115

Linear Accelerators for Tritium Production* Mahlon T. Wilson

Los Alamos National Laboratory

116

117

Linear Accelerators for Tritium Production Mahlon T. Wilson

Los Alamos National Laboratory

Accelerators have the potential to address a serious problem in our defense establishment: the production of tritium. Tritium is an essential component of all nuclear weapons. Squeezing deuterium and tritium together by using the power of a fission bomb causes a tremendous release of energy, in much the same fashion as the sun is powered.

Tritium is a heavy isotope of hydrogen, indicated by the symbols 3H or T. Like hydrogen, it has one proton in the nucleus and one electron orbiting around outside. The addition of a neutron to the proton in the nucleus produces deu­terium, a naturally occurring stable isotope. The addition of a second neutron creates tritium; however, the nucleus becomes unstable and one of the neutrons decays by emitting an electron, converting itself into a proton. The resulting nu­cleus of two protons and one neutron is known as helium-three and is stable. The rate of decay of tritium is about 5.5 percent per year, or one-half disappears in 12.3 years. Tritium is, therefore, a perishable material. Nuclear weapons have to be cycled out of the stockpile for tritium replenishment. There must be a continuous source of tritium. A lack of replacement tritium amounts to unilateral disarmament.

The traditional source of tritium in the U.S. has been from heavy-water­moderated, highly enriched uranium reactors at the Savannah River Plant (SRP). Lithium in the reactor core absorbs neutrons and is converted into tritium and helium. These reactors were built in the 1950s and do not meet today's strin­gent safety requirements. They are also experiencing the fatigue that one would expect of a 40-year-old facility. The reactors are currently shut down, no tritium is being produced, and their restart awaits resolution of several perceived safety questions.

The Department of Energy (DOE) conductec a study on how best to address the situation, the result of which was the announcement by former Energy Sec­

118

retary Herrington that two different reactor types would be constructed. These reactors are generically known as new-production reactors (NPR) in contrast to the traditional old-production reactors. Reactors are basically simple devices,

therefore DOE's decision is a reasonable one.

Neutrons can also be produced by knocking them out of the nuclei of heavy elements by bombardment with high-energy protons from accelerators. Various accelerator-based schemes have been studied for the last 40 years at the Brook­haven, Los Alamos, and Livermore national laboratories, and Chalk River Labo­ratory in Canada. The work discussed here is the result of a recent collaboration between Los Alamos, Brookhaven, and the Westinghouse-Hanford Company. Los Alamos considered the accelerator; Brookhaven the beam handling, beam absorption, and the tritium production; while Westinghouse-Hanford looked at the bricks-and-mortar construction. The resulting concept was named the Ac­celerator Production of Tritium (APT). The rational for this study was that a tritium supply is essential, long-term stockpiling of tritium is not possible. new-production reactors may face environmental roadblocks, and significant ad­vances have recently been made in accelerator technology. This was a quick­look study to determine if current technology is adequate, what the parameters would be, and how much it would cost. The result of this study has already kindled national interest in the use of accelerators and prompted further inves­tigation of the concept.

Our study utilized lead as the target material to avoid the question of so­called "fission" waste. However, the heavier the nucleus, the more neutrons are produced per impacting proton. Uranium, thorium, and the reactor-produced transuranics would make prolific neutron sources, but could be perceived as being less safe. The neutrons knocked out of the lead would be absorbed by lithium, which decays into tritium and helium (Fig. I, the APT process).

The facility that resulted from this study is shown in Fig. 2 (38807060). Protons are accelerated from the above-right portion of the figure into either of two large vacuum tanks located in a sealed building adjacent to the the now­vacant Fuel Material Evaluation Facility in the Hanford 400 area. The vacuum tanks (Fig. 3, 38807086.3.3-JP) contain the proton-stopping, tritium-producing assemblies which are irradiated by the protons. One tank receives beam while portions of the previously-irradiated assembly are withdrawn for tritium extrac­

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The APT Process

SPAUATION EVAPORATlON CAPTURE

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

i"igure 1: The API' process.

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Figure 2: AFT facility.

121

Target Assembly Beam Expander and Vacuum Vessel

'igure 3: Target assembly beam expander and vacuum vessel.

122

tion and replaced with fresh material in preparation for its next irradiation cycle.

Cycle times are six months.

The proton beam is fanned out upon entering a vacuum tank and is swept uniformly over the surface of stainless steel pressure tubes. The tubes contain hundreds of aluminum-clad pins filled with either lead or an alloy of lithium and aluminum (Fig. 4, 33807086.3.5JPM) There are two lead-filled pins for each LiAl pin. Upwardly flowing water cools the pins and helps slow the neutrons for more efficient capture by the lithium. Tritium is recovered by removing a bank of pressure tubes, cutting off their end caps, and sorting out the lithium pins, which now contain tritium.

