ML19343C493

Safety Consideration for 24 Element Graphite-Reflected Core.

ML19343C493
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Site:	05000087
Issue date:	12/03/1980
From:	WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
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WESTINGHOUSE NUCLEAR TRAINING CENTER NUCLEAR TRAINING REACTOR Safety Ccnsiderations for the 24 Element Graphite-Reflected Core i December 3, 1980

1. As defined by the Technical Specifications, Section 4.0, operation of the 24 element graphite-reflected core (Figure 1) is an " experiment".

Therefore, operations to evaluate the 24 element reflected core are authorized by the current facility operating license.

2. However, if the 24 element graphite reflected core is to be considered the normal configuration, then the reactor, as described in

, -the Safety Analysis Report, will-be changed. 10CFR50.59 states that this may be completed without prior NRC approval provided that there is no unreviewed' safety question involved. The identification of'an unreviewec

safety question willinecessitate'an amendment to the operatinag 1.icense.

Establishing [thatthere_is-nounreviewedsafetyquestioninvolves

{ -demonstrating that: .(i)lthe' probability or the consequences of any-analyzed accident are not-increased, -(ii) there is no possibility ofian unanalyzed accident occuring, and (iii) no margin for safety in any.

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.TechnicaliSpecification basis-is reduced.-

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3. Probability and' Consequences of Analyzed Accidents The following accidents are analyzed in the Safety Analysis Report:

Flood Earthquake

' Windstorm Fire

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Loss of electrical power Uncontrolled rod withdrawal Uncontrolled moderator insertion Failure of. experiments Reactoriloadings Temperature coefficient measurements Veio coefficient measurements Importance function measurements Flux distribution measurements and power calibration Reactor physics' parameter measurements Irradiation experiments Operational demonstrations and experiments

. Mechanical' damage and failure Reactor Maintenance Radiation conditions and analysis

Normal. operating levels Loss of shielaing water.

Radioactive material storage areas Fuel _ storage Radioactive sample and source storage

-Fuel element handling and irradiated material handling

. Maximum creditable accident Most of these analyses would.be unaltered by the 24 element graphite reflected core. Exceptions'are 'iscussed d below.

3.1 Fire TheLgrapnite comprising the reflector ~ rods'is not normally considered

- a combustible material. -Graphite does undergo oxidation in air in a reaction

- the rate of which-is-increased with' temperature. Normally the graphite rods

would be: located- underwater. Even when out of the water the scarcity of other

flammable materials insidelthe building makes-it unlikely that the graphite could ever reach the threshold temperature of about 800*F for-the oxidation reaction..

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Graphite has a very small neutron absorption cross-section (on the order of .003 barns for tnermal neutrons), tnerefore, the reflector rods will not become activated during reactor operation and consequently would not contribute to a radiological nazard in the unlikely event of a fire.

3.2 Uncontrolled Moderator Insertion Inserting the reflector rods has the same effect on the reactor as a moderator insertion. The rods are inserted by hand, which means that the reactor must be snut down to make the insertion. Since the fully reflected core is still substantially subcritical with all control rods inserted, the insertion of reflector rods does not represent a reactivity addition hazard.

The-Safety Analysis Report considers it incredible that a core be loaded without control rods present.

3.3 Temperature Coefficient Measurements

.The reflector rods will each displace approximately 260 in3 gf moderator-shield water for a total of about 52003 in = 3 ft 3

- 22.5 gal-

lons, this represents less than 1 percent of the approximately 2800 gallons of water-in the reactor-' tank with.the level at about the top of the upper reflector region of the core. Since. graphite has a specific heat on the same order.of magnitude as that of aluminum, it will have a negligible effect on the rate of temperature change during temperature coefficient measurements.

3.4 Void-Coefficient and Importance Function Measurements The void tube and absorber material specimens could conceivably be

-worth'more than'O.8% of reactivity near the center of the smaller core. None

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of these materials,'however, is subject to a catastrophic failure which could insert 0.83 of reactivity in a positive step.

3.5 Irradiation Experiments Reduction of.the Tcel inventory in tne core from 37 to 24 fuel ele-

.ments would raise the peak thermal neutron flux at full power from about

-8 x 10 10 n/cm2 /s to about 37/24 x 8 x 1010 = 1.2 x 10" n/cm2 /s, Lslightly greater than'the 10 lI n/cm2 j3 ass'umed in the Safety Analysis 3

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. Report (page 7-7). There is, however, no limit on the flux, only on the induced activity, ano no proolem is forseen in adnering to the limit, es-

'pecially'since only small gold and indium foils are ever routinely irradiated, producing activities on the order of only microcuries.

