ML12109A063
| ML12109A063 | |
| Person / Time | |
|---|---|
| Site: | Kansas State University |
| Issue date: | 04/09/2012 |
| From: | Geuther J Kansas State University |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| Download: ML12109A063 (10) | |
Text
9 April 2012 K~STATE. KansaStateUn_________
Department of Mechanical and Nuclear Engineering 3002 Rathbone Hall US Nuclear Regulatory Commission Manhattan, KS 66506 -5205 Document Control Desk 785-532-5610 Fax: 785-532-7057 Washington, DC 20555-0001 Re: License R-88, Docket 50-188 To Whom it May Concern:
This letter is a request for an upgrade to the allowed 235U inventory for possession and use at the Kansas State University TRIGA Mk-II nuclear reactor facility (License R-88, Section 2.B.2), and for the approval of a proposed change to the facility Technical Specifications Chapter 5.1.
Background Information According to the facility license renewal paragraph 2.B.2., the University is licensed 'to receive, possess, and use up to 4.20 kilograms of contained uranium-235 at enrichments less than 20% in connection with operation of the reactor and up to 90 grams of uranium-235 at any enrichment for fission chambers and reactor experiments." Amendment 17 to the facility license allowed the receipt an possession, but not use, of up to 350 g of fissile material in the form of 12%-loaded fuel elements (paragraph 2.D.2 of the facility license). This change allowed the receipt of the fuel pending additional safety analysis to demonstrate that the fuel could be safely used in the core.
Request for License Amendment A detailed safety analysis (attached) demonstrates that fuel loaded to 12% uranium by weight can be used safely in certain locations in the core (specified in the proposed amendment to the Technical Specifications). We therefore request that paragraph 2.D.2 to License R-88 be removed, and for the possession and use limit of 4.20 kg of fissile material in connection with the operation of the reactor to be increased to 4.55 kg to accommodate the fuel that will no longer be covered under 4.D.2.
Request for Amendment to Technical Specifications In accordance with the findings of the attached safety analysis, we request that the facility Technical Specifications chapter 5.1 be amended as attached. Specifically, we request the approval to use up to four fuel elements of up to 12.5% uranium by weight in the E-or F-rings of the core, in locations not adjacent to control rod or water channels.
Reactor Safeguards Committee Action The proposed amendments to the License and Technical Specifications have been approved by the Reactor Safeguards Committee.
I verify under penalty of perjury that the foregoing is true and correct.
Sincerely, Jef rey A. Geuther, Manager KSU Nuclear Reactor Facility (785)532-6657 112 Ward Hall, Manhattan, KS 66506 cc: Spyros Traiforos, Project Manager Attachments: 1. Safety basis for proposed revision to license and technical specifications
- 2. Technical Specifications 5.1 (proposed revision)
KSU REACTOR CORE DESIGN WITH 12%-LOAD FUEL INTRODUCTION The Kansas State University TRIGA reactor facility has recently received a shipment of six fuel elements with 12% loaded fuel. (The fuel is 12% uranium mass a fraction of total fuel mass). The fuel has a maximum enrichment of 20%, as does the fuel currently in use at the facility. This letter documents design checks which were performed to verify that the fuel can be safely used in the reactor core, as a safety basis for modifying Technical Specification 5.1.3(1), "...[Fuel elements] shall contain a maximum of 9.0 weight percent uranium which has a maximum enrichment of 20%."
Analysis presented below demonstrates that four 12%-loaded elements may be added to the E- or F-rings of the core in lattice positions not adjacent to control rod channels while remaining within the bounds of the facility Safety Analysis Report (SAR) and Technical Specifications (TS). The addition of the higher-load fuel is expected to increase the achievable reactor power to approximately 900 kW, above the current maximum power of 600 kW, but well within the license limit of 1.25 MW.
DESIGN CONSTRAINTS The Technical Specifications and Safety Analysis Report were carefully examined to determine what parameters may be affected by the change in fuel loading. The following design constraints in the Technical Specifications and SAR must be satisfied:
- 1. Minimum stuck-rod shutdown margin = $0.50 in optimum conditions (TS 3.1.3(2));
- 2. Maximum excess reactivity = $4.00 in optimum conditions (TS 3.1.3(1));
- 3. Maximum element-to-average power peaking = 2.00 (SAR 13.2.4);
- 4. Changes in fuel composition must not result in significantly different performance during reactor pulsing.
- 5. Mechanical properties of the fuel must remain unchanged or otherwise must not result significantly reduced performance during hypothetical accident conditions.
In addition to the above constraints, the following design criteria will also be met:
- 1. 8.5%-loaded elements in the B-ring must have the highest temperature (i.e., power) in the core, since the only instrumented fuel elements are 8.5% loaded elements in the B-ring.
