Oral Presentation/Viewgraphs Summary:

A Direct Drive Gas-Cooled (DDG) reactor core simulator has been coupled to a Brayton Power Conversion Unit (BPCU) for integrated system testing at NASA Glenn Research Center (GRC) in Cleveland, OH. This is a closed-cycle system that incorporates an electrica 11 y heated reactor core module, turbo alternator, recuperator, and gas cooler. Nuclear fuel elements in the gas­cooled reactor design are replaced with electric resistance healers to simulate the heat from nuclear fuel in the corresponding fast spectrum nuclear reactor. The thermodynamic transient behavior of the integrated system was the focus of this test series. I n order to belter mimic the integrated response of the nuclear-fueled system, a simulated reactivity feedback control loop was implemented. Core power was contro II ed by a point kinetics model in which the reactivity feedback was based on core temperature measurements; the neutron generation time and the temperature feedback coefficient are provided as model inputs. These dynamic system response tests demonstrate the overall capability of a nonnuclear test facility in assessing system integration issues and characterizing integrated system response limes and response characteristics.

https://ntrs.nasa.gov/search.jsp?R=20120016534 2020-03-23T01:19:30+00:00Z

National Aeronautics and Space Administration

Testing of an Integrated Reactor Core Simulator and Power Conversion System

with Simulated Reactivity Feedback

Authors:

Shannon M. Bragg-Sitton Texas A&M University, Dept. of Nuclear Engineering, sitton @tamu.edu

David S. Hervol NASA Glenn Research Center

Thomas J. Godfroy NASA Marshall Space Flight Center

Outline AJlM ' !~~SR ~~M I

• Overview of nonnuclear test approach

• Test objectives

• Modeling reactor dynamics

• Previous hardware tests

• Brief overview of current test hardware: - Direct Drive Gas-Cooled reactor core simulator (DOG)

- Brayton Power Conversion Unit (BPCU)

- Instrumentation

- See companion paper (Hervol, et al) for more info

• Test matrix and hardware limitations

• Results of simulated feedback testing

• Conclusions and future work

.,

Overview of Nonnuclear Testing

• Allows evaluation of integrated system performance without use of nuclear materials - Validation of thermal hydraulic codes

- Assess thermal hydraulic behavior in various regimes

- Characterize stress/strain during operation

- Verify integration processes

• So, what's the catch? - NO NEUTRONS!

- System feedback is not characteristic of a fully fueled system without introduction of models to mimic the dynamic, neutronic response

Realistic Nonnuclear Testing: Objectives

• Integration of thermal hydraulic hardware tests with simulated neutronic response to bridge electrically heated testing and full nuclear testing

• Demonstration of representative neutronic response of the fission system via computational models

• Demonstration of fission system response to changes in the state of the integrated power conversion system

• Dynamic system response tests can be used to: - Assess system integration issues

- Characterize integrated system response times

- Characterize integrated system response characteristics

- Assess / enable potential design improvements at a relatively small fiscal investment

Realistic Nonnuclear Testing

en.-, EAi I lu ••• Exit Bra"ton ~.--- I Plc~um ~ .1 SnpIlnn

COle-

Simulators

/ " Pow~ /' / Fw!tbro·J~S

Tnl ... r Plcncm

tltt fimg<

-.... G"" Tnln From ,.. .... Brt.yton

• • t

• • t

· .' // /

./ DO\vn:otu('r

~

1 Axial TC Prolxs

• Goal: Implementation of advanced test methodology with realistic feedback components.

• Analysis of key nuclear feedback components in a specific reactor design and development of corresponding hardware control algorithms.

• Application of computational models to hardware in-the-Ioop tests.

Modeling Reactor Dynamics

dP,h (t) = p - P P (t) + ~ A.C. (t) dt A th L.. I I

I

dCi(t) = Pi P (t)-A.C.(t) dt A fh I I

• Description of reactor dynamics using the point kinetics model - Derived from neutron transport and diffusion theory - No mechanism to describe neutron energy effects or structural

details - Good representation of a fast spectrum reactor

(small size, no moderator to slow neutrons to lower energy)

• Model Assumptions / Parameters: - 1 delayed neutron group:

• Weighted average one group decay constant (/...) /... = 0.0767 sec-1

• Total delayed neutron fraction (~) ~ = 0.00642

- Prompt neutron lifetime (A): • Fast Reactor, A = 10-7 sec

Reactivity Feedback

• Reactivity feedback in a fast spectrum reactor is relatively simple and negative - Mostly due to thermal expansion (which is a function of temperature)

• Use of a temperature based feedback model removes the variable of how this core expands relative to a different core concept (tested core is non-prototypic of any particular design)

• Reactivity feedback can be represented by a single temperature feedback coefficient (aT):

- Bulk core temperature feedback coefficient for most reactors of this class --0.1 to -0.3 cents / degree K -7 -0.2 cents / oK is assumed

•

.'

