ML20216D418
| ML20216D418 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 07/23/1999 |
| From: | Mckibben J, Schoone A MISSOURI, UNIV. OF, COLUMBIA, MO |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9907280109 | |
| Download: ML20216D418 (4) | |
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4 Research Reactor Center Research Park coiomi,;,,.uo 6n ii University of Alissouri-Columbia nioxr (573) 882-4211 w
nx(573)882 3443 July 23,1999 Document Control desk U.S. Nuclear Regulatory Commission Washington D.C.,20555
REFERENCE:
Docket No. 50-186 University of Missouri Research Rector License R-103
Subject:
Response to the NRC Request for Additional Information Dated May 21,1999.
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The University of Missouri Research Reactor (MURR) provides the following responses to the Nuclear Regulatory Commission letter of May 21,1999, requesting additional information required to evaluate our license amendment request dated May 4,1999. As per a phone conversation with Mr. Marvin Mendonca on July 7,1999, as extension in the response time to within 60 days cf receipt of the letter was approved.
The specific request for information and our responses to the questions are attached. If you have any further questions, please contact Anthony Schoone (573-882-5296) or Charlie McKibben (573-882-5204).
Sincerely, Endorsement:
Reviewed and Approvgd Y thuf Y ct%A
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Anthony R. Schoone J. Charles McKibben Reactor Manager Associate Director attachments xc:
Reactor Advisory Committee RAC Safety Subcommittee Dr. Edward Deutsch Mr. Thomas Burdick, U.S. NRC, Region III Mr. Alexander Adams, U.S. NRC, Washington, D.C.
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4 1.
Please describe the methods used to estimate and/or measure the reactivity worth of samples irradiated in the center test hole and the accuracy of the methods used.
The worth of samples irradiated in the center test hole is estimated by a one-energy group importance function calculation based on reactivity type and position factor utilizing perturbation theory. The position factor is the one-energy group reactivity importance function of the flux in the space where the sample is being irradiated. Therefore, the position factor for an experimental can is a function of the sample can length and diameter, and vertical position in the center test hole. In perturbation theory, the one energy group importance function is proportional to the relative flux squared. For each of the different reactivity
- types,i.e., significantly different composition of sample and can materials, there is a type worth. The type j
worth is an effective macroscopic cross section for the overall sample and can materials. The type worth for each sample type is calculated from a sample reactivity measurement for that sample.
Reactivity worth of an individual sample irradiated in the flux trap region of the reactor is measured by calculating a reactivity difference for the presence and absence of the sample. This is done by comparing the worth of the sample against that of a water-filled (flooded) can of the same size. A flooded can is placed in the Center Test Hole in the irradiation position where the sample will eventually be irradiated. A reactor startup is performed: the Shim and Regulating blade positions, and the pool and primary coolant temperatures, are recorded after reaching low power, steady state operation. Low-power operation is required in order to minimize the production of xenon. After the blade and temperature values are j
recorded, the reactor is shutdown and the flooded can is removed and replaced with the encapsulated sample whose reactivity worth is being measured. Then a second reactor startup is performed and again the blade positions and coolant temperatures are recorded. The reactivity worth due to the sample can be calculated from the changes in the blade positions and coolant temperatures.
This reactivity worth of the sample is calculated based on the known blade worth curves and from the difference in blade positions for the two startups. A reactivity conection is made for any coolant temperature changes by using the measured temperature feedback coefficients.
When new sample types need to be irradiated, the type worth is estimated by comparing the new sample to l
previously run samples with similar microscopic cross sections and sample size. The type worth is
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estimated and used to calculate the perturbation theory estimation. The new sample reactivity worth is then measured by the above method while loading the sample at a position and with a combinatmn of other samples so that even a significant error in the type worth estimation would not cause the total reactivity worth of samples and experiments in the center test hole to exceed the technical specification limit.
The accuracy of the sample worth measurements depends on the accuracy of a host of factors including the accuracy of the blade worth curves, the blade position indicators, the temperature indicators and temperatur: feedback coefficients, and finally the accuracy of having the reactor critical (Keff = 1.0) when the blade positions and coolant temperatures are recorded. Of all the above contributors, probably the biggest uncertainty arises from the blade worth curve. The total worth of the regulating blade is typically in the range of 0.002 to 0.003 AK. In the most reactive region of the regulating blade (between 8 and 16 inches withdrawn), the uncertainty is of the order of 0.0002 AK.
