ML20138J775
| ML20138J775 | |
| Person / Time | |
|---|---|
| Site: | La Crosse File:Dairyland Power Cooperative icon.png |
| Issue date: | 12/12/1985 |
| From: | DAIRYLAND POWER COOPERATIVE |
| To: | |
| Shared Package | |
| ML20138J769 | List: |
| References | |
| NUDOCS 8512180032 | |
| Download: ML20138J775 (9) | |
Text
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-2.7.3 Control Rods 2.7.3.1 Twenty-nine movable control rods shall be provided as shown in Figure 1. Coolant flow past the control rods shall be guided by the control rod channels formed by the fuel shrouds and by 52 vertical posts made of zirconium alloy.
2.7.3.2 The control rods shall br: of cruciform shape and shall conform to either the Allis-Chalmers (A-C) tube-sheath design or the ASEA-ATOM (AA) plate design with horizontally drilled absorber holes. The neutron absorber section of each control rod shall have a nominal length of 83 inches.
2.8 CONTROL ROD DRIVE SYSTEM 2.8.1 Twenty-nine drive mechanisms shall operate the control rods over a l- _ minimum travel range of 82 in. and shall fix their position in the reactor as required. Individual rod positions derived from synchro pairs and from reed switches shall be indicated continuously.
2.8.2 Each control rod shall be connected to a push rod by a positive latch, and the push rod shall be moved and positioned vertically by means of a roller nut assembly and a leadscrew which shall be rotated by an electric motor. All rod motion shall be accomplished against a mechanical brake which maintains the rod-in position when the drive motors are not operating. Interlocks shall prevent withdrawal of more than one rod at a time and shall prevent control rod withdrawal while the speed of either of the forced-circulation pumps is increasing.
2.8.3 A hydraulic motor on each drive mechanism shall cause the control rod to be rapidly driven upward upon receipt of a scram signal until it is fully inserted in the core. The electric drive motor on each drive mechanism shall independently drive the control rod into the reactor core upon receipt of a scram signal if the hydraulic motor fails to do so.
Amendment No. 11 8512180032 DR 851212 ADOCK 05000409 PDR .
.4.2.4.3 Deleted.
4.2.4.4 The reactor shall not be operated at a power level above 1 Mwt with
-less than 72 fuel assemblies installed in the reactor.
4.2.4.5 The reactor shall have a negative temperature coefficient for all moderator temperatures from ambient to.557,F or shall be preheated to obtain a negative temperature coefficient prior to withdrawal of control rods.
4.2.4.6_ The core shall be suberitical by a least 0.5% delta k/k in the cold clean condition with the rod of maximum worth in its fully withdrawn position and with all other rods fully inserted.
4.2.4.7- The' reactor may be operated with control rod drives inoperable provided that the associated rods are secured in the fully inserted or fully-withdrawn' position by electrically disconnecting the clutch and drive member and provided that all other requirements of these specificiations are met.
-4.2.4.8 If an unexplainable change of greater than 0.6 percent in core reactivity-is observed in a core which has undergane no physical change, the reactor shall be shut down. Except as required to investigate the reactivity change, the reactor shall not be operated until a satisfactory reason for~the change has been found.
4.2.4.9 Following core reloading and/or core rearrangement, operation will be discontinued if the difference between the observed and predicted value of core reactivity exceeds 2 percent. If this difference is exceeded, the effect of the difference on further operation will be evaluated before operation is continued.
Amendment No.'11 E~
4.2.5. Reactor Control 4.2.5.1 The total scram time of each control rod as measured by the rod scram timer shall not exceed 3.0 sec.
4.2.5.2 The control rod configurations used during reactor operation shall be such that-the maximum increase in reactivity that would be caused by the complete withdrawal of any one control-rod from a critical core would not exceed 2.5% delta k/k.
4.2.5.3 The maximum rate at which reactivity shall be. increased by movement of control rods when keff of the reactor exceeds 0.99 shall' not exceed 18 cents /sec.
4.2.5.4 The maximum rate at which forced circulation flow rate may be increased by changes in forced circulation pump speed shall be 1200 gpm/sec.
4.2.5.5 The reactor shall not be operated above 10-5 of full power unless at least one forced circulation pump is operating at or above its set minimum speed (34 percent of full rated speed).
4.2.5.6 The speed of the forced circulation pumps shall be manually controlled.
4.2.5.7 Except during plant startup and shutdown, the main steam bypass valve control shall be set to begin opening the bypass valve at an indicated pressure corresponding to a turbine inlet pressure not more than 15 psi above its nomint? value.
4.2.5.8 The rod position indication system, i.e., the synchro pairs, the reed switches, and the limit switches which indicate full rod insertion or withdrawal, shall be available for indication of individual rod positions, except that one of these systems may be removed for maintenance for a time period not exceeding 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
4.2.6 Safety Instrumentation 4.2.6.1 The safety instrumentation shall proalde scram, isolation acticn. and other safety actions as specified in Table 1 of these specifications.
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Attachment to LAC ll317 J
THE ASEA-ATOM CONTROL ROD FOR THE LACBWR i
The ASEA-ATOM (AA) control blade for U.S. boilng water reactors is described in detail in the Topical Report TR-UR-E S-225, "BWR Control Blade for US BWR's" submitted to the NRC in October-1985. The AA control blade (rod) described in this report has been slightly modified in order to fit the La Crosse Boiling Water Reactor (LACBWR) core lattice geometry, to closely match the reactivity worth of the original (A-C) LACBWR control rods, and to be compatible with the LACBWR control rod drives and control rod handling equipment. Table I summarizes the AA control rod design data along with
- comparative data for the original A-C control rod.
MECHANICAL DESIGN The cruciform absorber section is. formed by welding four solid stainless steel plates together at the center. High purity (reduced Si, P, N and Co
' impurities) Type SS 2352-28 (AISI 304L) stainless steel is used for these plates. The intermittent center weld-joint ensures straightness and required.
stiffness while permitting 17 cut-out sections which results in significant weight savings. Thus, the total weight of the control rod is slightly less than that of the original A-C rod, in spite of the slightly heavier absorber-containing part of the AA rod. i The rod wings are 8.05 mm (0.317 in.) thick. The absorber section of
. each wing has 245 horizontally drilled 6 mm (0.236 in.) diameter holes filled with B 4C powder and 19 similar holes containing Hf-pins. The holes are spaced at a pitch of 8 mm (0.315 in.) resulting in a total absorber section length of 2110 mm (83.071 in.). The holes are 104 mm (4.09 in.) deep. The absorber
- part of the control' rod coincides with the active core height when the control rod is fully inserted. The top 101 mm (3.976 in.) of the rod is free of holes. This section constitutes a'" grey" nose which reduces local power changes, thus. mitigating fuel duty.
The horizontal holes are filled with natural B C4 by vibratory co=paction
. to a packing density of 710 + 3% of:the theoretical density. Each control rod contains about 5.2 kg (11.5 lbs.) of B4 C plus 2.9 kg (6.4 lbs.) of hafnium.
The holes are closed at the outer blade edge but are connected thrcugh a narrow slit. This design allows gas pressure' equalization between holes but prevents any significant displacement of the B4C powder. The horizontal holes also render any further B 4 C densification after initial filling quite insignificant. Each wing forms a separate pressure enclosure which is pressure and leak tested after welding.
A lifting handle, cut from the same material as the control rod wings, is
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-welded to the top end of the absorber section. This handle is designed to fit the grapple used for installing the control rods, and corresponds to the A-C handle design. At the top end of the rod are guide pads, one on each wing, l which prevent direct contact between the control rod and the adjacent fuel l shrouds. The guide pads are made of Inconel X-750 which contains less than l 0.1 weight-% cobalt. The control rod absorber section and the extension at
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the bottom are welded together. The extension corresponds to the previous design for LACBWR control rods.
The AA control rod is designed for an internal over pressure of 15 MPa (2175 psi). The internal gas pressure is a function of the average boron depletion in a rod wing. Due to pressure equalization, all absorber holes in a wing are_under the same pressure. The hafnium tip control rod will have a slightly lower gas pressure buildup than an all B C4 rod due to the reduced axial burnup variation in the B C4 and no gas release from hafnium. During neutron exposure, gas pressure will buildup mainly due to helium production from neutron capture in B-10, and as a result of radiolysis of any water traces present. The maximum moisture content of the B4 C is specified as 350 ppm at filling in order to limit the water pressure contribution. Other reaction products in the gaseous form may be neglected. Gas pressure buildup is not expected to be life-limiting for this control rod design.
The control rod is provided with a 150 mm (5.9 in.) hafnium tip. The uppermost 19 holes in each wing are filled with hafnium metal rods instead of B4 C powder. The hafnium is of reactor grade with 4.5 w/o zirconium. Hafnium exhibits no irradiation induced swelling and proper tolerances ensure that the hafnium will not induce any stresses in the stainless steel plate during irradiation. The total weight in air of the control rod (including extension) is calculated to be 74 kg (163.16 lbs). Thus, it is slightly lighter than the A-C control rod which weighs approximately 77 kg (170 lbs.).
NUCLEAR DESIGN The reactivity worth of a control rod is directly dependent on the rate of neutron absorption in the rod materials-B4 C/Hf and stainless steel. The total absorption rate depends on several factors: amount and spatial distribution of the materials in the rod, and the neutron energy spectrum which in turn depends on the core conditions (temperature, void fraction, fuel assembly type and its exposure history). Eventually, the depletion of the highly neutron absorbing material will reduce the absorption rate, and hence, the control rod reactivity worth.
The reactivity worth of the AA control rod for the LACBWR relative to the original A-C control rod was calculated with the two-dimensional lattice depletion code PHOENIX for different core conditions. From a neutronics point of view, the design of the AA control rod differs from that of the original LACBWR f od mainly by its 35% larger volumetric inventory of neutron absorber (B4 C/Hf). As a result of this difference, the B C4 region of the AA control rod has a consistently higher reactivity worth, for the all rods inserted condition, of about 2.6-3.0% (relative) for the various reactor conditions.
_ Corresponding values for the AA control rod tip (5.9 in.) with Hf are 5.0-5.9%
(relative) less worth than that of the tip of the A-C rod. The over all reactivity worth of the AA control rod, in the all rods inserted condition, is about 1.3-1.8% (relative) greater than that of the existing A-C control rod for the various reactor conditions. In the hot, fullpower, 20% moderator void case with 75% or more of the control rods withdrawn from the core (average operating condition) the relative reactivity worth of the AA control rod is 0.0-0.3% less than the A-C rod.
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The increased neutron' absorption rate in the AA control rod is essentially entirely due to the volume absorption of epithermal neutrons in B 4 C.' Other differences between the two control rod designs, such as the amount and distribution of stainless steel, have minor influence on the reactivity worth differences.
The models used in the PHOENIX two-dimensional calculations represented either the cross section of one fuel assembly or + control cell with or without appropriate control rods inserted. The pt sical and neutronic properties of the LACBWR Type III fuel and other ccJe components were accurately represented. Reflective boundary. conditions rendered the calculated-reactivity quantities valid for an infinite core. For the purpose of assessing the reactivity worth of control rods, especially with respect to
. relative differences, calculations with PHOENIX will give reliable results.
Calculations were performed for the following reactor conditions using fresh xenon-free . fuel:
a) HFP, zero void, (i.e. near the core inlet) b) HFP, 20% and 40% void c) Cold, clean, zero power (shutdown condition).
T'he dependence of the relative worths of the AA and A-C rods on xenon and fuel burnup was also studied and the effect of these parameters was found to be quite negligible.
The difference in neutron absorption rates in the various control rod designs will have a small effect on the neutron flux density distribution in fuel assemblies near the control rods. The greater neutron absorption rate in the B4C region of the AA control rod, compared to the A-C control rod, causes
. a sl,ightly greater suppression of the power density in fuel rods near the AA control rod and a slightly greater power peaking'in fuel rods away from the AA control rod. In the 2D infinite lattice model, the maximum increase in fuel rod power density (relative to that with A-C ~ control rods) is less than 0.05
-- kw/f t' for the hot, full power, 20% void, no xenon case.
For the AA control rod Hf-tip section, the suppression of the power in the fuel near the control rod is less than that for an A-C control rod. The power density distribution in the fuel in the vicinity of the Hf region of AA control rods is actually flatter than with A-C rods and' peak fuel power densities are lower.