Accelerator technology has made major advances with the development of the structures used in the Los Alamos Meson Physics Facility (LAMPF) ac­celerator, with the low-energy structure invented by the Russians, and evolu­tions funded by the National Cancer Institute, the Department of Energy, and the Strategic Defense Initiative's Neutral Particle Beam program. Equally im­pressive advances have been made in the physics understanding of beams and of accelerator structures behavior, the ability to calculate performance, the creation of beam-diagnostic instrumentation, and packaged control system software that contain tool kits to simplify applications programming.

The parameters for the accelerator developed in this study are shown in Fig. 5. The accelerator layout is shown schematically in Fig. 6. Note that the proton beam begins in two low-energy accelerators whose beams are interleaved and injected into the main accelerator, thereby achieving twice the current. The accelerator sections will be discussed in sequence from the creation of the protons to their deliverance into the tritium production assemblies.

A bottle of pure hydrogen gas is the source of the protons. The gas is bled into a small chamber that contains an electrical arc which ionizes the gas. The chamber is held at a high voltage and properly placed electrodes and current­carrying coils extract the protons and deflect away unwanted ions. Additional voltages as high as 750,000 volts have been used to increase the speed of the protons enough to be accepted by the next acceleration stage. The accelerator community is fortunate that Kapchinskii and Teplyakov in Russia invented a completely new method of accelerating low-velocity particles which has be­

123

Cross Section of Pressure Tube

L1AI Rod

./Pressure Tube

r..-- Lead Rod

TUbe Diam. -30 em Rod Dlam.-1.1 em

No. of Rods -570 Pb/Li AI Rods -2: 1

gure 4: Cross section of pressure tube.

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>F.n~1 'ITl CCl

RFQ

Reference Accelerator Parameters

.nj. RFQ Funnel OTl CCl Total

Energy MeV 0.125 2.5 100 1600

Current Amp 2 x 1/8 2 x 1/8 2 x 1/8 1/4 1/4 1/4

Frequency MHz DC 350 350noo 100 1400

length M 10 4 3 51 983 1051

Beam Power MW 0.03 0.6 0 22.4 315 400

ACPower MW 0.2 2.6 0.1 46 696 146

Efficiency % 15 24 0 49 54 54

# Klystrons 2 1/655 32 450

Structure Cost M$ 6.4 6.2 3.1 63.5 231 1200

rfSys Cost M$ 4.4 3.1 58 118 of'

Structure Cost % 0.5 0.5 0.2 5.5 20.0 100

rf Sys Cost % 0.4 0.2 5.0 61.6

+ Indudes Control System $70.5 M =6%

Figure 5: Reference accelerator parameters.

125

gure 6: APr accelerator layout.

126

come universally adopted and is known as the radio-frequency quadrupole

(RFQ).

The RFQ is a simple hardware item that shapes electrical fields so that the garden-hose-like stream of protons from the source that enters one end emerges as packets in the string-of-beads configuration that is required by linear ac­celerators. The inside of an RFQ is shown in Fig. 7 (RN 82026015). The sig­nificance of the RFQ is seen in Fig. 8 (38807086.2.19-S0S) which shows the LAMPF proton source which is powered by a 750-kV Cockroft-Walton. The Fermilab injector is quite similar. On the table in the foreground is the RFQ, and its ion source, that was recently sent into space on the nose of a rocket. This particular RFQ delivers I-MeV protons, which is a higher energy than that provided by the large machinery in the background. The APT accelerator RFQs will each deliver 1/8 A of protons at an energy of 2.5 Mev. They require 2.5 MW of electricity which is converted into 350 MHz radio-frequency power and delivered to the RFQs to provide the electric fields to do the proton-beam shap­ing and acceleration.

Low-energy beams are relatively difficult to handle, therefore we chose to utilize two systems (each with one-half the required current) for creating the proton beam and for the initial acceleration. The beams are brought together in a funnel and directed into a drift-tube linac (DTL) for the next step of accelera­tion. The funnel consists of bending and focusing magnets to direct the two beams into the same path and radio-frequency cavities to shorten the length of each beam packet. This shortening is required as the two 1I8-A 350-MHz beams are interleaved into a 1/4-amp 700-MHz beam, and the 700-MHz DTL will only accept proton packets one-half as long as are accepted by 350-MHz systems.

