ML20049A130
| ML20049A130 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 11/13/1978 |
| From: | Counsil W NORTHEAST UTILITIES |
| To: | Reid R Office of Nuclear Reactor Regulation |
| References | |
| TAC-46174, NUDOCS 7811210232 | |
| Download: ML20049A130 (6) | |
Text
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- ::='r :r: " '2llll November 13, 1978 Docket No. 50-336 Director of Nuclear Reactor Regulation Attn:
Mr. R. Reid, Chief Operating Reactors Branen #4 U. S. Nuclear Regulatory Commission Washington, D.C.
20555
References:
(1)
W. G. Counsil letter to R. Reid dated June 6,1978.
(2)
W. G. Counsil letter to R. Reid dated July 31, 1978.
Gentlemen:
Millstone Nuclear Power Station, Unit No. 2 Neutron Shielding In Reference (1), Northeast Nuclear Energy Company (NNECO) provided a conceptual description of the proposed neutron shield design.
It was also indicated that detailed design work and supporting analyses were being completed in support of installation during the second refueling outage.
Since the date of Reference (1), many of the design details were finalized.
In an attempt to conclusively and decisively address historical Staff concerns in neutron shield designs, the complexity and structural rigidity of the Reference (1) design increased dramatically. The net effect of this complication is that this shield is impractical to install, principally because of ALARA considerations.
In response to this situation, NNECO has devoted considerable resources in develop-ment of an alternate shield design, which is discussed in Attachment 1.
In recog-nition of the ALARA philosophy, this shield requires minimal modification of existing plant hardware to install, and subsequent removals and installations can be accomplished without significant personnel exposure. As described in detail in the Attachment, the shield cor.sists of segmented tanks of water, which span the gap between the reactor vessel and primary shield wall.
In spite of the decreased amount of time available to design, procure, and in-stall this shield, it remains the intention of NNECO to have the shield in use prior to the start of Cycle 3 operation. Your expeditious review and comment of the attached material will greatly facilitate this endeavor. Verbal communi-cations with the Staff will continue in support of the target installation date.
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. o Concerning the requirements of 10CFR170, the NNECO comments expressed in Reference (2) remain applicable.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY
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yrfA m r o W.'G. Coun511 Vice President Attachment
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_3-Shield Activation The neutron shield will be composed of stainless steel. Large amounts of this material are used for the refueling cavity liner plate in the same areas as the shield. This small quantity of activated stainless steel will not cause a signi-ficant amount of additional personnel exposure.
Tank Leakage A vertical sheet metal stainless steel weir between the shield and the head insula-tion and the top of the cavity insulation form a drop pan which will channel any leakage away f rom the vessel nr the uninsulated primary piping. If a more serious leak developed, the bulk of tL ' akage would still be channeled off through the leak chase. Any minor leakage ther tgh seams in the insulation would vaporize due to the elevated temperatures in ta. eactor cavity before coming in contact with the vessel.
The tanks are compartmented so that leakage cannot drain a whole tank.
There are approximately 160 gallons per compartment.
ALARA Considerations The neutron shield is designed to minimize personnel exposure during initial installa-tion and subsequent removal and installation for refueling. No modifications or setting of anchor bolts is required in the high radiatior. areas around the reactor head. This will minimize personnel exposure during initLal installation. By climinating the need for bolting or unbolting the shield, it can quickly be moved for maintenance or refueling thus minimizing plant personnel exposure.
The only plant modification associated with this task is the construction of a shield storage platform on top of the steam generator shield wall. This is a low radiation area and this task will not result in a significant personnel expos ur e.
Storage of the shield on top of the steam generator shield wall in an area remote from areas which are normally occupied during outages will also minimize personnel exposure from activation of the shield tanks.
l Cavity Ventilation The neutron shield will have no adverse ef fect on cavity ventilation or ambient containment temperatures.
l
ATTACHMENT 1 MILLSTONE UNIT NO. 2 PROPOSED NEUTRON SHIELDING DESCRIPTION The proposed neutron shield for Millstone Unit No. 2 is composed of two (2) compartmented water tanks and is located at the reactor vessel flange elevation (see Figure 1).
