Review of the State of Criticality of the Three Mile Island Unit 2 Core and Reactor Vessel
- TMI-2 Safety Advisory Board, GPU-Nuclear, Middlelown, PA 17057 (United States)
The events during the early hours of the Three Mile Island Unit 2 (TMI-2) accident on March 28, 1979 caused the fuel in the reactor core to crumble or disintegrate, and then subside into a rubble structure more compact than its normal configuration. The present height of the core is about seven feet, five feet less than its normal configuration of 12 feet. With the same boron content and some or all of the control rod and burnable poison rod material as the normal core configuration, the collapsed structure is calculated to be more reactive. However, the reactor is assuredly subcritical at present because of the extraordinarily high boron concentration maintained in the coolant water. Many studies of the neutron multiplication of the TMI-2 reactor have been completed since 1979. These efforts culminated in the ''Criticality Report for the Reactor Coolant System at TMI-2'' (October 1984). In that report, a conservative model was chosen, which assumed the fuel to be in the lower head with the most highly enriched fuel (approximately 3% {sup 235}U) surrounded by a mixture of the remaining fuel. The bottom surface was reflected by the steel vessel, while the upper surface was somewhat rounded. Optimum moderation was chosen, and all fuel cladding and solid control materials were removed. This ''incredible'' model was found to be subcritical even with allowances for calculational uncertainties estimated by computer code benchmarking studies. After the development of this model, the first video examination of the lower plenum showed that as much as 1000-2000 kg of core material may have become liquefied. The model has been modified conservatively to allow for this effect, with the result that the neutron multiplication factor (k{sub eff}) is estimated to be slightly less than the earlier model (in other words, k{sub eff} < 0.99). Four additional and different physical models are discussed briefly in the report to illustrate the margin of subcriticality, to provide a better estimate of the neutron multiplication factor, and to provide some understanding of the criticality effects of the important parameters. The first two are infinite arrays of fuel pins (to represent a compacted configuration) and infinite arrays of UO{sub 2} spheres (to represent rubble). The two methods, with their different and conservative geometries, agree well. As a result, it can be deduced that the multiplication factor is about 0.91 for an infinite array of average enrichment fuel (2.57%). The multiplication factor of the finite system will be less than this value. Two different finite, cylindrical models of a collapsed core are also presented in this report. Both are believed to be conservative, but these models are much more realistic than the licensing or the infinite models, and are calculated to have multiplication factors between 0.86-0.90 for a boron concentration of 4350 ppm. The actual multiplication factor of the TMl-2 core is expected to be lower than this estimate by 5 or 10%. Also, because the concept of criticality or neutron multiplication is not obvious or well known, some additional background information is provided, along with related ideas leading to a measure of the margin of safety. To assure that an adequate liquid level and boron concentration are maintained in the reactor vessel, certain measurements must be made periodically, and equipment must be in place prior to defueling activities. The approach of the TMI-2 recovery program to these matters has been found to be adequate and is discussed briefly here. The conclusion of this review is that the reactor is now very far subcritical with a boron concentration of 4350 ppm or more, and no conceivable rearrangement of fuel can create a critical state. Careful administrative control to maintain the boron concentration of the reactor coolant close to 5000 ppm, and controls to rigorously exclude addition of unborated water to the primary system, provide additional assurance that subcriticality will be maintained. The immediate corollary is that the defueling of the reactor vessel can proceed as planned, with complete confidence that such operations will remain subcritical. (author)
- Research Organization:
- U.S. Department of Energy, Nuclear Criticality Information System, Lawrence Livermore National Laboratory, Livermore, CA (United States)
- DOE Contract Number:
- Contract W-740S-Eng-48
- OSTI ID:
- 22082899
- Report Number(s):
- DOE/NCT-01; TRN: US13R0001041917
- Resource Relation:
- Other Information: Country of input: International Atomic Energy Agency (IAEA)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
BURNABLE POISONS
CLADDING
COMPUTER CODES
CONTROL
COOLANTS
FISSION PRODUCTS
FUEL PINS
MATHEMATICAL MODELS
MULTIPLICATION FACTORS
REACTOR ACCIDENTS
REACTOR COOLING SYSTEMS
REACTOR CORES
REACTOR VESSELS
REVIEWS
SAFETY MARGINS
SLIGHTLY ENRICHED URANIUM
THREE MILE ISLAND-2 REACTOR
URANIUM 235
URANIUM DIOXIDE