DESIGN OF Д CALORIMETER FOR MEASURING GAMMA RADIATION IN AN EXPERIMENTAL REACTOR

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1 ZJE V. Лпмйк, J. KoU, J. Plániíka, S. Ttran DESIGN OF Д CALORIMETER FOR MEASURING GAMMA RADIATION IN AN EXPERIMENTAL REACTOR SKODA WORKS MKIMT Powtr CoMtrediofl DtperiiMflt, Infomiaiioii Смита PLZEŇ - CZECHOSLOVAKIA

2 ZJE V. Jiroušek, J. Kott, J. Plénička, S. Teren DESIGN OF A CALORIMLTER FCR MEASURING GAÍ3SA RADIAÍICN IN AN EXPERIMENTAL REACTOR SKODA WORKS Nuclear Power Construction Deportment, Information Centre, PLZEN, CZECHOSLOVAKIA

3 - 2 - ABSTRACT The report describee briefly the calculation technique and the design of calorimeters for in-core measurement of gamma dose rete within a region of 10 5 up to 10 9 red/hour. Two alternatives, i. e. an axial and a radial arrangement of the calorimeter are considered. The materiel of calorimetrical bodies are tungsten and platinum. ттт^штшчтф/щ

4 LIST OF SYMBOLS <p (W/cm 2 ) H Л /* G K(T) т т T t> p T, L г r с Or P P Q zc*) V (W/g) (cm 2 /g) (1/cm) (-) (W/g. C) ( K) ( K) ( C) ( C) (cm) (W/cm. C) (g/ca 3 ) ( C) (-) (cm) (ca) (W) (f/cm. C) ( /cm) («> photon field intensi»y dose rate mass effective cross section for the absorption ot energy of gamma radiation linear effective croee section for the abeorption of energy of gamma radiation self-shielding factor of the sody coefficient of heat transfer from radial calorimeter temperature of calorimeter body temperature of calor -.&<.. *.<.r -icket temperature of the free end of the body of conductivity-type calorimeter temperature of the fixed end of the body length of the body coefficient of thermal conductivity of the body specific weight of the body Sutherland constant Graahof number inner radiu* of calorimeter jacket radius of calorimeter bod? mount of beat removed from tne body into the jacket by conduction thermal conductivity of gas equivalent thermal conductivity of the gap at a given gradient mase of the body

5 - 4 - INTRODUCTION The importance of in-e^re measurement, both in experimental end power reactors, has increased markedly in the recent years* The conception of in-core measurement, sensors and instruments was established towards the end of sixties. We have concentrated our effort predominantly on the calorimetrical method of detecting reactor radiation. As may be found in ref. 1, 2, 3» 4, and 5, a number of successful measurements have been done with calorimeters of cur design and manufacture» These measurements have been in Czechoslovakia as well as Abroad, As a /sensor of reactor radiation, a calorimeter can be employed for a continuous in-core measurement at full power, with the amount of absorbed radiation energy being determinable by means of thermal quantities. The report presented here has been worked out as an answer to a request for calorimeters for measuring gamma radiation dose rate within a region from Kr up to lcr rad/hour e It is possible to us3 Individual calorimeters for simple monitoring of reactor power, which will be done for ensuring stabilized conditions throughout extended irradiation experiments in the core, as well as to make use of multiple oalorimetrical eyeterns for performing epectrocalorimetrical measurements in experimental channels with diameters of 38, 41, 62, 80, and 100 ma. OPERATIONAL HEfllON OF THE CALORIMETERS As stated hereinabove, the working region of the calorimeters is stipulated within 10* and 10 9 red/hour. This 7 11 corresponds to 10' up to 10 eryg.hour, or to mv/g up to 2.78 W/g in the air.