A DTL is used to accelerate the beam from 2.5 to 100 MeV. The inside of a DTL is shown in Fig. 9 (CN86 467). The acceleration takes place in the gaps between the lollipops, since adjacent lollipops are at opposite polarity. The beam coasts (that is the origin of the expression "drift-tube") within the lollipop for a distance long enough to allow time for the lollipop polarity to reverse so that acceleration will again occur when the proton packet emerges into the next gap. Focusing magnets are located within each lollipop to squeeze the beam di­

127

Figure 7: RFQ internal configuration.

128

Figure 8: LAMPF Cockroft-Walton injector compared to an RFQ structure.

129

gure 9: Drift tube linac internal components.

130

ameter down, as it wants to spread out since all the protons in the packet have

like charge and repel each other.

There are many DTLs around the world. Figure 10 (eN7I 1758) shows the 200-MHz drift-tube linac at LAMPF. Most high-energy circular accelerators use DTLs as injectors; for example Fermilab, Brookhaven, and CERN.

While accelerating structures appear to be rather simple, they represent the state-of-the-art in their ability to calculate shapes, provide high-conductivity materials, and accomplish precision fabrication. Good vacuum must be main­tained within the structures to permit high voltages to exist between the adjacent metal parts without electrical arcs occurring, and to reduce interference with the passage of the protons by air molecules. The temperatures of the structures must be accurately maintained because the resonant frequency of the parts change with temperature. Adequate cooling is essential as a fraction of the radio-frequency electrical power that is used to create the accelerating voltages is lost through resistive heating of the structure walls. Consequently, there is an assortment of electronic and electrical apparatus, vacuum pumping, and water­cooling hardware that attaches to an accelerator to make it work. Figure 11 (CN86 1882) shows the Accelerator Test Stand at Los Alamos. The slender tank on the right is an RFQ which increases the proton energy from 60 kV to 2.5 MeV, while the fat tank in the middle is a DTL that increases the energy to 5 MeV. Instrumentation to diagnose the beam location and shape is positioned between accelerator sections, and complete beam characterization is obtained with the equipment to the left in the photo.

As the proton packets advance through a DTL, they pick up speed and travel further during each radio-frequency power cycle. Therefore, each accelerating gap and each drift space must progressively increase in length and consequently the electrical resistance increases per accelerating gap. The faster proton pack­ets also require fewer focusing magnets to keep the beam small. After a certain energy, typically 100 MeV, other accelerating structures become more efficient than a DTL. These structures usually operate at a frequency two or more times faster than the DTL and utilize individual accelerating cavities that are electri­cally coupled together. Structures of this type are known as coupled-cavity linacs (CCL).

131

'igure 10: LAMPF drift tube linac.

132

Figure 11: Accelerator test stand.

133

A favorite CCL accelerator structure has been the side-coupled hnac (SCL) structure invented at Los Alamos, in part by Ed Knapp who is a past president of Universities Research Association, Fermilab's governing organization. A portion of the LAMPF facility half-mile-long SCL is shown in Fig. 12 (38807086.2.21-S0S). Several other SCLs have been built and Fig. 13 (CN85 4779) shows a section of one assembled for the racetrack microtron at the Na­tional Bureau of Standards. The cooling requirements of this machine are actu­ally more stringent than required for APT. Figure 14 (CN83 263) shows the simple internal design of this structure. The cooling passages are seen in the lower-left disk while the other two show the field-shaping cavity that provides beam acceleration. This cavity requires a good surface finish and its volume must be very precise, a task that is easily accomplished during machining by us­ing radio-frequency test apparatus. The structure is made of oxygen-free high­conductivity copper that is machined to a few mils tolerance and is brazed to­gether in a hydrogen-atmosphere furnace. This structure is rapidly produced on numerically-controlled milling machines and lathes.

A 1400-MHz coupled-cavity linac like the SCLs built at Los Alamos is pro­posed to accelerate the beam from 100 to 1600 MeV. APT requires 8600 ac­celerating cavities, which compares with LAMPP's 4300. This portion of the accelerator dominates all of the lower-energy portions in length, power con­sumption, and cost.

The beam is guided from the end of the accelerator to the vacuum tanks by a series of magnets which periodically refocus the beam, followed by bending magnets which direct the beam into one of the two vacuum tanks. Prior to en­tering the tank, the beam is shaped by magnets into a vertical fan which is swept across the face of the banks of pressure tubes within the tank. Magnet designs are under consideration which will shape the beam to uniformly illuminate the face of the pressure tubes without the need for a sweep magnet.

The I-kilometer length of the APT accelerator is slightly longer than the LAMPF accelerator, and about one-third the length of SLAC at Stanford University. Optimization studies must be performed since lengthening an ac­celerator reduces the amount of electrical power that is lost in the structure walls, but the buildings and accelerator support become more expensive. The beam power is 400 MW which is only about 30 times more than the LAMPF

134

Figure 12: LAMPF DTL and SCL structures.

