XYLENE POWER LTD.

FNR ASME Code Issues

By Charles Rhodes, P.Eng., Ph.D.

Today you raised the issue of ASME requirements for material traceability, because ASME is integral to significant bodies of law. First let me say that I encountered this issue in a previous life when I was involved in manufacture of stainless steel fired pressure vessels containing pressure tubes. I am more than aware of how onerous it is, because I was the one who had to certify that the alloys used in each pressure vessel were compliant with relevant ASME standards.

With respect to FNRs to achieve economy we need to minimize ASME involvement. There are a number of categories of materials. Please indicate those that you think will require source traceability and why.

1) The fuel tubes. These fully enclose the fuel rods. Since they touch the fuel rods source traceability will almost certainly be required.

2) The fuel bundle. Each fuel bundle holds either 280 and 416 fuel tubes in an assembly that weighs several tonnes. There are two issues with the fuel bundles. Sustained strength under fast neutron radiation and swelling due to fast neutron radiation. The material is Fe + 12%Cr. A fuel bundle structural failure could potentially lead to an accident. For that reason I would want to closely monitor the fuel bundle material and fuel bundle strength tests, whether or not such monitoring is required by ASME.

ALL THE SUBSEQUENT ITEMS ARE PROTECTED FROM CUMULATIVE NEUTRON DAMAGE

3) The steel lattice. The steel lattice physically supports the fuel bundles in the primary liquid sodium pool. It is more than 3 m from the reactor core zone. It is important that it not structurally fail but otherwise the main concern is that it be compatible with high temperature liquid sodium and have a predictable thermal coefficient of expansion. In essence it serves the same purpose as the legs on a commercial boiler. I do not know what position if any ASME would take with respect to it. However, maintaining source traceability would be a real pain in the neck due to the multiple parties being involved in its manufacture.

4) The stainless steel pool liner. This liner is fabricated from 3/4 inch thick 304L stainless steel plate that is preshaped but field welded with deep penetration welding. There are a lot of tonnes of that plate and source tracing all the plates and maintaining knowledge of which plate came from where would be very expensive. It makes sense to x-ray all the welds but beyond that quality control is less important because that liner has two backup liners, so the public safety consequences of an interior liner failure are essentially zero. However, an interior liner failure would still cost a lot of money, so some control on the 304L material is justified.

5) The liquid sodium. Here my concern is primarily with difficult to remove suspended impurities that would become neutron activated. Impurities that sink or float are much easier to remove. I do not know if there are any applicable ASME codes.

6) Intermediate fuel bundle heat exchange tubes. These tubes are exposed to both the primary and secondary sodium. The material is a high nickel alloy in order to withstand thermal stress. These tubes normally operate with a pressure difference of 0,1 MPa. However, in the event of a steam generator tube failure this pressure difference could briefly reach 4.6 MPa. I do not know how ASME would characterize this except to require a safety pressure test of about 8 MPa.

7) The intermediate heat exchange manifolds. These are cast pieces that are subsequently precision machined in a manner similar to the engine blocks of a large truck. They would likely be made in a similar plant using similar equipment. I do not know what position ASME would take with respect to these manifolds. A leak is of little consequence unless it occurs simultaneous with a steam generator tube failure.

8) Secondary sodium pipe. This pipe should be off the shelf 12.5 inch OD schedule 80 stainless steel pipe that is factory welded to the intermediate heat exchange manifolds. The major issue is that the weld must reliably withstand a 4.6 MPa pressure surge.

9) Steel columns located in the primary liquid sodium pool that support the weight of the intermediate heat exchange bundles. It is contemplated that off-the-shelf structural steel would be used.

Please give me your view as to the applicability of nuclear specific ASME codes to each of the above items. I have serious concerns that unwarranted application of ASME nuclear codes to non-nuclear items might seriously threaten the entire project.
 

This web page last updated March 16, 2020