U.S. NUCLEAR REGULATORY COMMISSION March 2009

Revision 2 

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

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 agency’s 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 NRC’s public Web site under the 

Regulatory Guides document collection of the NRC’s Electronic Reading Room at http://www.nrc.gov/reading rm/doc 

collections/ and through the NRC’s Agencywide Documents Access and Management System (ADAMS) at 

http://www.nrc.gov/reading rm/adams.html, under Accession No. ML082810208. 

REGULATORY GUIDE 1.125 

(Draft was issued as DG-1198, dated May 2008) 

PHYSICAL MODELS FOR DESIGN AND OPERATION OF 

HYDRAULIC STRUCTURES AND SYSTEMS FOR 

NUCLEAR POWER PLANTS 

A. INTRODUCTION 

This guide describes the detail and documentation of data and studies that an applicant should 

include in the preliminary and/or final safety analysis report (PSAR/FSAR) to support the use of physical 

hydraulic model testing for predicting the performance of hydraulic structures and systems for nuclear 

power plants that are important to safety. Hydraulic structures are defined as anything that can be used to 

divert, restrict, stop, or otherwise manage the natural flow of water. The regulatory position of this guide 

is applicable only to physical models used to predict the action or interaction of surface waters with 

features located outside the containment. The recommendations of this guide do not apply to internal 

plant systems or structures. 

Title 10, Paragraph 50.34(a)(3)(ii), of the Code of Federal Regulations (10 CFR 50.34(a)(3)(ii)) 

requires that the PSAR include information on the design bases of the facility and the relation of the 

design bases to the principal design criteria. In part, 10 CFR 50.34(a)(4) requires a preliminary analysis 

of the adequacy of structures, systems, and components provided for the prevention of accidents and the 

mitigation of the consequences of accidents. Additionally, 10 CFR 52.47(a) and 10 CFR 52.79 describe 

the required technical content of the final safety analysis report (FSAR) that must accompany applications 

for early site permits, standard design certifications, and combined licenses for nuclear power plants. 

This regulatory guide contains information collection requirements covered by 10 CFR Parts 50 

and 52 that the Office of Management and Budget (OMB) approved under OMB control numbers 3150-

0011 and 3150-0151 respectively. The NRC may neither conduct nor sponsor, and a person is not 
Rev. 2 of RG 1.125, Page 2 

required to respond to, an information collection request or requirement unless the requesting document 

displays a currently valid OMB control number. 

B. DISCUSSION 

Physical hydraulic models are often used to predict prototype performance in designing and 

rehabilitating hydraulic structures. The physical modeling studies ultimately increase the safety of the 

hydraulic structures by identifying and eliminating potential problems, thus reducing construction and 

maintenance costs. They are particularly useful where hydraulic structures and systems are of unusual 

design or configuration and hydraulic parameters cannot be adequately evaluated by state-of-the-art 

analytical or computational methods. Furthermore, physical models will incorporate the appropriate 

governing equations without the simplifying assumptions that are often necessary in analytical or 

numerical models. Physical hydraulic models may also be used to establish conservative and reasonable 

design or operating bases for sites, structures, or systems involving thermal and erosional problems. 

Examples of types of physical modeling studies of hydraulic structures and systems for nuclear 

power plants include, but are not confined to, the following: 

• intake structures and pump impeller flow characteristics , 

• discharge structures, 

• energy and wave dissipation structures, 

• spillway and tailwater ratings for dams (water level and discharge relations), 

• release of water resulting from dam failures, 

• wave propagation and runup on a coastal structure, including tsunami effects, 

• stability of structure when exposed to waves and protection from waves, 

• erosion and deposition in streams and other water bodies and protection from these processes, 

• flow patterns and dispersion and dissipation of heated or contaminated effluents in receiving 

water bodies, 

• heat dissipation in receiving water bodies, 

• response of moored floating bodies to incident wave systems, and 

• response of harbors to waves. 

The applicant should incorporate the information described in the regulatory position in the 

PSAR/FSAR as required. However, the staff recognizes that it will not always be possible to incorporate 

such information in the initial application for a reactor license, since studies of this type may not be 

undertaken until after the safety analysis report is submitted. Such information may be added to the 

PSAR/FSAR, either by reference to separate reports or by insertion into the PSAR/FSAR. 