- 2. 12% elements may not be placed near control rod channels, to avoid local power peaking effects during pulsing.
Since the fuel cladding remains 20 mil stainless steel, the mechanical properties of the fuel cladding remain unchanged from the 8.5% loaded elements. The thermal conductivity, thermal expansion, and hydrogen dissociation pressure of the fuel is known to have no major dependence on uranium content [1].
MODEL The reactor core was modeled in MCNP5 as a 3D model with all rods out. Fission products were not included in the fuel composition for the 8.5%-loaded fuel elements. However, the composition and density were adjusted to approximate core depletion. Fuel elements selected for replacement with
12% loaded fuel were modified by replacing the material card with a material card featuring a uranium content and density appropriate for the higher-load fuel.
RESULTS AND ANALYSIS
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.................I........................
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SHUTDOWN MARGIN AND MAXIMUM EXCESS REACTIVITY The kcode output from MCNP5 can be used to compare the multiplication factor between a control case (the existing core) and the perturbed cases (i.e., with 12%-loaded fuel in certain lattice locations). For this analysis, four 12%-loaded elements were placed in either the D-, E-, or F-rings, with approximate radial symmetry and the restriction that the 12% element must not be located near a control rod channel. The change in reactivity can then be added to the known values of maximum excess reactivity and subtracted from the known minimum shutdown margin to determine if the perturbed case would satisfy the limiting conditions of operation (LCOs) in the Technical Specifications. The known values of excess reactivity and shutdown margin are based on a reactivity balance following two weeks of shutdown, thereby ensuring a xenon-free condition.
TABLE 1 - EFFECT OF FOUR 12%-LOADED ELEMENTS ON LIMITING CONDITIONS OF OPERATION Parameter LCO Current D-Ring +/- E-Ring _ F-Ring +/-
Value Max. Excess <$4.00 $2.50 $3.75 $0.05 $3.14 $0.05 $2.90 $0.05 Reactivity Shutdown Margin >_$0.50 $1.54 $0.28 $0.05 $0.90 $0.05 $1.13 $0.05 The values in Table 1 reflect the radial "cosine"-shaped flux density in the core. The error values are one sigma and are based on MCNP counting statistics, with no uncertainty on the measured parameters, i.e., the "Current Value" column. As Table 1 shows, the criterion than the stuck-rod shutdown margin be at least $0.50 is the most limiting LCO with respect to core reactivity. In particular, the addition of four 12%-loaded elements to the D-Ring would reduce the stuck-rod shutdown margin to below the acceptable level. However, four elements can be placed within the E or F rings with a large margin to the LCOs. It is probable that three elements in the D-ring could meet the shutdown margin requirement, but this analysis would need to be redone for the three element case in advance of installing 12% fuel in the D-ring. Either in the case of the E-ring or F-ring receiving four fuel elements with 12% loading, there will be ample margin to account for both the calculation uncertainty and additional uncertainty on the measured reactivity, rod worth, etc.
Further calculations performed with four 12.5%-loaded fuel pins in the E-ring, the maximum loading allowed under the proposed amendment to the facility Technical Specifications, would decrease the minimum safety shutdown margin to $0.77 and increase the maximum excess reactivity to $3.27. In other words, assuming the all of the fuel elements are higher than the expected nominal 12% uranium by weight, the LCOs pertaining to core reactivity would not be challenged.
MAXIMUM POWER PEAKING The effect of adding higher-load fuel to a ring does not challenge the SAR assumption that the maximum element-to-average power and fission density ratio can be no greater than 2.0. In fact, the effect on the flux distribution is minimal, as shown in Figure 1. The actual heat generated in each fuel element experiences a somewhat higher perturbation due to the increase in the
macroscopic cross section for fission in the 12%-loaded elements relative to the balance of the core, but as Figure 2 shows, the B-ring still has the highest heat per fuel element, such that the temperature indicated by the 8.5%-loaded IFEs in the B-ring will be conservative for the entire core. Figure 2 also more clearly shows the expected result of adding higher-load fuel to the outer region of the core - the fission power density is depressed somewhat in the center (B-ring and C-ring), and is elevated in the perturbed ring (D-ring). Relative error on the track-length estimates of flux and heating used in this calculation were very small, <0.4%, and are not included in the results.
1.7 B-Ring 1.5
- C-Ring 4)
D-Ring 1.3 S S SI 00 E-Ring E 2.2..
