ML20205Q488
| ML20205Q488 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 05/07/1986 |
| From: | Dean R GENERAL ATOMICS (FORMERLY GA TECHNOLOGIES, INC./GENER |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20205Q479 | List: |
| References | |
| TAC-60400, NUDOCS 8605280375 | |
| Download: ML20205Q488 (9) | |
Text
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b G A Technologies Inc.
1025 Connectcut Avenue N.W.
Washington. D.C. 20036 (202) 659-3140 May 7, 1986 Mr. Harold R. Denton Director Office of Nuclear Beactor Regulation Nuclear Regulatory h insion
-- -7920 Norfolk Avenue Bethesda, Maryland 20555
Dear Mr. Denton:
We recognize that the press of business and your travel schedule prevented our meeting with you to discuss the Chemobyl incident and
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the lack of similarities between Chernobyl and the high tenperature gas reactor concepts currently being developed in the United States.
EM M, for your information, is material that we have prepared 1
in San Diego and that we would have used as a talking paper had we had
-the opportunity to meet with you. We would still like to discuss this matter with you at your ocrivenience.
S ly, ae i _
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! Richard A. Dean Senior Vice President PAD /sb Enclosure l
8605280375 860514 PDR ADOCK 05000267 p
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5/1/86 THE HTGR AND THE CHERNOBYL DISASTER Graphite, which is stable at high temperatures and provides a i
high heat storage capacity, has long been considered a means for improving reactor performance and safety.
Currently, a Modular High Temperature Gas-Cooled Reactor (MHTGR) is being designed which takes advantage of 'these graphite properties to attain a
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passively safe system, i.e, the safety of the public is assured under all conditions without requiring any operator action or the operation of any active systems (such as pumps, valves, motors, etc.).
The Chernobyl nuclear disaster and the subsequent fire has raised questions regarding the design similarities and differences between the RBMK (Chernobyl Type) Boiling Water Graphite Moderated Reactor, the METGR, and other reactor systems.
In particular, the repofter graphite fire has drawn attention to the MHTGR which also uses graphite as a moderator.
Study of the Chernobyl reactor design, and of the information available on the accident indicate that graphite was not the cause 5 of the accident.
Further, the consensus is that even if graphite hid not been used as a. moderator on that reactor, the accident, explosion, and release of radioactivity would still have occurred.
The Chernobyl RBMK reactor is a pressure-tube type of reactor using zirconium alloy clad uranium oxide pellets as fuel, boiling light water as the coolant, and graphite as the moderator (Figure 1). ~ The fuel assemblies which are similar to light water reactor fuel assemblies consist of a cluster of 18 clad fuel pins located in vertical zirconium alloy process tubes through which the cooling water is circulated (Figure 2).
The reactor is designed to perform.on-line refueling.
The reactor coolant inlet and outlet temperatures and pressures are essentially identical to those for U.S.
boiling water reactors. The approximately 1700 process tubes which serve as the pressure boundary for the system pass through stacked graphite moderator blocks.
The reactor is surrounded by concrete and steel shielding and support structures.
In general, the RBMK reactor is similar to the Hanford "N"
reactor.
However, the "N"
reactor is a Pressurized Water Graphite Moderated Reactor.
In addition, the "N"
reactor has horizontal pressure tubes and the refueling operation is performed off-line.
The HTGR uses ceramic coated particle fuel, helium gas as a coolant, and graphite as the moderator and core structural material (Figure 3).
The fuel particles consist of microspheres of uranium carbide (or uranium oxy carbide) clad with pyrolytic l
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carbon and silicon carbide, which form a barrier and pressure vessel to retain fission products.
These ceramic coatings are stable to very high temperatures.
The coated particles are bonded together with a graphitic binder and inserted into large graphite fuel element blocks.
The helium gas passes through coolant holes in the fuel element blocks.
Although the coolant pressure is lower than that for RBMK, "N",
or BWR type reactors, the use of the inert helium allows operation of the reactor at a higher temperature.
The pressure boundary for the M'odular HTGR system is a steel reactor vessel which is contained in a below ground silo 4
(Figure 3).
(In the case of the Fort St. Vrain HTGR, the reactor vessel is a massive, thick-walled Prestressed Concrete Reactor vessel.)
N Is indicated above (and on Table 1), the RBMK and HTGR are significantly different types of reactors.
These differences are al-so--reflected in their susceptibility and response to accident conditions which could lead to the release from the plant of radioactivity or to a graphite fire.
Key factors in the different accident responses of the
, reactor systems include:
Heatuo Rate With Loss of Coolant The heatup rate of the fuel in the HTGR is very slow since the fuel and moderator are integral and the heatup rate is determined by the heat capacity of
- - the massive graphite moderator.
The RBMK heatup rate of the fuel and cladding is rapid since the heat capacity of the fuel assemblies is small and the fuel assemblies are thermally isolated from the graphite moderator.
Temperature stability and Marcins The ceramic pyrocarbon and silicon carbide fuel particle coatings (i.e., cladding) in the HTGR are stable and rgtain fission groducts to temperatures of over 3200 F, about 1200 F above the normal peak I
operating temperatures.
Tge graphite core structure gains in strength to 4500 F and is structurally 1
sound to over 5400,F.
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The zirconium alloy fuel cladding of the RBMK. loses strength and relgaces fission produgts at approximately 1300 F, approximately 800 F above the normal peak operating temperature.
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Chemical Interactions 1
1 In the HTGR, the helium coolant is inert and does I
not react with the graphite, fuel coatings, or other system components.
l In the RBMK, an exothermic (heat producing) chemical reaction between the water coolant and the zirgonium i
alloy fuel cladding is initiated at about 1800 F.
It is difficult to induce a self-sustaining burning of the dense, massive graphite shapes used in either the HTGR or RBMK type reactors.
Studies of the graphite combustion process (supported by the DOE since-1969) have shown that even under ideal controlled conditions in which an ample supply of gxygen or air is available, graphite temperatures in excess of 1300 F are required.
These conditions do not exist in normal operation of the systems and would be expected to result only as an ultimate consequence of the major accidents which could both raise the temperature of the graphitt moderator and cause multiple breaches the barriers, thus permitting air contact with the graphite.
(A double breach of the steel pressure vessel would be required in the MHTGR.)
The buildup of Wigner energy in graphite, which can lead to a rapid. buildup in temperature upon release, is not a consideration
.' in'the HTGR and likely not in the RBMK reactor (assuming it was operated at the reported temperature).
Both reactors normally operate at temperatures at which the energy is annealed out of the graphite continuously.
The accidents which would result in major releases of fission products are those which could result in a failure of the fuel cladding (or particle coatings) and the failure of the pressure.
boundary (pressure vessel or pressure tube).
The accidents of major concern are those involving a loss or blockage of coolant for water-cooled systems or depressurization accidents for gas-cooled reactors.
In the case of the RBMK, coolant system blockages or breaks are the most likely conditions which would lead to a loss of coolant accident.
The limited heat storage capacity of the RBMK fuel rods (and the thermal isolation from the graphite mass) can lead to failure of the zirconium alloy clad and the release of fission products into the system in a matter of minutes.
Further damage to the cooling system and/or exothermic chemical reactions (such as the zirconium water reaction) could occur which could lead to a breach of the primary pressure boundary, to radiation release from the plant, to an increase in graphite temperature, to air ingress, and ultimately to graphite burning.
In addition, it also appears that the RBMK reactors are i
susceptible to reactivity accidents (a rapi,d increase in power with loss of water) which could lead to rapid steam formation and the rapid overpressurization of the system.
The loss of coolant accidents of concern for the HTGR involve j
the depressurization of the core, along with the loss of all l
active means of circulating the coolant or of removing heat from the core.
Since in the HTGR the fuel particles are physically and thermally coupled with the massive core and reflector graphite, the fuel temperature increases very slowly.
Further, sufficient
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heat is removed from the core by passive radiation and conduction to the surroundings that the Modular HTGR core temperatures never reach temperatures at which the ceramic cladding will fail or i
release fission products within the system.
(At Fort St. Vrain hours are available to take corrective action before the initial failure of the fuel coatings.)
chemical reactions which could lead to an explosion are essentially precluded in the HTGR since the system remains blanketed with the inert helium coolant and the core components do not chemically react with each other.
The introduction of water into the system could lead to the formation of hydrogen and carbon monoxide; however, this reaction is endothermic (absorbs heat) and is self-limiting.
T A large sustained '. air-graphite reaction is also precluded since the Modular HTGR is designed to be contained within a below ground silo.
This type of installation limits the quantity of air that would be available to sustain an air-graphite reaction.
(In the case of Fort St. Vrain, the amount of air is limited by the massive prestressed concrete reactor vessel.)
Further, studies of the combustion of HTGR fuels have shown that the fuel particles retain their capibility to retain fission products even if the graphite has been burned away.
Thus, the nature of the fuel system, coolant, and plant arrangement for the HTGR, which are dramatically different from the RBMK type system, essentially preclude the possibility of a sustained graphite fire such as reported to have occured at Chernobyl and, more importantly, prevent accident consequences which would result in significant release of radioactivity.
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