Pebble Bed Modular Reactor

Eric Glatstein, May 8, 2003

Pebble Bed Modular Reactor

• nuclear fission reactor

• uranium or thorium fueled

• graphite “moderated”

• high temperature operation

• helium cooled

• small, “modular” construction

• intrinsic safety

Electrostatics

“+” and “–” charges like charges repel opposite charges attract

Plum-pudding

Since atoms evidently are electrically neutral,

+ / - charges must be uniformly distributed.

Rutherford scattering (c.1911)

(Really Marsden and Geiger.)

Rutherford scattering

“+” concentrated in small volumes

So, what’s holding “nucleus” together against “+” electrical repulsion?

So, what’s holding “nucleus” together against “+” electrical repulsion?

Strong Nuclear Force

The New Physics

• Niels Bohr (1913)– hypothesizes quantized angular momentum

• Erwin Schrödinger (1925)– describes electron orbits with “wave equation”

• Werner Heisenberg (1927)– ties it all together with the “indeterminacy principle”

• Karl Llewellyn (1930)– describes the law with quantum jargon

Neutrons

• James Chadwick (1932)

– in addition to positively charged “protons,” nuclei contain neutral “neutron” particles.

– together, called “nucleons.”

Atomic mass

neutron number, N, v. proton number, Z

N

Z

Missing mass

0

50

100

150

200

250

300

0 50 100 150 200 250 300

mass-measured (amu)

ma

ss

(a

mu

)

measured

calculated

Missing mass

The weight of a nucleus is less than the sum of its parts.

-0.0120

-0.0100

-0.0080

-0.0060

-0.0040

-0.0020

0.0000

0 50 100 150 200 250 300

mass-measured (amu)

mis

sin

g m

as

s p

er

nu

cle

on

(a

mu

)

delta / A

0

50

100

150

200

250

300

0 50 100 150 200 250 300

mass-measured (amu)

ma

ss

(a

mu

)

measured

calculated

Fe

U

H

Nuclei (like everything else) try to approach lowest energy level:

lowest point on graph

• light nuclei undergo “fusion”

• heavy nuclei undergo “fission”

Missing mass released from “Binding Energy” between nucleons.

(E = m c2 )

Stability

Fission• Otto Hahn (1939)

– bombarded uranium with neutrons

– light “fragments” and heat unexpectedly appear

• more neutrons out than put in

• energy largely released as “Kinetic Energy” (gets Hot)

Neutrons “Fast” and “Slow”

Most emitted neutrons travel too fast to cause another uranium fission:

do not remain near nucleus long enough to cause instability

For self-sustaining “chain” reactions, a “moderator” is need to slow emitted neutrons.

• light -- transfer momentum

• low neutron absorption

Self-sustaining “chain” reactions

• Oklo, Gabon,

1.7 – 1.5 BYBP

• Chicago, Illinois,

61 YBP (1942)

Enrico Fermi

(1st artificial)

Common moderators

• Graphite

• Hydrogen-2 (deuterium; “heavy” water)

• Hydrogen-1 (“light” water)

• Boron

Useful materials

Atomic Energy Act of 1954

42 U.S.C. § 2014.  Definitions. (z) The term "source material" means (1) uranium, thorium, or any other material which is determined by the Commission . . . to be source material; or (2) ores containing one or more of the foregoing materials, in such concentration as the Commission may by regulation determine from time to time.   (aa) The term "special nuclear material" means (1) plutonium, uranium enriched in the isotope 233 or in the isotope 235, and any other material which the Commission . . . determines to be special nuclear material, but does not include source material; or (2) any material artificially enriched by any of the foregoing, but does not include source material.

“Fissile” materials

23592U (naturally occurring at ~ 0.7% ) t½ = 700 million yr

23392U (effectively zero in nature)

10n +

23892U 239

92U 23993Np 239

94Pu

t½ = 24 min t½ = 2.4 day t½ = 24,000y

10n +

23290Th 233

90Th 23391Pa 233

92U

t½ = 22 min t½ = 27 day t½ = 160 thous yr

Coolants• Air

– O2 can support corrosion / combustion

• Carbon dioxide

• Water– “light” and “heavy”

• Liquid metals, e.g., sodium

• Inert gases, e.g., helium– good heat transfer– no corrosion

Nuclear “reactor”

(“batch” configuration)

Reactor control

• control rod withdrawal and insertion

• reactivity “poisoning” at higher temperatures

Pressurized Water Reactor

Pressurized (Light) Water Reactors

USS NAUTILUS (SSN-571)Launched: January 21, 1954

Byron Generating StationOperating license (first unit) granted: 1985

Optimism . . .

“Our children will enjoy in their homes electrical energy too cheap to meter.”

Lewis L. Strauss, Chairman, Atomic Energy Commission,

September 16, 1954.

Cynicism . . .

Problems for reactors to overcome . . .

– hard to build and operate well– “run-away” danger– radioactive leaks– waste / proliferation risk– low temperature; low thermal efficiency– output not easily dispatchable– outages necessary for regular refueling– uranium-235 is rare– people make mistakes– still waiting for the waste repository . . .

Benefits . . .

