ML20203H738
| ML20203H738 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 02/18/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20203H727 | List: |
| References | |
| NUDOCS 9902230087 | |
| Download: ML20203H738 (4) | |
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UNITED STATES l j . NUCLEAR REGULATORY COMMISSION I l
9 $ - WASHINGTON, D.C. 30886 4001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION FOR TOPICAL REPORT FOR THE INSERTION OF LEAD TEST ASSEMBLIES -
CONTAINING U-238 INTO THE SEQUOYAH UNIT 2 CORE DOCKET NO. 50-328 1.0 .NTRODUCTION By letter dated August 7,1998, as supplemented by leMr dated November 5,1998 Tennessee Valley Authority (TVA) proposed to insert four lead test assemblies (LTAs) containing downblended uranium into the Sequoyah Unit 2 core. The initial submittal included topical report BAW-2320, " Blended Uranium Lead Test Assembly Design Report,"
which was prepared by Framatome Cogema Fuels (FCF). This uranium would contain i higher amounts of uranium-236, uranium-234, and uranium-232 than specified per i American Society for Testing and Materials Standard C996-96 for commercial grade uranium hexafluoride. The LTAs are part of a study between TVA and the,U.S. ,
Department of Energy (DOE) to demonstrate the feasibility of using downblended uranium j as fuel in commercial nuclear power plants. '
. 2.0 EVALUATION The licensee requested U.S. Nuclear Regulatory Commission (NRC) approval for use of lead test assemblies containing downblended uranium in the Sequoyah core. The ,
submittal contained a chemical and neutronic evaluation of the effects of using !
downblended uranium which ccatains higher amounts of uranium-236, uranium-234, and i uranium-232 than enriched commercial grade uranium. The analysis enclosed with the !
topical report was performed for the insertion of four fuel assemblies into the core as clarified in the supplementary letter.
Uranium-234 and uranium-236 are neutron absorbers. To counter the effect of the isotope
' neutron absorption, the lead test assemblies would utilize an increased uranium-235 enrichment. The enrichment increase is obtained by matching the end of cycle fuel reactivity so a similar amount of energy is obtained from the core.
The change in the uranium isotopic composition changes the neutronic response of the fuel but does not effect the chemical fuel properties. Therefore, the mechanical properties of the fuel pellet which are governed by the chemical properties of the fuel will not differ from the mechanical properties of standard commercial uranium fuel of the same enrichment.
, The mechanical properties of the fuel were evaluated using FCF design codes which have
! received NRC approval to ensure that they meet the current FCF commercial uranium fuel specification. Similarly, the material properties of the fuel pellet which differ between i
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2 standard uranium fuel and downblended fuel are all changes created from the higher uranium-235 enrichment and not produced by the higher uranium-236 and uranium-234 l
. isotopes.
- As stated above, uranium-234 and uranium-236 are neutron absorbers. The uranium-234 isotope will absorb a neutron to become uranium-235 which is a fissile isotope and will contribute to the energy producing potential of the fuel as it is bumed. The uranium-236 l isotope undergoes a series of transformations ending in its final state as plutonium-238. I This transformation chain is a series of thermal neutron absorption interactions which acts !
as a poison and will not contribute to the energy production of the fuel. Confirmatory I calculations using the ORIGEN-S code for bumup depletion fed into the CASMO-3 code versus bumup depletion with the CASMO-3 code demonstrated that a 70 per cent milli-rho l (pcm) difference can be introduced through using the CASMO-3 code for bumup depletion.
l MCNP 48, which is a Monte Carlo based method, was used to evaluate the effect of using ;
CASMO-3 for the pin cell reactivity. The results demonstrated that the maximum reactivity !
, ' difference was 334 pcm. This indicates that if the MCNP results are more accurete than !
t the CASMO-3 results, then the core reactivity difference would be 7pcm. This is !
acceptable for the addition of four LTAs to a core; however, additional bencNnarking needs !
