Peter s. Winokur, Chairman

Jessie H. Roberson, Vice Chairman

John E. Mansfield

Joseph F. Bader

DEFENSE NUCLEAR FACILITIES

SAFETY BOARD

Washington, DC 20004-2901

April 3, 2012

Mr. David Huizenga

Senior Advisor for Environmental

Management

U.S. Department ofEnergy

1000 Independence Avenue, SW

Washington, DC 20585-0113

Dear Mr. Huizenga:

On December 17, 2010, the Board issued Recommendation 2010-2, Pulse Jet Mixing at

the Waste Treatment and Immobilization Plant [WTP]. The Recommendation states: "Any use

of computer simulations must be technically defensible, and the limits of each computational

fluid dynamics simulation need to be well understood to prevent potential safety issues from

arising during operations." Additionally, Recommendation 2010-2 included a subrecommendation; "Complete verification and validation of any computational models used by

the WTP project team (e.g., Low Order Accumulation Model and FLUENT) based on the results

from the large-scale testing." Based on its review ofthe verification and validation (V&V) plan

and associated data gap analysis, the Board is concerned that the project's FLUENT model will

not be validated over the complete range ofWTP mixing conditions.

The FLUENT model is to be used to confirm that the performance ofthe WTP mixing

systems will meet design requirements. The Board's review ofthe V&V activities for WTP

revealed several deficiencies. For example, the data gap analysis neglected physical parameters

that may be necessary to accurately model full-scale mixing behavior in the WTP. As a result,

data collected during planned small-scale mixing tests may be insufficient to validate the model

adequately. This and other issues are documented in more detail in the enclosed report.

Therefore pursuant to 42 U.S.C. § 2286b(d), the Board requests a report and a briefing

within 45 days ofreceipt ofthis letter identifying the approach to be used to resolve the issues

described in the enclosed report so the FLUENT model will be appropriately validated prior to

use for mixing system design confirmation.

Sincerely,

~J?~W,J)... Peter S. Winokur, Ph.D.

Chairman

Enclosure

c: Mr. Dale E. Knutson

Mr. Scott Samuelson

Mrs. Mari-Jo Campagnone
DEFENSE NUCLEAR FACILITIES SAFETY BOARD

StaffIssue Report

February 27, 2011

MEMORANDUM FOR:

COPIES:

FROM:

SUBJECT:

T. J. Dwyer, Technical Director

Board Members

P. Meyer, A. Poloski

Basis for Selection ofValidation Data Set, FLUENT Model,

Waste Treatment and Immobilization Plant

Background. This report documents issues identified by the staffofthe Defense

Nuclear Facilities Safety Board (Board) related to verification and validation (V&V) activities

for the FLUENT computational fluid dynamics (CFD) model for the Waste Treatment and

Immobilization Plant (WTP). The FLUENT model is to be used to confirm that the

performance ofthe plant mixing systems utilizing pulse jet mixers (PJMs) meet design

requirements. The WTP V&V plan uses American Society ofMechanical Engineers (ASME)

Standard ASME V&V 20-2009, Standardfor Verification and Validation on Computational

Fluid Dynamics and Heat Transfer, which provides a methodology for assessing the accuracy

of a given computational simulation.

On December 17, 2010, the Board issued Recommendation 2010-2, Pulse Jet Mixing at

the Waste Treatment and Immobilization Plant. As part ofthe continuing review in support of

this Recommendation, the Board's staffreviewed the WTP V&V plan and associated gap

analysis for experimental data. The purpose ofthe gap analysis was to identify any additional

mixing tests required to complete validation ofthe FLUENT model. This report focuses on the

technical basis used for determining requirements for validation test data.

WTP Project Team's Approach for Determining the Sufficiency of Validation Test

Data. The WTP V&V plan specifies test variables that will be used for the validation process.

The plan also presents a technical basis for determining the range of experimental conditions

(validation points). This basis involves a dimensional analysis ofthe WTP mixing systems. The

resulting set ofdimensionless parameters provides the basis for defining the suite oftests

required for validation. The dimensional analysis in the V& V plan identifies three dominant

parameters important to mixing. The data gap analysis presents a different analysis based on the

governing equations in FLUENT, with five dimensionless parameters being identified as

important. The recommendation resulting from the data gap analysis is that new mixing tests

should be conducted in an 8 ft vessel to obtain data that span the plant-scale parameter ranges,

completing a set of experimental data adequate for V&V.
Observations. Based on this review, the Board's staff identified the following issues

related to the technical basis for selection of the validation data set.

Consideration ofVessel Length Scales-The dimensionless parameters derived in the

V&V plan and the data gap analysis do not incorporate characteristic vessel length scales, such

as vessel diameter and fill level. The length scales incorporated in the analysis are limited to the

jet nozzle diameter and characteristic particle size. While the choice of dimensionless

parameters is not unique, inclusion of the vessel length scales in the analysis results in two

additional independent dimensionless parameters. The staff is concerned that by neglecting large

vessel scales, the resulting set of dimensionless parameters may not appropriately describe

coupling between large-scale and localized mixing phenomena. For example, the transport of

solids in large circulatory flow patterns may be coupled to the clearing of solids off the bottom of

the vessel during PIM discharge, contributing to the vertical distribution of solids near the

suction inlet of the vessel. Tests defined from the limited parameter set that excludes vessel

length scales may not capture important mixing phenomena characteristic of plant mixing

systems.

