Regulatory Guide 1.24
| ML083300020 | |
| Person / Time | |
|---|---|
| Issue date: | 03/31/1972 |
| From: | Office of Nuclear Regulatory Research |
| To: | |
| References | |
| RG-1.024, Rev 0 | |
| Download: ML083300020 (7) | |
U.S. NUCLEAR REGULATORY COMMISSION
March 1972 Revision 0
REGULATORY GUIDE
OFFICE OF NUCLEAR REGULATORY RESEARCH
REGULATORY GUIDE 1.24 (Draft was issued as Safety Guide 24)
ASSUMPTIONS USED FOR EVALUATING
THE POTENTIAL RADIOLOGICAL CONSEQUENCES
OF A PRESSURIZED WATER REACTOR
RADIOACTIVE GAS STORAGE TANK FAILURE
A. INTRODUCTION
Section 50.34 of 10 CFR Part 50, Contents of Applications: Technical Information, requires that each applicant for a construction permit or operating license provide an analysis and evaluation of the design and performance of structures, systems, and components of the facility with the objective of assessing the risk to public health and safety resulting from operation of the facility. Radioactive gas storage tanks are used at pressurized water power reactors to permit decay of radioactive gases as a means of reducing or preventing the release of radioactive materials to the atmosphere. The accidental release of the contents of one of these tanks resulting from a rupture of the tank, or of an inlet or discharge pipe, or because of operator error or valve malfunction, is one of the postulated accidents used to evaluate the adequacy of these components with respect to the public health and safety. This safety guide lists acceptable assumptions for use in evaluating the radiological consequences of this postulated accident. In some cases unusual site characteristics, plant design features, or other factors may require different assumptions which will be considered on an individual case basis.
B. DISCUSSION
Radioactive gas storage tanks are used at pressurized water power reactors to permit decay of radioactive gases as a means of reducing or preventing the release of radioactive materials to the The NRC issues regulatory guides to describe and make available to the public methods that the NRC staff considers acceptable for use in implementing specific parts of the agencys regulations, techniques that the staff uses in evaluating specific problems or postulated accidents, and data that the staff needs in reviewing applications for permits and licenses. Regulatory guides are not substitutes for regulations, and compliance with them is not required. Methods and solutions that differ from those set forth in regulatory guides will be deemed acceptable if they provide a basis for the findings required for the issuance or continuance of a permit or license by the Commission.
This guide was issued after consideration of comments received from the public.
Regulatory guides are issued in 10 broad divisions: 1, Power Reactors; 2, Research and Test Reactors; 3, Fuels and Materials Facilities; 4, Environmental and Siting; 5, Materials and Plant Protection; 6, Products; 7, Transportation; 8, Occupational Health;
9, Antitrust and Financial Review; and 10, General.
Electronic copies of this guide and other recently issued guides are available through the NRCs public Web site under the Regulatory Guides document collection of the NRCs Electronic Reading Room at http://www.nrc.gov/reading-rm/doc-collections/ and through the NRCs Agencywide Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html, under Accession No. MLXXXXXXXXX.
RG-1.24, Page 2 atmosphere. Several tanks are normally provided to afford operating flexibility and allow one or more tanks to be isolated from the rest of the system for an extended period of time.
Most of the gas stored in the decay tanks is cover gas, generally nitrogen, displaced from the liquid waste holdup tanks. The radioactive components are principally the noble gases krypton and xenon, the particulate daughters of some of the krypton and xenon isotopes, and trace quantities of the halogens. With the exception of krypton-85, the longest half-life of the principal noble gas radionuclides present in reactor effluents is 5.27 days (xenon-133). Thus, storage of these gases for a period of 60 days will essentially eliminate by decay all of the radionuclides except krypton-85.
The probability of a gas decay tank rupturing is low. However, the probability of an accidental release resulting from such things as operator error or malfunction of a valve or the overpressure relief system is considered to be sufficiently high that the calculated offsite whole body exposures that might result from a single failure during normal operation should be substantially below the guidelines of
In considering the probability and consequences of such a single failure occurring, it is recognized that greater volumes of radioactive gases will be generated by the larger plants presently being constructed than the volumes generated by most presently operating plants. This increased quantity of radioactive gas will necessitate a substantial increase in the number or size of gas storage tanks used.
Considering the potential which exists for an inadvertent release and the high noble gas content of the tanks together with the fact that such gas decay tanks are normally located outside the reactor containment, every reasonable effort should be made to reduce the probability of such an accidental release. Thus, there is a need for a strong quality assurance program, as required by Appendix B to 10
CFR Part 50, to provide control over activities affecting the quality of the identified structures, systems, and components, to an extent consistent with their importance to safety The program shall take into account the need for special controls, processes, test equipment, tools, and skills to attain the required quality, and the need for verification of quality by inspection and test. The program shall provide for indoctrination and training of personnel performing activities affecting quality as necessary to assure that suitable proficiency is achieved and maintained. In the following paragraphs, some potential problem areas are noted which are considered to merit special attention in the design of a radioactive gas storage system:
1.
If there is a potential for hydrogen to build up in the gas storage system, special care should be taken to prevent air in-leakage to assure that an explosive mixture of hydrogen and oxygen does not accumulate in the decay tanks.
2.
The gas storage system should be designed so that the tanks are isolated from each other during use to limit the quantity of gas released in the event of an accident by preventing the flow of radioactive gas between tanks.
3.