Previous Hardware Tests

Heat pipe cooled core simulator SAFE-100 and 100a test articles

Tested at NASA MSFC with reactivity feedback (2004, 2005)

• Thermal expansion

• Average core temperature

Transients stabilized to new steady state after -20-30 min

720

700

~ 6S0

~ ~ a !.! 660 ~ 0. E ~

~ 640

620

Average Core T emp.raMe

Total Core Power

Average HX Upstream I Downstream T emp.ratute

19000

18000

17000

16000 i[ ~

~

15000 ~ .. ~

14000 2; U

"'----~ 13000 ~ ~

'-.--;r;-;:c"'o,-... - g---,l 12000

• T HX Upstream avg • T HX Downstream avg 11000

600 0:00:00 0:08:38

+:~~~~-r _ _ ~_~~~~r~ol~.,~co~" ~P~~~·~'==dl " 10000

0:17:17 0:25:55 0:34:34 0:43:12 0:51:50 1 :00:29

0 .025

0 .02

0.015

i!E ~ 0.01

:~ u 0.005 " ~ 0:

-0.01

-0.01 5

Elapsed Time [hrs:min:sec]

T etal Core Power

0:17:17 0:25:55 0:34:34

Elapsed Time [hrs:min:sec]

0:51 :50 1

18000

17000

16000 i[ i

15000 ~ .. ~

14000 is U

Previous Hardware Tests

• Direct drive gas-cooled reactor core simulator DDG testing at MSFC without power conversion system Initial demonstration of feedback testing prior to disassembly and refurbishment for testing at NASA GRC

Reference: S.M. Bragg-Sitton and T.J . Morton, Dynamic Response Testing in an Electrically Heated Reactor Test Facility, STAIF 2006.

0.06 ,--------<r 30 - Reactivity • T ota I Core Power

0.04 25

Negative ~

Reactivity ~ 2O~ Insertion -

0.02

~ i ~ 0 -H:---/r-H--f,---4~{.,-:.h------=tr1--!--t 15 ~ 1il 3 :1 4324 ~ ~ u

-0.02 10 c;;

-0.04

-0.06

Reduced

Gas Mass Posrtive Flow Rate Reactivity

Insertion

Dynamic Control Disabled;

Reactivity Set to Zero 5

Exp. Terminated; 0

1; ~

Elapsed Time (mln :sec) Power/Flow Set to Zero

Current Test Hardware: DDG Core

*Minimallnstrumentation* Heater elements NOT instrumented.

TCs allow measurement of gas temperature and a rough "block" temperature.

Previous hardware tests allowed cosine radial power distribution; current wiring scheme does NOT allow for radial power shaping.

DDG Instrumentation r.

II TC8 D I ,

j " ,

~ TC3 TC :z to • • fI,

TC6 TC 5 TC - ------~

7 . .

• State estimator for reactivity feedback control: TC-1 - Time constant associated with "block" temperature too long

- Poor estimator of "fuel" temperature

- Applies to all profile probe TCs

• State estimator: TC-8 - Outlet gas temperature

- More tightly coupled to changes in heater element temperature

• R uire instrumented heater elements for future a lications

Integrated DOG - BPCU

Front View - DDG

I

~ I !~~SR ~~M

Rear View from Tank 6-BPCU

Test Matrix with Feedback

Test Identifier Shaft Speed Initial DDG Initial (krpm) Electrical Power Temperature

(W) (TC 8, °C)

A Variation of BPCD shaft speed @ 555°C outlet.

37 -3700 555

46

37

B Positive reactivity insertion.

37 -3900 555

C Variation of BPCD shaft speed @ 580°C outlet.

37 -4300 580

46

52

46

37

D Negative reactivity insertion.

• 37 -4230 580

Transient Operation (555°C out)

600

595

590

585

U 580 ~ ., ~

:l 10 575 ~ ., Q.

E {!! 570

, , , . ---. ,

• Core Block (TC-01)

• Exit Gas (TC-08)

• Reactor Power (W)

Shaft Speed (rpm/10)

.- ._,--------------,------ -------~---~~---------------- .---

, ,--, , , ,

---,_._-- -- -- -_. T- -- --- -----,--- ----------,

r 5000

4500 0-

~ Q.

.:.

."