6 - 5 - The technique of calculation presented hereinafter requires t.o know the corresponding intensity of tlí photon field /i. f. of the flow of energy/. This can be expressed es, H <p = /1/ The mess effective cross section pi^ for the absorption c: gamma energy in the air exhibits only a weak dependency, or- 'he energy and equals cm /g. After inserting into the foregoing relationship for the dose rate ran/ring from 2.78 * 10" 4 up to 2.78 W/g, and neglecting the self-shie". in air, ф will range from 9-93 mw/cm 2 up to 99.3 W/cur 2 >r after some simplification, /2/ It is seen that the upper limit is considerably high and corresponds to a level expectable in great power reactors. For comparison purposes let us mention a value of ф - 50 W/cm which is valid for the VVR-SM core of the EWA-10 reactor in Poland at a power level of 8 VS. THE CHOICE OF MATERIALS *'0R CALORIMETRICAL BODIES Because the request involves measurement of gamma radiation in the reactor core, it is necessary to select for the csir,- riaetrical bodies materisle sensitive especially to garrrra radiation. These will ba mainly heavy elemente. Furthermore a heating of these materials due to other reactions occurring in the reactor core should be stated as another weighty requirement. It is also necessary to take into ec. * that the calorimeters must be operable in a broad tempers.' region practically within 0 and 600 <j f and must comply n a number of additional, mainly technological demands f ^ their machinability and weldebility. They must be also -

7 - 6 - of being brazed with other structural materials and thermocouples. In the cose of so called spectrocalorimeters there exists en additional requirement calling for a high effective cross section for the absorption of energy /i. e. the self-shielding of the samples of various diameters must be the highest/. It whould be also stated that this cross section must be strongly dependent on energy. Some heavy elements seem also suitable from this viewpoint. Considered in this light, the best materials are platinum end tungsten. THE TECHNIQUE OF CALCULATION As an introduction it should be stated that it is impossible for one calorimeter to be used for measurement within a range of four orders. Because both radial and axial calorimeters measure a temperature difference on the jacket and body or. on Joth ends of the body with the help of sheathed thermocouples andibecause it is possible in operational measurement to channel and evaluate with an acceptable accuracy a temperature difference /by means of a data logger/ within 0.1 up to 10 mv /whi r ch corresponds to a temperature difference between 1 up to 100 C with a nearly linear dependency of temperature difference on the dose rate/, is it possible to cover with one calorimeter a two-order region of the measured quantity. The dimensions of calorimeters should be regarded as a compromise among sensitivity, space, heet transfer, and the technological possibilities of manufacture and assembly. On the basis of stipulated requirements and experience, the external dimensions of the calorimeters have been chosen as foliové: oiameter of calorimeter jacket 14 mm, and the length of the calorimeter 70 up to 80 mm. The diameters of bodies correspond approximately to a geometrical series with a quotient of 1.33* and have the following values: 3*9» 5-2, 6.9» end 9.1 mm. For measurements within a power region of four orders are proposed two types of calorimeters.

8 The radial calorimeters will be employed for the lower orders, while the axial ones are intended for the higher orders. Now follows a short description of both types: The radial calorimeter is represented pictorially in fig. 1 The heat generated in the body due to the action of gamma radiation is transferred via the radial gas gap. The received dose may be obtained on the jacket and the body using equations from ref. /6/ and /7/ : н - К(Т)С т т ~ T^ /3/ K/T/ is temperature-dependent coefficient of heat transfer from the calorimeter. The sensitivity may be modified within certain limits either by the width of the gap or by gases with different thermal conductivities. The axial type of calorimeter is shown in. fig. 2. The heat generated in the body due to the action of gamma rsdiatior is conducted through the materiel of the body into the cooler which is metalically connected to the base of the body. The sensitivity is dependent mainly on the lenght of the body /for a given material/. The gap is either filled by a noble gas, with a small thermal conductivity, or evacuated. Dose rata will be obtained in terms of equation /4/ taken over from ref. /8/, H - zz. - T T l 1 /4/ The calculation of calorimeters will be therefore divided into two sections, on* dealing with radial arrangement of сеюriaetert, and the latter solving questions associated wit v. * -». design of axial ones.