A special effort should be made in the design of the overpressure relief system to minimize the likelihood of an inadvertent release occurring because of operator error or valve malfunction and to route piping to minimize any possible radiation exposure to onsite personnel from any gas vented from the system.
4.
The gas storage system components should be located so as to minimize the likelihood of any system damage that could result in the release of stored gas due to the occurrence of a common industrial accident such as a vehicle out of control, a maloperating crane, a dropped object, etc.
RG-1.24, Page 3
5.
Since missiles generated externally by high winds are a potential cause of gas storage system damage, they should be considered even though the radiological consequences of such an accident would be mitigated by the high wind speed required to generate such missiles.
C. REGULATORY POSITION
1.
The assumptions related to the release of radioactive gases from the postulated failure of a gaseous waste storage tank are:
a.
The reactor has been operating at full power with one percent defective fuel and a shutdown to cold condition has been conducted near the end of an equilibrium core cycle.
As soon as possible after shutdown, all noble gases have been removed from the primary cooling system and transferred to the gas decay tank that is assumed to fail.
b.
The maximum content of the decay tank assumed to fail should be used for the purpose of computing the noble gas inventory in the tank. Radiological decay may be taken into account in the computation only for the minimum time period required to transfer the gases from the primary system to the decay tank.
c.
The failure is assumed to occur immediately upon completion of the waste gas transfer, releasing the entire contents of the tank to the building. The assumption of the release of the noble gas inventory from only a single tank is based on the premise that all gas decay tanks will be isolated from each other whenever they are in use.
d.
All of the noble gases are assumed to leak out of the building at ground level over a two hour time period.
2.
The atmospheric diffusion assumptions for ground level releases are:
a.
The basic equation for atmospheric diffusion from a ground level point source is:
z y
u
1 Q
/
=
Where:
= the short term average centerline value of the ground level concentration (curies/m3)
Q
= amount of material released (curies/sec)
u
= windspeed (meters/sec)
y
= the horizontal standard deviation of the plume (meters) [ See Figure V-I,
Page 48, Nuclear Safety, June 1961, Volume 2, Number 4, Use of Routine Meteorological Observations for Estimating Atmospheric dispersion, F. A. Gifford, Jr.]
z
= the vertical standard deviation of the plume (meters). (See Figure V-2, Page 48, Nuclear Safety, June 1961, Volume 2, Number 4, Use of
RG-1.24, Page 4 Routine Meteorological Observations for Estimating Atmospheric Dispersion, F. A. Gifford, Jr.]
b.
For ground level releases, atmospheric diffusion factors1 used in evaluating the radiological consequences of the accident addressed in this guide are based on the following assumptions: (a) windspeed of 1 meter/sec; (b) uniform wind direction; (c)
Pasquill diffusion category F.
c.
Figure 1 is a plot of atmospheric diffusion factors (/Q) versus distance derived by use of the equation for a pound level release given in regulatory position 2.a. above under the meteorological conditions given in regulatory position 2.b. above.
d.
Atmospheric diffusion factors for ground level releases may be reduced by a factor ranging from one to a maximum of three (see Figure 2) for additional dispersion produced by the turbulent wake of the reactor building. The volumetric building wake correction as defined in Subdivision 3-3.5.2 of Meteorology and Atomic Energy-1968, is used with a shape factor of 1/2 and the minimum cross-sectional area of the reactor building only.
3.
The following assumptions and equations may be used to obtain conservative approximations of external whole body dose from radioactive clouds:
a.
External whole body doses are calculated using Infinite Cloud assumptions, i.e., the dimensions of the cloud are assumed to be large compared to the distances that the gamma rays and beta particles travel. The dose at any distance from the reactor is calculated based on the maximum ground level concentration at that distance.
For an infinite uniform cloud containing curies of beta radioactivity per cubic meter, the beta dose rate in air at the cloud center is:2
=
E
457
.0
D
D
= beta dose rate from an infinite cloud (rad/sec)
E
= average beta energy per disintegration (MeV/dis)
= concentration of beta or gamma emitting isotope in the cloud (curie/m3)
Because of the limited range of beta particles in tissue, the surface body dose rate from beta emitters in the infinite cloud can be approximated as being one-half this amount or:
=
E
23
.0
D
1 These diffusion factors should be used until adequate site meteorological data are obtained. In some cases, available information on such site conditions as meteorology, topography and geographical location may dictate the use of more restrictive parameters to insure a conservative estimate of potential offsite exposures.
2 Meteorology and Atomic Energy-1968, Chapter 7.
RG-1.24, Page 5 For gamma emitting material the dose rate in air at the cloud center is:
=
E
507
.0
D
Where:
D
= gamma dose rate from an infinite cloud (rad/sec)
E
= average gamma energy per disintegration (MeV/dis)
However, because of the presence of the ground, the receptor is assumed to be exposed to only one-half of the cloud (semi-infinite) and the equation becomes:
=
E
25
.0
D
Thus, the total beta or gamma dose to an individual located at the center of the cloud path may be approximated as:
=
=
E
25
.0
D
E
23
.0
D
or Where is the concentration time integral for the cloud (curie sec/m3).
b.
The beta and gamma energies emitted per disintegration, as given in Table of Isotopes,3 are averaged and used according to the methods described in ICRP Publication 2.
3 C. M. Lederer, J. M. Hollander, and I. Perlman, Table of Isotopes, Sixth Edition (New York: John Wiley and Sons, Inc., 1967).
RG-1.24, Page 6
Figure 1. Ground Level Release Atmospheric Diffusion Factors
RG-1.24, Page 7
Figure 2. Building Wake Correction Factor