Five Decades of TRIGA Reactors

Helmuth BÖCK Technische Universität Wien-Atominstitut Stadionallee 2, 1020 Vienna, E-mail: [email protected]

Mario VILLA, Robert BERGMANN Technische Universität Wien-Atominstitut Stadionallee 2, 1020 Vienna, Austria [email protected]; [email protected]

ABSTRACT

In the late 1950’s to the end of the 1960’s, TRIGA1 reactors were mainly commissioned in the US and Europe while later Asian countries as well as Latin America followed. Most of these reactors were used for the training and education of engineers and scientists to develop a national nuclear program, at universities for academic training or at hospitals for radioisotope production. Since the commissioning of the first TRIGA in 1958, a total of 66 TRIGA reactors have been built, some of which were converted from MTR type fuel to TRIGA fuel. The following decades during the 1990’s and beyond are characterized by TRIGA reactors being shut down or decommissioned due to changes in the national nuclear programs, under- utilization or simply lack of funds. In addition, the US fuel return program which started in 1996 put pressure on many TRIGA reactors to return any HEU fuel to the US. In many countries, this program initiated TRIGA shut down processes due to reasons mentioned above. Today 38 TRIGA fueled reactors remain operational. Presently the main concern of the TRIGA community is the continuous supply of TRIGA fuel; it is presently suspended due to necessary post-Fukushima safety and security investments at the fuel factory located at Romans/. Other concerns are costly refurbishments due to new safety and security requirements, obsolescence of original parts, and underutilization. After a brief history of TRIGA reactors, the paper covers the present situation of the TRIGA community and gives an outlook of problems to be solved during the next decade for further successful TRIGA operation.

1 HISTORICAL DEVELOPMENT OF TRIGA REACTORS

The TRIGA (Training, Research, Isotope Production General Atomics) concept has its origins in August 1955, when a large international conference was held in Geneva, Switzerland. One of the two US organizers of that meeting was Frederic de Hoffman, a nuclear physicist employed by General Dynamics Corporation in San Diego, California, USA.

1 TRIGA is a registered trademark of General Atomics (USA)

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After the conference, de Hoffman convinced General Dynamics that it was time for the commercial development of nuclear reactors and nuclear energy. General Dynamics responded by creating the General Atomic Division in 1956 (now known as General Atomics, GA), with de Hoffman as its first president.

In June of 1956, then General Atomic Division of General Dynamics convened a group of scientists, to design a ‘safe reactor’ which must be one that “could be given to a bunch of high school children to play with, without any fear that they would get hurt”. The reactor fuel itself should have inherent safety characteristics even for fast reactivity insertion events. This special feature is enabled due to the large prompt negative temperature coefficient of reactivity of the UZrHn fuel. The prototype reactor, named TRIGA Mark I, achieved first criticality on 3 May 1958 at the General Atomic division’s new facilities in La Jolla, near San Diego, California, USA. In 1964, full patents on the TRIGA reactor were granted to its early designers [1], [2].

Over the years several TRIGA fuel types that differ in cladding, enrichment, weight percent uranium, size, and burnable poisons were developed. In total 66 TRIGA research reactors (RRs) have been constructed in 23 countries and by September 2016, 38 RRs are still in operation. The prototype TRIGA reactor at General Atomics in San Diego was closed down permanently in 1995 and was previously designated by the American Nuclear Society in 1986 as a Nuclear Historic Landmark. The citation highlighted its role in pioneering the use of unique, inherently safe capabilities in nuclear reactors. A second TRIGA, at the University of Illinois (now decommissioned) also received this distinction in 2016 for its pioneering work.

2 THE TRIGA CONCEPT [3], [4]

The TRIGA reactor was developed and offered to customers in several standard designs. The below-ground TRIGA Mark I reactor is extremely simple in its physical construction. It has a graphite-reflected core installed near the bottom of an aluminium tank and typically operates at power levels up to 1 MW with pulsing capability typically to 1000 MW and average in-core thermal flux levels of 1E+13 neutrons/cm2s. Surrounding earth and demineralized water provide most of the required radial and vertical shielding. No special containment or confinement building is necessary and installation in existing buildings has often been possible. Core cooling is adequately achieved just through natural convection. Each Mark I reactor is equipped with various irradiation facilities including a central thimble for high-flux irradiations, pneumatic rabbit with in-core terminus, and a rotary specimen rack for uniform irradiations of up to 80 sample containers.

The above-ground TRIGA Mark II reactor has a core that is identical to that of the Mark I but is located in a pool surrounded by a concrete biological shield that is above the reactor room floor level. The pool water provides natural convection cooling for operation up to 2 MW, or up to 3 MW with a down-flow forced cooling. In addition to the Mark I’s irradiation facilities, the Mark II includes four horizontal beam ports extending through the concrete shield to the surface of the reflector, and a graphite thermal column providing a source of well-thermalized neutrons suitable for physical research or biological irradiations. In the early TRIGA Mark II reactors, a separate thermalizing column was included together with an associated water-filled pool for shielding studies. In recent times, users have converted these for other applications, such as dry neutron radiography facilities with built-in shielding. More

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A later design option, the TRIGA Mark III provided a movable reactor core, supporting both steady-state (up to 2 MW) and pulsing operations, but with greatly increased operational flexibility. The core can be moved to one end of the pool for experiments in an adjacent dry, walk-in exposure room or to the opposite end for experiments involving the thermal columns and beam ports, or used in the centre of the pool for isotope production and other applications.

Special purpose TRIGA reactors have also been built. These include:

• The dual core TRIGA reactor in , licensed at 14 MW, is the highest power TRIGA built (GA has designed TRIGA reactors to power levels of 25 MW) and is primarily employed for power reactor fuel testing. The dual core feature includes a second annular core that pulses to 22 GW. A similar annular core pulsing reactor was constructed at Tokai Mura, , which has been extensively used for transient testing of LWR fuels. • The 2 MW TRIGA reactor now operated by University of California-Davis was originally designed and constructed for the U.S. Air Force, equipped with unique robotic and neutron camera facilities to conduct real-time neutron radiography for detecting corrosion in military aircraft wings. • The last TRIGA to be constructed near Rabat, is a 2 MW TRIGA Mark II, commissioned in 2007, and including built-in forced cooling features that would allow a future upgrade to 3 MW.

Instrumentation and Control (I&C) systems for all new TRIGA reactors have now evolved into compact, microprocessor-driven systems. As with previous generations of the I&C systems, they are designed to enable inexperienced students and non-technical personnel to operate the reactor with a minimum of training, with simplicity afforded as a result of the inherently safe characteristics derived from the physical properties of the UZrH fuel.

Four operating modes are typically available: manual, automatic, pulsing, and “square wave,” the latter being a one-button start-up sequence for bringing the reactor up quickly (a few seconds) to its operating steady-state power level. TRIGA reactors have also been licensed to operate in unattended mode, again as a result of the protection afforded by the safety characteristics of the UZrH fuel.

3 THE TRIGA COMMUNITY

In February 1970 the first US TRIGA Users Conference took place in Denver/Colorado followed by the First European TRIGA Users Conference in Otaniemi/ with a large number of overseas participants from the USA and Asia as well as representatives from General Atomics. At this conference topics such as reactor operation, experience with reactor systems, reactor utilization and safety issues were discussed. These regular TRIGA meetings were continued on a two-year basis both in the USA and in Europe. While the US TRIGA

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Conference was later on embedded into the US-TRTR Conference, the European TRIGA conference continued up to 2008.

Due to lack of participation, the European TRIGA Users Conference was then embedded into the annual Fuel Management conference (RRFM) where the TRIGA community met in a side event to discuss common problems. The subjects were mainly fuel procurement and fuel back-end, topics which were of high interest for all TRIGA reactors. As of September 2016 in Europe only 7 TRIGA reactors (Mainz, Vienna, Ljubljana, Pitesti, Roma, Pavia and Istanbul) were in operation; the TRIGA contacts nowadays rely mainly on personal, direct contacts among the TRIGA operators as they know each other since many years. In addition on the initiative of the IAEA recently a document has been issued[5] which covers the past, present and future of TRIGA Reactors and which also includes a CD with all papers presented at the US and European TRIGA Users Conferences from 1970 to 2010.World-wide TRIGA Situation.

4 WORLD-WIDE TRIGA SITUATION

According to the IAEA Research Reactor Data Base (RRDB) [6] totally 69 TRIGA reactors of all types has been constructed or converted from another fuel type to a TRIGA core, as of Sept.1 2016 38 TRIGA type reactors are still in operation while 31 reactors are shut down or decommissioned. A detailed list is shown in [5].