9 - 8 - RADIALL CALORIKETEKS This chapter deels with the determination of operational range for the calorimeters with proposed dimensions and bodies' materials /i. e. for tungsten and platinum/. Leading parftculars for radial calorimeters are presented in table 1. The heat transfer from radial calorimeters has been solved using Sutherland's technique. This method does not involve radiation, but.within e region up to 500 Q quickly furnishes results with an adequate accuracy. This method is applicable for closed spaces and for such a range of free convection where the transfer of heat energy between the heat-transferring surfaces is accomplished predominantly by conduction. In addition, the following condition for the gap is to be fulfilled f О 2 3.Se where the determining dimension is the width of the radial ges gap /R - R^/, end h = L/(R^ - Rp). Because condition /5/ is fulfilled, it is possible to proceed to solve the transfer of heat. Taking advantage of Sutherland's theory, the coefficient of heat conductivity may be expressed in the following terms: z «>~ TT7JF ">< ^ = ^ (0] ~^^ /7/ Z fo) being the thermal conductivity of gee at a temperature of 0 C. Stai&ng from the assumption that the amount of heat being transferred through any concentric surface of the round gap into the jacket is the same, the basic equation will assume the form as follows:

10 - 9 - ^7 -l&yf'-zzr* t r Q /8Л Inserting for У and into equation /6/ will result in the following final shape of the equation or Wr /9/ ^ Щ /io/ Ш where V(%/cmj expresses en equivalent thermal conductivity of calorimeter gap with the given gas and the temperatures of both heat-transferring media T and T-, while the fraction expresses the influence of gap geometry.. The maximum heat transfer /under given circumstances/ will be determined by the maximum permissible temperature of body surface T T * 600 C, and the temperature of calorimeter jacket T.. Two.ltematives have been investigated, with temperatures of calorimeter jacket 60 C, and 100 C /i. e. for T_ s 333 or 373 КЛ As a gee occurring in the gap has been supposed nitrogen. The minimum heat transfer from the calorimeters has been considered for a 5 C temperature difference between the the temperature of body and that of the jacket. The results obtained for individual calorimeters with different diemeters are presented in teble 1. The dependency of heat transfer in the gap filled with gas on the body temperature at a constant temperature of the jacket /independently on the geometry/ is presented for the maximum heat transfer /i. e. for a great value of T' T T - T / in' fig. 3, while for small differences of temperature in fig. 4. Both these fipur?a are valid fn* jftefcfit. temperatures 333 end 373 K.

11 The operational range of the photon field intensity restricts the amount of heat which can be transferred via the gap. This operational range of phoron field intensity can be obtained on the basis of the following equations 4 irtax ттах и, (h. *Z- /11/ r ^"* /f-f-<s The results of the calculation as obtained for the calorimeters containing tungsten are presented in table 2, whereas table 3 presents those valid for platinum calorimeters. In determining G, an effective energy of 0.5 MeV of the gamma spectrum has been considered. The more favourable case has been considered for the jacket temperature of 333 K» When using the instruments as spectrocalorimeters, the upper limit of the photon field intensity is to be related to the body dia 6.9 ой» while the lower one to the body dia 3.9 mm. Practically speaking, this results in an operational range of фе (O.C55 up to 6.59 W/cm 2 } for tungsten calorimeters, and for platinum ones in the range of Ф*(о.ОАб up to 5.69 W/cm 2^ otherwise the minimum temperature range would not be at least 5 C, and the maximum temperature of the body would exceed 600 c. The regions stated above correspond to approximate values of 5*5 x 10 ^ up to 6.6 x 10 rad/hour for tungsten and to 4.6 x 10* up to 5.7 x 10' rad/hour for platinum, respectively. Because it is required to use the calorimeters at the photon field intensity of 100 W/cm 2 /= 10'rad/hour/, it is necessary to broaden the range of apnlicability towerds higher dose rates. One way of doing it is to fill the calorimeter gap with a gas having a higher thermal conductivity. In our case it is helium. The results of calculation carried out similarly as in the case of using nitrogen, are presented in tables 1, 2, and 3, and in figures 3 and 4. Evidently, helium will made it possible, under the same conditions, to increase by 500 % the removal of heat and, /12/

