1. Burst Disc Review
Closeout
21 May 2019
J.G. Weisend II, Chairman

2. Burst Disc Review - J.G. Weisend II
General Comments
The FREIA Team did a thorough analysis of the event and presented a very open explanation of the event and its causes.
The conditions of the CM during the FREIA burst disc event were: 4 K operation, the RF was on for 20 seconds after the cavity quench. The mechanical safety valves were only sized to have a 2 – 3 g/s. In the series, the safety valves are sized for
70 g /s.
During the event FREIA was monitoring the forward power of one cavity while exciting a different cavity. The transmitted power was measured on the correct cavity but a software error resulted in a incorrect power calculation. The result of these errors is that the active cavity was overpowered and quenched.
FREIA has put in a number of mitigations: including more instrumentation, better parameter summaries, additional interlocks including high speed pressure interlocks to the RF power, operation at 2 K and additional trained staff.
May 2019
Burst Disc Review - J.G. Weisend II

3. Burst Disc Review - J.G. Weisend II
General Comments
In the FREIA test stand TAOs were on the He pumping line which has a larger control valve than the series valve boxes. This has been fixed by the addition of another volume to the circuit. It is hoped that we will not see the TAOS at the series version of the valve boxes which have a smaller control valve.
The explanation of the CEA events is that due to the operation of the heat exchanger with a 2 phase flow, the system was very unstable (this is a theory) which leads to a desire to fill the 2 phase line line very full. This has likely resulted in over filling the two phase line results in cold He coming in contact with a warm wall which then results in a rapid pressure rise.
There is a very detailed report that explains the burst disc opening of the elliptical prototype cryomodule. This report will be released soon by CEA.
May 2019
Burst Disc Review - J.G. Weisend II

4. Answers to Charge Questions
Do we understand the reasons for the opening of the burst discs at both the CEA test stand and the FRIEA test stand?
Not yet. There are plausible explanations for both events with more details available for the FREIA event. Additional analysis includeing the release of the CEA report is needed
2) In each case, did the burst discs function properly i.e. open at the correct pressure and protect the 2 K circuit and SRF cavity from damage?
Yes
May 2019
Burst Disc Review - J.G. Weisend II

5. Answers to Charge Questions
3) Is it likely that we will see similar events during operations of the series cryomodules at test stands and in the linac? Do we understand what type of protection functions we need to make sure are in place at ESS to prevent the unexpected opening in absence of a safety issue? Are the plans for their implementation in place?
Its likely that we may see these events again. We do believe that the various suggested solutions will help prevent these problems but these need to be verified at the test stands.( FREIA and ESS TS2) Further details of the implementation need to be developed.
May 2019
Burst Disc Review - J.G. Weisend II

6. Answers to Charge Questions
4) Is the relationship between nominal operating pressure, relief valve opening pressure, burst disc opening pressure, cavity material limits and Maximum Allowable Working Pressure correct or can it be improved? Any possibility of increasing the set pressure of the burst discs should be considered carefully.
May 2019
Burst Disc Review - J.G. Weisend II

7. Answers to Charge Questions
5) Are there any design or operating changes that can be made which will reduce the possibility of the burst discs opening while still maintaining safety? Yes, a number of changes have been proposed and further studies and tests are needed.
May 2019
Burst Disc Review - J.G. Weisend II

8. Answers to Charge Questions
6) Have any outstanding safety issues been identified? No
May 2019
Burst Disc Review - J.G. Weisend II

9. Burst Disc Review - J.G. Weisend II
Recommendations
Increase the data acquisition speed of the pressure measurements in FREIA to faster than 1 Hz.
Consider not using a Self Excited Loop for testing even if the LLRF is not ready. Use the more tedius but less risky approach at FREIA.
The CEA report on the burst disc events should to be finished and released within the next 2 weeks
Optimize the pressure setting of the control valve that protects the burst disc,. The impact of this setting including the impact of a power outage would need to be carefully considered.
Test the pressure setting of the control valve prior to cooldown
FREIA should replace its spring loaded safety valve on the pototype spoke cryomodule with a larger one than can protect from a 50 gram per second flow
CEA should model the effect of adding kW’s into the helium surrounding the cavity to see how this affects pressure rise.
Consider the impact of not pressure testing the spoke cavities, provide the logic supporting this decision
May 2019
Burst Disc Review - J.G. Weisend II

10. Burst Disc Review - J.G. Weisend II
Recommendations
ESS should should develop a quench detection system
Consider the use of other mechanical reclosable devices
May 2019
Burst Disc Review - J.G. Weisend II

11. Burst Disc Review - J.G. Weisend II
One Last Comment
The Chair recognizes and thanks the ESS, CEA, IPN Orsay & FREIA team for all their hard work both in conducting the analysis and in preparing for the review.
The Chair also thanks the committee for their service and time in participating in this review
May 2019
Burst Disc Review - J.G. Weisend II