to be performed before an entire reload core of blended uranium is used. l Fuel load studies using the NEMO code show that the LTAs placed in the anticipated core l
- l. locations will not become the most limiting fuel assemblies in the core during the first fuel load cycle. Additionally, the margin to the core peak pin is great enough to absorb any additional peaking created by the reactivity difference predicted between MCNP 4B and CASMO-3 without having the LTA become the limiting fuel pin. The peak pin criteria will be analyzed for each fuel reload pattem to encure that the down blended LTAs do not become l the most limiting peak pin at any time during their residence in the core. H l
- l. Core analysis for the proposed reload core were performed to verify that the addition of the l LTAs to the Sequoyah Unit 2 core would not exceed any of the key safety analysis l parameters used in the analysis of record. The moderator and doppler power coefficienis for the LTA added core were calculated and found to be bounded by the original safety analysis values. Reexamination of the rod ejection accident in a core loaded with the LTAs found that the addition of the LTAs to the core resulted in a more conservative result with the addition of LTAs than with a standard uranium core. The shutdown margin at the end i of core life (typically the most limiting for Sequoyah) was reduced by 13 pcm with the addition of the LTAs to the core; however, the margin is still bounded by the safety analysis limit. The delayed neutron fraction remained unchanged with the addition of the LTAs to !
the core and is within the safety limits.
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The thermal-hydraulic performance of the fuel assemblies will not be affected by the
- addition of the blended uranium because the only difference between the assemblies is the use of blended uranium with uniform axial enrichment.
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- Limiting safety parameters were used in the originalloss-of-coolant accident and steam
- generator tube rupture analyses for Sequoyah Chapter 15 analyses. These limiting values !
bound the effects introduced through the addition of the LTAs to the core. The parameters of interest for these transients are the reactivity coefficients, pellet radial power profile, fuel i material properties, fuel temperature and fill gas pressure, and the decay heat. Most of '
these parameters are affected by the increased uranium-235 enrichment and not directly l changed by the uranium-234 and uranium-236 isotopes of the blended uranium fuel or are l bounded by the original safety analysis. The exception is the decay heat produced by the blended fuel. The decay heat of the blended fuel will be slightly higher than standard :
uranium fuel; however, the increase is bounded by the original plant safety analysis. I i
The overcooling, overtemperature, decreased flow, reactivity anomaly, increases and j decreases in reactor coolant transient classes were reevaluated to ensure that the original '
safety parameters used in the licensing analysis bounded the parameters generated with the safety parameters used in the licensing analysis bounded the parameters generated with the use of blended uranium fuel. It was found that the differences in the safety parameters introduced from the use of the blended uranium were mostly introduced from the higher uranium-235 enrichment used in the LTAs. The original safety parameter values were bounding for all the overcooling, over temperature, decreased flow, reactivity anomaly, increases and decreases in reactor coolant transients.
An evaluation of the plant radiological consequences was performed. Source term activity of the LTAs at a maximum assembly bumup of 1500 effective full-power days was l
calculated to be lower then the activity of standard uranium fuel. Therefore, the evaluations !
of all the radiological release accidents are bounded by the original analysis performed with standard uranium fuel. ,
i The preliminary core operating limits evaluation for the Sequoyah reload with the LTAs l
. demonstrated that the core operating limits would not be violated by including the LTAs in the reload. Additionally, analysis showed that the LTAs would not be the most limiting fuel assemblies in the core even when the core was being operated at the limits specifed in the !
core operating limits report. Based on the preliminary evaluation, the core operating limits l
will not be exceeded with the addition of the LTAs and none of the LTAs will be the most limiting fuel assembly. A final core operating limits evaluation must be performed prior to l core reload with the LTAs to verify that the operating limits are not violated nor any of the LTAs will be the most limiting fuel assembly in the core.
3.0 CONCLUSION
S Based on our review, the staff conch Ms bat the proposed topical report, for the addition of a limited number of LTAs containing bl6nded uranium fuel, is acceptable for Sequoyah
}. Unit 2 because the changes introduced from the addition of the LTAs to the core are mostly from the higher uranium-235 enrichment of the fuel, The changes directly related to the j higher content of uranium-234 and uranium-236 are bounded by the original safety analysis parameters. This acceptance is subject to the following conditions:
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- 1. The down blended uranium lead test assemblies may not be the most limiting fuel assemblies in the core at any time during their use in the core.
- 2. A final core operating limits evaluation must be performed prior to core reload to verify that the fuel design criteria and specified acceptable fuel design limits are met for the lead test assemblies for the anticipated specific in-core conditions.
- 3. The use of these lead test assemblies must be included in the plant technical specifications.
- 4. ' The use of this topical report is valid for the insertion of a maximum of four lead test assemblies into the core at any time during their residence in the core as described in the supplementary letter from TVA on November 5,1998.
The staff has concluded, based on the consideration discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operat' .1 in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: U.Shoop Dated: February 18, 1999
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