135

8'igure 13: N85 Ractrack microtron SCL section.

136

Figure 14: Components of a side coupled linac structure.

137

beam when it is pulsed on (note that the LAMPF beam is on only 10 percent of the time, which saves power and cooling). The LAMPF beam is capable of burning through any object placed in its path and care must be exercised that the beam is properly dispersed. The high power of the APT machine requires con­trolled beam manipulation and adequate safety interlocks. An accelerator can be shut off within microseconds after fault detection, which is usually soon enough to prevent beam bum-through.

The frequency and the amount of electrical power are determined by the di­mensions and the electrical resistance of the structures and by the power that is put into the beam. The electrical pathways in modern accelerator structures are typically tens of centimeters long, requiring that the polarity of the electrical voltage be reversed hundreds of millions of times per second, i.e., a frequency of hundreds of megahertz. The amount of power required is equal to that car­ried away by the beam plus that lost to resistance heating in the structure walls. APT requires 458 MW of radio-frequency (rf) power.

The klystron has been the traditional device utilized to convert direct current electricity to rf for powering accelerators. Klystrons contain an electron source which is held at a high voltage and emits a constant stream of electrons. The electrons pass down a pipe which has a few gaps along its length. These gaps open into cavities formed by cans which are sealed around the outside of the pipe. A drive signal having the proper frequency, amplitude, and phase is intro­duced into the first cavity, creating a time-varying voltage across the gap. This voltage modulates the velocity of the electrons streaming by. As they travel down the pipe, the faster electrons tend to get ahead and the slower tend to fall back, converting the constant stream from the source into a spatially modulated beam which excites the second cavity. The excited second cavity further modu­lates the beam, and so on. The electrons give up much of their kinetic energy in traversing the last gap, from which rf power is withdrawn for use in the ac­celerator structure. The remaining kinetic energy is absorbed on a water-cooled stop and represents an inefficiency, which may be as low as 35 percent in a well-matched tube.

Klystrons capable of high-power continuous operation would need to be de­veloped at 700 and 1400 MHz for APT. If they can be provided in I-MW packages, 32 are required for the DTL and 450 for the SCL. These are big num­

138

bers for klystrons and reliability would be essential. The most successful klystrons for LAMPF were built by Varian 20 years ago. They have a mean­time-between-failure in the range of 60,000 to 80,000 hours; some have run for over 100,000 hours. The accelerator community has a long history of successful operation of large numbers of klystrons; LAMPF has 44 on line and SLAC has 244. The SLAC klystron gallery is shown in Fig. 15 (38807086.2.20-S0S).

The klystrons require direct-current power at approximately 100 kV. The electrical power delivered from the grid will be 60 Hz and in the range of 115 to 500 kV. The grid power will be stepped down in voltage, rectified into direct current, and distributed to each klystron. Protection will be provided to prevent damage to a klystron should it develop an internal fault and an arc be created. This portion of the power conversion has typical inefficiencies of 15 percent; however, APT may provide the incentive for improvement. The best that one can expect is the 0.75 percent inefficiency experienced by the utilities with the AC-DC-AC plants that tie electrical grids together.

The APT requires 770 MW of electrical power, which is utilized as shown in Fig. 16. Power-reducing efforts would have the biggest payoff in the area of conversion of electrical power from the transmission lines into usable radio­frequency power delivered to the accelerator structure. A 1 percent improve­ment reduces the electricity cost by $1.6 million a year and saves $4 million in the initial capital cost. A 4 percent improvement over the 40-year lifetime of the plant is a savings of half a billion dollars in the electric bill.

The APT power requirement of 770 MW is comparable to the output of a large nuclear generating plant. So why not just use a reactor instead? The re­sponse involves acceptance and interpretation of risk. Production reactors re­quire highly enriched uranium in their cores to provide enough neutrons to keep themselves critical since they contain a lot of neutron-grabbing lithium. Should cooling fail and the core get hot, one could imagine the lithium boiling off, thereby removing that neutron absorber and resulting in a supercritical core. Two electrical grids that contain large hydroelectric sources, Bonneville Power and Tennessee Valley Authority, have indicated adequate capacity to handle the APT load. Bonneville power is quoted at 32 mils per kilowatt hour and is the basis of the $162-million per year electrical bill shown in Fig. 17 as being the largest component of the APT annual operating cost of $269-million per year.

139

~re 15: SLAe Klystron gallery.

140

Where Does The 770MW Electric Power Go?