4000 ~ .l!­iij 6i

3500 ~ co !5 i

565 "' __ 111..~ , .... ~_-.... - 1 3000 j 560

555 1jIII __

!5 M ., r 2500 a:

550 +-----.-----i-------i----+----..--------"- 2000

120 140 160 180 200 220 240

Elapsed TIme (min)

Approximate time constant: - 1 hour to reach stea state

"

Positive Reactivity Insertion

620 • .... c .. • .. ·- ... , ...... • .. ·c· .. . ..... .... 9000

610

600

U !... 590 ., ~

~

11 ~ ., E 580 ~

570

560

• Core Block (TC·Ol)

• Exit Gas (TC·08)

......... ~ ................................... ~ ........... ~ ........... " . .. Reactor Power (W)

: i Shaft Speed (rpmjlO)

----------~---

; .. : .. : " : "

---------~--.. --. .. : " : ... : " ~ -. : "

, " -----------~-----------~-----------~-----------~-----------~----. .. , ., , .,

, , . ,

~",~- ---- ---- --~---- --- -- .-~ ----- --- -- -f --. --. ----

. -1------- ____ .. ___ _

8000 0-

~ Q. ~

"C

7000 j 'Ii Oi ::l

6000 ~ co 15

~ 5000 I

Q.

15 ~

4000 ~

550 +----i----i----,----;----,----;.---+---+---'- 3000

240 250 260 270 280 290 300 310 320

Elapsed Time (min)

Approximate time constant: - 1 hour to reach steady state

• Core Block (TC-Ol)

: • Exi t Gas (TC-08) ~ 605 +----------------. - --.- -------.~--jliii.fiI'.",~" -- - - -.~- -.-- .. Reactor Power (W)

:.. : 5500 0-. Shaft Speed (rpm/10) ....

600

U .!... 595 ., ~

:J 1'ii ~ ., E 590 ~

585

580

;...-~---~.--.~~-.-----------.--,------ -- -- ---,

r- o ____________ p

-~---------------. .

E Q.

.!:-

" 5000 g: ~ i 51

4500 a Q. CD

Is

! 4000 I

"-Is ~ ., a: -.Ii 3500

575 +-----'-----+---------+-----+---- 3000

320 360 400 440

Elapsed TIme (sec)

480 520 560

A roximate time constant: - 1 hour to reach steady state

' .

Negative Reactivity Insertion

650 • Core Block (TC-01)

• Exit Gas (TC-08)

... Reactor Power (W)

625 ~1iIiiii-.---iii- -.--.---... ------ ------ ---- -- "--- --- ------ ---- ----,---- ----.--0 - Shaft Speed (rpm/10)

U 600 ~ .. ~

:J 1;j ~ .. 0.

~ 575

550

5000

4500 0-... E-o. ~

4000 ::c .. .. ~ 'i

3500 6i a "-'"

3000 ~ i ~

~ 2500 "-

2000

~ y .. a:

525 -f------+-------,--------f----------,------+ 1500

560 580 600 620 640 660

Elapsed Time (sec)

A"""""""v'mate time constant: - 1 hour to reach stea state

.'

What did we learn?

• Integrated system test can be conducted to demonstrate - System time constants

- Response characteristics

- Possible control algorithms

BUT: The results are only as good as the model and the data used in that model!

• Refining the model is necessary for improved test fidelity - 6-group PKE

- Characterizing multiple feedback components (fuel , core block reflector)

• Instrumentation is the key to success - Type, Location

- Noise reduction

'.

Instrumentation Needs

• Need to adopt a highly realistic approach to system control (with neutronic feedback) early in the design phase to allow appropriate selection of core instrumentation

• Good estimation of "fuel" temperature - Heater elements with embedded instrumentation

- Thermal simulator design that mimics the dynamic response of a nuclear fuel pin

• Highly distributed instrumentation - Require multiple temperature (or deflection) measurements to

introduce multiple feedback components in the dynamic model

. .

Improved Data Acquisition

• Noise reduction techniques in the data acquisition system: - Selection of a feedback control signal from all available TC data or

an average of those data points

- Application of a moving time average to the measured temperature (or deflection) data (i.e. measured temperature input into the control could be the average of the previous 5 or 10 data points, as applied in SAFE-100 testing)

- Application of a computational filter, such as a Kalman filter, in the control system

, <

Enhanced Control System

• Tested control system simulated inherent transient response of an integrated reactor core and power conversion system, but. .. - NO constraints were applied to the magnitude of the prescribed

adjustments in the core power level

- Step insertions of reactivity resulted in large oscillations in the core power level

• Additional controller can be modeled and introduced to the control system to: - Limit the maximum power level or maximum change rate in the

power level following a control maneuver to reduce the potential for system damage

- Provide a test bed for candidate autonomous reactor control systems

~ • 1·

Acknowledgements

The work in this paper was supported by and performed for the NASA Exploration Technology Development Program and the Fission Surface Power Project. The authors would like to thank:

David Riquelmy (BAE Systems), Kenny Webster (NASA MSFC), Maxwell Briggs (NASA G RC) and A. Karl Owen (NASA GRC)

for their assistance with software development and system testing. The opinions expressed in this paper are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration.