ML19221A854

Reg Guide 1.24, Assumptions Used for Evaluating Potential Radiological Consequences of PWR Radioactive Gas Storage Tank Failure

ML19221A854
Person / Time
Issue date:	03/23/1972
From:	NRC OFFICE OF STANDARDS DEVELOPMENT
To:
References
REGGD-01.024, REGGD-1.024, NUDOCS 7907100115
Download: ML19221A854 (6)

Text

{{#Wiki_filter:.. 3/23/7 SAFETY GUIDE 24 ASSUMPTIONS USED FOR EVALUATING THE POTENTIAL RADIOLOGICAL CONSEQUENCES OF A PRESSURIZED WATER REACTOR C RADIOACTIVE GAS STORAGE TANK FAILURE A. Introduction krypton and xenon, the particulate daughters of some of the krypton and xenon isotopes and Section 50.34 of 10 CFR Part 50, trace quantities of the halogens. With the "Co n t e n t s of Applications: Technical exception of krypton-85, the longest half-life of Information," requires that each applicant for a the principal noble gas radionuclides present in construction permit or operating license provide reactor effluents is 5.27 days (xenon-133). Thus, an analysis and evaluation of the design and storage of these gases for a period of 60 days performance of structures, systems, and will essentially ehminate by decay all of the components of the facility with the objective of radionuclides except krypton-85. assessing the risk to public health and safety The probability of a gas decay tank resulting from operation of the facility. ruptunng is low. However, the probability of an Radioactive gas storage tanks are used at accidental release resulting from such things as pressurized water power reactors to permit operator error or malfunction of a vab e or the decay of radioactive gases as a means of reducing overpressure relief system is considered to be or preventing the release of radioactive materials sufficiently high that the calculated offsite to the atmosphere. The accidental release of the whole body exposures that might result from a contents of one of these tanks resulting from a single failure durmg normal operation should be rupture of the tank, or of an inlet or discharge substantially below the guidelines of 10 CFR pipe, or because of operator error 'or valve Part 100. malfunction, is one of the postulated accidents in considering the probability and used to evaluate the adequacy of these consequences of such a single failure occurrmg, components with respect to the public health it is recognized that greater volumes of and safety. This safety guide lists acceptable radioactive gases will be generated by the larger assumptions for use in evaluating the plants presently being constructed than the radiological consequences of this postulated volumes generated by most presently operating accident. In some c a ses unusual site plants. This increased quantity of radicactive gas characteristics, plant design features, or other will necessitate a substantial increase in the factors may require different assumptions which number or size of gas storage tanks used. will be considered on an individual case basis-Considering the potential which exists for an inadvertent release and the high noble gas B. Discussion content of the tanks together with the fact that such gas decay tanks are normally located Radioactive gas storage tanks are used at outside the r eactor containment, every pressunzed water power reactors to permit reasonable effort should be made to reduce the decay of radioactive gases as a means of reducing probability of such an accidental release. Thus. or preventing the release of radioactise materials there is a need for a strong quality assurance to the atmosphere. Several tanks are normally program, as required by Appendix B to 10 CFR provided to afford operating flexibihty and Part 50, to " provide control over activities allow one or more tanks to be isolated from the affecting the quahty of the identified structuren rest of the system for an extended period of systems, and com ponents. to an extent time. consistent with their importance to safety. Most of the gas stored in the decay tanks is The program shall take into account the need cover gas, generally nitrogen, displaced from the for special controls, processes, test equipnent. liquid waste holdup tanks. The radioactive tools, and skills to attain the required quality. components are principally the noble gaws and the need for verification of quahty by 125 115 24.I %N