5 FUEL FRONT-END SITUATION

From the initial period in 1950’s up to mid-1990’s the TRIGA fuel elements were manufactured by General Atomics in the USA. In 1996, GA entered into a joint venture with CERCA of France to relocate its fuel manufacturing facility from the USA to the same site in Romans, France where CERCA already manufactured MTR fuels. The joint venture, TRIGA International SAS, now owns the technology rights and know-how previously held exclusively by GA. Since its establishment, the new TRIGA fuel production facility at CERCA/France delivered fuel elements for all the HEU to LEU core conversions both in the US and internationally as well as reload fuel for other existing TRIGA reactors worldwide. This conversion campaign ended in 2014, whereupon CERCA stopped its TRIGA fuel production temporarily to make post-Fukushima safety upgrades mandated by the French regulatory authority. Currently fuel fabrication facility is expected to resume operations in late 2018 or early 2019 [8].

Therefore, many TRIGA reactors around the world are now in a difficult fuel situation as many of them had very little or no spare fuel elements and may run out of fuel very soon such as , and Morocco. As a result a number of TRIGA reactors either run at reduced power or operate only a few hours per week to save fuel and to prevent shut down and/or lose their users or customers.

6 FUEL BACK-END SITUATION

Originally in the 1960’s, when most of the TRIGA reactors were commissioned, the TRIGA fuel was on loan from the then USAEC. The fuel assemblies and the contained special nuclear material (SNM) belonged to the USA and had to be returned at the end of its use to the USA. This policy changed in 1970’s where each country had to purchase the

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TRIGA fuel with the guarantee that the USA takes back ultimately the spent nuclear fuel for final storage and ultimate disposal.

The next policy change happened in the early 1990’s when the USDOE stated that this take back guarantee starts in 1996 and ends by 2006. Each TRIGA user has to shut down its TRIGA by May 13, 2006 and has three years time to return his fuel to the US otherwise the host country has to take care for a national fuel storage facility. This created panic not only among TRIGA users but within the whole research reactor community as this would lead to a massive shut down of well utilized research reactors. A US consultancy company was then retained by a research reactor users group (AFR Arbeitsgemeinschaft Forschungsreaktoren ) to intervene at the DOE to extend or abandon the take back limitation.

This intervention was successful leading to an extension of another 10 years from May 13, 2006 to May 13, 2016. This would be the last day to operate a research reactor, with a follow-up fuel return period to May 13 2019. Any facility operating beyond May 13 2016 has to store their research reactor fuel in their national storage facility.

For low-income countries the fuel return shipment was paid by the USA, for other countries the return shipment and ultimate fuel storage fee which is in the range of several thousands of US $ per kg fuel, has to be paid by the country.

In addition political pressure was placed on all operators to return all TRIGA HEU fuel otherwise TRIGA LEU fuel would not be accepted. For several countries operating HEU cores, high U density TRIGA LEU fuel already developed and qualified by General Atomics (GA) in the 1980s, and approved by USNRC [5], was provided by DOE in exchange. This allowed these reactors to maintain comparable performance with the LEU fuel.

An interesting procedure satisfying US DOE demands to return both HEU and LEU fuel in time to the USA and continue operation with almost new TRIGA fuel, took place in late 2012 at the TRIGA reactor Vienna. As this reactor is the closest research reactor to the International Atomic Energy Agency and heavily used by this organization, an interesting fuel exchange took place in November 2012 after almost 2 years of negotiations. The DOE offered almost fresh 8,5 wt% TRIGA fuel stored at Idaho National Lab. (INL) in exchange to all HEU and LEU fuel used since about 50 years at the TRIGA reactor Vienna. The fuel at INL originated from the Musashi TRIGA reactor (Japan) which was decommissioned while the fuel burn-up was less than 0,5 % and still of high value in another TRIGA reactor core. According to the contract, this fuel may be used until 2025 and must then be returned to the USA or the contract has to be renegotiated. The actual fuel swap procedure from the arrival of the INL fuel at the TRIGA Vienna to the fuel transport away from the TRIGA Vienna took only about two weeks [7].

7 AREAS OF TRIGA APPLICATIONS

Recently there have been several reports being published or are under preparation, which covers extensively the application of research reactors, one devoted uniquely to TRIGA reactors [5], [9]. Therefore below only a short survey of TRIGA applications is given and more details can be extracted from the references.