3.6 Mechanical Damage and Failure The support structure of the core bears less total weight in that the gain in weight from displacing water by graphite is more than made up by the reduction in the numoer of fuel elements. This should more than make up for any displacement in loading caused by the asymetrical placement of the Core in

-the reactor tank.

The control rods weigh on the order of 80 pounds apiece. Any change in'the deflection of the rod drive platform and support beams brought about by removing four control rods from the south side of the platform should De negligible.

The graphite'of the reflector rods is fairly strong, but soft. Care

. must be taken to avoid marring their surfaces. Under extremely rough hand-l

. ling, pieces of graphite could'conceivabl) be dislodged. At' worst, these might lodge in the-seats of the fast or~ fine dump valves, preventing their complete closure, which is.a failure in the safe direction of moderator removal.

Graphite tends to' change its. physical properties to a remarkable extend under neutron irradiation, becoming harder, more brittle and swollen in

- size. The onset of these changes, however, requires exposure to an integrated neutron flux on the order of 10 I9 nvt. Even if the thermal neutron flux at

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the reflectorErods.were as high.as 10 10 nv at full power, the safety limit

- of-200 kilowatt hours'per year would mean that it would still take on tne

- order of 19 10 4 10

- 4 x 10 years-for~tnese effects to manifest themselves, l10 ix x 3600-

so.that theylare of no_ concern in the NTR.

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i Fuel element insert adapters will be used to hold the reflector rods in' locations 6-3 ana 6-9. The reflector rods will weign more than the fuel 4

elements for wnich the adapters were designed, by a ratio of about 20 pounds to 14 pounds = 1.4. It is conceivable that the insert adapters could fail under this extra weight, particularly if a reflector rod were accidentally dropped into tne adapter-tune. Tne worst consequences woula be for adapter and rod to creak on impact, an event having no safety implications. A re-flector rod woula be removed from its location, adding negative reactivity to the system, which is failure in the safe direction.

3.7 Radiation Conditions and Analysis

'The graphite reflector rods represent poorer gamma and neutron shields than the water they displace. Consequently, radiation levels at the

side of the reactor tank may be expected to increase slightly, especially at I' the north side'where the core'itself is closer to the tank wall. Radiation

. levels outside the reactor tank are nominal in any event, and it is doubtful

~that any increase-would be measureable.

-Since the neutron absorption cross-section of graphite is so low (on

'the order of .003' barns for thermal neutrons), the reflector rods will not i

become activated and tnerefore will not represent a radioactive hazard in themselves nor contribute to-the radiation hazard during a loss of shielding water accident.

Because the critical water height will be higher in the 24. element core, the reactor will. shut itself down sooner on a loss of shielding water accident.

~3.8 Maximum Credible Accident

, Under.tne postulated conditions of the maximum credible accident, the graphite reflector rods would'be subject to a steam and water environment. Of 1 concern under.tnese. conditions are the possibilities of-oxidation of the graphite with consequent release of-hydrogen and carbon monoxide gases and of-annealing.the graphite with the consequent. release of Wigner energy as heat.

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1 The temperature for the onset of the oxidation reaction is on the order of 600*C. Since tne hottest temperature in tne reactor is estimated (SAR, page 7-17) to be about 360*C (slightly higher in the slightly smaller 24 element core), the graphite rods will not increase the consequences of tne accident.

The integrated neutron flux required for the builaup of Wigner energy due to radiation damage is on the order of 10 I9 nvt. As explained in sec-tion 3.6 of -tnis analysis, integrated fluxes of this magnitude can not cred-ibly be achieved. Therefore, the graphite rods will not increase the con-sequences of the accident.

4. -Probability of an Unanalyzed Accident The change in core configuration and addition of graphite reflector rods idoes'not admit the possibility of any unanalyzed accident occurring. Graphite

.is chemically' inert and insoluble and would neither degrade tne moderator-shield water quality nor react with the aluminum core structure. References on graphite as a reactor material-are included in the appendix to tnis-analysis. Purchasing documents must specify an adequate grade of graphite purity.

5. ~ Technical Specification Margins for Safety

~The principle items _ pertaining to safety discussed in the Technical Speci-(fication bases are listed in this section. Most require no comment since they-would essentially be~ unaffected by.the proposed change in core loading and reflection. Comments, where made,- are underlined. The applicable section of the Technical Specifications are parenthized.