~ ~. _- inspection and test. The program shall provide defective fuel and a shutdown to for :ndoctrination and training of personnel cold condition has been conducted performing activities affecting quality as near the end of an equilibriur; me necessary to assure that suitable proficiency is cycle. As soon as possibit atter achieved and maintained." In the following shutdown, all noble gases have paragraphs, some potential problem areas are been removed from the primary noted which are considered to merit special cooling system and transferred to attention in the design of a radioactive gas the gas decay tank that is assumed to fail. storage system: b. He maximum content of the 1. If there is a potential for hydro' a decay tank assumed to fail should build up in the gas storage sum, be used for the. purpose of special care should be taken to prevent computing the noble gas inventory air in-leakage to assure that an in the tank. Radiological decay explosive mixture of hydrogen and may be taken into account in the oxygen does not accumulate in the computation only for the decay tanks. minimum time period required to 2. He gas storage system should be transfer the gases from the primary designed so that the tanks are isolated system to the decay tank. from each other during use to limit the c. The failure is assumed to occur quantity of gas released in the event of immediately upon completion of an accident by preventing the flow of the waste gas transfer, releasing the radioactive gas between tanks. entire contents of the tank to the 3. A special effort should be made in the building. He assumption of the design of the overpressure relief system release of the noble gas inventory to minimize the likelihood of an from only a single tank is based on inadvertent release occurring because of the premise that all gas decay tanks operator error or valve malfunction and will be isolated from each other to route piping to minimize any whenever they are in use. possible radiation exposure to onsite d. All of the noble gases are assumed personnel from any gas vented from the to leak out of the building at system. ground level over a two hour time 4. De gas storage system components period. should be located so as to minimize the 2. The atmospheric diffusion assumptions lixelihood of any system damage that for ground level releases are: could result in the release of stored gas a. The basic equation for atmospheric due to the occurrence of a common diffusion from a ground level point industrial accident such as a vehicle out source is: of centrol, a maloperating crane, a j X/Q= dropped object, etc. 7tua a YZ 5. Since missiles generated externally by high winds are a potential cause of gas Where: storage system damage, they should be the short term average = x considered even though the radiological centerline value of the ground consequences of such an accident IeV*I c o n c e n t ratio n would be mitigated by the high wind 3 (curies /m ) speed required to generate such missiles. Q = amount of material released C. Regulatory Position (curies /sec) 1. The assumptions related to the release u = windsp (meters /sec) of radioactive gases from the postulated the horizontal standard o = failure of a pseous waste storage tank y ceviation of the plume are: a. Therc has been operating at (meters) [See Figure V-1, Page full pu. with one percent 48, Nuclear Safety, June 125 116 24.2

1961. Volume 2 Number 4, radioactive clouds: Use of R o u tine a. External whole body doses are M e teorological Observations calculated using " Infinite Cloud' for Estimating Atmospheric assumptions. i e. the dimensions dispersion," F.A. Gifford, Jr.] of the cloud are assumed to be o = the vertical standard deviation large compared to the Jnt.mccs 7 of the plume (meters) [See that the gamma rays and beta Figure V-2. Page 48, Nuclear particles travel. The dose at ant Safety. June 1961, Volume 2. d istance from the reactoi Number 4. "Use of Routine calculated based on the maumam M e t eorological Observations ground level toncentration at ti.at for Estimating Atmospheric distance. Dispersion.' F. A.

Gifford, J r. ]

For an infinite uniform cloud containing x curies of beta b. For ground level

releases, radioactisity per cubic meter. the a t mospheric diffusion factors' beta dose rate in air at the cloud used in evaluating the radiological center is:-

co nsequences of the accident addressed in Ihis guide are based gU '. = 0.45 7 I x g on the following assumptions: (a) windspeed of I meter /sec: (b) uniform wmd direction: Where: (c) Pasquill diffusion category F. D. beta dose rate f rom an c. Figure I is a plot of atmospheric O,, = finite cloud (rad 'sec) in diffusion factors (x/Q) versus distance derived by use of the E beta energy per equation for a ground level release g = aserace dismtegration (Mev' dis) given m regulatory position 2.a. above under the meteorological x = concentration of beta or conditions gis en in regulatory gam ma emitting isotope m position 2.b. above. 3 the cloud icurielm ) d. Atmosphenc dif f usion factors for gro und lesel releases may be Because of the limited rangt of reduced by a factor ranging from beta particles m tissue. the surtase one to a maximum of three (see body dose rate from beta enutters Figure 2) for additional dispersion in the infinite cloud can be produced by the turbulent wake of ~ pproximated as being one-half the reactor b u il d i n g. The this amount or: voIu m e t rie building wake correction as defined in D ~ = 0.23 Ex Subdivision 3-3.5.2 of Meteorology 0 e and Atomic Energy-1968. is used For gamma emitting material the with a shape factor of 'i and the dose rate m air at the cloud center minimum cross-sectional area of is: the rentor building only. D; = 0.507 E x 3. The following assumptions and y equations may be used to obtain Where: conservative a p p r o x i m a t ions of e x te rnal whole body dose from D' = gamma dose rate from an 7 inimite cloud (rad /sec) ' These diffusion futors should be used until adequate site meteorolopeal data an obtained. In some cases, available information on such site conditions as E aserage gamma energy per = y meteorology. topography and geographical location may dismt egration ( Mes 'did dictate the use of more restacuve parameters to insure a 2 conservative estimate of potential offsne exposures. Meteorology and Atomic Energy-1968. Chapter 7 125 117 24.3

Ilowever, because of the presence Where @ is the concentration time of the ground, the receptor is integral for the cloud (curie 3 assumed to be exposed to only sec/m ). o n e -h alf of the cloud (semi-infinite) and the equation b. 'Ihe beta and gamma energies becomes: emitted per disintegration, as given in Table of Isotopes 3 are averaged D '= 0.25 E X 7 7 and used according to the methods Thus, the total beta or gamma uose to an individual located at the center of the cloud path may be approximated as: 3C. M.12derer, J. M. Hollander, and I. Perlman. 0 = 0.23 E @ or Table of Isotopes, Sixth Edition (New York: John Wiley D O D = 0.25 E 0 and sons. Inc.,1967). y 7 9 125 118 m

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