C. REGULATORY POSITION 

Because not all hydraulic design problems can be resolved by the mechanics of similitude and 

because there are limitations to hydraulic modeling, the applicant should supply the NRC staff with 

certain documentation for any structural, thermal, erosional, or other physical hydraulic models that it 

used to establish design or operating bases. The regulatory position contained in this guide applies only 

to physical models used to predict the action or interaction of surface waters with hydraulic features 

located outside the containment. 
Rev. 2 of RG 1.125, Page 3 

Generally, Regulatory Positions 1 and 2 describe information that should be furnished before the 

building and testing of the physical model and Regulatory Positions 3 through 5 describe information 

needed after testing has been completed. It is desirable for the applicant to solicit staff reviews and 

recommendations before model construction and following or coincident with the submittal of the 

information listed in Regulatory Position 1, as well as to invite appropriate members of the NRC staff to 

be present periodically during model testing to observe the performance of the model. In addition, the 

applicant should provide partial test results for staff review and acceptance during the course of testing to 

allow for the consideration of those parameters that partial data collection shows to be important. 

1. Regulatory Position 1 

Before constructing the physical model, the applicant should submit a test plan that includes the 

following information: 

• the problem(s) to be resolved; 

• reasons for selecting the physical hydraulic model chosen to resolve the problem(s); 

• expected results and how these results will solve the stated problem(s); 

• a detailed description of the model, including a description of materials, instrumentation, and 

methods used to measure parameters, including resolution and error of instrumentation, scale 

relations, and other physical characteristics of the model; 

• a detailed description of the testing facilities; 

• methods that will be used to analyze the data obtained from the model studies; and 

• a schedule of expected tests, including proposed start and completion dates, and estimated dates 

for submitting information for NRC staff review. 

2. Regulatory Position 2 

In addition, the applicant should furnish documentation describing how it considered the various 

conditions of geometric, kinematic, dynamic, and thermodynamic similitude that take into account the 

physical properties and flow state of the fluid (i.e., Froude, Reynolds, Euler, Cauchy, Weber, and other 

related numbers). Because certain forces may act differently in a model than in a prototype, the 

documentation provided should justify the neglect of any forces by showing that these forces (1) are of 

negligible magnitude, (2) compensate for other neglected forces in such a manner that the effects of both 

forces are negligible, or (3) are such that their neglect leads to conservative model results and the 

establishment of conservative design or operating bases. 

The applicant should document the methods used to satisfy the equations of similitude in the 

model and the effects of scale distortions on data obtained from the model studies. Where applicable, the 

document should describe model adjustment and verification procedures and furnish information on the 

validity of the model over a range of likely flow conditions, heat regimes, atmospheric conditions, and 

other physical parameters. The document should demonstrate, where applicable, that the model will 

simulate known flow conditions and provide this verification when measured historical data are available. 

3. Regulatory Position 3 

Where full-scale structures or systems having characteristics similar to those being modeled exist 

and information about the observed or measured performance of the existing structures or systems is 

available, the applicant should compare the physical model results with the available information. If 

testing is or has been performed on existing full-scale structures or systems, the applicant should describe 

such tests and their results. The document should address the applicability of such tests to the problem in 
Rev. 2 of RG 1.125, Page 4 

question and discuss any conclusions derived from the tests. If using the results of other model tests, the 

applicant should justify the use of these results and verify and document the ability of these other models 

to reproduce or predict prototype performance. 

The applicant should provide detailed documentation of data obtained from existing full-scale 

structures and systems, including: 

• instrumentation used; 

• description of the data collection network; 

• frequency of collection; 

• methods of collection; and 

• physical parameters existing at the time of collection, such as heat regimes, flow conditions, and 

atmospheric conditions. 

4. Regulatory Position 4 

The applicant should discuss any changes to the original design of the prototypes that result from 

the model test, documenting the designs that were modeled and the basis for modifying the design. The 

discussion should address any undesirable flow characteristics or failure modes for the design tested, as 

well as any other relevant problems. 

5. Regulatory Position 5 

The report covering the completed model tests should provide the data obtained from the tests, as 

well as the (1) instrumentation used, (2) description of the data collection network, (3) frequency of 

collection, and (4) method of collection. Figures, drawings, photographs, and text submitted as 

documentation for Regulatory Positions 2, 3, and 4 should be sufficiently detailed to allow the staff to 

evaluate independently the applicability of the model to the design problem in question. This report will 

provide the basis for the interpretation of model results and for any conclusions reached. The applicant 

should not dismantle the models until the staff has reviewed the submittals. Preserving the model for a 

maximum of 1 year after the submission of the model documentation to the NRC will be acceptable in 

most cases. 

D. IMPLEMENTATION 

The purpose of this section is to provide information to applicants and licensees regarding the 

NRC’s plans for using this regulatory guide. The NRC does not intend or approve any imposition or 

backfit in connection with its issuance. 

In some cases, applicants or licensees may propose or use a previously established acceptable 

alternative method for complying with specified portions of the NRC’s regulations. Otherwise, the 

methods described in this guide will be used in evaluating compliance with the applicable regulations for 

license applications, license amendment applications, and amendment requests.