12 consequently, the dose rete in the calorimeter. So, the operational ranges of helium calorimeters will be as follows /a spectrocalorimeter containing four bodies with chosen diame- I ters is concerned/: Ф /0.337 up to 39.5 W/cm 2 / for tungsten, and ф /0.282 up to 34 W/cm 2 / for platinum. It can be therefore implied that even helium is not capable of withdrawing so much heat as to enable the calorimeter to operate with a rate of 100 W/cm 2. There are two additional possibilities of intensifying the heat transfer. The first lies in diminishing the radial gap of gas, while the latter consists in using conductivity-type calorimeters where it is possible to take advantage oi higher thermal conductivity of the metallic body for the transfer of heat. Seen technologically,further decrease of the gap does noseem possible, because the proposed calorimeters have gap size of 1.75 mm and the diameter of body is 9.1 mm. The dependency of heat transfer on the size of gas gap is presented in fig. 7. The results have been obtained by solving equation /10/ for various radii of the jacket. AXIAt_C4b0RIMET HS, As mentioned above, is it necessary to adjust the sensitivity /and thereby the operational range of the calorimeter/ by the length of the body. The equation for body lenght will be obtained by combining equations /4/ and /1/. This yields: " ф р /13/ Similarly as in the case of radial calorimeters the calculation will be performed under the assumption that the least measured difference of temperatures is Ъ С, and the maximuji temperature of the body 500 C For every diameter of the body will be obtained a certain length. For conductivity-type calorimeters containing platinum this range will be between 2.03 and 2.37 cm, whereas for tungaten bodiee between 3.О6 and 3.51 cm. After unification according to the shortest body, the final

13 length will be 2 cm for platinum end 3 cm tor tungsten. In this way it will be mede sure tnat for 0 = ICO W/cm body temperature will not exceed 500 C. On the basis of temperature restrictions mentioned hereinabove it will be now possible to obtain from e modified shape of equation /13/ the operational region of the calorimeters within the following limits: +~>-1$ГГ (71 - П) ~ч /14/ ф "'- = '/»«в М- 7 ')* *> w with jacket temperature being assumed of the same value of T = 100 C /373 K/ as for radial calorimeters. The results of the celcubtion are given in tables 4 and 5- In the case of simultaneous operation of four calorimeters with different diameters of bodies in one spectrocalorimetric unit the operational region of platinum calorimeters will be from 1.75 up to W/cm 2, while the range of tungsten calorimeters lies between 1.71 and W/cm 2. Roughly speaking, 7 this corresponds to 1.75 ř 1'03 rad/hour or 1.71 x 10 up to 1.04 x 10^ rad/hour. The calculation neglects the loss of heat into the surrounding in radial direction /via the gas gap/. Nevertheless, this loss involves usually only a small portion of heat flowing parallel with the longer axis of the body /1 up to 5 %/ Its neglecting in a preliminary design seems to be justified. Comparison cf ranges of dose rates for axial and radial calorimeters shows that, the proposed types provide an overlapping of 1.71 up to Ó.58 W/cm 2 for tungsten bodies and 1.75 up to 5.69 W/cm 2 for platinum ones, thereby affording continuity in the measurement and e possibility to compere the calorimeters. It should be taken into consiaerbtion that the conductivity type calorimeters are capable of being used in the reactor for measuring within the region of all the four requested orders /with the two lower orders beinp uncepeble of being evaluated/,