In response to questions raised by the Board's staff about incorporating characteristic

vessel length scales into the dimensional analysis, WTP personnel responded that they addressed

the length scales by considering inter-PIM spacing. For example, WTP personnel assert that if

the distance between a PIM nozzle and the wall of a test vessel is comparable to the inter-PIM

distance in plant vessels, the effects of vessel size are incorporated into the testing. The Board's

staff is concerned that this assertion focuses principally on localized jet behavior and discounts

any coupling with the larger vessel scales. For example, academic studies of multiphase flow

processes have shown that the largest scales are important.1 Currently, WTP personnel have no

plans to obtain the experimental data needed to support their assertions. Further, this approach

leads to justifying non-prototypical mixing tests to close perceived gaps in test data.

Physical and Rheological Properties-The WTP V&V plan and gap analysis use highly

simplified waste properties as the basis from which to derive dimensionless parameters. The

Board's staffis concerned that the selected dimensionless parameters do not reflect more

realistic physical and rheological properties important to mixing performance. For example, the

dimensional analysis assumes monodispersed particles and neglects consideration of continuous

particle size and density distributions. The particle size distribution has been shown to be an

important dimensionless parameter for similar multiphase systems.1 The dimensional analysis

does not treat the ratio of solids density to liquid density as an independent dimensionless

parameter, but incorporates it into the definition of other parameters. The Board's staffbelieves

that the sufficiency of this approach should be verified. Further, the dimensional analysis does

not address the technical complexity of actual waste solids morphology. To account for this

complexity, variables describing particle morphology should be included in the dimensional

analysis. The dimensionless parameter space also is limited to Newtonian fluids. Recent test

results show that yield stress plays an important role in bottom-clearing processes and cannot be

ignored. The Board's staff and Department of Energy, Office of River Protection and Bechtel

National, Incorporated representatives have discussed these preliminary data and the concerns

1 L.R. Glicksman. Scaling Relationshipsfor Fluidized Beds. Chern. Eng. Sci. Vol. 39, Issue 9,1984,1373-1379.

2
raised by the results (e.g., bottom clearing is dependent on yield stress). The inclusion of slurry

yield stress and settled layer cohesion requires additional dimensionless parameters that imply an

expanded technical basis for the V&V program. The Board's staffbelieve this expanded

technical basis is appropriate for non-Newtonian processes vessels as well as any Newtonian

process vessels where pulse jet mixing results in localized regions that exhibit non-Newtonian

properties.

Completeness ofTest Data for Spanning Relevant Parameter Space-The gap analysis

uses the absolute minimum and maximum dimensionless parameter values to demonstrate the

range of values covered by various tests. However, these ranges neglect the combinatorial set of

parameter values required to show that a given test matches relevant plant scale conditions.

Further, the analysis does not address or identify the specific combinations of parameter values

that represent the most challenging mixing conditions. The Board's staff believes that validating

the FLUENT model for the most challenging mixing conditions is necessary to ensure that the

performance of the plant mixing systems will meet design requirements.

Non-prototypical Testing-Based on the results of the gap analysis, the WTP project

team is planning tests in an 8 ft vessel, which the team argues eliminates remaining gaps in

validation test data. Compared to a 38 ft diameter WTP vessel, an 8 ft diameter test vessel

represents approximately one-fifth scaling. The recommended tests utilize full-scale jet velocity

(12 m/s) and full-scale (4 in) and half-scale (2 in) nozzles to achieve full- and near-full-scale jet

behavior. However, the combination of a full-scale jet nozzle and a small-diameter vessel results

in non-prototypical test geometries, such as test configurations with only one PJM. Important

geometric similarity parameters are not preserved with these configurations. For example, the

dimensionless ratio ofjet nozzle diameter to vessel diameter for the test geometries is much

larger than it is for many of the plant mixing systems. These test geometries, together with the

planned operational conditions, are non-prototypical mixing systems with relative power and

pulse volumes that are larger than those ofsome plant geometries. Further, the characteristic

settled bed depth for fast settling solids will be much shallower in the test vessels than it will be

in the more challenging plant vessels. Consequently, these tests may exhibit more intense, nonprototypical mixing of solids compared with plant designs. To provide data applicable for

validation, the staff believes that these tests may need to incorporate reduced jet velocity.

However, with reduced jet velocity, any perceived benefit of achieving full-scale jet conditions

will be lost.

Conclusions. The Board's staffbelieves that the FLUENT model will not be validated

over the complete range of conditions expected for mixing system operations in WTP. This

concern is based on the following observations:

• The dimensionless parameters derived in the V&V plan and the data gap analysis do

not involve characteristic vessel length scales such as vessel diameter and fill level.

WTP personnel are not planning to obtain additional test data to support their assertion

that these large length scales are insignificant.

3
4 

• The WTP V&V plan and data gap analysis consider simplified waste properties in the 

development of the dimensionless validation parameter space. The analysis and 

resulting proposed validation tests exclude particle size and density distributions, 

particle morphology, and non-Newtonian rheological properties. 

• The gap analysis uses the absolute minimum and maximum dimensionless parameter 

values to demonstrate the range of values covered by various tests. The gap analysis 

did not consider the combinatorial set of variable values required to show that a given 

test matches the relevant or the most challenging plant scale conditions in defining the 

test requirements. 

• Planned tests in an 8 ft vessel are non-prototypical and are insufficient to eliminate 

remaining gaps in validation test data.