular region shielding is being E
investigated at this time..
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