CONDITION OF RESEARCH REACTOR
SPENT NUCLEAR FUEL IN WET STORAGE1
Lambert J.D.B., 2Maksimkin O.P. 1Argonne National Laboratory, Chicago, USA. 2Institute o f Nuclear Physics, Almaty, Kazakhstan
INTRODUCTION
The Department of Energy established the Integrated Research Reactor Safety Enhancement Program (IRRSEP) in 2002 to support U.S. nonproliferation goals by (a) implementing safety upgrades, or (b) assisting with the safe shutdown and decommissioning of foreign test and research reactors presenting security concerns. This review of spent nuclear fuel (SNF) grew out of IRRSEP collaborative work with the Institute of Nuclear Physics, Kazakhstan (1).
SCOPE OF REVIEW
The condition of SNF in wet storage at ten Soviet-designed research reactors (Table 1) was reviewed in the light of international experience in order to identify possible safety issues. These ten reactors use Al-clad UO2-AI or U-Al alloy dispersion fuels of >20% enrichment fabricated in Russia (2). The ten reactors began operation over 1957-74 and today five are shut down permanently. Although SNF was originally sent to Russia for reprocessing much of it generated over the last 25-30 years has been stored at the reactors in fuel storage pools (FSPs) of variable water
1 Article created by the University o f Chicago as Operator o f Argonne National Laboratory under contract No. W-3I-
109-ENG-38 with the U.S. Department o f Energy. The U.S. government retains for itself and others acting on its behalf a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by and on behalf o f the Government.
quality. Table 1 includes research reactors in Brazil and Argentina of similar vintage, which use Al- clad dispersion plate fuel of U.S. origin. Data on SNF condition was obtained from site visits for IRRSEP and other programs, and workshops (3), conference proceedings and journal articles. Particularly valuable information came from an IAEA Coordinated Research Project on research reactor SNF performed over 1997-2003 (4).
The A1 cladding of research reactor fuel irradiated in water at 60-80°C is rapidly oxidized to give a layer of Boehmite or AI2O3.H2O (5), which will protect it from further degradation, Fig. 1 (6). After discharge to an SFP, the condition of SNF during subsequent wet storage is then largely determined by the stability of this Boehmite layer.
Table 1: Research reactors in this review (in order of startup)
Country Reactor Startup Permanent Power U-235 IRRSEP Assembly
(Upgrade) Shutdown (MW) (%) Interest Type
Romania WWR-S 1957 1997 3.5 10, 36 Yes EK-10, S-36
Brazil IEA-R1 1957 2 20, 90, Plate type
20
Poland WWR-S 1958 1995 2 10 Yes EK-10
(EWA) (1968) 6-10 36 WWR-SM
Yugoslavia RA 1959 1984 6.5 2(80) RA
Hungary WWR- 1959 2 10 EK-10
SM (1968) 5 36 WWR-SM
(1992) 10 36 WWR-M2
Uzbekistan WWR- 1959 10 90/36 Yes IRT-2M/3M
SM 20 IRT-4M
Ukraine WWR-M 1960 10 36
WWR-M2/M5
Latvia IRT-5000 1961 1997 2 10 Yes EK-10
(1973) 5 90 IRT-2M/3M
Bulgaria IRT-2000 1961 1989 2 10 EK-10
(1985) 5 36 S-36
Kazakhstan WWR-K 1967 10 36 Yes WWR-TS
1998 6
Argentina RA-3 1968 5 20 Plate type
Poland Maria 1974 15 80 Yes MR-5/MR-6
Spent Fuel Condition
The external condition of the wet-stored SNF assemblies from the reactors under review varied from significant failure due to galvanic corrosion that was driven by poor water quality, through gradual pitting caused by slightly impure water, to a stable condition of no observable change in the oxidized A1 alloy surface of the irradiated fuel. SNF stability in wet storage appears to depend on
Boehmite layer Resin mounting
compound
Fig-1 Reactor-formed Boehmite layer
three factors: (a) A1 being the sole metal in the FSP to avoid galvanic action; (b) good water chemistry2 to suppress attack of the Boehmite layer by aggressive ions, and (c) careful SNF handling to limit damage to the Boehmite layer. If one of these factors is not satisfied, SNF will degrade; if more than one factor is not satisfied, failure of the A1 cladding may occur (see Table 2). But even SNF failure does not seem to raise significant safety issues—for example, after drying, failed SNF from the RA-3 reactor was recently shipped to the U.S. for storage without any difficulty (4). A possible exception is where galvanic corrosion and very impure water has caused massive fuel failure, as at the RA reactor. Figure 2 shows the excellent condition of WWR- K SNF, where water quality is high and the FSP contains only Al.