5.1 The average moderator temperature increase at 20 kw is less than 10*F.. (2.1) 5.2 The radiation' level immediately outside the reactor rocm at 20 kw is.

less than 20 mrem / hour. As discussed in-section 3.7 of this analysis, any possible increase in the radiation levels would likely be so small-as~to be negligible. _ (2.1);

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5.3 The estimated error in absolute neutron flux measurement by acti-vation metnods is 20 percent. The hot channel factor and fuel mass constants used in tne flux determination will oe different out will not effect the accuracy of the determination. (2.2) 5.4 The maximum error in the nonlinearity of the neutron flux monitoring instruments is 35 percent. The characteristics of the instrument channels will be unchanged. The neutron flux at the detectors will increase for Channel A and probably decrease for Channels E and F, due to the Geometry of the core and detectors (Figure 1). Analysis of the core must provide this assurance before the proposed configuration is operated. The prediction must be verified early in the testing programs. (2.2) 5.5 The minimum flux level specified will prevent a source-out startup.

.(2.2) 5.6 A startup transient would be terminated in less tnan 200 milliseconds after a period trip. (2.2) 5.7 .Tne gamma level channels will assure that increasing radiation levels will be detected before they become excessive when the reactor is operated at moderator-shield water. heights other than.the normal le/el. (2.2) 5 '. 8 There is adequate shutdown capability even for the stuck control rod condition. Analysis of the core must provide tnis assurance before the pro-posed configuration is operated. The prediction must be verified early~in the evaluation program. (3.1.1) 5.9 The maximum excess reactivity for the reactor is 153. The same comments apply as'for 5.8,'above, although it may be noted that the excess reactivity available in-the N-37-S core is conside'rably less than 153, and will be still less in the 24~ element reflected core. (3.1 Bases, Paragraph 2) 5.10 The maximum contral rod withdrawal and moderator-shield water reac-tivity addition rates, far irom and near criticality, assure that the reac-tivity addition. rate is.less than that analyzed in-the maximum credible acci-dent. : The same comments apply as for 5.9,- above.- ( 3.1. 2 ) __

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5.11 The control rod insertion time from fully withdrawn assures that the assumed time for establishing the minimum period LSSS is satisfied. (3.1.3) 5.'12 Control rod withdrawal prior to adding moderator-shield water as-sures that reactor trip will have tne capat'ility of adding negative reactivity during reactor startup. (3.1.4) 5.13 Tne auxiliary reactor trip assures that there is a secondary mode of shutdown available during reactor operations. The minimum time for it to add negative reactivity limits the consequences of a potential power transient.

Negative reactivity will be added sooner in the 24 element reflected core because'of its higher minimum critical water level. (3.1.5) 5.14 The normal moderator-shield water level assures an adequate shield during maximum power operation. -At-lower moderator levels, reduced trip settings are required to further reduce the possibility of operating with a high neutron and' gamma. radiation field. Controlling the reactor by moderator.

level near criticality only after ther reactor is first made critical by control rod' movement assures that the' control rod is the primary mode of reactivity control in the critical reactor. (3.1.6) 5.15 The manual reactor trip assures that the trip may De activated readily by either the operator-or individuals in the. reactor room. (3.1.7) 5.16 Electrical-power interruption provides an additional mode to manually trip.the reactor. ~~(3.1.8) 5.17 The minimum safety system channels provide a high degree of. redun-dancy.to assure that human or mechanical failures will not endanger the reac-

. tor _ facility or.the general public. (3.1.9) 5.18 The interlock system assures that only authorized personnel can operate the reactor, that the proper sequence of. operations is performed,.that-no<one can accidentally enter the reactor room and.that'the reactor' room is entered.witn proper conditions prevailing when the master key is on. L(3.1.10)

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5.19 The minimum absolute value of the temperature coefficient of reac-tivity assures that an adequate innerent negative reactivity effect takes place when the reactor temperature increases above the value where the coef-ficient become negative. (3.2.1) 5.20 The minimum absolute value of the void coefficient of reactivity assures that the negative reactivity insertion due to void formation is greater than that which was calculated to occur in the SAR. (3.2.2) 5.21 -The moderator-shield water quality assures adequate corrosion con-trol in the reactor environment. Grapnite will not reduce the water auality as discussed in section 4 of this analysis. (3.2.3) 5.22= Area radiation monitors assure warning of the existence of any abnormally high radiation levels. Tne availability of instruments to measure air and water activity assists in monitoring fuel clac integrity and assures continued compliance with the requirements of 10CFR20. The availability of portable monitors provides assurance that personnel will be able to monitor potential radiation fields before an area is entered. (3.3) 5.23 To assure that experiments are well planned and evaluated prior to being performed, detailed written procedures for all new experiments must be