14 while the radial calorimeters can occur in the cor-г only a- -werpowers, for at higher rates of reactor radiation they would ;,«destroyed. DESIGN As already mentioned in the chapter devoted to calculation the following basic dimensions of radial calorimeters have с chosen: external diameter of jacket 14 am, length of jacke- 70 mm, wall thickness 0.7 mm, diameters of bodies 3.9, 5.2, 6.9, and 9.1 mm, and the length 36 nun. For monitoring reactor power can be used any arrangement of radial celori meter /i. е.. with any diameter of the bcdy/. For performing spectrocalorimetx:~řl measurements, a simultaneous application of all four calorimeters attached to a common suspension tube is assumed. The design makes it possible for the measurement to be done for channels with a diameter as small as 38 mm. The design involves calorimeters operating directly i v. *'-a environment of the cooling water flowing through the reactor core. The temperature of every body is monitored by means of sheathed thermometers die 1 mm, while the temoerature of jackets, which under cooling by water will be practically the зала es that of water, would by monitored at least at two points for all four calorimeters. For the sake of lengthening the li'e span of the thermocouples, sheathed thermocouples will be -Ла:в<5 in a suspension tu'.-e in order to avoid their contacting with water. Fig. 8 shows a sectional view of one spectrocalorimeter. Proposed arrangement of the bodies, i. e. the alternation of smaller and larger diameter has been dictated by the nef t? diminish the self-shielding effect of thicker bodies. suspension facility makes it Possible for the cel'-rimete.-s to be roteteble. In this manner it is possible to map the сгforrr.otion of the photon field. Almost everything valid for the radial calorimeters n'.r.-.,e also apnlied for the other arrangement. The only differejv.-c- ; > in the length of the bodies /2 cm for ol*tinum end 3c:: tungsten/, and in the lenrht of calorimeters which wil ; - r.m

15 At every end of the body is one thermocouple die 1 mm. Measurement of the jacket temperature may be dispensed with in this arrangement. CONCLUSIONS These calorimeters cen be used in reactor core within the dose rate region from 10 up to 10' rad/hour. Covering four orders of power range cells for using two types of calorimeters, namely the ax^al calorimeter end the radisl one. The design stems from an arrangement of the bocy-ty-ve reactor calorimeters which can be considered nowadays r-.v classical one*. For the lower limit of the dose rite of 10^ red/hour /which corresponds to a tharnal output of JVR-M'. core of 1.6kW/, the above-mentioned design cannot le used for the purpose of in-core measurement, for the level of gam;;.a radiation after '.hutting the reactor down from a level of several megawatts is comparable, even after several days, with a gamma rediation with the reactor opeří ti-v with a Dower of several tens of kw. That is the reason why the proposed calorimeters have the lower thresholds i>.5 x 1С К red/hour for tungsten bodies, and 4.6 x 10'rad/hour for platinum ones. Shifting the lower thresholds of sensitivity towards lower values would require complicated modifications, mainly increasing the radial gap /and thereby increasing external dimensions of the calorimeters/. It ie also possible that it would call for evacuating the calorimeters, in order to reduce heat losses from body into the.jacket. (r. the other 't.?nd, broadening the range of applicability t.ov.ř.;rd i irher /flues by filling the gap with helium cannot be cor.gi <'rтч<. for rr, economical solution. The >.:.ь Iculr.t i on has been performed using simplified relationships, ťor instance, the following items have been ignored: the effect of radiation and of heat losses into the suspension of radial calorimeters, losses of heat by virtue of conduction.through the gas gap of axial calorimeters, etc. i'ecause even a detailed calculation cannot provide sufficiently precise value of the beat tr&nsfer coefficient, the calorimeters «re individually cahbrsted by ьеьпь of a heating c#il wound