Each tank is semi-circular in shape and bridges the gap between the reactor vessel flange and the refueling cavity floor. The inside edge of each tank is equipped with a "C" clamp which slips under the reactor vessel flange and retains the tanks during a postulated LOCA. Once the two tanks are in place, they are fastened together with four (4) pins. This connection gives the entire structure hoop integrity.
The sides of the tank are constructed of one-half inch thick stainless steel plate.
Stainless steel gusset plates are installed in the tank at regular in-tervals. These plates provide the tank with structural strength and also divide each of the semi-circular tanks into eight (8) individual water-tight compartments.
This arrangement prevents loss of-the entire inventory of the tank should leakage develop in any one compartment. The bottom of the tanks are constructured of one-eighth inch stainless steel plate. A notch is machined into the bottom plate of each comparmment. Should a LOCA occur in the reactor cavity, the dif ferential pressure across the plate will cause the plate to rupture and relieve the pressure in the cavity. The shielding structure itself will be retained in place by the "C" clamp under the vessel flange. The top of the tank will be covered with a notched plate similar to the bottom plate.
The bottoms of the tanks are insulated with reflective thermal insulation to maintain the water inventory below 212*F.
The insulation has joints at the edge and in the middle of each tank compartment. During a LOCA, the insulation is displaced through the ruptured tank bottom clearing the cavity for venting.
A small air gap is provided between the insulation and the shield tanks. Reactor cavity cooling air is directed between the insulation and the tanks and serves as the principle cooling source to maintain the water inventory below 212*F.
Other design features and their relationship to potential concerns are discussed below.
DESIGN BASES / CONSIDERATIONS Reactor Cavity Pressurization Peak post-LOCA reactor cavity pressure have been calculated using RELAP 4 to en-sure that placement of the shield at the reactor vessel flange elevation will not cause post-LOCA cavity pressure to exceed design pressure. Nodal sensitivity _
studies were performed to ensure that the reactor cavity has been divided into a sufficient number of nodes.
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A conservative double-ended cold leg guillotine break with a one-millisecond break opening time was determined to be the design break.
l Missile Generation As described above, the bottoms of the shield tanks rupture to provide a post-LOCA vent space for the cavity. The shield structure itself is retained in place and is not a potential missile. The thermal insulation which is displaced through the ruptured taak bottoms is light weight (approximately thirty pounds per panel) and has little structural integrity. Because of this, thermal insula-tion is not considered a credible missile.
In addition, the bulk of the insula-tion will be stopped by the missile shield. The missile question for reactor cavity thermal insulation was addressed in a letter from D. C. Switzer to V. Stello, Jr., dated February 22, 1978.
Sump Clogging As stated above, the shielding structure is retained following a LOCA and will not migrate to the containment sump area. Most of the thermal insulation which is displaced to clear the cavity will be stopped by the missile shield. Any insula-tion which escaped the refueling pool would most likely come to rest on the opera-ting floor, or some intermediate level in the containment since it is a long and tortuous path from the operating floor to the sump.
Should any insulation manage to migrate to the sump, it will rapidly fill with water and harmlessly sink.
Fire Hazard The neutron shield is composed entirely of stainless steel and water. No fire hazard exists.
Hydrogen Generation Fhterials such as aluminum, zinc, or other materials which generate hydrogen in a post LOCA environment are not employed in this shield design.
I Neutron Attenuation l
Approximately sixteen inches (16") of unborated water will be used in this shield design.
Calculations have shown that this amount of water will result in a neutron attenuation factor greater than 40.
Utilization of the shield minimizes neutron radiation levels throughout the containment and permits discretionary personnel access to the containment during power operations.
Containment Pressurization The effect of the water contained in the proposed neutron shield design on the containment peak pressure analysis has been assessed in some preliminary calcula-tions using the Contempt LT-016 computer code.
Two (2) analyses have been run, one assuming all water is added to the containment vapor region, and one assuming all water is added directly to the sump. A conservative amount of water (2500 gallons) at a conservative temperature (200*F) was added within 0.3 seconds of the start of i
the postulated accident. A maximum change in pressure of 0.26 psi occurred due to the presence of the water. This will not cause post LOCA containment pressure l
to exceed design pressure.
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