-prepared, reviewed by RSC and approved by the Facility-Manager. This t

. requirement will be met for the 24 element graphite reflected core. (4.1) 5.24 Since the car ~ ol rods enter the core by gravity and are required by

.other Technical Spect ions to be operable, no experiment,should be allowed to interfere with theli  : ions. To assure that specified power limits are

.not exceeded, the nuclear urumentation must be capaole of accurately moni-toring core parameters. The 24 element graphite reflected core will alter the flux seen by the nuclear instrument detectors but will in no way interfere with their operation or function. See section 5.4 of-this repcrt. (4.2) 9

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-5.25 A maximum reactivity change is established for the remote posi-tioning of experimental samples and devices during reactor operations to assure.that the reactor controls are readily capable of controlling tne reactor. Tne experiment will not include remote oositionina of experimental samples and devices. (4.3) 5.26 All experimental apparatus placed in the reactor must ce properly

. fabricated and made physically secure in the reactor. In consideration of

. potential' accidents, the reactivity effect must ce limited to the maximum accidental steo reactivity insertion analyzed in the SAR. In actual practice,

-no single-unit of experimental apparatus will be placed in tne reactor anich has'a reactivity worth greater than 0.803 wnich is less tnan the reactivity addition accident analyzed in tne SAR. Refer to section 3.4 and 3.6 of this analysis. (4.4)

'5.27 Restrictions'on irradiations of explosives and highly flamnable materials are-imposed to minimize the possibility'of explosions or fires in the vicinity of the reactor. To minimize the possibility of exposing facility personnel or:the public to radioactive materials, no experiments will be

-performed with materials that could result in-a violent chemical reaction

-and/or produce airborne: radioactivity. -Refer to sections 3.1 and 3.5 of this analysis. (4.5)'

6. Conclusion Since it has_ been established that.(i) the proDability or the consequences of'any analyzed.' accident are not increased, (ii)'there is no possibility of an unanalyzed accident occurr_ing, and (iii) no margin for safety _in any Technical

-Speciffcation basis is reduced, it_is very unlikely that there.are any unreviewed safety questions involved in the operation of the 24 element' graphite reflected, core. Consequently, there is no need for NRC approval prior to; operation and no.need to amend the NTR Facility operating license.

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REFERENCES ON GRAPHITE AS A REACTOR MATERIAL S. Glasstone and A. Sesonske, " Nuclear Reactor Engineering", Von Nostrand Reinhold, New York, 1967, pages 438-442 H. Etherington (ed.), " Nuclear Engineering Handbook", McGraw-Hill, 1958 (First Edition) pages 12-70 to 12-74, 10-55 to 10-59,10-109 to 10-115,13-180 C.'R. Tipton, Jr. (ed), " Reactor Handbook Volume I - Materials", Interscience, New~ York, 1960 (2nd Edition)

Chapter 43, Graphite, by L. D. Loch, pages 888-896 Chapter 53, Bulk Shielding Data by R. O. Schamber, et. al., pages 1119 and 1123 Cnapter'41, 3eview of Moderator Materials by E. M. Simons, pages 835-837 Proceedings of the Internat %nal Conference on Peaceful Uses of Atomic Energy, United Nations, N.Y.,1956 Volume 7, " Nuclear Chemistry and the Effects of Irradiation" Irradiation Damage to Artificial Graphite by Woods, et. al., pages 455-471 The Effects of Irradiation on Graphite by G. H. Kinchin, pages 472-478 Volume 8, " Production Technology of the Materials Used for Nuclear Energy" The Production and Properties of Graphite for Reactors by Currie, et. al.,

pages 451-473 T. J. Tnompson and J. G.'Beckerly (eds.), "The Technology of Nuclear Reactor Safety",

'The-M.I.T. Press, Cambridge, Mass., 1964 Volume 1, " Reactor Physics and Control", pages 633-636 Volume 2, " Reactor' Materials and Engineering", pages 436, 464, 469, 45-A. R. Kaufman (ed.), " Nuclear Reactor Fuel Elements Metallurgy and Fabrication",

Interscience, New York, 1962, page 251 (NOTE: .The following sources were not consulted in the-preparation of n; NTR safety

. analysis,-but are listed for completeness.)

J.-H. W. Simmons, " Radiation-Damage in Graphite", Pergamon Press, New York, 1965 W. G. O'Driscoll and J. C. Bell, Graphite: Its Properties and Behavior, Nuclear Engineering 3: 479-489-(Nov. '58) ana 533-537 (1958)

R. F. Nightingale (ed.), " Nuclear. Graphite",' Academic Press, 1962 007SC F