Tritium and Safety Issues for Power Plants

1. Tritium and Safety Issues for Power Plants Lee Cadwallader Fusion Safety Program ARIES Meeting Bethesda, MD April 4-5, 2011

2. Tritium and Safety Issues to Address from October 2010 ARIES meeting • Guidelines for tritium release, concentrations, inventory • Compare these to ITER • ARIES and these guidelines • Define the confinement boundary • Impact on cost of components • The vacuum vessel is a safety-related component • What others should be safety-related components?

3. Routine Tritium Releases • US fission power plants release, on average, ~130 Ci (13 mg or ~0.2 μSv) of airborne HTO and ~260 Ci of liquid HTO each year. • ITER’s present routine release goal is < 2 g-T/year, usually HT but assumed to convert to HTO, stacked release to the environment. • To put this in perspective, a rough estimate for a 1.2 km distance, 30-m stack, releases of airborne HTO would be ~6.25 g-T/year to yield the ITER goal of 0.1 mSv/year. • There are other radioisotopes that must be included in this release calculation to obtain the total annual dose, such as activated air (N-16, Ar-41, etc). ALARA principles keep releases as low as possible.

4. US Regs and ITER Goals

5. Tritium Inventories in Storage • The US NRC does not have any specific regulations on tritium storage, but DOE HDBK-1129 directs that in handling tritium it is to be captured, contained and cleaned up. Proposed methods: • Multiple confinement barriers • A tritium removal system associated with the second barrier • No one segmented inventory of tritium should be more than the tritium release amount that would give 10 mSv (1 rem) dose at the site boundary (for ITER, a stacked accidental release of ~120 g-T as HTO gives 10 mSv at a distance of 1.2 km) • Inventories must be isolated from each other so multiple inventories cannot be released simultaneously.

6. A Large DOE Tritium Facility • The largest tritium inventory limit at a DOE facility is the SRNL Tritium Extraction Facility (TEF) at 7 kg-T. The facility produces 2-3 kg-T/year and has a similar amount of storage on site (this tritium processing plant similar in many aspects to the ITER tritium plant). The TEF came on-line in late 2006, $500M cost. • The TEF tritium inventory is kept in elemental form, and it is segmented within the various processing steps and on Z-beds in the facility. • The T2 quantities in all segments of the plant are not published. The TEF hazards assessment gives the largest single inventory of 450 g-T, and for its dose calculations used an accident release of 750 g-T. Given the TEF remote site (>7 mile site boundary), 750 g-T results in ~5 mSv dose to a member of the public at the SRS boundary.

7. ITER Tritium Inventories in Various Confinement Zones • ITER tritium confinement principles are similar to those called out given in DOE HDBK-1129 . The highest, most mobile inventories require 2 strong barriers. • For ITER, the largest threat to tritium inventories in the tritium building is fire. ITER has mandated that no one room in the T plant can contain more than 700 g-T, and no fire zone can contain more than 70 g-T mobilizable. • Note that the maximum inventories are not all present at the same time

8. ARIES Meets these Guidelines • ARIES AT estimate of Vacuum Vessel inventory (from Petti et al., FE&D, v80, 2006, p111-137) was: • 745 g-T, only 205 g-T is easily mobilized in an accident • ARIES CS estimate of Vacuum Vessel inventory (from Merrill et al., FS&T, v54, 2008, p838-863) was: • 1,590 g-T, assuming first wall T implantation, 205 g-T easily mobilized • ARIES AT met the no-evacuation 10 mSv dose limit if the release is isolated within 30 minutes. Po-210 from Bi in LiPb was a larger safety concern than the tritium. ARIES AT did not include on-line Bi or Po-210 removal. • ARIES CS met the 10 mSv dose limit if the release is isolated within 2 hours (for ITER, confinement building isolation occurs within 30 s based on pressure/radiation set-points being reached or one hour manually after an accident begins). • ARIES safety assumptions compare well with ITER plans, although ARIES focused on the nuclear island and did not include the tritium processing plant; perhaps in future assessments it should be included.

9. Define Confinement Boundaries • Confinement boundaries are boundaries derived from the defense-in-depth confinement strategy called out in the MFE Fusion Safety Standard. They are typically any one of several enclosures/structures placed between radioactive inventories and the general public. Structures include the vacuum vessel, reactor confinement building, glove boxes, etc. • These boundaries are either a strong boundary, a low failure probability structure designed to withstand large forces (e.g., gas/dust explosions), or a weak boundary, a higher failure probability structure designed to accommodate lower forces (e.g., a glove box). The number and classification of required confinement boundaries depends on the amount of radioactivity to be confined and the frequency of, and type of, events that challenge these boundaries.

10. ITER Confinement Boundaries Set a Precedent ITER’s 2nd boundary (building) is also considered a strong boundary ITER’s 1st boundary (VV) is considered a strong boundary

11. Impact on Cost for Confinement Boundaries • The ITER VV is made of 316LN. The VV will be licensed according to the French Order for Equipements sous Pression Nucleaire (ESPN), a cost driver (ITER team believes 10-20% cost increase over the French Pressure Equipment Directive vessels), but probably comparable in cost to ASME N-Stamp equipment. • The ITER VV procurement arrangement value with the EU is 150M Euros (~$215M). VV ports are another 80M Euros (~$115M). ROK is another ~65M Euros. This totals to nearly 6% of the project capital cost of 5B Euros. • In comparison, the US NGNP primary helium system (reactor vessel of 2¼ Cr 1Mo steel, hx, circulator, piping) would be about $250M, out of a total project capital cost of $3.5B, or ~7%. A 1995 LWR cost estimate showed the primary system was ~$280M or 6% of the estimated plant project cost of $4.6B.

12. Conclusions • Facilities in general do not approach their legal limits for releases. Designing for routine releases at the annual legal limit is not demonstrating ALARA. • ITER has set conservatively low release and exposure goals, like ARIES, which sets a precedent for future facilities. • Individual tritium inventories should be set below that amount which would give the 10 mSv no-evacuation dose limit. • ITER confinement strategy has matured over time and is standing up to regulatory scrutiny. • Vacuum vessel safety provisions add to the cost but not more than an estimated 10 to 20%.

13. Future Directions • Because the cost adder for normal industrial equipment versus ASME N-Stamped/ESPN equipment, and safety importance components are important to the ongoing systems code effort, I am going to perform a more in-depth study of this area to see if more accurate estimates can be developed. • Work on the US DCLL test blanket module has shown higher than desired maintenance personnel doses for the LiPb system. The TBM work has also given insight to construction and inspection requirements. • Based on these findings, I would like to have the ARIES team help me in investigating PHTS design options that improve liquid breeder PHTS maintenance safety of these systems, compared to ceramic breeder systems, including the impact on plant costs for the systems code.

14. Future Directions • ARIES-CS analysis showed permeation rates for implanted tritium were high, more tritium permeating through the blanket first wall (FW) than bred in the blanket. • We should first verify the FW implantation flux is really as high as O. V. Ogorodnikova claims, and if correct, look at ways to reduce all permeation paths into the helium coolant of a DCLL concept, e.g., a thin clad of W on the FW and walls of the breeder channel, design options like a vacuum permeator inside the VV …. • An ASME Section III code committee is developing design rules for HTGRs and Fusion power plants. We expect the NRC will adopt these rules. ARIES should be involved in guiding this development process to preserve design flexibility and allow for future innovation within these design rules. • The INL FSP is involved in this work. It would be a good idea for the ARIES design team to have a presence on the design rules committee.