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Graphite and Xenon Behavior and Their Influence On GRAPHITE AND XENON BEHAVIOR AND THEIR INFLUENCE ON KEYWORDS: molten-soh re- REACTOR DESIGN ocfors, breeder reocfors, rQ' MOLTEN-SALT diotion effecls, temPeroture, grophi te moderoto_r, _sfresses, DUNLAP SCOTT and P. EATHERLY oak Ri'd,ge -ing,porosity, xenon't15, Poison' bubbles, desig_n_, Nationat Laboratory, Nuclear Di,uision, Oak Ridge, Tennessee 37830 helium, ec-onotics, reoctor core, MSBR Received August 4, 1969 Revised October 2, 1969 maintenance requirements; i.€., the downtime f€- quired for scheduled maintenance should coincide Existing data on di,mensional changes in gra- with that for graphite replacement. Hence, to graph- phite haue been to parabolic temperahrre' avoid load-factor penalties associated with fitted f €- sensiti,ue ct(r?)es. From these , the graphite life , ite replacement, we have set as a minimum two radiation-induced s tre s s € s, and, permissible quirement that the graphite have a life of geometries haae been calculated. It is concluded years; at the same time, a longer life would be Zxi,sting materials can be utitized in a molten-salt desirable and consistent with the power industry's reactor which has a core graphite life of abant objective of increasing the time interval between Thus, in the four years, without serious cost penalty. turbine maintenance operations. Fission product xenon can be remoued by reference design the reactor performance is con- sparging the salt with helium bubbles and strained to yield a graphite lif e of about four fuet ,paper basis that remoaing them after enrichment. With rea,sonable years, and this points out the for transfer coefficient and value. aalues of salt-to-btrbbte 135Xe puts graphite permeabitity , the Pernlty to breeding The removal of from the core also a graphite. The fuel salt must be ratio can be red,uced to l.. .0.5%. constraint on the excluded from the graphite to prevent local over- heating and also to decrease fission-product poi- soning, and this, in turn, requires that the graphite pore diameters not exceed one micron. limitation, for the xenon INTRODUCTION This, however, is not a removal will be shown below to require a gas and One of the attractive a^speets of the molten-salt permeability of the order of 10-8 cm'/sec, 0.1 coneept is that even the most stringent of sugh a value requires pore diameters of - P. reactor graph- the present materials or process limitations per- Even though xenon is excluded from the mit reactor designs having aeeeptable economic ite, it needs to be removed from the salt stream performance. In this paper we conSider, aS if the desired neutron economy is to be attained. examples, the effects of finite graphite lifetime This removal is accomptished by iniecting helium and xenon poisoning on MSBR design. Finite bubbtes into the flowing salt, which transfers the graphite lifetime implies periodic replacement of xenon from the salt to the bubbles and effectively the graphite; neutron economy requires the f€- removes xenon from the core region. ttuxe discuss in moval of the bulk of from the core to keep In the following sections we shall existing the xenon poison fraction below the target value some detail: the method of analysis of data on radiation damage to permit prediction of of LVo' The major economic penalty associated with the graphite lifetime in MSBR cores; the applica- graphite replacement would be the load factor tion of these damage rates and the radiation- in penalty associated with taking the reactor off - induced creep to calculate induced stresses in the stream. This cost can be Circumvented by aSSUf - the graphite; the considerations involved 135Xe other noble ing that the graphite will maintain its integrity for distribution and removal of and at least the time interval between normal turbine gases; and last, the method proposed for safely 179 NUCLEAR APPLICATIONS & TECHNOLOGY VOL. B FEBRUARY 1970 Scott and Eatherly GRAPHITE AND XENON BEHAVIOR collecting and disposing of these gases. we con- and clude that graphite and the removal of xenon um -L2.0 + 8.92 x 10-3 present no questions of feasibility, but require = T Vo , only minor extensions of existing technology. where T is the temperature in oc, valid over the range 400 to 800o. At 700oC, these yield Qr - 3.1 x !022 with a goTo double-sided confidence GRAPHITE LIFETITUIE limit of *,0.2 x LO", and um = -5 .8Vo, with the limit t0 .2 . Figure 1 shows It has been recognized for several years that the behavior of the linear distor- under prolonged radiation exposure, graphite tion as a function of fluence with temperature as be- parameter. gins to swell extensively, even