16 onto the surface of the body. In this case it will be possible to carry out the reactor experiments with an accuracy of some 5 %. REFERENCES /1/ J. Kott, V. JirouSek, J. Němec, К. Dach, В. Hadak: Spectrocalorimetric measurements in the core of the heavy-water reactor KA at vince. Bulletin of the BKI of Nuclear sciences, Vol. 21, Chemistry, No. 2, Jugoslavia, /2/ J. Kott, v. Jirouěek, a. Teren, J. němec, M. Czerniewski, A. Janikovski, J. Kot, L. Labno: Photon flux intensity measurements in EWA-10 reactor radiation field by means of reactor calorimeters. Heport IBJ No. 1017/IXA/PR, /3/ J. Kott, J. Němec, V. Jirouěek, K. Dacii, a. Teren, A. Janikoweki, J. & 0 t: Celorimetric measurements on reactor EWA-10 in March and December, Report IBJ No. 1183/1XA/PR. /4/ J. Kott, J. Kot, M. Labrousse, L. Labno: Intercomparison measurements of the SKODA and INR calorimeters on the EWA-10 reactor. Report IBJ No. 1319/IXA/PR, /5/ Proceedings of the International Seminar of Intercomparieoh of Reactor Calorimeters, in awierk, Poland, May, Report ZJE-137, /6/ J. Kott: The technique of measuring radiation-induced heating of non-fissile reactor material» by.mesne of reactor calorimeters /in Czech/. Thesis. The Institute of Nuclear Research, Czechoslovak Academy of Science, 1969* /7/ J. Kott: Jaderná energie H /1968/ No. 2. /8/ V. Jirouatk, J. Kott: Analysis of the applicability of conductivity-type reactor calorimeter for measuring fission heat in the KS-150 reactor /in Czech/. Skoda Work» internal report, 1974.

17 I Table 1 MAIN PARAMETERS OF RADIAL CALORIMETERS diameter of body /mm/ body length /cm/ body volume /cnr/ pap width /cm/ Or /N 2 / Gr /He/ , h 0 ' * * ***/ % Q max in nitro en ; /W/ 0* Л Q min in nitrogen,/mw/ Q max in helium, /W/ Q min in helium,/mw/

18 Table 2 RADIAL TUNGSTEN CALORIMETERS diameter of body /mm/ body weight /g/ /ъп г П C.29.0 G /-/ ,763 Q max in nitrogen Q min in nitrogen Q max in helium Q min in heliím Table 3 RADIAL PLATINUM CALORIMETERS diameter of bod У /mm/ body weight /g/ /»V T (~) /-/ *710 Q max in nitrogen Q min in nitrogen Q max in helium Q min in helium

19 - ив - Table 4 AXIAL PLATINUM CALORIMETERS diameter of. body /mm/ body length /cm/ body volume /cnr/ body weight /#/ Ю G /-/ Q max (НГ/стУ Q min '(M*S) r Q тех /И/ Table 5 AXIAL TUNGSTEN CALORIE ETEKS diameter of body /mm/ body length /cm/ body volume /cnr/ body weight /g/ /-/ Q тех Q min (W/ш 1 ) Q тех /W/

20 Fig. 1. Arrangement of a radial calorimeter and bodies die 3.9 mm die 9.1 mm

21 řig. 2. Arrangement of an axial calorimeter and bodies dia 9.1 mm dia 3.9 mm

22 íl Fig. 3. Dependency of heut conduction (after Sutherland) in a round gap filled with nitrogen омз?з*к ом жоо too 900 WOO *т

23 Pig. 4. Dependency of heat conduction (after Sutherland) in a round gap filled with nitrogen (valid for asall Л = Xj - T ) ш +*> Z pkj

24 ш Pig. 5. Dependency of heat conduction in a round gap filled with helium (after Sutherland) 0.9 O.A 0.7 J*»'* A Ct- ЛЛ-».1 lóo t ЛОР r 900 *»o WiQ

25 i «яг - Obr. 6» Dependency of beat conduction in e round gap filled with helium (valid for a»all л7 * Tj - * p ) ^f /T P -339"K.*»" / /, /Tf,-W«a/M< а»- «а»- / / у / я v эю *** x» э+ф js» же т зве эх моо» но т г М

26 о Ы ко Fig. 7. Dependency of heat transfer on the width of round gap filled with nitrogen. Body dia Q.l ш, L = 36 mm, m T = - COO c.r\r\ W Оя C, «n - cr\»n I P- 60 "C R p O r fmmj

27 Fig. 80 Arrangement of a spectrocalorimeter in experimental channels dia 38 and 62 mm. á\a 2.1 u» d'.a 36 dia 6& d.a 9J