As TRIGA reactors allow two modes of reactor operation applications are separated in steady state mode and transient mode:

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In the steady-state mode of operation, TRIGA reactors provide much the same research and training capabilities as other types of research reactors. These include

. academic education and training in all nuclear fields . neutron activation analysis . radiochemistry . radiation protection and dosimetry . radioisotope production . neutron transmutation . neutron-beam applications, including neutron radiography and BNCT. . nuclear technology

In addition, however, TRIGA reactors offer the unique and added capabilities to produce pulsed bursts of neutrons. This capability has provided scientists and researchers a wide variety of additional areas of research applications such as:

• studies of biomedical effects in pulsed radiation fields • transient radiation effects in electronic components • tests of power reactor fuel under simulated accident conditions • production of very short-lived radionuclides for radiochemistry • nuclear physics studies using 250 to 2000 MW pulses and pulses as high as 20 GW in specialized TRIGA reactors.

8 REFURBISHMENTS

Many of the operating TRIGA reactors have been refurbished in the past, safety requirements for research reactors have been changed, new security requirements are implemented and also the utilization and application of TRIGA reactors changed.

One major area of refurbishment is the I&C system of TRIGA reactors. The original I&C system was of electronic tube type design, followed by the next generation of transistorized I&C systems. Both types were easy to maintain as every dedicated radio amateur could detect failures and replace failed components.

At the early 1990ties digital, micro-processor controlled I&C systems were offered to customers which in some cases replaced the previous analogue I&C systems. However due to the fast development of computers and digital engineering the typical life time of these I&C systems is about one decade, further the usual spare part guarantee is also limited to ten years. Therefore today the next generation of digital I&C systems is available (Fig.1) but many aged TRIGA reactors have to decide if this investment (several M€ ) can still be financed. [10]

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Figure 1: The new I&C system of the TRIGA reactor Vienna

9 CONCLUSION

Although most of the TRIGA reactors are now of “senior age”, many of them are still “going strong”. Evidently TRIGA reactors cannot be compared with complicated high flux reactors; however it should be mentioned here that many experiments around high flux reactors have their roots at one of the TRIGA reactors around the world. In many cases these roots cannot be traced back clearly to its origin but two typical examples are the following:

o The first successful neutron interferometer which operated in 1974 at the TRIGA reactor Vienna and was later transferred to the High Flux Reactor in Grenoble (ILL)) [11] more recently. o The demonstration that ultra-cold neutrons can be produced very efficiently during transient operation of the TRIGA reactor Mainz [12].

10 REFERENCES

[1] McReynolds, A.W., Nelkin, M.S., Rosenbluth, M.N., And Whittemore, W.L., Neutron thermalization by chemically bound hydrogen and carbon, UN/P/1540, Second United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1– 13 September 1958. [2] Simnad, M.T., International Journal of Applied Radiation and Isotopes, I, 1956, pp 145-171. [3] Wallace, W., Simnad, M.T., Fuel Element, US Patent No. 3119747 filed 8 June 1960, granted 28 January 1964.

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[4] Taylor, T.B., Mcreynolds, A.W., And Dyson, F.J., US Patent No. 3127325, filed May 9, 1958, granted March 31st, 1964. [5] History, Development and Future of TRIGA Research Reactors IAEA Tech.Report Series 482 (2016) [6] IAEA Research Reactor Data Base: https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx [7] M.Villa, R.Bergmann, A.Musilek, J. Sterba, H. Böck C. Messick “The Core Conversion of the TRIGA Reactor Vienna” NENE 2013, Bled 9-12.9.2013 [8] J. Razvi private communication July 2016 ‘Applications of Research Reactors’ [9] IAEA Nuclear Energy Series No. NP-T-5.3, ‘Applications of Research Reactors”. Vienna (2014) [10] M. Villa, R. Bergmann, H. Böck, M. Kroc, M. Proks, V. Valenta, M. Kase, J. Herrmann, J. Matousek: The New I&C System of The Triga Mark II Reactor Vienna RRFM 2016 Berlin 13-17.3.2016 [11] H. Rauch, W. Treimer, U. Bonse: “Test of a simple crystal neutron ingerferometer”- Phys. Letter A 47 (1974) 369 [12] “Safety Evaluation Report on High-Uranium Content, Low-Enriched Uranium- Zirconium Hydride Fuels for TRIGA Reactors,” U.S. Nuclear Regulatory Commission, NUREG-1282 (August 1987)

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