Balance of Plant 25MW 3.2%

Heating of the

Accelerator Structure 57.6MW

7.5%

288.4MW

Figure 16: Where does the 770 MW electric power go?

141

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

Annual Operating Costs, 1988 Dollars

• Plant Operations

• Target Fab/Recovery, Waste Management

• General Support

• Capital Upgrades

• Electrical Power (770 MWe) (750/0 availability)

Total

Subtotal, S.M $32

41

17

17

162 $269 M/yr

38807086.4.19M-JH

~ure 17: Annual operating costs, 1988 dollars.

142

The Paducah end of the Tennessee Valley grid had a gigawatt of excess power available one-third of the time last year at a price of 14 mils. It might be practi­cal to utilize APT as a base-load leveler for a power grid by operating it at power levels between about 250 MW and the maximum 770 MW. The lower levels merely reduce tritium production.

A cost estimate by component is shown in Fig. 5 and by area in Fig. 18. These figures show that the major costs occur in the rf systems. The compli­cated, physics driven, mystical accelerator structures may determine the beam delivering capability of the accelerator, but they certainly do not dominate the costs. This indicates that the accelerator-structures costs should be increased to enhance reliability, reduce radiation levels, and increase efficiency. Designs de­veloped in future studies will emphasize the operational aspects and one would expect the structures costs to increase as frequencies are lowered and beam­passage apertures are increased to reduce the chance of scraping the beam, thereby increasing the radiation activation.

A comparison of the total life-cycle costs of the NPR alternatives is shown in Fig. 19. The HTGR is the high-temperature gas-cooled reactor concept pro­posed for siting in Idaho and it is capable of generating electricity which is sold to partially offset its high capital and operating costs. The HWR is a heavy­water reactor similar to the traditional production reactors to be located at Savannah River. The WNP-1 is a proposal to complete the defaulted Washington Nuclear Power station 1 at Hanford, but with the core modified to also produce tritium. The life-cycle cost of the APT is shown to be in line with the other proposals.

The reactor-based proposals face licensing requirements that are expected to be more rigorous than for APT. APT contains no fissionable material so there can be no criticality accident and fuel reprocessing does not have to be con­sidered, APT has low enough afterheat that a loss-of-coolant accident should not be serious, and APT generates a lower radioactive isotope inventory than reac­tors thereby reducing the high-level-waste disposal problem.

APT facility construction should be accomplished faster than reactor con­struction since the containment building is smaller and possible overpressures are considerably less. Much less hardware must be placed within the tight con­fines of the pressure vessel. The long length of an accelerator allows construc­

143

Accelerator Cost Breakdown

Control ($37M)

Accelerator (INJ, RFQ,

Total - $1.28

proBram Mgmt DTl, CCl) ($163M) ($7 M) 38807088.2.1011-505

igure 18: Acclerator cost breakdown.

TOTAL LIFE CYCLE COST COMPARISON

Cost Category HTGR HWR WNP-1 APT

Pre-operational 546 208 514 279

Capital 5,314 3,040 1,954 2,270

O&M 20,063 16,544 17,591 9,893

Total Costs 25,924 19,792 20,059 13,186

Electric Revenue 10,840 0 11,080 0

Discounted Values

Costs 11,476 8,076 8,736 5,850

Revenue 3,769 0 4,333 0

Net Cost 7,707 8,076 4,403 5,850

ure 19: Total life cycle cost comparison.

144

tion to occur at many locations simultaneously, thereby reducing interference. An accelerator consists of hundreds of very similar parts which favors mass production, automated machining, and assembly line techniques.

The reduced licensing requirements, coupled with ease of construction, sug­gests a fairly short time period is required for completion of the APT project. Figure 20 presents a schedule which indicates that tritium production could be­gin nine years after authorization and adequate funding.

The APT collaboration believes that the use of an accelerator for the produc­tion of tritium is viable and should be investigated further as a backup to the proposed new production reactors. It is recommended that funding be provided to construct hardware and perfonn tests to prove the adequacy of the technology and the process.

145

LEVEL 1 APT SCHEDULE

1. Preconceptual Design

2. Plant Design, EIS

3. Component Development/ Optimization

4. Accelerator Construction

5. Lattice Construction

6. Plant Construction

7. Plant Start-up

FY 88 I FY -4 I FY -3 / FY .2/ FY·1 / FY 1 / FY 2 I FY 3 , FY 4 , FY 5

---. (CoqIl''''

tx 6.

tx ~

Aulhorlallon

6 6.

6 ~

RIC

~~ 6 6 Sch44

fr---6

~]IS. Goal Tritium I Production

Figure 20: Level 1 APr schedule.

146