– near zero greenhouse emissions

(at least from generating stations)

– Burn-up decommissioned bomb materials

– maintain an electrical generating mix

Policy . . .

• Appropriate regulation

– set rigorous standards

– enforce standards consistently

– let new technologies thrive or die on their own

Hard to build and operate well

The Nuclear Regulatory Commission believes that standardization of nuclear power plant designs is an important initiative that can significantly enhance the safety, reliability and availability of nuclear plants [and encourages] . . . plant designs which:

-- Are essentially complete in both scope and level of detail;

-- Cover plant design, construction, and quality assurance programs;

-- Satisfy regulatory requirements before construction begins; and

-- Can be referenced for individual plant applications.

52 FR 34884, September 15, 1987

Hard to build and operate well (cont.)Experience has shown that the "one-of-a-kind" approach to reactor design, construction, and operation has led to an operating reactor population of great variability and diversity, even among reactors from the same vendor.* * *This variability has introduced significant differences in the licensing and operation of these plants, in the transfer of experience from one reactor to another, in technical specifications, in operating procedures, and in backfitting considerations.

Thus . . . 10 CFR PART 52 — EARLY SITE PERMITS;

STANDARD DESIGN CERTIFICATIONS; AND COMBINED LICESNSE FOR NUCLEAR PLANTS

March 24, 2002

DONOR WON PRAISE IN ENERGY REPORT

By DON VAN NATTA Jr. 

WASHINGTON, March 23

In Chapter 5 of Vice President Dick Cheney's [May 2001] national energy report, executives of the once-moribund nuclear power industry were probably thrilled to read that the White House supported "the expansion of nuclear power in the United States as a major component of our national energy policy." * * *One such proposal was the development of a new nuclear reactor designed to produce electricity -- a gas-cooled reactor built on tennis-ball-size graphite spheres -- that the report said "has inherent safety features." * * *

                                                       

Cont.Exelon, the nation's largest nuclear energy company, is the only American corporation developing a design for the pebble-bed reactor, which it says will lead to a new generation of cheaper, smaller and more efficient nuclear reactors. The company says the pebble-bed reactor will be safer, too, though environmentalists in the United States and in other countries have sharply disputed this, calling the pebble-bed reactor a failed system vulnerable to terrorist attack. * * *The administration's endorsement of Exelon's technology was learned through interviews and documents provided to The New York Times by the corporation itself. * * *Exelon provided The New York Times with two documents that the company submitted to the task force that had not been made public by Mr. Cheney: a pamphlet describing the pebble-bed reactor and a one-page description of the reactor's benefits.

The document begins, "Exelon Corporation believes that we have found a technology that possesses the characteristics necessary to successfully compete in a deregulated environment in the P.B.M.R., a design under development in South Africa."

The document argues that the reactor is "safe, economic and clean."

“Pebble Bed Modular Reactor”

(“plug flow” configuration)

Reactor interior

(AVR reactor)

“Pebble” fuel elements

“Pebble” fuel handling

AVR reactor• 360,000 “pebbles” in core • 120 day detention• 15 cycles

Fuels

•PBMR developed for Thorium cycle (Germany 1967)

•Similar HTGR configuration operated with uranium

– PECO Peach Bottom unit 1 (Lancaster, Pa.)

(1967 - 1974)

SafetyProponents

• Each pebble self-contained• Fuel stable to 2,800°C

– “Design basis” 1,600°C accident

• Releases limited from one “module”

• No need for “defense in depth”• No need to cool pebbles before

disposal

Opponents

• Manufacturing defects

• Graphite burns

• Keep the “defense in depth”

• Pebbles large relative to fuel rods

“Bomb grade” fuels

• Highly enriched uranium (80% 235U)

– 500 tons of HEU from 30,000 dismantled Russian nuclear weapons to be blended to “proliferation-resistant” low-enriched uranium (LEU) by 2013.

• Mixed Oxide (“MOX”)

– DOE believes that half of 100 ton plutonium stockpile is excess of military needs.

“Light Water” / “Pebble Bed”

PWR PBMRThermal power (MW) 3,000 300

Burn-up (MWDt / MT) 60,000 80,000Power density (MW / m3) ~100 ~8

Reactor exit temperature (°C) 300 900Thermal efficiency 33% 45% - 50%

Capital cost (per Watt e) $75 (?) $1 (?)

Thermodynamic efficiency

Hot Cool

work

Effmax = 1 - ——TC

TH

April 16, 2002

Exelon Generation Not Proceeding with PBMR

Exelon Generation has announced today that it will not be proceeding with Pebble Bed Modular Reactor (PBMR) project beyond the completion of the current feasibility study phase. Exelon Generation has advised PBMR Pty (Ltd.), the entity managing the project, that it will continue to devote technical personnel and executive leadership to the project for the time being. As of June 30, 2002, Exelon Generation’s support of the project will total

about $20 million.

Exelon recently conducted its regular review of investments to ensure a disciplined strategy focused on the fundamentals of generation, power marketing and distribution. Becoming a

reactor supplier is no longer consistent with Exelon’s strategy.

Conclusions . . .

• Probably safer

• Probably cheaper

• It would be nice to beat the swords into plowshares

• Time will tell