0.9 F-Ring 0 Control 0.7 E i 2.2644 o12% Elements in D Ring 0.5 FIGURE 1- EFFECT OF ADDING FOUR 12%-LOADED FUEL ELEMENTS TO D-RING ON THE ELEMENT TO AVERAGE FLUX RATIO 1.8 B-Ring 1.6
- C-Ring
- 4) 1 D-Ring 0
0 1.4 88 .8 0 8
'U 1.2 ** e,8 8
0 ov*
S 4-r C 0 0 E-Ring 0 1 1,6 e.0 8, F-Ring 0.8
- Control
- 0 0.6 j o 12% Elements in D Ring 0.4 --------...........
FIGURE 2 - EFFECT OF ADDING FOUR 12%-LOADED FUEL ELEMENTS TO D-RING ON THE ELEMENT TO AVERAGE HEAT RATIO.
1.8 B-Ring 1.6 - 8 C-Ring 0
1.4 D-Ring
- 88 E-Ring 1.2 0 E 0 0
oT 00 8,00 F-Ring 0
C 0.8
- a
- 8 18%!1 U,
.2 - Control 0.6 o 12% Elements in E Ring 0.4 FIGURE 3- EFFECT OF ADDING FOUR 12%-LOADED FUEL ELEMENTS TO E-RING ON THE ELEMENT TO AVERAGE HEAT RATIO.
1.8 B-Ring 1.6 - 8 C-Ring 8o
> 1.4 D-Ring
- 8 e *e8 8 1.2 E E-Ring i
a) o8O F-Ring o
-r 1 0
. 0.8 0
.2.
- Control U-0.6 S.80 80 o 12% Elements in F Ring 0.4 FIGURE 4- EFFECT OF ADDING FOUR 12%-LOADED FUEL ELEMENTS TO F-RING ON THE ELEMENT TO AVERAGE HEAT RATIO.
PERFORMANCE DURING REACTOR PULSING Research performed by General Atomics [1] indicates that "the pulse response of uranium-zirconium hydride TRIGA fuel is independent of the uranium content of the fuel and is dominated by the behavior of the zirconium hydride, along with the prompt negative temperature coefficient of reactivity." The work cited featured uranium loading of up to 45% by weight. Therefore, physical effects due to changing fuel loading from 8.5% to 12% by weight need not be considered with regard to pulse performance.
There is some concern that local power peaking effects in 12% fuel would result in unacceptably high local fuel temperatures if the fuel is located near a control rod channel, which becomes a moderator channel upon rod withdrawal. While the KSU reactor facility has not concluded whether such a problem exists, for the sake of expedience the Technical Specifications will restrict 12% fuel to locations not adjacent to control rod channels.
CHANGES TO THE MAXIMUM HYPOTHETICAL ACCIDENT The only accident discussed in the SAR that can be credibly affected by increasing the fuel loading per element is the maximum hypothetical accident. The maximum hypothetical accident (MHA) for the KSU TRIGA reactor is the failure of a single fuel element in air. The analysis (SAR 13.2.4) includes an assumption of a maximum hot channel factor of 2.0, which is in accord with the design constraints discussed above. However, the analysis also assumes that the failed fuel element has 39g of 235U. Since 12%-loaded elements contain more fuel (-56g) than 8.5% -loaded elements
(-39g), they can be depleted more and therefore have a larger inventory of fission product radioisotopes prior to failure.
The reactivity of a fuel element during depletion is dominated by the product of the reproduction factor, rj, and the thermal utilization factor, f. Each term is a ratio of macroscopic cross sections (with the fission cross section in rl multiplied by the average number of neutrons produced per thermal fission, 4).
+z3 z23 5 +~Zrz238~ f 77fa *a a a a a + a +'a The product rjf gives the average number of neutrons produced per thermal neutron absorbed in the core, and decreases with depletion due to the increase in poison concentration and decrease in fuel concentration. (The concentrations are multiplied by the microscopic cross section for the poison or fuel to yield the macroscopic cross section). However, the thermal fission and absorption cross sections for 235U are much higher than the absorption cross sections for 238U, the moderator, or the fission product poisons, such that the reactivity of the fuel element changes in rough proportion to the concentration of the fissile nucleus. 235U. Therefore, to first order, we can approximate the useful lifetime of a 12%-loaded fuel element as being greater than the lifetime of a 8.5%-loaded element by the ratio (12%/8.5%) = 1.41. (For a 12.5%-loaded element, the maximum load allowed under the proposed amendment to the Technical Specifications, the useful lifetime would be expected to increase by 1.47; for a 12.3%-loaded element, the factor would be 1.45).
The Safety Analysis Report includes a conservative approximation that the maximum element-to-average power peaking for a failed fuel element is 2.0. This factor is included in the calculation of the worst-case fission product release. According to Figure 2 through Figure 4, the maximum element-to-average power peaking for 12%-loaded, 12.3%-loaded, and 12.5%-loaded fuel will actually be:
Calculated Maximum El. to Avg. Power Peaking Ring 12%-Load 12.3%-Load 12.5%-Load E 1.I 1 .19..1.26s.i.
E 1.19 1.19 1.26
The maximum fission product buildup will therefore be approximately:
Calculated Maximum Element-to-Average Fission Product Inventory Ring 12%-Load 12.3%-Load 12.5%-Load
'D 1._8 Not j Iý tedv f Noesti ated E 1.68 1.72 1.85 According to this analysis, the MHA developed in the SAR is still bounding for 12.5% fuel elements placed in the E or F rings.
CONCLUSION Four fuel elements with 12% uranium loading may safely be added to the E- or F-rings of the KSU TRIGA reactor while satisfying all relevant design limits:
- 1. Minimum stuck-rod shutdown margin = $0.50 in optimum conditions (TS 3.1.3(2));
Actual shutdown margin calculated to be at least$0.90.
- 2. Maximum excess reactivity = $4.00 in optimum conditions (TS 3.1.3(1));
Actual excess reactivity calculatedto be at most $3.14.
- 3. Maximum element-to-average power peaking = 2.00 (SAR 13.2.4);
Actual maximum power peaking is calculated to be less than 1.2for the new elements;
- 4. Changes in fuel composition must not result in significantly different performance during reactor pulsing.
Published literature [1] has shown no significant difference in pulse performance between different types of TRIGA fuel.
- 5. Mechanical properties of the fuel must remain unchanged or otherwise must not result significantly reduced performance during hypothetical accident conditions.
By restricting the 12%-loaded fuel to the E- and F-rings of the core, the SAR analysis for maximum fission product releasefrom a failed element will still be bounding after accounting for the potentialincrease in fission product inventory versus an 8.5%-loaded element.
- 6. 8.5%-loaded elements in the B-ring must have the highest temperature (i.e., power) in the core, since the only instrumented fuel elements are 8.5% loaded elements in the B-ring.
Analysis has shown that the maximum element-to-averagefission heating will be significantly higher in the B-ring (1.61) than in the E-ring (1.19), even with 12%-loaded elements in the E-ring.
- 7. 120/6 elements may not be placed near control rod channels, to avoid local power peaking effects during pulsing.
This restriction is in place in the proposed revision to the Technical Specifications.
The initial loading of the new fuel elements will be done conservatively: each element will be added one at a time, with shutdown margin and maximum excess reactivity measurements taken at each step. If the reactivity is unexpectedly high, fuel addition will be halted, and high-load fuel will be removed if necessary to satisfy the Limiting Conditions of Operation. The rod worth curves will be re-generated following the loading of the final element, and the core reactivity LCOs will be re-checked using the new rod worth curves to verify that the LCOs are still satisfied. High-load fuel will be removed from the core if the new rod worth curves indicate that the LCOs are not being met.
SAFEGUARDS COMMITTEE REVIEW AND APPROVAL The Reactor Safeguards Committee has reviewed and approved this submittal.
I swear under penalty o hat the foregoing is true and correct.
ffre A. Geuther, Reactor Facility Manager REFERENCES
- 1. NUREG-1282, "Safety Evaluation Report on High-Uranium Content, Low-Enriched Uranium-Zirconium Hydride Fuels for TRIGA Reactors," US NRC, 1987.
TECHNICAL SPECIFICATIONS
- 5. Design Features 5.1 Reactor Fuel 5.1.1 Applicability This specification applies to the fuel elements used in the reactor core.
5.1.2 Objective The objective is to ensure that the fuel elements are of such a design and fabricated in such a manner as to permit their use with a high degree of reliability with respect to their mechanical integrity.
5,1.3 Specification (1) The high-hydride fuel element shall contain uranium-zirconium hydride, clad in 0.020 in.
of 304 stainless steel. It shall contain a maximum of 12.5 weight percent uranium which has a maximum enrichment of 20%. There shall be 1.55 to 1.80 hydrogen atoms to 1.0 zirconium atom.
(2) For the loading process, the elements shall be placed in a close packed array except for experimental facilities or for single positions occupied by control rods and a neutron startup source.
(3) Up to four elements with greater than 9.0 weight percent uranium may be installed in the core. These elements may only be placed in the E- and F-rings of the core lattice, and may not be adjacent to control rods or water channels.
5.1.4 Bases These types of fuel elements have a long history of successful use in TRIGA reactors.
Calculations show that 12%-load fuel in the E- and F-rings will not exceed the temperature of 8%-load instrumented elements in the B-ring. Additionally the power peaking and fission product inventory assumptions in the SAR will not be challenged by 12% fuel in the E- and F-rings. Local power and temperature peaking effects during pulsing are avoided by prohibiting placement of the 12%-load fuel near water and control rod channels.
K-State Reactor TS-38 Original (12/11)