point a to the of As pointed cracking and breaking into fragments. However, out in the introduction, it is nec€s- sary that the graphite data at high fluences and within the operating exclude both salt and xenon. The requires graphite temperature ranges anticipated for molten-salt first the to have no pores larger than ,-,L breeder reactors were Iargely nonexistent. p in diameter; the second, as will be seen below, r€euires graphite Nevertheless, by using existing datq it was possi- the to have a permeability to xenon ble to estimate graphite behavior over the range of the order of 10-8 emz/ of MSBR conditions. For a large group bt sec. CIearIy, as the graphite expands and visible cracking (u commercial graphites - including British Gilso- occurs ) +3%ol, these requirements graphi,te, Pile Grade A, and the American grades will have been lost. Laeking definitive datq we AGOT and csF it was found that the volume have made the ad hoc assumption that the pore - size and permeability distortion, u, could be related to the fluence, O, requirements will be main- by a parabolic curve, tained during irradiation until the time when the original graphite volume is reattained, namely, u = Aa + Bez , (1) when t = r as defined above. where A and B ate functions of temperature only. To estimate the lifetime of an MSBR core, we The fit of this equation to the experimental data is musi take into aeeount the strong dependence of r excellent for the isotropic Gilso-graphite, but the relation is approached only asymptoticatly for the anisotropic graphites. we may write the fluence a^s the product of flux and time, i.€., O = Qt. The 850oc / 8000 ttsco behavior of u is to decrease to a minimal value, u*, and then increase, crossing the v = 0 axis at a defined ti,me, r, given by 0- AEr+B(W)' (21 Clearly, in terms of um and r, Eq. (1) can be rewritten as I o z0 u=4u*+(t (Zal 9 *) uF o For isotropic graphite, the linear dimensional F an changes will be given approximately by one-third 6 the volume change. If the graphite is anisotropic, u- _l the preferred c-axis direction will expand more zUJ quickly, the other directions more slowly. This f will induce a more rapid deterioration of the material in the preferred c-direction. As a consequence, we require that the graphite be iso- tropic, and can rewrite Eq. (zal in terms of the linear distortior5 G, G=+u=++(t (zb) ) -3 and this relation will be used hereafter. 0r23 FLUENcE Ox rc-22 (E >50 kev) The best values for the parameters Qr and v* were found to be Fig. 1. Graphite linear distortion as a function of W = (9.36 - 8.93 x 10-3 f) x LOz'nvt (E >to keV) fluence at various temperatures. r80 NUCLEAR APPLICATIONS TECHNOLOGY VOL. 8 FEBRUARY 19?O Scott and Eatherly GRAPHITE AND XENON BEHAVIOR of the on temperafure, and the temperature of the core radius q,. Finally, since the heat capacity io keeping graphite will depend on the fuel salt temperatures, salt is a weak function of temperature, power density the heat transf er coeffieient between salt and with the sinusoidal variation of graphite, and the gamma, beta, and neutron heat along the a>ris, the salt temperature, T o, becomes generation in the graphite. In all core designs + (rr rr) cos (8) which have been analyzed, the power generation r o =*lr,aL r; T) (i.e., fission rate) has been found to vary closel.y and T7 are the entering and exiting as sin (n z/t ) along the urial centerline, wher e z/t in which Ti fractional height, and / is the effective total temperafures, respectively, of the salt. is the determine core height. Thus, the heat generation rate in the Equations (3) through (8) completely graphite. As we sha1l see graphite will varY as the temperatures in the below, the radiation-induced strain in the graphite (3) q= eo + Qt sin(nz/l), along the z-a)<is, i.€., A^ z/ z, is given by where eo approximates the rate due to delayed gamma, beta, and neutron heating, and Qt approxi- y = tGz)rdr ' manimum prompt heating. R epresent- # mates the (2b)' A graphite core prisms as cylinders where G (T) is the damage function of Eq' ing the actual A with internal radius a and external radius b, we similar expression applies to the radial strain. can readily calculate the internal temperature negligible error is introduced by qPproximating G(TI, where 7 ts the aver- distribution assum ing q is not a function of radius ' the right-hand side by section; there- The result i.sl age temperature over the cross fore, + = c(7) (e) (4) Since T varies with z fi, each point along the (Tl, defined Tb surface temperatures at cylinder will have a different life, r where To and are the these r (Tl K the graphite thermal 6 d,z/ z = 0.
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