The overall accuracy of the perturbation theory estimation (to assure it is within technical specifications limit of not exceeding 0.006 AK) can be periodically checked by performing a total cumulative reactivity worth of all experiments in the flux trap region of the reactor. The cumulative reactivity worth is also measured by calculating the reactivity change between two different low power reactor startups. A reactor startup is performed without the center test hole installed, but with a strainer installed to prevent anything from falling through the flux trap region. The Shim and Regulating blade positions and the pool and primary coolant temperatures are recorded after reaching low-power, steady-state operation. After the blade and temperature information is obtained, the reactor is shutdown and the center test hole is installed for which the reactivity worth has to be determined. Then a second reactor startup is performed and again the blade positions and coolant temperatures are recorded. The total reactivity worth of the center test hole and the samples loaded can be calculated from the changes in the blade positions and coolant temperatures.
l Computer programs can also be used to estimate the reactivity worth of samples. When needed, the Monte Carlo program (MCNP), developed by the Los Alamos National Laboratory, is generally used to model the core and to estimate the reactivity of samples. Criticality calculations are performed with and without the sample present. The accuracy of MCNP criticality calculations is of the order of 0.001 AK. If the sample is too small (reactivity worth too small), the worth of a larger sample of the same type is first calculated and then the worth of the actual sample is estimated. A linear relationship between size and worth is assumed unless there is significant self-shielding in the sample.
After receiving the approval for running unsecured and movable experiments in the center test hole, the
. reactivity worth of a new sample type, which is to be a movable experiment, will be measured as an unsecured experiment by doing two low power startups with a xenon-free core. The reactor will be taken critical with and without the " unsecured" experiment installed. The reactivity worth of the sample will be calculated from the changes in the blade positions and coolant temperatures. After validating the experiment can be classified as a movable experiment, the experiment can be either inserted or withdrawn to cause a positive reactivity addition with the reactor critical at low power and no xenon. The doubling time for the positive reactivity addition can be used to obtain a more accurate reactivity worth for the movable experiment.
2.
Please describe your experience with estimating the reactivity of samples placed in the center test hole. Over the past 10 years have there been any samples where the pre-irradiation reactivity estimate and/or measurement was in error to the extent where it would have caused a violation of reactivity limits considering the requested changes to the license?
Periodic reactivity measurements have been routinely performed. Over the past 10 years, no measured cumulative reactivity worth for experiments being irradiated in the center test hole has exceeded the technical specifications limit of 0.006 AK. MURR uses an administrative maximum limit on the estimated cumulative reactivity (less than the Technical Specifications limit) allowed to be irradiated to prevent the differences between the estimated worth and the measured worth from causing a violation of the reactivity limits. Our methodology to estimate the cumulative reactivity worth utilizing the one energy group importance function calculation has enabled us to operate within the technical specification reactivity limits. MURR also has administrative controls that limit the acceptable error in determining overall reactivity changes. These administrative controls limit the control rod height difference between estimated critical position and actual critical position. If significant differences are found between the estimated cumulative reactivity worth and the measured worth, additional individual sample can reactivity worth measurements are made to update the type worths to improve the estimated worth.
Adding the capability to irradiate unsecured and movable experiments in the flux trap will not alter our ability to maintain good agreement between the estimated cumulative worth and the measured worth.
Additionally, our conservative approach to defining an experiment as " movable" coupled with the reactivity worth of movable experiments being measured more accurately than for secured and unsecured i
experiments should mean that this change will not increase the potential for a violation of reactivity limits.
Periodic measurement of the cumulative reactivity worth of all experiments in the flux trap region of the reactor will continue to be performed. These measurements can be used to ensure that the cumulative reactivity is within the technical specification limits.
3.
In TS 3.6 g. you changed the first word of the word of the TS from " Experiments" to
" Experiment." If this is not a typographic error please explain why you are making this change.
This was a typographical error; there was no intention of changing " Experiments" to " Experiment" in Technical Specification 3.6 g. A corrected Technical Specification sheet is attached.
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TECHNICAL SPECIFICATION
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UNIVERSITY OF MISSOURI RESEARCH REACTOR FACILITY Number 3.6 Page 2 of 5 I
Date
SUBJECT:
Exneriments (continued) l air borne concentration of radioactivity averaged over a year will not exceed the limits of Appendix B, Table I of 10 CFR Part 20. Exception: Fueled
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experiments (See Specification 3.6.a).
d.
Explosive materials shall not be irradiated or allowed to generate in any experiment in quantities over 25 milligrams.
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e.
Only movable experiments in the center test hole shall be removed or installed with the reactor operating. All other experiments in the center test hole shall be removed or installed only with the reactor shutdown. Secured experiments shall be rigidly held in place during reactor operation.
f.
Experiments shall be designed and operated so that identifiable accidents such as loss of reactor coolant flow, loss of experiment cooling, etc., will not result in l
a release of fission products or radioactive materials from the experiment.
g.
Experiments shall be designed. such that a failure of an experiment will not lead l
to a direct failure of other experiments, a failure of reactor fuel elements, or to interference with the action of the reactor control elements or other operating components.
h.
Cooling shall be provided to prevent the surface temperature of a submerged irradiated experiment from exceeding the saturation temperature of the cooling medium.