Fig.2. Surface condition of WWR-TS SNF from WWR-K reactor
Insight into the history of FSP water quality came from the condition of the unalloyed Al liners used in most FSPs. Unlike Al cladding which forms its stable Boehmite layer in-reactor, Al liners remain unprotected and prone to significant corrosion in impure water. In the absence of much information regarding water chemistry in the early years of reactor operation, the condition of the FSP liners was used as an indication of past water quality; i.e., where water quality was poor, significant liner corrosion would occur, and versa.
Based on this interpretation of water quality, Table 2 summarizes our understanding of the condition of the SNF reviewed here.
The condition of the FSP liner in several of the reactors raises the safety issue of losing water from FSPs due to liner failure. Where the risk of failure is high, the pool liner should be replaced, as was done at the WWR- SM reactor in Budapest in 1986 (after 27 years use) and at the EWA reactor in 1997 (after 39 years use). Meanwhile the Al liners at the WWR-M and WWR-SM reactors are exhibiting significant pitting corrosion after 43 and 44 years of use and should be evaluated. Figure 3 shows the condition of the WWR-M liner.
2 IAEA recommended water chemistry values are:- electrical conductivity: 1-10 uS/cin: pH: 5.5-6.5; Cl' ions: <1-5 ppm; S 0 4‘ ions: <10 ppm; Cu, Hg, Ag: <0.02ppm; Fe, Al, N 0 3 N 0 2: <1 ppm; and hardness: <60 ppm (4).
Table 2: Summary of SNF condition in wet storage
Mode of Attack Reactor Water Quality Dissimilar Metals Cs-137 Activity in FSP (Bq/L) SNF Failure Comments Galvanic corrosion in impure water RA Y ugoslavia Bad Yes 140,000 (1996)
Yes Steel storage barrels leak Cs-137 activity RA-3 Argentina Poor Yes -120,000 (1999) Yes IER-R1 Brazil Fair Yes <5 (2002)
Yes Low Cs activity due to cleanup Pitting corrosion in impure water WWR-SM Uzbekistan Poor No NA No WWR-M Ukraine Poor No 6,600 (1999) No Cs activity from WWR-M2 failures in-reactor in 1991 WWR-SM Hungary Poor No NA No WWR-S Romania Fair Yes* 50,000 (1997)
No Likely bad water early; galvanic corrosion now IRT-2000 Bulgaria Poor Yes NA No None apparent WWR-K Kazakhstan Good No 50-70 (2003) No No failures in-reactor or in wet storage * Thermocouple elements SUMMARY
The stability of SNF in wet storage depends on good water chemistry, an absence of dissimilar metals in the FSP, and gentle handling—measures that will minimize damage to a protective layer of Boehmite (AI2O3.H2O) that forms on Al-clad fuel in reactor. Although departure from these measures occurred in the early operation of some research reactors, their importance has been stressed over the last decade at workshops and meetings and is now well known. Degradation of SNF in wet storage is therefore becoming a phenomenon of the past. In contrast, the unalloyed A1 liners of FSPs are not so protected by an oxide layer, and their safety should be evaluated.
Fig.3. Pitting corrosion of the A1 liner of the FSP at the WWR-M reactor REFERENCES
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