The burnup of the neutron absorber in a control rod will lead to
- decreasing neutron absorption and hence, reduced control strength. The rate of reactivity decrease as a function of neutron exposure depends on the
~ initial' absorber inventory (i.e. on relative effective consumption of absorber nuclides). With about 50% greater B4 C content per unit length than the A-C Econtrol rod, the AA~ control' rod will have a corresponding exposure time advantage for any given relative reactivity decrease or relative depletion of B-10. The reactivity worth of the hafnium region of the AA control rod 'will
,. decrease even slower:than the B4 C region. This is due to the fact that
. natural'Hf consists of several isotopes, where the most important are Hf-177, Hf-178, Hf-179 and Hf-180. All these isotopes have significant absorption cross-sections. An absorbed neutron will give rise to a new Hf isotope which W3.6
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in' turn may absorb a neutron. See Figure 1 for a comparison of the depletion characteristics of Hf and B C4absorbers as a function of absorber diameters and exposure time.
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RELATIVE NORMALIZED NEUTRON ABSORPTION RATES FOR CYLINDRICAL ABSORBERS y (HAMMER CALCULATIONS) ua Q 1.01 cc z
9 4 F I 's s o N s
E O*9- N's s l 4 Hf - DIAM. 6 mm o N s m
> s'N g N J
w ;
cc 0.8 - -o- DENOTES APPROX.
40% B-10 BURNUP B4C - DIAM. 6 mm l B4C - DIAM. 3.5 mm l 0.7 - . i j 0 5 10 15 20 TIME STEPS f
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FIGURE 1
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t-TABLE I CONTROL ROD DESIGN DATA (ASEA-ATOM AND ORIGINAL A-C)
ASEA-ATOM ROD ORICINAL A-C ROD MATERIALS Neutron Absorber Top 5.9 in. - Hafnium metal rods with 4.5 . Sintered B4C pellets, approximately 68% of
. w/o Zr in horizontal holes. Remaining 57.1" theoretical density, in vertical Inconel
- BAC powder, vibratory compacted to 70% of 600 tubes.
theoretical density, in horizontal holes.
Control rod Wings and Handle SS 2352-28 (AISI 304L) stainless steel. Inconel 600 tubes in a perforated AISI 304 Comax = 0.05 w/o, Baax = 5 ppe, reduced Si, stainless steel sheath welded to a central P and N. spider of AISI 304 stainless steel. AISI 304 stainless steel handle.
Extension SS 2352-27 (AISI 304L) stainless steel with AISI 304 stainless steel.
Co.,x = 0.05 w/o and Naax = 0.08 w/o.
Guide Pads / Rollers Guide Pads - Inconel X-750 with Comax = 0.1 w/o. Rollers 4PH stainless steel.
End Stud 17-4PH stainless steel. 17-4PH stainless steel.
t DIMENSIONS AND WEICHTS l
Rod Span 247.9 +0/-1.5mm (9.7M +0/ .059 in.) 9.750 1 0.10 in.
Rod Wing Thickness 8.05 mm (0.317 in.) 0.315 in.
Guide Pads / Roller Thickness 10.9 +0/ .1mm (0.429 +0/ .004 in.) Roller 0.430 +0/ .003 in.
Length of Absorber Section 2110 mm (83.071 in.) 83.001 03 in.
Overall Length 4983.5 1 3.2 ar. (196.200 1 126 in.) 196.2 + .125 in.
Length of " Grey" Tip 101mm (3.976 an.) 2.131 03 Absorber Hole Diameter 6 +.150/-Osna (.236 +.006/-0 in.)
Absorber Hole Depth 104 +1.5/ .5mm (4.095 +.059/ .020 in.)
Absorber Hole Pitch 8 mm (0.315 in.)
l No. of Holes Per Rod 4 x 264 Hf Rodlet Diameter 5.97 +0/ .05 mm (.235 +0/ .002 in.)
Absorber Pellet (B4C) Diameter 0.157 in.
Absorber Tube ID/0D 0.164/0.206 in.
Absorber Tube Pitch approximately 0.206 in.
No. of Absorber Tubes Per Rod 4 x 20 B4 C Inventory 5.2 +.20/ .14 kg. 3.74 i .15 kg.
Hf Inventory . 9 kg (6.39 lbs.)
Total Weight 74 kg. (163.17 lbs.) Approximately 170 lbs.
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